Quaternary Magmatism in the Cascades—

Geologic Perspectives

Professional Paper 1744

U.S. Department of the Interior

U.S. Geological Survey

U S S G 
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Quaternary Magmatism in the Cascades—

Geologic Perspectives

By Wes Hildreth

Professional Paper 1744

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia 2007

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Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, July 19, 2007

Text edited by James W. Hendley II

Layout and design by Judy Weathers

Cover—Distributed arc volcanoes in the Cascades of central Oregon. At least 23 separate vents are discernible in this 

southwestward view from low on the southern slope of South Sister. Vent belt in this continuous reach of the northsouth arc is about 40 km wide, east to west. The numerous small volcanoes here consist predominantly of basaltic 

andesite, but many are basaltic, a few are dacitic, and (in contrast to most of the arc) several dozen are rhyolitic. In 

foreground, Rock Mesa rhyolite coulee and an adjacent rhyolite dome, both about 2,200 years old, each contain 73.5 

percent SiO2. Partly bare cone just beyond the rhyolite flow is early postglacial LeConte Crater, a scoria cone at the 

source of an extensive apron of mafic lava flows (54.5% SiO2

). The Cascade arc includes more than 2,300 Quaternary 

volcanoes, at least 2,050 of them independent but fewer than 30 of them the andesite-dacite or mafic stratovolcanoes 

conventionally thought to represent the arc. (Photograph by author taken in July 2001.

Frontispiece—Burney Mountain, 37 km north-northwest of Lassen Peak, is by far the largest of ~340 Quaternary lava 

domes recognized in the Cascades. The middle Pleistocene composite dacite dome has a volume of ~9 km3

, which is 

more than three times as large as Lassen Peak or (Shasta) Black Butte (each ~2.5 km3

) and exceeds the amount of dacite 

erupted from such andesite-dacite stratovolcanoes as Mount Adams or Mount Baker. Along with 110 shield volcanoes 

and ~1,850 monogenetic (mostly mafic) vents, individual lava domes greatly outnumber the 30-odd Quaternary stratovolcanoes and major silicic complexes in the Cascades. Relief visible in the image is ~850 m. (Photograph by L.J.P. Muffler, 

taken in May 1981 from near the summit of Doyle Butte, 4 km northeast of the summit of Burney Mountain.)
iii

Foreword

The Cascade magmatic arc is a belt of Quaternary volcanoes that extends 1,250 km from Lassen 

Peak in northern California to Meager Mountain in Canada, above the subduction zone where the 

Juan de Fuca Plate plunges beneath the North American Plate. This Professional Paper presents a 

synthesis of the entire volcanic arc, addressing all 2,300 known Quaternary volcanoes, not just the 

30 or so visually prominent peaks that comprise the volcanic skyline. 

Study of Cascade volcanoes goes back to the geological explorers of the late 19th century and 

the seminal investigations of Howel Williams in the 1920s and 1930s. However, major progress 

and application of modern scientific methods and instrumentation began only in the 1970s with 

the advent of systematic geological, geophysical, and geochemical studies of the entire arc. Initial

stimulus from the USGS Geothermal Research Program was enhanced by the USGS Volcano Hazards 

Program following the 1980 eruption of Mount St. Helens. Together, these two USGS Programs 

have provided more than three decades of stable funding, staffing, and analytical support. This 

Professional Paper summarizes the resultant USGS data sets and integrates them with the parallel

contributions of other investigators. The product is based upon an all-encompassing and definitive 

geological database, including chemical and isotopic analyses to characterize the rocks and 

geochronology to provide the critical time constraints. 

Until now, this massive amount of data has not been summarized, and a systematic and uniform 

interpretation firmly grounded in geological fact has been lacking. Herein lies the primary utility of 

this Cascade volume. It not only will be the mandatory starting point for new workers, but also will 

provide essential geological context to broaden the perspectives of current investigators of specific 

Cascade volcanoes. 

Wes Hildreth’s insightful understanding of volcanic processes and his uncompromising scientific 

integrity make him uniquely qualified to present this synthesis. During more than three decades 

of volcanological studies, he has carried out comprehensive investigations of Mount Adams, 

Mount Baker, the Three Sisters, and the Simcoe Mountains Volcanic Field. He also brings a broad 

experience in other volcanic arcs, having conducted integrated field and laboratory investigations 

at several major volcanic centers in the Andes and the Aleutian arcs. His expertise and perspective 

have been further enhanced by in-depth petrologic studies of caldera environments, primarily in Long 

Valley, California, and Yellowstone. On the basis of all these field and laboratory investigations and 

exhaustive literature searches, he has published three definitive petrologic syntheses addressing the 

passage and transformation of basaltic magmas from their mantle sources through the crust to form 

the many types of volcanic manifestations at the Earth’s surface. 

A major strength of this Professional Paper is that it adheres to data first and foremost, and 

only then correlates these data with relevant theories. Petrological and geophysical interpretation is 

left to the later sections of the volume, and even there is never allowed to stray from the pertinent 

databases. Hildreth’s interpretations are not just idle speculations, but are carefully reasoned 

inferences firmly based on his thorough evaluation of the observational geological data. 

Professional Paper 1744 should not be skimmed lightly, in the hope that the salient points will 

quickly rub off. Instead, every section, indeed every paragraph, presents scholarly observations and 

insightful interpretations that demand careful and thoughtful study. This volume will influence and 

guide the course of Cascade investigations for decades to come. 

Patrick Muffler

Western Regional Geologist, retired
iv 

Contents

Foreword ........................................................................................................................................................iii

Introduction.....................................................................................................................................................1

Terminology and Limitations ...............................................................................................................1

Volcanoes and Vents............................................................................................................................3

Distribution of Quaternary Cascades Volcanoes .....................................................................................4

Garibaldi Volcanic Belt ........................................................................................................................4

Mount Baker.................................................................................................................................9

Glacier Peak..................................................................................................................................9

Mount Garibaldi..........................................................................................................................10

Garibaldi Lake Volcanic Field...................................................................................................11

Mount Cayley..............................................................................................................................11

Mount Meager............................................................................................................................11

Garibaldi Volcanic Belt—Overview........................................................................................13

Mount Rainier to Mount Hood Segment .........................................................................................13

Cascade Axis..............................................................................................................................13

Goat Rocks.........................................................................................................................15

Mount Adams ....................................................................................................................16

Mount Hood .......................................................................................................................17

Forearc.........................................................................................................................................17

Mount Rainier....................................................................................................................17

Mount St. Helens ..............................................................................................................19

Indian Heaven Volcanic Field .........................................................................................19

Diffuse Forearc Volcanism..............................................................................................20

Contrasts with Garibaldi Volcanic Belt .........................................................................21

Oregon Cascades Segment ..............................................................................................................22

Jefferson Reach (I)....................................................................................................................22

Mount Jefferson ...............................................................................................................23

Sisters Reach (II) .......................................................................................................................23

Tumalo Volcanic Field (Bend Highland) ........................................................................27

Broken Top .........................................................................................................................28

Three Sisters .....................................................................................................................28

Mount Bachelor................................................................................................................30

Wickiup Reach (III)....................................................................................................................31

Thielsen Reach (IV) ...................................................................................................................31

Mazama Reach (V) ....................................................................................................................32

Mount Mazama .................................................................................................................32

McLoughlin Reach (VI) .............................................................................................................33

Contrasts Between Oregon and Rainier-to-Hood Segments .............................................34

Shasta Segment..................................................................................................................................34

Mount Shasta .............................................................................................................................35

Lassen Segment..................................................................................................................................37

Lassen Volcanic Center............................................................................................................39
 v

California Comparisons......................................................................................................................40

Contemporaneous Volcanic Fields Behind the Arc ......................................................................41

Simcoe Mountains Volcanic Field...........................................................................................42

Newberry Volcanic Field ..........................................................................................................42

Medicine Lake Volcanic Field..................................................................................................43

Characteristics of Quaternary Volcano Distribution..............................................................................44

Types of Volcanoes.............................................................................................................................44

Vent Redundancy................................................................................................................................48

Distribution Patterns of 2,050 Independent Volcanoes ................................................................53

Breadth of the Cascade Arc .............................................................................................................54

Volcanic Front and Forearc Volcanoes ...........................................................................................55

Spacing of Major Volcanic Loci .......................................................................................................56

Vent Density.........................................................................................................................................57

Along-arc Gaps ...................................................................................................................................57

Why are Rear-Arc Volcanic Fields Few and Clustered? ..............................................................59

Eruption Rates and Episodicity..................................................................................................................62

Output of Distributed Mafic Vents vis-à-vis Long-Lived Evolved Centers.................................63

Eruption Rates at Individual Loci......................................................................................................64

Postglacial Eruptions in the Cascades............................................................................................65

Start-up Times and Major Growth Episodes ..................................................................................72

Outstanding Questions.......................................................................................................................72

Productivity of the Cascades vis-à-vis Other Continental Arcs..................................................75

Inferences Concerning Future Eruptive Activity ...........................................................................77

Parental Magmas.........................................................................................................................................78

High-Alumina Olivine Tholeiite (HAOT) ...........................................................................................78

Intraplate Basalt..................................................................................................................................79

Arc Basalt.............................................................................................................................................79

High-Mg Basaltic Andesite...............................................................................................................79

Shoshonitic Arc Magma....................................................................................................................80

Rhyolite and Dacite.............................................................................................................................80

Sr-Rich Andesite .................................................................................................................................81

Inferences about Mantle Magmatism.............................................................................................81

Intracrustal Evolution of Arc Magmas .....................................................................................................82

MASH Model .......................................................................................................................................82

Shifts of Focus.....................................................................................................................................84

Basaltic Andesite................................................................................................................................84

Andesite and Dacite...........................................................................................................................87

Rhyodacite and Rhyolite....................................................................................................................89

Arc Plutons and Upper Crustal Magma Chambers.......................................................................92

Tectonic and Geophysical Insights...........................................................................................................94

Concave Curvature of the Margin....................................................................................................94

Translation and Rotation of the Forearc..........................................................................................95

Slab Depths and Discontinuities ......................................................................................................96

Young Warm Slab and Influences on Magmatism........................................................................97

Cascadian Basement and Sr-Isotope Overview............................................................................99
vi 

Vent Distribution and Basement Influences ..................................................................................99

K2O Asymmetry Across the Cascade Arc? ...................................................................................101

Continuing Quest for Clarity .....................................................................................................................102

Acknowledgments.....................................................................................................................................104

References..................................................................................................................................................105

Figures

1. Quaternary Cascades volcanic arc ......................................................................................................2

2. Contrasting degrees of erosion of Cascades volcanoes near opposite ends of arc ...................5

3. Garibaldi Volcanic Belt vent map..........................................................................................................8

4. Kulshan caldera and Mount Baker.......................................................................................................9

5. Glacier Peak............................................................................................................................................10

6. Mount Cayley..........................................................................................................................................12

7. Mount Meager........................................................................................................................................12

8. Rainier-to-Hood segment vent map....................................................................................................14

9. Goat Rocks volcanic cluster ................................................................................................................15

10. Mount Adams from the southeast ......................................................................................................16

11. Mount Hood and Sandy Glacier volcano...........................................................................................18

12. Mount St. Helens in 1972 ......................................................................................................................20

13. Indian Heaven Volcanic Field ..............................................................................................................21

14. Oregon segment vent map: northern reaches.................................................................................24

15. Oregon segment vent map: southern reaches ................................................................................25

16. Mount Jefferson.....................................................................................................................................26

17. Little Belknap shield ..............................................................................................................................27

18. Broken Top ..............................................................................................................................................29

19. South Sister summit region..................................................................................................................30

20. Mount Thielsen.......................................................................................................................................32

21. Shasta and Lassen segments vent map ............................................................................................36

22. Goosenest shield....................................................................................................................................38

23. Profiles of selected Cascade volcanoes (two facing pages).........................................................46

24. Mount Adams from the northeast with peripheral vents................................................................53

25. Tectonic setting of the Quaternary Cascade arc..............................................................................60

26. Shifts of eruptive focus.........................................................................................................................85

27. Black Buttes and Mount Baker...........................................................................................................86

28. Chilliwack composite batholith compared with the Lassen volcanic segment .........................94

29. Sr-isotope data for the Quaternary Cascades plotted versus latitude.......................................100

30. K2O versus SiO2 contents for a volcanic transect across southern Washington .....................103

Tables

1. Distribution and spatial density of Quaternary vents.........................................................................6

2. Elevation, relief, and volume of selected Cascade edifices .............................................................7

3. Major fall units from the Quaternary Cascades................................................................................49

4. Spacing of main Quaternary volcanoes.............................................................................................58

5. Gaps along Quaternary Cascade arc .................................................................................................59
 vii

6. Some volumetric eruptive rates for the Quaternary Cascades .....................................................66

7. Postglacial eruptive episodes in the Cascades ...............................................................................67

8. Pulses of elevated eruptive output ....................................................................................................73

9. Start-up times, predecessors, and shifts of eruptive focus ...........................................................74

10. Comparison of three Quaternary continental arcs ..........................................................................76

11. Silicic products at Cascade volcanoes..............................................................................................90
Quaternary Magmatism in the Cascades—

Geologic Perspectives

By Wes Hildreth

Introduction

The geologic view of Cascade magmatism presented here 

is meant to complement flourishing geochemical and geophysical investigations of convergent-margin processes. Advances in 

understanding the behavior of arc volcanoes and the tectonic, 

thermal, and petrologic processes that contribute to arc magmatism have been substantial. Geophysical and geochemical models have clarified much (Gill, 1981; Takahashi, 1986; Jarrard, 

1986; Hildreth and Moorbath, 1988; Tatsumi, 1991; Peacock, 

1991, 1996, 2003; Davies and Stephenson, 1992; Gill and Condomines, 1992; Woodhead and others, 1993; Pearce and Parkinson, 1993; Stolper and Newman, 1994; Zhao and others, 1994; 

Kay and Kay, 1994; Pearce and Peate, 1995; Kirby and others, 1996; Luhr, 1997; Plank and Langmuir, 1998; Sisson and 

Bronto, 1998; Schmidt and Poli, 1998; Grove and others, 2002; 

Hacker and others, 2003; Hildreth, Fierstein, and others, 2004), 

but the geophysical insights are limited by heterogeneities on all 

scales at inaccessible depths, and the geochemical insights are 

blurred by polybaric fractionation, reaction during ascent, and 

mixing of magmatic contributions from varied source rocks that 

partially melt at many depths.

Geologic input has been comparatively neglected. One 

still encounters, for example, convergent-margin models in 

which a magmatic arc is rendered as one or two single-file 

chains of evenly spaced stratovolcanoes. The distribution, 

storage, ascent, and eruption of arc magmas are, of course, 

much more complex, resulting in an awesome variety of 

compositions and regional patterns—widely disregarded in 

modeling. Such patterns provide information about the spatial 

availability of sub-arc magma, its occasional concentration 

in large chambers, and the relative penetrability by magma 

of geologically contrasting segments of the arc crust. Providing reliable data, however, for all the volcanoes in a chosen 

time interval, outlining their distribution, their compositional 

ranges and affinities, and the time-volume-compositional 

evolution of each center, requires authentic geologic mapping, 

petrography and petrochemistry, and integration of map-based 

stratigraphy with detailed geochronology—all of which are 

labor-intensive and time-consuming. Recent advances along 

these lines in the Cascade arc, stimulated principally by 

the U.S. Geological Survey (USGS) Volcano Hazards and 

Geothermal Research Programs, make possible this progress 

report and analysis. Opportunities now exist to treat the timespace distribution of eruptive centers more systematically, to 

examine how such data clarify or delimit the nature of subarc processes, and to integrate such information into better 

geophysical-geochemical models of arc magmatism.

The subject transcends the academic. Volcanic chains on 

populous continental margins are of growing importance, economically and existentially, to expanding civilizations. In the 

Cascades (fig. 1), no fewer than 12 U.S. volcanic centers (Lassen, Shasta, Medicine Lake, Mazama, South Sister, Newberry, 

Hood, Adams, St. Helens, Rainier, Glacier Peak, and Baker) 

are rated as “high threat” or “very high threat” volcanoes in 

the framework for a National Volcano Early Warning System 

(Ewert and others, 2005), and the three large Canadian centers 

(Garibaldi, Cayley, and Meager) should be as well.

I first give an explanation of methodology and terminology and then provide a north-to-south summary of what is 

known of the eruptive history and vent distribution of Cascade 

volcanoes. These basic data are then discussed from the points 

of view of vent-distribution patterns, volumetric eruption rates, 

parental and derivative magmas, and the tectonic framework 

that modulates the magmatism.

Terminology and Limitations

The Cascade magmatic arc includes more than 2,300 

Quaternary volcanoes (table 1). Amplification of this observation at the outset serves to introduce the conventions and 

definitions adopted in this paper. “Volcanoes” are simply sites 

of emission of volcanic products (and the distinction between 

independent and redundant vents is addressed below). Nearly 

all Cascadian volcanoes are magmatic or phreatomagmatic, 

and most constructed positive landforms (cones, rings, shields, 

domes) around their vents. Fewer than 30 of the 2,300 are the 

high peaks (mostly andesite-dacite or mafic stratovolcanoes) 

that provide the popular image of the Cascades. Most mafic 

magmas, however, which tend to retain more information 

about subcrustal or deep-crustal processes, produce small 

shields or scoria cones that are independent of the large centers and are far more numerous.

Owing to the emphasis placed here on volcano distribution, analysis is limited (by and large) to the last million 

years. The Cascade arc has been magmatically active since the 
2 Quaternary Magmatism in the Cascades—Geologic Perspectives

49oN

42oN

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PACIFIC OCEAN

Figure 1. Quaternary Cascades volcanic arc. Red areas encompass more than 2,300 vents for 

more than 2,000 independent volcanoes of Quaternary age, as elaborated in figures 3, 8, 14, 15, and 

21 and discussed in the text. Despite continuity of subduction beneath the 1,250-km-long arc, the 

several gaps in the vent belt are real, containing no or few Quaternary volcanoes. Compositionally 

evolved major centers are indicated by numbered triangles: 1, Mount Meager; 2, Mount Cayley; 

3, Mount Garibaldi; 4, Mount Baker; 5, Glacier Peak; 6, Mount Rainier; 7, Mount St. Helens; 8, Goat 

Rocks; 9, Mount Adams; 10, Mount Hood; 11, Mount Jefferson; 12, Middle Sister; 13, South Sister; 14, 

Broken Top; 15, Cappy Mountain; 16, Mount Mazama (Crater Lake); 17, Rainbow Mountain; 18, Mount 

Shasta; 19, Snow Mountain; 20, Lassen volcanic center; 21, Maidu volcanic center; and 22, Dittmar 

volcanic center. Within the arc vent belt, five distributed volcanic fields are enclosed and indicated 

from north to south as follows: I, Indian Heaven; P, Portland (Boring); T, Tumalo; B, Bachelor chain; 

and C, Caribou. Vent zones for four extensive rear-arc volcanic fields just east of the Cascades are 

enclosed and identified as follows: SM, Simcoe Mountains; N, Newberry; M, Medicine Lake; and H, 

Hackamore. Mafic stratocones, including such large ones as North Sister, Diamond Peak, Mount 

Thielsen, and Mount McLoughlin, are shown on figures 14 and 15, not here. Red arrows offshore 

indicate (latitudinally varied) plate-convergence rate.
Introduction 3

Eocene, but documentation of pre-Quaternary volcanic vents is 

far from complete. Many Tertiary vents are recognized (Luedke 

and others, 1983; Smith, 1993; Sherrod and Smith, 2000), but 

owing to erosion and burial, most are not. Restricting consideration to the Quaternary assures that most vents are accounted 

for and avoids unresolved questions about secular migration of 

the arc. The beginning of the Quaternary is variously placed at 

1.65, 1.8, or 2.6 Ma (Richmond and Fullerton, 1985; Morrison 

and Kukla, 1998; Ogg, 2004; Pillans, 2004), and the boundaries 

between early, middle, and late Pleistocene are now generally 

agreed to be ~780 ka and ~126 ka (Gradstein and others, 2004). 

As a practical matter, most Quaternary vents recognized in 

the Cascades (table 1) are younger than ~1 Ma, ranging from 

perhaps 75 percent of those in California to >90 percent in 

Washington and British Columbia. Delineation and dating of 

Quaternary volcanoes older than 1 Ma is incomplete (owing 

to erosion and burial) and latitudinally very uneven (owing to 

progressively more severe glacial erosion northward).

Quaternary volcanoes of the “Cascade magmatic arc” are 

distributed from southwestern British Columbia to northern 

California (50.9 ̊ to 40.3 ̊N), specifically from ~30 km north 

of Mount Meager to ~25 km south of Lassen Peak (fig. 1). 

The Garibaldi Volcanic Belt (Mount Meager to Glacier Peak) 

is thus included, but the sketchily known Franklin Glacier and 

Silverthrone volcanic fields (Green and others, 1988), far to 

the northwest and ambiguous in their affinity, are not. 

Treated somewhat separately in this analysis are three 

major rear-arc complexes adjacent to but inboard of the northsouth arc belt (fig. 1). Each of these is a “central volcano” in 

the sense of Walker (1993), characterized by a large mafic 

shield, a central rhyolite complex, and a peripheral field of 

hundreds of (compositionally varied) monogenetic vents, 

many of them independent of the centered magma-supply system. These are the Simcoe Mountains Volcanic Field, partly 

Pliocene but overlapped by a Quaternary field of monogenetic 

intraplate alkali-basalt scoria cones and shields (Uto and others, 1991; Hildreth and Fierstein, work in progress), and the 

Newberry and Medicine Lake Volcanic Fields, each centered 

on a great mafic shield-form edifice in the transition between 

the Cascade arc and the Basin and Range Province, and each 

marked by a caldera, numerous peripheral vents, and abundant 

low-K tholeiitic basalt.

The Quaternary Cascade arc extends 1,250 km northsouth, and its belt of vents ranges in width from less than 25 

km to more than 100 km. Elevation of the pre-Quaternary 

basement beneath the volcanic chain (which commonly forms 

drainage divides and provides interior rainshadows) ranges 

from 1,200 to 1,500 m in much of Oregon to more than 2,000 

m in northern Washington. Among the major cones, a dozen 

exceed 3,000 m in elevation but only Rainier and Shasta 

exceed 4,000 m. 

Arc plutons are a fundamental component of any magmatic arc, but only one pluton of Quaternary age is so far 

exposed in the Cascades (Hildreth and others, 2003). Nevertheless, many are likely to be at various stages of growth and 

consolidation, so I present below inferences relevant to Quaternary intracrustal magmatic processes, drawn in part from 

Cascade plutons of Tertiary age and from Quaternary granitoid 

ejecta brought up in explosive eruptions. 

Not synonymous with the Cascade magmatic arc, the 

“Cascade Range” is the mountainous physiographic province 

limited by convention to the United States. It is a composite 

terrain that, in addition to constructional topography provided 

unevenly by Quaternary volcanoes, consists of rugged topography cut on Mesozoic or Tertiary batholiths and on Mesozoic 

and older metamorphic rocks, as well as on Tertiary volcanicarc assemblages.

Volcanic-rock nomenclature in this paper is kept simple, 

principally based on SiO2

 contents—basalt has 47-52%, 

basaltic andesite 52-57%, andesite 57-63%, dacite 63-68%, 

rhyodacite 68-72%, and rhyolite >72% SiO2. For reasons of 

convenience or uncertainty, I commonly use the terms “mafic”

to lump basaltic andesite with basalt, “intermediate” for 

andesite-dacite, and “silicic” to cover the range of rhyodacite 

and rhyolite. Compositional varieties of Cascade basalt are 

discussed in a later section, but the acronym “HAOT” (Hart 

and others, 1984) will be used repeatedly for MORB-like lowpotassium high-alumina olivine tholeiite, which has fed many 

low-viscosity lava flows within the Cascades from Washington 

to Lassen as well as more widely in the adjoining Basin and 

Range Province. I employ the simple term “arc basalt” for the 

common calcalkaline to weakly tholeiitic basalts that carry the 

familiar geochemical signature of slab-derived contributions 

(the “subduction component”); and I use the term“intraplate 

basalt” for those carrying the so-called OIB trace-element 

pattern—not uncommon in parts of the Cascades but typically 

mildly alkalic where found there.

Volcanoes and Vents

Ambiguities arise in counting volcanoes (table 1). Some 

eruptions issue from more than one vent, as when a chain of 

domes or cinder cones is fed concurrently by a single dike 

system. Other eruptions issue from flank vents of large cones 

and shields, representing lateral breakouts of magma from a 

central conduit system that intermittently feeds a main summit 

vent. Should domes, flows, and conelets erupted from such 

flank vents or along fissure alignments be counted as separate 

volcanoes? Ideally, probably not, if the aim (as here) is to 

delineate the extent and variety of magma available beneath 

the region. Without detailed study, however, there may be little 

justification for assuming that adjacent or aligned volcanoes 

erupted concurrently or are magmatically related (Conway 

and others, 1997). Documented examples in the Cascades are 

surprisingly few (for example, Scott, 1987; Gardner, 1994; 

Muffler and others, 1994; Hildreth and Fierstein, 1995). Moreover, the notion of so-called “parasitic” flank vents has been 

overextended and widely misapplied (Hildreth and Lanphere, 

1994). Cinder cones peripheral to arc shields and stratocones 

typically exhibit compositional contrasts, usually less evolved, 

and most of them probably have independent conduits from 
4 Quaternary Magmatism in the Cascades—Geologic Perspectives

the deep crust or mantle. This is in noteworthy contrast to 

large oceanic shields, where many flank vents are indeed fed 

laterally by dikes issuing from a central conduit system (for 

example, Eaton and Murata, 1960; Dieterich, 1988; Chadwick 

and Howard, 1991).

More troublesome sources of error in reckoning the 

number and distribution of Quaternary volcanoes are concealment by younger lavas and obliteration by glacial erosion 

(which increases in severity northward along the arc). At 

Mount Adams, for example, at least 30 vents for discrete units 

of mafic lava (cropping out around the periphery) are covered 

either by till or, in greater measure, by an apron of andesitic 

lava flows that extends as far as 17 km radially from the stratocone’s summit (Hildreth and Fierstein, 1995). Likewise, mapping of the Mount Bachelor volcanic chain (Scott and Gardner, 

1992; Gardner, 1994) illustrated that, for mafic shield alignments as well, many independent vents can be wholly buried 

by compositionally distinguishable products of nearby vents 

that may be only thousands or hundreds of years younger. 

Recent mapping of the glaciated district around Mount Baker 

raised the number of recognized Quaternary vents from 4 to 

25, but mapping of ~110 dikes suggests that there had been 

many more (Hildreth and others, 2003). Owing to the climatic 

gradient along the arc, a middle Quaternary lava-cone that 

would be colluvium-mantled but largely intact in California might be sculpted to ridgecrest remnants in Oregon and 

glacially reduced to a plug or dikes in northern Washington or 

British Columbia (fig. 2).

Along-arc comparability of the vent-distribution data 

summarized in table 1 thus requires identification of eroded 

vent remnants, judicious counting of concealed vents for 

plausibly independent eruptive units, and consistency in dealing with the vent-redundancy problems. Available data are too 

uneven regionally to discriminate redundancy with adequate 

consistency, so table 1 simply records all known Quaternary 

vents, although I have kept track separately of all identified 

and probable cases of redundancy (as discussed below in the 

section entitled “Vent Redundancy”). Inclusion of (1) authentic flank vents, (2) aligned cinder cones and domes that could 

have erupted concurrently, and (3) lava domes clustered above 

shallow magma reservoirs no doubt inflates the vent total but, 

I think, by no more than 290 (~12%). There are compensating 

factors: Stratovolcanoes can add flank vents but they also bury 

independent peripheral vents, and caldera-forming reservoirs 

concentrate clusters of vents but also destroy many of them 

during collapse. Moreover, additional vents will no doubt be 

recognized as geologic mapping becomes more uniformly 

detailed. Mapping of the Quaternary Cascades is already fairly 

advanced, however, and it seems unlikely that future additions 

to table 1 would exceed a few hundred vents, most of which 

would probably be of early Quaternary age.

A total of 3,416 Quaternary vents is given in table 1. 

Among these, nearly 1,100 are rear-arc volcanoes customarily 

associated with the Cascades. Of the 2,339 counted for the Cascade magmatic arc itself, more than 200 are forearc volcanoes 

distributed well seaward of the arc’s main axial belt. This is as 

accurate a tally of sites of magmatic emission as current data 

avail, although more uniformly detailed mapping will no doubt 

eventually add dozens (if not hundreds) more. Nearly 90 percent 

of the vents identified are independent volcanoes in the sense 

that each possesses its own conduit system stemming from a 

melting source or magma reservoir in the crust or the mantle.

Distribution of Quaternary Cascade 

Volcanoes

At first glance, the Quaternary Cascade arc (fig. 1) 

consists dually of a broad north-trending swath of myriad 

close-set vents (the Lassen region to Mount Rainier) and a 

narrow NNW-trending chain of well-separated vent clusters 

(the Garibaldi Volcanic Belt). Guffanti and Weaver (1988) 

combined a seismicity-based subduction model with the (5 to 

0 Ma) vent-distribution data compiled by Luedke and others (1983) to define five plausible Cascade arc segments: (1) 

The Garibaldi Volcanic Belt, (2) Mount Rainier to Mount 

Hood, (3) the rest of the Oregon Cascades, (4) the Mount 

Shasta region, and (5) the Lassen region. Although the extent 

to which the segments of Guffanti and Weaver (1988) reflect 

lower-plate processes remains nebulous (and indeed differential lithospheric extension and block rotation may be more 

important), I concur with their primary utilization of regional 

vent distribution and their deemphasis of the stratovolcano 

alignments conventionally employed in segment models. In 

table 1 and several vent-distribution maps (see below: figs. 

3, 8, 14, 15, and 21), the Quaternary vent pattern is subdivided still more finely, principally for sharpening analysis 

and discussion, taking into account several local changes in 

vent density, alignment, or distribution. These subdivisions 

are not “segments,” in the sense of reflecting some inferred or 

postulated relationship to either crustal tectonics or subduction 

geometry, but are simply volcanic fields or empirically coherent tracts of convenience—several having diffuse boundaries. 

For 34 of the larger edifices in the Cascades, some basic physical data are summarized in table 2.

Garibaldi Volcanic Belt

It is clear from the vent-distribution maps that little of the 

Cascade arc conforms to the stereotyped festoon of solitary 

stratovolcanoes. Even the Garibaldi Volcanic Belt (GVB; Glacier Peak to Mount Meager and Bridge River), although closer 

to the stereotypical image and often referred to as a “chain of 

isolated stratovolcanoes,” is not that at all (fig. 3). Of the five 

major volcanoes in the GVB, four are silicic dome complexes 

with stubby ice-contact lava flows surrounded by predominantly pyroclastic aprons; and the one that really is an andesitic stratovolcano (Mount Baker) is a modest young cone on 

the margin of an early Quaternary caldera filled by rhyodacitic 

ignimbrite and lava domes (Kulshan Caldera; Hildreth, 1996; 
Distribution of Quaternary Cascade Volcanoes 5

Figure 2. Examples of contrasting degrees of erosion of Cascade volcanoes near opposite ends of the arc. A, Pleistocene 

andesitic cones in the Lassen segment, California. View is southward up Hat Creek Valley, floored by the 24-ka Hat Creek 

Basalt (HAOT), toward snowclad Lassen Peak and Chaos Crags, 37 km away on the far skyline. In middle distance (15-17 

km away), three cones of pyroxene andesite are, left to right, Sugarloaf Peak (2,000 m), Wilcox Peak (1,970 m), and (faulted) 

Logan Mountain (2,210 m). Sugarloaf and Wilcox each rise ~800 m, and Logan 1050 m, above the valley floor. Sugarloaf is of 

late Pleistocene age, and the other two are probably of middle Pleistocene age. (Photograph by author taken in summer 1995.) 

B, Mount Cayley (2,377 m) on central skyline, rising above glacially incised crystalline basement rocks of the Coast Plutonic 

Complex, British Columbia. View is westward 25 km from Whistler Mountain. Bicolored pyramid on central skyline is Mount 

Cayley, a multistage edifice dominated by dacitic lavas, pyroclastic deposits, and a summit-forming dacite intrusion. Crags to its 

left are Vulcan's Thumb and Pyroclastic Peak (see fig. 6), also parts of the Cayley cluster. Powder Mountain icefield forms right 

skyline. Canyon of Brandywine Creek at left descends to join the Cheakamus River (foreground) at ~425 m elevation. Although 

much of Mount Cayley is thought to be middle Pleistocene or older, it may also have erupted in the late Pleistocene, as lavas on 

its south flank descend 1,500 m down the floor of an adjacent canyon (Souther, 1980), and several subglacial flank vents erupted 

during the last main glacial interval (Kelman and others, 2002). (Photograph by author taken in August 1997.)
6 Quaternary Magmatism in the Cascades—Geologic Perspectives

Vents Area (km2) Area (km2)

per vent

Region

23 160 7 Basalts of Salal Creek to Bridge River upland

24 250 10 Meager Creek Volcanic Field (Lillooet R. to Elaho R.)

21 100 5 Mt. Cayley Volcanic Field (Tricouni to Crucible dome)

10 90 9 Garibaldi Lake Volcanic Field (The Table to Loggers Lake)

16 240 15 Mt. Garibaldi area (Watts Point to Glacier Pikes)

25 220 9 Mt. Baker and Kulshan Caldera

8 — — Glacier Peak to Skykomish River

127 Garibaldi Volcanic Belt total (fig. 3)

6 — — Greenwater to Mt. Rainier

30 570 19 Bumping Lake to Goat Rocks, axial 

121 1,150 9 Mt. Adams Volcanic Field (Walupt Lake to Guler Mtn)

15 250 17 Gilmer shield to Underwood Mtn, axial

40 1,200 30 Mt. Defiance to Mt. Hood to Barlow Pass, axial

52 300 6 Indian Heaven Volcanic Field

61 1,850 30 Forearc: Battle Ground to Mount St. Helens to Blue Lake

87 2,500 32 Forearc: Portland to Wind River (53 in Oregon; 34 in Washington)

412 Rainier to Hood segment total (fig. 8)

176 1,150 7 Frog Lake Buttes to South Cinder Peak (I, fig. 14)

466 3,250 7 South Cinder Peak to Crane Prairie Reservoir (II, fig. 14)

118 1,850 16 Crane Prairie Reservoir to Cowhorn Mtn (III, fig. 14)

86 1,225 14 Cowhorn Mtn to Timber Crater (IV, fig. 15)

175 1,100 6 Mt. Mazama area (Timber Crater to Big Bunchgrass)(V, fig. 15)

33 925 28 Mt. McLoughlin area (Pelican Butte to Aspen Lake)(VI, fig. 15)

1,054 Oregon segment total (figs. 14, 15)

3 — — Copco cones (HAOT in Oregon-Shasta Gap)

32 460 14 Mt. Shasta area (west of Cascade axis)

223 2,070 9 Cascade axis (and east to Sharp Mtn and Doe Peak; fig. 21)

225 Shasta segment total (fig. 21)

12 — — Brushy Butte and Timbered Crater (HAOT in Shasta-Lassen Gap)

43 1,000 23 Lassen forearc

250 2,200 9 Cascade axis (west of Caribou Volcanic Field and Butte Creek Rim

112 300 3 Caribou Volcanic Field

71 2,100 30 Diffuse backarc (east of Caribou Volcanic Field and Butte Creek Rim)

476 Lassen segment total (fig. 21)

2,339 TOTAL — CASCADE ARC (proper)

Adjacent rear-arc volcanic fields

35 360 10 Simcoe Mountains Volcanic Field (not including ~150 Pliocene vents)

>450 1,630 <4 Newberry Volcanic Field

580 1,700 3 Medicine Lake Volcanic Field

12 500 40 Hackamore Volcanic Field (Basin and Range HAOT)

1,077 Rear-arc total

3,416 GRAND TOTAL—CASCADIAN QUATERNARY VENTS

Note: Principal data sources are compilations by Blakely and others (1997), Guffanti and others (1990, 1996), Luedke and others (1983), 

Scott (1987), Sherrod and Smith (2000), Sherrod and others (2004), and Smith (1993), as well as area studies by Bacon (1990), Bacon and 

Nathenson (1996), Clynne and Muffler (1990, in press), Conrey (1991), Donnelly-Nolan (in press), Green and others (1988), Hammond 

and Korosec (1983), Hildreth and Fierstein (1995), Hildreth and others (2003), Hildreth, Lanphere and others, 2004), Lawrence and others 

(1984), MacLeod and Sherrod (1992), Read (1990), Roddick and Souther (1987), Scott and Gardner (1992), Smith (1988), Stasiuk and 

Russell (1989), Taylor (1978, 1987), Uto and others (1991), and author’s work in progress. See also references for tables 6 through 8. 

Table 1. Distribution and spatial density of Quaternary vents.

[Areas estimated are defined by vent fields, not by lavas or other outflow deposits. For example, Medicine Lake lavas cover an area greater 

than 2000 km2 (Donnelly-Nolan and others, 2003), but all or nearly all of the vents are within an area of 1,700 km2, the area used here for 

vent-density calculation. HAOT is high-alumina olivine tholeiite, as elaborated in the text]
Distribution of Quaternary Cascade Volcanoes 7

Edifice Summit

elevation (m)

Highest older 

rocks (m)

Proximal 

relief (m)

Draping relief (m) Main Edifice

Composition Volume (km3)

Meager 2,679 1,980 700 2,100 D>A>R 20

Cayley 2,377 1,830 550 2,070 D>A 15

Garibaldi 2,678 1,375 1,300 2,375 D>A>R 20

Baker 3,286 2,315 970 2,675 A 15

Glacier Peak 3,213 2,410 800 2,330 D>A 30

Rainier 4,392 2,332 2,060 3,510 A>D 130

Goat Rocks 2,494* 2,200 300 1,520 A>D 40

Adams 3,742 1,440 2,300 3,300 A>D 210

St. Helens 2,950 1,540 1,410 2,270 D>A>M 25

Hood 3,425 1,650 1,775 2,790 A>D 50-70

Olallie Butte 2,199 1,490 710 835 M 5

Jefferson 3,199 1,710 1,490 2,100 A>D 20

Three Fingered Jack 2,390* 1,790 400 1,400 M 10

Washington 2,376* 1,550 825 1,025 M 15

Belknap shield 2,095 1,680 415 1,350 M 10

Black Butte OR 1,962 1,200 760 960 M 10

Black Crater 2,210 1,900 310 930 M 7

North Sister 3,074 1,680 1,400 1,565 M 15

Middle Sister 3,062 2,560 500 1,940 A>D~M 12

South Sister 3,157 1,980 1,175 1,500 A~R~D 20

Broken Top 2,800* 2,040 760 1,800 M>D~R 10

Bachelor 2,763 1,950 815 1,270 M 25

Maiden Peak 2,383 1,700 680 900 M 12

Diamond Peak 2,665* 2,050 615 1,815 M>A 15

Odell Butte 2,143 none 760 760 M 7

Cappy Mountain 2,252* ~1,650? 600 730 A>R>D 25

Thielsen 2,799* 2,200 600 1,220 M 10

Bailey 2,549 1,850 700 1,300 M>A 15

Mazama 3,700? 2,145 1,550 2,250 A>D~R 120

McLoughlin 2,894 1,770 1,125 2,040 M 13

Shasta 4,316 1585 2,730 3,420 A~D 450

Magee 2,646* 1,805 840 1,050 M~A>R 10

Lassen Peak 3,187 2,680 500 1,050 D 2.5

Brokeoff 2,815* 2,225 590 1,470 A~D 50

Table 2. Elevation, relief, and volume of selected Cascade edifices.

[Listed north to south. Proximal relief is difference between summit elevation and that of highest exposure of older rocks overlain by the edifice. Draping 

relief is difference between summit elevation and that of lowest distal lavas of the edifice (not including distal pyroclastic or debris flows). Volume and Composition refer to the present-day main cone only, not including peripheral vents; M, mafic; A, andesite; D, dacite; R, Rhyodacite and Rhyolite. See text for discussion of eruptive volumes estimated, which are generally much larger than those preserved.]

*

 Means that the original elevation of an eroded edifice was >100 m higher. St. Helens and Mazama summit elevation estimates are pre-collapse.
8 Quaternary Magmatism in the Cascades—Geologic Perspectives

Hildreth, Lanphere, and others, 2004). Glacier Peak, Mount 

Garibaldi, and Mount Cayley are dominantly dacitic; rhyodacite is also common at Garibaldi and Cayley and abundant at 

Mount Meager; high-silica rhyolite (>75% SiO2) is uniquely 

present at Garibaldi; and subordinate andesite erupted at all 

four, apparently relatively early in the development of each 

center (Matthews, 1952a, 1957; Tabor and Crowder, 1969; 

Green and others, 1988; Stasiuk and Russell, 1989; Green, 

1990; Read, 1990; Souther and Yorath, 1991). 

GLACIER

PEAK

HC

KC

GC

0 50 100 km

Garibaldi Lake

PACIFIC

OCEAN

Vancouver

Seattle

MEAGER

CAYLEY 

GARIBALDI

Olympic

Peninsula

Vancouver

Island

51°

50°

BRITISH COLUMBIA

WASHINGTON

Pemberton-Chilliwack

Tertiary

arc

Watts Point

Salal Creek cones

Bridge River cones

Fraser River

Bridge River

Skagit River

Lillooet River

BAKER

49°

48°

124° 121°

124° 121°

Figure 3. Garibaldi Volcanic Belt in southwest British Columbia and northwest Washington, showing 

distribution of 127 Quaternary volcanic vents. Dots represent volcanoes of basaltic to andesitic composition; 

crosses represent volcanoes of dacitic to rhyolitic composition; and triangles represent major edifices. Ovoids 

depict calderas: KC, 1.15-Ma Kulshan Caldera; HC, 3.7-Ma Hannegan caldera; GC, inferred Gamma Ridge 

Caldera, probably early Pleistocene. Axis of deeply eroded Tertiary magmatic arc inboard of the Quaternary arc 

is shown as Pemberton-Chilliwack belt.

Eruptive volumes and production rates in the GVB have 

not been reconstructed with much accuracy, owing to (1) 

difficult access, (2) severe recurrent glacial erosion of all five 

long-lived centers, (3) sparsity of precise geochronologic and 

detailed stratigraphic data for most centers, (4) predominance 

of glassy rocks, chilled by snow and ice, rendering precise 

dating difficult, and (5) unknown proportions of silicic fallout 

and fragmental flows deposited directly on, within, or against 

glaciers and removed by advances of the Cordilleran ice sheet. 
Distribution of Quaternary Cascade Volcanoes 9

Mount Baker

The Mount Baker locus (fig. 4) is probably the most 

productive (~160±50 km3; fig. 18 of Hildreth and others, 

2003), having erupted more than 80 km3 of early Quaternary 

rhyodacite from Kulshan Caldera, about 30 km3 of andesite 

from Black Buttes stratovolcano (500-300 ka), at least 15 km3

of andesite from the active cone (Mount Baker), and more 

than 10 km3 (possibly much more) from several peripheral 

vents now glacially ravaged. Detailed mapping and compositional data, volume estimates for 77 mapped eruptive units, 

and high-precision K-Ar and 40Ar/39Ar dating established an 

eruptive chronology (1.3 Ma to Holocene) and time-volumecomposition relations for the whole Mount Baker Volcanic 

Field (Hildreth and others, 2003; Hildreth, Lanphere, and others, 2004). Nonetheless, the lost eruptive volume represented 

by ~110 mapped Quaternary dikes, most of them cutting 

glaciated basement rocks or intracaldera tuff, remains poorly 

known. The towering andesitic edifice of Mount Baker, highest peak in the North Cascades at 3,286 m, was built largely 

in the interval 40 to 12 ka, although it overlies a few (apparently confocal) glaciated andesite remnants as old as 100 ka. 

Its icecap, an acid-altered region around its steaming crater, 

and its steep peripheral valleys, which drain to the populous 

Puget Lowland, make the cone a significant debris-flow 

hazard. The youngest magmatic eruptions were the basaltic 

scoria cone and zoned lava-flow apron (Green, 1988) from the 

peripheral Schreibers Meadow vent (9.8 ka) and a subplinian 

shower of andesitic ash from Mount Baker's summit at 6.5 ka 

(Hildreth and others, 2003).

Glacier Peak

The volume of Glacier Peak (3,213 m) inferred from 

the late Pleistocene and Holocene deposits exposed is ~30 

km3 (Tabor and Crowder, 1969; Beget, 1982a,b; Sherrod and 

Smith, 1990), but the eruptive volume is likely to have been at 

Figure 4. Kulshan Caldera and ice-clad Mount Baker (3,285 m), fumarolically active andesitic stratocone, viewed southwestward from 

Table Mountain. The cone has had only one documented magmatic eruption in the Holocene, a 6.5-ka subplinian shower of andesitic 

ash. Pale gray and tan deposits flooring middleground bowl are ash-rich sedimentary strata that rest on white intracaldera ignimbrite of 

Kulshan Caldera (1.15 Ma). Dark ridge at right and crag at center (below Mount Baker) are andesite lavas, 500 ka and 300 ka, respectively, 

that overlie the caldera fill. Rugged ridges at left include three postcaldera rhyodacite domes (1.13-1.01 Ma). Ice-filled Carmelo Crater on 

the summit is breached to north (right) by glacial outflow. Partway down the south (left) slope of Mount Baker, Sherman Peak forms south 

wall of steaming Sherman Crater. Relief visible in image is 1,900 m. (Photograph by author taken in summer 1995.)
10 Quaternary Magmatism in the Cascades—Geologic Perspectives

least twice that much. The edifice is principally a dacite lava 

dome-and-flow complex (fig. 5), but it is surrounded by an 

enormous pyroclastic apron, tongues of which extended 100 

km to Puget Sound. Moreover, remnants of an early Quaternary (2.0-1.6 Ma) intracaldera fill appear to be present at 

Gamma Ridge on the east flank (Tabor and Crowder, 1969; 

Tabor and others, 2002). By analogy with Kulshan Caldera 

near Mount Baker (Hildreth, 1996; Hildreth, Lanphere, and 

others, 2004), more than half of the caldera-forming eruptive 

volume is likely to have been eroded away.

Several episodes of postglacial eruptive activity have 

been identified at Glacier Peak. Two plinian eruptions (~13 ka) 

each released ~2 km3 of magma as downwind fallout (Porter, 

1978; Gardner and others, 1998), along with much larger 

volumes as pyroclastic flows (Beget, 1982a). Several substantial pyroclastic eruptions in the middle and late Holocene also 

produced major pyroclastic-flow and debris-flow deposits, 

although the fall deposits accompanying these events were thin 

and of modest volume (Beget, 1982a,b; Foit and others, 2004). 

Little is known, however, of the earlier eruptive history, which 

apparently dates from the middle Pleistocene, because much 

of the record has been glacially ravaged and buried proximally 

Figure 5. Glacier Peak (3,213 m) viewed northeastward from White Chuck cinder cone, 7 km southwest of summit. Disappointment 

Peak dome (2,973 m) forms shoulder 700 m right (south) of summit dome. Pale bare apron is mostly dacite lava flows, veneered by

dacitic pyroclastic deposits that thicken into canyons below (not visible here). Darker rocks of steep lower walls and foreground ridge 

are Mesozoic schists and Miocene granitoids. Relief visible in image is about 1,700 m. (Photograph by author taken in August 1988.)

by the young dome complex. Reconnaisance K-Ar dating by 

M.A. Lanphere (Tabor and others, 2002) identified numerous 

late and middle Pleistocene lavas, the oldest about 600 ka. 

Magnetically reversed lavas (>780 ka) were nowhere found 

by Tabor and Crowder (1969). Steeply glaciated radial ridges 

are mantled by the 13-ka pumice-fall deposits up to about 

2,500 m, and the large Disappointment Peak dacite dome (~1 

km south of the summit) appears likewise to be older than 13 

ka (Beget, 1982a). The summit and much of the upper 700 m 

of the edifice, however, apparently represent a younger dome 

cluster, probably sources for the thick middle and late Holocene parts of the valley-filling pyroclastic apron.

Mount Garibaldi

Similar uncertainties exist concerning the eruptive histories and volumes of the ice-clad Garibaldi, Cayley, and Meager 

volcanic centers. Surviving products of the Mount Garibaldi

(2,678 m) dome complex (56-77% SiO2

) sum to only 16 to 20 

km3 (Matthews, 1952a, 1957, 1958), but they represent many 

episodes of activity spread across the long interval from about 

670 ka to Holocene (Green and others, 1988; Green, 1990), 
Distribution of Quaternary Cascade Volcanoes 11

during which the edifice was probably torn down several times. 

The pyroclastic apron is large, deeply eroded, and little studied. 

Andesite-dacite lavas and their pyroclastic accompaniments 

from several vents first filled paleovalleys having as much as 1.8 

km of relief glacially incised into the Coast Plutonic Complex 

basement. Several dacite domes and derivative pyroclastic material next built the main edifice (~260-200 ka), draping relief cut 

in the older assemblage. A deeply eroded obsidian dome remnant at Lava Peak (77% SiO2

) is the only Quaternary high-silica 

rhyolite identified anywhere in the Cascades north of the Three 

Sisters. The south summit of Mount Garibaldi (Atwell Peak), a 

large dome of hornblende-biotite rhyodacite that shed a major 

pyroclastic apron, may postdate the Last Glacial Maximum 

(LGM; 21-18 ka), as may the Glacier Pikes hornblende-dacite 

dome a few kilometers north of the summit. Of the several 

glacially sculpted (still ice-clad) summits, the hornblendebiotite dacite Dalton Dome may be the youngest. The only unit 

unequivocally postglacial is the 18-km-long Ring Creek lava 

flow (~4.5 km3

) of hornblende-biotite dacite (63-65% SiO2) that 

erupted at Opal Cone on the southeast flank at ~10 ka (Brooks 

and Friele, 1992).

A small but independent locus of intermediate magmatism, 25 km southwest of Mount Garibaldi at the head 

of Howe Sound near the town of Squamish (Green, 1994b), 

includes the only known eruptive sites between Garibaldi and 

Mount Baker. Facing each other across the sound are the 200-

m-thick Watts Point pile of dacite lavas and breccias (63-65% 

SiO2) cut by thick dikes, and the 300-m-thick Monmouth 

Creek pile of hornblende andesite-dacite lavas and breccias 

(57-64% SiO2), likewise cut by several dikes, which support 

pinnacles that tower above the complex (called “The Castle” 

by Mathews, 1958). Both are glacially devastated and the 

original volumes impossible to reconstruct. The Watts Point 

center is dated at about 100 ka (Green and others, 1988) and 

may have been erupted subglacially (Bye and others, 2000).

Garibaldi Lake Volcanic Field

The Garibaldi Lake Volcanic Field, which extends 20 

km northward from the north toe of Mount Garibaldi, is a 

scattering of small, compositionally diverse (alkali basalt 

to dacite) volcanoes (Mathews, 1952a, 1957, 1958; Green, 

1981, 1982, 1990; Green and others, 1988; Waddell and 

Green, 1993). Excluding vents on Mount Garibaldi itself, 

the field has at least 14 distributed vents, including three 

that erupted dacite, eight andesite, two basaltic andesite, 

and one alkali basalt and hawaiite. At least four valley-floor 

basalts (49-51% SiO2

) that flowed south into the volcanic 

field from unidentified vents more than 28 km farther up the 

Cheakamus River enclose sediment radiocarbon dated at 34.2 

ka. The largest edifice in the field is the andesitic multivent 

Mount Price-Clinker Peak complex (~4 km3; Mathews, 1958), 

from which two 7-km-long lava tongues distally abutted the 

Cheakamus valley glacier during post-LGM ice recession 

(Mathews, 1952b). Many units in the volcanic field exhibit 

ice-contact features or were erupted subglacially. The Table, 

a 300-m-thick multitiered pile of hornblende andesite lavas, 

melted out its own intraglacial cavity (Mathews, 1951), 

possibly also during the post-LGM deglaciation. 

A few volcanic remnants in the field are early Pleistocene (as 

old as ~1.2 Ma), but most appear to be late Pleistocene (Green 

and others, 1988).

Mount Cayley

Mount Cayley (2,377 m), at least as old as middle 

Pleistocene, today represents 15 to 20 km3 of glacially eroded 

crags (fig. 2), clearly only a modest fraction of its total output 

of silicic eruptive products (Souther, 1980; Souther and 

Dellechaie, 1984; Green and others, 1988). Three precipitous 

summits (Mount Cayley, Wizard Peak, and Pyroclastic Peak; 

fig. 6) are sculpted from a multivent andesite-to-rhyodacite 

edifice (57-69% SiO2) that has nearly 2 km of relief, rests on 

ruggedly eroded granitoid basement, and consists of domes, 

shallow intrusions, lava flows, and voluminous breccias and 

tuffs, cut by numerous dikes and sills. The complex was 

divided by Souther (1980) into three unconformable eruptive 

sequences, but alteration and ice-contact chilling have made 

precise geochronology difficult (and some published dates 

should be considered suspect). Off-edifice peripheral vents 

range from the biotite+quartz-bearing dacite lava flows and 

craggy intrusion of Mount Fee (65-68% SiO2

), through a 

north-trending chain of pyroxene- or hornblende-bearing 

andesite/dacite domes and tuyas, to a southeastern cluster of 

six olivine-bearing andesitic domes at Ember Ridge (Green 

and others, 1988; Kelman and others, 2002). Among the 

youngest units may be a pair of biotite-dacite domes at the 

east and southeast margins of the Cayley edifice and a 5-kmlong valley-filling lava tongue related to one of them. That 

lava flows from both Mount Cayley and Mount Fee extend 

down the floors of present-day tributary drainages nearly to 

adjacent canyon bottoms suggests that each remained active 

well into the late Pleistocene. Because the precipitous Cayley 

complex is rich in coarse proximal pyroclastic deposits, some 

of them hydrothermally altered, it is especially prone to slope 

failure and debris avalanches (Clague and Souther, 1982; 

Evans and Brooks, 1991).

Mount Meager

Mount Meager (fig. 7) likewise had numerous eruptive phases (58-70% SiO2

) during a lifetime that could be as 

long as 2.2 Myr (Read, 1978, 1990; Green and others, 1988; 

Stasiuk and Russell, 1989, 1990). At the south end of the 

compound 9x13-km edifice, basal andesite-dacite lavas and 

pyroclastic deposits (~2 Ma) rest on ruggedly eroded Mesozoic batholithic basement and are draped by a 500-m-thick 

pile of altered rhyodacite tuffs, lavas, and shallow intrusions 

(~2 to 1 Ma) that suggest a vent complex south of Pylon Peak. 

These are overlain by a large pile of silicic andesite and dacite 

(~1 to 0.5 Ma) as thick as 1,200 m, centered at “The Devasta-
12 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 6. Mount Cayley cluster viewed toward the northwest. Spire at left is Vulcan’s Thumb, and three main summits (from left 

to right) are Pyroclastic Peak, Mount Cayley, and Wizard Peak (Souther, 1980). Relief visible in image is about 900 m. 

(Photograph by Paul Cordy; courtesy of Professor J.K. Russell.)

Figure 7. Mount 

Meager complex 

viewed toward the west. 

Capricorn Mountain and 

Capricorn Glacier at left. 

Two high summits are 

Mount Meager at center 

and Plinth Peak to its 

right. Relief visible in 

image is about 1,800 m. 

(Photograph courtesy of 

Professor J.K. Russell.)
Distribution of Quaternary Cascade Volcanoes 13

tor” and distributed radially around the compound edifice. 

This part of the edifice is in turn overlapped on the north by a 

late Pleistocene complex ≤150 ka) of biotite-quartz (± hornblende) rhyodacite that forms four additional ice-clad summits 

(Capricorn, Job, Meager, and Plinth) and consists of numerous 

domes, felsic intrusions, lava flows, and steeply dipping pyroclastic aprons. Although basement rocks are exposed locally 

as high as 1,800 m, products of the Meager edifice drape them 

to elevations as low as 600 m from the 2,679-m peak. The 

severely glaciated complex that survives today is a composite 

of deeply eroded remnants of many episodes and has a total 

estimated volume of ~20 km3. Some 2 to 3 km3 of rhyodacite 

magma was released in the only known Holocene eruption, the 

2.4-ka Bridge River plinian fall (identified as far downwind 

as Alberta) and an accompanying 9-km-long valley-filling 

pyroclastic-flow and lava-flow assemblage (Nasmith and others 1967; Stasiuk and Russell, 1990; Read, 1990; Stasiuk and 

others, 1996; the Pebble Creek Formation of Hickson and others, 1999). Because of their longevities and repeated ravaging 

by the Cordilleran ice sheet, it should be no surprise if Mount 

Meager and each of the other principal GVB centers are 

ultimately shown to have been as productive as comparably 

explosive and silicic Mount St. Helens (which has produced an 

eruptive volume of at least 75±15 km3

).

Garibaldi Volcanic Belt—Overview

Mafic andesite and basalt issued from independent 

(mostly monogenetic) vents peripheral to all five principal 

GVB centers. At least four basaltic vents are scattered just 

south of Glacier Peak (Tabor and Crowder. 1969), the White 

Chuck scoria cone being the northernmost vent for HAOT-like 

magma in the Cascades (Green and Sinha, 2005). Six basaltic 

vents of many ages (716 to 9.8 ka) are mapped around Mount 

Baker (Hildreth and others, 2003). Relatively young mafic 

and andesitic lavas crop out adjacent to the glacial valleys at 

the south and southeast base of Mount Garibaldi (Green and 

others, 1988). The Garibaldi Lake Volcanic Field contains 

vents for basaltic andesites and for both alkaline and subalkaline basalts. A chain of small, subglacially erupted, andesitic 

volcanoes extends 25 km north-south through Mount Cayley 

(Souther, 1980; Green and others, 1988), and basaltic andesite 

erupted near both Crucible Dome (Ring Mountain) to the north 

and Tricouni Peak to the south (Kelman and others, 2002). 

Numerous vents for varied basalts, alkaline and subalkaline, are 

dispersed to the north, south, and west around Mount Meager 

(Stasiuk and Russell, 1989; Read, 1990; Green and Sinha, 

2005), including a 25-km-long set of young basalts (51% SiO2

) 

that flowed south down the floor of the Elaho River valley. 

Extending 35 km north from Mount Meager along Salal 

Creek to north of Bridge River, a field of at least 20 scattered 

mafic monogenetic volcanoes (46-51% SiO2

) terminates the 

Quaternary GVB. Although one or two may be postglacial, 

most are plugs, necks, dikes, tuyas, lava-flow remnants, and 

parts of small cones and shields, badly eroded and showing 

ice-contact features. Often called collectively the “Bridge 

River Cones,” all or most consist of alkali basalt, hawaiite, or 

basanite (Lawrence and others, 1984; Roddick and Souther, 

1987; Souther, 1990; Green and Sinha, 2005). 

The GVB has at least 127 known vents (fig. 3; table 1), 

is 375 km long, trends N35 ̊W in the south but N20 ̊W in the 

north, suggesting internal segmentation (Green and Harry, 

1999), and exhibits very uneven spacing of its five main 

centers—the maximum being 147 km (Baker to Garibaldi), 

the minimum 35 km (Garibaldi to Cayley). Extensive ice 

cover and the large number of dikes that cut both basement 

rocks and glacially scoured early edifice deposits suggest that 

unrecognized (or obliterated) vents are more numerous in the 

GVB than farther south in the Cascades and that GVB eruptive 

volumes are likely to be correspondingly underestimated. True 

basalts (47-52% SiO2

) erupted within 10 km of all five main 

GVB centers during late Pleistocene or Holocene time (Green 

and Sinha, 2005). Extraordinarily for the Cascades, however, 

as much as 75 percent of the eruptive volume of the GVB is 

dacite and rhyodacite.

Mount Rainier to Mount Hood Segment

The contrast between the Garibaldi Volcanic Belt (fig. 

3) and the broad region of Quaternary volcanism in southern 

Washington and northernmost Oregon (fig. 8) could scarcely 

be greater. A virtually continuous array of at least 150 

Quaternary volcanic vents extends 170 km along the main 

Cascade axis from Mount Hood (45 ̊22’N) to Bumping Lake 

(46 ̊54’N) (Luedke and others, 1983; Smith, 1993). Trending 

N10 ̊E and having no vent-free gap longer than 10 km, this 

axial belt includes Mount Hood, Mount Defiance, Underwood 

Mountain, Gilmer shield, King Mountain, Mount Adams, 

Goat Rocks, Hogback Mountain, and Tumac Mountain; it 

ends abruptly against a 120-km-long gap that separates it 

from the next Quaternary vent cluster just south of Glacier 

Peak. In addition, a broad forearc region includes Mount 

Rainier, Mount St. Helens, and scatterings of mafic shields 

and monogenetic vents that, near Portland, reach halfway 

to the Pacific coast. East of the Cascade axis, the rear-arc 

Simcoe Mountains have at least 35 cinder cones and maars 

of Quaternary age (largely hawaiite and alkali basalt), which 

augment a more extensive Pliocene volcanic field (subalkaline 

and alkaline) arrayed around a rhyolite-cored mafic shield 

volcano (Sheppard, 1967; Uto and others, 1991; Hildreth 

and Fierstein, work in progress). Whereas the widest array 

of Quaternary vents across the Garibaldi Volcanic Belt is 

less than 25 km, the Quaternary volcanic zone of southern 

Washington stretches 160 km west to east (Portland to 

Goldendale) and would still be 110 km wide if the intraplate 

Simcoe Mountains Volcanic Field were excluded (fig. 8).

