U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1822

Late Holocene Volcanism at Medicine Lake Volcano, 

Northern California Cascades
COVER PHOTO

Aerial view looking west across the ~950-yr-old Glass Mountain lava flow. Snow-covered Mount Shasta is on the 

horizon. Medicine Lake can be seen beyond and to the left of dome of Glass Mountain proper. The lake lies within the 

Medicine Lake caldera at the center of Medicine Lake volcano.
Late Holocene Volcanism at Medicine Lake 

Volcano, Northern California Cascades

By Julie M. Donnelly-Nolan, Duane E. Champion, and Timothy L. Grove

Professional Paper 1822

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Donnelly-Nolan, J.M., Champion, D.E., and Grove, T.L., 2016, Late Holocene volcanism at Medicine Lake volcano, 

northern California Cascades: U.S. Geological Survey Professional Paper 1822, 59 p., 

http://dx.doi.org/10.3133/pp1822.

ISSN 2330-7102 (online)

ISSN 1044-9612 (print)

ISBN 978-1-4113-4037-4
 iii

Contents

Abstract...........................................................................................................................................................1

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

The ~1-ka Episode..........................................................................................................................................8

Glass Mountain .....................................................................................................................................8

Previous Work ............................................................................................................................12

The Glass Mountain Eruption ..................................................................................................12

Inclusions....................................................................................................................................17

Storage and Extrusion of Magma ...........................................................................................17

Little Glass Mountain..........................................................................................................................23

Basaltic Andesite of Paint Pot Crater..............................................................................................29

Callahan Flow ......................................................................................................................................30

Hoffman Flows.....................................................................................................................................32

The ~3-ka Episode........................................................................................................................................35

Burnt Lava Flow...................................................................................................................................35

Basalt of Black Crater and Ross Chimneys....................................................................................37

The ~5-ka Episode........................................................................................................................................39

Dacite of Pit Craters ...........................................................................................................................39

Medicine Lake Glass Flow.................................................................................................................41

Identifying the Timing of the Eruptions Using Paleomagnetism ..........................................................42

Crease Structures...............................................................................................................................44

Near-vent Welded Tephra .................................................................................................................44

Quenched Magmatic Inclusions (QMI)...........................................................................................44

Prismatically Jointed Peripheral Blocks.........................................................................................47

Lava Levee Environments Including Channel Walls and Shear Planes ....................................47

Spatter Vents and Flow Surfaces in Mafic Lava Flows................................................................47

Results of Paleomagnetic Sampling................................................................................................47

Geophysical Data and Implications..........................................................................................................48

Magma Sources and Processes...............................................................................................................48

Late Holocene Mafic Lavas .............................................................................................................49

Late Holocene Silicic Lavas..............................................................................................................49

Episodic Eruptive Activity and Tectonics........................................................................................55

Conclusions...................................................................................................................................................56

Acknowledgments.......................................................................................................................................56

References....................................................................................................................................................56
iv 

Figures

1. Shaded relief location maps showing major volcanoes of the Cascades volcanic arc, and a 

part of northern California showing Mount Shasta, the Cascades axis, and Medicine 

Lake volcano..................................................................................................................................2

2. Photographs showing view south across basaltic lava in Lava Beds National Monument 

toward the main edifice of Medicine Lake volcano, and aerial view of Medicine Lake in 

the central caldera of the volcano.............................................................................................3

3. Shaded relief map of Medicine Lake volcano adapted from Donnelly-Nolan and others.........4

4. Maps featuring Holocene lavas at Medicine Lake volcano ...........................................................5

5. Plots showing chemical composition of late Holocene lavas and magmatic inclusions compared to the range of all other lavas erupted from Medicine Lake volcano ......................7

6. Photograph taken from rim of late Holocene Paint Pot Crater showing the black and red coloring of this youngest mafic cinder cone at Medicine Lake volcano mantled by white 

pumice from the adjacent and slightly younger Little Glass Mountain eruption...............8

7. Aerial view looking northwest across the upper part of the Glass Mountain flow ....................9

8. Aerial view looking west-southwest across the central part of Medicine Lake volcano 

toward Mount Shasta dimly seen through haze on the horizon.........................................10

9. View westward toward the dome of Glass Mountain from eastern zone of mingled and 

spine-covered rhyodacitic lava................................................................................................10

10. Views east across the northern Glass Mountain vent fissure....................................................11

11. Photo of Glass Mountain tephra taken in 1987 during active quarrying adjacent to and east 

of dome 10....................................................................................................................................13