Cascade Axis 

The axial belt (fig. 8) includes four andesite-dacite centers, one modest (Jennies Butte) and three large (Goat Rocks, 
14 Quaternary Magmatism in the Cascades—Geologic Perspectives

47°

46o

WASHINGTON

OREGON

Columbia

River

46°

122° 121°

122° 121°

Frog Lake Buttes

Barlow Pass Badger Butte

Mt Defiance

Bumping

Lake

Spiral Butte

Tumac Mtn

Randle

Greenwater

Hood River

Glenwood

Laurel

Goldendale

The Dalles

Vancouver

Portland

ST HELENS ADAMS

INDIAN

HEAVEN

HOOD

SIMCOE

MOUNTAINS

GOAT ROCKS

RAINIER

Underwood Mtn

Gilmer shield

Battle

Ground

Hogback Mtn

Walupt Lake

Jennies Butte

Spring Creek shield

0 10 20 30 40 50 km

Blue

Lake

King Mtn

Potato

Hill

Wind River

LR

Hood River

Figure 8. Rainier-to-Hood segment of the Cascade arc, showing distribution of 412 Quaternary volcanic vents, 319 of 

them in Washington and 93 in Oregon. Two colored fields depict limits of vent distributions that define Indian Heaven 

Volcanic Field (middle Pleistocene and younger) and Simcoe Mountains Volcanic Field (4.5-0.6 Ma), within which only the 

~35 Quaternary vents are indicated. LR, Lemei Rock shield. Dots represent volcanoes of basaltic to andesitic composition; 

crosses, those of dacitic to rhyolitic composition; and triangles, major edifices.
Distribution of Quaternary Cascade Volcanoes 15 

Adams, Hood), as well as ~15 basaltic shields, and at least 130 

lesser (mostly monogenetic) Quaternary volcanoes, among 

which basalt predominates over basaltic andesite (proportions 

opposite those in the Oregon Cascades). Quaternary dacites 

and andesites from vents independent of the stratovolcanoes 

are few, modest, and largely concentrated along the 35-kmlong axial strip north of Goat Rocks (Clayton, 1980; Smith, 

1993). Quaternary rhyolites are absent.

Goat Rocks

The Goat Rocks stratovolcano complex (fig. 9) has been 

glacially reduced to ridge-forming remnants of radially dipping 

stacks of andesite (mostly 58-62% SiO2) that surround altered 

cores laced with plugs and dikes (Swanson and Clayton, 1983; 

Swanson and others, 1989). The eruptive volume of the edifice 

was estimated by G.A. Clayton (Smith, 1993) to have been 

about 60 km3

, which seems conservative in view of the extent 

and thickness of the remnants, which represent products of a 

cluster of vents (Swanson and Clayton, 1983; Smith, 1993). 

The Goat Rocks may well have been as voluminous as presentday Mount Hood, likewise an andesite-dominated center. Like 

the Mount Baker locus, the Goat Rocks edifice was a high 

andesitic cone cluster built at the margin of an older silicic 

complex—in this case the rhyolite complex of Devils Horns 

(~3.2 Ma; Swanson and Clayton, 1983), a pile of ignimbrite, 

lavas, and (caldera-collapse) breccias, exposed for more than 

900 m vertically and evidently representing the fill within a 

caldera roughly 5×8 km in area (Smith, 1993). How long after 

the Pliocene rhyolitic episode the Goat Rocks cone began its 

growth is not well established, but parts of the andesitic edifice 

are certainly of early Quaternary age and younger. Intracanyon 

lava flows from the Goat Rocks center include the 80-km-long 

Tieton Andesite (59-60% SiO2; perhaps the longest andesitic 

flow in the world), which erupted at ~1.0 Ma (Ellingson, 1972; 

Swanson, 1978; Hammond, 1980). The youngest major vent 

for pyroxene andesite dates from ~0.7 Ma, and intracanyon 

flows of hornblende andesite issued still later from vents near 

the volcano's core during the middle (and perhaps late) Pleistocene (Clayton, 1983; Swanson and others, 1989). 

Although mid-Pleistocene waning of central activity at 

Goat Rocks was roughly concurrent with the beginning of 

andesitic cone growth at modern Mount Rainier 40 km to 

the northwest, the axis of Cascade magmatism did not shift. 

Along-axis eruptions of basalt and basaltic andesite took 

place before, during, and after construction of the Goat Rocks 

Figure 9. Goat Rocks volcanic cluster viewed northwestward from Jennies Butte (with ice-clad Mount Rainier far behind, 60 km 

distant). In foreground, uppermost Klickitat River canyon heads on Cispus Pass. At right, Klickton Divide culminates in pyramidal Gilbert 

Peak (2,494 m), which stands above the cliffy Goat Rocks (proper). In distance, left of center, Ives Peak and Old Snowy Mountain mark 

the Pacific crest divide. All the named peaks are broadly andesitic and are thought to be early Quaternary. The Pliocene Devils Horns 

rhyolitic complex is out of view to the right, 4 km northeast of Gilbert Peak. Relief visible in image is about 1,200 m. 

(Photograph by author taken in August 2001.)
16 Quaternary Magmatism in the Cascades—Geologic Perspectives

stratocone. About 20 post-Goat Rocks mafic and andesitic volcanoes have erupted along or near the Cascade crest, north and 

south of the Goat Rocks center (from Tumac Mountain to Jennies Butte and Potato Hill), several of them as recently as 30 to 

15 ka (Ellingson, 1972; Swanson and others, 1989, Swanson, 

1990; Smith, 1993; Hildreth and Fierstein, 1997). Moreover, 

while Mount Rainier was growing in virtual isolation 30 km to 

the west, as many as ten additional middle and late Pleistocene 

vents along the Cascade axis between Goat Rocks and Bumping Lake contemporaneously produced hornblende andesite 

and dacite (Clayton, 1980; Smith, 1993).

Mount Adams

Mount Adams (3,742 m), an active 200-km3 andesite-dacite stratovolcano (fig. 10) and the most voluminous Quaternary 

volcano in Washington, lies on the Cascade axis 35 km south 

of Goat Rocks and 90 km north of Mount Hood. The great 

cone stands near the center of a 1,250-km2

 coeval volcanic 

field that contains at least 120 separate vents (fig. 8). About 25 

of these, all within 6 km of the summit, are considered flank 

vents because they erupted andesite and dacite similar to that 

from the stratocone’s central vent complex. The rest, however, 

are peripheral eruptive centers that probably (on compositional 

and spatial grounds) have independent conduits from magma 

reservoirs in the deep crust or mantle. Distributed on all sides 

of Mount Adams, between 5 and 25 km from its summit, the 

majority of these peripheral volcanoes are basaltic; others consist of basaltic andesite, but only eight nonflank vents (6 km 

or more from the summit) ever produced andesite or dacite. 

The greatest concentration of peripheral vents defines a northtrending (355o) belt only 6 to 7 km wide but more than 50 km 

long that includes several shields, numerous scoria cones, and 

the multivent stratovolcano itself. 

Detailed mapping and compositional data, volume estimates for 124 mapped eruptive units, and high-precision K-Ar 

dating have established the eruptive chronology and time-volume-composition relations for the whole volcanic field—stratoFigure 10. Upper half of Mount Adams (3,742 m) viewed northwestward from the air. Mazama Glacier at left; Battlement Ridge at 

center, flanked by debris-covered Klickitat and Rusk Glaciers. Black-over-brown unconformity at lower left places 33±14 ka lavas 

from South Summit (snow-topped shelf to left of summit) atop the dike-laced 500-ka lavas of a large predecessor edifice (Hellroaring 

Volcano). Small ejecta cone at summit, partly agglutinated, is dated at 15±8 ka. Black knob standing against the summit icecap at top 

of Battlement Ridge is “The Castle” (255 m lower than the summit), which is also visible in figure 24 (where seen from the north). Relief 

visible in image is about 1900 m. Image taken in 1987, a year of unusually strong summer melt-off. 

(Copyrighted photograph by and courtesy of Darryl Lloyd.) 
Distribution of Quaternary Cascade Volcanoes 17

cone and periphery (Hildreth and Lanphere, 1994; Hildreth and 

Fierstein, 1995, 1997). Eruption of basalt began about 940 ka, 

and inception of the stratocone near the center of the distributed 

volcanic field took place about 520 ka. Although the cone grew 

principally in three episodes centered at 500, 450, and 50-30 

ka, it remained recurrently active between pulses, never really 

shutting down. Andesite-dacite activity in the focal region and 

dominantly basaltic activity around the periphery have coexisted 

for 520 kyr, and their products are interstratified. During the 

520-kyr lifetime of the stratovolcano, basalt erupted many times 

within 5 to 10 km of its summit, but only once (at 63±14 ka) 

did basalt penetrate the andesitic focus itself and erupt centrally. 

Vents for calcalkaline, arc-tholeiitic, arc-alkaline, intraplate-alkaline, low-K HAOT, and ocean-island-type basalts are mutually 

interspersed and lie within 5 to 25 km of the central conduit system for the coeval andesite stratovolcano (Hildreth and Fierstein, 

1995). Substantial shields adjacent to Mount Adams include 

Goat Butte (~159 ka) and King Mountain (100-160 ka), both arc 

basalt; Lemei Rock (~22 ka) and Spring Creek (late Pleistocene), 

both HAOT; and Smith Butte (14 ka), alkali basalt. The cumulative eruptive volume of the volcanic field is 315 + 84 km3

, of 

which ~85 percent issued focally from central or proximal-flank 

vents. Of the total eruptive volume, basalt constitutes 9 to 15 

percent, dacite only 1 to 2 percent, and andesite plus basaltic 

andesite 84 to 89 percent (Hildreth and Lanphere, 1994).

Mount Hood

Mount Hood (3,425 m) is another active andesite-dacite 

stratovolcano on the Cascade axis. The present cone has been 

active since the middle Pleistocene (~0.5 Ma), but it overlaps 

remnants of at least two early Pleistocene centers (Scott and 

others, 1997; Keith and others, 1985; Swanson and others, 

1989; Wise, 1968, 1969). These include the mafic-to-andesitic 

Sandy Glacier Volcano (~1.3 Ma) beneath the west flank of 

Mount Hood (fig. 11) and the buried source(s) of andesitic lava 

flows (1.5-0.8 Ma) that crop out beneath products of the modern cone to the southeast, northeast, and near Vista Ridge to the 

northwest. There may have been an interval of low productivity 

(0.8-0.5 Ma) prior to inception of the modern cone ~500 ka.

Present-day volume of Mount Hood was estimated by 

Sherrod and Smith (1990) to be ~50 km3, but, taking into 

account recurrent glacial erosion, an eruptive volume of 70 

to 100 km3

 is likely for the Quaternary locus. Compared to 

Mount Adams where products of central and proximal-flank 

vents range continuously from 52 to 68 percent SiO2, Mount 

Hood is compositionally intermediate, 57 to 64 percent SiO2

(Wise, 1969; White, 1980; Scott and others, 1997). Although 

the stratocone consists mostly of phenocryst-rich andesite, low-silica dacite (63 to 64% SiO2

) erupted sporadically 

throughout its construction (White, 1980) and dominated products of the last 50 kyr, which have largely been summit domes 

and derivative block-and-ash flows that built extensive radial 

fans (Crandell, 1980; Scott and others, 1997). Despite the 

importance of dacitic pyroclastic activity, no widespread fall 

units are known to have erupted from Mount Hood. Concurrent with the central andesite-dacite activity, basaltic andesite 

(55-58% SiO2) erupted from several peripheral vents nearby, 

including the prominent Cloud Cap (~424 ka) and Pinnacle 

(~128 ka) centers, respectively only 5 km northeast and north 

of Mount Hood’s summit (Scott and others, 1997). True basalt 

is not known to have erupted at Mount Hood, nor anywhere in 

its near periphery since the early Pleistocene.

Within a 25-km radius, an additional 40 to 45 peripheral 

volcanoes of Quaternary age (and many more of Tertiary age) 

surround Mount Hood on all sides (fig. 8; Sherrod and Scott, 

1995; Sherrod and Smith, 2000). Most consist of basaltic 

andesite, a few are andesitic or basaltic, and all are modest 

in volume. Among the larger ones are the mafic shields of 

Lost Lake Butte to the northwest, Aschoff Butte to the west, 

Grasshopper Point to the southeast, and Fir Mountain to the 

northeast, as well as andesitic Badger Butte to the southeast. 

The youngest peripheral vent (7.7 ka), near Parkdale (12 km 

northeast of Mount Hood's summit), produced a basaltic andesite lava tongue that flowed 6 km down the Hood River Valley.

Farther north, the axial strip stretching between Mount 

Hood's peripheral array and the King Mountain-Mount Adams 

vent corridor is an area of relatively reduced vent density. 

This 40-km-long reach (Hood River Valley to Laurel; fig. 8) 

contains only about 20 Quaternary vents, but among them are 

substantial mafic shields at Gilmer, Underwood Mountain, and 

Mount Defiance, the latter in part andesitic.

Forearc

West of the Quaternary axis lie four extraordinary components of the volcanic Cascades: Two major foci at Mount 

Rainier and Mount St. Helens, a concentration of about 50 

mafic vents making up the Indian Heaven Volcanic Field, and 

a diffuse scattering of compositionally varied forearc volcanoes in the Portland region (fig. 8). The term “forearc” is used 

in a straightforward geographic sense (fig. 8) with no compositional implications.

Mount Rainier

Mount Rainier (4,392 m), the highest peak in the Cascades and second most voluminous stratocone in Washington, stands in splendid isolation 35 km west of the northern 

terminus of the continuous Cascade axis (fig. 8). The great 

cone consists of eruptive products of exclusively normal 

magnetic polarity, mostly of pyroxene andesite-dacite in 

the range 59 to 65 percent SiO2

, with minor amphibole in 

some dacites. Its growth has been fed largely by a persistent 

east-west summit-vent alignment from which radial dikes 

have occasionally produced voluminous flank effusions from 

vents partway up the edifice (Fiske and others, 1963; Sisson, 

1995; Sisson and Lanphere, 2000). Some 140 to 200 km3

 of 

rapidly erodable material has issued from a conduit system 

fixed beneath the cone for ~500 kyr (Fiske and others, 1963, 

1964; Luedke and others, 1983; Swanson and others, 1989; 

Lanphere and Sisson, 1995). 
18 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 11. West face of Mount Hood (3,425 m) viewed eastward up Muddy Fork of Sandy River. Sandy Glacier 

at center; Reid Glacier at upper right, below Illumination Rock (2,909 m) on right skyline. Darker cliffs beneath the 

glaciers are mafic-to-andesitic lavas and fragmental deposits of the early Pleistocene Sandy Glacier Volcano 

(~1.2±0.1 Ma) that dip eastward beneath Mount Hood and are cut by numerous dikes. Above the leftward-sloping 

unconformity, Mount Hood lavas supporting the glacier shelves are middle Pleistocene (~450-130 ka), whereas the 

summit consists largely of dacite domes younger than 50 ka (Scott and others, 1997). Relief visible in image is 2,200 m. 

(Photograph by W.E. Scott taken from Bald Mountain in August 1990.)

Mount Rainier is unusual among Cascade stratocones 

in having so few peripheral vents around it. Leaving aside 

the high dikes radial to the summit vent system, the only 

ones identified are a pair of basaltic-andesite vents (~105 ka; 

Lanphere and Sisson, 1995; McKenna, 1994) on the northwest 

flank (fig. 54 of Fiske and others, 1963), the hornblende-dacite 

lava of Bee Flat (~130 ka; 11 km north of the summit), and 

two much older vents for mafic lavas (48-53% SiO2

; Reiners 

and others, 2000) near Greenwater, 30 km to the north (fig. 8). 

Basaltic andesite is nonetheless common as chilled enclaves in 

the intermediate lavas of the edifice and is probably the main 

parent magma that feeds into Rainier's mid-to-upper crustal 

reservoir (Sisson and others, 2001). Petrologic studies suggest 

upper-crustal magma storage, differentiation, and recharge in 

complex multilevel reservoirs, commonly resulting in mixed 

or zoned eruptions (Venezky and Rutherford, 1997; Stockstill 

and others, 2002). 

The Rainier edifice is constructed principally of stacks of 

lava flows and of fragmental deposits derived from them. Large 

tephra eruptions have been rare, although remnants of two 

substantial pumice falls (~380 ka and 190 ka), each likely to 

have exceeded 1 km3

 in volume, have been recognized (Sisson 

and Lanphere, 2000). In postglacial time, at least 10 pumiceous 

tephra falls of limited volume (0.001 to 0.1 km3) erupted from 

Mount Rainier (Mullineaux, 1974), and as many as 25 layers 

of poorly vesicular (lithic) fallout identified by Vallance and 

Donoghue (2000) were deposited locally by phreatomagmatic 

eruptions or by ash clouds rising from the block-and-ash flows 

that have swept down nearly every radial valley. At least ten of 

these fall layers are younger than 2.6 ka, four may be younger 

than 2.2 ka, and most may have been accompanied by debris 

flows. Because Mount Rainier was built upon a high basement 

terrain already deeply dissected, however, it failed to construct 

a ring plain of fragmental deposits like those around Mount 

Shasta, St. Helens, and Glacier Peak. Block-and-ash pyroclastic 

flows and debris flows from Mount Rainier have been abundant 

but tend to be funneled into the many radial canyons, where 

they are readily eroded and reworked as alluvium. Because of 

its elevation (4,392 m), relief, hydrothermal alteration, icecap, 

glacier-fed radial valleys, and proximity to encroaching suburbs 

of the Seattle-Tacoma metropolis, Mount Rainier is the most 

threatening volcano in the Cascades, owing less to eruption 

itself than to its potential for sector collapse, magma-water-ice 

interaction, and downvalley debris flows (Crandell, 1971; Scott 

and others, 1995; Vallance and Scott, 1997). 

Several mid-Pleistocene andesite-dacite lava flows are 

of large volume (3 to 9 km3

) and as thick as 350 m (Sisson 

and others, 2001; Stockstill and others, 2002). Extending 
Distribution of Quaternary Cascade Volcanoes 19

from flank vents high on the edifice, some are as long as 15 

to 22 km and are preserved as an array of great radial ridges 

(Lescinsky and Sisson, 1998). Episodes of elevated eruptive 

output have been identified for the intervals 500 to 420 ka 

and 280 to 190 ka; and productivity has again been fairly high 

since ~40 ka (~0.4 ± 0.1 km3/kyr), including refilling of the 

amphitheater left by a major flank collapse at 5.6 ka (Vallance 

and Scott, 1997; Sisson and Lanphere, 2000).

A large predecessor volcano marked by hornblenderich andesite grew during the interval 1.3 to 1.0 Ma on or 

near the site of the present edifice, but it was largely eroded 

away prior to inception of Mount Rainier ~500 ka; remnants 

survive east of the modern volcano but principally as fluvial 

gravel and mudflow deposits of the Lily Creek Formation, 25 

to 35 km northwest of it (Crandell, 1963; Sisson and Lanphere, 2000).

Mount St. Helens

Mount St. Helens is a predominantly dacitic forearc 

volcano 50 km west of the Cascade axis (fig. 8). One of the 

most active young centers in the Cascades, it has erupted at 

least 75±15 km3 of magma—more than half of it in the last 28 

kyr. At least 90 percent of its products are dacite (along with 

minor rhyodacite), mostly emplaced fragmentally as fallout, 

pyroclastic flows, and derivative debris flows (Crandell, 1987; 

Scott, 1988, 1989; Mullineaux, 1996). The largest single 

eruption recognized took place ~3.5 ka, issuing ~4 km3 of 

magma as plinian fallout and pyroclastic flows. Andesite and 

basalt are mainly limited to the young composite stratocone, 

which was constructed during the last 2.5 kyr, laced with 

dacite domes (fig. 12), and eviscerated by sector collapse in 

1980 (Lipman and Mullineaux, 1981; Hopson and Melson, 

1990). Partly concealed by the young cone are remnants of 

episodically emplaced dacite dome complexes as old as 28 ka, 

sources of a stratigraphically complex fragmental apron that 

has permitted exceptionally detailed documentation and agecalibration of the volcano’s eruptive history (Mullineaux and 

Crandell, 1962, 1981; Hyde, 1975; Crandell, 1987; Hopson 

and Melson, 1990; Mullineaux, 1996). It was long thought 

that the earliest activity at Mount St. Helens was about 40 to 

50 ka, but recent fieldwork and dating has identified remnants 

of dacite domes and fragmental deposits from at least two 

older episodes at 300 to 250 ka and 160 to 35 ka (Clynne and 

others, 2005, in press). Extending as far as 10 km southwest 

of the modern edifice, these older dome clusters were ravaged 

by glacial erosion and by a late Pleistocene debris avalanche 

larger than that of 1980. In addition to the amphiboles and 

pyroxenes typical of younger St. Helens dacites, the products of these early episodes (preserved as fallout, domes, 

fragmental flows, and avalanche deposits) commonly also 

contain quartz and biotite phenocrysts, both of which are rare 

elsewhere in the Cascades. Most rocks of all ages at Mount 

St. Helens are distinctively phenocryst-rich, carrying 20 to 

40 percent plagioclase, and most products of the volcano are 

notably rich in Na.

Mount St. Helens dacites provide one of the clearest 

examples in the Cascades of focused intracrustal melting 

(Hildreth, 1981a; Halliday and others, 1983; Smith and Leeman, 1987, 1993; Dawes, 1994; Green, 1994a; Conrey, 2002). 

Extensive partial melting of deep young mafic crust results in 

extraction of voluminous dacitic magma, entrapment of most 

batches of the mantle-derived basalt that promotes the melting, and repeated replenishment-mixing that maintains a zoned 

or heterogeneous reservoir and produces the hybrid andesite intermittently permitted to erupt (Hopson and Melson, 

1990; Smith and Leeman, 1993). Petrologic studies suggest 

upper-crustal magma reservoirs at 6 to 12 km depth (Pallister 

and others, 1992), with water-rich cummingtonite dacites at 

temperatures <800°C and hornblende-hypersthene dacites at 

850 to 920°C with lower water contents (3 to 5 wt%) (Rutherford and others, 1985; Geschwind and Rutherford. 1992; 

Gardner and others, 1995). Gabbroic inclusions, many of them 

carrying interstitial glass, are common in St. Helens dacites 

(Heliker, 1995); some are probably partially melted fragments 

of Tertiary plutonic rocks (M.A. Clynne, work in progress), 

and some may be cumulates related to the mafic magmas that 

began penetrating the shallow dacitic reservoir during the last 

2,500 years. 

In considering the complex distribution of volcanoes 

within this segment of the Cascades, it should be kept in mind 

that during the early eruptive histories of nearby Rainier, 

Adams, Hood, and Indian Heaven, Mount St. Helens had not 

yet come into existence; and, although its eruptive activity 

may have started around 300 ka, its output rate remained low 

until after 28 ka (Clynne and others, 2005, in press).

Indian Heaven Volcanic Field

Indian Heaven (fig. 13), centered 30 km southwest of 

Mount Adams, is a 450-km2 volcanic field consisting principally of coalescing mafic shields and a scattering of cinder 

cones and tuyas (Hammond and Korosec, 1983; Korosec, 

1989). Although vents are also dispersed some 20 km east to 

west, about half of the 50 known vents (including the most 

productive ones) define a 30-km-long constructional highland 

(fig. 13) that trends N10 ̊E, roughly parallel to (but 15-20 km 

west of) the Cascade axis of southern Washington and northern Oregon (fig. 8). Eruptive activity extended from middle 

Pleistocene to Holocene, the latest eruption (9 ka) being 

that of Big Lava Bed (0.9 km3

) at the south end of the field. 

Contrary to older estimates of far greater longevity, virtually all Indian Heaven units exposed have been shown to be 

normally polarized (Mitchell and others, 1989), thus younger 

than 780 ka, the only exception recognized being the lowest lavas in the eroded core of Gifford Peak (Korosec, 1989). 

The volcanic field has an eruptive volume of 60 to 80 km3

, at 

least 80 percent of which is true basalt, making Indian Heaven 

the most voluminous field of Quaternary basalt anywhere in 

the Cascades north of the Newberry Volcanic Field in central 

Oregon. Indian Heaven basalts include low-K HAOT, calcalkaline, shoshonitic, and alkaline intraplate varieties. Subor-
20 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 12. North side of Mount St. Helens (2,950 m) in 1972, before it was wrecked by the sector collapse and ensuing eruption of 18 

May 1980. The elegant cone had been constructed by central eruptions of andesitic (and fewer mafic) lavas only during the last 2,500 

years, succeeding what had long been a dacite domefield. Several dacite domes continued to be emplaced during late Holocene 

growth of the cone; those visible in this image are Dogs Head dome on left skyline; Sugar Bowl dome directly below it; Goat Rocks dome 

right of center, halfway up; and West dome on right skyline. The Summit dome, exposed as the thin dogleg cleaver (The Boot) extending 

to the central skyline, was emplaced in the 17th Century. Goat Rocks dome and the fan of pyroclastic deposits and leveed andesitic lava 

flows below it were emplaced in the 19th Century, representing the last pre-1980 eruptive episode. The two midslope (andesitic) cleavers 

between Dogs Head and Goat Rocks domes were called the Big and Little Lizards. Relief visible in image 1,850 m. 

(Copyrighted photograph by and courtesy of Darryl Lloyd; taken in 1972.)

dinate basaltic andesite and sparse andesite (as much as 59% 

SiO2) erupted from several vents along the crest, but dacite is 

lacking (Hammond and Korosec, 1983; Smith, 1984; Leeman 

and others, 1990, 2005; Hildreth and Fierstein, 1995). In some 

respects, Indian Heaven is the “inverse” of nearby Mount St. 

Helens, in that voluminous basalt has passed through the crust 

in numerous conduits without engendering crustal melting on 

a large scale.

Diffuse Forearc Volcanism

Distributed forearc volcanism is as well developed in 

this arc segment as anywhere in the world, certainly more 

strikingly than elsewhere in the Quaternary Cascades. Scattered as far as 90 km west of the Cascade axis, the abundant 

forearc volcanoes provide perhaps the strongest volcanological 

rationale for postulating a segment boundary at the southern 

limit of their distribution (fig. 8). Radiometric dating indicates 

sporadic but persistent activity ranging from about 2.5 Ma to 

as young as 57 ka (Conrey and others, 1996; R.C. Evarts, work 

in progress, 2007). Most of the nearly 150 forearc vents can be 

lumped broadly into three main areal groups:

(1) a large region of at least 60 scattered vents extends eastwest on both sides of the Columbia River and through the 

city of Portland (Allen, 1975; Conrey and others, 1996); (2) 

southwest of Indian Heaven, a southeast-trending 40-km-long 

diffuse belt of at least 22 vents includes Marble Mountain, 

Bare Mountain, West Crater, and Trout Creek Hill and has had 

at least three postglacial eruptions (Wise, 1970; Hammond, 

1990); and (3) a northeast-trending diffuse zone of about 30 

small volcanoes extends 90 km from Battle Ground maar (past 

the Mount St. Helens vent cluster) to an array of basaltic vents 

near the Cispus River and Blue Lake (Swanson and others, 

1989; Swanson, 1994, 1996; Evarts and Swanson, 1994). 

A few of these 150 scattered forearc volcanoes are small 

shields, thus perhaps polygenetic, but most are monogenetic 

cones and lava flows. Although many consist of basaltic 

andesite and a few are andesitic (Swanson, 1989; Smith, 1993; 

Conrey and others, 1996; Leeman and others, 2005), most are 

basaltic—including HAOT, alkaline, calcalkaline, shoshonitic, 

and intraplate varieties. Tumtum Mountain, a small rhyodacite 

dome (68-70% SiO2), which stands alone 32 km southwest 
Distribution of Quaternary Cascade Volcanoes 21

Figure 13. Indian Heaven Volcanic Field, viewed southwestward from ~1,850 m elevation on southwest slope of Mount Adams (~1 km 

east of Horseshoe Meadow). The three skyline peaks with traces of snow are (left to right) Lemei Rock shield (also called Lake Wapiki 

volcano; 1,806 m), Bird Mountain (1,739 m), and Sawtooth Mountain (1,632 m); to the left of Lemei Rock are Berry Mountain (1,525 m) and 

(at left edge) Red Mountain (1,513 m); all five consist of middle or late Pleistocene basalt and belong to the north-trending vent zone that 

dominates the volcanic field (fig. 8). East Crater and Gifford Peak, additional members of the alignment, are hidden by Lemei Rock. Broad 

swell in middle distance is called Sleeping Beauty; it consists of Tertiary intermediate volcanic and sedimentary rocks and is marked by 

a prominent intrusive plug (at left end), four fine examples of clearcuts, and several inconspicuous Pleistocene scoria cones marginal to 

the Indian Heaven field (Hildreth and Fierstein, 1995). Forested foreground is Stagman Ridge, which consists of lavas from Mount Adams 

and is separated from Sleeping Beauty by the canyon of the White Salmon River. Relief visible in image is ~800 m.

 (Copyrighted photograph by and courtesy of Darryl Lloyd; taken in 1970.)

of Mount St. Helens, was formerly thought to be Pleistocene 

(Mundorff and Eggers, 1988) but is now dated as Miocene 

(R.C. Evarts, work in progress).

Contrasts with Garibaldi Volcanic Belt 

As defined by vent distribution, the across-arc width of 

the Quaternary volcanic region of southern Washington and 

northernmost Oregon is 5 to 10 times greater than the GVB 

(figs. 3, 8). The number of Quaternary volcanoes is correspondingly greater, with 127 vents identified in the 375-

km-long GVB segment and 412 in the 190-km-long Rainierto-Hood segment (table 1). The trend of the GVB roughly 

parallels the strike of the subducting plate, but any lower-plate 

influence on volcano distribution in southern Washington 

is obscure at best. Two stratocones, a concentrated basaltic 

volcanic field, and 150 scattered forearc volcanoes lie west of 

the broad and continuous Hood-Adams-Goat Rocks-Tumac 

Mountain axis, whereas virtually no vents lie significantly 

seaward of the narrow and discontinuous GVB axis. Vent 

alignments and elongate zones of high vent density are locally 

conspicuous in southern Washington, suggesting the influence 

of upper-crustal stress field and structure on magma transport. 

Such trends are close to northerly in the Mount Adams, Indian 

Heaven, and central Simcoe Mountains Volcanic Fields but 

N35°E near Mount St. Helens and N35°W for Quaternary 

cinder cones in the southern Simcoe Mountains. 

Dacite and rhyodacite are the principal eruptive products 

of the GVB, but in the Rainier-to-Hood segment Quaternary 

rhyodacite is almost absent and dacite is dominant only at 

Mount St. Helens. Eruptive volumes are hard to compare, 

owing principally to the greater severity of glacial erosion and 

the prevalence of explosive silicic volcanism in the GVB. At 

least two-thirds of the material erupted by Mount St. Helens 

was dispersed as fallout or deposited beyond the stratocone 

as pyroclastic and debris flows. The same is likely for all five 

major GVB centers, where fallout and fragmental flows would 

commonly have been deposited on the Cordilleran ice sheet or 

removed by its next advance. More than half (perhaps twothirds) of the Quaternary material erupted from the Mount 

Baker-Kulshan Caldera locus has been stripped (Hildreth, 

1996). Farther south, I estimate that the preserved edifice 

of Mount Adams represents 50 to 70 percent of the volume 

erupted, and comparable surviving fractions seem reasonable 
22 Quaternary Magmatism in the Cascades—Geologic Perspectives

for Hood and Rainier. Sherrod and Smith (1990) tabulated 

present-day volumes of 651 km3 for Quaternary products of 

the Rainier-to-Hood segment and 194 km3 for the GVB, a 

ratio of 3.4 to 1. The ratio of eruptive volumes may have been 

smaller than 2 to 1 and perhaps close to unity.

Oregon Cascades Segment

At least 1,054 Quaternary volcanoes (table 1) define an 

uninterrupted vent belt 25 to 50 km wide that straddles the 

Cascade crest for 340 km south of Mount Hood (figs. 14, 15). 

An additional 450 (or more) Quaternary vents are present 

on the great Newberry edifice just east of the Cascades. The 

continuous axial belt ends 30 km north of the California border, where a 64-km gap (containing many Tertiary but hardly 

any Quaternary volcanoes) separates it from the Quaternary 

volcanic region around Mount Shasta. The north end of the 

Oregon segment is placed at Frog Lake Buttes (~17 km south 

of Mount Hood), within a limited stretch of the Cascade axis 

where few centers are younger than early Quaternary and 

seaward of which the region of widespread forearc volcanism 

terminates (Sherrod and Smith, 2000). In contrast to the Portland region, in all of Oregon south of 45 ̊15’N only about ten 

Quaternary forearc volcanoes have been recognized (compare 

fig. 8 with figs. 14, 15; Sherrod, 1990a; White, 1992). About 

midway along the Oregon segment, with no discontinuity or 

change in width, there is a slight right-stepping dogleg in the 

Quaternary vent belt (fig. 14), from north-south in northern 

(and southern) Oregon to N15-20°E for the 40-km reach south 

of Crane Prairie Reservoir (43°46’N). 

For a convergent-margin volcanic belt, the density of 

Quaternary volcanoes in the Oregon Cascades is extraordinary 

(1,054 vents in ~9500 km2

; table 1). The only comparably 

extensive areas with such concentrations of Quaternary arc 

volcanoes are in Michoacán (Mexico) (Hasenaka and Carmichael, 1985; Hasenaka, 1994), southern Washington and northern California (table 1), and a few short reaches of the Chilean 

Andes (Moreno-Roa, 1976; Servicio Nacional de Geología y 

Minería, 1982).

Most of the Oregon Cascade volcanoes are scoria cones, 

small shields, and fissure-fed lava fields, the great majority 

composed of basaltic andesite. Basalt and andesite are common as well but subordinate. More than 50 of the Oregon 

volcanoes are substantial shields 3 to 10 km across that exhibit 

a range of profiles from gentle to steep. Steeper shields grade 

to lava cones and, with increasing accumulation of fragmental ejecta at the core, into mafic composite stratovolcanoes. 

Stratocones consisting principally of basaltic andesite are 

common in this segment, including Mounts McLoughlin, 

Thielsen, Bachelor, and Washington, Three Fingered Jack, 

Black Butte, Black Crater, and North Sister. Stratocones that 

likewise contain substantial basaltic andesite but also extend to 

andesitic or more silicic products include Broken Top, Middle 

Sister, Diamond Peak, and Mount Bailey. Major stratocones 

made up predominantly of andesite and more silicic material 

are Mount Jefferson, South Sister, Cappy Mountain (Williams, 

1957), and Mount Mazama (as well as Mount Hood, just north 

of the segment). 

Lesser andesitic centers, uncommon in Oregon, include 

Badger Butte and Frog Lake Buttes in the north (fig. 14), 

Brown Mountain and Pelican Butte in the south (fig. 15), 

Crater Butte at 43 ̊30’N, and minor clusters near Santiam 

Pass (44 ̊25’N) and south and west of Summit Butte (45 ̊N) 

(Sherrod and Smith, 2000; Webster, 1992). Isolated dacite 

volcanoes are fewer still, limited to Frog Lake Buttes, dome 

clusters near Mount Jefferson, First Creek (southeast of Three 

Fingered Jack), Bench Mark Butte, three tiny domes near 

Wickiup Reservoir, Mule Peak, and flows near Salt Creek (fig. 

14) west of Maiden Peak (Taylor and others, 1987; Sherrod, 

1991; Conrey, 1991; Sherrod and Scott, 1995; Sherrod and 

Smith, 2000). As in Washington, most dacite erupted from the 

flanks or summits of large stratovolcanoes. 

Quaternary rhyodacites (and minor rhyolite) are important in the Oregon segment, much less so than in the Garibaldi 

Volcanic Belt but far more so than in southern Washington. 

Such evolved products are restricted, however, to a few major 

centers—Mount Jefferson, Tumalo, Three Sisters, Cappy 

Mountain (Burn and Clover Buttes), and Mount Mazama 

(Taylor and others, 1987; Scott, 1987; Bacon and Druitt, 1988; 

Conrey, 1991; Hill, 1991; Nakada and others, 1994; Sherrod 

and Smith, 2000). In contrast to the arc itself, rhyolites are 

numerous in the rear-arc Newberry Volcanic Field and elsewhere across much of the Oregon Basin and Range Province.

For convenience of discussion, the Oregon segment is 

here divided into six reaches or subsegments (figs. 14, 15) 

designated I to VI from north to south.

Jefferson Reach (I)

The northern 75 km of the Oregon segment (fig. 14), 

from Frog Lake Buttes (45 ̊13’N) to South Cinder Peak

(44 ̊34’N), contains at least 175 Quaternary volcanoes but is 

the narrowest stretch of the Quaternary vent zone anywhere 

south of Mount Rainier. Only 25 km wide, this reach contrasts drastically with the adjacent region to the north, where 

Quaternary volcanoes are scattered from east of Mount Hood 

to west of Portland, 105 km from east to west. The northernmost 30 km of this narrow axial reach is also atypical (for 

the Oregon Cascades) in having few centers younger than 

early Pleistocene and in having several minor andesitic and 

dacitic volcanoes scattered within the dominating array of 

mafic shields (Sherrod and Smith, 2000). Just south of the 

andesite-dacite dome cluster at Frog Lake Buttes, the axis is 

dominated by an array of coalescing basaltic andesite shields 

that include Clear Lake Butte, Mount Wilson, North Wilson, 

Wests Butte, and Summit Butte, among which are scattered 

~20 mafic and andesite-dacite scoria cones and domes (Sherrod and Scott, 1995), few of which are likely to be younger 

than 500 ka. Because the Cascade crest here is atypically low 

(1,100-1,500 m) for Oregon, these older centers are heavily 
Distribution of Quaternary Cascade Volcanoes 23

soil-mantled and only locally glaciated (Sherrod, 1990b). 

From Pinhead Buttes (44 ̊56’N) southward, however, the rest 

of this 75-km reach (and indeed the next 250 km of the Cascade axis) is capped by a continuous array of middle Pleistocene and younger volcanic centers, generally glaciated, 

among which the largest mafic shields are Sisi and Olallie 

Buttes (Sherrod, 1990b).

Mount Jefferson

Mount Jefferson (3,199 m), largest volcano in this reach 

but one of the smaller andesite-dacite stratocones in the 

Cascades, was built in several stages between ~280 ka and 

latest Pleistocene, much of it around 70 ka (Conrey, 1991). 

The youngest additions to the main cone may have been in the 

interval 30 to 20 ka (fig. 16). Glaciated remnants of distributed 

andesite-dacite precursors east and west of the modern edifice, 

however, suggest longevity of at least 1 Myr for the Jefferson 

andesite-dacite locus; moreover, a few other evolved loci of 

pre-Quaternary age are recognized within 10 km (Conrey, 

Hooper, and others, 2001). Present-day edifice volume was 

estimated to be ~14 km3 by Sherrod and Smith (1990) or 25 

km3 by Conrey (1991), but in view of recurrent glacial erosion 

and dispersal of rhyodacitic plinian fallout and pyroclastic 

flows (Yogodzinski and others, 1983; Conrey, 1991), eruptive 

volume could well have been as much as 30 km3. 

Around the main cone (which is largely andesite-dacite 

with 58-64 percent SiO2), and as far as 13 km north and 7 km 

south of it, andesite and dacite also erupted from at least 35 

lesser vents, building many domes, small shields, and lava 

aprons. About 8 additional vents produced rhyodacite lavas 

and unknown volumes of pyroclastic ejecta that have been 

extensively stripped. North and south of this 20-km-long 

zone transecting Mount Jefferson, the eruptive dominance of 

basaltic andesite resumes, as exemplified by Olallie Butte, 

Three Fingered Jack, and dozens of lesser volcanic centers. 

Conrey (1991) emphasized the longevity of the andesite-dacite 

anomaly where, within a 20×8-km axial strip, mafic lavas 

have largely been excluded—not only during the lifetime of 

the Mount Jefferson edifice but throughout the Quaternary 

(and perhaps during much of Pliocene time as well). Conrey, 

Hooper, and others (2001) interpreted the strip as a long-lived 

focus of intracrustal melting and magma storage and provided 

evidence for ubiquitous mixing between mafic and silicic 

magmas in producing the andesites and dacites. Eruption of 

postglacial basaltic andesite from four monogenetic vents 6 to 

12 km south of Mount Jefferson (Scott, 1977) suggests that the 

system remains potentially active.

Sisters Reach (II)

From South Cinder Peak (44 ̊34’N) to Crane Prairie Reservoir (43 ̊47.5’N), the next reach of the Quaternary vent zone 

broadens markedly, expanding abruptly to 35 km, then gradually to 45 km toward its south end—where most vents lie well 

east of the Cascade crest (fig. 14). This 90-km-long subsegment contains at least 466 Quaternary volcanoes. It continues 

the high vent density of the narrower Jefferson reach and has 

roughly double that of the neighboring reach to the south 

(table 1). Nowhere in the Cascades are vent alignments more 

conspicuous than in this reach, where lines of mafic volcanoes 

(and even chains of rhyolite vents) form several north-trending 

arrays (Bacon, 1985; Hughes and Taylor, 1986; Scott, 1987; 

Scott and Gardner, 1990, 1992). At the leading western end 

of the propagating crustal melting anomaly that has crossed 

Oregon since the mid-Miocene (MacLeod and others, 1975; 

Jordan and others, 2004), this reach is also extraordinary (for 

the Quaternary Cascades) in having numerous eruptive units 

of true rhyolite, which is rare elsewhere along the arc proper 

(though common at rear-arc centers).

Among the many mafic volcanoes, basaltic andesite is 

again dominant, but true basalt is more common in this reach 

than elsewhere in the Oregon Cascades. More than 30 mafic 

shields and stratocones, ranging in age from early Pleistocene 

to Holocene, are continuous along the entire length of the subsegment. Larger ones include Three Fingered Jack, Maxwell 

Butte, Hoodoo Butte, Cache Mountain, Mount Washington, 

Belknap Crater (fig. 17), Scott Mountain, Black Crater, Black 

Butte, Trout Creek Butte, North Sister, Little Brother, The 

Husband, Substitute Point, Sphinx Butte, The Wife, Burnt 

Top, and the shields of the Bachelor chain, as well as Tumalo, 

Koosah, Horse, Packsaddle, Williamson, Irish (30 km3

), Lookout, and Cultus Mountains (Williams, 1957; Taylor and others, 

1987; Sherrod, 1990b; Sherrod and Smith, 1990, 2000). 

Postglacial mafic eruptions, more common here than 

anywhere else in the Cascades, include a lava flow adjacent to South Cinder Peak, the Nash Crater-Lost Lake cone 

cluster, Sand Mountain chain, Inaccessible Cone chain, Blue 

Lake cone, Belknap shield complex (fig. 17), and several 

additional monogenetic scoria cones or chains with associated lava flows (Williams, 1957; Taylor, 1968; Black and 

others, 1987; Gardner, 1994; Sherrod and others, 2004). In 

the area of McKenzie and Santiam Passes (fig. 14), more 

than a dozen separate mafic eruptions are radiocarbonbracketed between 4.5 and 1.1 ka (Sherrod and others, 

2004), representing a 15-km3

 distributed mafic pulse in the 

late Holocene (Taylor, 1968). Moreover, just south of there, 

peripheral to the Three Sisters, several additional mafic units 

that include Sims Butte, Cayuse Crater, LeConte Crater, 

the Mount Bachelor chain, the Egan Cone cluster, and the 

Katsuk-Talapus chain may all have erupted in the interval 18 

to 8 ka, during deglaciation or early postglacial time. Near 

the south end of the Sisters Reach, a postglacial basaltic lava 

flow erupted on the east flank of (much older) Sitkum Butte.

Much of the andesite and dacite in this reach erupted 

from central or high flank vents of four large volcanic centers—Broken Top, Middle and South Sisters, and the midPleistocene Todd Lake Volcano south of Broken Top (Taylor, 

1978; Taylor and others, 1987; Webster, 1992), but several 

small dispersed intermediate volcanoes are present as well. 

Isolated among the coalescing mafic volcanoes, phenocrystpoor intermediate domes include dacitic Bench Mark Butte 
24 Quaternary Magmatism in the Cascades—Geologic Perspectives

0 10 20 30 40 50 km

South Cinder Peak

Sisi Butte

JEFFERSON

HOOD

Olallie Butte

Pinhead Buttes

Summit Butte

Frog Lake Buttes

Barlow Pass

Detroit

Santiam Pass

TFJ

Sand Mtn chain

Sisters

MW

BB

OB

Scott Mtn BC

Belknap shield

McKenzie Pass

122°

Newberry

Volcanic Field

Newberry

Caldera

LaPine 

Bend

Crescent

CAPPY

MTN

Irish Mtn

Packsaddle Mtn

Williamson Mtn

Horse Mtn

Nash Crater

Oakridge

Triangle

Hill

Tumalo

Volcanic Field

THREE SISTERS

NS

MS

SS BROKEN TOP

Warm

Springs

TMR

SB

TLV

Mt Bachelor

SM

BMB

LM

WR

Waldo Lake

The Twins

Salt Creek

Fuji Mtn

Maiden Peak

Mt Yoran

Diamond Pk

Cowhorn Mtn

Lakeview Mtn

Crater Butte

HB

DM

CPR

I

II

III

121°30' 121°

45°

44°30'

44°

43°30'

121°30' 121°

Figure 14. Northern part of Oregon segment of the Cascade arc, showing distribution of 760 

Quaternary volcanic vents and subdivided into reaches (I, II, III, by gray dashed east-west lines), 

as discussed in text. Vent symbols as in figures 3 and 8. Axial dashed line is Cascade topographic 

crest. Small green ovoid encloses vents for silicic units attributed to Tumalo Volcanic Field. Large 

orange ovoid is generalized outline of rear-arc Newberry vent field, which encompasses at least 450 

additional Quaternary vents, after MacLeod and others (1995). Note the atypical abundance of silicic 

vents (predominantly rhyolite) in the Cascade arc northwest of Newberry. Abbreviations from north 

to south: TFJ, Three Fingered Jack; BB, Black Butte; MW, Mount Washington; BC, Black Crater; TMR, 

Tam McArthur Rim; NS, North Sister; MS, Middle Sister; SS, South Sister; TLV, Todd Lake volcano; SM, 

Sheridan Mountain; BMB, Bench Mark Butte; SB, Sitkum Butte; LM, Lookout Mountain; CPR, Crane 

Prairie Reservoir; WR, Wickiup Reservoir; DM, Davis Mountain; HB, Hamner Butte; OB, Odell Butte.
Distribution of Quaternary Cascade Volcanoes 25

0 10 20 30 40 km

Goose Nest

Ashland

Medford

Brown Mtn

Mt McLoughlin

Pelican Butte

Mtn Lakes

volcanic

cluster

Little

Aspen

Butte

Devils Peak

Big Bunchgrass

shield

Klamath

Falls

Klamath

Graben

Union Peak

MAZAMA

Crater Lake

Caldera

CAPPY

MTN

Mule Peak

Chemult

Howlock Mtn

Mt Thielsen

Mt Bailey DL

Timber Crater

pre-Mazama

rhyodacite field

Yamsay

Mtn

Cowhorn Mtn

Sawtooth Mtn

OREGON

CALIFORNIA

42o30'

42o

43o

122o 121o30'

Diamond

Peak Lakeview Mtn

Crescent

Copco

cones

43o30'

122o 121o30'

numerous Pliocene volcanoes

X

III

IV

V

VI

Figure 15. Southern part of Oregon segment of the Cascade arc, showing distribution of 294 Quaternary volcanic 

vents and subdivided into reaches (IV, V, VI, by gray dashed east-west lines), as discussed in text. Vent symbols as in 

figures 3 and 8. DL, Diamond Lake. Axial dashed line is Cascade topographic crest (poorly defined in Crater Lake area 

and southward). Green field encloses vents for mid-Pleistocene pre-Mazama rhyodacites. Yamsay Mountain is a major 

volcanic locus of late Tertiary age, and the Mountain Lakes volcanic cluster (pink field) may be of both Pliocene and 

Quaternary age. Area around and south of Klamath Falls has numerous well-preserved mafic shields and scoria cones, 

all or most older than 2 Ma (Sherrod and Pickthorn, 1992).
26 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 16. North face of Mount Jefferson (3,199 m) and north and northwest ridges of the edifice, viewed toward the south-southeast. 

Upper part of mountain is carved from thick andesite-dacite lavas younger than 70 ka and some as young as the last major glacial 

interval. Dome complex on far left skyline (28±9 ka) is overlain by uppermost package of summit lavas, which gave a 40Ar/39Ar age of 

20.5±4 ka (R.M. Conrey, unpub. data, 2007). There have been no postglacial eruptions of the main edifice. True summit is largely hidden 

slightly beyond the apparently uppermost crags of the north summit. Relief visible in image is 1,500 m. 

(Photograph by author taken in August 1988.)

(65.5% SiO2) in the south and andesitic Hogg Rock and 

Hayrick Butte (both 60% SiO2

) near Santiam Pass, all three 

of late Pleistocene age (Taylor, 1981; Black and others, 1987; 

Hill and Priest, 1992). In upper First Creek, just northeast 

of Santiam Pass, an early Pleistocene phenocryst-poor lava 

and tuff include both dacite and andesite, and several middle 

Pleistocene andesitic scoria cones are present in the vicinity of 

Triangle Hill (Sherrod and others, 2004). Kokostick Butte, a 

phenocryst-rich dacite coulee with several satellite vents (63% 

SiO2

), erupted 7 km south of South Sister at ~27 ka. The Fourin-One Cone chain of six andesitic scoria cones (56-59% SiO2

) 

erupted 5 km northwest of North Sister at ~1.9 ka. Finally, 

the Collier Cone (at the northwest toe of North Sister), one of 

the youngest eruptive units in the reach (~1.5 ka), released an 

apron of lava flows as long as 13 km that range from 56 to 65 

percent SiO2

 (Schick, 1994).

The middle part of this reach contains the second most 

extensive field (behind Lassen) of silicic volcanic rocks in 

the Quaternary Cascades (fig. 14). Within a 20×25-km area 

extending from the Three Sisters to well east of Broken Top 

and Mount Bachelor, there are no fewer than 50 known or 

concealed eruptive sites for rhyolitic and rhyodacitic lavas and 

ejecta (Taylor, 1978, 1987; Taylor and others, 1987). Most of 

the silicic units are middle and late Pleistocene, but ~20 vents 

adjacent to South Sister produced Holocene rhyolite during 

two or three eruptive episodes at 2.3-2.0 ka (Scott, 1987). 

Because many of the more easterly silicic units are middle 

Pleistocene (Hill, 1991), still more silicic vents may be hidden by younger lavas and till. Although rhyodacite (68-72% 

SiO2

) is fairly common in the Cascade arc, Quaternary rhyolite 

(72-77% SiO2) is rare, significant only at Mount Garibaldi, 

in the Lassen region, and here in the Three Sisters-Broken 

Top-Tumalo area. As widely noted for “bimodal” extensional 

volcanic fields elsewhere, the elevated proportions of true 

rhyolite and true basalt in the Sisters reach (at the expense of 

the intermediate compositions dominant in adjacent reaches) 
Distribution of Quaternary Cascade Volcanoes 27

Figure 17. Little Belknap shield (~3 ka) flanked by 150-m-high Belknap Crater scoria cone on left and middle Pleistocene Mount 

Washington in right distance. Little Belknap is a flank shield built on the much larger Belknap shield, which had several secondary vents 

in addition to the summit scoria cone and was active both before and after growth of Little Belknap (from before 3 ka until about 1.3 ka). 

Summit of Belknap is 500 m higher than the camera and summit of Little Belknap 350 m higher. The two represent examples of shields 

with and without significant scoria cones, perhaps because the magma that fed Little Balknap had extensively degassed prior to shallow 

lateral transport to the lower flank vent. Mount Washington (2,376 m), 8 km north and nearly 800 m higher than the camera, is a glacially 

sculpted mafic stratocone (like North Sister, Three Fingered Jack, and Mount McLoughlin) with a broad apron of mafic lavas; its summit is 

part of a 400-m-wide central intrusion surrounded by an eroded collar of radially dipping lavas and ejecta cut by many dikes. 

(Photograph by author taken in August 1988.)

probably reflects penetration of the Cascade arc by whatever 

combination of mantle flow and lithosphere extension has 

guided rhyolitic volcanism westward across the High Lava 

Plains and Newberry since the Miocene (MacLeod and others, 

1975; Jordan and others, 2004).

Tumalo Volcanic Field (Bend Highland)

The area containing distributed silicic lavas east of 

Broken Top was called the “silicic highland” by Taylor (1978, 

1987) and later termed the “Tumalo volcanic center” by Hill 

and Taylor (1989, 1990). The 15×20-km area (fig. 14) includes 

andesitic and mafic scoria cones as well as numerous rhyolitic 

and rhyodacitic lava flows and domes. It is thought to be the 

source of several rhyolitic to andesitic ignimbrites and plinian fall deposits exposed near the towns of Tumalo and Bend 

(Taylor, 1981; Hill and Taylor, 1990; Mimura, 1992; Taylor 

and Ferns, 1994) that erupted between ~650 ka and ~250 ka 

(Sarna-Wojcicki and others, 1989; Gardner and others, 1992; 

Lanphere and others, 1999; Sherrod and others, 2004). Owing 

to extensive cover by younger mafic lavas and glacial deposits, 

neither the integrity of the Tumalo volcanic “center” nor its 

western limit is well defined. Because silicic lavas of Todd 

Lake Volcano and Tam MacArthur Rim, both adjacent to the 

Broken Top edifice (Williams, 1944; Taylor, 1978) erupted 

during the same time interval (Hill, 1991) as the Tumalo volcano center (as defined by Hill and Taylor, 1990), they could 

likewise be considered parts of an extensive mid-Pleistocene 

silicic volcanic field. Of the four nearby stratovolcanoes, 

South and Middle Sisters are now known to be wholly younger 
28 Quaternary Magmatism in the Cascades—Geologic Perspectives

(Fierstein and others, 2003) than the distributed activity in the 

“silicic highland” (Taylor, 1978, 1987, 1990a; Hill and Taylor, 

1990; Sherrod and others, 2004), but the eruptive lifetimes of 

North Sister and Broken Top overlapped with it.

Volumes of three extensive ignimbrites, patchily but 

widely preserved in the periphery of the highland, are poorly 

known but likely to be in the range 5 to 10 km3 each. Such volumes are near the usual threshold for caldera formation, so the 

Tumalo Volcanic Field might or might not contain an unrecognized filled caldera, obscured by products of younger eruptions. 

Following Hill and Taylor (1990), Sherrod and others (2004) 

suggested a 5-km-wide buried source area centered on Triangle 

Hill (fig. 14), whereas Conrey and others (2002) suggested a 

location 6 km farther west (near Tam MacArthur Rim) as a 

possible source for one of them. Pinning down the source vents 

for the three Tumalo ignimbrites (the Desert Springs, Tumalo, 

and Shevlin Park Tuffs) remains one of the more challenging 

unresolved problems in Cascades volcanology.

Broken Top

Only 5 km southeast of South Sister, Broken Top was 

a modest middle Pleistocene stratovolcano (eruptive volume 

7-10 km3), active in the interval from about 300 to 150 ka and 

long extinct. Its mafic apron lavas bank against Todd Lake 

Volcano (460±30 ka; Hill, 1991), and they sandwich thick 

rhyodacite lavas at Tam MacArthur rim (213±9 ka; Hill, 1991) 

and at Squaw Creek Falls (169±2 ka; USGS work in progress). Although dominantly basaltic andesite (Webster, 1992), 

subordinate products of Broken Top range from andesite to 

rhyodacite. Internal structure of the ice-sculpted edifice (fig. 

18) is well exposed on cirque headwalls, radial ridges, and in 

a large southeast-facing amphitheatre of uncertain origin (Taylor, 1978; Grubensky and others, 1998). Cayuse Crater is an 

unrelated postglacial basaltic (50-52% SiO2) complex of lava 

flows and scoria cones that erupted ~11 ka on the southwest 

apron of Broken Top. Like Mount Baker, Goat Rocks, and 

Brokeoff, Broken Top was a stratocone built at the margin of a 

large silicic complex. 

Three Sisters

These three modest contiguous stratocones (Williams, 

1944), conventionally lumped sororally, could hardly display 

less family resemblance. North Sister is a middle Pleistocene 

monotonously mafic edifice, Middle Sister a basalt-andesitedacite cone largely built in the interval 37 to 14 ka, and South 

Sister a bimodal rhyolite-andesite edifice that has alternated 

compositional modes from 178 ka to 2 ka. For each of the 

three, the eruptive volume is likely to have been in the range 

15 to 30 km3

, but such estimates are fairly uncertain, owing not 

only to glacial erosion but to doubts about the affinities and 

vent locations for varied apron lavas that issue from beneath 

bases of the two younger cones (see Taylor and others, 1987). 

North Sister (3,074 m) is a glacially dissected stratocone 

of fairly uniform basaltic andesite (52.5-55% SiO2; Taylor, 

1987; Schmidt and Grunder, 2003), sufficiently eroded to 

expose hundreds of dikes and sills internally. Many are zigzag 

or inconsistently oriented within the great pile of thin rubbly 

lava flows, but Schmidt (2005) observed a long-term shift 

from edifice-influenced radial diking toward later dominance 

of north-south dikes controlled by the regional stressfield. Oldest of the Three Sisters, its period of construction overlapped 

with that of Broken Top. A few of the distal lavas from North 

Sister overlie the northwesternmost remnant of the Shevlin 

Park Tuff, which at ~260 ka is the youngest ignimbrite of the 

Tumalo Volcanic Field (Lanphere and others, 1999). The oldest lava flow exposed at the south toe of North Sister yields an 

age of 311±23 ka (USGS work in progress), whereas higher 

parts of the multistage edifice may be as young as 100 ka 

(Schmidt and Grunder, 2003; Schmidt, 2005). It is noteworthy that so long-lived an edifice remained so compositionally 

monotonous (4.5-6% MgO; 0.6-0.8% K2O) throughout its 

eruptive history. Much younger are late Pleistocene northtrending chains of mafic scoria cones and dike-fed agglutinate 

that erupted through and unconformably drape the North Sister 

edifice and extend ~11 km farther northward (collectively 

called the “Matthieu Lake fissure” by Schmidt and Grunder, 

2003; Schmidt, 2005). 

Middle Sister (3,062 m) is the youngest cone of the 

three, the present edifice having been built during the interval 

37 to 14 ka, mostly 25 to 18 ka (USGS work in progress), 

although glaciation (persistent from late Pleistocene to the 

present) has sharply steepened its eastern face. The young 

cone has issued a range of mafic, andesitic, and dacitic lavas 

(52-65% SiO2

) from its central vent, as well as dacites from 

six flank vents (three of them high on the edifice) and andesites from three more. The largest dacite flank vent, which fills 

the saddle between North and Middle Sisters, built a substantial pile (informally called Prouty Point, the Black Hump, or 

Step Sister) of at least five coulees (64% SiO2) dated at 27 

to 18 ka. Distal mafic lavas from a buried older edifice that 

extend eastward beyond the limit of lavas from the modern 

cone yield ages of 180 to 160 ka, and a western apron of 

young andesite-dacite lavas (30 to 19 ka) from Middle Sister 

banks against glaciated mafic centers dated at 170 to 150 ka. 

There thus appears to have been an extended period (>100 kyr 

long) with little or no activity on the site prior to initiation of 

modern cone growth, for which eruption of the high-silicarhyolite coulee of Obsidian Cliffs (38±2 ka) at the northwest 

toe of the subsequent Middle Sister edifice may have been the 

thermal harbinger.

South Sister (3,157 m), formerly considered the youngest stratocone in the cluster (Wozniak, 1982; Clark, 1983), 

began its growth well before Middle Sister. A distal dacite lava 

flow southeast of the cone yields an age of 178±1 ka (USGS 

work in progress), and an age of 93+11 ka was reported 

by Hill (1991) for an andesite lava flow to the northeast, 

although each of these units predates construction of the 

modern cone. During the interval 50 to 30 ka, South Sister 

became a rhyolite volcano consisting of numerous and varied 

(aphyric to phenocryst-rich) rhyolitic lava flows and domes 

(72-74% SiO2

) that crop out radially on the cone to eleva-
Distribution of Quaternary Cascade Volcanoes 29

Figure 18. Broken Top mafic stratocone, inactive since 150 ka or longer, viewed northwestward. Radially dipping strata of the cone 

include subequal proportions of lava flows and flow breccias and of proximal agglutinate and scoria falls, nearly all having 54 to 56 

percent SiO2

. Numerous mafic dikes and sills intrude the cone, and an apron of compositionally similar mafic lavas extends far outward 

from the cone. Skyline crags behind buttress at right are top of a central intrusion 400 m wide, also with 54 percent SiO2

. Altered core 

of the edifice was eviscerated by glacial erosion, probably by enlargement of an old crater (suggested by internal unconformities). 

Unusual orientation of the cirque, open to the southeast, might suggest sector collapse, but no avalanche deposit is recognized. 

Neoglacial moraine obscures small ice remnant. Orange layer on left wall is 20-m-thick dacitic pyroclastic deposit, which (along 

with two thinner such layers) interrupted the mafic monotony of the cone-building sequence. Relief visible in image is about 600 m. 

(Photograph by author taken in September 2001.)

tions within 500 m of the present summit. Around the base 

of the edifice, two large rhyolite domes (Kaleetan Butte and 

Devils Hill) at the south toe of South Sister, one near Green 

Lakes at the east toe, another near Chambers Lakes at the 

north toe, and a coulee along Squaw Creek northeast of the 

cone erupted during this same time interval; all five have 74 

percent SiO2. Resumption of intermediate magmatism began 

with radial outflow of several rhyodacite and dacite lavas in 

the interval 38 to 32 ka, alternating with the rhyolite eruptions, 

followed by construction of a broad cone of silicic andesite 

lavas (62-64% SiO2

), and culminating at 27+3 ka in growth 

of a steeply dipping summit cone of agglutinate-dominated 

basaltic andesite (54-57% SiO2). Emplacement of a multiphase 

andesitic intrusive complex (17+2 ka) culminated in formation 

of a summit crater 800 m wide (fig. 19). Subsequent filling of 

the large crater by thick dacite lava and more than 150 m of 

dacitic pyroclastic ejecta, followed by draping of the summit 

by a final thin sheet of andesitic agglutinate (derived from the 

small present-day summit crater), constitute a sequence that 

may have extended to the very end of the Pleistocene.

After about 10,000 years of inactivity, the South Sister 

locus underwent yet another compositional reversal, yielding 

two complex but discrete rhyolitic eruptions ~2.2 and 2.0 ka 

(Taylor, 1978; Scott, 1987). The first episode produced the 

0.5-km3

 Rock Mesa coulee and subordinate tephra and satellite 

domelets (altogether ~0.53 km3; all ~73.5 percent SiO2) at the 

southwest toe of the South Sister edifice. The second episode 

produced the dike-fed 5-km-long Devils Chain of 16 vents that 
30 Quaternary Magmatism in the Cascades—Geologic Perspectives

cone-building

andesite lavas intrusive

complex true summit

mafic agglutinate

coarse dacite ejecta

dacite lava flow

atop unconformity

cone-building

andesite lavas

Neoglacial moraines

Figure 19. Northeast face of South Sister. Neoglacial moraines in foreground conceal small Prouty 

Glacier. At left and right, radially dipping gray-and-red lavas and ejecta define an andesitic stratocone 

(27±3 ka), the top of which ends abruptly at a subhorizontal unconformity. Craggy mass just left of center is 

an intrusive complex dated at 17±2 ka. Above central snow chute, a subhorizontal but convolutely foliated 

dacite lava flow at least 60-m-thick fills the paleocrater, rests on the unconformity, and banks against 

the intrusive complex. The dacite lava is overlain by 150 m of poorly to crudely stratified gray-to-tan 

phreatomagmatic dacite ejecta of different composition, capped in turn by a red-and-gray ledge of maficandesite agglutinate 5-to-10 m thick at the true summit. Relief visible above the lower snowfield is 290 m. 

(Photograph by author taken in September 2006.)

cuts both the 30-ka Devils Hill dome and the southeast flank 

of the South Sister edifice itself. North of a 3-km-long gap 

bypassing the summit, another 1.2-km-long parallel chain of 

four additional minor vents runs down the northern slope of 

the edifice and may have been fed concurrently by the same 

dike system (Scott, 1987). All 20 vents of the younger episode 

(about 0.32 km3 altogether) produced rhyolite lava or tephra 

with 72.5 percent SiO2, compositionally clearly distinguishable from rhyolite of the Rock Mesa episode. The northernmost vent of the Devils Chain proper lies at 2,400 m, well up 

the South Sister edifice and close to vents for several of the 

late Pleistocene rhyolites. That some 60 percent of the magma 

released during the younger episode issued from this uppermost vent (the Newberry Flow) supports the likelihood that the 

rhyolitic magma reservoir lies under the edifice itself (Bacon, 

1985; Scott, 1987).

Taylor (1990a) proposed that a plexus of varied magma 

reservoirs has underlain the whole Tumalo-Broken TopThree Sisters region. Hill (1991) advocated a wide range in 

the proportions of crustal and mantle contributions to intermediate magmas of the region, and he inferred lithologically 

distinct crustal sources for the silicic magmas around the 

Three Sisters vis-à-vis those in the Tumalo-Broken Top area. 

Bacon (1985) and Scott (1987) addressed the implications 

of rhyolite vent patterns for the nature, size, and depth of the 

silicic magma reservoir that probably still lies below South 

Sister. Eruption of numerous postglacial and late-glacial 

basaltic-to-andesitic lavas and cinder cones nearby indeed 

suggests that magmatism beneath the Three Sisters region 

remains potentially as vigorous as ever (Williams, 1944; 

Taylor and others,1987; Scott and Gardner, 1990)

Mount Bachelor

Mount Bachelor, a 2,763-m mafic cone-atop-shield 

volcano 15 km southeast of South Sister, is the largest center 

along a 25-km-long chain of ~50 vents that produced 40+5 km3

of basalt and basaltic andesite (49-57% SiO2) in a few major 

episodes during the interval 18 to 8 ka (Scott and others, 1989; 

Scott and Gardner, 1990, 1992; Gardner, 1994). Most of the 

vents lie along a north-south alignment marked by numerous scoria cones, but most of the magma issued effusively to 
Distribution of Quaternary Cascade Volcanoes 31

build shields and lava fields. Five main eruptive episodes were 

defined (stratigraphically and paleomagnetically) and related 

to late-glacial features, suggesting that most of the material 

erupted between 15 ka and 12 ka and that as much as 25 km3

of it may have erupted in less than 1,500 years. 

The comprehensive study of the Bachelor chain illustrates several important processes: (1) that rapid eruptive 

pulses built much of the Quaternary Cascades of Oregon; 

(2) that the north-south vent alignments common in the 

Oregon Cascades are parallel to the arc and to the maximum horizontal compressive stress in the segment (Zoback 

and Zoback, 1980; Bacon, 1985; Scott and Gardner, 1992); 

(3) that eruption rates for mafic shields (on 103 to 104 year 

timescales) can match or exceed those for stratovolcanoes; 

and (4) that coeruption and mixing of independent magmatic 

lineages indicate complex shallow magmatic plumbing and 

compartmentalized reservoirs (Gardner, 1994). A comparable 

effusion rate for basalt zoned to basaltic andesite marked 

the late Holocene just north of the Three Sisters, where the 

6 to 9 km3

 multivent Belknap shield was constructed in less 

than 1,500 years (Taylor, 1965, 1968). Only slightly older 

than Belknap (fig. 17), a nearby chain of scoria cones that 

includes Nash Crater and Sand Mountain (fig. 14) produced 

2 to 3 km3 of Holocene mafic lavas and ejecta in about 1,000 

years (Taylor, 1968; Sherrod and others, 2004).

Wickiup Reach (III)

From near Crane Prairie Reservoir (43 ̊47.5’N) as far south 

as Cowhorn Mountain (43°24’N), the next 40-km reach of the 

Oregon Cascades (fig. 14) is distinguished by a modest (15-20 

km) right-stepping offset of the Quaternary volcanic zone 

(Williams, 1957; Sherrod and others, 1983b; Sherrod, 1991; 

MacLeod and Sherrod, 1992; Sherrod and Smith, 2000). As the 

Cascade crest also swings west (from Maiden Peak to Diamond 

Peak), most of the Quaternary vents remain east of the drainage divide, as in the Mount Bachelor region to the north. The 

Quaternary vent belt remains 40 to 50 km wide, but it trends 

N15-20°E in this reach, whereas its strike is northerly for the 

175 km between Mount Hood and the Bachelor chain. Although 

at least 118 Quaternary vents are recognized in this oblique subsegment, its vent density is only half that in the reach next north 

(table 1), and vent alignments are present but far less common.

Along with scattered cinder cones, middle and late 

Pleistocene mafic shields are numerous here. At least 15 

shields are prominent (Williams, 1957), and several (for 

example, The Twins, Hamner Butte, Maiden Peak, Mount 

Yoran, and Davis, Fuji, Sawtooth, and Cowhorn Mountains) 

have eruptive volumes greater than 10 km3

. In general, the 

shields east of the Cascade crest are well-preserved, whereas 

most of those on or west of the crest are glacially sculpted. 

A few of the mafic shields (for example, Odell Butte) are 

unusually steep, and (with increasing amounts of fragmental material at the core) some grade from shields to modest 

stratocones like Mount Yoran, Lakeview Mountain, and 

2,665-m Diamond Peak (~15 km3

). 

Basaltic andesite greatly exceeds basalt along this reach, 

but products still more evolved are especially sparse. Much of 

the upper part of Diamond Peak and adjacent Crater Butte are 

andesitic (Webster, 1992), and dacitic to rhyolitic lavas and 

tuffs were mapped by Sherrod (1991) in Salt Creek ~10 km 

west of Maiden Peak. Crossing the cluster of mafic shields 

south of Wickiup Reservoir, a 20-km-long north-south alignment of three separate Holocene andesitic lava flows (the only 

postglacial eruptive units in the Wickiup Reach) may have 

been mutually dike fed. Also nestled among the steep shields 

south of Wickiup Reservoir are three small isolated andesitedacite domes—McCool, Ranger, and Eaton Buttes (with 62.1, 

63.9, and 62.6% SiO2

, respectively)—that are thought to be of 

early Pleistocene age (Sherrod, 1990b; MacLeod and Sherrod, 

1992; Sherrod and Smith, 2000).

Thielsen Reach (IV)

From Cowhorn Mountain (43°24’N) to about 43°05’N, 

just north of Timber Crater, the next 40-km-long reach of the 

Quaternary vent zone narrows slightly to ~35 km. With only 

86 Quaternary vents recognized, its vent density is similar 

to that of the Wickiup reach to its north but much lower than 

in the Mount Mazama area to its south (table 1). The belt 

resumes the roughly north-south trend otherwise so characteristic of the Oregon segment but interrupted by the Wickiup-Cowhorn right-stepping offset. Vent alignments are not 

pronounced in this subsegment, although crude arrays trending 

N15°E do run through Mounts Bailey and Thielsen. 

As in most of the Quaternary Oregon Cascades, basalt 

is subordinate to basaltic andesite, which makes up numerous scoria cones, shields, and a few steep cone-atop-shield 

complexes that include Cowhorn, Sawtooth, and Howlock 

Mountains, and 2,799-m Mount Thielsen, the “lightning rod 

of the Cascades” (Williams, 1933, 1957; Sherrod, 1991). Like 

290-ka Mount Thielsen (fig. 20), the intrusive spire of which 

is so starkly exposed, the other three large centers are glacially 

dissected and probably likewise of middle Pleistocene age. 

Each of the four edifices may have released as much as 15 

to 20 km3

 of magma. Less eroded but comparably voluminous (~15 km3

), the Mount Bailey edifice appears to be late 

Pleistocene, as are ten or more scoria and lava cones along the 

reach. Among these smaller cones, well-preserved Red Cinder 

Butte, Windigo Butte, Tenas Peak, Kelsay Point, and Thirsty 

Point are among the youngest, probably latest Pleistocene, and 

Cinnamon Butte may have erupted during or after deglaciation 

(Sherrod and others, 1983a; Sherrod, 1990b).

Andesite is limited to Timber Crater (Bacon, 1990), parts 

of Mount Bailey (Barnes, 1992), and a deeply eroded middle 

Pleistocene stratovolcano centered on Cappy Mountain

(43°18.5’N, 121°59’W) and adjacent Tolo Mountain. Rhyodacitic lavas (69-71% SiO2

) are mapped around the flanks of 

the Cappy Mountain center (at Mule Peak; Burn, Hemlock, 

and Clover Buttes; Williams, 1957; Sherrod and Smith, 2000). 

With preserved relief of 700 m and a width of 12 km, the 
32 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 20. Mount Thielsen, the “lightning rod of the Cascades,” a 300-ka mafic cone, viewed eastward. Relief visible in image is 1,100 

m, of which about 600 m is above timberline. Physically similar to Goosenest (fig. 22) and many steep mafic shields in the Cascades, 

Thielsen's interior has been unusually well exposed by glacial erosion. Several radial arêtes consist of modestly dipping stacks of 

scores of thin rubbly lava flows that interfinger proximally with a fragmental core facies of loose-to-agglutinated stratified ejecta that 

thickens and steepens inward from ~10° to as much as 35°. The pyroclastic core is laced with dikes, is widely altered hydrothermally, 

and is locally deformed by a composite intrusion that supports the summit spire. 

(Photograph by author taken in September 2001.)

glacially devastated andesitic stratocone and its flanking silicic 

units represent an evolved-magma anomaly comparable in 

importance to Mount Jefferson or South Sister. Its eruptive 

volume is poorly known but may have exceeded 30 km3.

Mazama Reach (V)

The Mazama reach of the Oregon segment, extending 40 

km from Timber Crater (43°04’N) to Big Bunchgrass shield 

(42°42’N), contains ~175 Quaternary vents in a belt only 25 

to 30 km wide (fig. 15). Although the vent belt is relatively 

narrow for the Cascades, vent density is unusually high (table 

1), as is the proportion of silicic products erupted (Williams, 

1942; Bacon and others, 1994; Bacon and Lanphere, 2006). 

Except close to the large Mount Mazama edifice itself (Bacon, 

1985), many shields and monogenetic vents lie along northsouth alignments, some in association with north-trending 

normal faults of the Klamath Graben system that strikes 

toward and past Mount Mazama (Bacon and Nathenson, 1996; 

Bacon and others, 1999; Bacon, 2007). Straddling the Cascade 

axis, Mount Mazama itself is a cluster of overlapping stratovolcanoes, by far the most voluminous Quaternary volcanic 

system in the Oregon Cascades, and the second largest (behind 

Newberry) Quaternary volcanic locus in Oregon.

Mount Mazama

Mount Mazama is a large compound edifice, active quasicontinuously since ~420 ka, built largely of andesite and dacite (55-68% SiO2

) with very minor basaltic andesite (Bacon, 

1983; Bacon and Lanphere, 1990, 2006). Limited exposures 

of early and middle Pleistocene (1.8-0.6 Ma) volcanic precursors include as many as ten mafic and andesitic units peripheral to the edifice and several dacites and rhyodacites beneath 

and just southeast of it. As a silicic focus, the area developed 

slowly, with scattered dacites as old as 1.28 Ma and a few 

rhyodacites erupted during the long interval 725 to 500 ka 

(Bacon and Lanphere, 2006). Activity then intensified to produce a 16×24-km domefield (~20 km3

) containing as many as 

40 rhyodacite lava domes and flows (470-410 ka) that directly 

preceded growth of the stratocone cluster (Nakada and others, 

1994). Beginning with dacitic Mount Scott (420-350 ka), the 

andesite-dacite edifice then grew piecemeal, covering the 

west margin of the rhyodacite domefield. It became a cluster 

of contiguous and overlapping cones and shields, each active 

briefly or for as long as 70 kyr. Among some 50 separate components identified by Bacon (2007), the largest (~5-11 km3

each) include Mount Scott and the andesitic edifices of Kerr 

Notch (340-300 ka), Applegate Peak (270-210 ka), Llao Bay 
Distribution of Quaternary Cascade Volcanoes 33

(150-100 ka), and the West Wall (~70 ka). Although andesite 

exceeds dacite in the compound edifice by roughly 2:1, dacite 

magma erupted recurrently, at least ten times during growth of 

the edifice. During this 400-kyr interval, however, not a single 

rhyodacite was produced until ~27 ka, shortly after the final 

eruption of dacite at ~35 ka. Subsequently, several rhyodacite 

eruptions took place, as the rhyodacite magma reservoir grew 

by an average of 2.5 km3/kyr for ~20 kyr leading up to the 

caldera-forming eruption at 7.7 ka.

Surrounding and contemporaneous with growth of Mount 

Mazama are two substantial shields and at least 35 monogenetic vents within ~12 km of the caldera rim. The shields are 

mafic Union Peak (~6 km3, ~164 ka) and andesitic Timber 

Crater (~9 km3, ~137 ka). Most of the lesser peripheral vents 

(~300-10 ka) erupted basaltic andesite (for example, Desert 

Cone, Bald Crater, Red Cone, Oasis Butte, Maklaks Crater, 

Whitehorse Bluff, and Scoria Cone); about six produced 

andesite (for example, Crater Peak); but none produced dacitic 

or more silicic magmas, which are strictly limited to the focal 

edifice. Several of the youngest peripheral vents (~35-10 ka), 

which lie west of the edifice, erupted relatively primitive arc 

magmas (51-54% SiO2

) and HAOT (48-50% SiO2), plausibly representing part of an episode of elevated thermal and 

material input beneath the contemporaneously accumulating 

rhyodacite reservoir nearby (Bacon and Lanphere, 2006). 

Structural collapse at 7.7 ka during catastrophic eruption 

of ~50 km3 of magma (zoned from rhyodacite to mafic cumulate mush) produced Crater Lake Caldera, which engulfed 

summits as much as 1 km higher than its rim (Bacon, 1983). 

Strata exposed on the caldera wall include andesite-dacite 

cone-building lavas, two small mafic shields, and dacitic 

pyroclastic deposits, all alternating recurrently rather than 

following any systematic compositional progression. Near the 

rim, the wall truncates some of the rhyodacite lavas that leaked 

from the shallow zoned reservoir during the 20-kyr interval of 

preclimactic magma accumulation (Williams, 1942; Bacon, 

1983; Bacon and Druitt, 1988; Bacon and Lanphere, 1990, 

2006; Bacon 2007). Postcaldera eruptions on the caldera floor 

involved four vents that released ~4 km3 of andesite soon after 

collapse as well as a small rhyodacite dome emplaced ~4.8 ka 

(Bacon and others, 2002). Along with Kulshan Caldera adjacent to Mount Baker and Rockland Caldera at Lassen, Crater 

Lake is one of only three Quaternary calderas recognized in 

the Cascades, an anomalously small number for a continentalmargin arc (Hildreth, 1996). 

Precollapse volume of the Mazama edifice was roughly 

112 km3

 (Bacon and Lanphere, 2006), but, considering glacial 

erosion, the volume erupted might well have been greater than 

130 km3. Moreover, the 50-km3 magma volume estimated 

for the climactic eruption is probably an underestimate, as it 

takes no account of concealed intracaldera ignimbrite. A total 

eruptive volume since 420 ka (neglecting the pre-Mazama 

rhyodacite domefield and its early Pleistocene intermediate 

platform) may thus have been greater than 180 km3

, suggesting that Mount Mazama rivals Rainier and Lassen as the third 

or fourth most productive Quaternary center in the Cascades. 

U-Th model ages of zircons from (variably melted) granodiorite blocks that were plucked from the walls of the magma 

chamber and ejected radially during the caldera-forming 

eruption record a 300-kyr history of freeze-thaw fluctuations 

during assembly of a composite pluton (Bacon, 1992, 2005; 

Bacon and others, 2000, 2005; Bacon and Lowenstern, 2005). 

Multimodal age spectra for individual blocks are interpreted 

as reflecting recycling of zircon antecrysts from earlier intrusions and from crystallizing stalled mush batches. The main 

zircon age clusters (200-50 ka) for granodiorite blocks ejected 

in different radial sectors correspond to K-Ar ages of dacites 

erupted in those sectors, suggesting assembly of many modest 

silicic batches crystallizing piecemeal in constructing a multiphase pluton at least as wide as the 30-km2

 subsided floor.

Other important centers in the Mazama reach are (1) 

the 10-km-wide andesitic Timber Crater shield just north of 

Mount Mazama; (2) the 7-km-wide mafic shield of Union 

Peak southwest of Mount Mazama, topped by a glaciated 

central intrusive spire comparable to that of Mount Thielsen 

(Williams, 1942); (3) a chain of andesitic scoria cones ~13 

km south of Crater Lake that includes Goose Nest, which 

produced a 60-km2

 fan of lava and fragmental flows (Smith, 

1983; 1988); and (4) a cluster of mafic cones around the Big 

Bunchgrass shield (fig. 15). Altogether, at least 80 vents in 

the reach erupted products ranging from HAOT to andesite 

(47-62% SiO2) before or during construction of the Mazama 

edifice (Williams, 1942; Bacon, 1990, 2006; Bacon and others, 1994; Bacon and Lanphere, 2006). Despite an abundance 

of basaltic andesite cones and shields, the Mazama reach is 

unique in the Quaternary Cascades in having erupted almost 

no true basalts (<52% SiO2, noncumulative) other than HAOT 

(Bacon and others, 1994). Equally remarkable is the absence 

of true rhyolite; although at least 75 km3 of rhyodacite was 

released in more than 50 mid-Pleistocene to Holocene eruptions at or adjacent to Mount Mazama, not a single unit having 

>73 percent SiO2 has been reported (Bacon and Druitt, 1988; 

Nakada and others, 1994; Bacon and Lanphere, 2006).

McLoughlin Reach (VI)

 The 50-km-long McLoughlin reach, from Big Bunchgrass shield (42 ̊42’N) to Little Aspen Butte (42°17’N), 

contains ~33 Quaternary volcanoes in a vent zone only 

15-25 km wide (fig. 15). Vent density is far lower than in the 

adjacent Mazama reach (table 1), and Quaternary eruptive 

volume (<100 km3

) is low for the Cascades. There may be a 

slight westerly (right-stepping) offset of the vent zone relative to the Mazama reach, but thick sedimentary fill in the 

Klamath Graben just east of the zone makes both the apparent 

shift and the width of the zone equivocal. In striking contrast 

to the Mazama reach, no dacitic or more silicic Quaternary 

volcanic rocks have erupted in this reach. Although basalts are 

sparse, both HAOT and arc types are present (Mertzman and 

others, 1992). In the number of volcanoes, basaltic andesite 

dominates (as everywhere in Oregon), but a few of the main 

centers in the reach are andesitic: (1) Pelican Butte shield, the 
34 Quaternary Magmatism in the Cascades—Geologic Perspectives

largest Quaternary center at ~20 km3; (2) Brown Mountain 

(~5 km3), which is probably the youngest; and (3) Devils Peak 

(>10 km3), a glacially dissected cone-on-shield of middle 

or early Pleistocene age that consists of both andesite and 

basaltic andesite (Smith, 1983, 1988). Mount McLoughlin is 

a towering (2,894 m) but volumetrically modest (~13 km3) late 

Pleistocene stratocone built almost wholly of basaltic andesite 

(54-57% SiO2; Maynard, 1974), more like Mount Bachelor 

or North Sister than the andesite-dacite stratovolcanoes with 

which it is often confused. 

The glacially excavated Mountain Lakes volcanic cluster 

(Greylock Mountain-Whiteface Peak-Aspen Butte) is a major 

eruptive complex (>50 km3) and dominantly andesitic. It has 

been thought to be mostly of Pliocene age (Smith, 1983; Sherrod and Smith, 2000), but several mafic and andesitic flank 

vents are probably Quaternary. Much of the Cascade crest in 

this reach is wilderness that has been studied only in reconnaissance. It should come as no surprise if detailed mapping 

and dating were to show that many more of the glacially dissected edifices in the Mountain Lakes, Sky Lakes, and Seven 

Lakes Basin areas are Quaternary.

Little Aspen Butte, a mafic shield on the south flank 

of the Mountain Lakes cluster, is the southernmost Quaternary volcano in Oregon. South of it, a 64-km stretch of the 

Cascades almost lacking in Quaternary volcanoes separates 

the Oregon segment from the vigorously productive volcanic region around Mount Shasta. Although the gap is rich in 

Pliocene mafic centers, the small Copco scoria cones on the 

Klamath River (Williams, 1949; Hammond, 1983) may be the 

only Quaternary volcanoes within it. The southern termination 

of the Quaternary volcanic belt in Oregon (fig. 15) coincides 

with an abrupt change in strike of Basin and Range faults of 

the Klamath Graben system, from northerly trends adjacent 

to the Quaternary volcanoes to northwest-southeast trends 

in the volcanically inactive region farther south (Walker and 

MacLeod, 1991; Blakely and others, 1997).

Contrasts Between Oregon and Rainier-to-Hood 

Segments

Scarcity of forearc volcanoes south of 45°15’N, unbroken continuity of the vent-zone axis, and narrowness of the 

Quaternary volcanic belt (one-third as wide as in southern 

Washington) distinguish the Oregon segment from the adjacent one to the north. Moreover, in contrast to the quadrilateral 

of stratocones in southern Washington (fig. 8), all the main 

stratovolcanoes of Oregon lie on or very close to the Cascade 

axis. Although axial vent-densities are comparable (table 1), 

some broad compositional differences also distinguish the segments: (1) importance of rhyodacite at several major centers in 

Oregon; (2) predominance of basaltic andesite among the many 

mafic shields, cones, and lava fields in Oregon but of true basalt 

among those in the Rainier-to-Hood segment; and (3) absence 

in the Oregon Quaternary Cascades of the mildly alkalic basalts 

that are scattered across southern Washington. Because the 

crust beneath the Oregon Cascades is thought to be younger, 

less mature, and less continental than beneath adjacent parts of 

the Washington and California Cascades, the scarcity of basalts 

relative to those areas is an apparent paradox.

Although eruptive volumes are hard to estimate and 

compare with much accuracy, a first-order contrast in volumeproportions of the stratocones vis-à-vis the surrounding mafic 

volcanoes nonetheless stands out between these segments. If 

consideration is limited to the Brunhes Normal-Polarity Chron 

(for the sake of better preservation and definition of the evidence), then the data of Sherrod and Smith (1990) suggest that 

during the last 780 kyr no more than one-third and perhaps as 

little as 10 percent of the material erupted along the Oregon 

Cascades issued from the major evolved centers; that is, 67-90 

percent of the mid-Pleistocene and younger volume of the 

Oregon segment originated in mafic shields and cones. For the 

Rainier-to-Hood segment, virtually the opposite is clear—about 

80 percent of the volume younger than 780 ka erupted from four 

andesite-dacite stratovolcanoes, whereas more than 150 mafic 

vents (including Indian Heaven) accounted for only ~20 percent.

The volumetrically dominant form of Quaternary volcanism of the Oregon Cascades— eruption of a nearly continuous belt of coalescing mafic shields and cones—had already 

been characteristic of the Oregon segment in the Pliocene, and 

it may have begun locally as long ago as 7 Ma (Sherrod and 

Smith, 2000). This long-lived mafic swath along the Oregon 

rangecrest, which includes the “High Cascade platform” of 

Taylor (1990a), has no counterpart in southern Washington, 

where Pliocene shields (for example, Hogback Mountain and 

the Simcoe Mountains shield) are few and far between. Except 

for the rear-arc Simcoe Mountains Volcanic Field, very little of 

the widespread basalt erupted in southern Washington is preQuaternary. In contrast to the dispersed and mostly Quaternary 

mafic volcanism of the Rainier-to-Hood segment, the mafic 

axis of the Oregon Cascades has been a stable, concentrated, 

and persistent eruptive belt some 25 to 50 km wide for several 

million years.

Shasta Segment

The Quaternary volcanic zone containing the huge Mount 

Shasta center is separated from the Oregon and Lassen segments by extensive gaps (figs. 1, 21) where little Quaternary 

volcanism has taken place. Along the arc, from Goosenest

(41°43’N) to Signal Butte (41°17’N), the Quaternary Shasta 

segment is only 50 km long (fig. 21), no longer than the gaps 

north and south of it. West-to-east, however, the Quaternary 

vent zone is extraordinarily wide, extending without interruption for at least 120 km, from west of Mount Shasta across the 

Cascades and far into the Basin and Range Province, embracing the great Medicine Lake Volcanic Field (Anderson, 1941; 

Donnelly-Nolan, 1988) and the adjacent Hackamore basalt 

field (Donnelly-Nolan and others, 1996), both of which include 

abundant HAOT. Not counting these dense inboard arrays of 

Quaternary vents (table 1) as parts of the main arc belt, the 
Distribution of Quaternary Cascade Volcanoes 35

Shasta segment of the Cascades proper is still 40 to 50 km wide 

and contains at least 255 Quaternary vents (fig. 21).

Volcanism was even more extensive here in the Pliocene, 

when it was characterized principally by mafic shields and fissure-fed plateau-capping basalts that extend from the Cascades 

to far east of Medicine Lake and the Klamath Graben (Luedke 

and Smith, 1981; McKee and others, 1983; Mallin and Hart, 

1991; Sherrod and Pickthorn, 1992; Sherrod and Smith, 2000; 

Carmichael and others, 2006). Before the Oregon-Shasta 

gap fell quiescent in the late Pliocene, an axial belt of mafic 

shields was continuous across it, extending from southern 

Oregon to the Eagle Rock and Willow Creek Mountain shields 

(fig. 21). The trend of the Cascade axial belt changes near 

Goosenest (fig. 21) from the north-south orientation characteristic of Oregon to the southeasterly trend typical of the 

adjacent part of the Basin and Range Province, passing well 

inboard of the subsequent site of Mount Shasta and extending as far southeast as the Black Fox Mountain mafic-shield 

cluster (fig. 21). Because erosion of mafic shields is relatively 

slow in the southern Cascades, the constructional topography 

established largely in the Pliocene remains the physiographic 

axis of the Cascades here (Williams, 1949). Since then, the 

active Cascade axial belt has contracted latitudinally, the enormous Medicine Lake and Mount Shasta systems have developed east and west of the axis (largely during and since the 

middle Pleistocene), and scores of mafic to silicic volcanoes 

have built an across-axis highland between them (Christiansen, 

1996; Donnelly-Nolan, in press). Quaternary arc volcanism 

has thus been limited to an equant (50x50 km) tract of the 

Cascades that overlaps inland with the contemporaneous (and 

areally comparable) Medicine Lake Volcanic Field, which 

is fundamentally extensional and powered in part by HAOT 

(Donnelly-Nolan, 1988). 

Mount Shasta

Mount Shasta (4,316 m) is by far the most voluminous 

stratovolcano in the Quaternary Cascades, having a diameter 

of 30 km, relief of 3.3 km, and an eruptive volume greater than 

450 km3

. Active since ~600 ka, the central cone complex has 

recurrently produced silicic andesite and dacite (57-67% SiO2; 

Christiansen, 1982, 1985, 1990; Grove and others, 2002). An 

ancestral andesite edifice (Sand Flat cone) was destroyed by 

sector collapse, probably between 400 and 300 ka; a remnant 

left in place gave a K-Ar age of 593±41 ka (Crandell, 1989). 

The hummocky debris avalanche deposit is one of the world's 

largest (~45 km3

), having flowed at least 50 km northwest 

of the present summit and today covering about 675 km2. 

The great compound edifice present today was subsequently 

constructed during at least four main episodes that built four 

overlapping stratocones, the dome-filled craters of which are 

separated by only 1 to 3 km (Christiansen and others, 1977). 

Middle Pleistocene lava flows from the oldest of the four 

locally overlap the debris avalanche deposit, and their source 

edifice (Sargents Ridge cone, ~85-km3

) is overlapped in turn 

by a substantial late Pleistocene edifice (Misery Hill cone, 

~60 km3). The two youngest cones, Shastina and the modern 

summit edifice (Hotlum cone), together represent at least 60 

km3 of eruptive products and were built entirely during the 

Holocene (Miller, 1980; Christiansen and Miller, 1989; R.L. 

Christiansen, written commun., 2006). 

In addition to central dacite domes culminating each 

andesite-dacite cone-building episode, at least 16 flank domes 

are exposed, nearly all of them likewise dacitic (Christiansen 

and others, 1977). A small remnant (72% SiO2) just south of 

dacitic McKenzie Butte, low on the south flank, is the only unit 

at Mount Shasta known to have more than 67 percent SiO2, and 

it may be the only Quaternary arc rhyodacite between Mazama 

and Lassen. Haystack dome (64% SiO2) on the northwest 

has geochemical characteristics that suggest derivation by 

partial melting of MORB-like source rocks (Kay and others, 

1993; Feeley and Hacker, 1995), perhaps part of the ophiolitic 

Klamath basement. Black Butte dome (65-66% SiO2), 13 km 

southwest of Shasta’s summit, is the largest dome exposed here 

(~2.5 km3

), consists of four steep extrusions of hornblende 

dacite, and is only ~9,500 years old (Miller, 1980). Extensive 

pyroclastic-flow fans from Black Butte and the dome cluster 

atop Shastina overlap at the west toe of Mount Shasta; both 

fans were emplaced ~9,500±300 years ago, the one from Black 

Butte being slightly younger (Miller, 1978). Another large apron 

of pyroclastic and debris flows, derived from the Hotlum cone 

(mostly 9 to 4 ka), spreads around the north and east sides of the 

great edifice.

The scattering of varied silicic domes as far as 10 to 15 

km to the north, south, and west (but none to the east) of the 

summit suggests that partial melting is distributed beneath an 

area somewhat wider than the principal melting zone that has 

supplied exceptionally voluminous andesite-dacite magma to 

the central conduit cluster, which itself shifted less than 3 km 

in 600 kyr. Together with nine of the domes, several andesitic 

to mafic cinder and spatter cones on Shasta’s north and south 

flanks define a narrow north-trending array of vents (fig. 21) of 

various ages and compositions that probably reflects regional 

stress or structural influence rather than local edifice control 

(Williams, 1932a; Christiansen and others, 1977). One of the 

mafic cones, Green Butte (53-54% SiO2

, MgO > 8%; Anderson, 

1974; Baker and others, 1994), lies only 4 km south of the summit at an elevation of 2,800 m, thus ranking among the highest 

mafic vents in the Cascades (Christiansen and Miller, 1989). A 

vent location so nearly central (on an andesite-dacite edifice) 

for magma so mafic suggests complex crustal plumbing and 

dispersed (nonintegrated) silicic magma bodies beneath Mount 

Shasta. Several more cinder cones north, south, and northwest 

of Mount Shasta (Christiansen and Miller, 1989) erupted Mgrich basaltic andesite or basalt (Baker and others, 1994; Grove 

and others, 2002); some represent the most primitive magma 

associated with the Shasta system whereas others are hybrids 

(Streck and others, 2007). In addition, sheets of HAOT contemporaneous with the edifice are widespread in the lowland 

periphery of Mount Shasta (Williams, 1949), as well as far into 

the Basin and Range interior of northern California and southern Oregon (Hart and others, 1984).
36 Quaternary Magmatism in the Cascades—Geologic Perspectives

0 10 20 30 40 km

MH CM LD

HB OM

SM

WH

GM DM

DP

ACB

MB BFM

McCloud Bartle

caldera

Medicine Lake

Volcanic

Field

Sheep Mtn

Tulelake

Klamath Graben OREGON

CALIFORNIA

TW

Copco cones

Eagle Rock

Willow Creek

Mtn

Yreka

Goosenest

RAINBOW MTN

Haystack

SHASTA

Weed

Shastina

Black Butte GB

Everitt Hill

Signal Butte

BB

TC

Hackamore Hackamore Volcanic Field

McArthur

Burney

BM

MV

LM SP

HC rim

BC rim

ChM

HC

TM

BV Dvc

Mvc

MH

CrB

Chester

SM

Yana

vc

Latour

vc

FB

BKM

LASSEN

Redding

Black Butte

Inskip Hill

Tuscan Buttes

HM

RP FP

CC

PP

Caribou

Volcanic Field 

Susanville

CiB

SM vc Eagle Lake

122o

121o

42o

41o30'

41o

40o30' LP

(all HAOT)

122o

121o 122o

x

x

X

X

rbr

rmcp

rlcp
Distribution of Quaternary Cascade Volcanoes 37

← Figure 21. Shasta and Lassen segments of the Cascade arc, showing distribution of 746 Quaternary volcanic vents. Vent symbols 

as in figures 3 and 8. Lassen region vent distribution from Guffanti and others (1990, 1996) and Clynne and Muffler (2007). Generalized 

extents of rear-arc edifice of Medicine Lake vent field (green) and adjacent Hackamore vent field (encompassing ~580 and ~12 additional 

Quaternary vents, respectively) are adapted from Donnelly-Nolan (1988, in press) and Donnelly-Nolan and others (1996). Also outlined 

are Caribou Volcanic Field (violet; enclosing ~112 Pleistocene vents), Lassen volcanic center (orange; >40 vents), and the caldera formed 

during the 610-ka Rockland eruption and later filled by the Lassen domefield. Major volcanic centers (vc) active in the early Quaternary 

include Rainbow Mountain, Snow Mountain (SMvc), Dittmar (Dvc), and Maidu (Mvc); large Yana vc and Latour vc (both indicated by X) 

were built during the Pliocene. The Cascade axis (dashed line), well defined only in the north, is in part marked by two well-preserved 

mid-Pliocene mafic shields (x), Eagle Rock and Willow Creek Mountain. Unless indicated, all other vents identified are Quaternary. Shasta 

segment abbreviations: ACB, Ash Creek Butte; BB, Brushy Butte; BFM, Black Fox Mountain; CM, Cedar Mountain; DM, Deer Mountain; 

DP, Doe Peak; GB, Green Butte; GM, Garner Mountain; HB, Horsethief Butte; LD, Little Deer Mountain; MB, McKenzie Butte; MH, Mount 

Hebron; OM, Orr Mountain; SM, Sharp Mountain (Pliocene); TC, Timbered Crater; TW, The Whaleback; WH, Wild Horse Mountain 

(Pliocene). Lassen segment abbreviations: BC, Butte Creek Rim; BM, Burney Mountain; BKM, Barkley Mountain dacite chain; BV, Brokeoff 

Volcano; ChM, Chalk Mountain; CiB, Cinder Butte; CrB, Crater Butte; CC, Cinder Cone; FB, Finley Butte; FP, Fairfield Peak; HC, vents for Hat 

Creek Basalt and nearby fault-scarp rim; HM, Hat Mountain; LM, Logan Mountain; LP, Lassen Peak; MH, Mount Harkness; MV, Magee 

Volcano; PP, Prospect Peak; RP, Raker Peak; SP, Sugarloaf Peak; TM, Table Mountain. Large rhyolite lava flows (in yellow) around Maidu 

volcanic center form Blue Ridge (rbr), Mill Creek Plateau (rmcp), and Lost Creek Plateau (rlcp).

Elsewhere in the Shasta segment, as in the Oregon 

segment, a majority of the Quaternary volcanoes consist of 

basaltic andesite. In contrast to Oregon, however, the volumetrically dominant composition erupted during the Quaternary 

is andesite—owing not only to massive Mount Shasta but to 

Rainbow Mountain, Goosenest, and a few lesser volcanoes. 

Centered about 20 km northeast of Mount Shasta and predating it, the Rainbow Mountain center was one of the largest 

andesite-dacite stratovolcanoes in the Cascades during the early 

Pleistocene, measuring ~20 km across (R.L. Christiansen, work 

in progress; Guffanti and others, 1994). Called Haight Mountain 

volcano by Williams (1949, p. 39), the glacially dissected pile 

probably had an eruptive volume well in excess of 100 km3. 

On the Cascade crest 35 km north of Mount Shasta, Goosenest 

(Williams, 1949) is a 10-km3 late Pleistocene andesitic cone (up 

to 58.5% SiO2) with ~1,400 m of relief (fig. 22). Andesite contributions (accompanied by basaltic andesite and HAOT) also 

make up significant parts of the early Pleistocene Deer Mountain vent complex (southeast of Goosenest; Williams, 1949) and 

the middle Pleistocene Garner Mountain cone and fissure-vent 

system (Hughes and Mertzman, 1976; Hart and others, 1979) 

~25 km farther east. The Fisk Ridge to Pumice Stone Mountain alignment of about 20 vents (parallel but 8 km south of the 

northeast-striking Garner Mountain chain) includes a compositionally wide array of (early to late Pleistocene) HAOT, mafic 

and silicic andesite, dacite, and the 1.0-Ma rhyolite of Red 

Cap Mountain (Mertzman, 1982; Donnelly-Nolan, in press). 

Nonetheless, most of the many Pleistocene shields and cones 

scattered between Medicine Lake and Shasta consist of basaltic andesite—including Horsethief Butte, Mount Hebron, and 

Sheep, Cedar, Orr, and Little Deer Mountains. Likewise basaltic 

andesite but of Pliocene age are nearby Sharp and Wild Horse 

Mountains (R.L. Christiansen, work in progress). Adjacent to 

Mount Shasta and contemporaneous with its construction, three 

middle Pleistocene shields (Everitt Hill, Ash Creek Butte, and 

The Whaleback) each produced 5-10 km3

 of basaltic andesite. 

Additional mafic shields may lie buried beneath the great stratovolcano itself (Christiansen and others, 1977).

Lassen Segment

The Quaternary volcanic zone in the Lassen region (fig. 

21) extends at least 110 km east-west (Eagle Lake to Inskip 

Hill) and 80 km north-south, from 41 ̊N to 40 ̊15’N, where 

the Quaternary Cascade arc terminates. The Lassen segment 

is separated from the Shasta segment by a 50-km gap having 

hardly any Quaternary volcanoes and relatively few of Pliocene age (Luedke and Smith, 1981; Blakely and others, 1997). 

Within the Lassen segment, at least 476 vents are thought to 

be younger than 2 Ma (table 1), and 121 more were reported 

by Guffanti and others (1990) for the pre-Quaternary interval 

7 to 2 Ma.

A 25-km-wide swath of ~200 close-set volcanoes trending north-northwest through the Lassen volcanic center (fig. 

21) constitutes the voluminous axial zone of the Quaternary 

Cascades. At least 17 forearc vents lie as far as 40 km southwest of the axial belt (fig. 21; Helley and others, 1981; Helley 

and Harwood, 1985). About 200 more Quaternary shields, 

cones, and fissure vents lie as far as 60 km east of the axis 

(Guffanti and others, 1990), either concentrated in the Caribou Volcanic Field (Guffanti and others, 1996) or scattered 

less densely across a wide inboard region (table 1; fig. 21). 

Although much of the Lassen segment is marked by Basin and 

Range extensional faults and associated vents for HAOT, its 

calcalkaline arc products (basalt to dacite) are not subordinate 

to HAOT except perhaps far inboard near Eagle Lake. For 

this reason, the expanse of Quaternary volcanoes well east 

of the Cascade axis in the Lassen segment is treated here as 

Cascadian, though its eastern portion might well be considered another rear-arc volcanic field (fig. 1). Compared to the 

Caribou-Eagle Lake rear-arc area, however, the Hackamore 

and Medicine Lake Volcanic Fields (equivalently contigu-
38 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 22. Goosenest shield viewed southward (with lower flank of Mount Shasta in left distance). Transitionally mafic-to-silicic andesite, 

this latest Pleistocene nonglaciated edifice shows smooth moderate slopes typical of numerous Cascade shields, as well as modest 

asymmetry that commonly results from growth on a preexisting slope. The summit cone of scoria and agglutinate, which rises 250 m above 

the shield, is not a subsequent cinder cone but, rather, the youngest part of a core facies that typically interfingers radially with the flank 

lavas throughout growth of such an edifice. Relief visible in image is about 1,200 m. 

(Photograph by author taken in September 1991.)

ous behind the Shasta segment; fig. 1), appear to have still 

larger proportions of the HAOT and HAOT-derivative magmas, reflecting a dominance of dry decompressive (Basin 

and Range) mantle melting where extensional unloading has 

impinged on the subduction-dominated Cascade arc.

Pliocene volcanism was even more widespread, well 

established along what is now the Quaternary axis but 

extending farther inland—as far as 120 km east of Lassen 

Peak. Contraction of activity during the Quaternary (Guffanti 

and others, 1990) entailed accentuation of the north-side 

gap, extinction of a large Pliocene volcanic region south 

of 40 ̊15’N (principally the Yana volcanic center; fig. 21; 

Clynne, 1990a,b), and shrinkage of the wide volcanic region 

east of the axis, part of a longer-term westward migration 

of the eastern limit of active volcanism (Grose and McKee, 

1986; Grose and others, 1990, 1992). At first glance, figure 1 

seems to show a 40 to 50 km left-stepping offset between the 

Shasta and Lassen segments (Luedke and Smith, 1981). The 

offset is less profound than apparent, however, if account 

be taken of (1) the gap between segments, (2) the deviation 

(north of Mount Shasta) of the Cascade axis from a southerly 

to a south-southeasterly trend (fig. 21), and (3) the position 

of Mount Shasta itself, well west of the axis.

Mafic volcanoes predominate numerically in the Lassen segment, as they do in all parts of the Cascades south 

of Mount Rainier. Fewer than 20 percent of the Quaternary 

volcanoes here are andesitic, and fewer than 10 percent are 

silicic (Guffanti and others, 1990). In contrast to the Oregon 

and Shasta segments, where basalt is sparse and basaltic 

andesite far more dominant, as many as 30 percent of the ~350 

Quaternary mafic vents in the Lassen segment erupted true 

basalt (<52% SiO2

, either calcalkaline or HAOT) (Guffanti and 

others, 1990; Clynne, 1993). Some 13 units of HAOT younger 

than ~1 Ma are exposed across the Lassen region (Clynne and 

Muffler, in press), and numerous older examples have been 

mapped just east of the Quaternary arc (Grose and others, 

1990, 1992). A generally westward progression of scattered 

HAOT magmatism has accompanied westward advance of 

Basin and Range extension into the arc (Hart and others, 1984; 

Guffanti and others, 1990). The youngest and best-known representative is the Hat Creek Basalt (24±6 ka; 48-49% SiO2

), a 

30-km-long, tube-fed lava-flow complex (~5-km3) that erupted 

from a multivent fissure ~18 km north-northeast of Lassen 

Peak (Anderson, 1940; Anderson and Gottfried, 1971; Anderson, 1971; Muffler and others, 1994; Turrin and others, 2007).

Arc (calcalkaline) basalts are also scattered among the 

more numerous basaltic andesite cones and shields across the 

Lassen segment, although they are missing within the Lassen volcanic center itself, presumably intercepted at depth by 

a long-lived reservoir or crustal column of evolved magma 
Distribution of Quaternary Cascade Volcanoes 39

(Hildreth, 1981a; Hildreth and Moorbath, 1988; Guffanti and 

others, 1996). At least 50 vents for relatively primitive arc 

basalts have been identified and their products analyzed and 

discussed by Clynne (1993), Borg and others (1997), and 

Clynne and Muffler (in press). These include Inskip Hill and 

Cold Creek, Soap, and Black Buttes in the forearc; parts of the 

compositionally varied major shields at Sifford Mountain and 

Mount Harkness along the arc axis; and the Swain Mountain 

shield and numerous scoria cones of the Caribou field east of 

the axis. At least a dozen vents have also been identified for 

primitive high-Mg basaltic andesite, most of them west of the 

axis (Clynne, 1993; Borg and others, 1997). 

Volumetrically, however, as opposed to numbers of vents, 

andesitic and silicic eruptive products, taken together, exceed 

the Quaternary output of the more numerous mafic volcanoes 

in the Lassen segment. The Lassen volcanic center alone has 

produced more than 200 km3 of material in the last 825 kyr 

(Clynne, 1984, 1990a; Clynne and Muffler, in press), ~95 percent of it andesitic or more silicic. Three additional large centers (intermediate stratovolcanoes with late-stage peripheral 

silicic lavas) were established nearby around 2.5 Ma and may 

have remained active until ~1 Ma, that is, through much of the 

early Quaternary (Clynne, 1990b; Guffanti and others, 1990; 

Clynne and Muffler, in press). These are the Snow Mountain, 

Dittmar, and Maidu volcanic complexes (fig. 21), centered 40 

km northwest, 15 km southeast, and 18 km south-southwest, 

respectively, from Lassen Peak. Several lava plateaux flanking 

the huge Maidu complex represent by far the most voluminous 

occurrence of Quaternary rhyolite lava in the Cascades; the 

1.2-Ma Blue Ridge flow alone (75% SiO2

), though eroded, 

still covers ~90 km2 and is as thick as 150 m (Gilbert, 1969; 

Clynne and Muffler, in press). Eruptive volumes of these 

eroded centers are difficult to reconstruct, but edifice dimensions estimated by Clynne (1990b) permit crude reckoning 

of volumes (ignoring dispersed pyroclastic ejecta) that might 

reasonably be expected for such complexes: Snow Mountain 

>25 km3, Dittmar >150 km3, and Maidu >200 km3. In addition, 

a middle Pleistocene stratocone, the glacially gutted Magee 

Volcano 25 km northwest of Lassen Peak, consists mostly of 

andesite and basaltic andesite (53-63% SiO2

) but produced a 

late-stage lava flow and plug of rhyodacite (Macdonald, 1963; 

Borg, 1989; Borg and Clynne, 1998). Eruptive volume of the 

Magee center may have been 15±5 km3, comparable to that of 

South Sister. 

Away from these big centers, Quaternary rhyolite and 

rhyodacite vents are unknown, and the only substantial occurrences of dacite are Burney Mountain (~9 km3

), a steep middle 

Pleistocene composite dome ~900 m high (40 km northnorthwest of Lassen Peak; see Frontispiece), and the early 

Pleistocene coulees of Huckleberry Mountain (20 km northwest of Lassen Peak). Scattered across the segment, however, 

as many as 50 Quaternary cones and shields are wholly or 

partly andesitic, including such substantial edifices as Prospect, West Prospect, and Sugarloaf Peaks; Table, Sifford, Red 

Lake, Badger, and Logan Mountains; Mount Harkness shield, 

and several chains and clusters of lesser cones and lava fields 

such as Tumble Buttes, the Sugarloaf chain, and the Red Cinder chain in the Caribou Volcanic Field (Guffanti and others, 

1996; Clynne and Muffler, in press)—altogether contributing 

some 150 to 200 km3 of nonmafic eruptive products. 

Excluding the major evolved centers, Sherrod and Smith 

(1990) gave what they called a conservative volume estimate of 

425 km3 for products of regional cones and shields younger than 

2 Ma in the Lassen segment. At least 25 percent (and conceivably 40 percent) of this total is likely to be andesitic and the 

rest mafic. It is unlikely that mafic products (basalt and basaltic andesite) of the last 2 Myr, even if underestimated, could 

approach in volume the 700 km3 or more of andesite-to-rhyolite 

magma erupted in the Lassen region during that interval.

Quaternary vent density for the axial belt (and for the 

Lassen segment as a whole) is similar to that of the Cascades 

in central Oregon (table 1), but vents are locally spaced more 

densely—particularly in the Lassen domefield and in the 

Caribou Volcanic Field (25 km east of Lassen Peak). The Lassen concentration reflects the numerous silicic lava domes that 

leaked recurrently from a long-lived crustal magma column 

under the Lassen Peak area, whereas the Caribou concentration reflects several crowded mafic-vent alignments controlled 

by Basin and Range extension within the arc. The 50-km3

Caribou Volcanic Field consists of basalt to andesite (basaltic 

andesite predominating), and a majority of its ~140 middle 

and late Pleistocene cones and shields are aligned north-northwest, along or parallel to regional extensional faults (Guffanti 

and others, 1990, 1994, 1996; Clynne and Muffler, in press). 

The extraordinary vent density in the Caribou Volcanic Field 

(<3 km2 per vent; table 1) is comparable to that of Medicine 

Lake and Newberry fields, where Basin and Range extensional 

faulting is likewise important. Although Basin and Range 

faulting has encroached into the Cascades east of Mount 

Shasta and south of Crater Lake as well, in the Lassen segment the faulting affects not only the eastern approaches but 

completely overlaps the main axial belt of the magmatic arc 

(Guffanti and others, 1990, 1994).

Lassen Volcanic Center 

Surrounded by the distributed mafic-to-andesitic volcanic field, the 400-km2

 Lassen volcanic center (LVC) is the 

southernmost large active center in the Cascades. Since ~825 

ka (perhaps since ~1 Ma), it has remained a major magmatic 

focus, recurrently one of the foremost silicic volcanic areas in 

the Cascades (Williams, 1929, 1932b; Clynne, 1984, 1990a; 

Bullen and Clynne, 1990; Clynne and Muffler, 1990, in press). 

The LVC consists of four main components: (1) a precaldera 

and caldera-forming silicic sequence (825 to 600 ka); (2) an 

andesite-dacite stratocone (590 to 390 ka); (3) a dacite domefield containing ~35 domes that erupted during two main episodes at 300 to 190 ka and 70 to 0 ka; and (4) a field of hybrid 

intermediate cones and lava flows peripheral to the domefield 

but contemporaneous (313 to 0 ka) with it.

The earliest phase, the “Rockland sequence”, produced 

a 15-km-wide ring of 15 precaldera dacite domes and lava 
40 Quaternary Magmatism in the Cascades—Geologic Perspectives

flows (64-68% SiO2) that surround the filled and buried 

caldera source of the 600-ka Rockland Tuff (>75 km3), a 

silicic ignimbrite and far-flung ashfall known from Utah to 

the Pacific Ocean (Sarna-Wojcicki and others, 1985; Meyer 

and others, 1991; Lanphere and others, 2004). Unique among 

the domes, the youngest (Raker Peak ~600 ka) is rhyolitic 

(73.5% SiO2) and compositionally similar to the dominant 

pumice in the ignimbrite (71-74.5% SiO2) (Clynne and Muffler, in press). Soon after the Rockland eruption, a successor 

stratocone, long-lived Brokeoff Volcano (590 to 390 ka), 

began to grow atop the southern part of the caldera, producing ~80 km3

 of andesite and dacite (55-68%, mostly 57-63% 

SiO2). The present-day amphitheatre of the gutted stratovolcano is not itself a caldera (as advocated by Williams, 1932b) 

but simply reflects glacial erosion of its altered core (Clynne 

and Muffler, 1989, in press). 

Filling and obscuring of the Rockland Caldera was 

subsequently completed by growth of the Lassen domefield, 

an 8×15-km assemblage of about 35 contiguous dacite and 

rhyodacite (63-72% SiO2

) lavas and pyroclastic deposits 

emplaced in at least 20 events (Clynne, 1990a) between 

~300 ka and 1.1 ka, clustering into two main episodes dated 

at 300-190 ka and 70-0 ka (Clynne and Muffler, 1989, in 

press). Youngest is the Chaos Crags cluster of six domes 

that erupted ~1,100 years ago (Crandell and others, 1974; 

Heiken and Eichelberger, 1980; Clynne and others, 2002; 

Christiansen and others, 2002). Highest point in the whole 

segment is Lassen Peak (3,187 m), which is not a stratocone 

(as some assume) but a 2.5-km3 rhyodacite dome (63-70% 

SiO2) more than 800 m high (Williams, 1929, 1932b), one of 

the youngest members of the domefield at ~27 ka (Turrin and 

others, 1998). Many of the postcaldera domes contain quartz 

phenocrysts, which are rare elsewhere in the Quaternary Cascades, in addition to biotite, hornblende, plagioclase, sparser 

pyroxenes, and commonly mafic enclaves.

The postcaldera Lassen domefield represents at least 50 

km3 of silicic magma (Guffanti and others, 1996), approaching the magma volume released as fallout and outflow by 

the caldera-forming eruption. Because neither the volume of 

intracaldera Rockland ignimbrite nor that of tephra dispersed 

by recurrent postcaldera dome-producing episodes is known, 

however, the nominal total of ~125 km3

 of silicic magma 

erupted during the last 600 kyr of activity at the LVC is likely 

to be an underestimate. As the nearby Maidu volcanic center 

(fig. 21) also produced abundant rhyolite and dacite (probably >50 km3

) during the early Pleistocene, the Lassen district 

volumetrically rivals and probably exceeds Mazama, Tumalo, 

St. Helens, Kulshan, and Meager in Quaternary eruption of 

silicic magma. For eruption of rhyolite alone (>72% SiO2), 

the Maidu and Lassen volcanic centers are unrivalled in the 

Quaternary Cascades.

Peripheral to and within the Lassen domefield, ten 

volcanoes contemporaneous with it produced at least 10 km3

of “hybrid andesites” (including “quartz basalts” of early 

investigators), interpreted by Williams (1932b, p. 373) as 

mixtures of mafic and dacitic magmas, a hypothesis confirmed 

and elaborated analytically by Clynne (1989, 1990a, 1999). 

The hybrids include products of such centers as Hat Mountain, 

Crater Butte, and Raker and Fairfield Peaks and appear to 

entail varied proportions of regional mafic magmas blended 

with rhyodacite magmas like those of the Lassen domefield. 

Also assigned to the hybrid group by Clynne and Muffler (in 

press) are the complex mid-17th century eruption of Cinder 

Cone (53-60% SiO2

; Clynne and others, 2000) and the small 

andesite-dacite mixed eruptions of May 1915, which fortuitously broke out through the top of the 27-ka Lassen Peak 

dome (Clynne, 1999; Christiansen and others, 2002). 

 Conspicuous in most of the silicic lavas of the domefield itself are chilled enclaves and blebs of mafic and andesitic magma (Williams, 1932b; Clynne, 1989, 1990a), as well 

as unequilibrated phenocrysts dispersed from such inclusions. Clynne (1990a) showed that dispersal, disaggregation, 

and mixing of such material into the resident rhyodacite 

magma reservoir extends the centrally eruptible compositional range down to dacite and even to silicic andesite. The 

ubiquity of mafic inclusions in the many domefield lavas 

suggests that the LVC rhyodacite reservoir is thin and susceptible to wholesale convection when injected by batches of 

mafic magma. Vents for more homogenized hybrid andesites 

are scattered from the domefield to as far as 12 km outside 

it (fig. 21), indicating distribution of additional silicic melt 

pockets well outside the focal reservoir.

California Comparisons

The Lassen and Shasta segments are rather different from 

the rest of the Cascades, having some features in common 

with other segments but each being unique in its combination 

of characteristics. The volcanically inactive gaps north and 

south of the Shasta segment provide a first-order contrast with 

the unbroken belt of close-set volcanoes that stretches more 

than 500 km from central Washington to southern Oregon. 

The great width of the Quaternary volcanic zone in California resembles that of southern Washington and northernmost 

Oregon, but it contrasts strongly with the discontinuous vent 

zone of the Garibaldi Volcanic Belt and with the continuous 

but narrower Oregon Cascades (fig. 1). In California, the main 

Cascade axis trends north-northwest, parallel to many sets of 

Basin and Range faults, in contrast to the northerly (or N10 ̊E) 

trend of the axis in Oregon and southern Washington. Shasta 

and Maidu are huge centers that lie west of the Cascade axis 

in California, but elsewhere in the Cascades the only large 

forearc edifices are Rainier and St. Helens. Likewise, the only 

thing comparable to the scattering of small forearc volcanoes southwest of Lassen is in the region around Portland 

and southwest Washington. Elsewhere in Oregon, only a few 

isolated vents of Quaternary age lie significantly west of the 

main belt (Sherrod, 1990a), and in the Garibaldi Volcanic Belt 

no forearc vents are recognized.

Clustering of the voluminous Latour (3.5 to 3.0 Ma), 

Yana (3.2 to 2.4 Ma), Dittmar (2.4 to 1.4 Ma), Maidu (2.4 to 1 
Distribution of Quaternary Cascade Volcanoes 41

Ma), and Lassen (1 to 0 Ma) volcanic centers within 15 to 25 

km of each other is unparalleled elsewhere in the Quaternary 

Cascades. The volume erupted by each may have matched that 

of the entire Mazama cluster. For comparison, spacing is much 

greater among the Hood, Adams, St. Helens, Rainier, and Goat 

Rocks stratovolcanoes (a not dissimilar regional array of scattered major centers; figs. 1, 8), ranging from 35 to 100 km.

Counting the small volcanoes, however, Quaternary vent 

densities along most of the Cascade axial region are generally 

comparable (8-30 km2/vent; table 1). Vent density is significantly greater around the Lassen and Mazama evolved centers, 

and it attains maximum values at the three extensional, 

predominantly mafic, foci farther inland—Caribou, Medicine 

Lake, and Newberry (table 1).

The major evolved foci account for 60 to 75 percent of 

the Quaternary eruptive volume in the Lassen segment and 

more than 80 percent in the Shasta segment, as compared to 

only 10 to 33 percent along the Oregon segment. In southern 

Washington, the proportion is about 80 percent and in the 

Garibaldi Volcanic Belt more than 98 percent. From a different perspective, however, major andesitic-to-silicic volcanic 

centers number only six among the more than 740 Quaternary 

vents in the California Cascades, only six among the more 

than 1050 in the Oregon segment, five among the 412 in the 

Hood-to-Rainier segment, and five among the 127 in the 

Garibaldi Volcanic Belt (tables 1, 2).

Considering compositional output more narrowly, 

the eruptive volume of true rhyolite in the Lassen segment 

(mainly Maidu and Rockland) is unapproached elsewhere 

in the Quaternary Cascades, that of the Tumalo-South Sister 

area being a distant second. Moreover, production of Quaternary rhyodacite at Lassen volcanic center is rivalled only by 

the Mazama/pre-Mazama, Tumalo, Jefferson, Kulshan, and 

Meager systems. In the Shasta segment, on the other hand, 

rhyolite is absent and rhyodacite nearly so. Dacite is plentiful 

at the major centers in both California segments. Accurate 

estimates are not feasible, but a total volume of dacite greater 

than 50 km3

 is likely at Mount Shasta and likewise at the 

Lassen volcanic center, where much of it may result from 

mixing of rhyodacitic and mafic magmas. Such volumes may 

exceed the amount of dacite erupted from such dacite-dominated volcanoes as Glacier Peak, Mount Cayley, and Mount 

Garibaldi, and they rival that of Mount St. Helens. Away 

from the major foci, however, Quaternary dacite is rare in the 

Lassen and Shasta segments, even sparser than the isolated 

dacite occurrences in the Cascades of Oregon and Washington. Burney Mountain, however, one such rarity in the Lassen segment, consists of ~9 km3

 of phenocryst-poor dacite, a 

volume surpassing the amount of dacite produced by Mount 

Adams or Mount Baker and rivalling the dacite output of 

Mounts Hood and Jefferson. Volumetrically, silicic andesite 

is the dominant composition at Mount Shasta, probably also 

at Rainbow Mountain, and therefore in the Shasta segment 

as a whole. Andesite is probably the leading Quaternary 

composition erupted in the Lassen segment as well, owing to 

the large contributions of the Dittmar, Maidu, and Brokeoff 

edifices, even though it is exceeded in volume at the Lassen 

volcanic center itself by still more silicic products. In the 

Cascades of California (as in Oregon but not in Washington), 

a majority of the individual Quaternary volcanoes consist, 

however, of basaltic andesite, reflecting the great number of 

shields, cones, and modest lava fields distributed regionally. 

The Lassen segment is distinctive, nevertheless, in that small 

scattered andesitic and basaltic volcanoes are also well represented; although still numerically subordinate to basaltic 

andesite, both are much more common than in the Oregon 

and Shasta segments of the arc.

Among true basalts in the California Cascades, HAOT is 

widespread, and its vents are widely but sparsely distributed. 

In the Lassen segment, arc basalt is much more common than 

HAOT, in both eruptive volume and number of vents (Guffanti 

and others, 1990), but in the Shasta segment the mafic calcalkaline rocks are virtually all basaltic andesite (>52% SiO2

). 

HAOT is the hallmark of extension-related magmatism in the 

parts of California and Oregon inland of the Cascades, and 

HAOT appears to be the leading parental magma at the Medicine Lake Volcanic Field and probably at Newberry, as well. 

As westward-encroaching Basin and Range faulting invaded 

the Cascades and even reached the Cascade axis, so did venting of HAOT. The volcanically inactive gaps north and south 

of the Shasta segment are not longstanding but are marked by 

Pliocene mafic shields and cones, abundant in the northern 

gap, sparse in the southern. Development of the quiescent 

gaps in the southern Cascades is a Quaternary phenomenon, 

concurrent with northward contraction of the arc's southern 

terminus, westward drift of the eastern limit of the active belt, 

and westward penetration of extension-related faulting and 

magmatism to the heart of the Cascade arc.