12. Photo shows elongate pinkish band of pumice breccia, easily identified by the trees that 

grow in this zone of broken, brecciated, and in part oxidized pumice and lava..............13

13. Map showing compositional variation within the Glass Mountain flow and domes..............14

14. Photomicrographs of Glass Mountain lavas in order of decreasing silica content ...............15

15. View looking east along old bulldozer road that extends across the eastern part of the main 

Glass Mountain flow ..................................................................................................................16

16. Photograph of a single block of Glass Mountain mingled lava, shows quenched magmatic 

inclusions, porphyritic dacite, and a band of aphyric rhyolitic obsidian...........................16

17. Map showing the distribution of the variety of inclusions found in Glass Mountain lavas...18

18. Photomicrographs of quenched magmatic inclusions in Glass Mountain lavas ....................19

19. Photomicrographs of cumulate-textured inclusions in Glass Mountain lavas........................20

20. Photomicrographs of plutonic inclusions in late Holocene silicic lavas ..................................21

21. Plots showing the full range of compositions found in Glass Mountain...................................22

22. Plots show the highest-silica compositions of the Glass Mountain eruption..........................23

23. Aerial view of Little Glass Mountain, looking west, with snow-covered Mount Shasta on 

the horizon ...................................................................................................................................24

24. Aerial view looking northeast across the rocky and unvegetated Crater Glass flows ..........25

25. Aerial view looking southwest over the Crater Glass flows toward Little Glass Mountain 

along the entire length of the ~1,000-yr-old rhyolite eruptive fissure................................25

26. The Little Glass Mountain eruption was accompanied by the opening of cracks along, and 

parallel to, the vent fissure........................................................................................................25

27. Map showing lavas erupted from the Little Glass Mountain fissure and vents ......................26

28. Photomicrographs of the Little Glass Mountain host lava and a variety of inclusions ..........27

29. Photograph showing typical occurrence of granitic inclusions found in Little Glass Mountain lava ........................................................................................................................................28
 v

30. Largest sampled magmatic cumulate inclusion in Little Glass Mountain sits among blocks 

of rhyolite at the edge of the flow............................................................................................28

31. Plots comparing mafic magmatic inclusion suites in the Glass Mountain and Little Glass 

Mountain lavas............................................................................................................................29

32. Aerial view looking northeast across the late Holocene Paint Pot Crater flow and cone ....30

33. Photomicrographs of the late Holocene mafic host lavas ..........................................................31

34. View northeast across the southern Callahan Flow and its main vent, Cinder Butte ............32

35. Aerial view west across the northern domes of the Glass Mountain fissure in the foreground to the northern part of the Hoffman flows in the center of view...........................33

36. Photomicrographs of the host lava and inclusions from the Hoffman flows ...........................34

37. Photograph of High Hole Crater, vent for the ~3-ka Burnt Lava Flow, as seen across a part 

of this blocky andesite flow.......................................................................................................35

38. Plots showing compositional variability in the Burnt Lava Flow................................................35

39. Photomicrographs of quenched basaltic inclusions in the Burnt Lava Flow...........................36

40. Photograph of the uppermost Ross Chimneys spatter vents in northern Lava Beds National 

Monument....................................................................................................................................37

41. Outline map of the ~3-ka basalt of Black Crater and Ross Chimneys .......................................38

42. Paleomagnetic drilling in a crease structure on the surface of the Medicine Lake Glass 

Flow...............................................................................................................................................39

43. Photomicrographs of the ~5-ka dacitic lavas and inclusions.....................................................40

44. View west-southwest from Mount Hoffman, the highest point on the rim of Medicine Lake 

caldera, toward Mount Shasta.................................................................................................41

45. Photograph showing the use of a sun compass to orient core samples collected for paleomagnetic study............................................................................................................................42

46. Map showing paleomagnetic site locations listed in table 3......................................................43

47. Paleomagnetic drilling in crease structures on the southern of the two Hoffman flows ......44

48. Photograph of welded near-vent agglutinate located adjacent to Glass Mountain 

dome 3 ..........................................................................................................................................45

49. Photograph of drilling in a crease structure on dome 6, the smallest of the Glass Mountain 

domes ...........................................................................................................................................45

50. Photographs of paleomagnetic sampling at big crack in the center of the main Glass 

Mountain flow .............................................................................................................................46

51. Paleomagnetic drilling at Ross Chimneys in mafic lava showing evidence of fluid 

morphology ..................................................................................................................................46