Contemporaneous Volcanic Fields Behind the Arc

Three large Quaternary volcanic fields conventionally 

associated with the Cascade arc occupy discrete areas just 

east of it (figs. 8, 14, 21) and, although contiguous with the 

arc, they exhibit tectonic and compositional features distinguishable from typical arc volcanism. All three may be 

regarded in part as “central volcanoes” in the sense of Walker 

(1993) in that significant volumes of rhyolite are intruded 

and erupted near the central focus of a distributed, multivent, 

fundamentally basaltic volcanic field in which intermediate compositions are subordinate and mostly hybrid. Each of 

the three has commonly been referred to as “a volcano” (for 

example, Newberry Volcano), a usage I regard inappropriate 

and volcanologically misleading. Each of the three volcanic 

fields consists of hundreds of volcanoes. Although some vents 

are indeed laterally dike-fed at shallow levels, from the central 

magma-supply system out onto the flanks (as on some oceanic 

shields), many or most peripheral vents instead have independent conduit systems from the deep crust or mantle, and 

only a modest fraction of the vent array is concentrated in the 

(rhyolite-rich) summit region of the shield.
42 Quaternary Magmatism in the Cascades—Geologic Perspectives

Simcoe Mountains Volcanic Field

The Simcoe Mountains Volcanic Field extends ~45 km 

eastward from the (mostly younger) Mount Adams Volcanic 

Field in south-central Washington (fig. 8). It also extends ~75 

km northwest from Goldendale as far as Jennies Butte, draping two of the preexisting east-west-trending anticlinal ridges 

(consisting predominantly of Miocene flood-basalt units) that 

form the Yakima Foldbelt (the southern of the two being the 

Simcoe Mountains proper). The volcanic field lies largely in 

the Yakama Indian Reservation, and only its southern third 

drapes the Simcoe Mountains Anticline (atop which the 

conspicuous multivent Simcoe shield was constructed). New 

mapping (author's work in progress) has raised the number 

of vents identified to 184, of which at least 35 are of early to 

middle Quaternary age, the remainder Pliocene. A majority 

of the vents occupy the trough between the anticlines. Most 

are basaltic scoria cones and fissure-fed lava fields, although 

about 10 scattered units are intermediate (53-67% SiO2

), and 

five rhyolites (73-75% SiO2) are exposed near the top of the 

shield. Predominant among the mafic rocks (46-53% SiO2) 

are intraplate (rather than arc) trace-element signatures, and 

most are moderately alkalic (some carrying harzburgite, 

dunite, or lherzolite xenoliths), although subalkaline and 

even a few HAOT units are also present. In the southern and 

central parts of the field (as roughly partitioned by the two 

east-west anticlines), numerous vent cones are clustered, and 

some are locally aligned roughly north-south or NNW; but in 

the northern third of the field, vents are fewer, more widely 

spaced, and small shields dominate. K-Ar ages determined by 

Uto and others (1991) range from 4.5 to 0.6 Ma, and extensive 

40Ar/39Ar dating currently underway (2007) in concert with our 

mapping confirms that range. The Simcoe Mountains Volcanic 

Field covers at least 1,400 km2, with an estimated volume of 

~200 km3, of which about half may be younger than 2 Ma and 

roughly 10 percent younger than 1 Ma.

Newberry Volcanic Field

The Newberry Volcanic Field in central Oregon (fig. 14) 

contrasts with the Simcoe field in being subalkaline, younger, 

and far more voluminous. Although Newberry takes the form 

of a great shield volcano 35×65 km across, it is a profoundly 

hybrid edifice. Capped by a 5×7-km late Pleistocene caldera, 

the edifice has a rhyolite-rich core surrounded by a gently dipping apron sprinkled with more than 400 mafic scoria cones 

and fissure vents along with about 20 silicic flank domes 

(MacLeod and others, 1995). Many more domes and cones 

are likely to be concealed beneath the mantle of thin mafic 

lavas and mafic-to-silicic pyroclastic deposits that smooths the 

surface and conveys a deceptive integrity to the edifice, which 

is in large part a distributed volcanic field that surrounds and 

interfingers with products of the central volcano. 

The Newberry Volcanic Field occupies a tectonic singularity where the age-progressive rhyolite trend of the High 

Lava Plains (Walker, 1974; MacLeod and others, 1975) and 

the northwest prong of Basin and Range extensional faulting 

both impinge upon the Cascade arc (Walker and MacLeod, 

1991; Jordan and others, 2004). Many vents are northeastaligned on the south flank and northwest-aligned on the north 

flank, reflecting extensional fault belts that intersect beneath 

the edifice and probably helped instigate the major magmatic 

focus here. North-south elongation of the edifice and eastwest elongation of the caldera may both reflect the influence 

of net east-west extension—on mafic dike propagation and on 

growth of a shallow silicic magma reservoir, respectively.

The caldera is likely to be a composite structure that 

resulted from two or three syneruptive subsidence events, 

probably related to pyroclastic deposits that crop out patchily on most flanks of the edifice (MacLeod and others, 1995). 

Silicic ignimbrites (~300 ka) on the east flank may be related 

to an early subsidence, and compositionally zoned pyroclastic deposits (75-80 ka) on the west flank may be related to a 

younger collapse (Donnelly-Nolan and others, 2004). The high 

south wall of the caldera cuts the rhyolite lava of Paulina Peak, 

which has been dated at 83±5 ka (Donnelly-Nolan and others, 2004). Several more late Pleistocene rhyolitic lava flows 

are cut by the caldera walls, and a dozen intracaldera rhyolite vents (mostly Holocene) have erupted lavas and pumice 

(Williams, 1935; MacLeod and Sherrod, 1988; MacLeod and 

others, 1995; Jensen, 2006). Recurrent intracaldera eruptions 

of phenocryst-poor rhyolite of similar composition (73-74% 

SiO2

, 790-850 ppm Ba), the latest one as recent as 1.3 ka, and 

the scarcity of mafic eruptions on the caldera floor support the 

likelihood of a shallow rhyolitic magma reservoir (MacLeod 

and Sherrod, 1988), as does the 600oC/km conductive thermal 

gradient measured for the bottom 300 m of a 932-m-deep 

drillhole on the caldera floor (MacLeod and Sammel, 1982). 

Eruption of Little Crater and other mafic tuffs on the caldera 

floor, probably in the early Holocene, suggests, however, that 

the rhyolitic magma reservoir is modest and either distributed 

or transient. Relatively young mafic lavas (53.5-57% SiO2) 

have also erupted nearby, in the early Holocene along the east 

rim and on the east flank, as well as around 7 ka from at least 

two vents south of the caldera and a 23-km-long chain of vents 

northwest of it (MacLeod and others, 1995; Donnelly-Nolan 

and others, 2004; Jensen, 2006). As currently understood, 

products erupted at Newberry are strongly bimodal, silicic 

andesite and dacite being sparse while basaltic andesite, basalt, 

and rhyolite-rhyodacite are abundant.

The Newberry edifice itself covers ~1,600 km2

, but a 

(vent-poor) low-gradient apron of fluid tube-fed lava flows adds 

another 1,000 km2, principally in a broad thin lava field extending ~50 km northward from the toe of the edifice. Eruptive 

volume is at least 500 km3—not counting lost downwind tephra, 

which may be substantial (Kuehn, 2002). Basaltic andesite 

is strongly represented among the abundant scoria cones and 

lava flow fields at Newberry, but at least two kinds of relatively 

primitive basalt (48-50% SiO2) are also present—HAOT like 

that of the High Lava Plains (of eastern Oregon) and arc basalt 

rich in Ba and Sr (J.M. Donnelly-Nolan, work in progress).
Distribution of Quaternary Cascade Volcanoes 43

Medicine Lake Volcanic Field

Another great edifice adjacent to but inboard of the 

Cascade arc is the Medicine Lake Highland (Anderson, 1941; 

Donnelly-Nolan, 1988, in press) in northernmost California, 

centered 55 km east-northeast of Mount Shasta (fig. 21). Like 

Newberry, it takes the form of a gently sloping central shield 

dotted by hundreds of scoria cones and fissure vents, topped 

by a caldera, and surrounded by a low-relief apron of fluid 

basaltic lava flows (Donnelly-Nolan and Champion, 1987; 

Waters and others, 1990). Also like Newberry, it occupies an 

area of Basin and Range faulting, where net east-west extension may have influenced both north-south elongation of the 

vent zone and east-west elongation of the caldera. Vents of 

the volcanic field encompass an area of 40×55-km (fig. 21) to 

which the surrounding apron of outflow lavas adds half again 

as much more. About 66 percent of the surface exposure is 

basalt, 13 percent basaltic andesite, 15 percent andesite, and 

only 6 percent dacite and rhyolite (Donnelly-Nolan and others, 2003). With an eruptive volume of ~600 km3

, the Medicine Lake Volcanic Field is the most voluminous Quaternary 

eruptive locus in or adjacent to the Cascades, approached in 

volume only by Newberry and Shasta.

A dozen geothermal drillcores (Donnelly-Nolan, 2006) 

reveal, however, that the central core of the edifice contains 

numerous 475-to-300-ka rhyolite lavas, which are largely 

concealed by the veneer of mafic and intermediate lavas that 

shingle off toward the apron, conveying the deceptive appearance of a simple mafic shield. Like Newberry, therefore, the 

Medicine Lake Volcanic Field has as its focus a “central volcano” (in the usage of Walker, 1993), a multivent mafic shield 

centered on a rhyolite-rich core, but its flanks and periphery 

are also studded by hundreds of widely distributed monogenetic vents, many of which are fed by separate conduits from 

separate source volumes, independent of the shield's central 

magma-supply system. Donnelly-Nolan (in press) further distinguished 13 rhyolitic units that crop out on the upper flanks 

of the edifice, many of them of middle Pleistocene age like 

those in the drillcores; two more are late Pleistocene, and two 

erupted in the late Holocene. 

Although several smaller mafic shields, a few early Pleistocene rhyolites, and probably the buried source of a 1-Ma 

rhyolitic ignimbrite already occupied the site, construction of 

the present-day Medicine Lake edifice began ~500 ka (Donnelly-Nolan and Lanphere, 2005), roughly contemporaneous 

with initiation of Newberry and several intermediate Cascade 

stratocones (Baker, Rainier, Adams, Mazama, and Shasta). 

After 200 kyr as a bimodal (mafic-rhyolitic) center, Medicine 

Lake became a predominantly mafic eruptive center around 

300 ka and has remained so since.

The caldera, which has rim dimensions of 7×12 km, is of 

uncertain origin. Only one ignimbrite is exposed on the edifice 

(Anderson, 1941; Donnelly-Nolan and Nolan, 1986). Dacitic 

in pumice composition (63-67% SiO2

), the tuff is 10 to 30 m 

thick in drillcores beneath the summit, crops out widely but 

thinly (less than 5 m) on the flanks, is dated at ~180 ka, and 

appears to be too small (5-10 km3

) to account for the caldera 

(Donnelly-Nolan, 2006). It may be that mafic magma withdrawal toward flank vents (repeatedly?) and eruption of the 

dacite tuff both contributed to subsidence, creating ring faults 

that subsequently provided conduits for a ring of andesitedacite lavas (~100 ka) that surrounds and thus accentuates the 

depression (Anderson, 1941; Donnelly-Nolan, in press).

The Medicine Lake edifice has had 17 postglacial eruptions (Donnelly-Nolan and others, 1990). A sequence of eight 

events released ~5.3 km3 of HAOT, basalt, and basaltic andesite ~13 ka, by far the largest being the zoned Giant Crater lava 

field (~4.3 km3). After a subsequent lull, nine more events 

beginning at 4.4 ka produced HAOT, basalt, basaltic andesite, 

andesite, dacite, and rhyolite. Of the nine, the most recent 

(Glass Mountain) eruption (in the 11th Century) was the most 

voluminous (~1 km3), as well as the most silicic (zoned up to 

74.6% SiO2) and explosive (Eichelberger, 1975; Heiken, 1978; 

Donnelly-Nolan and others, 1990; Grove and others, 1997). 

Vent alignments are numerous and conspicuous on the 

Medicine Lake edifice, rivalled only by Newberry and Bachelor in their importance. Generally (but not strictly) reflecting 

east-west tectonic extension, most alignments strike within 30o 

of north, augmented by a few that are influenced by the caldera or other edifice effects or by a preexisting vent alignment 

that extends east-northeast from Rainbow Mountain through 

Garner Mountain and beneath the Medicine Lake edifice 

(Donnelly-Nolan and others, 1990).

Smith and Donnelly-Nolan (2005) identified 525 Quaternary vents mapped on or near the Medicine Lake edifice, 

noting also that vents for an additional 56 map units have been 

buried by younger units. They report that 20 percent of the vents 

released HAOT, 53 percent other basalts and/or mafic andesite, 

11 percent andesite, 3 percent dacite, and 9 percent rhyodacite 

and rhyolite. Because many map units have multiple vents, vent 

redundancy is far greater here than anywhere in the Cascade arc 

proper, reflecting the abundance of fissure eruptions of dike-fed 

basaltic lavas influenced by Basin and Range extension. 

Primitive magmas at Medicine Lake are predominantly 

HAOT, which is widespread in the Basin and Range Province 

of northeastern California and the High Lava Plains of Oregon 

but is also sprinkled sparingly along the Quaternary Cascade 

arc from Lassen to Goat Rocks. Thought to be produced under 

tectonic extension by dry decompression melting of shallow 

MORB-source mantle, HAOT magmas at Medicine Lake are 

water-poor (Sisson and Layne, 1993), as impoverished in K2

O 

as 0.07 percent, and have repeatedly induced upper-crustal 

melting, which has led to mixing, assimilation, and eruption 

of extensive lava flows zoned from primitive HAOT to basaltic 

andesite and andesite (Grove and others, 1988; DonnellyNolan and others, 1991; Baker and others, 1991; Bartels and 

others, 1991). Nonetheless, hydrous arc basalts containing 3 

to 6 percent H2

O have also erupted at Medicine Lake (Wagner 

and others, 1995; Kinzler and others, 2000), two as recently 

as the 9th Century, attesting to the influence of a slab-derived 

fluid flux (however attenuated) that affects some domains of 

the mantle wedge well into the rear-arc.
44 Quaternary Magmatism in the Cascades—Geologic Perspectives

The Hackamore Volcanic Field (fig. 21), which extends 

from beneath the Medicine Lake apron for at least another 

25 km eastward, is a HAOT-dominated Quaternary lava field 

formerly lumped with the Tertiary HAOT lavas it overlies 

(Donnelly-Nolan and others, 1996). Covering at least 850 

km2, the field has a dozen or more vents, most of them early 

Quaternary, but a few are middle Quaternary and as young as 

~600 ka (Donnelly-Nolan and Lanphere, 2005). Like Medicine Lake, Hackamore is a Basin and Range volcanic field, 

and it may be far enough inboard to be completely beyond the 

influence of the slab-derived fluid contributions sporadically 

or locally effective beneath Medicine Lake.

Characteristics of Quaternary Volcano 

Distribution

Types of Volcanoes

Of the 2,339 Quaternary volcanoes identified in the Cascade arc proper (table 1), only 19 are andesite-dacite stratovolcanoes in the usual sense of being steep composite cones 

(solitary or clustered) built up of interstratified lava flows and 

fragmental deposits. Ranging in eruptive volume from ~10 to 

450 km3 each, these are Baker, Rainier, Goat Rocks, Adams, 

St. Helens, Hood, Jefferson, Broken Top, South Sister, Middle 

Sister, Cappy, Mazama, Rainbow, Shasta, Snow Mountain, 

Magee, Maidu, Dittmar, and Brokeoff. The apparent cone 

height of these edifices (excluding apron deposits that extend 

down radial valleys cut in older rocks) ranges from 1,300 

m for South and Middle Sisters, to ~2,000 m for Jefferson, 

Baker, and pre-1980 St. Helens, to ~2,300 m for Hood and 

Rainier, to 2,650 m for Adams, and 3,250 m for Mount Shasta. 

For more details, see table 2. Among the other major centers, 

Glacier Peak, Garibaldi, Cayley, and Meager are in large part 

well-focussed silicic dome clusters surrounded by aprons of 

lavas and pyroclastic flows, whereas Lassen, Kulshan, and 

Tumalo are distributed silicic domefields, as was the preMazama rhyodacite field. Mount St. Helens added its modest intermediate-to-mafic stratovolcano to its volumetrically 

dominant silicic domefield only during the past 2,500 years, 

whereas the long-lived Mazama locus underwent alternating 

episodes of domefield and stratocone growth. Mount Meager 

may also have had an early stratocone-bulding phase. The Lassen region has produced a succession of intermediate stratovolcanoes, each culminating in peripheral eruption of rhyolite 

or rhyodacite.

To the list of large evolved edifices should be added at 

least 11 large mafic cones (predominantly basaltic andesite; 

all in Oregon) that are sufficiently composite, steep, and high 

(700 to 1,300 m relief) to qualify as stratocones. These are 

McLoughlin, Bailey, Thielsen (fig. 20), Odell Butte, Diamond Peak, Bachelor, North Sister, Black Crater, Black Butte 

(Oregon), Mount Washington (fig. 17), and Three Fingered 

Jack (each having an eruptive volume in the range 5-to-25 

km3

). There is a morphological continuum, however, from 

these steep mafic cones through transitionally conical shields 

(such as Olallie Butte, Maiden Peak, Hamner Butte, Davis 

Mountain, and Goosenest (fig. 22) into a spectrum of flatter 

mafic shield volcanoes (fig. 17) cored or capped by a fragmental facies. Table 2 summarizes some physical dimensions of 

most of the major edifices in the Cascades.

At least four times more abundant than stratovolcanoes in 

the Cascades are shield volcanoes, the number of Quaternary 

shields totalling about 110 (~30 in California, ~60 in Oregon, 

~20 in Washington, and none in the GVB). Most are basaltic 

andesite or calcalkaline basalt, but a few consist of HAOT 

(for example, Lemei Rock, Spring Creek) or andesite (Timber 

Crater, Pelican Butte, Goosenest). Some of the smaller shields 

(0.1-2 km3) may be short-lived or even monogenetic, but many 

larger ones (2-35 km3

) are probably polygenetic, erupting 

recurrently for centuries or millennia. Several of the largest 

Cascade shields (including King Mountain, Lemei Rock, Irish 

Mountain, The Twins, Fuji Mountain, Mount Yoran, Cowhorn 

Mountain, Sawtooth Mountain, Howlock Mountain, Timber 

Crater, Union Peak, Pelican Butte, Whaleback, and Ash Creek 

Butte) have volumes in the range 7 to 35 km3, thus rivalling 

or surpassing in size several of the smaller stratovolcanoes. 

Slope gradients of shields range greatly, from 2o for the Spring 

Creek shield (north of Mount Adams), which consists of 

fluid HAOT, to 7 to 10o for many basaltic and mafic andesitic 

shields like King Mountain, Trout Creek Butte, Maiden Peak, 

Davis Mountain, and Everitt Hill, to 15 to 20o for Odell and 

Black Buttes (Oregon), which would better be called cones 

than shields. Mount Bachelor illustrates a common ambiguity whereby a later cone (~20o) grew with apparent eruptive 

continuity from the same vent system atop an earlier shield (4 

to 7o) of similar composition. Magma viscosity is an obvious 

influence, but prevalence of low-intensity eruptions of spatter 

and scoria that pile up proximally would also steepen shields 

into cones. Topographic profiles of many types of Cascade 

volcanoes are assembled in figure 23.

Lava domes (including equivalent stubby viscous flows) 

account for about 15 percent of the vents, approximately 340 

such units among 2,339 Quaternary vents in the arc. Nearly all 

domes consist of dacite or rhyodacite, but a few are andesite 

(for example, Hogg Rock) or rhyolite (Three Creek Butte). 

The tallest domes are (Shasta) Black Butte (730 m), Lassen 

Peak (~800 m), and Burney Mountain (900 m), and two or 

more ice-mantled eroded domes atop Mount Meager are probably of comparable size. For comparison, Ptarmigan dome at 

Kulshan caldera is 300 m high, Butte Camp dome at the southwest toe of Mount St. Helens is 430 m, Goat Mountain (10 

km southwest of Mount St. Helens) is ~550 m, Spiral Butte is 

~520 m, the 1980-86 dome in the crater of Mount St. Helens 

is 267 m, and Crater Rock dome near the top of Mount Hood 

is less than 200 m. Volumetrically, very few domes exceed 0.5 

km3

, and some are smaller than 0.01 km3. Among the largest, Spiral Butte approaches 2 km3

, Lassen Peak dome and 
Characteristics of Quaternary Volcano Distribution 45

(Shasta) Black Butte are each ~2.5 km3, and ice-mantled Pali 

dome north of Mount Cayley may be of comparable volume. 

By far the largest Quaternary dome in the Cascades is dacitic 

Burney Mountain, at ~9 km3 (Frontispiece).

Most domes are clustered on or around a few main centers. Of the 22 Cascade domes identified in British Columbia, 

17 are closely related to the major edifices, Garibaldi, Cayley, 

and Meager. Of 36 in the Washington Cascades, at least 15 

are on or near Mount St. Helens and six are inside Kulshan 

Caldera. Of ~190 domes recognized in the Oregon Cascades, 

at least 40 lie close to Mount Jefferson (Conrey, 1991), ~22 

are scattered in the Tumalo “silicic highland” and its southward extension, 28 or more are around the Three Sisters, and 

no fewer than 63 are on, around, or beneath Mount Mazama. 

Of ~90 domes in the California Cascades, ~45 belong to the 

Lassen domefield (~14 precaldera and more than 30 postcaldera), at least 5 belong to the Maidu volcanic center, and 18 

are on or around Mount Shasta. In contrast to such concentrations, relatively few domes are isolated, clearly separated from 

major clusters or evolved centers. Examples include Spiral 

Butte in Washington and Frog Lake Buttes, Hayrick Butte, 

and Bench Mark Butte in Oregon. In not being related to any 

major edifice, the early Quaternary array of 18 dacite domes 

that extends 15 km southeastward from Barkley Mountain at 

the south end of the Lassen segment (fig. 21) is unparalleled in 

the Cascades. Another anomaly is the absence of lava domes 

for 140 km along the Cascade arc between Mount Mazama 

and the base of Mount Shasta.

Setting aside the 37 stratovolcanoes and major silicic 

dome complexes, the 110 shield volcanoes, and the 340 

lava domes, more than 1850 additional Quaternary vents are 

recognized in the Cascades, nearly all of them monogenetic 

(as are most domes and some shields). A majority of these 

lesser vents are marked by scoria, spatter, or lava cones, but 

several fissure-fed lava flows, lava fields, and lava fans left no 

conspicuous edifice or crater at all. Monogenetic tuyas and 

comparable near-vent piles of englacial mafic lava and hyaloclastite are recognized in the Bridge River, Cayley, Garibaldi 

Lake, Baker, Indian Heaven, Jefferson, Mazama, and Caribou 

Volcanic Fields (and ice-contact lava flows are present at most 

of the main stratocones). Maars and tuff cones are rare in the 

Cascades; the few examples include Battle Ground, Carp, and 

Pothole Lakes in southern Washington, clusters of maars in 

central Oregon north of Wickiup Reservoir and east of Lookout Mountain, and tuff rings near the lakes in Newberry Caldera. Deep Hole is a mid-Pleistocene phreatic explosion crater 

~800 m wide that lies 8 km west-northwest of Lassen Peak. 

Some monogenetic vents produced lava fields as voluminous 

as 1-5 km3

, but most erupted less than 1 km3 of lava or ejecta, 

and several yielded less than 0.01 km3

. Among them, however, 

are vents for nearly all the relatively primitive magmas erupted 

in the Cascades.

Lengths of lava flows are influenced by magma discharge 

rate, viscosity, and external factors such as slope, water and 

ice, topographic channeling, and ponding. Walker (1973) 

argued that the first is often the most important. Most of the 

longest lava flows recognized in the Cascades are partly or 

exclusively HAOT: the Spring Creek (>35 km) and Ice Caves 

(43 km) basalts near Mount Adams; Plutos Cave basalt northwest of Mount Shasta (>30 km); Hat Creek Basalt (fig. 2A; 20 

km); an intracanyon flow along Deer Creek southwest of the 

Lassen volcanic center (~45 km), and several more in the Lassen segment (30 to 55 km). Likewise, HAOT or eruptive units 

zoned to HAOT extend 45 km from Giant Crater (Medicine 

Lake) and ~50 km northward from the slopes of Newberry. 

Among the few arc basalts and basaltic andesites as long as 

15 km (most of them intracanyon flows) are the Elaho Valley 

flow (~25 km) south of Mount Meager, the Cheakamus Valley 

basalts (>35 km) northwest of Mount Garibaldi, the Glaciate Butte flow (25 km) at Mount Adams, the Cave Basalt (15 

km) at Mount St. Helens, the Big Lava Bed (15 km) at Indian 

Heaven, the late Holocene McKenzie River flow (15 km) from 

Belknap shield, several mafic flows of the Bachelor chain (15 

to 22 km), the Hootman Ranch flows west of Lassen Peak, and 

a remarkable intracanyon flow along the Sacramento River 

from Everitt Hill south of Mount Shasta (~60 km). The few 

comparably long lava flows of andesite or dacite were (like 

many of the lengthy basalts) channeled along canyons or along 

the margins of glaciers occupying the canyons (Lescinsky 

and Sisson, 1998)—for example, the Ring Creek dacite flow 

south of Mount Garibaldi (18 km), the Glacier Creek andesite 

flow northwest of Mount Baker (>8 km), a thick intracanyon 

andesite (>20 km) along the Klickitat River east of Mount 

Adams, an intracanyon dacite (>22 km) along Cascade Creek 

southwest of Mount Adams, and several thick ice-marginal 

andesite-dacite flows (now forming ridges) radial to Mount 

Rainier, of which some were as long as 15 to 22 km (Fiske 

and others, 1964; Lescinsky and Sisson, 1998). In a class by 

itself is the 1-Ma Tieton Andesite, which flowed 80 km eastward from the Goat Rocks (Swanson, 1978). Few rhyolite or 

rhyodacite lava flows in the Cascades surpass 3 km in length, 

exceptions being the Glass Mountain zoned dacite-rhyolite 

flow (>5 km) at Medicine Lake and the gigantic Blue Ridge 

rhyolitic coulee (14 km) at Maidu. Not surprisingly, many (not 

all) of the longest units, whether principally mafic, intermediate, or silicic, are compositionally zoned (Eichelberger, 1975; 

Green, 1988; Donnelly-Nolan and others, 1991, 2005; Schick, 

1994; Swanson, 1994; Grove and others, 1997; Stockstill and 

others, 2002).

Among the hundreds of Cascade scoria cones that are 

uneroded or only modestly so, most are 50 to 150 m high, only 

about 20 exceed 200 m, and only Border Mountain southeast 

of Medicine Lake is higher than 300 m. Of those that exceed 

200 m, nearly all are in the Lassen region, the Caribou Volcanic Field, and the Mazama and Sisters reaches of Oregon. 

Only two of the many Simcoe Mountains scoria cones reach 

200 m, and none of the superabundant scoria cones at Newberry is that high. Remarkably, one of the highest ejecta cones 

(210 m) in the region—the Central Pumice Cone at Newberry 

Caldera—is not mafic, consisting instead of Holocene rhyolite. 

Irrespective of composition and volume, global data compilations show that the original height of scoria cones is close 
King Mtn 

Pelican Butte 

Mount Bachelor 

North 

Sister 

Middle 

Sister 

South 

Devils 

Sister 

Step Hill 

Sister 

Chaos Crags 

Black Butte (Shasta) Burney 

Lassen Peak 

Glacier Peak 

Gamma Rid

Lillooet 

R. 

Fall Creek 

Plinth Pk Capricorn Mtn Pylon Pk 

Meager 

Creek Mount Meager complex 

Odell Butte 

Mount McLo

SW 

SW NE 

NW SE 

NNW 

N S N 

SSE 

SW 

NNE SSW 

NW 

N S 

NW 

Mount Washington Belknap Crater 

N S 

 caldera 

5 km 
Mount Rainier

Echo Rock

Mount Shasta

Shastina

Mount Hood

Sandy Glacier volcano

Mount Adams

Goat Butte

Black Buttes

Mount Baker

The Portals

Kulshan caldera fill

Table Mtn

W 

W 

NW SE

SW NE

N S 

5 km

to debris avalanche

0

Figure 23. Profiles of selected Cascades volcanoes, all at the same scale, illustrating a variety of sizes and slopes. For only a few is the configuration (hachures) well known. Free-standing lava domes shown are Lassen Peak and adjacent Chaos Crags, Black Butte (Shasta), and Burney Mountain, the in the Cascades. Major stratovolcanoes shown are Rainier, Adams, Hood, and Shasta, which is the most voluminous in the Cascades. Although such edalso have vent domes, Glacier Peak is transitional in being dominated by a prominent summit dome complex that feeds an extensive apron of lavas and of contiguous edifices shown are Meager, Baker, and the Three Sisters. Mafic cones and shields intergrade morphologically: King Mountain and Belknshields; Odell and Pelican Buttes are steep shields; Mount Bachelor has been called a mafic cone atop a shield (of similar composition); Mounts McLouThielsen; fig. 20) are steep and high enough to be called mafic cones with proportionally important pyroclastic and intrusive core facies.
48 Quaternary Magmatism in the Cascades—Geologic Perspectives

to 0.18 × basal diameter, that nearly all such eruptions last 

between a week and a year, that most scoria cones include layers, lenses, or rim facies of agglutinate, and that most produce 

lava flows (Wood, 1980).

Pyroclastic deposits in the Quaternary Cascades are 

varied, superabundant, and (with only a few exceptions) 

small. Mafic scoria and spatter cones are most common, but 

only a few are known to be associated with large downwind 

ashfall blankets—for example, Collier Cone (Oregon), Smith 

Butte (Adams), Blue Lake crater (Oregon), and Cinder Cone 

(Lassen) (Taylor, 1987; Hildreth and Fierstein, 1997; Sherrod 

and others, 2004; Clynne and others, 2000); erosion of such 

ash is rapid, and the preservation potential poor. Crudely 

stratified phreatic and phreatomagmatic deposits surround 

the summit vents of ice-capped andesitic stratocones including Baker, Rainier, and Adams. Block-and-ash-flows (poorly 

pumiceous pyroclastic flows) have commonly been deposited 

around many Cascade stratovolcanoes and preserved most 

prominently at Glacier Peak, St. Helens, Hood, and Shasta, 

but such deposits have been rapidly scoured from the radial 

drainages surrounding Mount Rainier and Mount Adams. 

Pumiceous pyroclastic flow deposits (ignimbrites) of small 

to moderate scale are recognized at Lassen, Shastina and 

Black Butte (Shasta), Jefferson, St. Helens, Glacier Peak, 

Garibaldi, and Meager, but large-scale Quaternary ignimbrites (>10 km3

) are recognized only at Lassen, Crater Lake, 

Tumalo, and Kulshan.

Far-flung silicic fallout sheets, many of them resulting from plinian eruptions, are listed in table 3, which also 

gives estimates of age and volume. Absence from the list of 

any tephras predating the Last Glacial Maximum that issued 

from the silicic Canadian centers surely reflects removal by 

the Cordilleran ice sheet rather than incapacity to produce 

explosive eruptions. The great number of plinian eruptions 

from Mount St. Helens (table 3; Mullineaux, 1996) probably reflects major contributions from crustal sources, low 

temperatures, and high magmatic water contents, which also 

promoted crystallization of cummingtonite in many units 

(a mineral unknown elsewhere in the Cascades). Adams, 

Rainier, Hood, and Shasta have each erupted numerous dacite lavas, many as silicic as the pyroclastic products of Mount 

St. Helens, but none of these volcanoes has a significant 

record of plinian eruptions. 

Vent Redundancy

Every lava dome and scoria cone marks a discrete volcanic vent and thus represents, in the broadest sense, a separate 

volcano. Of greater interest, however, is whether each one 

has its own conduit system from some magma reservoir in 

the crust or mantle and thus represents a truly independent 

volcano. Owing to the obsolete but once widespread view 

that most small volcanoes reflect lateral magma leakage from 

big ones, it may be useful to expand here on distinguishing 

independent volcanoes from redundant ones. Four main types 

of vent redundancy can be envisaged—(1) flank vents (of 

stratovolcanoes and some shields) fed laterally by shallow 

breakouts of the magma supplying the central conduit system, 

(2) secondary rootless vents fed by lava tubes or other breakouts on surfaces of lava-flow fields and shields, (3) chains of 

domes or pyroclastic cones fed concurrently from a common 

dike, and (4) clustered overlapping lava domes central to a 

large edifice or inside a caldera, fed in succession from a common reservoir of similar magma through essentially the same 

conduit, fissure, or ring-fracture system. 

Cascade examples of type 1 (lava flows fed from authentic flank vents) are the Lava Park flow at Shastina (Williams, 

1932a; Miller, 1980) and several postglacial andesite flows 

around Mount Adams (Hildreth and Fierstein, 1995, 1997). 

Flank vents at Adams, Shasta, and Mount St. Helens and abundant dikes at (deeply eroded) North Sister reflect the tendency 

of a high edifice to develop its own gravity-driven stressfield, 

favoring radial fracturing. Such edifice effects may likewise 

have influenced emplacement of caldera-wall dikes and preclimactic rhyodacite vents around Mount Mazama (Bacon, 1983, 

1985). The relative scarcity of authentic flank vents elsewhere 

in the Cascades may reflect the importance of regional deviatoric stress. Throughout much of the arc, an along-arc northsouth direction of maximum horizontal compression may rule 

against edifice dominance and in favor of somewhat deeper 

control of the abundant north-south vent alignments.

Examples of type 2 redundancy are present on the King 

Mountain shield in Washington (Hildreth and Fierstein, 

1995), along the Bachelor chain in Oregon (Scott and Gardner, 1992), and on the Plutos Cave HAOT apron northwest of 

Mount Shasta. Examples of type 3 include the Devils Chain of 

rhyolite domes near South Sister (Scott, 1987), the Sharp Peak 

rhyodacite dome chain near Mount Mazama (Bacon, 1983), 

the spatter-cone vent chain for the Hat Creek Basalt (Muffler and others, 1994), the Red Crater scoria-cone chain west 

of Mount Bachelor (Scott and Gardner, 1992), and the Smith 

Butte chain of scoria and spatter cones south of Mount Adams 

(Hildreth and Fierstein, 1995, 1997). Cascade examples of 

type 4 redundancy are the succession of 1980-2006 intra-crater 

domes at Mount St. Helens and the summit-dome clusters at 

Shastina, Glacier Peak, and Mount Garibaldi. In contrast to 

such tight clusters, however, most domes in the distributed 

Lassen and pre-Mazama domefields probably had independent vents, because they were emplaced over protracted time 

intervals through separate conduits from secularly evolving 

reservoirs (although within such domefields, a few chains like 

the eight rhyodacite domes of Sunflower Flat north of Lassen 

Peak may well be examples of type-3 redundancy).

In counting Cascade volcanoes, I tried to identify all 

cases of vent redundancy, demonstrated or likely. Tube-fed 

secondary vents on the flanks of mafic shields were not 

included at all. Keeping track of type-4 redundancy is relatively simple, because the only Cascade examples of summitdome clusters are Meager, Cayley, Garibaldi, Glacier Peak, 

Hood, Shastina, and pre-1980 St. Helens. Solitary multipulse 

domes such as Burney Mountain and (Shasta) Black Butte 

are treated as single volcanoes. Type-1 redundancy (authentic 
Volcano Unit Age Magma (km3) Notes 

Meager Bridge River tephra 2.4 ka 2 Broad easterly dispersal (British Columbia, Alberta) 

Kulshan Lake Tapps tephra 1.15 Ma ~30 Intracaldera tuff also ~30 km3; outflow eroded away 

Glacier Peak GP layer A 2 ka <0.01 Phreatomagmatic ash associated with dome collapse 

GP Dusty Creek tephras 5.8 ka <0.01 Four thin fall layers; ash clouds from pyroclastic flows GP layer D 6.1 ka <0.01 Ash cloud associated with dome collapse GP layer B 13 ka 2 Plinian pumice fall; dispersal axis to east-southeast 

GP layer M 13 ka 0.4 Plinian pumice fall; southeast dispersal axis GP layer G 13 ka 2 Plinian pumice fall; easterly dispersal axis 

Rainier MR layer C 2.2 ka 0.1 Biggest of 10 postglacial vesicular falls; mixed andesite-dacite 

MR layer D ~6.8 ka 0.01 Subplinian pumice showers east of the edifice MR layer L ~7.2 ka 0.01 Subplinian pumice shower southeast of the edifice MR layer R ~9.7 ka 0.01 Subplinian pumice and scoria fall Biotite-bearing pumice fall 380 ka >1 2 m thick at Sourdough Ridge; sole biotite-rich unit at Rainier White pumice fall 190 ka >1 20 m thick at Sunset Amphitheater 

St. Helens MSH 18 May 80 1980 CE 0.3 Composite of plinian and coignimbrite ash contributions 

MSH layer T 1800 CE 0.4 Narrow lobe to NE. Goat Rocks period; layer has opx>hb>cpx MSH layer We 1482 CE 0.4 Broad easterly dispersal. Kalama period; Set W has opx+hb MSH layer Wn 1480 CE 2 Narrow lobe to northeast MSH layer Pu ~2.6 ka 0.2 Subplinian; dispersal to east. Pine Creek period; Set P has opx+MSH layer Ps ~2.9 ka 0.1 Subplinian; dispersed to northeast. MSH layer Ye ~3.5 ka 0.9 Strong easterly dispersal. Smith Creek period, Set Y has cm+hMSH layer Yn ~3.5 ka 4 Most voluminous eruption at MSH. Dispersal to north-northeaMSH layer Yb ~3.9 ka 0.3 Dispersal to north and east. Has minor biotite (+cm+hb) MSH layer Jg ~12 ka ~0.3 Subplinian; thickest to WSW. Andesitic (58% SiO2; hb+opx+cp

MSH layer Jb ~12 ka ~0.5 Swift Creek stage, Set J has hb+opx. Jb only to east and southeMSH layer Jy ~12 ka ~1 Discrete lobes to southeast and northeast MSH layer So ~14 ka ~0.5 Thickest to NE and east. Swift Creek stage; Set S has cm+hb±oMSH layer Sg ~14 ka ~1 Most voluminous layer in Set S MSH layer Mm ~20 ka <0.3 Thickest layer of Set M on east side. Cougar stage; cm, hb, opx 

MSH layer Mp ~20 ka <0.1 Subplinian. This (or another M layer) may = Summer Lake ash 

MSH layer Mc ~20 ka <0.3 Coarsest layer in Set M; thickest to southeast 

MSH layer Mo ~20 ka <0.05 Subplinian; limited to east and southeast 

MSH layer Mg ~20 ka ~0.1 Subplinian or plinian; limited to east and southeast 

Table 3. Major fall units from the Quaternary Cascades.[Many additional fall units exist, either small and local, or old, eroded, and poorly documented. Mineral abbreviations: bi, biotite; cm, cummingtonite; hb, hornblende; opx, orthopyroxene; cpx, clinopyroxene. S, south; S-C; south central]
Volcano Unit Age Magma (km3) Notes 

MSH layer Cs 35-50 ka >1 Ape Canyon stage, Set C has cm, bi, hb. Cs is thickest to souMSH layer Cy 35-50 ka >1 Thickest to E. Cy may = Cs and both may = Summer Lake asMSH layer Cm 35-50 ka ~1 Thickest to east and southeast MSH layer Cw 35-50 ka ~1 Coarsest layer in Set C MSH layer Ct 35-50 ka <0.3 Subplinian. Restricted to southeast MSH pre-Set C 80±20 ka <1 Cummingtonite-dacite ash in eastern Washington MSH pre-Set C ~300 ka >1 Biotite-bearing ash layer OO at Summer Lake 

Jefferson MJ layer E 50-100 ka ~1 Older than last major glaciation. Accompanied by ash flows 

MJ layer U 50-100 ka ~0.1? Slightly older than layer E. Accompanied by ash flows 

Tumalo Shevlin Park Tuff 260 ka >5 Ignimbrite near Bend, Oregon. May = Summer Lake ash JJ 

Pumice of Columbia Canal ~300 ka? ~1? Plinian fall near Bend, Oregon. May = Summer L. ash NN Loleta ash 440 ka >5? Bend pumice fall and Tumalo Tuff; = ash 6 at Clear Lake Rye Patch Dam ash ~650 ka >1 Matches Desert Spring Tuff and ash T321 in Tulelake core 

South Sister Devils Chain tephra 2.0 ka 0.02 Accompanied by ash flows; followed by rhyolite domes 

Rock Mesa tephra 2.2 ka 0.03 Accompanied by ash flows; followed by rhyolite coulee 

Mazama Cloudcap Road pumice fall >85 ka? ~1? May = ash T2382 in Tulelake core and ash 4 at Summer Lake 

Pumice Castle ash ~72 ka >0.5 Caldera wall to Summer Lake ash 6 (and to Palouse?) 

Wono ash 31 ka >0.1 Preclimactic rhyodacite; ash F at Summer Lake. Preclimactic rhyodacite ~30 ka >0.1 Ash E1 at Summer L.; ash T2438 at Tulelake. Trego Hot Springs ash 27 ka >0.1 Preclimactic rhyodacite; ash 18 at Summer L.; T2307 at TulelLlao Rock plinian 7.9 ka 2 Tsoyawata ash in NV; ash T2309 at Tulelake Mazama ash 7.7 ka 33 Caldera-forming eruption; accompanied by large ignimbrite 

Shasta Red Banks pumice 11 ka >1 Predates Shastina edifice; fallout sheet extends NE, NW, and 

Lassen Rockland ash 610 ka >30 Caldera-forming eruption; accompanied by large ignimbrite 

Chaos Crags fallout 1.1 ka <0.01? Fall deposit associated with domes identified for 30 km east 

Rear-arc centers Newberry Big Obsidian plinian 1.3 ka 0.1 Followed by ash flows and rhyolite coulee 

Olema ash ~75 ka >5? May = youngest caldera-forming eruption at Newberry Older plinian pumice falls >75 ka ? Several plinian deposits not well dated or correlated. Tuff of Tepee Draw 280-300 ka ~10? Caldera-forming (?) rhyolite ignimbrite; fall equivalent uncert

Medicine Lake Glass Mountain plinian 0.9 ka 0.03 Followed by domes and flows of rhyolite and dacite 

Tuff of Antelope Well ~180 ka >3? May = Tulelake ash T2023. [Does not = Summer Lake ash K

Table 3. Major fall units from the Quaternary Cascades.—Continued[Many additional fall units exist, either small and local, or old, eroded, and poorly documented. Mineral abbreviations: bi, biotite; cm, cummingtonite; hb, hornblende; opx, orthS, south; S-C; south central]
Note: In addition to these, many uncorrelated Quaternary tephras potentially derived from the Cascades, Newberry, or Medicine Lake have been found in nearby drill cores. 

cm thick or contain coarse ash or lapilli, few are likely to represent plinian falls of more than local dispersal. Of 54 tephra layers logged (14) in exposures and cores at Summer only 12 have been assigned to source volcanoes; 19 are unstudied because they are too wispy, reworked, or mixed; and 23 have been subjected to microprobe analysis of glass sprovenance. In the 331-m Tulelake (northern California) core (38), at least 12 uncorrelated tephras beneath the 610-ka Rockland ash are well distributed through the age range have high-silica glass potentially related to rhyolite eruptions near Medicine Lake, whereas half have rhyodacite glass, as likely to be related to units at Mazama, Shasta, or LasLake area. Higher in the Tulelake core, ~28 ash layers (all but four with high-silica glass) are clustered in the age range ~360 to 140 ka; half of these are 3-8 cm thick, and six c(38), suggesting nearby sources. Five more unassigned tephras in the top 17 m of the Tulelake core are intercalated with several from Mazama and are likely to come from ther22 middle and late Pleistocene tephras were logged in lake-sediment cores at six sites between Mount Shasta and Klamath Lake, of which more than half are uncorrelated. On many pumice-fall deposits have been identified in isolated excavations (25); some are plinian, others may be subplinian or lesser falls related to domes or pumice cones, and sevor mixed and appear to be complexly lobate; although some are likely to have reached Summer Lake, ages and convincing correlations are not yet available. 

No widespread fall units have been identified from Hood, Adams, Rainier, Baker, Garibaldi, or Cayley, though each of these major centers has erupted dacitic or rhyodacitic 

Deposition atop (and erosion by) the Continental ice sheet may account for absence of Pleistocene fall deposits from Meager to Baker. Despite proximity of lake cores, only onfied from Shasta. References: 1. Bacon, 1983. 2. Bacon and Lanphere, 2006. 3. Beget, 19 81. 4. Beget, 1984. 5. Benson and others, 1997. 6. Busacca and others, 1992. 7. Carey and othe9. Christiansen and others, 2002. 10. Clynne and others, 2007. 11. Conrey, Donnelly-Nolan, and others, 2001. 12. Criswell, 1987. 13. Davis, 1978. 14. Davis, 1985. 15. 2004. 16. Donnelly-Nolan, in press. 17. Foit and others, 2004. 18. Gardner and others, 1992. 19. Gardner and others, 1998. 20. Heiken, 1978. 21. Herrero-Bervera and otand Helsley, 1993. 23. Hildreth, 1996. 24. Hildreth, Lanphere, and others, 2004. 25. Kuehn, 2002. 26. Kuehn and Preppernau, 2005. 27. Lanphere and others, 1999. 28. LLeonard, 1995. 30. MacLeod and others, 1995. 31. Miller, 1980. 32. Mullineaux, 1974. 33. Mullineaux, 1996. 34. Nasmith and others, 1967. 35. Negrini and others, 2001990. 38. Rieck and others, 1992. 39. Sarna-Wojcicki and others, 1981. 40. Sarna-Wojcicki and others, 1985. 41. Sarna-Wojcicki and others, 1987. 42. Sarna-Wojcicki anWojcicki and others, 1989. 44. Scott, 1987. 45. Sherrod and others, 2004. 46. Sisson and Vallance, 2005. 47. Sisson and Lanphere, work in progress, 2007. 48. Sparks and Russell, 1990. 50. Yogodzinski and others, 1983. 51. Young, 1990. 

pre-Medicine Tulelake pumice T2119L ~600 ka ? Pumice (glass 76.7% SiO2) correlates to Clear Lake ash 7 

Lake Tuff of Box Canyon 1.0 Ma >1? May correlate to rhyolitic ash at 110.5 m in Tulelake core 

Tuff of Gillems Bluff 2.0 Ma >1? May correlate to silicic ash sequence at 181-174 m, Tulelake co

Orphan ashes (sources uncertain)S Cascades Summer Lake tephra V ~140 ka ? Rhyodacite ash (glass 73% SiO2) = Tulelake T1193 

S Cascades Rio Dell ash ~1.45 Ma >5? Ash in Humboldt Basin and DSDP cores 

S-C Cascades Summer Lake ash 2 ~75 ka ? 4-cm rhyolitic ash; may be from Newberry? 

Wadsworth ash ~160 ka ? May = Summer L. ash KK and Tulelake ash T261 Pringle Falls ash D ~218 ka ? 30-cm pumice fall; may be from Sisters reach or Newberry 

S-C Cascades Dibekulewe ash ~630 ka >5? Rhyolitic ash (glass 77% SiO2) widespread in California, Nevada, a
52 Quaternary Magmatism in the Cascades—Geologic Perspectives

flank vents) can be hard to identify on long-lived or deeply 

eroded edifices, but (perhaps surprisingly) it appears to be 

common at only a few centers in the Cascades. Ten unequivocal flank vents well up on the slopes of Mount Adams (fig. 

24) erupted andesite-dacite lavas compositionally similar 

to the magma supplied centrally to the summit vent, and at 

least 15 similar but generally older eruptive units (30-400 ka) 

issued from vents (now covered by younger lavas) that could 

either have been central or on the upper-to-medial flanks of 

the stratocone. Elsewhere, there may be a few authentic flank 

vents on each of the following stratovolcanoes—St. Helens, 

Jefferson, Middle Sister, South Sister, Mazama, and Shasta—

but none has been identified at Baker, Rainier, or Hood (at 

each of which relatively mafic peripheral vents are considered independent). In like manner, peripheral rhyolite lavas 

that erupted around such intermediate cones as Maidu and 

Middle Sister are treated as independent, because they issued 

either from a separate reservoir or from one at a far different 

evolutionary stage than when the central cone was constructed. 

The total number of authentic flank vents in the Quaternary 

Cascades is therefore not known to exceed 50.

Relatively mafic small volcanoes on or near the flanks of 

andesite-dacite stratocones exemplify the class of peripheral 

vents that were fed from deep in the crust by independent 

magma batches that avoided entrapment by the complex 

crustal reservoir and conduit bundle that supplied the central 

edifice. The products of such small peripheral volcanoes are 

commonly more primitive than anything erupted from the 

main central volcanoes themselves (fig. 24). Far from being 

mere flank vents “parasitic” upon the major stratovolcanoes, 

small peripheral mafic cones are actually the more fundamental volcanoes magmatically, in that they erupt magma drawn 

more directly from the mantle or deep crust than do the stratovolcanoes, beneath which interception, mixing, and production 

of evolved hybrids is the magmatic norm (Hildreth, 1981a; 

Hildreth and Moorbath, 1988).

Type-3 redundancy (synchronously erupted vent chains) 

is the hardest to prove, a strong case requiring flow-by-flow 

mapping, paleomagnetic, chemical, and petrographic data, 

and (optimally) tephra stratigraphy. Alignment alone does not 

signify that vents are either comagmatic or contemporaneous (Conway and others, 1997). This point is illustrated, for 

example, by the linear vent zones (22 to 50 km long) that 

strike northerly through Bachelor, Shasta, Adams, Caribou, 

and Indian Heaven, along each of which compositionally 

varied products erupted recurrently over thousands (or hundreds of thousands) of years (Christiansen and others, 1977; 

Korosec, 1989; Gardner, 1994; Hildreth and Fierstein, 1995; 

Guffanti and others, 1996; Clynne and Muffler, in press). 

Within the Mount Bachelor area, however, several local 

subchains of scoria cones and maars (the Red Crater, Siah, 

Sheridan, and Wuksi alignments) probably do contain redundant vents, which could total as many as 35 of the 80-odd 

postglacial vents mapped along and near the Bachelor chain 

by Scott and Gardner (1992). Likewise, among the several 

chains of scoria cones in the Sand Mountain-Nash Crater 

area (southwest of Three Fingered Jack), as many as 10 vents 

could be redundant (Taylor, 1968, 1990b; Sherrod and others, 

2000). Five scoria cones are redundant near Mount Adams 

(Hildreth and Fierstein, 1995), and elsewhere in southwestern 

Washington another handful are likely to be so (Smith, 1993). 

In the periphery of Mount Mazama, as many as eight mafic 

to intermediate vents appear to be redundant (Bacon, 2006), 

and among the 476 Quaternary vents recognized in the Lassen 

segment about 50 are likely to be redundant—judging by the 

alignments, close spacing, and compositional data portrayed 

by Guffanti and others (1990) and Clynne and Muffler (in 

press). However, no mafic-vent redundancy is recognized in 

the Garibaldi Volcanic Belt and little in the Shasta segment 

(except along the Garner Mountain and Fisk Ridge chains 

and in the rear-arc Medicine Lake Volcanic Field). Although 

future work may add more examples of redundancy among the 

mafic monogenetic vents of the Cascade arc, the number now 

considered plausible is smaller than 180.

Redundancy among silicic domes warrants additional 

comment. Clustered summit domes (as atop Glacier Peak and 

Mount Garibaldi) may well be of several ages but, because 

they vented repeatedly within a limited area, they were presumably fed by a spatially restricted conduit system. Distributed summit domes (and intrusive equivalents) separated by 

3 to 5 km (as for Mount Meager) and flank domes high on an 

edifice but only a few kilometers outboard of a central vent, as 

for Mount Cayley and pre-1980 Mount St. Helens (fig. 12) are 

less clearly redundant. They could have been supplied either 

by shallow branches of a central conduit or by independent 

conduits from a crustal magma reservoir. Where a large uppercrustal silicic magma body develops, derivative lava domes are 

commonly distributed in space and time, and each typically 

has its own conduit, as, for example, many of the preclimactic 

rhyodacites around Crater Lake (Bacon and Druitt, 1988). 

Dike-fed chains of contemporaneously emplaced domes like 

the Sharp Peak (Bacon and Druitt, 1988) and Devils Chain 

(Scott, 1987) alignments seem to be exceptional. Intracaldera 

domes can be solitary and central (as beneath Crater Lake), 

arcuately controlled by ring faults (as at Valles Caldera in 

New Mexico), or partly dike-aligned and partly scattered (as 

in the Lassen domefield). Nearly all Cascade domes except 

the dike-linked chains and summit clusters are thought to 

have independent conduits, although such conduits generally 

issue only from mid- or upper-crustal reservoirs (in contrast to 

deeper reservoirs for many of the monogenetic mafic volcanoes). With these qualifications, then, I consider redundant 

only about 60 of the nearly 340 lava domes recognized in the 

Cascades.

Of the 2,339 Quaternary vents identified, therefore, fewer 

than 50 are redundant by being authentic flank vents of stratovolcanoes, and only ~60 domes and ~180 scoria cones are 

redundant by having been supplied either sequentially by the 

same conduit or synchronously with a neighbor along a single 

dike. This leaves the Cascade arc with about 2,050 independent Quaternary volcanoes, roughly two volcanoes for each 

kilometer of arc length.
Characteristics of Quaternary Volcano Distribution 53

Figure 24. Northeast side of 3,742-m Mount Adams stratovolcano, showing one late-glacial and two postglacial peripheral vents, in the 

foreground at 2,100 to 2,250 m. Red Butte scoria cone (at left bottom) produced slightly alkalic basaltic lavas (49-50% SiO2) unrelated to 

Mount Adams that flowed as far as 13 km northeast ~14 ka. To the right of Red Butte, a 2-km-long fissure vent that strikes toward Mount 

Adams is marked by black lavas, agglutinate, and a scoria ridge. Erupting in the middle Holocene, it represents a lateral flank vent of the 

stratocone that produced lava flows of pyroxene andesite (59-60% SiO2

) that extend as far as 10 km northward. The complex of lavaflow levees at left center is a pyroxene-andesite shield (58-59% SiO2

) that erupted at 37±8 ka and is only lightly ice-scoured (proximally); 

likewise a lateral flank vent, its lavas are compositionally similar to contemporaneous products of the central-vent stratocone. Upper 

end of shield is overlain by a Holocene debris-avalanche deposit (Devils Gardens), which in turn is overlain by a Little Ice Age moraine. 

Summit icecap of Mount Adams feeds several radial glaciers, separated by cleavers consisting of andesitic lavas and scoria (55-60% 

SiO2

) that erupted between 40 and 15 ka. For detailed eruptive history, see Hildreth and Fierstein (1995, 1997). High spur left of summit is 

“The Castle” (Rusk, 1924), which is also visible in figure 10 (where seen from the southeast). 

(Copyrighted photo by and courtesy of Darryl Lloyd, 2006.)

Distribution Patterns of 2,050 Independent 

Volcanoes

The approximately 2,050 independent Quaternary 

volcanoes in the Cascade arc include ~1,630 small mafic to 

intermediate volcanoes, ~275 domes, ~110 substantial shield 

volcanoes, 11 mafic stratocones, and 26 major evolved centers 

(of which 19 are andesite-dacite stratovolcanoes and seven are 

silicic dome complexes).

Virtually all of the 2,050 volcanoes are indicated on map 

figures 3, 8, 14, 15, and 21 (although a few vent symbols 

obscure each other at this scale). The map patterns illustrate 

several noteworthy features: (1) the concentrated clusters of 

volcanoes in the Garibaldi Volcanic Belt contrast with the diffuse but more continuous distribution elsewhere; (2) the close 

spacing of mafic volcanoes contrasts with the exceedingly 

irregular spacing of large evolved volcanoes; (3) the volcanic 

front is ragged or ill-defined and is not generally defined by 

stratovolcanoes or other major centers (except in the Garibaldi 

Volcanic Belt); (4) the breadth of the vent belt from Mount 

Rainier to California is extremely variable along the arc and 

of extraordinary width in southern Washington and California; 

(5) closely spaced mafic volcanoes form a belt of remarkable continuity along much of the Oregon segment; (6) dense 

concentrations of small volcanoes cluster tightly into several 

mafic volcanic fields (Indian Heaven, Mount Adams, Port-
54 Quaternary Magmatism in the Cascades—Geologic Perspectives

land, Bachelor, Caribou, and the big rear-arc fields); (7) a few 

gaps along the arc virtually lack Quaternary volcanoes despite 

contiguity with extensive segments having high vent-density; 

(8) forearc volcanoes are irregularly distributed, abundant in 

southern Washington and around Portland, sprinkled sparsely 

along the Oregon Cascades, absent in the Garibaldi Volcanic 

Belt, and common in California; (9) the arc volcanic belt 

overlaps eastward with three major contemporaneous rear-arc 

volcanic fields, which are discrete areas of high vent density 

that contrast with the sparsity of Quaternary rear-arc volcanism elsewhere; and (10) rhyodacite has limited local distribution in the Cascades, true rhyolite is rare except in the Lassen 

segment, central Oregon, and the rear-arc, and (by global arc 

standards) Quaternary calderas are remarkably uncommon.

Breadth of the Cascade Arc

In contrast to some single-file chains of variably spaced 

arc volcanoes elsewhere (for example, Alaska Peninsula, 

Aleutian, Lesser Antilles, and South Sandwich arcs), the 

belt of Quaternary volcanic vents is 25 to 100 km across in 

Oregon and wider still in the southern Washington and Lassen segments. Because the magma fed to most of the 2,050 

independent arc volcanoes is mafic and that fed to many of the 

others is thought to have evolved from basalt by fractionation 

and hybridism within the crust (Hildreth, 1981a; Hildreth and 

Moorbath, 1988), such breadth implies at least equally widespread (across-arc) penetration of mantle-derived basalt into 

the arc crust. How such magma subsequently evolves and rises 

through the crust are issues addressed later, but the critical 

point here is that basaltic magma is contemporaneously available from partially molten sub-arc mantle across an east-west 

belt 25-160 km wide from Lassen to Rainier (though apparently narrower beneath the GVB).

The question is seldom addressed whether the conduit 

system that feeds any particular mafic volcano taps (1) a circumscribed mantle melting column; (2) a family of melting 

(or magma-aggregation) loci, as within an upwelling region; 

(3) a trunk stream that blends tributary contributions (fed by 

porous, networked, or channelized flow) from various mantle 

melting loci; or (4) a storage reservoir (collection zone in 

the lower crust or upper mantle) capable of blending magma 

contributions from many domains. If long-lived and areally extensive enough, an upwelling domain or a collection 

zone could each be tapped at different times by more than 

one conduit and might thus conceivably feed more than one 

apparently independent volcano. Across the Quaternary Cascade arc, however, the great compositional variety of mafic 

volcanoes closely interspersed in space and time, including 

many of fairly primitive but diverse compositions (Bacon, 

1990; Leeman and others, 1990, 2005; Hildreth and Fierstein, 1995; Bacon and others, 1994, 1997; Clynne and Borg, 

1997; Borg and others, 1997), suggests that given magma 

bodies seldom supply more than one (nonredundant) mafic 

volcano. Compositional contrasts among neighboring mafic 

scoria cones and shields, adjacent to each other on 1-to10-km scales, illustrate a general failure of mantle-derived 

arc magmas to homogenize during collection and storage 

(except beneath large evolved centers), thereby indicating 

that “underplated” basaltic reservoirs are distributed, numerous, discrete, short-lived, and rarely extensive beneath arcs. 

Diffuse penetration of the crust by diverse mafic magmas 

from heterogeneous mantle sources thus maps out the surface 

projection, not of some vast underplated reservoir, but of the 

minimum region underlain by arc mantle sufficiently molten 

for magmas to segregate and ascend.

Production of arc magma beneath a volcanic belt as 

wide as 25 to 100 km (figs. 8, 14, 15, and 21) thus provides a 

picture of subduction-induced magma generation somewhat 

different from that of conventional models. Throughout the 

Quaternary, subcrustally derived magma has penetrated the 

Cascade arc crust in thousands of small batches distributed 

across this wide zone. During the Miocene and Pliocene, there 

was a gross westward drift of the arc volcanic belt in the Lassen segment and in northern Washington and British Columbia, antithetic to a broadly contemporaneous (Tertiary) eastward drift in Oregon and southern Washington. No systematic 

migration of activity, however, is recognized across the arc 

for the last 2 Myr, indicating that the unusual breadth of the 

magma-production zone has existed throughout much or all of 

the Quaternary. Areas of greatest eruptive volume are scattered, showing no preferred distribution toward the trenchward 

side of the arc (as implicit in some arc models). Apart from the 

narrow Garibaldi Volcanic Belt (fig. 3), the only major centers 

on the volcanic front are St. Helens, Rainier, and Shasta (and 

arguably Mazama; figs. 8, 15, 21). Voluminous centers like 

Adams, Hood, and Lassen are remote from the volcanic front. 

Overall, therefore, there is no evidence for a slab-derived 

narrow fluid curtain, for a linear belt of diapirs, for a sharp 

wedge corner, or for a constant depth of dehydration melting 

of downdragged (or complexly recirculated) hydrated peridotite; that is, there is no evidence for a narrowly linear zone 

of mantle melting, for which such mechanisms have been 

proposed to produce simple chains of arc volcanoes. Instead, 

basaltic magma ascends from a wide across-arc domain of 

the Cascadian mantle, and its eruptive distribution is scattered 

rather unsystematically in both volume and composition.

In studies of some arcs, parallel chains of arc volcanoes 

have been postulated, generally as a means of rationalizing arc 

breadth in terms of conceptual models of successively deeper 

but discontinuous zones of melting or of fluid release. Where 

postulated, however, such double or multiple chains seldom 

bear scrutiny. As in the Cascades, volcano distributions in the 

Japanese, Indonesian, Mexican, South American, Kurile, and 

Izu-Bonin arcs are better viewed as broad scatterings of hundreds of volcanoes, large and small. Imaginary parallel chains 

may appeal to wishful modellers or inveterate lineamentdrawers, but they are generally concocted only on the basis 

of selectively emphasizing large evolved centers and ignoring 

the many small, more primitive ones. A recent example is the 

set of arc maps presented by Tatsumi and Eggins (1995, p. 

29-41), wherein delineation of parallel volcanic chains vari-
Characteristics of Quaternary Volcano Distribution 55

ously entails neglecting many volcanoes, accepting one or two 

inboard centers as “chains,” or headlong assertion of imagined 

alignments using the eye of faith. Although local alignments 

of volcanoes (some of them oblique to the strike of the arc) 

certainly exist, contemporaneous arc-parallel chains extending 

side by side for hundreds of kilometers are nowhere plausible. 

It is clear that mantle-wedge melting is distributed across a 

broad inboard swath and not limited to one or two narrow 

strips that overlie some fixed pressure-sensitive reaction in the 

subducting plate. 

Because slab dehydration and plate interaction begin in 

the shallow forearc (Peacock, 1993; Schmidt and Poli, 1998), 

the megathrust zone is likely to start out as a thick brittle 

shear zone full of lenses, lozenges, and “knockers” of varied 

ultramafic, mafic, and metasedimentary rocks that remains 

heterogeneous into the deeper ductile regime. Such primary 

lithologic heterogeneity, along with spatially variable shear 

heating (Peacock, 1996), steep thermal gradients through the 

slab, uneven domainal serpentinization of lower-plate mantle 

peridotite, survival of serpentine and chlorite in the cooler 

slab interior to depths greater than in the downdragged base of 

the wedge (Ulmer and Trommsdorff, 1995; Peacock, 2001), 

persistence of diverse minor hydrous phases in slab eclogite 

and downdragged wedge peridotite, and solid-solution effects 

on progressive decomposition of such hydrous phases (Schmidt 

and Poli, 1998) all suggest that both initial slab release and 

later boundary-layer reliberation of the hydrous slab contribution into the mantle-wedge interior are likely to be smeared out 

unevenly over a considerable depth range beneath and beyond 

the volcanic front. Protracted down-dip release of hydrous fluid 

to the upper plate would be further enhanced if the blueschisteclogite transition were kinetically delayed until the slab 

reaches depths of 100 to 170 km (Kirby and others, 1996) or 

if a trapped fluid phase were to persist in the downdragged 

peridotite layer to sub-arc depths (Davies and Stevenson, 1992; 

Mibe and others, 1999). Heterogeneous but roughly continuous, polybaric fluid release, typically declining but persisting 

well behind the volcanic front, is more congruent with the 

world's variety of broadly scattered across-arc and rear-arc volcanism than is a model envisaging decomposition of particular 

hydrous phases that release fluid curtains only at fixed depths 

beneath fictively circumscribed frontal and rear-arc chains. The 

broad Cascade arc is one of the prime examples.

Volcanic Front and Forearc Volcanoes

The volcanic-front concept originated with Sugimura 

(1960), who characterised it as the trenchward limit of arc 

volcanoes—typically a line of polygenetic cones and usually (as in Japan and Alaska) the main chain of large centers. 

Application of the term has been quite uneven, however, even 

contradictory, as some use it for the main chain of stratocones 

(ignoring small forearc volcanoes) and others for the array of 

most trenchward vents (ignoring volume and duration). Behind 

volcanic fronts (of many arcs), extensive fields of mafic and 

intermediate volcanoes carrying unequivocal arc-geochemical 

signatures (that is, not including alkalic intraplate basalts) show 

that magma-generating processes influenced by contributions 

from the subducting slab also persist far inboard. Less common 

globally but abundant in the Cascades are scatterings of forearc 

volcanoes, trenchward of the most productive volcanic belt. 

Their distribution challenges the very concept of a linear volcanic front for the Cascades and, together with the many rear-arc 

volcanoes, provides evidence that the processes promoting arc 

magmatism remain effective, though perhaps less robust, across 

a broad belt fore and aft of the main chain of stratovolcanoes.

Mechanisms advocated as contributing to the concentration 

of magmatism beneath volcanic fronts include: (1) pressuredependent breakdown of amphibole and chlorite at ~30 kb from 

a viscously coupled (downdragged) layer of hydrated mantlewedge peridotite superjacent to the subducting slab, releasing 

a “hydrous curtain” that fluxes mantle melting most intensely 

beneath the front (Tatsumi, 1986); (2) subduction-induced 

corner convection that sweeps the high-temperature core of 

the wedge to its shallowest level beneath the front (inboard of 

the wedge corner), thereby promoting highest melt fractions 

there, enhancing buoyant ascent and melt segregation (Kushiro, 

1987; Davies and Stevenson, 1992; Furukawa, 1993; Tatsumi 

and Eggins, 1995; Schmidt and Poli, 1998); (3) ablation of the 

base of the lithosphere beneath the volcanic front by impingement of the inclined ascending limb of the corner convection, a 

process enhanced by slab sinking and rollback (Hasegawa and 

Zhao, 1994); (4) gradients in orientation and magnitude of the 

deviatoric stress field within the upper part of the ascending 

limb (Furukawa, 1993), favoring subvertical fracturing, melt 

segregation, and transition from porous flow to channelized 

magma ascent, preferentially beneath the arc front; and (5) existence of a P-T threshold beneath the front for grain-boundary 

connectivity (dihedral angle <60 ̊) of interstitial aqueous fluid in 

peridotite, governing abrupt release of fluid previously trapped 

in the hydrated downdragged basal layer and its ascent into the 

hotter wedge interior (Mibe and others, 1999; Grove and others, 2002). Such mechanisms might all help focus magmatism 

beneath the front, but, to be realistic, their effectiveness must be 

spatially gradational, contributing also to arc-magma production in the rear arc (and less commonly or less robustly in the 

forearc). The abruptness of many arc fronts may in part reflect 

slab-promoted cooling of the wedge corner, freezing out a stagnant, cool (but buoyant and weak) viscous nose of serpentinized 

wedge peridotite just outboard of the front (Kincaid and Sacks, 

1997; Hyndman and Peacock, 2003; Brocher and others, 2003), 

consistent with sharp increases in heat flow and seismic attenuation from forearc to volcanic front.

Contrary to lingering myth, depth to the top of the 

inclined seismic zone beneath volcanic fronts is far from 

constant. Present-day range is about 80 to 150 km, plus a few 

deeper examples in the tectonically complex SW Pacific (Gill, 

1981). For the Cascades, the ragged and arbitrary nature of 

what might constitute the volcanic front renders the depth 

question unresolvable but, more generally, it is clear that slab 

depth ranges greatly beneath so broad an arc (see section entitled “Tectonic and Geophysical Insights”, below). From such 
56 Quaternary Magmatism in the Cascades—Geologic Perspectives

variability, I infer that fluid release by pressure-dependent 

decomposition of particular hydrous phases is only part of 

the volcanic-front story. An important role for mantle-wedge 

kinematics seems required.

In the classic Japanese model, the volcanic front is abrupt 

and linear, and both the total eruptive volume and the spatial 

density of volcanoes are greatest along the front, decreasing 

inboard (Sugimura, 1960; Aramaki and Ui, 1982). Of these 

four key features, none characterizes the Cascades south of 

Mount Rainier (figs. 8, 14, 15, 21). If the volcanic front were 

taken to be the seaward limit of Quaternary volcanoes, it 

would amount to an irregular zigzag of scattered centers of low 

volumetric output. If taken to be a line of large polygenetic 

stratovolcanoes, any line chosen would be arbitrary, irregularly 

nonlinear, and flanked on the west by scores of volcanoes 

trenchward of the front so defined. Accordingly, for the Cascades, the supposedly global model of a volcanic front seems 

inapplicable and inappropriate. Generally more meaningful 

may be the concept of a “Cascade volcanic axis”, a belt of 

higher than average vent density and eruptive output that grades 

both east and west into flanking areas of lesser productivity. 

Difficulties with this viewpoint, however, include locations of 

the voluminous Rainier, St. Helens, and Shasta loci west of the 

apparent axis and the poor definition of any such axial belt in a 

few reaches of the Cascades (figs. 14, 15, 21).

Both the axial and frontal frameworks, however, beg the 

question of why so many volcanoes, large and small, are scattered far trenchward along the western side. If they are considered “forearc volcanoes” spatially, it needs to be noted that 

only a few of them consist of high-magnesian andesite from 

refractory mantle sources (for example, Clynne and Borg, 

1997; Borg and others, 1997; Leeman and others, 1990, 2005) 

and even fewer are tholeiitic. Many are typical calcalkaline 

arc basalt and basaltic andesite, while a few others are HAOT, 

andesite, dacite, shoshonite, or alkalic basalt. Unlike the classic model, no systematic across-arc compositional distribution 

is apparent; the varied compositions are closely interscattered 

in both axial and forearc regions. Irrespective of composition 

or volume, however, the existence of more than 250 independent Quaternary volcanoes irregularly distributed as far as 30 

to 70 km seaward of the main volcanic axis implies mantle 

melt production far into what is usually considered the forearc. 

It may be that a broad across-arc swath of the sub-arc mantle 

bearing partial melt is actually characteristic of most arcs 

but that such melt can coalesce and erupt widely only under 

extensional or transtensional upper-plate conditions. In more 

compressive arcs like the Alaska Peninsula or the Garibaldi 

Volcanic Belt, apparently only the most intense magmatic foci 

penetrate the arc crust.

The Garibaldi Volcanic Belt conforms better to the 

volcanic-front model than do the other Cascade segments, 

although (unlike Japan) it has little breadth, representing 

what is essentially a discontinuous chain of vent clusters (fig. 

3). Most small volcanoes in the GVB (all of its mafic ones) 

are peripheral to the five long-lived centers, which consist 

largely of andesite to rhyodacite. It is not clear whether the 

GVB cluster pattern reflects restriction of mantle melting to 

discontinuous domains or whether the foci of crustal melting 

and storage manifested by the main edifices provide localized 

stress fields that enable peripheral ascent of mantle magmas 

that may be more widely available beneath the arc but are 

elsewhere muffled. The Bridge River basaltic cluster lacks any 

focal edifice, however. Located at the termination of the arc, 

the more alkalic compositions and small eruptive volumes of 

the Bridge River-Salal Creek volcanoes (fig. 3) may reflect 

attenuation of subduction-related processes, with slab-absent 

mantle upwelling promoting lower degrees of mantle melting, 

little influenced by slab-derived fluid contributions.

The largest concentrations of forearc volcanoes are 

in the Portland-to-St. Helens and Lassen regions, beneath 

both of which the slab may be torn, distorted, or unusually 

stressed (Michaelson and Weaver, 1986; Benz and others, 

1992; Wilson, 1989, 2002). Anomalous slab geometry in 

these two regions might distort wedge corner convection or 

induce mantle upwelling more vigorous than elsewhere along 

the arc. Induced upwelling might be especially effective in 

the Lassen segment adjacent to the southern margin of the 

(northward-retreating) slab. The uncommonly warm thermal structure of the Cascadia subduction zone (Oleskevich 

and others, 1999) might favor partial melting of the mantle 

wedge farther trenchward than in most arcs, but this could 

be expected to be a general feature along the arc rather than 

pronounced principally in the Portland and Lassen regions. 

Impingement of the northward-migrating Oregon coastal 

block against the compressional backstop of Puget Sound, 

at the south margin of the region underlain by an arch in the 

slab, may induce upper-plate stresses conducive to forearc 

magma ascent (see section entitled “Translation and Rotation 

of the Forearc”, below). Confounding the search for a unifying explanation of forearc magmatism is the close association of water-poor and water-rich mafic magmas in both the 

Portland and Lassen regions. Appeal to anomalous slab-edge 

processes is also complicated by the prominence of high-Mg 

andesite and HAOT in the Lassen segment (at the south 

edge), the dominance of alkali basalt along Bridge River (at 

the north edge), and the great variety of mafic magma types 

around Portland and across southern Washington (where 

there may be a torn edge). 

Spacing of Major Volcanic Loci

The oft-repeated old myth of evenly spaced arc volcanoes 

40 to 70 km apart, sometimes employed in idealized models, 

doesn't bear scrutiny in any arc, but it would verge on absurdity 

if advanced for the Cascades (table 4; figs. 3, 8, 14, 15, 21). Even 

if we neglect the fact that the most primitive mantle-derived 

eruptive products issue from small monogenetic vents scattered all over the volcanic belt and focus instead on the major 

long-lived eruptive centers, there is clearly no systematic, even 

roughly periodic, spacing. Table 4 (column 1) shows that the 

main Cascade edifices exhibit spacings that range from 35 to 170 

km, with a mean of 87 km, and a standard deviation of 45 km 
Characteristics of Quaternary Volcano Distribution 57

(with spacings for off-axis variations listed parenthetically). The 

middle column (table 4) augments the list of main centers with 

nine second-rank centers that also consist wholly or in part of 

andesite-dacite, yielding a range of spacings from 5 to 150 km, a 

mean of 54 km, and a standard deviation of 38 km. The righthand column (table 4) similarly augments the list of main centers 

(left-hand column) with nine large mafic centers, yielding a 

range from 7 to 150 km, a mean spacing of 56 km, and a standard deviation of 44 km. No manipulation of centers included or 

statistical contrivance can produce systematic spacing.

Vent Density

It can be calculated from table 1 that the mean spacing 

for all 2,339 Quaternary vents is actually less than 5 km, that 

is, an hour's walk between vents. It was explained above that 

as many as 2,050 of these vents are independent volcanoes 

with their own conduits from the deep crust or mantle. Measuring the areas enclosing about 25 empirically selected vent 

fields (not including the surrounding areas of lava flows or of 

other outflow deposits) permits tabulation of vent densities 

for the constituent segments or reaches of the entire Cascade 

arc and its rear-arc volcanic fields (table 1). Vent density 

thus approximated ranges from 3 km2

 per vent in the Caribou 

Volcanic Field to 6 km2 per vent for Indian Heaven to 32 km2

per vent for the Portland forearc. Note that a vent density of 

16 km2 per vent would signify an orthogonal vent spacing of 

4 km. The vent densities given for the GVB apply only to the 

several individual clusters (fig. 3), but from Bumping Lake 

(Washington) to Mount McLaughlin (Oregon) the values 

tabulated in table 1 represent a continuous swath 515 km long 

of closely neighboring volcanoes (figs. 8, 14, 15). Inspection 

of figures 3, 8, 14, 15, and 21 shows that vent density reaches 

maxima around Mount Adams, in Indian Heaven, in several 

parts of the Oregon Cascades, in the axial strip east of Mount 

Shasta, and in the Lassen-Caribou region, as well as in the 

three rear-arc volcanic fields. The dense clusters around Jefferson, Mazama, and Lassen Peak are dominated by vents for 

dacitic and silicic products, whereas most other areas of high 

vent density are predominantly mafic, marked by numerous 

vent alignments but including few or no evolved products.

A realistic model of arc magmatism needs to encompass, 

in addition to the major centers, the densely but irregularly 

distributed sites of eruption of the mafic magmas that come 

most cleanly from the mantle. Spatially restricted fields of mafic 

monogenetic vents (scoria cones, small shields, fissure-fed lavas) 

have sometimes been attributed to localized mantle upwellings (diapirs), but the regional scale and along-arc continuity 

of monogenetic vent distribution in the Cascades cast doubt on 

such a model. The 515-km-long swath in Oregon and southern 

Washington, as well as the size and complexity of the Lassen, 

Shasta, and three major rear-arc vent fields, suggest instead that 

partially molten mantle is a general condition beneath the entire 

Quaternary volcanic region. Variation in relative penetrability of 

the lithosphere, not in the availability of basaltic magma, apparently limits and governs the distribution of volcanism. 

Along-arc Gaps

Despite the unusually high vent density and breadth of so 

much of the Quaternary Cascade arc, its continuity is nonetheless interrupted by several long gaps containing few or no Quaternary volcanoes (table 5). Ostensibly, the contrast between 

extensive reaches of high vent density and intervening nonvolcanic gaps would seem to be in conflict with the standard 

model of arc magmatism, which presumes secular fluxing of 

continuously convecting sub-arc mantle by contributions from 

a subducting slab continuous along the strike of the arc.

Major gaps north and south of the Shasta segment are 

especially anomalous in view of the high vent density in adjacent areas (fig. 21). A 64-km-long reach to the north is blanketed by Tertiary arc volcanics, and the small low-K HAOT 

scoria cones near Copco Dam are the only known Quaternary 

volcanic rocks. Similarly, a 50-km-long reach south of the 

Shasta segment exposes Mesozoic basement rocks partly overlapped by Tertiary volcanics, and a young cluster of undated 

HAOT vents (Timbered Crater and Brushy Butte; fig. 21) at the 

eastern extremity of the gap produced the only volcanic rocks 

known to be of Quaternary age. The gap would be slightly 

wider if Chalk Mountain and other undated vents at the north 

end of the Lassen segment prove actually to be of Pliocene age.

A third reach ~17 km long, between Pelican Butte and Big 

Bunchgrass shield (fig. 15), has relatively low vent density and 

may or may not amount to a Quaternary gap in the present context. The reach has ~10 undated mafic vents, most of which are 

either Pliocene or early Quaternary. From here north, however, 

the Quaternary vent belt is uninterrupted for 500 km until, at 

the latitude of Mount Rainier and Bumping Lake (fig. 8), the 

continuous belt of close-set vents ends abruptly. 

The Glacier Peak vent cluster, which overlies the slab 

arch that accommodates the change in trend of the convergentmargin system, is isolated by two major vent-free gaps. To 

the south, the 150-km separation of Rainier and Glacier Peak 

is shortened to a gap of 105 km by inclusion of two isolated 

mafic vents north of Rainier and a mafic vent cluster just 

southwest of Glacier Peak (figs. 3, 8). To the northwest, a 

74-km gap separates Glacier Peak from the basaltic vents 

peripheral to Mount Baker. Finally, the longest vent-free gap 

of all extends 136 km from the Baker locus to Watts Point 

(fig. 3). As so delineated (table 5), the gaps in Washington 

and Canada consist of pre-Cenozoic basement terranes, with a 

fraction of Tertiary volcanic and plutonic rocks that increases 

southward toward Mount Rainier, but with no Quaternary 

vents, dikes, or other remnants.

Persistence for at least 2 Myr of these vent-free reaches 

along a Quaternary volcanic belt otherwise virtually continuous raises some fundamental questions about arc magmatism. 

If subduction is continuous along the length of the arc, and if 

some combination of slab-fluid flux, mantle-wedge convection, and extensional decompression promotes melting (and 

magma coalescence and ascent) beneath a volcanic belt 1,250 

km long (and generally many tens of kilometers wide), then 

is the failure of magma to erupt within the gaps attributable 
58 Quaternary Magmatism in the Cascades—Geologic Perspectives

Table 4. Spacing of Main Quaternary Volcanoes.

Large Andesitic-to-Silicic Centers Basalt and Basaltic Andesite

km Major Pairs* Include 9 Lesser Centers* km Include 9 Large Mafic Centers* km

118 Lassen-Shasta Lassen-Burney 38

Burney-Shasta 82

170 Shasta-Mazama Shasta-Goosenest 35 Shasta-McLoughlin 115

Goosenest-Pelican 88 McLoughlin-Mazama 58

Pelican-Mazama 47

43 Mazama-Cappy Mtn Mazama-Bailey 25 Mazama-Thielsen 24

Bailey-Cappy Mtn 26 Thielsen-Cappy Mtn 18

Cappy Mtn-Diamond Pk 28

89 Cappy-S. Sister Diamond Pk-S. Sister 71 Cappy-Bachelor 78

S. Sister-M. Sister 5 Bachelor-S. Sister 15

64 S. Sister-Jefferson M. Sister-Jefferson 59 S. Sister-N. Sister 7

(7) (S. Sister-Broken Top) N. Sister-Washington 19

Washington-Three Fingered Jack 17

(67) (Broken Top- Three Fingered Jack-Jefferson 22

 Jefferson)

78 Jefferson-Hood Jefferson-Frog Lake Buttes 61 Jefferson-Olallie Butte 16

Frog Lake Buttes-Hood 17 Olallie-Hood 62

94 Hood-Adams Hood-King Mtn 78

(99) (Hood-St Helens) Hood-Defiance 32 King Mtn-Adams 18

(54) (Adams-St Helens) Defiance-Adams 62 Hood-Indian Heaven (Lemei Rock) 72

35 Adams-Goat Rocks Indian Heaven (Lemei Rock)-Rainier 93

(75) (Adams-Rainier)

(80) (St Helens-Rainier)

47 Goat Rocks-Rainier

150 Rainier-Glacier Pk

91 Glacier Pk-Baker

147 Baker-Garibaldi

35 Garibaldi-Cayley Garibaldi-Price 7

59 Cayley-Meager Price-Cayley 29

Summary of Calculated Spacings for the Cascade Arc

14 Number of Pairs 23 22 20

35–170 km Range of Spacings 5–150 km 7–150 km 7–150 km

87 km Mean Spacing 54 km 56 km 61 km

45 km Standard Deviation 38 km 44 km 46 km

(via Mount Adams)* via Lemei Rock)*

*

 Left column gives spacings among 17 large, compositionally evolved, Quaternary volcanic centers in the Cascade arc. Central column augments these with 

9 lesser centers that are also composed all or partly of silicic andesite or more evolved products. Right column augments the left (not the central) column by 

including 9 of the largest Quaternary mafic centers. Combining all 3 columns (not shown) would yield 30 pairs having a mean spacing of 42±37 km and a spacing range of 2-150 km. 

Mounts Thielsen and Bailey lie directly across-arc from each other, as do South Sister and Broken Top (Tumalo), and likewise Mount Adams, Indian Heaven, 

and Mount St. Helens; for consistency, therefore, only one of each group is used in calculating the along-arc mean spacings. In the left column, parenthetical 

entries are not included in the spacing calculations. In the right column, spacing was calculated along alternative north-south paths, by way of Mount Adams or 

by way of Indian Heaven (Lemei Rock).
Characteristics of Quaternary Volcano Distribution 59

Table 5. Gaps along the Quaternary Cascade arc.

[HAOT is high-alumina olivine tholeiite, as elaborated in the text]

Gap Length Region Latitude Details

44-52 km South of Mount Shasta 41.0—41.3 Chalk Mtn to Signal Butte. Gap is wider if Chalk 

Mtn is Pliocene. Young HAOT vent cluster lies 

at east side of gap

64 km North of Mount Shasta 41.7—42.3 Goosenest to Little Aspen Butte. Copco HAOT 

scoria cone cluster is lone Quaternary exception in gap

17 km North of Pelican Butte 42.5—42.7 Pelican Butte to Big Bunchgrass shield. Eroded 

mafic vents within this potential Quaternary 

gap are undated; probably either Pliocene or 

early Quaternary 

105 km North of Mount Rainier 47.1—48.0 Dalles Ridge (Greenwater) to Wards Pass (N. 

Fork Skykomish River)

74 km Northwest of Glacier Peak 48.1—48.6 Glacier Peak to Bear Creek cone near Lake Shannon (southeast of Mount Baker).

136 km Northwest of Mount Baker 48.8—49.7 Cougar Divide to Watts Point.

to (1) locally reduced slab-derived contributions, inadequate 

to promote robust mantle melting; (2) locally cooler thermal 

structure or more refractory composition of the mantle wedge; 

or (3) lithospheric impedance of mantle magma ascent? No 

discontinuity, asperity, or fracture zone on the oceanic Juan de 

Fuca Plate system (Wilson, 2002) is recognized to coincide 

with or project beneath the gaps. For the two Washington gaps, 

a case for mantle control might postulate that the slab arch 

hinders mantle-wedge convection—except that the Glacier 

Peak locus lies right on the arch crest where it projects eastward from beneath the Olympic Peninsula (fig. 3). No lithospheric structures are recognized that might impede magma 

ascent differentially beneath any of the gaps, and indeed 

mid-or-late Tertiary magmas had penetrated four of the gaps. 

For the two gaps north of Glacier Peak and Mount Baker, the 

Tertiary (Chilliwack-Pemberton) magmatic belt had lain far 

inboard, well east of the Quaternary alignment (fig. 3). 

Altogether, it seems unlikely that the gaps reflect alongstrike reaches of the mantle wedge that are poorly fluxed, 

stagnant, refractory, or otherwise deficient in partial melt. 

More plausible is that the slab dehydrates and the wedge 

convects and melts along the whole subduction margin from 

Lassen to Meager, but that the arc lithosphere (including a 

40-km crust) is capable of stalling, suppressing, and locally 

completely defeating mantle melts that intrude it, within large 

domains, along and across arc, for millions of years at a time. 

The intra-arc tectonic extension expressed as vent alignments 

and faulting, virtually continuous from Lassen to Adams, 

terminates at the latitude of Bumping Lake and Mount Rainier, 

implicating a compressive (or at least nonextensional) lithospheric stress regime as a possible explanation of the gaps 

farther north, in and adjacent to the GVB. As for the gaps in 

Quaternary volcanism north and south of the Shasta segment, 

it was pointed out by Blakely and others (1997) that the gaps 

are dominated by northwest-southeast dextral shear but each 

of the adjacent volcanic segments by extensional north-south 

normal faults. Because the Lassen and Shasta segments have 

in common widespread abundances of both arc basalt and 

HAOT, the mantle and its melt-generating behavior beneath 

them are probably similar, rendering it doubtful that the 

50-km-long reach of mantle separating them should be very 

different. These observations suggest again that extension 

favors magmatic penetration of the lithosphere and that its 

absence suppresses such penetration.

Why are Rear-Arc Volcanic Fields Few and 

Clustered?

The three large rear-arc volcanic fields are likewise separated by long gaps that lack rear-arc volcanic rocks of Quaternary age. The 200-km reach separating the Simcoe Mountains 

and Newberry Volcanic Fields contains a few Pliocene vents in 

the south but otherwise lacks volcanic rocks younger than the 

Miocene flood basalts. Similarly, the 180-km reach separating the Newberry and Medicine Lake Volcanic Fields contains 

numerous late Miocene and Pliocene vents but virtually none 

of Quaternary age (Luedke and others, 1983; Sherrod and 

Smith, 2000).

In the Lassen segment, the Caribou Volcanic Field and 

the many Quaternary vents scattered eastward to Eagle Lake 

(fig. 21) might well be treated as another rear-arc volcanic 

field, largely basaltic. During the Pliocene, the field had 

extended farther north as well as an additional 50 km east of 

Eagle Lake (Luedke and Smith, 1981; Guffanti and others, 

1990). During the Quaternary, however, this inboard mafic 

vent field has been separated by a 50-km north-south gap from 

the rear-arc Medicine Lake and Hackamore fields (fig. 21).
60 Quaternary Magmatism in the Cascades—Geologic Perspectives

HCG 

PL 

WV 

HR 

GR 

SFZ 

LG 

WR 

ML 

HC 

Yakima Foldbelt 

Siletzia 

KLAMATH BLOCK

Coast Plutonic Complex

North 

Cascades 

Olympic 

Peninsula 

12-13 mm/yr 

SIERRA NEVADA 

GREAT VALLEY 

Walker Lane (6 mm/yr) 

COAST RANGES 

M 

T 

J 

H 

SH 

A 

R 

GP 

B 

G 

MZ 

DP 

MS 

SS 

S 

LVC 

ML

M

N

T

J

H

SH SM

A

R

GP

B

G

MZ

DP

MS

SS

S

LVC

MV 

HL 

WS 

L.

Tahoe

Pyramid

L.

Strait of Georgia

Vancouver 

Seattle 

Portland 

Eugene 

Medford 

Redding 

Susanville 

Klamath 

Falls 

Bend 

Burns 

Alturas 

Cedarville 

SAF 

50

mm/yr

BFZ

124o 123o 122o 121o 120o

41o 

49o

50o 120o 122o 124o

48o

47o

46o

45o

44o

43o

42o

41o

40o

39o

2

4

6

8 10 Ma

0 100 200 km 

60

50

40

30 km

PACIFIC OCEAN

NEVADA CALIFORNIA

OREGON

WASHINGTON

BRITISH COLUMBIA
Characteristics of Quaternary Volcano Distribution 61

← Figure 25. Tectonic setting of the Quaternary Cascade arc. Selected large volcanoes indicated in red. For complete 

Quaternary vent distribution, see figures 3, 8, 14, 15, and 21; for offshore plate system, see figure 1. Main features are the 

following: (1) Basin and Range extensional region has expanded westward since the late Miocene to overlap the subductioninduced magmatic arc in California and Oregon; black lines (with ticks on downthrown sides) indicate main faults. (2) Plateboundary shear traction principally drives northwestward translation of rigid Sierra Nevada/Great Valley microplate, accounting 

for ~25 percent of relative motion between Pacific Plate and stable North America (Dixon and others, 2000), most of which 

takes place along San Andreas Fault (SAF). Dextral shear on Walker Lane (belt of rheologically weak lithosphere) accounts 

for about half the microplate translation and merges diffusely northwest into region of intra-arc normal faulting. (3) Microplate 

compression through Klamath block helps (along with oblique plate convergence) to drive northward translation and clockwise 

rotation of Oregon forearc block, contributing to intra-arc extension along its trailing edge. Oregon forearc block consists 

dominantly of ~30-km-thick Siletzia terrane of accreted oceanic crust; its northward motion compresses the accretionary 

Washington forearc against the Canadian batholithic buttress (Wells and others, 1998). North Cascades and Coast Plutonic 

Complex consist predominantly of crystalline basement terranes of Mesozoic and older metamorphic and plutonic rocks. Red 

arrows illustrate rotation of Cascadia; directions (not velocities) are generalized from hundreds of GPS sites referenced to 

stable North America (McCaffrey and others, 2007). (4) Seaward-concave plate margin promotes slab arch with axis plunging 

toward 48oN; slab depth contours (solid orange lines, 30 to 60 km) inferred from seismicity and tomography by Crosson and 

Owens (1987). (5) Proximal forearc depression occupied by Strait of Georgia, Puget Lowland (PL), and Willamette Valley (WV), 

50 to 100 km seaward of active arc, may reflect densification of slab at eclogite transition; its absence south of Eugene may 

reflect upper-plate deformation by the northwest-translating Klamath block. (6) Formation of Yakima fold and thrust belt, which 

principally deforms lavas of the Columbia River Basalt Group, began ~15 Ma during their main eruptive interval and continues 

today; green lines with opposing arrows indicate anticlinal axes; Mount Adams, Mount Hood, and Simcoe Mountains Volcanic 

Field overlie the folds. (7) In central Oregon, isochrons (red dashed lines, 10-2 Ma) depict westward progression of initiation 

of rhyolitic volcanism across the High Lava Plains (MacLeod and others, 1975; Jordan and others, 2004); in the Quaternary, 

eruptions of true rhyolite (rare elsewhere in the Cascades) have further advanced through Newberry and as far northwest as 

South and Middle Sisters. Abbreviations for volcanoes (red triangles) from north to south: G, Mount Garibaldi; B, Mount Baker; 

GP, Glacier Peak; R, Mount Rainier; SH, Mount St. Helens; A, Mount Adams; SM, Simcoe Mountains; H, Mount Hood; J, Mount 

Jefferson; MS, Middle Sister; SS, South Sister; N, Newberry; DP, Diamond Peak; T, Mount Thielsen; MZ, Mount Mazama (Crater 

Lake); M, Mount McLoughlin; S, Mount Shasta; ML, Medicine Lake; LVC, Lassen volcanic center. Faults: HR, Hood River Fault; 

GR, Green Ridge Fault; SFZ, Sisters (Tumalo) Fault Zone; BFZ, Brothers Fault Zone; HCG, High Cascades Graben; LG, LaPine 

Graben; HC, Hat Creek Fault; WR, Walker Rim; WS, Warm Springs Fault; HL, Honey Lake Fault; MV, Mohawk Valley Fault. Purple 

shading in HCG and LG indicates areas of deep graben fill.

Behind the northern part of the arc, a 400-km-long reararc gap separates the Simcoe Mountains Volcanic Field from a 

contemporaneous (Pliocene to Quaternary) diffuse scattering 

of alkali basalt scoria cones in south-central British Columbia (Canil and Scarfe, 1989). The southernmost concentrated 

Quaternary back-arc volcanic field in Canada is the Clearwater/Wells Gray field of alkali basalts (Hickson and Souther, 

1984), mostly younger than 500 ka and about 600 km north 

of the Simcoe field. Except for the Bridge River-Salal Creek 

cones at the north end of the Quaternary arc (fig. 3), however, 

the Canadian alkali basalt fields lie more than 200 km inboard 

of the arc, in contrast to the arc-adjacent rear-arc fields of Simcoe, Newberry, Medicine Lake, and Caribou-Eagle Lake.

Why do only four reaches of the Cascade arc have 

adjacent Quaternary rear-arc vent fields? Although widely 

separated from one other, each field protrudes ~50 km 

inboard from the main volcanic belt, each extends ~50 km 

north-south, each is fundamentally basaltic, and each has a 

reduced proportion (relative to the arc) of basalts that have 

arc geochemical signatures, suggesting attenuated slab flux 

inboard. If slab contributions were a controlling factor, it 

seems unlikely that their effectiveness would extend inboard 

of the main volcanic belt at only a few discrete loci. The 

following discussion thus addresses the tectonic elements 

(fig. 25) that may uniquely control the siting of each of these 

rear-arc anomalies.

The mafic rear-arc vent field in the eastern part of the 

Lassen segment occupies a tectonic singularity where the 

main outboard faults of the northern Walker Lane dextral 

shear zone (which bounds the northwest-translating Sierran 

microplate) end abruptly. From the Reno/Lake Tahoe area, 

the Walker Lane follows the trend of the Miocene-Pliocene 

Cascade arc, cutting into the otherwise rigid Sierra Nevada 

batholith where its crust may have been thermally weakened 

by the Tertiary arc magmatism. The major dextral strands 

here, the Mohawk Valley and Honey Lake Fault Zones 

(fig. 25), strike west-northwest toward Lake Almanor and 

Susanville, where the dextral shear terminates and merges 

diffusely into a system of north-to-northwest-striking normal 

faults that transect the Caribou/Eagle Lake rear-arc vent field 

(Blakely and others, 1997; Dixon and others, 2000). Farther 

north, the Medicine Lake and Hackamore rear-arc fields 
62 Quaternary Magmatism in the Cascades—Geologic Perspectives

likewise lie at a structural knee where a set of more inboard 

northwest-striking strands of the dextral Walker Lane system 

end against an array of north-striking normal faults. Because 

the rigid Klamath and Siletzia blocks lie next northwest (fig. 

25), their backstop effect requires the Walker Lane to splay 

out northward into a broad diffuse array of normal faults 

across central Oregon, where strike-slip components are 

subordinate until the Brothers Fault Zone east of Newberry 

provides a northern transform margin to the extended terrain 

(fig. 25). The structural knees adjacent to the Caribou/Eagle 

Lake and Medicine Lake rear-arc vent fields both represent 

zones of transfer of horizontal strain from northwest dextral shear to east-west extension. In both fields, eruption of 

HAOT is proportionately more common farther inboard, 

indicative of shallow upwelling of hot dry mantle and freer 

transit of basaltic magma through the extending crust.

The Newberry Volcanic Field occupies a comparably 

unique rear-arc tectonic position (fig. 25), where the ageprogressive rhyolite trend of the High Lava Plains and the 

northwest corner of Basin and Range extensional faulting 

both impinge upon the Cascade arc (Walker and MacLeod, 

1991; Jordan and others, 2004). Separating Newberry from 

the Cascades is the westernmost basin of the Basin and Range 

Province, the profound LaPine Graben, which underwent ~700 

m of structural subsidence contemporaneous with growth of 

the Newberry edifice. The graben is bounded by the Walker 

Rim Fault Zone, which strikes beneath and is buried by the 

Newberry edifice (Walker and MacLeod, 1991; Macleod and 

Sherrod, 1992). An abrupt change in strike (from northwest to 

northeast) of the regional extensional fault swarm takes place 

just where it approaches the southwest toe of the edifice, against 

the margin of the graben. At the northwest toe of the Newberry 

edifice, 50 km farther north, the northwest-striking Sisters (or 

Tumalo) Fault Zone emerges from beneath the young volcanic 

pile, indicating that the edifice buries either a continuous curvilinear fault zone or (more likely) another extensional structural 

knee. Moreover, the northwest-striking Brothers Fault Zone, a 

diffuse belt of distributed oblique dextral shear that here forms 

the northern limit of the main extended region (Lawrence, 1976; 

Pezzopane and Weldon, 1993), passes Newberry adjacent to the 

northeast toe of the edifice. 

The mid-Pleistocene to Holocene rhyolites so abundant 

at Newberry have numerous contemporaneous counterparts 

in the Three Sisters-Tumalo region just 15 to 40 km farther 

northwest. Because true rhyolites are elsewhere so rare in the 

Quaternary Cascade arc, it appears likely that the rhyolitegenerating crustal-melting anomaly beneath each of these 

areas is attributable to the same pattern of mantle flow and 

decompression responsible for the 10-Myr-long westward 

propagation of rhyolitic volcanism across the High Lava 

Plains, perhaps enhanced at the present-day leading edge by 

pre-heated arc crust. The great Newberry edifice has thus 

grown over the last half-million years at the unique conjuncture of a profound and complex locus of regional extension 

with the advancing front of a long-lived, virtually independent, propagating mantle melting anomaly. 

The rear-arc Simcoe Mountains Volcanic Field has 

developed in a setting no less unique. All 180-odd vents of the 

1,400 km2

 field erupted through and directly onto a 2-to-3-

km-thick pile of flood basalts (Columbia River Basalt Group; 

CRB) where they are folded into two of the major anticlines 

of the Yakima Foldbelt (fig. 25). The east-west-trending folds 

are asymmetrical north-vergent anticlinal ridges, which have 

sharp hingelines and oversteepened north limbs cut by highangle reverse faults, and they are separated by broad flat-floored 

synclinal valleys (Reidel and others, 1989; Watters, 1989). 

Although they dominate the modern topography, the folds generally have less than 600 m of structural relief. Beneath the foldbelt, the CRB conceals a long-lived basin previously filled by 3 

to 7 km of Eocene to mid-Miocene sediments (Campbell, 1989). 

Within and west of the Simcoe Mountains Volcanic Field, the 

folds are cut by numerous northwest-striking dextral strike-slip 

cross-faults of very small (1 to 30 m) displacement (Bentley 

and others, 1980; Walsh and others, 1987), probably accommodating curvature of the fold axes. Farther west, the folds in the 

CRB continue into the adjacent Cascade Range, where they are 

incompletely covered by late Miocene and younger arc volcanics (Walsh and others, 1987; Beeson and Tolan, 1990). The 

folding began by 15 Ma during outpouring of the largest CRB 

units, persisted into the Pliocene (thus overlapping in time the 

4.5-0.6 Ma Simcoe volcanism), and is apparently still ongoing 

locally (Campbell and Bentley, 1981; West and others, 1996). 

Contemporaneously, since at least the mid-Miocene, the Oregon 

forearc block (principally Siletzia; fig. 25) has been moving 

northward and rotating clockwise with a pole of rotation northeast of the Yakima Foldbelt (Wells and others, 1998). The northsouth shortening in the Puget Sound region today has its inboard 

counterpart in the foldbelt and in the north or north-northwest 

vent alignments within the Simcoe vent field. 

An additional anomaly in southern Washington is the 

probable distortion or tearing of the subducting plate beneath 

the region. Its influence on mantle-wedge convection may be a 

factor in the unusual width of the volcanic belt here, in the abundance within the arc of HAOT and other relatively dry basalts 

with slab geochemical signatures weak or absent (Leeman and 

others, 1990), and, farthest inboard, in the dominance of waterpoor alkaline intraplate basalts in the Simcoe field. In common 

with the alkalic basalts of British Columbia, several Simcoe scoria cones contain xenoliths of spinel peridotite (Draper, 1992; 

Brandon and Draper, 1996; Peslier and others, 2002), mantle 

cargo wholly absent among the ejecta of the Quaternary rear-arc 

(subalkaline) volcanic fields described farther south.

Eruption Rates and Episodicity

When dealing with rates of magmatism, one first must 

distinguish clearly among volumes produced, erupted, and preserved. Magma production is always greater than that erupted, 

whether the unerupted excess be a small fraction (as in dikes 

of primitive basalt) or a dominant fraction when large granitoid or gabbroic plutons crystallize. For example, fractionation 
Eruption Rates and Episodicity 63

to produce the 50 km3 of climactic rhyodacite erupted at Crater Lake Caldera (7.7 ka) may have left behind three times that 

volume of magmatic cumulates in the upper crust (Druitt and 

Bacon, 1989; Bacon and Nathenson, 1996). In any system, a 

significant fraction of the parental magmas may stall, partially 

crystallize, and remain within the mantle-crust transition, or at 

the depth of gas saturation, or at the brittle-ductile transition, 

or at any discontinuity in crustal density. Furthermore, the 

volume of any volcanic unit erupted is always greater than that 

preserved. Discrepancies between volumes erupted and those 

preserved result from syneruptive tephra dispersal and from 

secular erosion. Other things being equal, pyroclastic deposits 

are dispersed more widely and eroded far faster than lavas, 

phenomena generally likely to lower the fraction of silicic 

products identifiable among those preserved relative to those 

erupted. The north-south gradient in the severity of Quaternary 

glaciations along the 1,250-km Cascade arc is responsible for 

extreme asymmetry in preservation (fig. 2). In the Lassen segment, 60 percent or more of the Quaternary eruptive products 

may be preserved, whereas in Canada the fraction may be less 

than 30 percent.

By using the term “eruption rate” here, I refer to the 

volumetric rate of release to the surface of volcanic products 

(or their equivalents recalculated as dense rock or magma), 

integrated over a specified time interval. Some would prefer 

to equate the term “eruption rate” with the magma discharge 

recorded during an active eruption, which is typically a brief 

episode of immediate interest but not one that provides a rate 

representative of edifice growth. Others have used the term 

“extrusion rate” for longer-term volcanic output (for example, 

Sherrod and Smith, 1990), but because the word extrusion 

(of lava) is widely applied in contradistinction to explosive 

ejection (of pyroclasts), it seems inappropriate here. Another 

possibility, “accumulation rate,” invites ambiguities such as 

erosive interference with secular accumulation and whether to 

count downwind ash as having accumulated. For simplicity, 

therefore, I think “eruption rate” best expresses the estimated 

total magma volume reaching the surface, regardless of the 

mode, form, episodicity, or irregularity of eruptive events. The 

context can be made clear by specifying the volcano (or volcanic region) and the interval of interest, whether integrated, for 

example, over 1,000 years or over the lifetime of an edifice.

Rough volume estimates for preserved sections of Tertiary 

arc volcanics in the Cascades of central Oregon suggested to 

Verplanck and Duncan (1987) that the range-wide eruption 

rate declined about sixfold from 32 Ma to 3 Ma, as the plate 

convergence rate slowed fivefold and the obliquity of subduction increased (owing in part to clockwise rotation of the 

Oregon forearc block). For the Quaternary Cascades, Sherrod 

and Smith (1990) estimated volumes preserved for about 50 

centers (or along-arc reaches) and calculated volumetric rates 

(for the last 2 Myr) by unit linear length of various arc segments, ignoring the variations in erosion and arc breadth. They 

stated, in summary, that “extrusion rate” in the GVB was only 

10 to 15 percent of that in the Rainier-to-Hood segment, which 

in turn was less than half that of the Oregon and California segments. As described above for many of the individual Cascade 

volcanoes, however, differences between what is erupted and 

preserved may be as great as twofold from the southern Oregon 

Cascades northward to Mount Rainier, and from twofold to 

fivefold in the GVB, where the volcanoes have repeatedly been 

torn down by advances of the Cordilleran ice sheet.

Output of Distributed Mafic Vents vis-à-vis 

Long-Lived Evolved Centers

It has sometimes been asserted that the magma volume 

erupted by the hundreds of distributed small volcanoes, mostly 

mafic, greatly exceeds that of the few large long-lived evolved 

centers. This is true for the Quaternary Cascades of central 

Oregon but certainly not true for the volcanic regions that 

include the Shasta, Mazama, Hood, Adams, Goat Rocks, or 

Rainier edifices, nor for any reaches of the GVB.

In the GVB, the five major polygenetic multivent 

andesite-to-rhyodacite edifices (fig. 3) clearly account for at 

least 90 percent of the volumes erupted. For the Mount Baker 

locus and Kulshan Caldera, the proportion exceeds 95 percent 

(Hildreth and others, 2003). 

For the Rainier-to-Hood segment, the proportion remains 

above 75 percent, despite the presence of more than 400 

small volcanoes (table 1) and only five large ones (fig. 8). The 

Indian Heaven Volcanic Field (~70 km3

) is almost entirely 

distributed, whereas more than 80 distributed mafic volcanoes 

in the adjacent Mount Adams field contributed only 15 percent 

of a total eruptive volume of 315±84 km3 (Hildreth and Lanphere, 1994), about 85 percent having vented from the focal 

andesite-dacite edifice. Mount Rainier is virtually isolated, 

the central edifice having released more than 98 percent of the 

magma erupted in its district. Comparable focal-to-peripheral 

proportions can be estimated for Goat Rocks (75:25), St. Helens (75:25), and Hood (80:20), whereas the Portland forearc 

is entirely distributed (fig. 8). For the segment as a whole, I 

estimate that ~650 km3 out of 850 km3 (76%) erupted from the 

five main edifices.

The central Oregon Cascades yield the opposite proportionate relationship. Along reaches I through V (Jefferson to 

Mazama; figs. 14, 15) of the Oregon segment, the arc includes 

more than 1,000 close-set Quaternary mafic vents (table 1) and 

eight long-lived evolved loci. Sherrod and Smith (1990; their 

fig. 3) estimated that ~9 km3

 of Quaternary mafic volcanic 

rock is preserved for each linear kilometer along this 280-kmlong stretch, yielding ~2,500 km3

. Because most of these rocks 

have been glaciated, the total mafic eruptive volume is likely 

to have been 10 to 30 percent greater. Eruptive volumes of the 

long-lived evolved centers, as estimated here, are as follows: 

Jefferson 25 km3

, Middle Sister 12 km3, South Sister 20 km3, 

Broken Top 10 km3, Tumalo 25 km3, Diamond Peak (andesitic 

upper part) 5 km3, Cappy 20 km3, pre-Mazama domefield 25 

km3, Mazama edifice 130 km3, and Mazama climactic ejecta 

50 km3. The major evolved centers thus add up to ~322 km3, 

which amounts to less than 12 percent of the Quaternary total 
64 Quaternary Magmatism in the Cascades—Geologic Perspectives

output for reaches I to V, or probably less than 10 percent if 

erosion of the mafic lavas were taken into account.

The McLoughlin reach (VI; fig. 15; table 1) is inadequately investigated but appears to be transitional in the 

present context. Although none of its several andesitic centers 

(Pelican Butte, Brown Mountain, Devils Peak, and the Mountain Lakes cluster) is known definitively to be long-lived, 

their combined eruptive volume evidently exceeds that of the 

20-odd mafic vents, which include Mount McLoughlin (~13 

km3) and several small distributed volcanoes.

For the Shasta segment, Sherrod and Smith (1990) estimated a present-day volume of ~120 km3

 for ~200 distributed 

vents (not including the Medicine Lake Volcanic Field), most 

but by no means all of them mafic (fig. 21). Because Mount 

Shasta alone is estimated to have erupted ~450 km3 of andesite/dacite and Rainbow Mountain at least another 100 km3

, the 

myriad distributed vents account for less than 20 percent of the 

total eruptive volume of the segment—a relationship virtually 

antithetic to that of central Oregon.

For the Lassen segment, the Quaternary output of three 

long-lived evolved centers likewise volumetrically surpasses 

the total for 400-odd regionally distributed vents (fig. 21), 

though not as strikingly so as in the Shasta segment or as in 

Washington and Canada. Sherrod and Smith (1990) estimated 

at least 425 km3

 for distributed Quaternary cones and shields 

in the Lassen segment (of which as much as 150 to 200 km3

may be andesitic rather than mafic). For the long-lived evolved 

complexes, the estimated eruptive volumes are 215 km3 for 

the active Lassen volcanic center (Guffanti and others, 1996), 

more than 150 km3 for the early Quaternary Dittmar volcanic 

center, and more than 200 km3 for the Maidu volcanic center 

(of which half or more is younger than 2 Ma). Although the 

volume estimates are nebulous for the early Quaternary, it 

seems clear that more than half of the products erupted in the 

segment issued from the large andesite-to-rhyolite centers. 

Moreover, because so many of the distributed lesser vents 

are andesitic, the mafic output of the numerically dominant 

regional vents may amount to as little as 30-35 percent of the 

Quaternary eruptive volume in the Lassen segment.

Eruption Rates at Individual Loci

The volume proportion erupted from large centers visà-vis myriad distributed vents thus ranges widely along the 

Cascade arc, as does the total Quaternary volcanic production, 

whether integrated by segment, by reach, or by center. For a 

few 50-to-100-km-long reaches there has been no Quaternary 

volcanism at all. The common practice of estimating integrated volumetric eruption rates for particular time intervals 

along arcs thus seems conceptually peculiar, perhaps interesting for global comparisons of material and geochemical 

budgets of arcs in general but failing to provide perspectives 

useful for analyzing eruptive behavior within one. Even for 

global comparisons, the “cubic kilometers erupted per kilometer of arc length” approach is flawed by averaging the huge 

variations and by the enormous uncertainties in intrusive-toextrusive proportions of the magmas emplaced along arcs in 

both space and time. As elaborated in studies of the volcanic 

fields around Mount Adams and Mount Baker (Hildreth and 

Lanphere, 1994; Hildreth and others, 2003), discussion of 

volumetric eruption rates can be somewhat misleading without 

conceptual clarity about scale. The time scale and spatial scale 

should fit the process being investigated. Appropriate scales 

are different for assessing slab and wedge contributions to 

subduction-zone magma generation, or deep-crustal heating 

and melting, or growth and replenishment of upper-crustal 

reservoirs, or eruption hazards at stratovolcanoes and calderas. 

For understanding volcanism, a feasible and fruitful approach 

is to work out the volumetric eruptive outputs of specific centers and to identify through stratigraphic and geochronologic 

studies any episodicities within their lifespans.

The data summarized above for the Cascades show that 

most of the large evolved centers have lifespans in the range 

0.1 to 1 Myr and that such lifespans are sporadically punctuated by short episodes of greatly elevated volcanic output 

(Hildreth and Lanphere, 1994). The wider arrays of distributed 

vents, generally more mafic, which can sometimes be circumscribed as volcanic fields, usually have somewhat longer 

lifespans, but this need not always be so. The basaltic volcanic 

field peripheral to Mount Adams started up ~940 ka, the central stratovolcano ~520 ka, and both remain active (Hildreth 

and Fierstein, 1995, 1997). The mafic to intermediate distributed vent fields peripheral to Mount Hood and Mount Mazama 

date well back into the early Pleistocene but the focal edifices 

date only from the middle Pleistocene. Longevities for distributed volcanic fields that lack stratovolcanoes include ~4 Myr 

for the Simcoe field, ~2.5 Myr for the Portland field, ~800 kyr 

for Indian Heaven, and 400-500 kyr for the Caribou, Medicine 

Lake, and Newberry Volcanic Fields. On the other hand, for 

none of the central edifices of the GVB, nor for Kulshan and 

Hannegan Calderas (Hildreth, Lanphere, and others, 2004; 

Tucker and others, 2007), has a peripheral mafic volcanic field 

been volumetrically important. The variations are so great that 

the most informative approach is to estimate eruption rates for 

individual edifices or fields that are well defined, well studied, 

and well dated.

Table 6 assembles data for several Cascade systems 

where eruptive volumes and longevities have been reasonably 

well estimated, at least for growth of the present-day centers 

(excluding in some cases remnants of older subjacent edifices 

largely torn down). Averaged over the lifetime of the center, 

many of the better-studied evolved systems yield eruption 

rates of 0.1 to 0.6 cubic kilometers per 1,000 years (km3/kyr). 

Exceptional are Mount Shasta (largest cone in the Cascades), 

which gives 0.75 km3

/kyr, and Mount St. Helens (one of the 

youngest), the main well-preserved parts of which yield an 

eruption rate of ~2 km3/kyr. For the few long-lived distributed 

volcanic fields that are both well-delimited and adequately 

investigated, there is a marked contrast in time-integrated 

eruption rates between the Indian Heaven, Mount Adams, and 

Caribou fields (each ~0.1 km3/kyr), on the one hand, and the 
Eruption Rates and Episodicity 65

Newberry and Medicine Lake rear-arc fields (both ~1 km3/

kyr), on the other. 

Stratovolcanoes tend, however, to grow in spurts (Hildreth and Lanphere, 1994), so several examples of the higher 

eruption rates during such episodes are given in the lower section of table 6. At Mount Adams, it was shown that four major 

pulses produced 80 to 85 percent of the output of the whole 

volcanic field; that during those four episodes (each roughly 

25 kyr long) average eruption rates were 2 to 5 km3/kyr; and 

that during the much longer intervals separating the pulses, the 

system remained active at greatly reduced eruption rates averaging ~0.05 to 0.1 km3

/kyr (1 to 5 percent of the peak rates). 

Because durations of major growth spurts are seldom precisely 

enough established, however, the real eruption rates for peak 

episodes are usually only loosely approximated. For example, 

the Bachelor, Adams, and King Mountain pulses (1 to 4 km3/

kyr) cited in table 6 might have actually represented rates 

twice as high if the age limits of the episodes were known 

more precisely. Wherever ages are somewhat better constrained—as for the Belknap shield, episode 3 at Bachelor, the 

last 4 kyr at Mount St. Helens, and for the preclimactic rhyodacite buildup to the Mazama caldera-forming eruption, the 

rates are notably higher (3 to 25 km3

/kyr). The extraordinary 

pulse that built the huge Shastina cone (table 6) on the side 

of Mount Shasta in the early Holocene could not have been 

recognized without radiocarbon dating of pyroclastic deposits 

bracketing its products (Miller, 1980; Christiansen and Miller, 

1989). An ultimate peak eruption rate, of course, is a calderaforming eruption when tens or hundreds of cubic kilometers 

of magma are released in hours to days; but such an event 

represents abrupt loss of magma that had been fed to a shallow 

storage reservoir over tens of millennia, as the Kulshan and 

Mazama precaldera and climactic records illustrate.

Volumes of particular eruptive batches are often hard 

to determine, owing to dispersal and erosion of pyroclastic 

deposits and to uncertainty whether stacks of similar lavas represent single batches or several. The most voluminous batches 

erupted in the Quaternary Cascades were certainly the three 

caldera-forming eruptions (Mazama, Kulshan, and Rockland), 

which each released 40 to 80 km3

 of silicic magma. Eruptive 

batches of dacite and andesite seldom exceed 1 km3, with a 

few noteworthy exceptions. The largest Quaternary dome in 

the Cascades at ~9 km3, Burney Mountain may have extruded 

in several pulses or lobes, but it probably represents a single 

dacitic magma batch. Likewise, the Shastina dacitic edifice (30 

to 40 km3

) was clearly built by a series of extrusive events, but 

the sequence appears to be bracketed to an interval of a few 

centuries, suggesting that its products could have belonged 

to a single magma batch. Andesite edifices typically grow 

by incremental addition of flows and ejecta from many small 

eruptions (<1 km3

), but one intracanyon andesitic lava flow 

from the east side of Mount Adams exceeds 3 km3, and several 

ridge-capping flows radial to Mount Rainier are each in the 

range 3 to 9 km3. The incremental-release uncertainty as to 

what constitutes a coherent magma batch is still more severe 

for mafic lava fields, cones, and shields. Some mafic edifices 

as voluminous as 5 to 25 km3 may have been constructed in 

centuries or less, but limiting ages are rarely precise enough 

to prove it. Many (not all) such cones and shields are compositionally fairly uniform, consistent with derivation from 

a single magma batch. Moreover, for low-viscosity basaltic 

magmas, modest compositional variations distinguishable in 

some sequences of lavas could have developed within what 

started as a single magma batch during its fitful ascent through 

the crust. The Giant Crater lava field (Medicine Lake), which 

erupted as a coherently zoned (basaltic andesite-to-HAOT) 

sequence, amounted to ~4.4 km3 of magma—one of the largest, rapidly emplaced, mafic batches clearly identified as such 

(Donnelly-Nolan and others, 1990, 1991).

Postglacial Eruptions in the Cascades

Table 7 provides a compilation of some 137 postglacial 

eruptive episodes, of which 21 were in the GVB, 56 in the 

Rainier-to-Hood segment, 50 in the Oregon segment, and 10 in 

California. Many of those assigned to the interval 15 to 12 ka 

appear to have taken place during deglaciation. More refined 

geochronology may eventually permit subdivision of some of 

the listed episodes, notably the 13-ka and mid-Holocene episodes at Glacier Peak, the Polallie period at Mount Hood, the 

Sand Mountain chain in Oregon, the Hotlum episode at Mount 

Shasta, and several busy intervals at Mount St. Helens. 

Volumetrically, the estimates of table 7 sum to an 

arc-long postglacial magma output of ~290 km3

 in the last 

15 kyr, an average of 19 to 20 km3 per 1,000 years, not 

including the rear-arc centers. By segment, 16 percent of 

this erupted in the GVB, 21 percent in the Rainier-to-Hood 

segment, 39 percent in the Oregon segment, 24 percent at 

Mount Shasta, and only 1 percent in the Lassen segment. 

By eruptive center, 24 percent issued from Mount Shasta, 

20 percent from Mount Mazama (almost entirely between 

8 and 7 ka), 17 percent from Mount St. Helens, 12 percent 

along the Bachelor chain, and 11 percent from Glacier Peak. 

Altogether, 79 percent of the 290 km3 postglacial output was 

from 15 large evolved centers and only 21 percent (61 km3) 

from some 63 distributed or peripheral cones, shields, or 

chains of mafic and intermediate products. Within the latter 

category, the Bachelor chain and Belknap shield complex 

together account for 71 percent of the 61 km3

, and the 

Oregon segment as a whole for 86 percent.

During the postglacial interval, there have been only three 

great eruptive episodes in the Cascades—(1) the early-to-mid 

Holocene growth spurt of Mount Shasta (~66 km3

), which 

included enormous effusive outputs from the Shastina and 

Hotlum vents, only 2 km apart; (2) the 8 to 7 ka episode centered around the caldera-forming eruption of Mount Mazama 

(~58 km3

); and (3) the 15 to 11 ka outpouring of mafic lavas 

from the main part of the Bachelor chain (~35 km3). 

Besides these three great episodes, about 30 large 

postglacial eruptive episodes released 1 km3 of magma or 

more (table 7). These include three Holocene events in British Columbia (from Mount Meager, Clinker Peak, and Opal 
66 Quaternary Magmatism in the Cascades—Geologic Perspectives

Lifespans

Center km3 Erupted Interval km3

/kyr Compositions Notes References*

Mount Baker 70-100 500-0 ka 0.1-0.2 A>>B>D Includes Black Buttes, Cougar Divide, 

Ptarmigan Ridge, Table Mtn andesites.

18

Kulshan Caldera 50-100 1.18-1.0 Ma 0.3-0.6 R Ignimbrite, fallout, and lavas. 19

Mount Rainier 140-200 500-0 ka 0.3-0.4 A>D Much lost to erosion. Omits early Pleistocene 

predecessor edifice, largely stripped.

10, 27, 28

Indian Heaven 60-80 800-8 ka 0.1 B>>A Nearly 50 vents. All erupted during Brunhes 

Chron except core of Gifford Peak.

14, 20, 21

Mount St. Helens 40-50 23-0 ka 2 D>A>B Severe erosive loss of pyroclastic deposits. 

Omits Ape Canyon stage remnants (>35 ka).

7, 22, 23, 27

Mount Adams—focal 200-330 520-4 ka 0.4-0.6 A>>D>B Stratovolcano and its high flank vents. 15, 17

—peripheral 30-70 940-14 ka ~0.05 B>>A Surrounding volcanic field, largely mafic.

Mount Hood 50-100 500-0 ka 0.1-0.2 A>D Omits older Sandy Glacier and Vista Ridge 

centers nearby.

26, 27, 31, 32

South Sister 20 50-2 ka 0.4 A~R~D Includes stratocone and flank rhyolites. Omits 

remnants of a few mid-Pleistocene andesites.

16, 24

Mount Mazama 160-180 420-0 ka 0.3-0.4 A>R>D Omits subjacent pre-Mazama rhyodacites. 

Includes climactic eruption.

1, 2

Mount Shasta 450 600-0 ka 0.75 A>D>B Includes 45-km3 avalanche deposit. Omits 

peripheral mafic shields.

3, 4, 8

Lassen volcanic center 215 825-0 ka 0.25 A~R>D Includes Rockland caldera, Brokeoff 

stratocone, and postcaldera domefield.

5, 6, 13

Caribou Volcanic Field ~50 450-20 ka 0.12 A~B Compact area east of Lassen volcanic center; 

includes ~140 vents for small cones and shields.

6, 13

Newberry Volcanic Field >500 ~400-1 ka >1 B~A>R Large uncertainties in volume and start-up time. 10

Medicine Lake V.F. >600 500-1 ka 1.2 B>A~R Omits remnants of early Pleistocene units that 

predate and are largely buried by the edifice.

9

Eruptive Pulses

Mount St. Helens 

—late Holocene

>40 4-0 ka 10 D>A>B Smith Creek eruptive period to the present. 7, 22, 23

Mount Adams 

—modern cone

39-52 35-15 ka 2 A Late Pleistocene cone added 1.3 km elevation 

to deeply eroded older edifice.

15, 17

King Mountain chain 7-12 115-106 ka 1 B King Mtn to Quigley Butte compound shield. 15, 17

Belknap shield 6-9 3-1.5 ka 5 B~A Multi-vent shield only. Does not include 

peripheral cinder cones and lavas to S and W.

29, 30

Mount Bachelor chain 35-40 18-8 ka 4 B>A Four main eruptive episodes along a 25-kmlong chain of >35 vents.12, 25

—Episode 3 ~25 ~14-13 ka 25 B>A Main Mt Bachelor—Kwohl Butte phase

probably took <1000 years.

12, 25

Mount Mazama climax 60-80 30-7 ka 3 R>A Includes preclimactic domes and ejecta, 

caldera-forming ejecta, and intracaldera 

lavas & tuff.

1, 2

Shastina 30-40 9.7-9.4 ka 100 D>A Growth of large cone bracketed by radiocarbon-dated pyroclastic units. 

3, 4

Shasta (Hotlum cone) 25-30 9-4 ka 5 D~A Summit cone and large fragmental ringplain. 3, 4

*

References: 1. Bacon and Druitt, 1988. 2. Bacon and Lanphere, 2006. 3. Christiansen, 1982, and written commun., 2006. 4. Christiansen and others, 1977. 

5. Clynne, 1990a. 6. Clynne and Muffler, in press. 7. Crandell, 1987. 8. Crandell and others, 1984. 9. Donnelly-Nolan, in press. 10. Donnelly-Nolan, work in progress. 

11. Fiske and others, 1964. 12. Gardner, 1994. 13. Guffanti and others, 1996. 14. Hammond and Korosec, 1983. 15. Hildreth and Fierstein, 1995. 

16. Hildreth and others, work in progress. 17. Hildreth and Lanphere, 1994. 18. Hildreth and others, 2003. 19. Hildreth, Lanphere, and others, 2004. 20. Korosec, 1989. 

21. Mitchell and others, 1989. 22. Mullineaux, 1996. 23. Mullineaux and Crandell, 1981. 24. Scott, 1987. 25. Scott and Gardner, 1992. 26. Scott and others, 1997. 

27. Sherrod and Smith, 1990. 28. Sisson and Lanphere, 2000. 29. Taylor, 1968. 30. Taylor, 1981. 31. Wise, 1968. 32. Wise, 1969.

Table 6. Some volumetric eruptive rates for the Quaternary Cascades.

[Compositions: A, andesite (52-63% SiO2); B, basalt; D, dacite; R, silicic products (>68% SiO2). In Compositions column, > means “exceeds” and >> means 

“greatly exceeds”]
Center or reach Eruptive episode Age Comp. Volum

Bridge River •Postglacial intracanyon lava flows reported, not dated. (Most units glaciated) <15 ka? B <0

Meager (Plinth) •Plinian fall (Bridge River tephra), pyroclastic flows, lava flows 2.4 ka R 3 

Cayley •Shovelnose dome pair and intracanyon flows (K-Ar ages older; excess Ar?) <15 ka? D 0

Garibaldi Lake •Clinker Peak; 2 adglacial lava flows, each 7 km long 15-12 ka A 4 

•Price Bay scoria cone <15 ka A 0

•Cheakamus intracanyon lavas, youngest set of supra-till lava flows 15-12 ka B 0

Garibaldi •Opal Cone and Ring Creek lava flow 10 ka D 4

Baker •Glacier Creek intracanyon lava flow; summit-derived 14 ka A 0

•Boulder Creek assemblage: lava flows, pyroclastic flows, and lahars 13-12 ka A 1 

•Tephra Set RC (14C ages 10.3 to 10.7 ka) ~12 ka A <0

•Sulphur Creek lava flows from Schreiber Meadow scoria cone; ashfall layer SC 9.8 ka B-A 1 

•Subplinian ashfall layer BA; summit-derived (14C age 5.8 ka) 6.5 ka A 0

Glacier Peak •White Chuck scoria cone 15-12 ka B 0

•Lightning Creek lava flow 15-12 ka M 0

•Plinian Layers G, M, and B; 6 lesser ashfalls; lava domes and pyroclastic flows 13 ka D 15 

•Lava domes, pyroclastic flows, ashfall set D 6.1 ka D 15 

•Lava domes, pyroclastic flows, Dusty Creek coignimbrite ashfalls 5.8 ka D 

•Lava dome and pyroclastic flows ~2.8 ka D 0

•Lava domes, pyroclastic flows, fall layer A 2.0-1.8 ka D 0

•Pyroclastic flows (and dome?) ~1.1 ka D 0

•Small ashfall and debris flows ~0.3 ka D <0

Rainier •19th century period: Minor ashfalls (1820-1854; 1894); scoria fall layer X 0.15 ka A-D <0

•Deadman Flat period: Pyroclastic flows and thin ashfall to northeast 1.1 ka A-D? <0

•Summerland period: ~6 sets of thin hydromagmatic ashfalls; lava extrusions 2.6-2.2 ka A <0

•South Puyallup River pyroclastic flow 2.45 ka A <0

•Lava flows, at least two sets 2.4-2.2 ka A 0

•Subplinian pumice fall layer C 2.2 ka A-D 0

•growth of two small cratered summit cones 2.2-2.0 ka A <0

•Osceola period: •Fall layers B and H; post-Osceola lavas refill amphitheater 5.4-4.6 ka A-D 2 

Table 7. Postglacial eruptive episodes in the Cascades.[Some of the episodes listed are likely to be subdivided after more detailed investigations. Volume estimates, adjusted to approximate the amount of magma erupted, are either from the refauthor; some are rough approximations and might be improved greatly by tighter field constraints. Dash (—) in volume column means data are inadequate for an estimate. HAOT is high-arated in the text. In Composition (Comp.) column, abbreviations are as in table 6, except that M additionally separates mafic andesite (52-57% SiO2) or refers to products zoned from basal
Center or reach Eruptive episode Age Comp. Volum

Rainier •Tephra layer F; phreatomagmatic blasts and pumiceous ashfall 5.6 ka A-D <0

•Osceola Mudflow (3.8-km3 sector collapse); during layer F 5.6 ka — 

•Tephra layer S; phreatic blast beneath Osceola Mudflow 5.6 ka — 0 

•Tephra layer N; pyroclastic flows and lahars 5.8 ka A 0

•Cowlitz Park period; •Tephra layer D (subplinian hornblende-rich pumice/scoria) 7.0-6.7 ka A-D 0

•Tephra layer L (subplinian pumice fall) 7.3-7.1 ka A 0

•Tephra layer A ~7.5 ka A-D 0

•Tephra layer R; pumice and scoria fall; oldest postglacial eruptive unit known 9.9-9.7 ka A 0

Adams •Andesite of Mount Adams summit; lava flows, fragmental cone 15-5 ka A 0

•Basalt of Trappers Creek; lava flows; Red Butte scoria cone 15-14 ka B 0

•Andesite of Cunningham Creek; fissure/spatter vent; intracanyon lava flow 15-14 ka A 0

•Basalt of Smith Butte; chain of scoria cones, lava-flow apron 14 ka B 0

•Four ash and scoria falls 12-8 ka A <0

•Andesite of High Camp; lava flows; vent covered 7-5 ka A 0

•Andesite of Takh Takh Meadow; 10-km-long lava-flow field; vent covered 7-5 ka A 0

•Andesite of Muddy Fork; fissure-fed lava flows to 8 km long 7-5 ka A 0

•Seven ash and scoria falls; and seven hydrovolcanic crystal-lithic ashfalls 7-4 ka A <0

•Andesite west of Gotchen Glacier; fissure-fed lava flow 1 km long 5-4 ka A 0

•Aiken Lava Bed; fissure-fed flank flow 7 km long ~4 ka A 0

•Andesite of Battlement Ridge; two spatter-fed lava flows, each ~1 km long 3-1 ka M 0

•Four phreatic or phreatomagmatic ashfalls 2.5-1 ka A <0

Indian Heaven •Big Lava Bed; scoria cone pair, 15-km-long lava field, and 0.1-km3 ashfall 9 ka B 1 

S. Washington •West Crater; lava dome, 2 lava flows, each 4 km long (head of Trout Creek) ≤8 ka A 0

•Hackamore Creek scoria cone and short lava flow 9-8 ka A <0

•Puny Creek crater (S toe of Bare Mtn), scoria blanket, and (related?) lava flows <9 ka M <0

•Chinook Creek cone and intracanyon lava flows (6 km south of Swift Reservoir) <15 ka M <0

St. Helens •Swift Creek stage: •Lava domes, pyroclastic-flow fans; 5 fall layers of Set S 16-13 ka D 10 

•Four fall layers of Set J ~13-12 ka D 

•Smith Creek period: •Lava domes, pyroclastic flows; fall layers Yb, Yd 3.9-3.8 ka D 10 

•Pyroclastic flows & ashfalls; 5 more fall layers of Set Y 3.5-3.3 ka D 

•Pine Creek period: •Domes, pyroclastic-flow fans & ashfalls; 4 layers of Set P 2.9-2.6 ka D 3 

Table 7. Postglacial eruptive episodes in the Cascades.—Continued [Some of the episodes listed are likely to be subdivided after more detailed investigations. Volume estimates, adjusted to approximate the amount of magma erupted, are either from the refauthor; some are rough approximations and might be improved greatly by tighter field constraints. Dash (—) in volume column means data are inadequate for an estimate. HAOT is high-arated in the text. In Composition (Comp.) column, abbreviations are as in table 6, except that M additionally separates mafic andesite (52-57% SiO2) or refers to products zoned from basal
•Castle Creek period: •Lava flows and early ashfalls of Set B; emplaced radially 2.55-2.5 ka

A 20 

•Lava flows on N flank; more ashfalls of Set B ~2.2 ka

A 

•Lava domes, pyroclastic flows; ashfall layers Bd, Bi ~2.0 ka

D 

•Lava flows: 3 radial sets of lavas; ashfall layer Bu 2.0-1.9 ka

B 

•Cave Basalt; 14-km lava tongue down S flank; final event 1.9 ka

B 0

•Sugar Bowl period: Domes; pyroclastic flows; lateral blasts; fall layer D 750-800 CE

R 0

•Kalama period: •Domes, pyroclastic flows; 5 fall layers of Set W 1479-1500 CE

D 5 

•Many lava flows, pyroclastic flows; ashfall Set X 1510-1570 CE

A 

•Summit dome, pyroclastic flows; ash layer Z 1620-1720 CE

D 

•Goat Rocks period: •Plinian fall layer T 1800 CE

D 1 

•Lava flow down N flank 1801 CE

A 

•Goat Rocks lava dome with intermittent ashfall 1831-1857 CE

D 

•1980-1986 period: Lateral blast, fallout, pyroclastic flows, lava domes 1980-1986 CE

D 0

•2004-2007 period; small ash plumes and lava dome 2004-2007 CE

D 0

Hood •Polallie period; many pyroclastic flows, domes; began during LGM ?20-12 ka

D >5 

•Parkdale lava flow, 6 km long, from vent in Hood River valley 7.7 ka

M 0

•Timberline period; domes and pyroclastic flows 1.5 ka

D 1 

•Old Maid Flat period; Crater Rock dome and pyroclastic flows 0.2 ka

D 0

Jefferson reach •North Pinhead Butte; scoria cone and lava flow to E ~15 ka

M 0

•Badger Creek flow; 2 scoria cones (12 km E of Sisi Butte) <12 ka

M 0

•Scoria cone at S toe of The Table 8-6 ka

M <0

•Horseshoe scoria cone and Jefferson Creek lava flow ~7 ka

M 0

•Forked Butte scoria cone and two lava flows in Cabot Creek ~7 ka

M 0

•Scoria cone at S toe of South Cinder Peak and lava flow to W ~1 ka

M 0

Sisters reach •Sand Mtn chain; ~20 scoria cones and multi-flow lava apron 4-2.8 ka B-M

•Little Nash Crater; scoria cone and lava flow ~2.6 ka

M 3

•Nash Crater; scoria cone and lava-flow apron 4-3 ka

B

•Lost Lake chain; 4 scoria cones and lava-flow apron ~2 ka

B

•Blue Lake crater; phreatomagmatic ejecta apron; ~3.5 ka

M <0

•Spatter cone chain of 3 fissure vents (at northeast toe of Mount Washington) <1.3 ka

M <0

•Inaccessible Cone chain of 5 scoria cones and 2 lava aprons ~7-4 ka

B 0

•Twin Craters; scoria cone pair and lava apron (southwest of Belknap shield) ~2.8 ka

M 0

•Belknap Crater shield; scoria cone on top (includes South Belknap flank vent) 3-1.4 ka B-M 6

•Little Belknap shield 2.9 ka

M 1

•Yapoah Crater; scoria cone and several lava flows as long as 13 km ~2.7 ka

M 0

•Four-in-One chain of 6 fissure-fed scoria cones and lava apron 2.0 ka M-A 0
Table 7. Postglacial eruptive episodes in the Cascades.—Continued [Some of the episodes listed are likely to be subdivided after more detailed investigations. Volume estimates, adjusted to approximate the amount of magma erupted, are either from the refauthor; some are rough approximations and might be improved greatly by tighter field constraints. Dash (—) in volume column means data are inadequate for an estimate. HAOT is high-arated in the text. In Composition (Comp.) column, abbreviations are as in table 6, except that M additionally separates mafic andesite (52-57% SiO2) or refers to products zoned from basalt 

Center or reach Eruptive episode Age Comp. Volum

Sisters reach •Collier Cone; scoria cone and lava flows, one as long as 13 km <1.6 ka M-A-D 0

•Sims Butte; large scoria cone and intracanyon lava tongue as long as 14 km ~15 ka M 0

•Cayuse Crater; scoria cones (one large, 2 small) and lava flows as long as 4 km 11 ka B 0

•LeConte Crater; scoria cone and several lava flows as long as 14 km 15-10 ka M 0

•Katsuk and Talapus Buttes; vent chain, hyaloclastite, lava plateau, 2 scoria cones 18-15 ka B 1

•Chambers Lake coulee; bifurcating flow vented high on S slope of Middle Sister 14 ka D 0

•Rock Mesa coulee, satellite domelets, and pumice fall 2.2 ka R 0

•Devils chain; 20 aligned vents; small lava domes and flows and pumice fall 2.0 ka R 0

•Egan Cone; scoria cone and lava apron to Sparks Lake ~8 ka B-M 0

•Mount Bachelor summit cone and Tot Mtn flank vent 13-11 ka M 

•Mount Bachelor shield; lava flows as long as 22 km; 3 scoria cones at north toe ~13 ka B-M 

•Kwohl Butte shield; lava flows as long as 18 km; at least 7 scoria cones 15-13 ka B-M 

•Siah Butte chain; 25-km-long; >30 vents; many lava flows, scoria/spatter cones 15-14 ka B 3 

•Sheridan Mtn shield; many lava flows; ~8 flank scoria cones 18-15 ka B-M 7 

•Red Crater chain (six vents); scoria cones and lava apron (W of Mt Bachelor) 18-12 ka B 0

•Wuksi Butte-Twin Lakes chain; lava apron; 6 scoria cones and 4 maars 18-15 ka B-M 1 

•Sitkum Butte; fissure-fed lava apron on east flank of Pleistocene cone <10 ka B <0

Wickiup reach •Davis Lake lava flow and scoria cone 5 ka A 0

•Black Rock lava flow and scoria cone (south of Hamner Butte) ~5 ka A 0

•Black Rock Butte lava flow and scoria cone (northeast of Odell Butte) ~5 ka A <0

Thielsen reach •Thirsty Point scoria cone and lava apron (11 km north of Diamond lake) 15-12 ka M 0

•Cinnamon Butte scoria cone and lava apron (8 km north of Diamond Lake) 15-12 ka M 0

Mazama reach •Basalt of Castle Point; 3 HAOT vents; lava flows (7 km southwest of caldera) 15-8 ka B 0

•Llao Rock plinian fall and lava flow, Mount Mazama 7.9 ka R 3

•Crater Lake caldera-forming plinian fall, pyroclastic flows, and Cleetwood lava 7.7 ka R>A 50 

•Wizard Island scoria cone and lava-flow apron, Crater Lake ~7 ka A 2

•Central Platform lava plateau (Crater Lake floor) ~7 ka A 1

•Merriam Cone; lava and fragmental cone on floor of Crater Lake ~7 ka A 0

•East Basin sediment-buried lava flow on floor of Crater Lake ~7 ka? A? 0

•Rhyodacite dome on floor of Crater Lake 4.8 ka R 0

McLoughlin reach •Imagination Peak; scoria cone and lava flows (northwest of Pelican Butte) 15-12 ka B 0

 25 
•Brown Mountain; postglacial lava flows at west toe of shield 15-12 ka M-A 0

Shasta segment •Red Banks plinian fall and pyroclastic flows 11 ka D >1 

•Shastina cone; many large lava flows, domes, and pyroclastic flows 11-10 ka D 40 

•Black Butte; compound lava dome with pyroclastic-flow apron 10 ka D 2

•Hotlum (summit) cone; many large lava flows, pfs; vulcanian ashfalls 10-0.2 ka D-A 25 

Lassen segment •Twin Buttes; scoria cone pair and 5-km-long lava flow (E of Burney Mtn) 15-12 ka M 

•Andesite of hill 7416; scoria cone and 6-km-long lava flow 15-12 ka A 0

•Devils Rock Garden; scoria cone and 4-km-long lava field 15-10 ka A 0

•Chaos Crags; 6 lava domes, pyroclastic flows and fallout 1.1-1.0 ka R 2 

•Cinder Cone; compound scoria cone, 5 lava flows, far-flung ashfall 1666 CE M-A 0

•Lassen Peak; lava dome, 2 short lava flows, pyroclastic flows and fallout 1915 CE D>R~A 0

References: 1. Bacon, 1983. 2. Bacon and others, 1994. 3. Bacon and others, 2002. 4. Bacon and Druitt, 1988. 5. Beget, 1982a. 6. Beget, 1984. 7. Brantley and Myers, 21992. 9. Christiansen, 1990. 10. Christiansen and Miller, 1989. 11. Christiansen and others, 2002. 12. Clynne and others, 2000. 13. Clynne and others, in press. 14. Clynne Crandell, 1980. 16. Crandell and Hoblitt, 1986. 17. Criswell, 1987. 18. Fierstein and others, 2003. 19. Foit and others, 2004. 20. Green, 1981. 21. Green, 1988. 22. Greemond, 1990. 24. Hildreth and Fierstein, 1995. 25. Hildreth and Fierstein, 1997. 26. Hildreth and others, 2003. 27. Hyde and Crandell, 1978. 28. Korosec, 1989. 29. LipmMacdonald, 1963. 31. Macdonald, 1964. 32. Macleod and Sherrod, 1992. 33. Major and others, 2005. 34. Mathews, 1952b. 35. Mathews, 1958. 36. Miller, 1980. 37. 

lineaux, 1996. 39. Phillips, 1987. 40. Porter, 1978. 41. Roddick and Souther, 1987. 42. K.M. Scott, work in progress. 43. Scott, 1977. 44. Scott, 1987. 45. Scott and Gers, 1997. 47. Sherrod, 1990b. 48. Sherrod, 1991. 49. Sherrod and Smith, 2000. 50. Sherrod and others, 2004. 51. Sisson and others, 2001. 52. Sisson and Vallance, 2005. and others, 1982. 55. Souther, 1980. 56. Souther, 1990. 57. Stasiuk and Russell, 1990. 58. Stasiuk and others, 1996. 59. Tabor and Crowder, 1969. 60. Taylor, 1965. 611981. 63. Taylor, 1990b. 64. Vallance and Scott, 1997. 65. Venezky and Rutherford, 1997. 66. Williams and others, 2004. 67. Wise, 1969. 
72 Quaternary Magmatism in the Cascades—Geologic Perspectives

Cone) and two in California (Black Butte and Chaos Crags). In 

central Oregon, there were large mafic eruptive episodes at the 

Belknap shield complex, along the Sand Mountain chain, and 

at two chains satellitic to the Bachelor chain (Katsuk-Talapus 

and Wuksi-Twin Craters; table 7). The Big Lava Bed at Indian 

Heaven and the post-Osceola refilling of Mount Rainier's summit amphitheater also represent large outpourings of lava over 

limited time intervals. In addition, major pyroclastic episodes 

include the Polallie and Timberline periods at Mount Hood, at 

least four voluminous episodes at Glacier Peak, and as many as 

15 from Mount St. Helens. Of the 30 large eruptive episodes, 

the biggest well-delimited events appear to be the Opal ConeRing Creek lava (4.5 km3

) and fall layer Yn from Mount St. 

Helens (4 km3). However, several postglacial episodes at Glacier 

Peak and Mount St. Helens produced great pyroclastic-flow 

fans that are hard to quantify volumetrically because, when 

mobile, they degenerate into downstream debris flows and, after 

emplacement, they erode away rapidly. Many such pyroclastic 

episodes at Glacier Peak, St. Helens, and Hood are likely (in 

addition to more readily quantifiable fall deposits) to have generated fragmental flow deposits in the range 1 to 5 km3

 for each 

major interval.

Scott (1990), amplifying a compilation by Hoblitt and 

others (1987) comparable to that of table 7 (but also including 

phreatic events and debris flows not tied to magmatic eruptions), defined ~181 eruptive periods over the last 15 kyr. Of 

these, 24 were at Newberry and Medicine Lake (not dealt with 

here), the remainder in the Cascade arc proper. Scott called 

attention to the incomplete and ephemeral record left by some 

of the smaller events included in his count, noting that a third 

(60) of the eruptive periods compiled were in the past 3,000 

years. The youngest, best-preserved record thus suggests a 

recent average of two episodes per century; and there were in 

fact two in the 20th century—at Lassen Peak (1914-17) and 

Mount St. Helens (1980-86). With age, however, discerning the record and integrity of minor episodes becomes more 

difficult. It seems unlikely, say, 5,000 years into the future, 

that either the 1914-17 events atop Lassen Peak or the 2004-7 

augmentation of the crater-confined St. Helens dome would be 

discerned as discrete eruptive episodes.

Start-up Times and Major Growth Episodes

Although an open-minded skeptic is unlikely to see any 

pattern of along-arc synchroneity or episodicity in the postglacial eruptive inventory of table 7, one can further examine the 

eruptive record on the longer timescales of edifice longevity 

and spurts of elevated volumetric output (tables 8, 9). Within 

the growth histories of many major Cascade centers, intervals 

of heightened eruptive output have been identified. Although 

the age limits of such intervals are seldom tightly defined, 

some of the better constrained examples are listed in table 8. 

They include a wide range of nominal eruption rates, between 

0.2 and 10 km3

/kyr, but each represents a marked pulse at its 

particular volcano. Again, however, no coordinated episodicity 

is obvious along the Cascade arc.

The eruptive intervals selected for table 6 took place 

at a number of large Cascade centers where major activity appears to have begun between ~600 ka and 400 ka, 

an observation that might be interpreted to suggest that 

some tectonic-magmatic event or transition took place 

around that time, in the middle Pleistocene. Initiation of 

the Tumalo, Cappy, and Garibaldi centers may (or may 

not) also date from that interval. In order to examine more 

closely the foundations of such an inference, the start-up 

times for the main evolved (nonmafic) centers in the Cascades have been tabulated (table 9) along with the evidence 

for any older evolved predecessors, subjacent or nearby. 

Table 9 shows that the existing centers began growth at 

widely different times and that most of them overlap the 

eroded remnants of earlier edifices or of distributed vent 

fields that date from the middle or early Pleistocene. 

In particular, the following points can be noted: (1) the 

Rainier and Hood edifices, which started up ~500 ka, are 

directly superimposed on remnants of predecessors as old 

as 1.3 Ma, and the Jefferson and Mazama edifices overlie 

distributed silicic volcanic fields that similarly date from 

the early Pleistocene; (2) the Baker, Shasta, and Lassen 

loci, each of which first grew large in the mid-Pleistocene, 

did so following shifts of the local magmatic focus by 

only 10 to 20 km from the sites of large evolved centers 

that had been active in the early Pleistocene; (3) start-up 

times of the South Sister, Middle Sister, and Mount St. 

Helens edifices are much younger (≤50 ka), as are those of 

evolved predecessors underlying each one (300 to 150 ka); 

(4) the Goat Rocks edifice started up sometime in the early 

Pleistocene and overlies remnants of a Pliocene center; and 

(5) reconnaisance K-Ar dating (Green and others, 1988) 

suggests that the multiphase Meager and Cayley edifices 

both contain components as old as 2 Ma, though this needs 

to be checked by current 40Ar/39Ar methods. The evident 

complexity is inconsistent with the suggestion that many 

or most Cascade edifices began growth at about the same 

time. After consideration of mantle processes beneath the 

arc, possible inferences will be addressed below concerning the contrast between shifting and stationary magmatic 

foci (table 9) on the 2-Myr Quaternary timescale (see the 

section below entitled “Shifts of Focus”).

As eruptive histories become better defined and more 

precisely dated, the search for along-arc coordinated episodicities should be maintained. For now, however, the data for 

Quaternary start-up times, subsequent episodes of high output, 

and the more detailed postglacial record support only skepticism concerning the existence of any pattern of synchronous 

magmatic behavior along the Cascades. Individual evolved 

centers behave independently, as does the mafic magmatism 

distributed along the different reaches of the arc.

Outstanding Questions

Any discussion of eruption rates or episodicities brings 

up many unanswered but fundamental questions about mag-
Eruption Rates and Episodicity 73

Table 8. Pulses of elevated eruptive output.

Center Interval Notes References

Meager 100-2 ka Northern half of multivent edifice built (in at least 4 stages) 8

Baker: Black Buttes 500-300 ka Mafic to dacitic stratocone 10

Modern cone 40-12 ka Andesitic stratocone

Rainier 500-420 ka Principal growth spurts of great central edifice 13

280-190 ka

40-4 ka

Adams: Early edifice 520-450 ka Combines Hellroaring Volcano and adjacent edifice beneath modern cone 9

Modern cone 35-15 ka Present andesitic summit cone

St. Helens 23-0 ka Silicic domefield concentric with young (<2.5 ka) stratocone 5,11

Jefferson 100-50 ka Most productive interval during 280-kyr growth of modern cone 6

South Sister 50-2 ka Growth of modern rhyolite-dacite-andesite edifice 7

Bachelor chain 18-8 ka Multivent mafic alignment 12

Mazama 80-40 ka Last big growth spurt of late Pleistocene edifice 1

27-7 ka Growth, leakage, and evacuation of climactic rhyodacite magma reservoir

Shasta:

Sargents Ridge 200-150 ka Four successive overlapping cones that were built atop wreckage of larger

mid-Pleistocene Sand Flat edifice eviscerated by huge debris avalanche

2

Misery Hill 40-30 ka

Shastina and Hotlum 10-4 ka

Lassen:

Rockland Tuff 600 ka Caldera-forming eruption 3,4

Brokeoff cone 590-390 ka Postcaldera andesite-dacite stratovolcano

Younger domefield 66-0 ka Younger of two main intervals in growth of Lassen domefield

References: 1. Bacon and Lanphere, 2006. 2. Christiansen, R.L., written commun., 2006. 3. Clynne, 1990a. 4. Clynne and Muffler, in press. 5. Clynne and 

others, in press. 6. Conrey, 1991. 7. Fierstein and others, 2003. 8. Green and others, 1988. 9. Hildreth and Lanphere, 1994. 10. Hildreth and others, 2003. 

11. Mullineaux, 1996. 12. Scott and Gardner, 1992. 13. Sisson and others, 2001.
Table 9. Start-up times, predecessors, and shifts of eruptive focus. Center Initiation of Present Edifice Predecessor Edifice beneath Present Volcano? Shift of Main Eruptive Focus? Meager 150-100 ka Young multi-vent silicic pile caps northern two-thirds of composite edi- fice. Older components crop out 3-6 km southwest. Pylon complex (andesitic, 1-0.5 Ma), T

(silicic, 2-1 Ma), and scraps of an and(~2 Ma) form southern third of cente

Cayley uncertain None. At least 3 stages in growth of multi-vent edifice. No. Distributed vents show no systemaGaribaldi ~260 ka Round Mtn andesitic edifice (600±150 ka). No. Little or no systematic vent migraBaker 45 ka Remnant lava stack in Park Creek (140-80 ka) and Black Buttes cone (500-300 ka) centered 3 km southwest; overlapped by modern cone. Kulshan caldera (1.3-1.0 Ma); 7 km nor

Lake Ann stock (2.8 Ma); 13 km northe

Glacier Peak 600 ka Gamma Ridge caldera (2 to 1.6 Ma) lies 4-9 km northeast of modern summit vents. Modest (fig. 26). Rainier 500 ka Lily Creek volcano (1.3 to 1.0 Ma). No St. Helens 23 ka Modern domefield dates from 23 ka; stratocone from 2.5 ka. Older domefield (125-35 ka) underlaps the modern complex. Ape Canyon domefield (300-250 ka; 12

crops out 4-10 km southwest of modern 

Adams 450 ka Hellroaring stratovolcano (520-450 ka) offset 5 km southeast of younger cone (fig. 10). Modest (fig. 26). Distributed mafic fielHood 500 ka Vista Ridge cone (1.5-0.8 Ma) buried by Hood. Sandy Glacier volcano (~1.3 Ma) crops out beneath west flank of Hood (fig. 11). Lookout Mountain center (~3 Ma): 7-15 Jefferson ~280 ka Distributed andesite-dacite field since 1-1.5 Ma. Lionshead evolved center (2-3 Ma) ~10 Middle Sister 38 ka Remnant lava stack at east toe (180-160 ka). No. All older volcanoes nearby are maSouth Sister 50 ka A lone dacite lava (178 ka) extends southeast from beneath toe of present 

edifice.

Broken Top (300-150 ka) lies 6 km souTumalo Volcanic Field (650-260 ka) is 

Mazama 420 ka Distributed mafic-silicic field (1.8-0.6 Ma) underlies the modern edifice. Pre-Mazama clustered rhyodacite field 

edifice and extends as far as 12 km events toward the west-northwest has of the Mazama edifice.

Shasta 600 ka None. Five successive stages were centered within 3 km of each other. Rainbow Mountain center (~1 Ma) lies 

(fig. 26).

Lassen domefield

300 ka Brokeoff stratocone (590-390 ka). Rockland precaldera/caldera sequence 

(825-600 ka) underlaps modern domefield.

Maidu center (2.4-1.2 Ma); ~18 km souDittmar center (2.4-1.4 Ma); ~15 km so
Eruption Rates and Episodicity 75

matism. The uncertainties reflect principally our nebulous 

understanding of mantle convection and melting and of intracrustal magma transport and multistage storage. (1) What 

fraction of the magma in any particular crustal column ever 

erupts? We don't know what proportions are deposited intra

crustally as melt-depleted (cumulate) mafic and felsic 

intrusive rocks. (2) Why do so few eruptive loci develop into 

long-lived productive centers? Slab subduction is continuous along the Cascade arc for 1,250 km, but some profound 

modulating processes have restricted the number of large 

evolved Quaternary centers to fewer than 30 (table 2). (3) 

What causes gross differences in eruptive output along the 

same arc? Although of similar (andesite-dacite) composition 

and longevity (500 to 600 kyr), Mount Shasta has had five 

times the output of Mount Hood. (4) What limits longevities 

of most major arc volcanoes to 0.1 to 1 Myr? Possibilities 

include source depletion, shifts of focus, and relaxation times 

of mantle upwellings. (5) Why are continental-arc stratovolcanoes seldom large? Many are 10-100 km3

 but only a few 

erupt total volumes of 100-1000 km3. Many intraplate shields 

are far more voluminous, and some intraplate extensional 

systems release single eruptions of silicic or mafic magma 

greater than 1000 km3. Arcs never produce flood andesites. 

Eruptive batches are numerous and small, and even calderaforming eruptions from shallow arc reservoirs seldom reach 

100 km3. And (6) if there is little coordinated episodicity 

along the arc, is it reasonable to attribute volcanic behavior 

(as opposed to magmatism in general) to secular, apparently 

steady, plate motions?

Productivity of the Cascades vis-à-vis Other 

Continental Arcs

It has commonly been asserted that volcanic output 

along the Cascade arc is low relative to that of many arcs, but 

the basis of the assertion has never been clear. The estimated 

290 km3 of postglacial magma erupted along the Cascade 

arc proper (neglecting the rear-arc centers) is equivalent 

to ~5 percent of the volume of oceanic crust and sediment 

(if together ~8 km thick) potentially subducted during any 

15-kyr interval beneath the 1,250-km-long Cascadia margin 

at a rate of 4 cm/yr. If such an eruption rate had been steady 

throughout the Quaternary, this would have yielded more than 

19,000 km3

 of eruptive products in 1 Myr and 38,000 km3 in 

2 Myr. Accepting estimates of Sherrod and Smith (1990) for 

volumetric output in California and Oregon and using my own 

larger estimates for the more severely glaciated volcanoes 

of Washington and Canada, I get a total eruptive volume of 

~6,400 km3

 of Quaternary products for the Cascade arc. What 

is present today (or is reasonably reconstructed at well-known 

centers) is thus only about 17 percent of the 2-Myr eruptive 

volume expected based on the postglacial eruption rate. Either 

the postglacial rate is far greater than the Quaternary average 

or the erosion of early Quaternary products was far greater 

than anyone has estimated.

Nonetheless, even 6,400 km3 is not low relative to 

the Quaternary productivity of some other continental 

arcs. For the 1,100-km-long Central American arc (where 

glacial erosion is absent), the 39 volcanic-front centers 

(many of them multivent edifices) yield a total volume 

of 3,464 km3 (Carr and others, 2003), and in the rear-arc 

some 10 modest composite cones and several clusters of 

scoria cones and domes add no more than an additional 

10 percent. For the 1,000-km-long Northeast Japan arc 

(Myoko to Tarumai), Aramaki and Ui (1982) conservatively estimated a total volume of 1,309 km3

 for 45 edifices 

or volcanic clusters plus 177 km3 for distributed pyroclastic falls and flows. Glacial erosion in the Japanese arc has 

been insignificant. They stated that most of the volcanoes 

are younger than 0.5 Ma and very few as old as 1 Ma. Even 

if their estimate were quadrupled (to approximate Quaternary output for 2 Myr), the resulting volume of 5,944 

km3

 would be smaller than the 6,400 km3 estimated for 

the Cascades. For the 1,150-km-long Alaska Peninsula arc 

(including continental Unimak Island but not the oceanic 

Aleutian Islands chain where submarine volumes are difficult to gauge), my estimates for all 55 volcanoes yield a 

total volume between 2,000 and 3,000 km3 (depending on 

how liberally one reconstructs erosive losses). Although 

glacial erosion has been severe on the Alaska Peninsula (as 

likewise in the northern Cascades), the observation that the 

accountable Quaternary volcanic volume is less than half 

that of the Cascade arc suggests that the latter is not particularly underproductive. Likewise, volumetric comparison with the 1,400-km-long Southern Volcanic Zone (SVZ) 

of the Andean arc (33-46oS) suggests that the Quaternary 

volcanic output of Cascadia has been at least as vigorous. 

My volume estimates for all 80 large volcanoes and five 

distributed volcanic fields (altogether representing a total 

of 820 SVZ vents) yield a total of ~5,300 km3, ~83 percent of the Quaternary volume estimated for the Cascades. 

Owing to a latitudinal gradient in severity of glaciation (as 

in the Cascades), the total volume uncertainty for the SVZ 

is likely to be at least ±25 percent, a value thought similar 

to those for the Cascadian and Alaskan estimates because 

the same approaches to approximation and reconstruction 

were used for each arc (table 10). Early Quaternary volumes are likely to be underestimated, but this is true for all 

the arcs just mentioned as well as the Cascades.

It thus appears that the supposedly low Quaternary 

productivity of the Cascade arc is another myth that can now 

be abandoned. This longstanding misimpression about the 

Cascades may have been attributable to (1) overemphasis on 

Cascade stratocones, which are relatively few and mostly of 

modest volume; (2) underappreciation of the large distributed 

volume in the Oregon and Lassen segments; (3) failure to 

account for large losses to glacial erosion in the northern half 

of the arc; and (4) inapt comparisons with intraoceanic arcs, 

where submarine edifices may well be systematically larger 

but the extrusive/intrusive ratios much greater than for arcs 

on continental crust.
76 Quaternary Magmatism in the Cascades—Geologic Perspectives

Table 10. Comparison of three Quaternary continental arcs.

Characteristics Cascade arc Alaska Peninsula Andean SVZ1

Length of Quaternary chain (km)1 1,250 1,150 1,400

Quaternary vents identified (arc proper only) 2,339 245 820

Major evolved centers (stratocones)2 26 45 65

Major mafic centers2 9 3 15

Large centers on volcanic front 12 45 36

Range of spacings (km) of large centers3 2-150 4-75 5-98

Number of pairs considered 30 44 35

Mean spacing (km) 42 27 42

Standard deviation (km) 37 23 22

Three longest Gaps without vents (km) 136, 105, 74 70, 66, 60 98, 69, 65

Distributed volcanic fields4 29 5 5

Total area represented (km2) 30,700 1,680 2,150

Number of vents encompassed 3,388 127 356

Area (km2) per vent 3-40 5-20 1.5-15

Total Quaternary volume erupted (km3 ± 25%) 6,400 2,500 5,300 

1

Southern Volcanic Zone (SVZ) of the Andes extends from Tupungato to Hudson (33 ̊-46 ̊S); alkali-basalt fields east of the arc are excluded. Continental 

part of Alaskan arc extends from Hayes to Westdahl-Pogromni (152 ̊-165 ̊W). Cascade arc includes Garibaldi volcanic belt and extends from Lassen to 

Bridge River (40 ̊-51 ̊N; table 1); rear-arc centers are excluded here.

2Envolved centers have erupted andesite, dacite, or rhyolite. Major mafic centers consist of basalt and basaltic andesite; for Alaska, only Shishaldin, Westdahl, and Pogromni qualify; for the SVZ, Casimiro, Planchón, Cerro San Pedro, Aguirre, Campanario, Antuco, Carrán-Los Venados, Mencheca, Antillanca, 

Puntiagudo, Osorno, Hornopirén, Hualaihue (Apagado), Puyuhuapi, and Cay; for Cascade arc, see tables 2, 4.

3

For details of Cascade spacing, see table 4. For the SVZ, where the arc is commonly broad and cluttered with vents, only the along-arc spacing of 36 

volcanic-front centers is given. For Alaska, in contrast, all but 3 of the major centers lie along the front; only these three, Griggs, Amak, and Pogromni, are 

excluded from the spacing calculations.

4

Multivent fields of distributed vents are the norm in the Cascade arc and are subdivided as in table 1, including the rear-arc fields. The five such fields 

considered in Alaska are Pogromni-Westdahl, Fisher, Shishaldin, Emmons-Pavlof, and Veniaminof. Those in the SVZ are Descabezado-Calabozos, 

Campanario-Laguna del Maule-Ñirales, Carrán-Los Venados, Antillanca, and Puntiagudo-Cordón Cenizas. Areas cited are those defined by vent fields, not 

by outflow lavas or fragmental flows.
Eruption Rates and Episodicity 77

Inferences Concerning Future Eruptive Activity

If the postglacial Cascade average of one or two eruptive 

episodes per century (table 7) were to be maintained, what 

might be anticipated next?

The likely sources of future explosive episodes that 

could produce widespread fallout and/or valley-sweeping 

flow deposits include the usual suspects—Meager, Garibaldi, 

Glacier Peak, St. Helens, Hood, Mazama, Shasta, and Lassen, 

all of which had devastating eruptions in the Holocene. On the 

other hand, Cayley, Goat Rocks, Jefferson, Tumalo, and Cappy, 

all formerly capable of such eruptions, appear to have been 

long moribund. The three active andesitic edifices in Washington (Baker, Rainier, and Adams) are not likely to produce 

major explosive events, but their relief, icecaps, and masses of 

hydrothermally altered rock make them especially susceptible 

to avalanching and to flushing of radial valleys by debris flows.

Some might be tempted to regard the Mazama system as 

played out owing to its 7.7-ka caldera-forming eruption and 

its largely andesitic postcaldera aftermath, but this would be to 

ignore its history of dacitic and rhyodacitic episodes recurrent for more than 500 kyr. Moreover, it should be recalled 

that Quaternary intracaldera eruptive activity at the Rockland 

and Kulshan Calderas persisted for several hundred thousand 

postcaldera years.

At all three Quaternary calderas, the principal calderaproducing magma had evolved at least as far as rhyodacite 

(each >70% SiO2

). At the Pliocene Hannegan and Devils 

Horns Calderas, it was rhyolite, as was the 400-ka Tumalo 

Tuff, which may or may not have issued from a now-buried 

caldera source. In contrast, none of the numerous explosive 

eruptions of Quaternary Cascade dacite (63-68% SiO2) produced a caldera. There may be what amounts to a batch-size 

control, with dacite magma batches (seldom more voluminous than a few km3

) arising from mafic-to-andesitic parents 

in modest or dispersed reservoirs small enough to be susceptible to mafic recharge and convective remixing. Voluminous 

rhyodacite magmas, on the other hand, may typically require 

prior accumulation of a large reservoir of mushy dacite 

(granodiorite) or silicic andesite (tonalite), thick enough to 

buffer mafic inputs and to extract voluminous interstitial 

melt, while shallow enough to permit caldera subsidence. 

Whatever the control, the compositional correlation supports 

the inference that persistently dacitic volcanoes (Garibaldi, 

Glacier Peak, St. Helens, Hood, Shasta, and the Lassen 

domefield) are neither likely to have large shallow integrated 

reservoirs nor to undergo caldera-forming eruptions. On the 

contrary, Meager and Newberry have released substantial 

batches of Holocene rhyodacite or rhyolite, none yet voluminous enough to yield a caldera but permissive of the capacity 

to do so in the future. 

At Mount Garibaldi, the age of the Lava Peak rhyolite 

(table 11) is poorly known, but if the voluminous Opal ConeRing Creek dacite (63% SiO2

; ~10 ka) is the youngest unit 

there, as is generally accepted, this suggests that any rhyolitic 

reservoir has been superseded. 

The Mount St. Helens system generated (~1.2 ka) a small 

batch of rhyodacite (at 70% SiO2

, as silicic as any product 

known ever to have erupted there), but the numerous dacites 

and andesites that have subsequently erupted centrally indicate 

that the reservoir was since purged of any such highly evolved 

magma. No comparably unequivocal inference can be made 

about the Three Sisters magmatic system, from which several 

small rhyolites erupted during the interval 50-30 ka and again 

about 2 ka. Central eruptions of mafic, andesitic, and dacitic 

magmas from Middle and South Sisters during the 30 to 14 

ka interval (between the rhyolitic episodes) suggest that the 

earlier rhyolitic reservoir(s) had been purged. However, reestablishment of a rhyolitic reservoir beneath South Sister by 

the late Holocene and eruption of almost 1 km3 of phenocrystpoor rhyolitic magma in two (compositionally slightly different) batches about 2.2 and 2.0 ka provide a basis for concern; 

although pyroclastic phases of those eruptions were not regionally catastrophic, such modest behavior need not characterize 

the next one. 

Although seldom (if ever) caldera-forming in the Cascades (Gamma Ridge may be an exception), dacitic pyroclastic eruptions can nonetheless devastate edifice aprons and 

particular drainages, and major dacitic ashclouds can be a 

widespread environmental nuisance, a danger to aircraft, and 

can cause economic damage downwind. Dacitic eruptions, 

however, even those as voluminous as Shastina or as explosive 

as the Sugar Bowl (1.2 ka) and 1980 blasts from St. Helens, 

seldom directly threaten lives and infrastructure farther than 20 

km from vent. Andesitic eruptions pose still less of a regional 

threat directly, because the ashfall at 10 km downwind is seldom thicker than a few centimeters and because andesitic lava 

flows are typically slow-moving and valley-confined. For both 

andesitic and dacitic eruptions, of course, the main threat away 

from the edifice itself is from pyroclastic flows and, more 

so, from debris flows that can devastate valley floors for tens 

of kilometers downstream. Each of the major edifices from 

Meager to Hood, every one of them ice-clad, is capable of 

generating such valley-sweeping flows. Farther south, renewed 

eruptions of the Lassen domefield or summit eruptions of 

Shasta or South Sister can also be expected to produce radial 

flows, principally along existing drainages. A major eruption 

of Mount Mazama through its Crater Lake is capable of producing a wide range of mild through catastrophic phenomena 

(Bacon and others, 1997).

So irregular is the pace of magmatic processes that it 

would be rash, presumptuous, and misleading to forecast 

specific eruptions. It can be said, however, that there is no 

basis to expect that during the next millennium Mount St. 

Helens would cease to be by far the most frequently active 

Cascade volcano, as it has been for 4,000 years. Glacier 

Peak appears to have been the second most significantly 

active Cascade center during postglacial time, its explosive 

episodes recurring on a millennial timescale. The rest of the 

Garibaldi Volcanic Belt, from Mount Baker to Bridge River, 

erupts very infrequently, there being only four magmatic 

eruptions proven for the Holocene (table 7). Adams and 
78 Quaternary Magmatism in the Cascades—Geologic Perspectives

Rainier have each had at least a dozen postglacial eruptive 

episodes, apparently clustered into a few periods separated 

by thousands of years; Hood, South Sister, and Mazama 

have had still fewer (though more silicic and more devastating) postglacially active periods, and for none of these 

five volcanoes does it appear that such a pulse is currently 

ongoing. The same can be said for Mount Shasta, where a 

volumetrically enormous pulse about 10 ka was followed by 

sporadic summit-derived Holocene effusions (of uncertain 

age), none of which appear to have taken place during the 

last two millennia. On the other hand, although there have 

been few postglacial eruptive events in the Lassen segment 

(table 7), the two postglacial episodes inside the long-lived 

silicic domefield did take place in the last 1,100 years.

Attention is naturally drawn to the large polygenetic 

volcanoes owing to their relief, recurrent activity, potentially 

explosive behavior, and capacity for long-runout flows, but 

it needs reemphasis that the Quaternary Cascade arc has 100 

times more small volcanoes than big ones. Half or more of the 

postglacial eruptions (table 7) have been from monogenetic 

vents—distributed or peripheral cones, fissure-fed chains, or 

short-lived shields, some andesitic but most of them mafic. 

The 137 postglacial episodes listed in table 7 include only 8 

that were rhyolitic or rhyodacitic. Dacitic events from Mount 

St. Helens are likely to continue dominating Cascade volcanic activity numerically for the indefinite future, though rare 

explosive events at Meager, Garibaldi, Glacier Peak, Hood, 

South Sister, Shasta, and Lassen, and modest effusions of lava 

(with minor local ashfall but potentially devastating debris 

flows) at Adams, Baker, or Rainier are not to be discounted. 

The most likely and exemplary Cascadian eruptive event of the 

next millennium, however, would be growth of a scoria cone 

accompanied by mafic lava flows that block a road or river and 

tempt tourists to get too close.

Parental Magmas

Quaternary eruptive products in the Cascades include a 

vast variety of chemical compositions that range widely and 

continuously (at any SiO2 or MgO content) in alkalinity, Fe/

Mg, LILE/HFSE, and LREE/HREE. Isotope ratios likewise 

range considerably, though the absence of highly evolved 

values is in accord with expectation for the noncratonic 

setting; correlation between isotopic and chemical compositions is in general remarkably poor (see the section below 

entitled “Cascadian Basement and Sr-isotope Overview”). In 

addition to diverse mantle source compositions, much of the 

heterogeneity evidently results from polybaric crystal-liquid 

fractionation, contribution of varied crustal melts, and repeated 

magma mixing at different depths and scales. Intensive 

geochemical investigations have been undertaken by several 

groups attempting to identify distinctive types of relatively 

primitive (unevolved) eruptive products that represent distinguishably different magmatic lineages potentially parental to 

the heterogeneous arrays erupted (Hughes and Taylor, 1986; 

Hughes, 1990; Bacon, 1990; Clynne, 1990a; Bullen and 

Clynne, 1990; Leeman and others, 1990, 2005; Baker and others, 1994; Bacon and others, 1994, 1997; Conrey and others, 

1997, 2001b; Clynne and Borg, 1997; Borg and Clynne, 1998; 

Borg and others, 1997; 2000, 2002; Grove and others, 2002; 

Hart and others, 2002, 2003; Green and Sinha, 2005; Green, 

2006). A summary follows here of what appear to be primitive 

members of the main magmatic lineages.

High-Alumina Olivine Tholeiite (HAOT)

Also called low-K olivine tholeiite (LKOT), these fluid 

basalts build low shields, fissure-fed spatter ramparts, and 

tube-fed lava fields—some of great length. Major-element 

compositions are comparable to MORB, although Al2O3

contents (17-19%) are notably higher. The low-LILE traceelement compositions are likewise comparable, over a range 

from NMORB to EMORB, though most HAOT have modest 

relative enrichments in Ba, Sr, and Pb (± Th ± Cs), suggesting a slight slab contribution, however weak or indirect. 

Because such minor enrichments (relative to MORB) by 

a nominal subduction component also typify HAOT as far 

inboard of the Cascade arc as Idaho (Hart and others, 1984; 

Hart, 1985), it has been suggested that the component may 

come from the lithosphere, where it might have accumulated during Mesozoic or Tertiary arc magmatism. HAOT 

compositions are broadly similar but by no means uniform 

along the Quaternary Cascades; SiO2

 contents are in the 

range 47 to 50 percent, K2O contents 0.05 to 0.5 percent, 

and HAOT are generally richer in Ti and Fe in Washington 

than in California and southern Oregon. REE patterns are 

nearly flat (or even LREE-depleted), and the lack of HREE 

depletion has been widely interpreted as consistent with 

low-pressure final equilibration in garnet-free uppermost 

mantle. If the lack of garnet influence from the source residue were primary (and not established later during magma 

ascent through the uppermost mantle), then the onset of 

melting for HAOT could be inferred as shallower than ~75 

km. HAOT have been shown to be very poor in H2

O (Sisson 

and Layne, 1993), to have formed by 6 to 10 percent partial 

melting of nearly dry spinel lherzolite, and to have last 

equilibrated with the mantle at ~1,300oC and 11 to 15 kb, 

virtually at the base of the crust (Bartels and others, 1991; 

Baker and others, 1994; Elkins-Tanton and others, 2001). 

Rapid ascent from mantle depths is commonly inferred from 

high abundances of compatible elements and spinel inclusions in olivine that crystallized at pressures greater than 

10 kb (Clynne and Borg, 1997). It has been argued that the 

widespread MORB-type depleted-mantle source region of 

HAOT at least in part involves subcontinental lithosphere 

that had been secularly reenriched (notably in Al and Fe) by 

pyroxene addition (Clynne, 1993; Clynne and Borg, 1997), 

rendering the HAOT source more fertile (capable of basalt 

melt production) than source domains for most other Casca-
Parental Magmas 79

dian mafic magmas. Because HAOT are distributed almost 

entirely in areas of strong to weak lithospheric extension, 

along the Cascade arc from California to central Washington and inboard as far east as Idaho and Nevada, a general 

model of dry decompression melting of mantle domains 

little affected by any slab-derived flux is nicely consistent 

with a contribution by partial remelting of fertile mantle 

lithosphere during tectonic unloading.

Intraplate Basalt

Sometimes also referred to as within-plate basalt (WIP) 

or ocean-island basalt (OIB), primitive magmas of this type 

are LILE-enriched (relative to MORB and HAOT) but lack 

the Nb-Ta deficiency globally characteristic of the subduction component. For basalts on a continent, the term OIB 

seems misleading, not only for its malapropos marine insularity but owing to the diverse unresolved models for actual 

ocean-island basalts that variously postulate sources in deep 

mantle plumes, in convecting upper mantle, or in resurrected 

domains of old subducted lithosphere. For simplicity and 

clarity, therefore, I prefer the plain term “intraplate basalt,” 

recognizing that HAOT also erupts in intraplate settings as 

well as in the Cascade arc.

Beneath the Cascades and the adjacent extensional 

interior, sources of intraplate basalts are likely to lie in convecting mantle beneath the lithosphere, either as thick sheets 

or as discrete domains distributed within a MORB-source 

matrix, little affected in either case by percolation or streaming of the slab-derived flux. Some source materials may be 

drawn toward the arc by wedge-corner flow from mantle 

domains originally inboard of the arc; others may rise into 

the wedge through tears or around the edges of slab segments 

from sub-slab asthenoshere. Compared to MORB and HAOT, 

primitive intraplate basalts are enriched in LILE, HFSE, and 

REE (except HREE), but they range widely in incompatible element abundances. They include some hypersthenenormative compositions but most are nepheline-normative, 

the more alkalic types generally having higher Nb/Zr ratios 

and other such indicators of low degrees of partial melting, 

probably in the range 1 to 5 percent. Most intraplate basalts 

have in common low Sr/Ba, steep REE patterns, and modest (not high) Ba/Nb and other LILE/HFSE ratios. They 

are seldom as LILE-enriched and never as HFSE-depleted 

as the typical arc basalts marked by an ample subduction 

component. Intraplate basalts are dominant in the Simcoe 

Mountains and Bridge River-Salal Creek Volcanic Fields, 

common from Mount Adams across southern Washington to 

Portland, sparse along the Cascades of northern and central 

Oregon, but unknown in the Quaternary of southern Oregon 

and California. Although not erupted independently there, it 

has been proposed that they provide a contribution to high-K 

arc basalts in northern California (Clynne and Borg, 1997; 

Borg and others, 1997). The characteristic eruptive products 

of Cascadian intraplate basalts are scoria cones and associated lava fields of small to modest extent.

Arc Basalt

Commonly called calcalkaline basalt (CAB) or such 

varieties as low-K, medium-K, and high-K calkalkaline 

basalt, these magmas are the paradigmatic arc suite, enriched 

in LILE and LREE and depleted in HFSE and HREE. 

They form numerous shields, scoria cones, and lava fields 

throughout the Cascades and are the principal parent of many 

(not all) basaltic andesites. The wide geochemical range 

embraced by this class of basalts reflects the conjunction of 

two broad but continuous compositional spectra—degree of 

enrichment in slab-derived fluid contributions (the subduction component) and wide ranges of relative enrichment 

in mantle-derived (non-slab) incompatible-element abundances. The two spectra of enrichment are commonly linked 

inversely (Hickey and others, 1986; Luhr, 1992; Borg and 

others, 1997); that is, more intense slab flux enriches the 

mantle source in Cs, Rb, Ba, Sr, Pb, U, ±Th±K±LREE, and 

aqueous fluid, which promotes greater degrees of partial 

melting of the mantle-wedge peridotite, thereby lowering 

(by dilution) the contribution of HFSE and REE from the 

convecting mantle. If mantle melting is initiated deeper 

than ~75 km, residual garnet in the source region could also 

contribute to the characteristic depletion of arc basalts in 

HREE, Y, and Sc. Although arc basalts bearing the hallmarks 

of a slab contribution (Nb-Ta troughs, LILE enrichment, and 

steep REE patterns) are widespread throughout the Cascades, 

the intensity or effectiveness of the slab flux appears to 

diminish irregularly (relative to the wedge contribution) in at 

least three regions—(1) from west to east across the Lassen 

segment (Clynne and Borg, 1997; Borg and others, 1997); 

(2) from south to north along the GVB (Green and Sinha, 

2005; Green, 2006); and (3) within the broad volcanic zone 

from Portland across southern Washington, where numerous 

vents for arc and intraplate (and HAOT) basalts are scattered 

intimately (Leeman and others, 1990; 2005).

The term “high-alumina basalt” (HAB) should be abandoned, because it has been applied to at least two fundamentally different magmatic lineages, thus confusing generations 

of petrologists. It was widely applied to primitive (relatively 

dry) MORB-like HAOT in the Cascades and later to waterrich arc basalts globally that fractionate toward low-Mg 

high-Al basalt and basaltic andesite owing to delayed crystallization of plagioclase.

High-Mg Basaltic Andesite

Although most basaltic andesites evolve by fractionation 

and assimilation from basaltic parents, a few (noncumulative) 

Cascadian examples appear to be primitive, having maintained high enough Mg, Cr, and Ni abundances to have been 

in equilibrium with a mantle peridotite residue when the melt 

separated. These represent an extreme type of arc magma, 

enriched in the slab-derived LILE, strongly depleted in wedgederived HFSE and HREE, and particularly water-rich (Ander-
80 Quaternary Magmatism in the Cascades—Geologic Perspectives

son, 1974; Tatsumi, 1982; Baker and others 1994; Clynne and 

Borg, 1997; Grove and others, 2002). Not all high-Mg basaltic 

andesites (and andesites) are primitive, however; careful petrographic examination has shown at least one example from the 

Shasta segment (Anderson, 1974) to have resulted from basaltdacite mixing, augmented by ultramafic crystal debris (Streck 

and others, 2007).

Akin to Pacific arc boninites, primitive 52 to 58 percent SiO2

 magmas appear to reflect the conjunction of an 

intense supply of hydrous slab fluid and a notably refractory 

(earlier melt-depleted) mantle-wedge peridotitic source, the 

high fluid contribution being necessary to initiate and sustain 

a high degree of partial melting, yielding atypically silicic 

melt and a (clinopyroxene-exhausted) harzburgitic residue. 

Experimental investigations of such lavas from the Shasta 

segment (Baker and others, 1994) indeed indicate high melt 

fractions (20-30%) and elevated water contents in the melt (3 

to 6.5 wt% just prior to eruption). The scattered Cascadian 

examples of high-Mg basaltic andesite are accordingly largely 

in the forearc (Baker and others, 1994; Borg and others, 1997; 

Conrey and others, 1997; Leeman and others, 2005), where a 

strong flux of slab-derived fluid apparently induces melting of 

the most refractory part of the mantle wedge. The corner flow 

compelled by subduction may be accompanied by progressive 

east-to-west depletion (by incremental melt extraction) of the 

wedge peridotite dragged trenchward beneath the volcanic 

zone, rendering forearc domains the least fertile parts of the 

sublithospheric convecting mantle (Clynne and Borg, 1997; 

Grove and others, 2002).

Shoshonitic Arc Magma

Another uncommon but distinctive type of primitive 

arc magma, recognized principally in southern Washington 

and northern Oregon, belongs to the potassic shoshonite 

suite, mafic members of which (49-52% SiO2) are sometimes 

called “absarokites.” Primitive examples in the Cascades 

(Conrey and others, 1997; Leeman and others, 2005) have 

Mg-numbers ≥70, MgO contents of 8 to 9 percent, and Ni 

contents of 160 to 200 ppm, but they also have elevated 

K2

O (2-3%) and P2O5 (0.4-0.6%). More evolved shoshonitic 

basaltic andesites (54-59% SiO2) have been identified in 

southern Oregon (Bacon, 1990; Bacon and others, 1994). All 

recognized examples in the Cascades are lavas and scoria 

cones of small volume. The rocks have extreme slab signatures with great enrichments in LILE, especially in Ba and 

Sr (1,000-2,300 and 1,500-3,700 ppm, respectively), along 

with large LILE/HFSE ratios, deep Nb-Ta troughs, and steep 

REE patterns with very depleted HREE. They also have 

87Sr/86Sr ratios (0.7037-0.7039) among the most radiogenic in 

Oregon (Bacon and others, 1994; Conrey and others, 1997). 

The magmas are thought to be low-degree (<5 percent) 

partial melts of depleted-mantle peridotite that had, earlier 

and deeper, been infiltrated or veined by water-rich slabderived contributions that introduced phlogopite, amphibole, 

and pyroxene (± apatite ± garnet) to deep parts of the wedge 

(Carmichael and others, 1996; Luhr, 1997; Conrey and 

others, 1997; Hesse and Grove, 2003). When such shoshonitic melts ultimately separate from the peridotite matrix in 

the shallow mantle, they retain a more robust geochemical 

signature of the slab contribution than do any other magmas 

recognized in the Cascades, even when modified and diluted 

by reaction and melting of their envelope during upwelling 

through the hotter wedge core. It remains unclear whether 

the Quaternary examples are products of modern subductionzone infiltration and upwelling or of extension-induced 

remelting of veined lithosphere stagnant since episodes of 

Tertiary arc magmatism.

Rhyolite and Dacite

Some rhyolitic, rhyodacitic, and dacitic magmas may be 

produced by direct partial melting of crustal rocks, with or 

without subsequent crystal fractionation or mixing with mafic 

magmas that helped heat their source. In this sense, some silicic crustal melts can be at least as primitive (near-primary) as 

the basaltic melts that result from the conjunction of slab- and 

wedge-derived contributions and undergo prolonged melt-matrix reaction during many tens of kilometers of ascent. There 

is broad agreement that silicic crustal melts provide persistent, varied, and substantial contributions to intermediate arc 

magmas that range from basaltic andesite to dacite (Hildreth 

and Moorbath, 1988; Bullen and Clynne, 1990; Hill, 1991; 

Clynne, 1993; Smith and Leeman, 1987, 1993; Green, 1994b; 

Borg and Clynne, 1997; Conrey, Hooper, and others, 2001), 

but the petrogenetic challenge has always been to identify silicic eruptive products that approach near-primary crustal melts 

and to infer their source materials.

For the rhyolites at Three Sisters, Tumalo, and Broken 

Top, Hill (1991) showed geochemically that none could have 

evolved by crystal fractionation from the broadly contemporaneous dacitic or rhyodacitic magmas. He inferred that the 

Three Sisters rhyolites resulted from 20 to 30 percent dehydration melting of basaltic amphibolite and the Broken TopTumalo (slightly more sodic) rhyolites from 30 to 50 percent 

melting of Cascadian tonalite. Both rhyolitic suites have 

87Sr/86Sr in the range 0.7036 to 0.7037, favoring sources in 

Cenozoic Cascadian arc crust. Because both suites also have 

flat and relatively undepleted HREE patterns, residual garnet 

appears to be excluded, suggesting (Hill, 1991) that melt 

extraction took place in the middle crust rather than in the 

deepest arc crust, there as thick as 45 km. Conrey, Hooper, 

and others (2001) agreed that the fairly uniform suite of 

amphibole-bearing rhyodacites (87Sr/86Sr ~0.7032-0.7034) 

around Mount Jefferson could not have descended from 

any of the mafic or intermediate magmas erupted nearby, 

concluding instead that they were partial melts of mid-crustal 

basaltic amphibolite at 850 to 900°C. At Mount St. Helens, 

the varied dacites that have dominated its explosive eruptive 

history (mostly 62-66% SiO2; 87Sr/86Sr ~0.7035-0.7039) were 
Parental Magmas 81

shown by Smith and Leeman (1987) to have HFSE and REE 

contents similar to or lower than those of associated basalts 

and andesites, precluding a fractionation relationship. They 

inferred that there the primary crustal melt is dacitic, arising from partial melting of MORB-like amphibolite fluxed 

by LILE-bearing slab fluids, leaving residual amphibole, 

and providing the parent magma for rhyodacites by crystal 

fractionation and for St. Helens andesites by mixing with 

mafic magmas (Smith and Leeman, 1993). Indeed, partial 

melting experiments (Sisson and others, 2005) have verified 

that voluminous arc-type dacite, rhyodacite, and rhyolite 

melts can be generated from moderately hydrous medium-tohigh-K basaltic compositions at 825 to 925°C and 7 kb (midcrustal pressures), the melts becoming increasingly rhyolitic 

at higher oxygen fugacity or lower melt fraction.

Haystack, a phenocryst-poor dacite dome (64% SiO2) 

at the northwest toe of Mount Shasta, is unusual in having 

most of the compositional characteristics of “adakite.” These 

include high Sr (1,450 ppm), high Sr/Y (76), low K2

O/Na2O 

(0.2), and remarkably unevolved isotopic ratios—87Sr/86Sr = 

0.7029 and εNd = +7.4. This small dacite batch may have been 

a relatively simple partial melt of low-K MORB-like oceanic 

crust in the accretionary terranes beneath the Shasta segment.

In modeling geochemical evolution of the diverse regional 

suite of arc basalts and andesites in the Lassen segment, Clynne 

(1993) found that the crustal contributions are rhyolitic, added 

as melts (not bulk assimilants), and derived from young arc 

crust, not principally from the pre-Cenozoic basement. For the 

long-lived Lassen volcanic center (LVC), Bullen and Clynne 

(1990) concluded that geochemical trends of several successive 

suites each resulted from mixing between well-homogenized 

silicic melts and heterogeneous mafic magmas. They showed 

that the most silicic LVC products have Pb-Sr-Nd isotopic 

compositions within the ranges defined by the mafic magmas 

of both the LVC and regional suites, leading them to infer 

partial melting of young mafic lower crust and extraction of 

some batches of silicic melt cleanly enough to avoid mixing 

with contemporaneous mafic batches. Borg and Clynne (1998) 

expanded the investigation to rhyolitic and rhyodacitic products 

scattered all across the Lassen segment, concluding much the 

same about young mafic deep-crustal sources—apparently free 

of residual garnet here (on the basis of REE patterns) where the 

crust is as thin as 38 km. For generating the observed spectrum of silicic magmas (65-75% SiO2

), they inferred a range 

of partial melting conditions for deep-crustal basaltic amphibolite intruded by basaltic magmas of varied water content. 

Hotter drier melting exhausts residual amphibole, yielding 

pyroxene rhyodacites, whereas wetter melting at ≤900°C yields 

hb±bi±opx rhyolites marked by a MREE deficiency characteristic of residual amphibole. Very few silicic eruptive units 

were identified that show evidence (such as elevated Pb and 

O isotope ratios or steep REE patterns) for partial melting of 

upper-crustal granitoids, though modest contributions of such 

melts can be expected wherever mafic magmas are stored or 

transported shallowly (for example, Grove and others, 1988; 

Baker and others, 1991; Green, 1994b).

Sr-Rich Andesite

Another kind of intracrustal melt was advocated by 

Conrey, Hooper, and others (2001) to be near-primary, as its 

derivation has likewise proved impossible to model geochemically by crystal fractionation (or AFC) of any mafic 

Cascadian magma recognized. This Sr-rich andesite (58-62% 

SiO2, 800-1200 ppm Sr) has high Sr/Y and low HREE, Y, 

Ba/La, and 87Sr/86Sr (0.7028-0.7031). It too may arise by 

partial melting of mafic lower crust but under drier, hotter (~1,050°C), deeper conditions, leaving a garnet-bearing 

residue (Conrey, Hooper, and others, 2001). They further 

proposed that the MORB-like deep-crustal source domains 

may have underplated the arc crust during a pulse of HAOT 

magmatism that accompanied Pliocene extension of the High 

Cascades Graben (Smith and others, 1987; Conrey and others, 1997). Such rocks are scattered in the Mount Jefferson 

area (Conrey, Hooper, and others, 2001) and may also occur 

sparsely at Diamond Peak, near Mount Mazama, and in the 

Shasta and Lassen segments; the magma type is also thought 

to contribute as a mixing member to some basaltic andesites 

and to be related to coexisting high-Sr dacites. The thermal 

and material behavior of the deep crust in response to intrusion of basaltic magmas, under conditions leading to generation of rhyodacitic vis-à-vis andesitic partial melts, has been 

explored by Dufek and Bergantz (2005).

Inferences About Mantle Magmatism

Wherever a primitive mantle-derived magma erupts with 

little crustal contamination, it can be inferred that no intracrustal 

magma reservoir was present to impede its ascent. Because 

primitive basalts produce scoria cones, small shields, and 

discrete lava fields but virtually never erupt centrally at stratovolcanoes (or domefields), it is also presumptive that the latter 

sorts of systems do overlie crustal columns and reservoirs that 

intercept and process the mantle input.

The intimate spatial scale of vent locations for arc, intraplate, and shoshonitic basalts, HAOT, and primitive basaltic 

andesites—all recurrently within 5 km of each other and often 

very close to andesite-dacite stratocones as well—supports 

additional inferences about deep magmatism. The lithosphere 

is more widely a pincushion than a sponge. Some mantle 

magma batches that rise from independent source columns may, 

of course, merge, blend, stall, and fractionate, intensively so 

beneath stratovolcanoes, but many others penetrate the lithosphere independently without mutual interference. Arc magmas 

are often defined tectonically as evolving “above subducting 

lithosphere,” but in the Cascades (and elsewhere) they are 

closely accompanied by neighboring batches of HAOT and 

intraplate basalts that lack a geochemical subduction signature. 

It has sometimes been asserted that there is“no depleted mantle 

beneath continental arcs” (for example, Pearce and Peate, 1995, 

p. 274), but the widespread MORB-like HAOT in the Cascades 

demonstrates the opposite. There is still no consensus, however, 
82 Quaternary Magmatism in the Cascades—Geologic Perspectives

on the nature and distribution of the primary source regions for 

the HAOT and intraplate basalts, which are little affected by 

ascent of the slab-derived subduction component. Distributed 

domains (plums) within a depleted mantle-wedge matrix, layers 

of unfluxed convecting mantle drawn into the wedge-corner 

flow, deep mantle upwellings from beneath or inboard of a steep 

or segmented slab, and even the shallow mantle lithosphere have 

all been advocated as sources. Whatever the actual configuration, the following seem likely: (1) the slab-derived contribution 

does not permeate the entire wedge; (2) primary mantle source 

domains for the disparate primitive magmas, if distributed laterally, are separated by no more than a few kilometers; and (3) if 

sources are distributed vertically, some of the magma batches 

that each kind of source produces are able to pass upward 

through mantle domains of contrasting kind without loss of 

identity. It seems clear that the Cascade arc lacks a well-marked 

curtain of slab-fluid release governed by any major pressure-dependent downdip phase change, because (1) there is no abrupt, 

continuous, and voluminous volcanic front, (2) globally defined 

arc-type basalts are scattered across arc segments as wide as 50 

to 100 km, and (3) intraplate basalts and HAOT are likewise 

scattered from the rear-arc to the western (trenchward) edge of 

the Quaternary volcanic belt.

Volcanic fields dominated by monogenetic cones and small 

shields are sometimes attributed to decompression melting of 

ascending mantle diapirs that result in clustered ascent of many 

small magma batches. In the Cascades, much of the arc might 

be considered a field of such monogenetic vents, studded locally 

by a handful of large long-lived evolved centers. In any case, 

the belt of 2,000 lesser Cascade vents is generally continuous 

and elongate along the arc, as are many local vent concentrations and vent alignments within it, so the principal structural 

influence would appear to be along-arc lithospheric extensional 

tectonics rather than a belt of discretely focussed, intrinsically 

buoyant, mantle upwellings. 

Production of fundamentally contrasting types of basalt 

that erupt close together in space and time, whether as distributed vent fields or as arrays surrounding big andesite-dacite foci like Adams, Mazama, and Shasta, is difficult to attribute to large coherent diapiric upwellings, the more so owing 

to a lack of concentrically zonal compositional arrangements. 

Such diversity suggests, instead, coexistence of large- and 

small-degree melts from different depths with varied source 

materials and ascent paths, independently penetrating the arc 

crust in hundreds of small batches. The diversity of basalts 

reaching the surface also provides strong evidence against 

the existence of any regionally continuous intracrustal melt 

zone, an idea once boosted for the extensional parts of the 

Cascades on the basis of high and relatively uniform heat 

flow (see fig. 10 of Blackwell and others, 1990). 

Intracrustal Evolution of Arc Magmas

The mantle proposes; the crust disposes. Much (not all) 

of the variation among intermediate arc magmas arises within 

the crust, which frames a unique path for every batch, interposes traps for transient storage at many depths, contributes 

partial melts of its varied lithologies, and modulates the ascent 

of basalt and its derivatives. Basaltic magma impinges widely 

on the base of the Cascadian arc crust, but there are countless 

penetration pathways for some of its descendants to reach the 

surface. Fundamentally different basaltic parents can produce 

suites that converge intracrustally by mixing, partial remelting, 

and assimilation of common wallrocks, while identical basalts 

might produce divergent suites by polybaric fractionation and 

assimilation of contrasting wallrocks. There is perpetual tension between attenuation of heterogeneities (homogenization) 

by diffusion, convection, confluence of melts extracted from 

heterogeneous source domains, and blending of independent 

magmas in crustal reservoirs vis-à-vis diversification (differentiation) by injection of new basalt batches, crystal-melt fractionation, interactions with the heterogeneous crustal envelope, and 

upward escape of melt batches extracted from mush at successive transient magma traps. The simple proposition that most 

intermediate arc magmas are produced by varied combinations 

of crustal assimilation and fractional crystallization of a range 

of primitive basalts has outlived a number of alternative passing 

fancies: (1) that andesites are primary hydrous melts of mantle 

peridotite; (2) that arc magmas are slab melts; (3) that low-Mg 

high-Al basalts are a principal primary arc magma; and (4) that 

MORB-like HAOT is a universal arc parent.

MASH Model

A chemical and isotopic study of the 15 northernmost 

stratovolcanoes along the volcanic front of the Andean Southern Volcanic Zone (SVZ) revealed large crustal contributions 

for all volcanoes along the transect. There was shown to be 

a strong northward along-arc trend toward greater, deeper, 

and older crustal contributions, consistent with the regional 

gradients in upper-plate geology and a doubling of crustal 

thickness. The data led to formulation of the melting-assimilation-storage-homogenization (MASH) model of Hildreth 

and Moorbath (1988, 1991), wherein the baseline geochemical signature of each large arc volcano is set during protracted 

entrapment, storage, and reprocessing of mantle-derived 

magma within a focussed domain of remelting and hybridism 

in the deep crust. Cursory reading has led some to reduce the 

model to little more than lower-crustal assimilation, but the 

discussion (Hildreth and Moorbath, 1988, p. 480-485) is far 

more specific. Salient points include the following. (1) In addition to partial melting of older crustal protoliths, the MASH 

process entails extensive partial remelting of deep-crustal 

mafic intrusions and their differentiates, thermally induced by 

renewed pulses of basaltic intrusion and crystallization. (2) 

A MASH zone is not a magma chamber but, rather, a plexus 

of tabular and mushy differentiated intrusions, where ductile 

deformation promotes extraction, aggregation, and blending of 

melts. (3) MASH zones wax and wane in response to basaltic 

influx and to ascent of aggregated hybrids. (4) Each major 
Intracrustal Evolution of Arc Magmas 83

center has its own focal MASH zone, usually impenetrable by 

primitive basalts, whereas, peripheral to such foci, more primitive batches (not intercepted and hybridized) can ascend to 

produce monogenetic volcanoes. (5) Crustal thickness can be 

important by imparting to magmas the geochemical signature 

of residual garnet or amphibole and by increasing intracrustal 

path length, but age and composition of heterogeneous crustal 

lithologies are of comparable importance. (6) MASH zones 

intermittently feed hybrid melt to upper-crustal reservoirs, 

and some mobilize diapirically and segregate mush and melt 

to produce differentiated mid-crustal plutons. Assimilation in 

dikes or in subsequent mid-to upper-crustal reservoirs enriches 

magmas above MASH baseline ranges, enhancing diversity. 

Hildreth and Moorbath (1988; their fig. 6) dealt specifically 

with instances of important additional crustal contributions to 

ascending magmas, subsequent to the deep-crustal MASH process. And (7) in mature arcs, much of the arc-geochemical signature is acquired not only from proximate slab contributions 

but by repeated assimilative withdrawals from the secularly 

accumulated, deep-crustal, “arc-intrusive repository” (which is 

of far greater mass than the magmatic throughput). 

Persistent focussing at each MASH domain promotes 

thermal and mechanical feedback between entrapment of 

basalt, enhancement of lower-crustal ductility and melting, and 

maintenance of a buoyancy barrier. Such long-lived focussing 

is especially intense beneath large arc volcanoes but likewise 

beneath large intracontinental centers and at the deep-crustal 

staging reservoirs for flood basalts (many of which evolve 

in expanding well-mixed reservoirs to compositions well 

beyond the normal basaltic range). Crustal contributions to 

arc magmas are hard to quantify, owing in part to wide ranges 

in the proportions of slab and mantle contributions and in 

part to poor isotopic and chemical leverage provided by the 

young mafic lower crust progressively accreting. Nd-Sr-Pb-O 

isotopic contrast between mantle-derived magma and deep arc 

crust can actually decrease with time, as the average age of the 

lower crust becomes progressively younger, diluted by intrusive underplating by the arc basalt itself. A big change along 

an arc in mean age or composition of its basement is thus 

required to yield unambiguous evidence for large contributions 

from reprocessed lower crust, and such a change along the 

Chilean volcanic front was the basis of the transect experiment 

that led to the MASH model. Os-isotope data later sustained 

application of the MASH model to volcanic centers on thinner 

and younger mafic crust (in both Chile and the Cascades), 

permitting inference of a range of deep-crustal contributions 

that provided radiogenic 187Os/188Os ratios as much as five 

times greater than mantle values (Ruiz and others, 2000; Hart 

and others, 2003).

The apparent paradox that basalts are uncommon at 

the large Cascade stratovolcanoes but abundant and widely 

dispersed around and between them is readily understood if 

MASH zones are discrete domains, not a continuous deepcrustal underplate. Because small and diverse mafic volcanoes 

are so densely distributed throughout much of the Cascade 

arc, however, the process that really needs clarification is the 

focusing mechanism for MASH zones beneath the major centers. The 2,000-odd mafic volcanoes show that basalt impinges 

on the base of the crust almost everywhere beneath the Cascade arc but with intensity sufficient to promote large-scale 

partial melting of crustal rocks at only a few dozen scattered 

loci. The contrasting heat and mass balance demands of the 

adjoining, contemporaneous, and areally comparable Lassen and Caribou loci (fig. 21) were modeled by Guffanti and 

others (1996). They calculated that the flux of primitive basalt 

injecting and fractionally crystallizing in the lower crust was 

at least five times greater beneath the Lassen volcanic center 

(where extensive crustal melting contributes heavily to silicic 

and hybrid magmatism) than beneath the persistently mafic 

Caribou field (where more limited crustal melting is sufficient 

only to raise the average volcanic output into the compositional range of basaltic andesite).

As a first-order question, then, does such locally intense 

focusing ultimately reflect lower-plate processes, patterns of 

convection and magma aggregation in the mantle wedge, or 

the topology of magma entrapment in the lower crust? Arcmagma dynamics probably involves a series of batch processes. 

Although slab subduction and wedge corner convection may be 

virtually continuous, fluid or melt is likely to ascend batchwise 

from the interfacial region into the hot core of the wedge. Even 

if porous magma percolation is widespread in partially molten 

columns within the wedge, the aggregation, ascent, and arrival 

of large volumes of melt at the base of the crust is probably 

likewise ultimately a batch process, whether partly diapiric or 

by channelized flow. Crustal magma transport is even more 

certainly a batch process, too, as indicated by the sporadic 

nature of eruptive phenomena and the irregular compositional 

fluctuations at long-lived arc volcanoes. In fact, the mismatch 

between typical timescales of eruptive recurrence (102

 to 104

years) at stratovolcanoes and the documented longevities (105

to 106 years) for such major fixed centers weighs against the 

likelihood of MASH-domain focusing being fundamentally 

controlled by the properties or behavior of the crust, which 

appears to process magma throughput much more rapidly 

than the lifetime of the system. Such longevities are likewise 

difficult to reconcile with batch release of material from the 

subduction interface, which should be far more frequent as 

well as spatially more widespread than the sparsely distributed 

arc stratovolcanoes. It thus seems most plausible that MASH 

zones are fundamentally focussed by excess melt production in 

particular domains of the mantle wedge, speculatively where 

permeability and melt aggregation are enhanced by mantle 

upwelling or other forms of convective strain. 

After an exceptional mass of basaltic melt has locally 

intruded the base of the crust, the modified topology of the 

interface may itself induce changes in the buoyancy, strain 

field, and permeability of the system, promoting positive 

feedback that stabilizes the anomaly and promotes its growth. 

The great volumes of basalt erupted at a few of the largest 

arc volcanoes (such as Fuji and Klyuchevskoi) apparently 

represent extreme cases where throughput is rapid and MASHzone processing only modest; such examples provide further 
84 Quaternary Magmatism in the Cascades—Geologic Perspectives

evidence that localized concentration of mantle-derived melt 

is the key process in focusing large arc systems. From the 

distribution patterns of surface volcanism, it can be inferred 

that, in plan view, the intensely injected loci that develop 

MASH zones are typically 10 to 30 km across. That the major 

volcanic centers they feed are irregularly separated by 5 to 150 

km (table 4) probably mirrors spacing of the MASH domains 

themselves, which in turn is likely to reflect the convective 

upwelling structure of the mantle wedge.

Shifts of Focus

Numerous arc stratovolcanoes have died out, only to be 

promptly succeeded by an active neighbor, overlapping or 

nearby. Well-known pairs include Ixtaccihuatl/Popocatépetl 

and Nevado de Colima/Volcán de Colima in Mexico, and in 

the Andes, Sierra Velluda/Antuco, La Picada/Osorno, Pomerape/Parinacota, and San Pablo/San Pedro. Figure 26 illustrates 

some examples in the Cascades, several of which involve silicic domefields or calderas as well as stratocones. Seldom are 

such shifts of focus plausibly related to plate motions, either 

in terms of azimuth or timescale and distance of the jump, as 

seems clear for the inconsistent vectors of figure 26. The shifts 

are typically 5 to 20 km with no systematic directionality and, 

in most instances, they took place on timescales commensurate with normal longevities of the component volcanoes (fig. 

27). I thus infer that some change or instability in mantle or 

crustal elements of the magma system itself caused it not to 

die out but to relocate.

The shifts of individual volcanic foci illustrated in figure 

26 appear to have little or nothing to do with the migration of 

whole segments of the arc on longer timescales. For example, 

the westward jump from the Oligocene-Miocene PembertonChilliwack arc to the Quaternary Garibaldi Volcanic Belt (fig. 

3) and, since the late Miocene, the eastward drift of the Oregon segment and the westward drift of the Lassen segment, are 

all likely to reflect regional-scale plate interactions, subduction 

geometry, and upper-plate internal microplate motions. 

If the large, long-lived, evolved systems are individually rooted in deep-crustal MASH domains, the question 

of what allows some of them to jump is closely linked to 

the question of what pins most of them at fixed locations 

for million-year lifetimes. On the one hand, some lateral 

shifts of focus might simply represent relocation of a shallow crustal reservoir owing to magmatic exploitation of an 

intracrustal structural discontinuity. At the other extreme, a 

volcanic shift might mirror a modest lateral shift in the locus 

of mantle upwelling and thus in the site of intensified basaltic injection of the lower crust. A third possibility is successive buoyant protrusions from different parts of a common, 

thermally fluctuating, MASH zone.

If a MASH domain deep beneath a major volcanic center 

is a mushy to ductile, polyintrusive plexus of amphibolite, 

gabbro, diorite, tonalite, and protolith restites, all laced with 

leucosomes, cumulates, ephemeral magma pods, and mafic and 

felsic dikes, and if some such domains attain lateral diameters 

of 10 to 30 km, then, during thermally prograde intervals, 

discrete parts might blossom as buoyant domical protrusions 

(protodiapirs of Hopson and Mattinson, 1994). Locally intensified pulses of basaltic injection or pulses of ductile deformation 

might help destabilize particular regions within the broader 

mushy plexus, and secular melt depletion in long-active parts 

of the MASH domain might eventually favor shifting the buoyancy to adjacent regions. Successive mobilizations of different 

parts of the deep-crustal plexus would offset the staging area 

for the dikes that ascend to feed upper-crustal reservoirs, perhaps by the 5 to 20 km typical of the jumps in eruptive focus 

shown in figure 26. There is no conceptual conflict between 

buoyant ductile doming of a deep-crustal migmatized MASH 

zone and separation from it of a vanguard of dikes of hybrid 

magma that helps lower the viscosity of the deep crust directly 

above it while ultimately supplying and expanding the uppercrustal magma reservoir that feeds a volcano.

If major evolved volcanic centers can shift positions, or 

wholly new centers can arise, on timescales of 105

-106 years, 

then it might be asked how long it takes for all reaches along 

an arc to have been occupied by such edifices (and the shallow plutonic reservoirs supporting them). As suggested by the 

continuous composite batholiths exposed in Mesozoic arcs and 

by the 50-to-100-km-long plutonic reaches of Tertiary arcs 

exposed in the Washington Cascades (Chilliwack batholith and 

Snoqualmie-Index-Squire Creek batholiths), some magmatic 

arc chains do completely “fill in,” at least on 10 to 30 Myr 

timescales. The tentative answer from the volcanic Cascades 

appears to be more than 5 Myr, which is long enough to introduce the complication of lateral drift of whole arc segments. 

The axial portion of the Lassen segment could be said to have 

filled in between the Yana and Snow Mountain volcanic centers 

(fig. 21) over an interval of about 4 Myr, but no remnants 

of andesitic (or more silicic) centers younger than Miocene 

are recognized between the Lassen and Shasta segments, 

nor between Goosenest and the Mountain Lakes cluster, nor 

between Hood and Adams, nor between Rainier and Glacier 

Peak, nor between any of the five GVB loci. On the other hand, 

closely spaced mafic volcanoes have built a continuous strip 

more than 500 km long from southern Oregon to east of Mount 

Rainier on a timescale shorter than 1 Myr (figs. 8, 14, 15).

Basaltic Andesite

Several compositional varieties of basaltic andesite have 

been distinguished in the Cascades, as discussed by Hughes 

and Taylor (1986), Hughes (1990), Clynne (1990a), Bullen 

and Clynne (1990), Bacon (1990), Gardner (1994), and Conrey (1998). Although a few dozen of the thousand-odd basaltic 

andesite (52-57% SiO2

) cones and shields in the Cascades may 

represent fairly primitive descendants of primary melts of the 

mantle or the crust, the great majority are thought to evolve 

intracrustally from basaltic parents by varied combinations of 

crystal fractionation, remobilization of cumulates, deep-to-
Intracrustal Evolution of Arc Magmas 85

25 km

Glacier 

Peak 

0.6-0 Ma 

0.1-0 Ma 

1.0 Ma 

1.1 Ma 

2.8 Ma 

3.7 Ma 

0.45-0 Ma 

0.52-0.45 Ma 

0.65-0.25 Ma 

0.1-0 Ma 

0.5-0.3 Ma 

2-1.6 Ma 

0.5-0.4 Ma 

0.6-0 Ma

~1 Ma

<0.8 Ma

2.5-1 Ma

2-0.5 Ma 

2.5-1.5 Ma

3-2 Ma

3.2 Ma 

0.4-0 Ma 

0.42 Ma 

0.3-0.1 Ma 

<0.18 Ma 

0.4-0.2 Ma 

Mt Mazama 

Gamma Ridge 

Three 

Broken Top 

Mt 

Scott 

Pre-Mazama 

SHIFTS of FOCUS

Goat Rocks 

Devils Horns 

Caldera 

Caldera 

Maidu

Shasta Lassen

Dittmar

Yana

Rhyodacites 

Meager Meager 

Mount 

complex 

Plinth 

Pylon Peak 

Kulshan Caldera 

Mt Baker 

Black Buttes 

Lake Ann 

Hannegan Caldera 

Sisters N 

M 

S 

Stock 

Rainbow Mtn

Tumalo 

Mt Adams 

Volcanic Field 

Hellroaring 

Volcano 

Figure 26. Shifts of eruptive focus. Examples from nine areas in the Cascade arc where the principal site of eruption of evolved 

(andesite to rhyolite) magma migrated substantially during the Quaternary. All sketch maps at same scale. Red arrows generalize 

azimuths and distances of shifts. References are cited in text discussions of each center.
86 Quaternary Magmatism in the Cascades—Geologic Perspectives

Figure 27. Example of a late Pleistocene shift of eruptive focus: Black Buttes stratovolcano (500-290 ka) adjacent to southwest side of 

ice-clad Mount Baker (40-12 ka), viewed northeastward from the air. For Black Buttes, andesitic lavas and breccias of the two high crags, 

Lincoln Peak (left, 2,770 m) and Colfax Peak (right; 2,850 m) display opposing 30° dips that frame the gutted edifice. Extending below Lincoln 

Peak, an arcuate rugged ridge of outward-dipping flow-breccia sheets wraps the glacier-filled bowl where erosion has removed the 

hydrothermally altered fragmental core of the volcano. Shift of the central vent 3 km northeastward to the site of Mount Baker may have 

taken place by 100 ka, but the young cone (3,286 m) has grown largely since about 40 ka. Construction was complete by about 12 ka, and 

the only Holocene eruption of the cone was an andesitic ashfall about 6.5 ka. Summit crater is ice-filled; right shoulder below summit is rim 

of Sherman Crater, a lateral hydrothermal vent still fumarolically active. For details, see Hildreth and others (2003). Black Buttes edifice had 

twice or more the eruptive volume of Mount Baker. Relief visible in image is about 2,000 m. (USGS photograph by Austin Post, 1999.)

shallow crustal assimilation, backmixing of related batches 

of evolving magma, and cross-mixing of unrelated batches. 

(On the scale of an eruptive batch, there is no such thing as a 

“pure magma” as even most HAOT or rhyolitic magmas blend 

parental contributions and subsequently ingest distinguishable 

contaminants). Much of the variability among the mafic andesites is likely to reflect batch size, thermal budget, water content, and depths of transient storage, as well as compositions 

of primitive magmas and compositions of wall rocks partially 

assimilated. Critical influences during storage and ascent, 

sometimes compositionally detectable in the eruptive products, 

include the stability of garnet, plagioclase, and amphibole 

(each sensitive to pressure and water content of a magma reservoir) and the wallrock envelope, which might change during 

ascent successively from dry mafic granulite to amphibolite 

to tonalite-granodiorite. Great thicknesses of arc basalt and 

HAOT underplated during the Tertiary as gabbro and its differentiates, and their dehydrated or hydrated metamorphosed 

successors, may provide major Quaternary assimilants in the 

deep Cascade crust. 

The mechanisms of crustal assimilation by mafic magmas 

are no doubt varied and probably include all of the following: (1) Ductile kneading of partial melts from polylithologic 

deep-crustal wall rocks; (2) filter-pressing of interstitial melts 

by volume expansion during partial melting of (water-undersaturated) wall rocks at any depth; (3) melting and disaggregation of fragments stoped from the roofs of reservoirs or torn 

from the walls of dikes, thereby contributing both melts and 

xenocrysts; (4) fractional assimilation of melts and antecrysts 

from mushy mafic plutonic forerunners in the deep crust; (5) 

fractional assimilation of melts and antecrysts from felsic 

plutonic forerunners in the mid and shallow crust; and (6) 

reentrainment of cumulate crystals and associated melts at any 

depth during multibatch magma passage through long-lived 

mushy ascent columns (Philpotts and Asher, 1993; Green, 

1994b, Nakada and others, 1994; Dungan and Davidson, 2004; 
Intracrustal Evolution of Arc Magmas 87

Bacon and Lowenstern, 2005). Magma ascent through the 

40-km-thick Cascade crust almost inevitably incurs such interactions. Because of preferential incorporation of incompatible 

elements during multistage assimilation, it would be rare for 

an erupting mafic magma to retain the trace-element ratios or 

concentrations of its mantle-derived parent.

Because most arc basalts and basaltic andesites have 

only 4 to 7 percent MgO, it is commonly unclear what their 

primitive mantle-derived parents had been. Zoned mafic 

eruptions occasionally provide a clearer view; for example, 

the arc eruptions of Tolbachik 1975-76 in Kamchatka (10.5 

to 4% MgO), Jorullo 1759-74 in Mexico (9.3 to 4.3% MgO), 

and Galunggung 1982-83 in Java (11.5 to 3.2% MgO) were 

all zoned to low-Al high-Mg arc basalt as their most primitive 

product (Fedotov and Markhinin, 1983; Luhr and Carmichael, 

1985; Gerbe and others, 1992). In the Cascades, however, 

zoned basaltic-andesite eruptive units have been shown to 

grade down to either arc basalt or HAOT. Examples of the 

former include the Callahan flow at Medicine Lake, parts of 

the Bachelor chain, and the Sulphur Creek flow near Mount 

Baker (Kinzler and others, 2000; Gardner, 1994; Green, 1988; 

Hildreth and others, 2003). The best documented example of 

zonation involving a HAOT end member is the Giant Crater 

lava field (6.6 to 10.5% MgO; 53.4 to 47.7% SiO2

; 1.1 to 

0.06% K2O), also at Medicine Lake (Donnelly-Nolan and others, 1991; Baker and others, 1991).

Investigations of mafic lavas in the Cascades additionally 

support the following inferences. (1) Virtual absence of mantle 

xenoliths in the arc basalts and basaltic andesites (and HAOT) 

suggests magma collection, transient storage, and settling out 

of the crystal cargo in crustal reservoirs. In contrast, many 

rear-arc alkalic basalts of the Simcoe Mountains Volcanic 

Field (some with as little as 6 percent MgO) carry abundant 

mantle xenocrysts and nodules of lherzolite, harzburgite, and 

dunite, from which more continuous passage through the crust 

can be inferred. (2) Mafic shields in the Cascades probably 

remain active for something between 10 and 104 years, and 

most have volumes of 1 to 5 km3 (rarely to 15 km3 or more). 

Such longevities and volumes may suggest characteristic limits on the sizes of batches departing the mantle and fractionally 

crystallizing within the crust. (3) Most shields are constructed 

of mafic andesite and nonprimitive basalt, and although their 

lavas are seldom strictly homogeneous, they usually (not 

always) lack evidence for an upper-crustal contribution, leading to the inference that their proximate reservoirs typically 

lie in deep mafic crust. Clear exceptions marked by granitoid 

assimilants are well documented among the mafic products in 

the rear-arc Medicine Lake Volcanic Field (Grove and others, 

1988; Donnelly-Nolan and others, 1991; Baker and others, 

1991; Kinzler and others, 2000). And (4) although there are 

some fairly homogeneous mafic shields where one might infer 

rapid tapping of a single reservoir, there may in general be too 

much conceptual emphasis on “chambers” for holding mafic 

magmas in the crust. Widespread evidence for heterogeneity 

and mixing suggests that the crustal columns supplying mafic 

shields on millennial timescales may often consist of swarms 

of dikes (with a high combined surface area exposed to wallrock assimilation), as well as numerous transient magma pods 

that experience quasi-independent fractionation/assimilation 

histories. The basalts and basaltic andesites of the Bachelor 

chain, where Gardner (1994) distinguished ten different chemical groups (49 to 57% SiO2

, 8 to 3% MgO, 170 to 20 ppm Ni; 

0.5 to 1.25% K2O), erupted from vents along or adjacent to the 

same alignment within an episode only a few thousand years 

long, providing a well-documented illustration of the effects of 

such plumbing complexity.

Andesite and Dacite

Many silicic andesites (57-63% SiO2) evolved by rapid 

crystallization differentiation when their mafic parents reached 

water saturation in the upper crust, but most apparently also 

reflect continuation of hybridizing processes, having still more 

complicated downstream mixing histories. For every intermediate eruptive unit, basic questions concern the depths and 

wall-rock lithologies of preeruptive storage in single or successive intracrustal reservoirs, the mechanisms by which crustal 

contributions are added, and the extent to which the crystal 

cargo of numerous successive batches are mixed, partially 

resorbed, or re-entrained. One should beware the shoehorn of 

“end-member” oversimplification, because identifying coherent AFC trends (which may be occasionally feasible for a 

compact upper-crustal chamber with one dominant wall rock) 

is unrealistic for an intermediate magma batch traversing a 

40-km column of varied crustal lithologies, distributed pods of 

cumulates, and mushy differentiates of forerunning batches.

Tonalitic-to-dioritic plutons are the leftovers of andesitic 

magma bodies and demonstrate the reality of intermediate 

magma storage in the mid-to-upper crust. Granodiorite plutons 

likewise are evidence for shallow storage of dacitic magma. 

Because such granitoids usually lose interstitial silicic melt 

(to dikes, leucogranitoid bodies, and eruptions) during mushy 

stages of advancing crystallization, they end up somewhat less 

silicic than their former integrated bulk compositions. The 

relatively uncommon batches of crystal-poor silicic andesite 

sometimes erupted are likely to have been extracts of melt 

from comparably mushy bodies of crystallizing mafic magma. 

Although intermediate plutons can grow large by 

incremental aggregation of numerous batches of water-rich 

andesitic magma (which crystallize extensively when watersaturating in the upper crust), the dike or lens of eruptible 

magma at any stage is likely always to be small relative to the 

accreting mush body. Andesitic eruptive units are typically 

modest in volume, crystal-rich, and carry several generations 

of antecrysts and phenocrysts. That so many of them also 

carry chilled enclaves of more mafic magma suggests that 

their preeruptive holding cells within the larger crystallizing 

reservoir (pluton) had been fairly small when invaded by the 

enclave-generating batch, thus permitting convective dispersal 

of the disaggregated blobs throughout the cell and eruptive 

response to the invasion on a timescale faster than settling of 
88 Quaternary Magmatism in the Cascades—Geologic Perspectives

the blobs. Because rhyolites and rhyodacites typically carry 

few such enclaves (or lack them entirely), the preeruptive 

melt-rich lenses from which they erupt are likely to be well 

buffered by thicker mush reservoirs that protect them from 

such invasion. The equivalent storage cells (layers, lenses) 

from which dacite magma batches erupt would, following 

this logic, have crystallizing envelopes with a wider range of 

buffering capacity, since dacite lavas range from enclave-rich 

(as at Lassen) to aphyric and enclave-free (as at Adams). Great 

eruptions of flood rhyolite and flood basalt are widely documented, so the lack of flood andesites is yet another indication 

of the intimate nature of andesitic hybridism in complexly 

intricate plumbing systems.

If andesites were generally straightforward fractionation 

daughters of mafic magmas, it would seem a striking anomaly 

that so few of the numerous mafic shields in the Cascades 

grade into andesite. The few known to do so are Mount Defiance, Diamond Peak, Mount Bailey, Devils Peak, and Prospect 

Peak. At least 90 percent of the Quaternary andesite in the 

Cascades was erupted from 22 major evolved centers (Meager, Cayley, Garibaldi Lake, Garibaldi, Baker, Glacier Peak, 

Rainier, Goat Rocks, Adams, St. Helens, Hood, Jefferson, 

Middle Sister, South Sister, Broken Top, Cappy, Mazama, 

Rainbow, Shasta, Maidu, Dittmar, and Lassen). The remaining 

10 percent resides in nine substantial shields (Jennies Butte in 

Washington; Badger Butte, Timber Crater, Pelican Butte, and 

Brown Mountain in Oregon; Goosenest, West Prospect Peak, 

Badger Mountain, and Table Mountain in California) and in 

about 75 minor off-edifice units that include small shields, 

domes, cones, chains of cones, lava fields, and isolated flows. 

The 75 minor units of silicic andesite (57-63% SiO2) include 

eight north of Goat Rocks, five in southern Washington, 15 

scattered from Frog Lakes Buttes to the area around Summit Butte, five near Santiam and McKenzie Passes, six near 

Wickiup Reservoir, six south of Mazama, four in the Mountain 

Lakes cluster, six near Whaleback and Garner Mountain, and 

as many as 30 in the Lassen segment. It may be worth stressing the point again that in this reputedly “andesitic arc,” fewer 

than 10 percent of the 2,000-odd Cascade vents have erupted 

andesite. That most of the andesite is concentrated at only 22 

loci is taken to indicate that deep-crustal MASH processes are 

focussed in discrete domains. 

Many ordinary Cascade dacites are simply andesites 

further evolved, whereas the somewhat less common class 

of phenocryst-poor dacites is likely to have been interstitial 

melt that escaped from mafic-to-andesitic crystal mush. On 

the other hand, some crystal-rich dacites are mixtures of 

rhyodacite with various mafic-to-intermediate magmas, as in 

the Lassen domefield (Clynne, 1990a, 1999), and a few may 

be near-primary partial melts of mafic crust, as at Mount St. 

Helens (Smith and Leeman, 1987). At least 90 percent of the 

Quaternary dacite (63-68% SiO2

) in the Cascades erupted 

from the same 22 major volcanic centers that account for 

most of the silicic andesite. This is no surprise if most dacites 

descended from andesites that had been either fractionating 

parents or mixing members (with rhyodacite). If phenocrystpoor dacites represent interstitial melt extracted from crystal 

mush, the proximate parent may commonly be somewhat more 

mafic than silicic andesite, accounting for the not-uncommon 

bimodal arc association of dacite with basaltic andesite.

Dacite appears to be the volumetrically dominant product 

at Meager, Cayley, Garibaldi, Glacier Peak, St. Helens, and 

probably the Lassen volcanic center, where both precaldera 

and postcaldera dacites are especially numerous. Dacite is 

also a major constituent at Hood, Jefferson, and Shasta and a 

subordinate one at the remainder of the 22 main intermediate 

centers. Off-edifice domes and flows of dacite (not derived 

from vents within a major center) number about 35, fewer 

than half the number (~75) of comparably distributed andesitic 

units. These 35 units make up less than 10 percent of the total 

volume of Quaternary Cascade dacite; they include the Watts 

Point remnant in Canada, and Spiral Butte and the Snowden 

flow (which may be Pliocene) in southern Washington. In Oregon, there are at least 24 off-edifice dacites—a dozen separate 

domes and flows peripheral to Mount Jefferson, flows along 

First Creek (southeast of Three Fingered Jack), the Todd Lake 

Volcano and Kokostick Butte south of South Sister, a fraction 

of the zoned Shevlin Park Tuff in the Tumalo Volcanic Field, 

three small domes near Wickiup Reservoir, Mule Peak lavas 

(south of Cappy Mountain), and three pre-Mazama units just 

southeast of the Mazama edifice. Dacites in California include 

seven off-edifice domes west of Mount Shasta, the huge 

composite dome of Burney Mountain, lavas of Huckleberry 

Mountain (20 km northwest of Lassen Peak), and the Barkley 

Mountain chain (40 km southwest of it; fig. 21). 

What accounts for the great volume excess of andesite 

and dacite at just a few centers along the arc may best be 

understood in terms of discrete long-lived MASH domains 

at the base of the crust. Longevities of the big intermediate 

centers are typically at least two orders of magnitude greater 

than those of the mafic shields that dominate eruptive output 

along the reaches between them. Scrutiny of tables 2 and 4 

reveals no correlation between andesite-dacite edifice volume 

and spacing, so it seems clear that here (as in other arcs) there 

is no organized or periodic system of along-arc intracrustal 

upwelling instabilities. Because so many, small and varied, 

mafic volcanoes closely neighbor the major intermediate centers (table 1; figs. 3, 8, 14, 15, 21), it is also unlikely that an 

attractive process is involved, whereby some mantle columns 

suck in melt from a wide periphery. Instead, because mafic 

vent density is often elevated in the immediate periphery of 

the large intermediate centers (even in the GVB; fig. 3), one 

can infer the opposite—that the spatial density and intensity 

of independent basaltic magma batches reaching the base of 

the crust increase laterally toward the major foci that underlie 

MASH zones and the long-lived hybrid-intermediate volcanic 

centers they support. As for the productive mantle columns 

that power the foci, there is no evidence in the Cascades for 

any systematic spatial distribution that would suggest a regionally organized mantle structure of major convective upwellings or focussed melt channelization. Within the broad belt of 

subduction-influenced Cascade volcanism, 1,250 km long and 
Intracrustal Evolution of Arc Magmas 89

locally as wide as 100 km, the major (and minor) andesitedacite centers are scattered, with no systematic pattern to their 

mutual relative locations.

At risk of over-generalizing, many basalts and basaltic 

andesites are fairly phenocryst poor because relatively unimpeded ascent from the lower crust means that much of their 

preeruptive crystallization history took place in deep reservoirs 

where low melt viscosity allowed rapid removal of crystals. 

Arc andesites are typically rich enough in water that they 

exsolve an aqueous gas phase on ascent to the upper crust, 

which then compels extensive crystallization. Most andesites 

erupt rich in crystals because higher viscosity retards crystal 

removal and multi-batch recharge and mixing keeps crystals 

in circulation as well as sweeping up those of forerunning 

batches. Some andesite batches less disturbed by recharge may 

zone to dacite by gradual crystallization, while others that crystallize extensively may expel dacitic or even rhyodacitic melt 

from crystal mush. The widespread phenocryst-poor dacites are 

more likely to be the latter, whereas the phenocryst-rich dacites 

may either be andesite-rhyodacite mixtures or simply evolving 

andesites that erupted during advancing crystallization.

Rhyodacite and Rhyolite

Quaternary volcanic rocks more silicic than dacite are 

sparsely and irregularly distributed in the Cascades. None is 

present between the Jefferson and St. Helens loci, and except 

for a poorly exposed rhyodacite lava at the foot of McKenzie 

Butte (south of Mount Shasta), they are also lacking along the 

main chain between Mount Mazama and the Lassen segment. 

In contrast, the rear-arc Medicine Lake and Newberry centers 

have numerous rhyolites, including some in western peripheral 

areas transitional to the main arc. In the Washington Cascades 

south of Kulshan Caldera, no Quaternary rhyolites or rhyodacites are reported at Glacier Peak, and only one or two small 

units of rhyodacite are known at each of three other major 

centers, Adams, Rainier, and Goat Rocks (where the Devils 

Horns rhyolites are pre-Quaternary). At Mount St. Helens, 

only a handful of the many eruptive units had evolved as far as 

68 to 70 percent SiO2

, probably by fractionation of the volumetrically dominant crystal-rich dacites. 

Stratocones seldom erupt rhyolite or rhyodacite centrally, 

though Magee Volcano and South Sister (at 50-30 ka) provide 

apparent exceptions. More commonly, such silicic magmas 

issue from flank vents or independent peripheral vents (as at 

Shasta, Middle Sister, Jefferson, Adams, Baker, and the 2.4-ka 

Bridge River assemblage at Mount Meager), or, when shallow 

chambers grow large, they erupt as aligned dome chains or 

distributed domefields with or without calderas (as at Lassen, 

pre-Mazama, Mazama pre-climactic, Holocene South Sister, 

Tumalo, early St. Helens, and Kulshan). All three rear-arc 

(extensional) volcanic fields (Simcoe Mountains, Newberry, 

and Medicine Lake), however, built central rhyolitic complexes that began early in development of the three large 

shield-like edifices. The only rhyodacite or rhyolite units not 

clearly related to a major center are a few lavas as far as 8 km 

north of Mount Jefferson, the scattered Tumalo domes, and a 

few domes around the periphery of the Newberry edifice. The 

latter two sets of rhyolitic lavas lie near the leading edge of the 

age-progressive rhyolite trend of the High Lava Plains.

Table 11 gives the SiO2

 content of the most silicic eruptive product reported for each of the main evolved Cascade 

centers and for several minor ones. Of the 34 arc volcanoes 

listed, only 14 have SiO2 limits of 72 percent or higher; that 

is, only these have rhyolite (rather than rhyodacite or dacite) as the most evolved product erupted. High-silica rhyolite (75-77% SiO2

) is especially rare, limited to the Lassen 

segment, the Obsidian Cliffs lava flow near Middle Sister, 

and a single dome at Mount Garibaldi, but several lavas and 

pyroclastic units at each of the three large rear-arc centers are 

rhyolitic. Similarly, very few high-silica rhyolites are recognized in the Aleutian, Andean, and Japanese arcs, although the 

few include some voluminous pyroclastic units. Such scarcity 

is actually a global phenomenon for volcanic arcs, the upper 

SiO2

 limit at each evolved arc volcano being typically in the 

range 69 to 74 percent, in contrast to rear-arc and continentalinterior centers where high-silica (75-77%) rhyolite is widespread. The scarcity might reflect a need for either minimum 

melting of a quartz-bearing plutonic protolith or for expulsion 

of interstitial melt from already-evolved (dacite-to-rhyodacite) 

crystal mush. In the Cascades, rationalization of the scarcity 

of rhyolitic magmas in terms of a possible lack of continental 

basement might be advanced for northern Oregon but certainly 

not for the North Cascades or the Klamath-Shasta regions, 

where pre-Cenozoic crystalline rocks are widespread.

Many pyroclastic-flow deposits and far-flung tephras 

in the Cascades are dacitic, with rhyolitic, rhyodacitic, and 

andesitic units being common but subordinate. Table 3 lists 

more than 70 Quaternary fall units from Cascadian arc or 

rear-arc centers. In bulk (pumice) composition, about a 

dozen are rhyolitic, about 25 rhyodacitic, and most of the rest 

dacitic (though the glass phase that volumetrically dominates 

downwind ash is more silicic than dacite for all but the most 

phenocryst-poor dacitic fall units). Pyroclastic flows in the 

Cascades, whether pumiceous or the denser block-and-ash 

flows generally associated with dome collapse, are most commonly dacite (or silicic andesite) in bulk composition. Rhyodacitic pyroclastic flows are present, however, at Meager, 

Cayley, Garibaldi, Kulshan, Jefferson, Tumalo, and Mazama, 

whereas rhyolitic ones are identified only at South Sister, 

Tumalo, Lassen, and the three big rear-arc centers. Pyroclastic 

flows voluminous enough to be widely devastating (0.1 to 5 

km3

) have erupted at Meager, Cayley, Garibaldi, Glacier Peak, 

St. Helens, Adams, Hood, Jefferson, South Sister, Tumalo, 

Shasta, and the three rear-arc centers, but Quaternary ignimbrite eruptions great enough to be caldera-forming (>10 km3

) 

have been limited to Kulshan, Mazama, Lassen, Newberry, 

and (perhaps) Tumalo. Only a few Quaternary pyroclastic 

eruptions in the Cascades have been voluminous enough to 

reveal compositional zoning. At Tumalo, the Shevlin Park 

Tuff (260±15 ka) is compositionally bimodal, containing 
90 Quaternary Magmatism in the Cascades—Geologic Perspectives

Table 11. Silicic products at Cascade volcanoes.

[Listed is the weight percent SiO2 of the most silicic eruptive product recognized at each volcano, all the analytical totals having been normalized to 100 percent, 

volatile-free. The percent (%) values given in the Notes column likewise refer to SiO2. At most centers, the most silicic product belongs to a compositional continuum shared by less evolved products. At other volcanoes, however, rhyolites are separated from the main compositional array by a pronounced compositional 

gap. This pertains principally to high-silica rhyolites (75-77% SiO2) and is most conspicuous at Garibaldi, Middle Sister, and the three large rear-arc centers]

Volcano Highest % 

SiO2

Notes References

Meager 70 Bridge River 2.4-ka assemblage from Plinth subedifice 26

Cayley 69 Rhyodacites largely at Mount Cayley proper 12

Garibaldi Lake 65 Black Tusk. (Clinker Peak to 64%; Mount Price to 63%) 9, 10

Garibaldi 77 Lava Peak obsidian. (Silicic rocks here mostly dacite) 12

Watts Point 66 Lavas, tuffs, and dikes 11

Kulshan Caldera 73 Ignimbrite pumice and postcaldera lavas 65-73% 16

Baker 68.5 Boulder Ridge. (No other units >65%) 15

Glacier Peak 67 Domes and pyroclastic flows (mostly dacite) 17, 27

Rainier 68.5 Edifice mainly andesite-dacite; few rocks >66. 5

Spiral Butte 66 Large dome 4 km northeast of White Pass 3

Goat Rocks 68 Andesite-dacite stratocone cluster (Adjacent Pliocene rhyolites up to 75%) 3

Adams 68.5 Many dacite lavas from main stratocone 13, 14

St. Helens 70 Sugar Bowl dome (and mid-Pleistocene domes) 25

Indian Heaven 59 Forlorn Lakes. (Volcanic field mostly basalt) 19, 24

Hood 65 Postglacial pyroclastic flows 62-64% 23, 28, 29

Jefferson 72 Rhyodacites peripheral to andesite-dacite edifice 6, 7

Middle Sister 76.5 Obsidian Cliffs. (A dome at south toe has 74%) 17

South Sister 74 Many 50 to 30-ka rhyolites. (Holocene lavas 72-73.5%) 17

Broken Top 69.5 Coulees at Squaw Creek Falls and Tam McArthur Rim 17

Tumalo 74.5 Bend pumice fall and several lava domes 17, 18

Edison Butte 74.5 Dome 8 km southeast of Mount Bachelor 18

Benham Falls dome 71.5 Dome/flow 10 km southwest of Bend, Oregon 17

Bench Mark Butte 65.5 Coulee 16 km southwest of Mount Bachelor 17

Cappy Mountain center 71 Clover Butte. (Nearby Burn and Hemlock Buttes and Mule Mtn all 69-70%) 17

Mazama 72 Llao Rock episode. (Climactic pumice ≤71%) 1

Pre-Mazama domefield 73 Many dacite to rhyodacite lavas 22

Shasta 72 Lava south of McKenzie Butte. (Black Butte 65.7%; Haystack 65%; main 

andesite-dacite edifice up to 67.5%)

5, 17 

Burney Mountain 65 Large composite dome 35 km north of Lassen Peak 21

Magee 72 Late rhyodacite lava from mafic-to-andesitic cone 2

Lassen domefield 71.5 Many domes 64-70% 4

Brokeoff 68 Edifice dominantly andesitic 55-63% 4

Rockland caldera 74.5 Ignimbrite pumice zoned 71-74.5% 4

Maidu 76 Rhyolite coulees peripheral to andesite cone 4

Dittmar 75 Rhyolite lavas peripheral to andesite cone 4

Rear-arc centers

Simcoe 76 Ignimbrite pumice and lava flows (Pliocene) 17

Newberry 75 Pleistocene McKay Butte. (Holocene rhyolites 72-74%) 20

Medicine Lake 77 Pleistocene lavas. (Holocene rhyolites up to 74.5%) 8

References: 1. Bacon and Druitt, 1988. 2. Borg and Clynne, 1998. 3. Clayton, 1983. 4. Clynne and Muffler, in press. 5. Condie and Swenson, 1973. 6. Conrey, 1991. 7. Conrey, Hooper, and others, 2001. 8. Donnelly-Nolan, 1998. 9. Green, 1981. 10. Green, 1982. 11. Green, 1994b. 12. Green, N.L., unpubl. data, 

2005. 13. Hildreth and Lanphere, 1994. 14. Hildreth and Fierstein, 1995. 15. Hildreth and others, 2003. 16. Hildreth, Lanphere, and others, 2004. 17. Hildreth 

and Fierstein, work in progress. 18. Hill, 1991. 19. Leeman and others, 1990. 20. MacLeod and others, 1995. 21. Muffler, L.J.P., unpubl. data, 1981. 22. 

Nakada and others, 1994. 23. Scott and others, 1997. 24. Smith, 1984. 25. Smith and Leeman, 1987. 26. Stasiuk and Russell, 1989. 27. Tabor and Crowder, 

1969. 28. White, 1980. 29. Wise, 1969.
Intracrustal Evolution of Arc Magmas 91

black pumice (55-62% SiO2) and pale silicic pumice (64-68% 

SiO2) (Conrey and others, 2001); and at Medicine Lake, pumice in the Tuff of Antelope Well ranges from 63 to 67 percent SiO2

 (Donnelly-Nolan, 2006). Even in the voluminous 

Kulshan and Rockland (Lassen) pyroclastic units, the silicic 

pumice ranged through only 3 to 4 percent SiO2. The great 

volume of rhyodacite magma released in the caldera-forming 

eruption of Mount Mazama was nearly homogeneous, but 

the intermediate scoria and cumulate mush that closed the 

eruptive sequence provide important insights concerning 

magmatic parentage and intrachamber fractionation processes 

(Bacon and Druitt, 1988) that may be applicable to many 

intracrustal arc-magma systems. 

The phenocryst mineralogy of Cascadian rhyolites and 

rhyodacites is fairly simple, with plagioclase, orthopyroxene, 

and FeTi oxides ubiquitous and amphibole also present in 

many units. Clinopyroxene is commonly absent in these silicic 

rocks, although sparsely present in a few, especially within 

clots and enclaves. Quartz is common at Lassen and Meager 

and occurs sparsely at Kulshan, Maidu, Dittmar, and in many 

early eruptive units at St. Helens. Biotite occurs in a single 

pumice fall at Rainier, is present but uncommon at Kulshan, 

Maidu, Dittmar, and in the quartz-bearing early St. Helens 

units, and is common at Meager, Cayley, Garibaldi, and in 

many units of the Lassen domefield. Uniquely at Mount St. 

Helens, the Ca-poor ferromagnesian amphibole, cummingtonite, occurs in many units (mostly dacites) older than 3,000 

years and commonly coexists with hornblende. No sanidine 

has been reported in the Quaternary Cascades.

There are many roads to rhyolite, so summarizing the 

magmatic origins of the Cascadian silicic rocks is not straightforward. The following processes have been advocated: (1) 

Serial escape of silicic interstitial liquid from numerous successive batches of crystallizing intermediate magma that thereby 

contribute to concurrent accumulation of zones of crystal-poor 

melt and melt-depleted cumulate mush; (2) percolation of 

such interstitial silicic melt to the top of large upper-crustal 

reservoirs (plutons) of dacitic/granodioritic mush; (3) fractionation of parental dacitic magmas, many of which may have 

originated by intracrustal partial melting of mafic rocks; (4) 

fractionation from hybrid parents that are mixtures of basalt, 

andesite, and partial melts of granitoids, young or old; and (5) 

direct intracrustal partial melting of mafic rocks. No Cascade 

rhyolites have been identified that might be straightforward 

partial melts of metasedimentary or granitoid rocks, though 

the relatively elevated Sr-isotope ratios of some California 

rhyolites (as high as 0.7045) suggest contributions from such 

sources (see section below entitled “Cascadian Basement and 

Sr-isotope Overview”). A simple liquid line of descent, by 

which rhyolite fractionates continuously from a basaltic parent, 

is seldom plausible, volumetrically or volcanologically.

While anticipating complexity, a first-order question for 

any rhyolite is whether it arose principally by crustal melting or 

represented melt separated from a crystallizing reservoir of intermediate magma. Small batches of crustal melt might sometimes 

escape hybridization, but no large upper-crustal silicic magma 

body is likely to have been fed directly by melting of a lithologically simple source. Even where crustal melting may have begun 

within a fairly homogeneous, deep-crustal gabbroic underplate, 

growth of an upper-crustal chamber would integrate contributions from all along the crustal column as well as from its own 

envelope. A long-lived melting column is inevitably self-hybridizing, and the coalescence of partial melts of varied protoliths is 

intrinsically a mixing process. Moreover, beneath a large evolved 

volcanic center that lasts a million years, basaltic injection and 

crustal melting would be spasmodically fluctuating processes, 

waxing and waning, such that episodic fractionation would 

provide a range of partially remeltable deep-crustal rocks, from 

gabbros (with or without amphibole) to tonalite and lesser fractions of more silicic differentiates. Growth of large upper-crustal 

silicic magma bodies may involve at least three (conceptually 

discrete) major magma-transfer mechanisms—(1) incremental 

addition of mafic-to-andesitic magmas that ascend batchwise 

from the deep crust, then fractionate within the shallow reservoir 

itself; (2) separation and coalescence of broadly dacitic melt 

batches in deep mafic crust, followed by ascent and addition to 

the shallow reservoir; and (3) buoyant mobilization of partially 

molten MASH complexes that, during upwelling through the 

lower-to-middle crust, physically segregate into dike swarms and 

melt-rich silicic fractionates at the top, passing down through 

hybrid cumulate mush into cumulate and refractory dregs. Each 

and all may contribute to some upper-crustal magma reservoirs, 

and there are probably transitions among them.

Much of the compositional variation among andesitedacite magmas parental to rhyolites is likely to reflect fH2

O 

and fO2 conditions prevalent at sites of deep-crustal melting 

(Tepper and others, 1993; Sisson and others, 2005), which 

in turn may principally reflect the flux and water contents 

of mantle-derived basaltic intrusions that range in the Cascades from dry to water-rich (Sisson and Layne, 1993; Grove 

and others, 2002). Isotopic studies, particularly Nd-isotope 

systematics, have demonstrated repeatedly that most metaluminous (arc and intracontinental) rhyolites descend from 

hybrid parents that blend local crustal contributions with 

proportionately greater immature mantle inputs supplied either 

directly as contemporaneous basaltic magma or, second-hand, 

by partial remelting of Cenozoic gabbros and their differentiates in the deep crust (Mahood and Halliday, 1988; Hildreth 

and Moorbath, 1988; Hildreth and others, 1991; Johnson, 

1991). Few arc rhyolites are likely to be direct, unfractionated, 

crustal melts, and most probably represent melts that separated from intermediate parents with complex antecedence. 

The strongly peraluminous intracrustal magmas (for example, 

Andean macusanite and Himalayan leucogranite) are distinctive exceptions that prove the hybrid norm for silicic magmatism. Intracrustal melting is no doubt massive beneath arcs 

(and more so in areas of intracontinental extension), but partial 

melting of the predominantly mafic sub-arc protoliths generally yields not rhyolitic but intermediate magmas, from which 

most rhyolites subsequently evolve. If rhyolitic melt batches 

are sometimes produced directly, such as by partial remelting of deep-crustal tonalites or their differentiates, few such 
92 Quaternary Magmatism in the Cascades—Geologic Perspectives

batches are likely to be large enough to avoid hybridization in 

complex long-lived systems. Over a million-year timescale, 

crustal melting and crystal-melt fractionation, of course, both 

contribute to production of rhyodacites and rhyolites. The 

polarized interpretive dichotomy that contraposes melting visà-vis fractionation becomes moot when applied to a long-lived 

crustal column that fluctuates thermally in response to successive mantle inputs.

At some centers, rhyodacites are part of an unbroken 

compositional array with dacites and andesites, suggesting continuous fractionation (and/or back-mixing), whereas 

elsewhere some rhyodacites and many rhyolites are set apart 

from intermediate members by wide compositional gaps, 

suggesting that the silicic melt had evolved not progressively 

but by batchwise extraction from crystal mush. That most 

rhyolites are phenocryst-poor weighs in favor of melt extraction from pluton-scale mush reservoirs but weighs against 

both (1) conventional fractionation models of progressive 

compositional evolution by subtraction of crystals from a 

melt-dominant magma and (2) extraction of viscous rhyolitic 

minimum melts from restite-rich migmatites. The abundance 

of mafic and andesitic enclaves in andesite-dacite extrusions (and in tonalite-granodiorite plutons) contrasts with 

their rarity in phenocryst-poor rhyolites, further supporting 

the notion that rhyolitic melt lenses accumulate at the top of 

thick granitoid mush reservoirs that buffer the upper levels from new mafic inputs. A comparable inference can be 

drawn from the withdrawal sequences of many ignimbrite 

sheets that start with low-temperature gas-saturated crystalpoor rhyolite and end with higher-temperature crystal-rich 

intermediates (Hildreth, 1981a). Entrainment by many 

rhyolitic magmas of subpopulations of refractory zircon 

antecrysts as much as 50 to 250 kyr older than the eruption 

attests further to the generality of thermally waxing-waning, 

incrementally accumulating, mush reservoirs (Reid and 

others, 1997; Brown and Fletcher, 1999; Vazquez and Reid, 

2002; Bacon and Lowenstern, 2005; Charlier and others, 

2005). The “mush model” of rhyolitic magmatism was elaborated by Hildreth (2004) and discussed in detail by Hildreth 

and Wilson (2007).

Some high-silica rhyolites exhibit such extreme contrasts in trace-element abundances and ratios as compared 

with associated dacites and andesites (notably in REE, Y, and 

HFSE) that explanations of their origin are driven to invoke 

either fractionation of sparse accessory phases or direct 

low-degree melting of crustal rocks (Miller and Mittlefehldt, 

1984; Hildreth and Wilson, 2007). Such rhyolites are abundant intracontinentally, uncommon in arcs, and unrecognized 

in the Quaternary Cascades. 

Arc Plutons and Upper Crustal Magma Chambers

Granitoid plutons of Quaternary age are rarely exposed, 

the lone Cascadian example recognized being the small Bar 

Creek stock (~0.7 Ma) in a deep glacial trough at the margin 

of Kulshan Caldera (Hildreth and others, 2003; Hildreth, 

Lanphere, and others 2004). Granitoid xenoliths quarried by 

several different eruptions of Mount Mazama and of Medicine 

Lake Volcanic Field, however, indicate Quaternary growth 

of extensive composite plutons beneath both centers; U-Pb 

and U-Th dating of zircons from the xenoliths (and drillcore) 

yields several discrete clusters of Pleistocene crystallization 

ages (320-20 ka), some of which coincide with silicic eruptive intervals (Bacon and others, 2000; Lowenstern and others, 

2000, 2003; Bacon and Lowenstern, 2005; Bacon and others, 

2005). There can be little doubt that many other Cascadian 

volcanic centers with histories of episodic silicic-to-intermediate eruptive activity extending over 105

 to 106 years have likewise built multiphase composite plutons (of diorite to granite) 

in the upper crust beneath them. That Pleistocene granitoids 

underlie Meager, Cayley, Garibaldi, Kulshan, Glacier Peak, 

Rainier, St, Helens, Hood, Jefferson, South Sister, Broken Top, 

Newberry, Cappy, Shasta, Dittmar, Maidu, and Lassen seems 

inescapable. More interesting, but generally unknown, are the 

size and compositional-lithologic variability of the uppercrustal plutonic complexes now under construction.

Many plutons of Tertiary age, however, have been 

unroofed in the Cascades and may provide insights into those 

now forming, considering that they grew in an arc system and 

basement terranes much like those of the present. Owing to 

the northward increase in severity of glacial erosion, exposure 

of Tertiary plutons is much better in Washington and British Columbia than in Oregon, where the few Miocene stocks 

partly unroofed (Walker and Macleod, 1991) may in some 

cases represent windows into larger granitoid batholiths. No 

Tertiary granitoids are yet unroofed in the California Cascades. 

In the Cascades of central Washington where preservation of 

Tertiary volcanics and unroofing of Tertiary plutons are both

good, most of the numerous dacitic to rhyolitic ignimbrites 

were doubtlessly derived from magma bodies that helped 

construct some of the 27 to 13 Ma granitoid plutons, several 

of which are 10 to 40 km across (for example, Snoqualmie, 

Grotto, Cloudy Pass, Bumping Lake, Tatoosh, and Spirit Lake 

plutons; Tabor and others, 1993, 2000, 2002; Walsh and others, 1987). Most Cascade plutons consist of granodiorite and 

tonalite (or monzonitic equivalents) with lesser representation 

of granite and gabbro-diorite. Rhyolite-equivalent leucogranites (70-74% SiO2

) are few, and the sparse intrusives with 75 

to 77 percent SiO2 are mostly dikes or small masses of aplite 

or alaskite. Because of secular upward fractionation of the 

melt phase at all stages of crustal magmatism, eruptive losses 

result in many plutons being melt-depleted quasi-cumulates 

that on average are a few percent less silicic than either their 

net volcanic output or the integrated bulk composition that 

intruded the upper crust.

One of the best-exposed Tertiary plutonic complexes 

is the long-lived Chilliwack composite batholith, which is 

compared at equivalent scales in figure 28 with the series of 

Pliocene-to-recent andesitic-to-rhyolitic volcanic centers in 

the Lassen region. From the Chilliwack pattern, one can infer 

that (1) most volcanic deposits are rapidly stripped in glaciated mountainous terrain, with intracaldera fill being prefer-
Intracrustal Evolution of Arc Magmas 93

entially preserved; (2) calderas are commonly similar in size 

to some plutons but smaller than many; (3) some plutons are 

nested, others consist of several transitional phases or lobes, 

and still others sharply crosscut older ones; and (4) the mass 

of mafic magma that rose through the mushy crustal column 

to produce shallow gabbro-diorite bodies is a trivial fraction 

(1-5%) of the upper-crustal batholith. In the Lassen sketch 

(fig. 28), each of the volcanic centers outlined stayed active for 

~1 Myr and embraces dozens of vents for products that range 

from andesite to rhyolite. At least one caldera (Rockland) and 

possibly three or more are buried by younger volcanic rocks 

of the Lassen region. The upper-crustal plutonic roots of many 

long-lived volcanic centers are likely to be as complex as that 

depicted for equivalent areas of the Chilliwack batholith. If 

the 55 plutons distinguished along the 60-km-long Chilliwack 

exposure are representative of the upper arc crust, there could 

be more than 1,000 plutons (40-0 Ma) along the present-day 

Cascade arc and still more along its Tertiary continuation 

southeast into Nevada (Christiansen and Yeats, 1992). To the 

many substantial plutons (1-to-40-km across) must be added 

countless smaller plugs, dikes, and irregular intrusions, mostly 

gabbro-diorite or dacite-granodiorite.

Lower crustal domains, if preheated (as implicit in a 

long history of pluton growth), can be expected to be more 

responsive to renewed mantle input in generating new 

batches of hybrid to silicic magma, helping account for the 

longevity and recurrent rejuvenation of some magmatic loci. 

On a shorter timescale (104

 to 105 years), gradual preheating 

of the upper crust during an extended period of emplacement 

of intermediate magma batches could prepare the shallow 

system for eventual coalescence of a large reservoir of crystal mush, extraction of rhyolitic melt, and a caldera-forming 

eruption. A well-documented example is the 7.7-ka climactic eruption of Mount Mazama, where a shallow body of 

rhyodacite magma only began to segregate ~30 ka, following 

several hundred thousand years of distributed andesite-dacite 

injections (Bacon and Lanphere, 2006). Granitoid plutons 

should not be thought of solely as a magmatic dying stage 

but a culmination in productivity of the crustal column. A 

thick mass of crystal mush can for a time suppress eruption 

of all but its own silicic differentiates. As the upper end of a 

partially molten crustal column, such a mush reservoir can 

long remain susceptible to rejuvenation by amplification of 

the mantle-derived basaltic power supply. 

Many upper-crustal plutonic complexes appear to be 

products of piecemeal assembly spread out over the 105

-to-106

-year lifetimes of systems that wax and wane thermally. Some 

granitoid reservoirs grow large and fast enough to absorb and 

homogenize numerous ascending magma batches, whereas 

others (when batch addition is slow relative to crystallization) 

end up as composite assemblages of discrete intrusive bodies, related but distinguishable phases that may be contiguous, 

overlapping, or nested (fig. 28). Compositionally “andesitic” 

plutons (quartz diorite) are usually small, probably because 

such batches seldom crystallize independently but are instead 

typically blended into larger reservoirs that undergo secular 

fractionation into gabbroic cumulates and more silicic melts. 

The scarcity of large gabbro-diorite plutons in the upper arc 

crust must also reflect their inability to penetrate upward 

through the more buoyant granitoid mush that their thermal 

contribution helped engender. Large plutons of tonalite and 

granodiorite are commonly intruded by mafic sheets, dikes, 

and swarms of distributed derivative enclaves. Thick mushy 

plutons preferentially restrict penetration of mafic inputs to 

deeper levels, accounting for the relative scarcity of mafic 

enclaves in their felsic upper zones, as well as in voluminous 

rhyolites. Large pyroclastic eruptions of crystal-poor silicic 

magma sometimes end by tapping into viscous crystal mush 

(as at Mazama and Katmai), whereas small ones are commonly bimodal, reflecting mafic injection of small silicic 

chambers more susceptible to eruption-triggering invasion. 

There are hierarchies of input intensity and reservoir growth; 

upper-crustal chambers are but one manifestation of the magma 

processing along a column penetrating the whole crust.

Eruptive leakage from a tonalite-granodiorite pluton 

accumulating in the upper crust might for long intervals feed 

a limited range (58-65% SiO2

) of andesite-dacite magma to 

a growing stratocone (Baker, Glacier Peak, Rainier, Adams, 

Hood, Jefferson, Rainbow, Shasta). When such a reservoir 

grows large and mushy enough to separate a substantial roof 

zone of silicic melt (buoyantly resistant to convective remixing), leakage might variously feed central plinian eruptions 

(Llao Rock, Bridge River), central dome clusters (Meager, 

Garibaldi, Glacier Peak, St. Helens), dome chains (South 

Sister, Sharp Peak, Mono Craters), distributed domefields 

(Lassen, Tumalo, Kulshan), or pyroclastic withdrawals large 

enough to initiate caldera subsidence. In arcs, as elsewhere, 

caldera-forming eruptions typically follow lengthy intervals 

of andesite-dacite (or more silicic) eruptive activity, which 

provides the thermal conditioning of the upper crust leading 

to coalescence and growth of a large shallow magma reservoir. The precaldera volcanism may or may not construct 

an edifice (like Mount Mazama), but it usually does include 

extrusion of silicic lava domes. In northern Washington and 

Canada, however, recurrent glaciation has obliterated all but 

scraps of precaldera volcanic products and virtually all extracaldera pyroclastic deposits resulting from caldera-forming 

eruptions (for example, Berman and Armstrong, 1980; Clayton, 1983; Hildreth, Lanphere, and others, 2004; Tucker and 

others, 2007).

Coalescence of large shallow bodies of evolved magma, 

whether beneath arcs or intracontinentally, is apparently 

favored by tectonic extension. Intra-arc extensional caldera 

belts are well described in Kyushu, Guatemala, and the Taupo 

Volcanic Zone. The great Toba caldera complex in Sumatra 

formed along a major transtensional fault zone, and many 

of the big calderas in Mexico and the Andes are in weakly 

extensional rear-arc settings, behind the chains of stratovolcanoes. In the extensional interior of the western USA, there are 

more than 100 large calderas of Oligocene and younger age. 

Extensional unloading evidently favors mantle melt production, focusing of magma ascent, buoyant mobilization of 
94 Quaternary Magmatism in the Cascades—Geologic Perspectives

v 

10 km 10 km 

Latour 

vc 

Magee 

Volcano 

Lassen Region 

South Cascades 

Chilliwack Batholith 

North Cascades 

Dittmar 

vc 

Caribou 

mafic 

Volcanic 

Field 

Maidu vc 

Yana vc 

Brokeoff 

L dome assen vc

Lake

Almanor

Tv 

gr 

gd 

ton 

gbo-di 

121o30' 

121o30' 

121o45' 121o 

121o 

122o 121o 

48o30' 

40o 

40o40' 

49o 

122o 

Mount 

Baker 

Mount 

Shuksan 

gn 

gn 

v 

v 

v 

v 

v 

field 

Figure 28. Examples of long-lived clustering of Cascadian magmatism, comparing at the same scale shallow plutonic and volcanic levels 

of exposure—Chilliwack composite batholith in the glaciated North Cascades and major volcanic components of the Lassen region, all 

of which except Caribou are evolved (andesite to rhyolite) volcanic centers (vc). About 55 separate Chilliwack plutons (not all separable 

at this scale) range in age from 32 to 0.7 Ma, whereas all volcanic rocks in the Lassen regional sketch are younger than 4 Ma (the Lassen 

domefield being still active). Principal lithology of each Chilliwack pluton is given in map key: gr, granite; gd, granodiorite; ton, tonalite; 

gbo-di, gabbro and diorite; Tv, volcanic remnants in the roof of the batholith, largely pyroclastic, including at least three caldera fills 

(hachures); gn, inliers of basement gneiss. Compiled principally from Tabor and others (2003), Tepper and others (1993), Macdonald (1963), 

and Clynne and Muffler, in press.

deep-crustal MASH zones, and formation of magma traps for 

mid-to-upper-crustal pluton growth. In the brittle upper crust, 

extension naturally favors magma ascent, though at deeper 

crustal levels, ductile stretching might promote sill formation. 

Extension, however, is not the only factor influencing growth 

of large shallow crustal magma systems, which are scattered 

along the Quaternary arc from Lassen to Meager (that is, from 

its most to its least extensional segment), and some calderaforming reservoirs do develop along non-extensional volcanic 

fronts (for example, Kulshan and several along the Alaska 

Peninsula). The long persistence of mafic volcanic fields (Caribou, Hackamore, Indian Heaven, and reaches of the central 

Oregon Cascades) in areas of strong to weak tectonic extension, accompanied by little or no silicic volcanism, suggests 

that intensely localized crustal focusing of the basaltic magma 

flux from the mantle, not the extension itself, is the main prerequisite for growth of large reservoirs of silicic magma.

Tectonic and Geophysical Insights

Numerous geophysical and tectonic observations provide 

a variety of insights that help rationalize some of the varied 

patterns of volcano distribution in the Cascades (figs. 1, 25).

Concave Curvature of the Margin

The Cascadia subduction zone is unusual in that its 

northern half exhibits trenchward-concave curvature (fig. 

1). Accordingly, northeast-directed subduction of the small 

oceanic plates of the Juan de Fuca system (Riddihough, 

1984) results in nearly arc-normal convergence along the 

Garibaldi Volcanic Belt but oblique convergence elsewhere, 

which is apparently one of the factors that imparts a transtensional component to the Quaternary volcanic arc from 
Tectonic and Geophysical Insights 95

central Washington to California, probably contributing to 

its unusual breadth.

Because the concave-seaward margin of northern Cascadia is contrary to the natural convex curvature of a plate 

subducting into a sphere, the Juan de Fuca slab accommodates by bending into an apparently smooth arch (fig. 25), 

the crest of which plunges east (at about 48°N), as imaged by 

both seismicity and seismic tomography, beneath the Olympic Peninsula and Glacier Peak (Crosson and Owens, 1987; 

Weaver and Baker, 1988; Bostock and VanDecar, 1995). The 

relatively shallower slab over the crest of the arch implies a 

thinner mantle wedge there, which might reduce magma generation and thus provide an influence in limiting Quaternary 

volcanism along the 200-km reach between Rainier and Baker 

to the single locus around Glacier Peak. Moreover, if the slab 

beneath Glacier Peak were shallower, cooler, and wetter than 

to the north or south, that might help explain why that isolated 

volcano contrasts with its nearest neighbors in its abundance 

of explosive hornblende dacites.

Translation and Rotation of the Forearc

Tectonic deformation of Cascadia and adjacent areas is 

fundamentally driven by dextral shear between the Pacific 

and North American plates (Atwater, 1970) and by extension 

promoted by buoyant mantle beneath the western Basin and 

Range Province (Jones and others, 1996). Opposite the Cascade arc, most of the relative motion between the two megaplates is accommodated by spreading on the Juan de Fuca 

Ridge and subduction beneath Cascadia, but some 25 percent 

of the relative motion is distributed far inboard as block rotations and translations (Pezzopane and Weldon, 1993; Walcott, 

1993; Wells and others, 1998; McCaffrey and others, 2007).

In response to the dextral shear couple, and in concert 

with northwestward motion of the nearly rigid Sierra NevadaGreat Valley-Klamath microplate (Dixon and others, 2000; 

Hammond and Thatcher, 2004) and with Basin and Range 

extension inboard (fig. 25), the Oregon forearc has undergone 

clockwise rotation since at least the mid-Miocene (Wells and 

Heller, 1988; Wells, 1990; Wells and others, 1998). Along 

its trailing margin, rotation of the forearc block apparently 

contributes to the intra-arc and rear-arc east-west extension 

prominent along the Oregon Cascades, from the Klamath 

Graben (fig. 15) to the Columbia River (fig. 25). On the 

decadal timescale of GPS measurements, however, intra-arc 

extension there is currently limited to about 1 mm/yr or less 

(McCaffrey and others, 2007). Oblique subduction of the Juan 

de Fuca Plate probably also contributes by basal traction to 

northward motion of the Cascadia forearc. The transition from 

predominantly dextral shear in California to block rotation in 

Oregon lies between the Lassen and Shasta segments of the 

arc, coinciding with the transition offshore from a strike-slip to 

a subducting plate boundary (figs. 1, 25).

Northward motion of the forearc microplate promotes 

north-south shortening in the Washington forearc, may promote transpression along the arc north of Mount Rainier, and 

enhances compression along the Garibaldi Volcanic Belt, north 

of the restraining bend where the Coast Plutonic Complex forms 

a stable buttress. As shown by the GPS vectors of McCaffrey 

and others (2007), clockwise rotation of the Cascadia forearc 

is apparently coordinated with and merges smoothly into the 

nearly orthogonal plate convergence in northwest Washington 

and British Columbia. Northeasterly motion of the Mount Baker 

region may help account for the magmatic age progression of 

the Hannegan-to-Baker locus (figs. 3, 26; Hildreth, 1996).

Northwestward translation of the Sierra Nevada-Great 

Valley microplate involves a broad dextral inboard shear zone 

that extends from southern Nevada to northeastern California, 

where the northwest-trending strike-slip belt merges diffusely 

into an array of north-striking normal faults that reach northward through Mount Mazama. Faults of the dextral shear zone 

(Walker Lane belt) strike into the Lassen segment, and others 

may impinge on the Medicine Lake and Hackamore Volcanic 

Fields. In this region, Blakely and others (1997) outlined three 

right-stepping offsets where nonmagmatic gaps containing 

northwest-southeast dextral faults alternate with the Lassen, 

Shasta, and McLoughlin volcanic segments, within which 

north-south normal faults are more prominent. The pattern suggests that, where the regional belt of dextral shear overlaps the 

volcanic arc, east-west extension is capable of establishing the 

dominant structures principally within the crustal segments most 

weakened by magmatism.

Farther north, a belt of weaker northwest-trending 

dextral shear, the Brothers Fault Zone (fig. 25), provides a 

diffuse northern margin for the Basin and Range extended 

region (Lawrence, 1976). The Brothers Fault Zone likewise 

terminates diffusely against the Cascade arc, first stepping 

left (west) to form the northwest-trending Sisters (Tumalo) 

Fault Zone, which in turn merges with the north-striking High 

Cascades Graben (HCG in fig. 25; Taylor, 1981, 1990a; Smith 

and Taylor, 1983; Smith and others, 1987; Sherrod and others, 2004). Elements of this intra-arc belt of modest east-west 

extension, structurally active at least since ~5 Ma, may or 

may not follow the Cascade axis from Green Ridge (44.5oN) 

northward through the Jefferson reach to join the Hood River 

Fault (fig. 25) near the Columbia River (Conrey and others, 

1997; Walker and MacLeod, 1991), where discernible extension apparently peters out.

Inboard of both the Cascade arc and the Oregon Basin 

and Range, most of eastern Oregon and southern Idaho appear 

to be rotating clockwise as a large coherent block. As shown 

by the GPS vectors of McCaffrey and others (2007), the 

northwest end of this block moves directly northward against 

the Yakima Foldbelt and adjacent Mount Adams area. Northdirected compression there, with a small clockwise rotational 

component, is consistent with the east-west fold axes, minor 

dextral faults that crosscut the axes, north-vergent reverse 

faults, and several northerly vent alignments.

Differential extension and block rotation of the lithosphere may play a more important role than subduction 

geometry in controlling segmentation of the volcanic belt, in 
96 Quaternary Magmatism in the Cascades—Geologic Perspectives

defining its several distinguishable reaches in Oregon (figs. 

14, 15), and in promoting its exceptional width in California and southern Washington. The Cascade arc has been a 

well-defined continuous belt since the Eocene, but the dextral 

shear, block rotations, and Basin and Range extension were 

not imposed on the arc until the mid-Miocene. Had none of 

these external tectonic processes been superimposed on the 

Cascades, there would nonetheless still be a Cascade arc 

today, owing to the slab-derived contribution to mantle-wedge 

melt production. In all likelihood, however, there would be 

no HAOT, no rear-arc Simcoe, Newberry, Medicine Lake, nor 

Hackamore Volcanic Fields, and the volcanic belt in the Lassen segment would be much narrower.

Slab Depths and Discontinuities

Slab depths beneath the Quaternary volcanic belt are 

not well known, though the slab does appear to be segmented (Guffanti and Weaver, 1988; Bostock and VanDecar, 

1995). Except along the nearly aseismic Oregon subduction 

zone, seismicity within the slab is well recorded beneath the 

forearc, where it extends continuously to depths of about 60 

km in British Columbia, Washington, and California (Taber 

and Smith, 1985; Walter, 1986; Crosson and Owens, 1987; 

Weaver and Baker, 1988). Earthquakes within or along the 

slab at 60 to 80 km depths are rare and still deeper events 

absent. Imaging the slab by seismic tomography, however, 

suggests that slab dip increases abruptly from 5° to 20° 

beneath the outer forearc to 50° to 70° beneath the Willamette-Puget-Georgia forearc trough (fig. 25), the flexure 

(and breakaway?) typically lying 50 to 100 km seaward of 

the arc. The slab flexure and the topographic trough probably both reflect slab densification owing to basalt-to-eclogite 

phase changes at slab depths of 50 to 60 km (shallower 

than in older colder slabs). This pattern pertains from British Columbia to California (Michaelson and Weaver, 1986; 

Rasmussen and Humphreys, 1988; Harris and others, 1991; 

Benz and others, 1992; Bostock and VanDecar, 1995; Zhao 

and others, 2001). In contrast to the tomographic analyses just 

cited, however, that of Bostock and others (2002) suggests a 

lesser (30°) slab dip east of the flexure in central Oregon, so 

the actual shape of the seismically silent slab beneath Oregon 

evidently remains unresolved (McCrory and others, 2004).

If the steep, tomographically inferred, dips are real, then 

a 60° dip implies a slab-depth difference of 87 km across a 

50-km-wide volcanic belt (as in Oregon) or a 173-km difference across a 100-km-wide belt (as in southern Washington or 

the Lassen segment). For example, if a slab dipping 60° lies 

~75 km beneath the Portland-St. Helens forearc, then it might 

be 160 km below Mount Adams or as deep as 250 km beneath 

the Quaternary volcanoes farthest inboard near Goldendale 

(fig. 8). Moreover, if the slab were that steep beneath parts of 

the arc, then its vertical fall component might contribute to 

hingeline rollback, intra-arc extension, upwelling of mantle 

material little influenced by slab-derived fluid, and consequent 

production of some of the observed mantle magmas atypical 

of arcs (principally HAOT and alkali basalt).

On the south limb of the slab arch in southern Washington (fig. 25), there is likely to be some kind of structural discontinuity in the slab. In northern and central Washington, the 

deep slab is tomographically imaged to depths of 400 to 500 

km but only to 300 km in southern Washington and to only 

150-200 km beneath Oregon (Michaelson and Weaver, 1986; 

Rasmussen and Humphreys, 1988; Harris and others, 1991; 

Bostock and VanDecar, 1995). The discontinuity might take 

the form of an east-trending tear, fold, or monocline, and such 

a structure could be expected to distort mantle-wedge flow, 

which might in turn influence the compositional diversity and 

extraordinary width of the volcanic belt from Portland across 

southern Washington. On the basis of a linear trend in deep 

slab earthquakes beneath southwest Washington, McCrory 

and others (2003) indeed proposed just such a tear. Another 

possibility, an arc-parallel (north-south) tear, breakaway, or 

sharp hingeline where the Oregon slab steepens abruptly, 

would still require some kind of east-west disruption of the 

transition zone between the Oregon slab and that beneath the 

Rainier-to-Hood segment.

Reconstruction of 7-to-0-Ma motion of the Juan de Fuca 

microplate system (Riddihough, 1984; Wilson, 1989, 2002; 

Oppenheimer and others, 1993) indicates (1) slowing of its 

northeast-directed convergence from ~70 mm/yr to ~30 to 45 

mm/yr today, owing to the growing resistance to subduction 

of progressively younger (more buoyant) oceanic lithosphere, 

resulting in (2) detachment of the youngest (slowest) portion 

at ~3.5 Ma, creating an independent Explorer microplate that 

then virtually ceased subducting, and (3) subsequent stressing 

of the young buoyant southernmost portion of the plate, leading after 3 Ma to deformation, independence, further slowing 

(to 2 to 3 cm/yr), and potential locking of the so-called Gorda 

plate. Slowing and eventual Pliocene shrinkage of the Juan 

de Fuca Plate system at both ends may have influenced, over 

the same time interval, the westward jump of arc magmatism 

from the Pemberton-Chilliwack belt (fig. 3) to the Garibaldi 

belt in the north (Souther and Yorath, 1991) and the westward 

drift of volcanism in the Lassen segment in the south (Guffanti 

and others, 1990). In both areas, slowing of subduction may 

further have permitted a larger role for upwelling of poorly 

fluxed mantle, leading to production of alkali basalts in the 

north and abundant HAOT in the south, each impoverished in 

subduction components. Enhanced mantle upwelling past the 

edges (Lewis and others, 1997; Beaudoin and others, 1998) of 

the shrinking slab might also contribute to the crustal melting anomalies near the ends of the arc, expressed in the silicic 

Meager and Maidu centers and the Barkley Mountain dacite 

chain (figs. 3, 21).

Hingeline rollback and/or sinking of recently detached 

slab segments might well account for westerly arc drift at the 

north and south ends of the Cascades, but arc drift has been 

antithetically eastward in the center. From central Washington to southernmost Oregon, the Quaternary volcanic axis 

lies just east of the broad Miocene belt, the apparent cross-
Tectonic and Geophysical Insights 97

overs (where Quaternary and Miocene arcs coincide) being 

near Glacier Peak and the California border. In between, the 

eastward migration of Cascade magmatism is more likely to 

reflect the trenchward component of western Oregon's upperplate clockwise rotation than any shallowing of the steep 

sub-Oregon slab. In the Oregon Cascades, there is substantial 

overlap between the broad belts of Pliocene and Quaternary 

volcanism, so it appears that the last significant eastward 

shift of the Cascade volcanic axis took place around 7 to 5 

Ma, at the onset of intra-arc extension, which is linked to the 

rotation and has maintained the position of the axial magmatic belt ever since.

Young Warm Slab and Influences on Magmatism

The main part of the Juan de Fuca slab itself is, by global 

comparison, relatively young, warm, and buoyant, and its convergence with North America is relatively slow (30-45 mm/

yr) and initially (beneath the outer forearc) shallowly dipping 

(5-20°). Plate age at the deformation front (trench) is only 6-8 

Ma, contributing (along with an unusually thick 3-km blanket 

of sediments on the incoming oceanic plate) to a high initial 

temperature (225-260oC) on the megathrust at the (filled) 

trench (Oleskevich and others, 1999). Those investigators 

model downdip temperatures to reach 350oC at 12 km depth, 

450oC at 25 km, and 550oC at ~40 km, where the thrust plane 

dips below the forearc Moho. Slow subduction of hot young 

oceanic lithosphere might well influence mantle-wedge rheology and lessen cooling of the wedge nose, but it is hard to see 

how elevation of slab temperature by even as much as 100oC 

above normal could alone account for the unusual abundance 

of forearc volcanism (with eruption temperatures as high as 

~1,200oC) in parts of the Cascades. 

Isochrons of Juan de Fuca sea-floor age (Wilson, 

1988, 1989, 2002) suggest subducting slab ages of 14 to 15 

Ma beneath Puget Sound, 10 to 12 Ma beneath the Strait 

of Georgia (facing the Canadian volcanoes), 11 to 12 Ma 

beneath the coastline of southern Washington and northern 

Oregon, and 8 to 10 Ma beneath the coastline of southern 

Oregon and northern California, where deformation has 

made the sea-floor magnetic pattern especially complex. 

Owing to along-strike variation in the longitude and dip of 

apparently abrupt slab steepening farther inboard (presumably at the phase changes to eclogite) and to uncertainties in 

tomographic imaging of steep silent slab segments under the 

volcanic belt itself, slab depths directly beneath the Cascade 

arc remain poorly known (but revisit the slab-depth discussion, above; and see the slab-depth extrapolation of Harry 

and Green, 1999, discussed below). 

Because the Juan de Fuca slab is young and thus unusually warm, several studies have sought linkages between 

Cascadian thermal structure, slab dehydration, and compositions of magmas erupted. Widespread production of primitive 

H2

O-poor HAOT, from Lassen to Glacier Peak, is interpreted 

to indicate especially high temperatures (1,300-1,450oC) in 

parts of the mantle wedge (Bartels and others, 1991; Bacon 

and others, 1997; Elkins-Tanton and others, 2001). Since 

fairly primitive arc basalts that had contained several percent 

H2O at upper-mantle temperatures of 1,100-1,200oC (Baker 

and others, 1994; Sisson and Grove, 1993; Grove and others, 

2002) have erupted from vents situated close in space and 

time to those of HAOT, the thermal structure of the wedge 

cannot be simple. 

Largely on the basis of Li and B depletions, which they 

identified in basalts of all types and attributed to atypically 

early devolatilization of the hot Juan de Fuca slab beneath 

the forearc, Leeman and others (1990, 2004, 2005) argued 

that the slab contribution is weak or absent beneath all of the 

southern Washington Cascades. Assigning great weight to 

loss of these light volatile trace elements, they went so far as 

to propose virtually complete forearc dehydration of the slab, 

thus challenging the very model of arc magma generation 

fluxed by ongoing ascent of slab-derived fluids and proposing 

instead that the abundant arc basalts in southern Washington 

that do carry a typical slab signature had acquired it during 

passage through pre-Quaternary arc lithosphere. Problems 

with such proposals include (1) that advanced forearc dehydration of slabs (warm or cold) is a global phenomenon; (2) that 

the subduction-component geochemical signatures for Cascadian arc basalts are well within the normal range found for 

arcs above slabs of all ages; and (3) that the many HAOT and 

intraplate alkalic basalts that are closely interspersed with normal arc basalts in the Washington Cascades had traversed the 

same lithosphere without acquiring a comparable geochemical 

overprint. Slab dehydration always takes place predominantly 

beneath the forearc (Peacock, 1993; Schmidt and Poli, 1998; 

Bostock and others, 2002; Hacker and others, 2003; Hyndman 

and Peacock, 2003; Brocher and others, 2003). Most of the Li 

and B depletion of the subducting plate is expected to occur 

by collapse of porosity and dehydration of zeolites at 300 to 

500oC, therefore at slab depths shallower than 40 km. Globally, 

only a small fraction of the original slab water can ever reach 

the ~100-km depths beneath an arc, but the fraction remaining 

is obviously adequate to induce wedge melting and to yield 

hydrous magmas in all arcs. Many experimental and theoretical studies (summarized in Hildreth, Fierstein, and others, 

2004) indicate that diverse minor hydrous phases persist—in 

slab eclogite, in serpentinized lower-plate mantle peridotite, 

and in downdragged hydrated wedge peridotite—to depths of 

100 to 250 km. Progressive down-dip reliberation of hydrous 

fluid upward into the mantle wedge, smeared out over a great 

depth range owing to solid-solution effects on breakdown of 

several hydrous phases, helps account for the breadth of the 

Cascade arc, for crude across-arc gradients in intensity of the 

slab contribution identified in some reaches (Borg and others, 

1997; Bacon and others, 1997), and for production of some 

subduction-signature basalts even at the rear-arc Newberry 

and Medicine Lake Volcanic Fields (where HAOT is the 

dominant primitive magma). 

Opposite the Garibaldi Volcanic Belt, the slab youngs 

northwestward at the onset of its subduction beneath the 
98 Quaternary Magmatism in the Cascades—Geologic Perspectives

forearc, because the convergent plate margin is oblique to 

isochrons on the oceanic plate (Wilson, 1988). Thermal 

modeling, accordingly, suggests that the initial temperature 

of the downgoing slab is as much as 75oC hotter opposite 

Mount Meager than opposite Glacier Peak (Harry and Green, 

1999; Green, 2006). The obliquity likewise suggests that, 

downdip beneath the volcanoes, the slab youngs from 22 

Ma beneath Glacier Peak to 19 Ma under Mount Baker to 

15 to 14 Ma beneath the Mount Garibaldi to Bridge River 

reach (Green and Harry, 1999; Green and Sinha, 2005; 

Green, 2006). Arguing that the hotter younger (more northwesterly) oceanic lithosphere would dehydrate earlier and 

more thoroughly, Green and coworkers made a case for an 

along-arc correlation with basalt compositions. When the 

hot and drying subducting slab reaches a position beneath 

the arc, their logic continued, its fluid contribution would be 

relatively weaker to the northwest, where any fluid-induced 

melting of the mantle wedge would there be deferred to 

greater depths and would result in weaker slab geochemical signatures, lower melt fractions, and thus more alkalic 

magmas. The gradient in basalt geochemistry that they correlate with a younger hotter drier slab northwestward along 

the GVB depends heavily, however, on the two ends of the 

segment, as most of the basalts in between carry normal arctype signatures (Green and Sinha, 2005; Green, 2006). At the 

southeast end, the lone HAOT-like low-K basalt in the GVB 

(White Chuck scoria cone) is accompanied nearby by normal 

arc basalts; and it seems hard to fit a hot dry HAOT into the 

cooler wetter end of the model. At the northwest end of the 

segment, the alkalic basalts of Bridge River and Salal Creek 

support the model, but they are likely to represent poorly 

fluxed intraplate upwellings beyond the edge of the slab 

rather than products of hot dry subduction. 

Some intensification of forearc dehydration is reasonable 

for a slab younger and hotter than most, but total elimination of hydrous phases that could remain stable to slab depths 

and temperatures beneath the arc and rear-arc is unrealistic 

(Schmidt and Poli, 1998; Hacker and others, 2003). Moreover, 

even the hottest slab is a heat sink relative to the interior of the 

convecting mantle wedge, which may exceed 1,400oC (Tatsumi and others, 1983; Kushiro, 1987; Ulmer, 2001; Grove 

and others, 2002). An arc-magma generation model for the 

Cascades must account for intimately scattered eruption sites 

for relatively wet arc basalts and dry HAOT, and locally for 

H2

O-poor, low-melt-fraction, intraplate alkali basalts as well. 

It appears likely that once a slab-derived fluid batch induces 

partial melting of wedge peridotite, intrinsic buoyancy of the 

affected region induces domainal upwelling and/or porous 

upward flow of interstitial melt athwart the main corner convection of the wedge. Decompression and percolation through 

the high-temperature core of the wedge should both enhance 

melt fraction (and thus ascent rate) and, by progressive addition of melt at shallower levels, should gradually reduce 

initially high slab-derived water contents of the melt (Grove 

and others, 2002). HAOT generation, on the other hand, is 

probably related to dry decompression melting within the hot 

upper limb of the wedge corner convection, perhaps enhanced 

by extension and local entrainment of the (partially remelting) 

base of the overlying mantle lithosphere. Modelling of such 

corner flow suggests that the upper limb should shallow gently 

trenchward (Furukawa, 1993; Currie and others, 2004), as also 

suggested petrologically by an east-to-west traverse of HAOT 

generation depths across northern California (Elkins-Tanton 

and others, 2001). Because all the HAOT units investigated 

were thought to have first crystallized olivine and plagioclase 

virtually at the base of the crust (Bartels and others, 1991; 

Elkins-Tanton and others, 2001), each must either have originated by partial melting of mantle lithosphere or (more likely) 

have arisen from the hot convecting wedge and penetrated the 

lithosphere as a discrete batch of modest volume. Subsequent 

progress to the surface must have been rapid, because protracted storage of hot crystallizing HAOT in or adjacent to the 

lower crust (unless the envelope were refractory cumulates) 

would inevitably promote voluminous crustal melts, which 

might be expected to contaminate or suppress further ascent of 

the HAOT magma. 

If the trenchward-flowing upper limb of the convecting 

mantle wedge (with or without contributions from superjacent 

mantle lithosphere) is the principal source of HAOT, and the 

slab-fluxed deeper part of the wedge is the starting point for 

arc basalts, then batches of the latter must penetrate the upper 

limb in discrete columns, whether as upwellings, percolation columns of porous flow, fracture networks of channelized flow, or all of these sequentially. With such a discretely 

columnar configuration, the slab-derived geochemical signature should not overprint the whole wedge but only the 

ascent columns, at least where traversing the upper limb and 

lithosphere. In modeling wedge corner flow beneath Cascadia, 

Currie and others (2004) concluded that forced convection by 

slab traction (coupled downdrag) was insufficient to account 

for the high temperatures observed across the arc and rear arc. 

They suggested that an additional component of free convection by small-scale thermally buoyant upwellings is required 

to produce the across-arc thermal structure, consistent with my 

inference based on the closely spaced ascent paths of geochemically contrasting primitive basalts. 

As for the intraplate alkalic basalts, those of the Simcoe 

Mountains Volcanic Field and the Bridge River-Salal Creek 

field may result from low-fraction partial melting of unfluxed 

mantle upwellings, respectively just inboard of and just off 

the edge of the extensive wedge domain affected by contributions arising from the steeply subducting slab. The alkalic 

basalts interspersed with HAOT and arc basalt across southern 

Washington (and rarely elsewhere in the Cascades), however, 

are probably low-fraction partial melts of domains within the 

upper limb of the convecting wedge that remain uncontaminated by the nearby columns of slab-fluxed melt ascending 

through the wedge core. Alternatively, if the slab is broken 

or distorted into adjacent segments of different dip beneath 

southern Washington, the intraplate alkalic basalts might arise 

from columns of sub-slab asthenospheric mantle upwelling 

past the segment margins.
Tectonic and Geophysical Insights 99

Cascadian Basement and Sr-Isotope Overview

Geophysical investigations summarized by Mooney and 

Weaver (1989) show the crust beneath the Cascade arc in the 

USA to range only modestly in thickness (42±4 km), to reach 

an apparent minimum (~38 km) in the (incipiently extended) 

Lassen segment, and to be underlain by upper mantle having 

P-wave velocities of 7.7 to 7.8 km/s. Seismic refraction and 

reflection surveys (Cook, 1995; Clowes and others, 1995; Zelt 

and others, 1996) show the GVB volcanoes of southern British Columbia to overlie slightly thinner crust (35-38 km) and 

slightly faster upper mantle (~7.8-7.9 km/s). 

Despite only modest differences in total crustal thickness, 

the along-strike variation in upper-crustal basement lithologies 

is substantial. The Lassen segment (Berge and Stauber, 1987), 

Medicine Lake (Fuis and others, 1987), and the volcanoes 

in British Columbia (Wheeler and McFeeley, 1991) overlie 

Mesozoic batholiths with varied proportions of metamorphic 

wall rocks. In the Northwest Cascades, Mount Baker overlies a stack of nappes consisting of Mesozoic and Paleozoic 

metavolcanic and marine metasedimentary rocks (Tabor and 

others, 2003), whereas Glacier Peak overlies a terrane boundary between distinct packages of amphibolite-grade Mesozoic 

metasedimentary rocks intruded by discrete tonalitic plutons 

of late Cretaceous age (Tabor and others, 2002). From the 

Mount Rainier area for 500 km south to the Oregon-California 

border, basement rocks are wholly concealed (in the so-called 

Columbia Embayment) beneath Cascadian arc rocks of Eocene 

to Holocene age. A large window of basement rocks 20 km 

southeast of Mount Rainier (Miller, 1989) has age and lithologic affinities to the thrust sheets and mélange belts of both 

the Klamath Mountains and the Northwest Cascades. Consisting of tectonic slices and accretionary mélange of submarine 

fan deposits, hemipelagic sea-floor deposits, pillowed MORB 

greenstones, and tonalitic and mafic metaigneous K-poor 

arc rocks, all of Mesozoic age, the assemblage may be representative of the thick crust concealed beneath much of the 

“embayment.” Although it has been speculated that Mesozoic or Tertiary sea floor might underlie the “embayment,” a 

thick stack of varied accretionary rocks, thrust or underplated 

inboard and translated dextrally northward along the Mesozoic 

continental margin, is more consonant with the 44-km-thick 

sub-arc crust and with the absence of any discontinuity in the 

seismic-velocity structure along a refraction profile up the 

Oregon Cascade axis (Leaver and others, 1984), notably where 

the profile crosses the postulated Klamath-Blue Mountain lineament (Riddihough and others, 1986). Mount Shasta overlies 

the transition between the Mesozoic batholith inboard and a 

stack of thrust sheets of Paleozoic and Mesozoic, accretionary 

and ophiolitic, metasedimentary and metaigneous rocks that 

include the Trinity ultramafic sheet (Fuis and others, 1987). If 

undocumented Precambrian rocks are present anywhere in the 

structurally complex sub-Cascadian crust, they are likely to be 

metamorphosed marine sediments and proportionally trivial.

Crustal contributions to Cascadian magmas are unusually hard to quantify owing to the basement being mostly 

Mesozoic and Tertiary, much of it consisting of accretionary 

arc and oceanic terranes that provide (at best) modest isotopic and chemical contrasts with the mantle-derived magmas 

penetrating them. Such contrasts can actually diminish with 

time as the mean age of the lower arc crust becomes progressively younger owing to secular intrusive underplating by the 

arc basalt itself. Figure 29 presents a compilation of 87Sr/86Sr 

determinations for more than 660 samples of Quaternary eruptive products along the Cascade arc, illustrating a fairly limited 

range of values (0.7027-0.7046), a generally poor correlation between 87Sr/86Sr and bulk composition, and the largest 

isotopic diversity in California (where the basement includes 

Mesozoic granitoids) and in southern Washington (where the 

basement is not exposed but probably accretionary). Although 

more Sr-isotope data are needed for northern Washington and 

parts of Oregon, the along-arc coverage is far better than for 

Nd, Pb, O, or Os isotope data, which are adequate only locally.

Scrutiny of figure 29 shows the following. (1) Sr-isotope ratios for HAOT (47-50% SiO2

) in the Cascades range 

widely—0.7027 to 0.7037 in Washington, 0.7028 to 0.7036 in 

Oregon, and 0.7031 to 0.7044 in California, suggesting routine 

Sr-isotopic contamination of these low-Sr basalts in the arc crust 

and mantle lithosphere. (2) Arc basalts, basaltic andesites, and 

andesites all span wide and comparable ranges of 87Sr/86Sr in all 

segments, suggesting that assimilation of crustal Sr is unsystematic and certainly not a simple AFC process. In the Mazama 

and Jefferson areas, andesites show a wider range and extend 

to lower ratios than the mafic arc rocks. (3) Ratios for rhyodacites and rhyolites generally fall in the upper (more radiogenic) 

half of the range in each area investigated. This should be no 

surprise, since partial melts of crustal rocks are expected to be 

a leading contaminant across the whole range of compositions 

erupted and a proportionately larger one in the more silicic 

rocks. Values of 87Sr/86Sr for rhyolites are as high as 0.7046 in 

the Lassen segment but reach only 0.7036 in Canada, though 

in both areas the magmas erupted through Mesozoic crystalline 

basement. (4) Ratios for dacites range remarkably widely, from 

0.7028 to 0.7044 in California alone, supporting inferences 

discussed previously that Cascadian dacites include varieties 

that originated as andesite-rhyodacite mixtures, as near-primary 

partial melts of mafic crustal rocks, as straightforward fractionates of andesite, and as interstitial melts extracted from a range 

of mushy mafic to andesitic parents. And (5) whatever proportion of the Sr-isotopic diversity in the Cascades may have been 

inherited from mantle sources is at best obscure.

Vent Distribution and Basement Influences 

Although the Juan de Fuca Plate subducts northeast at 3 to 

4.5 cm/yr beneath the North American Plate (fig. 1), the maximum principal horizontal stress direction in the upper plate is 

margin-parallel, not in the convergence direction (Spence, 1989; 

Zoback, 1992; Ma and others, 1996; Hyndman and others, 

2003). Owing presumably to the combination of oblique convergence and northward push of the Sierra Nevada-Klamath block, 
100 Quaternary Magmatism in the Cascades—Geologic Perspectives

0.7046 

>68% SiO2 

63_68 

57_63 

52_57 

47_52 

HAOT 

0.7044 

0.7042 

0.7040 

0.7038 

0.7036 

0.7034 

0.7032 

0.7030 

0.7045 

0.7040 

0.7035 

0.7030 

0.7028 

51o 50o 49o 48o 47o 46o 45o 44o 43o 42o 41o 40o N 

LATITUDE

87Sr/86Sr

British

Columbia

Washington Oregon

California

Figure 29. Sr isotope data plotted versus latitude for the Quaternary Cascades. Explanation of symbols in inset: HAOT (LKOT) 47-50 

percent SiO2; other basalts 47-52 percent SiO2; basaltic andesite 52-57 percent SiO2; andesite 57-63 percent SiO2; dacite 63-68 percent 

SiO2

; rhyodacite and rhyolite >68 percent SiO2. Data sources: Bacon and others, 1994, 1997; Borg and others, 1997; Borg and Clynne, 

1998; Bullen and Clynne, 1990; Conrey, Hooper, and others, 2001; Green and Sinha, 2005; Grove and others, 2002; Halliday and others, 

1983; G.L. Hart and Hildreth, unpub. data; Hildreth and S. Moorbath, unpub. data; W.P. Leeman and Hildreth, unpub. data; Leeman and 

others, 1990; Schmidt, 2005; Smith, 1984.

the direction of compression is north-south in Washington 

and Oregon and as far south as Mount Shasta, within the arc, 

forearc, and parts of the rear-arc such as the Yakima Foldbelt 

and Simcoe Mountains Volcanic Field. This stress orientation is 

reflected in north-south vent alignments from Shasta to Mount 

Adams. Impingement of the inboard dextral shear system no 

doubt influences the many northwest-southeast vent alignments 

in the Lassen segment, and local changes in strike of extensional 

faulting in the rear-arc Medicine Lake and Newberry Volcanic 

Fields evidently contribute to varied vent alignments.

The unusual forearc location of Mount St. Helens and its 

low-K compositions and recurrently explosive behavior may 

reflect in part the structure of the crustal basement. Weaver 

and others (1987) proposed that the volcano is located above a 

right-stepping offset in the north-northwest-striking St. Helens 

seismic zone (inferred to be dextral on the basis of focal mechanisms), permissive of a local extensional domain focusing 

the upper-crustal magma reservoir. A second belt of persistent 

seismicity strikes east-northeast, perpendicularly crossing the 

first beneath the volcano, which appears to have grown within 

a small pull-apart basin. Moreover, Parsons and others (1998) 

interpret a discontinuity in seismic velocity structure beneath 

the St. Helens seismic zone to represent the inboard margin of 

Siletzia (fig. 25), an accreted Eocene basaltic seamount terrane 

as thick as 30 km that forms much of the Oregon-Washington 

forearc basement. Trace-element systematics (Smith and Leeman, 1987) indicate that the dacitic magmas so characteristic 

of Mount St. Helens are not fractionated from basaltic magma 
Tectonic and Geophysical Insights 101

but are produced by partial melting of hydrated basalt or gabbro in the deep crust. If a lower-plate influence is additionally 

important, such as some distortion or tearing of the slab that 

intensifies the flux of slab-derived fluid and/or wedge-derived 

wet basaltic magma into the lower forearc crust of Portland 

and southern Washington, then such effects do not extend 

50 km east of Mount St. Helens, where the andesite-dacite 

products of Mount Adams lack hydrous phenocrysts, generally 

erupt effusively, and are surrounded by an array of peripheral 

vents that include HAOT and alkalic basalt.

The area close to Mount Jefferson is one of the narrowest reaches in the Cascade arc, as defined by Quaternary vent 

distribution (fig. 14). An axial strip that includes the stratocone 

has maintained throughout the Quaternary what is probably 

the lowest proportion of mafic volcanic rocks in the Oregon 

Cascades (Conrey, 1991). Eruption of intermediate and silicic 

products along a high-vent-density (table 1) strip narrower 

than it is long (10x20 km; fig. 14), almost to the exclusion 

of mafic products for 2 Myr or more, supports the notion of 

tightly focussed mantle input engendering a long-lived, partially molten, MASH domain virtually impenetrable by basalt.

The regional “fabric” of volcanism—vent distribution, vent 

density, and the time-volume-compositional record of eruptions—

encourages certain inferences about deeper magmatic processes 

beneath the Cascades. (1) Vent alignments, where not edifice-influenced, are largely controlled by stress orientation and structure 

in the brittle upper crust. (2) Magmatic shadow zones operate at 

two levels—in the deep crust, where MASH domains intercept 

basalts at the base of long-lived magma columns, and in the upper 

crust, where intermediate magma reservoirs prevent throughput of all but their own differentiates. (3) Within mafic volcanic 

fields in and adjacent to the Cascades (Caribou, Medicine Lake, 

Bachelor, Belknap, Newberry, Indian Heaven, Adams periphery, 

Simcoe), no secular patterns or stages of compositional evolution 

are recognized (as, say, in Hawaii); typically, instead, reversals 

and alternations occur among tholeiitic, alkalic, and arc basalts, 

primitive basalts and mafic andesites, and even sporadic eruptions of silicic magmas. Batches from separate sources and with 

different paths of hybridization and fractionation remain concurrently available. (4) Most magmatism is local. Each volcanic field 

evolves independently of others along the arc. If a field is subjected to long-term extension or focussed mantle upwelling, some 

secular shift in source compositions or differentiation processes 

can be expected to modify the suite of magma types being erupted 

there. (5) There is no secular evolution at major stratovolcanoes 

from andesite to dacite; both recur throughout edifice lifetimes. 

(6) There is no systematic drift toward rhyolite production, which 

instead depends on establishment of large fractionating reservoirs 

of intermediate magma that in turn reflect fluctuations in the 

basaltic power supply. Rhyodacite or rhyolite appeared early at 

Lassen (Rockland), Baker (Kulshan), and Simcoe; late at Maidu, 

Garibaldi, and Meager; both early and late at Mazama, Newberry, Medicine Lake, and St. Helens; and recurrently during the 

so-far short history of South Sister. (7) Most of the true rhyolite 

(≥72% SiO2

) in the Cascades erupted either in the rear-arc centers 

or along an extension of the across-Oregon progression beyond 

Newberry to Tumalo, South Sister, and Obsidian Cliffs. (8) Mafic 

vent density is lowest in the compressional GVB and greatest in 

patently extensional areas, notably Indian Heaven, the BachelorBelknap region, Caribou, and the three big rear-arc volcanic fields. 

(9) Lack of extension in the GVB reduces also the number of 

evolved volcanic loci, both along and across the arc (fig. 3). And 

(10) the greater proportion of basalts across southern Washington 

(and Portland) and across the Lassen segment contrasts with the 

dominance of basaltic andesite among the Quaternary mafic rocks 

of the Oregon Cascades. It seems unlikely that a significantly more 

fusible crustal column would be responsible for diluting the mantle 

input preferentially beneath Oregon, because dacitic and silicic 

volcanic rocks are plentiful at the major centers in all three regions. 

Tectonically influenced vent alignments are also prominent in 

all three regions, though the observation that the proportion of 

basalt attains a maximum for Oregon in the Bachelor area, where 

such vent alignments are best developed, lends plausibility to the 

expectation that tectonic extension can favor unimpeded ascent of 

basaltic magma. The broader vent belts of southern Washington 

and northern California may simply reflect more widely distributed extension that provides more paths for basalt to the surface 

than does the more narrowly constrained extensional vent belt of 

the Oregon Cascades, where more concentrated basaltic flux may 

increase deep-crustal melting and impede magma transit, causing 

greater degrees of fractionation and reprocessing to be the norm.

K2O Asymmetry Across the Cascade Arc?

Ever since the suggestion that the potassium content 

(K) of arc magma suites might correlate with depth (h) of the 

subducting slab beneath their volcanic centers (Dickinson 

and Hatherton, 1967), the search for across-arc patterns in 

concentrations of K (and other LILE and isotope ratios) has 

been pursued by many investigators (for example, Hickey and 

others, 1986; Luhr, 1992; Stern and others, 1993; Ryan and 

others, 1995; Tatsumi and Eggins, 1995; Patino and others, 

1997; Hildreth, Fierstein, and others, 2004). In the Cascades, 

the search might seem futile because of the widespread 

distribution of low-K HAOT along, across, and inboard of the 

arc and because of the great range of K contents among the 

diverse basalts at several individual loci. For example, in the 

area close to Mount Adams, the K2O content of fairly primitive basalts (at 48 percent SiO2

) ranges from 0.1 to 1.6 percent 

(Hildreth and Lanphere, 1994). 

If a cross-arc K-h correlation were held to originate with 

the basaltic members of suites starting in the mantle, then 

processes potentially responsible could include, first, a set of 

those that are linked to slab depth itself—(1) progressively 

smaller melt fractions inboard from the volcanic front owing 

to a diminishing flux of slab-derived fluid with slab depth; (2) 

greater secular depletion of the wedge source below the volcanic front where the slab flux is most intensive; and (3) pressure-dependent breakdown of K-poor hydrous phases at shallower slab depths and K-richer ones (phengite, phlogopite) at 

greater depths. Second, a set of influences independent of slab 
102 Quaternary Magmatism in the Cascades—Geologic Perspectives

depth could contribute across-arc geochemical asymmetries 

to the magmas—(1) the sub-arc mantle wedge might inherit 

an initial lateral gradient from subcontinental inboard to more 

oceanic outboard; (2) the sub-arc lithosphere might similarly 

inherit a gradient from more potassic and thicker inboard to 

thinner oceanic or accreted terranes outboard; (3) the inboard 

mantle lithosphere might be older than that beneath the front 

and thus more enriched by secular veining from below; (4) 

rear-arc extension might promote decompression melting of a 

secularly enriched mantle lithosphere that contributes less to 

magmas that penetrate unextended parts of the arc; (5) rear-arc 

extension might promote separation of low-melt-fraction alkalic magmas from mantle upwellings unfluxed by slab contributions; and (6) the upper limb of the mantle-wedge convective 

counterflow might draw more potassic inboard mantle toward 

rear-arc and sub-arc melting domains, where melt extraction 

would then impose progressive depletion trenchward. All nine 

possibilities are feasible, so geochemical asymmetries across 

broad arcs should not be unexpected, but the second set of six 

involves conditions or processes not tied to slab depth; and 

intraplate alkali basalts (on which many supposed K-h correlations depend) are not arc magmas at all. In some arcs the nine 

potentially contributing factors are assembled more systematically than in the Cascades, where accretionary lithosphere, 

varied basement terranes, a warm warped slab, highly heterogeneous mantle, and latitudinally uneven rear-arc and intra-arc 

extension severely complicate the relative influences.

We need to recall, however, that the original K-h observation by Dickinson and Hatherton (1967) emphasized K contents of andesitic suites (normalized at 55 or 60% SiO2

), not 

basalts, as in those early years of the subduction paradigm many 

investigators thought that andesitic magmas were primary. If, 

instead, most andesites are hybrids that have assimilated crustal 

contributions, the possibility of across-arc gradients in age and 

composition of the crustal assimilants should be added to the list 

of factors potentially producing geochemical asymmetry.

In the Cascades, the segments where any pattern of 

across-arc asymmetry might be best discerned (through the 

confusion of mixed magmas and fields of interspersed vents 

for arc and non-arc basalts) could be expected to be the 

broadest ones, southern Washington and the Lassen segment. 

Indeed, ignoring HAOT, the mafic arc lavas of a 60-km-wide 

transect of the Quaternary Lassen segment show a rough tendency for K, Fe, P, HFSE, REE, Ba, and Th to increase eastward (Borg and others, 1997; Clynne and Borg, 1997). These 

investigators emphasized the dual importance of increasingly 

fertile (less depleted) mantle inboard and diminishing intensity of the slab-fluid flux inboard. On the other hand, for the 

southern Washington-to-Portland transect, where HAOT and 

alkali-basalt vents are closely interspersed with those for arc 

basalts across the entire 160-km width of the volcanic belt, 

Leeman and others (1990, 2004, 2005) found among the 

arc basalts only random scattering of K2

O contents and no 

systematic across-arc pattern at all in incompatible elements or 

radiogenic isotope ratios. Moreover, for the eruptive suites in 

southern Washington, a K2O-SiO2 plot (fig. 30) shows largescale overlap among the four west-to-east volcanic fields and 

a great range of K2

O among the basalts of each suite. In the 

basaltic-to-silicic andesite range (52-63% SiO2), it can generally be argued that the K2

O arrays increase inboard (fig. 30), 

but one needs statistics to affirm it.

Continuing Quest for Clarity

A great variety of investigations into convergent-margin 

processes has contributed over recent decades to improving our 

understanding of arc magmatism globally, as in the Cascades. In 

contrast to a generation ago, there is now considerable agreement (though seldom unanimity) that the following points are 

fairly well established. (1) The fundamental basis of plate-margin arc magmatism is mobilization from a subducting plate of 

a water-rich contribution that ascends into a convecting mantle 

wedge, where it enhances partial melting and buoyancy. (2) 

Because the wedge is cooler above, below, and at its outboard 

corner than within its core, melting paths are influenced by 

reversal of the thermal gradient as well as by decompression and 

by lateral and temporal variations in concentration of the slabderived contribution. (3) Decompression melting of drier mantle 

domains (little influenced by proximate slab contributions) 

takes place concurrently, more so in the Cascades than in other 

continental-margin arcs, presumably in response to pressure 

release by intra-arc and rear-arc tectonic extension. (4) Several 

primitive mantle-derived magmas coexist. Most are Mg-rich 

basalts, but a few Mg-rich andesites may also be near-primary 

partial melts of mantle peridotite (harzburgite) previously 

melt-depleted but strongly refluxed by water-rich slab-derived 

contributions. (5) When hot mantle magmas penetrate the crust, 

crustal contributions are inevitable and ubiquitous, but they 

range widely in magnitude. Basaltic magma intrudes the base of 

the crust all along and across the arc, but the crust modulates its 

transit and traps most of it. (6) The deep-crustal melting caused 

by injection of batches of basaltic magma might sometimes 

involve ancient rocks but principally entails remelting arc 

gabbros and their differentiates—the arc plutonic rocks that constructed the deep Cascadian arc crust during the past 40 Myr. A 

few arc andesites and dacites may be near-primary partial melts 

of such (relatively young) mafic crust, but most are complexly 

hybrid blends of crustal and juvenile basaltic contributions. 

Deep-crustal HAOT gabbro may likewise have begun to play 

such a role after the onset of intra-arc extension around 7 to 5 

Ma, perhaps contributing partial remelts to the LILE-deficient 

basaltic andesites common in the Oregon Cascades (Hughes, 

1990; Schmidt, 2005). (7) The few large mafic volcanoes as 

well as the myriad small ones are fed from deep-crustal reservoirs that incorporate only modest crustal contributions. And (8) 

the large long-lived andesite-dacite stratocones and domefields 

are supplied by crustal columns rooted in major deep-crustal 

domains of melting, fractionation, and hybridization that fluctuate thermally with variations in intensity of focussed mantle 

input. Most such columns also accumulate upper-crustal reservoirs, in which crystallization-differentiation is extensive when 
Continuing Quest for Clarity 103

3.0

2.5

2.0

1.5

5.0

5.5

4.5

4.0

3.5

1.0

0.5

45 50 55 60 65 70 75

Indian Heaven

St. Helens

Simcoe Volcanic Field

Mt. Adams 

postglacial 

15-70 ka 

pre-70 ka 

HIGH-K

MEDIUM-K

LOW-K

SiO2

K

2

O

Figure 30. K2O versus SiO2 contents (in weight percent, normalized to 100 percent volatile-free) of products of vents distributed 

across southern Washington (fig. 8), grouped by volcanic field—from west to east, Mount St. Helens (n>300), Indian Heaven (n>100), 

Mount Adams (n>700), and Simcoe Mountains Volcanic Field (n>700). K2

O contents of andesites (at 58 pecent SiO2) are 0.8 to 1.4 

percent at St. Helens, ~1.0 percent at Indian Heaven, 1.3 to 2.4 percent at Adams, and 2.2 to 4.4 percent in the Simcoe field. Data 

sources: Clynne and others (in press); Hammond and Korosec (1983); Hildreth and J. Fierstein, unpub. data; Hildreth and Lanphere 

(1994); Korosec (1989); Leeman and others (1990, 2005); Smith and Leeman (1987, 1993). Higher-K set of St. Helens basalts erupted 

centrally during the interval 2.5-1.9 ka, as did many of the lower-K basalts at that volcano.
104 Quaternary Magmatism in the Cascades—Geologic Perspectives

magmas attain water saturation; they build plutons and may or 

may not fractionate melt-rich caps of rhyodacite or rhyolite.

In contrast, understanding of the following matters could 

be said to range from limited clarity to downright poor; all 

deserve continued investigation. (1) What is the composition 

of the slab-derived contribution? If its release into the wedge is 

pulsed rather than continuous, how compositionally varied are 

the batches? (2) How widely is partial melt present in the subarc mantle wedge? Is wedge convection continuous or pulsed 

and how does its motion affect melting and melt coalescence? 

What causes melt-ascent paths that reach the lithosphere to be 

locally focussed or intensified? (3) Prior to melting or ascent, 

what were the spatial relations among the mantle domains that 

yielded contrasting HAOT, alkalic, and arc basalts? Were they 

dispersed randomly (like plums in pudding), layers at different 

depths, melting columns contrasting with ambient host mantle, 

or convecting asthenospheric mantle vis-à-vis remobilized 

lithosphere? How might the rheology of dry and wet upwellings 

differ? (4) How much pre-arc lithospheric mantle survives as 

such beneath a long-lived magmatic arc? Where partially melted 

and remobilized during arc magmatism, does such former lithosphere then participate in wedge convection and to what extent 

does it contribute to arc magmas? (5) Why is the intensely 

focussed basaltic flux that pins MASH domains (and the derivative crustal ascent columns that feed upper-crustal reservoirs 

and stratovolcanoes) maintained for million-year intervals but 

limited at a given time to only a few sites along an arc? (6) Why 

do so many mafic volcanic fields and intermediate stratovolcanoes alike have apparent longevities in the range 0.5 to 1.5 Myr? 

(7) Away from the basalt-entrapping MASH domains, what 

controls the rates of crustal throughput for batches of basalt? 

Why do a few erupt with still-primitive compositions while 

most stall, fractionate, and assimilate their envelope, evolving to 

low-Mg basalt or basaltic andesite during transient intra

crustal storage? (8) What causes a few long-lived andesite-dacite centers to initiate episodes of voluminous rhyodacite or rhyolite melt production while most do not? (9) Do the lengthy gaps 

along the arc (table 5) that lack volcanic activity for millions of 

years at a time reflect muffling of magmatism (as favored here) 

by a tight lithosphere? Or is there some property or behavior of 

the slab or wedge that temporarily reduces melt production or 

coalescence along such reaches? And (10) what proportion of 

the intracrustal magma system beneath a long-lived volcanic 

center fails to erupt? Is the eruptive/intrusive ratio far greater 

for a mafic stratocone or shield than for a major andesite-dacite 

center (as favored here)? Or does the former leave behind a 

proportionally large mafic-ultramafic intrusion in the lower 

crust? For a big evolved system, how is the total intrusive mass 

partitioned among deep-crustal cumulates, varied hybrid restites, deep to shallow plutons, and leftovers distributed as sheets, 

pods, and conduit-fills along the whole crustal column?

There are many uncertainties and interesting questions to 

be worked out by the next generation. Promising investigations 

include not only new theoretical, geochemical, and geophysical approaches to convergent-margin processes but detailed 

field studies to work out the time-volume-compositional evolution of Cascadian magmas, in systems large and small, mafic 

and evolved, volcanic and plutonic, Tertiary and Quaternary, 

and (whenever possible) with attention to measurable rates 

of concurrent tectonic and magmatic processes. The progress report and overview presented here nonetheless makes 

clear that modeling the Cascades as consisting of a few big 

andesite-dacite centers misses the essence of the arc, which is 

better expressed by the 2000 mafic vents that map out the wide 

distribution of sub-arc magmatism.

Acknowledgments

This overview was initiated at the invitation of George 

Bergantz, organizer of the Cascadia keynote symposium of the 

1994 GSA Annual Meeting in Seattle (Hildreth, 1994), and it 

was resurrected a decade later at the behest of Jeff Wynn, chief 

of the USGS Volcano Hazards Team. Reviews and comments 

that improved the clarity and accuracy of the long manuscript 

were provided by Bob Christiansen, Mike Clynne, Julie Donnelly-Nolan, Judy Fierstein, Don Swanson, and Willie Scott, to all 

of whom I am truly grateful. Meticulous scrutiny by USGS editor Jim Hendley polished the presentation and achieved suitable 

accommodation between stylistic convention and the author's 

eccentricities. I also thank Kelly Russell, Melanie Kelman, 

Patrick Muffler, and Willie Scott for providing fine photographs, 

and especially my friend and lifelong champion of Mount 

Adams, Darryl Lloyd, for his fine images of the Guardians of 

the Columbia. My perspectives elaborated in this paper are an 

outgrowth of three decades of authentic geologic mapping in the 

Cascades, principally by my colleagues in the USGS Geothermal Research and Volcano Hazards Programs, spearheaded and 

spurred foremost by L.J. Patrick Muffler. Major contributors 

have been Charlie Bacon, Duane Champion, Bob Christiansen, 

Mike Clynne, Rocky Crandell, Julie Donnelly-Nolan, Russ 

Evarts, Judy Fierstein, Cynthia Gardner, Norm MacLeod, Dan 

Miller, Patrick Muffler, Don Mullineaux, Kevin Scott, Willie 

Scott, Dave Sherrod, Jim Smith, Tom Sisson, Don Swanson, and 

Ed Wolfe. The perspectives, however, are my own, and those 

colleagues are not responsible for any errors, omissions, or misinterpretations. As new data come in, ideas will inevitably and 

properly change; all geologic studies are progress reports. The 

high-precision geochronology of Marvin Lanphere and Andy 

Calvert, particularly in dating late Pleistocene eruptive products, 

has been crucial in advancing our understanding of the rates of 

magmatic processes. Outside the USGS, the contributions of 

Rick Conrey, Nathan Green, Paul Hammond, Cathie Hickson, 

Cliff Hopson, Bill Mathews, Kelly Russell, Jack Souther, and 

Ed Taylor have been especially important. Howel Williams, of 

course, got it all started and covered more of the Cascades than 

most of us. During my 27 summers in the Cascades, scarcely 

a week went by when I didn't muse on the vitality, insight, and 

promise of my long-lost field pardner, Dave Johnston, who was 

swept away in the blast of May 18th, 1980 (see Hildreth, 1981b). 

In one sense, my field investigations in the Cascades and Alaska 

have always, I think, been dedicated to the recognition that I 

was privileged to carry on and Dave wasn't.
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