52. Plots of paleomagnetic directions for late Holocene units grouped by age............................48

53. Silica variation diagrams showing compositions of mafic lavas and quenched magmatic 

inclusions erupted in late Holocene time ...............................................................................50

54. Plot showing experimentally-determined compositional paths compared to the actual 

Glass Mountain compositions ..................................................................................................53

55. Illustration showing Holocene eruptive episodes color-coded by composition as in figure 

4A for late Holocene time..........................................................................................................54

56. Photographs showing examples of numerous open cracks at Medicine Lake volcano .......55
vi 

Tables

[Tables 1–3 are supplemental and online only, table 4 included in text]

1. Chemical analyses of late Holocene lavas and magmatic inclusions.

2. Chemical analyses of plutonic inclusions in late Holocene lavas.

3. Late Holocene Medicine Lake volcano paleomagnetic data.

4. Summary of late Holocene erupted materials at Medicine Lake volcano .................................52

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Abbreviations

HAOT high-alumina olivine tholeiite

ka thousands of years ago

MLV Medicine Lake volcano

PTRM partial thermal remanent magnetization

QMI quenched magmatic inclusions
Late Holocene Volcanism at Medicine Lake Volcano, 

Northern California Cascades

by Julie M. Donnelly-Nolan1, Duane E. Champion1, and Timothy L. Grove2

1

U.S. Geological Survey

2

Massachusetts Institute of Technology

Abstract

Late Holocene volcanism at Medicine Lake volcano in 

the southern Cascades arc exhibited widespread and compositionally diverse magmatism ranging from basalt to rhyolite. 

Nine well-characterized eruptions have taken place at this 

very large rear-arc volcano since 5,200 years ago, an eruptive frequency greater than nearly all other Cascade volcanoes. The lavas are widely distributed, scattered over an area 

of ~300 km2

 across the >2,000-km2 volcano. The eruptions 

are radiocarbon dated and the ages are also constrained by 

paleomagnetic data that provide strong evidence that the 

volcanic activity occurred in three distinct episodes at ~1 ka, 

~3 ka, and ~5 ka. The ~1-ka final episode produced a variety 

of compositions including west- and north-flank mafic flows 

interspersed in time with fissure rhyolites erupted tangential to 

the volcano’s central caldera, including the youngest and most 

spectacular lava flow at the volcano, the ~950-yr-old compositionally zoned Glass Mountain flow. At ~3 ka, a north-flank 

basalt eruption was followed by an andesite eruption 27 km 

farther south that contains quenched basalt inclusions. The 

~5-ka episode produced two caldera-focused dacitic eruptions. 

Quenched magmatic inclusions record evidence of intrusions 

that did not independently reach the surface. The inclusions 

are present in five andesitic, dacitic, and rhyolitic host lavas, 

and were erupted in each of the three episodes. Compositional and mineralogic evidence from mafic lavas and inclusions indicate that both tholeiitic (dry) and calcalkaline (wet) 

parental magmas were present. Petrologic evidence records the 

operation of complex, multi-stage processes including fractional crystallization, crustal assimilation, and magma mixing. 

Experimental evidence suggests that magmas were stored at 

3 to 6 km depth prior to eruption, and that both wet and dry 

parental magmas were involved in generating the more silicic 

magmas. The broad distribution of eruptive events and the 

relative accessibility and good exposure of lavas, combined 

with physical and petrologic evidence for multiple and varied 

mafic inputs, has created an unusual opportunity to understand 

the workings of this large magmatic system. A combined total 

of more than 25 intrusive and extrusive events are indicated 

for late Holocene time. Plutonic inclusions, some with ages 

as young as Holocene, were also brought to the surface in five 

of the eruptions. All eruptions took place along northwest- to 

northeast-trending alignments of vents, reflecting the overall 

east-west extensional tectonic environment. The interaction 

of tectonism and volcanism is a dominant influence at this 

subduction-related volcano, located where the west edge of 

the extensional Basin and Range Province impinges on the 

Cascades arc. Ongoing subsidence focused at the central 

caldera has been documented along with geophysical evidence 

for a small magma body. This evidence, combined with the 

frequency of eruptive and intrusive activity in late Holocene 

time, an active geothermal system, and intermittent longperiod seismic events indicate that the volcano is likely to 

erupt again.

Introduction

Medicine Lake volcano (MLV) is a large shield-shaped 

volcano (figs. 1 and 2) that lies east of the southern Cascades 

arc axis ~100 km above the surface of the subducting Juan de 

Fuca slab (McCrory and others, 2012). This subduction-related 

rear-arc Cascades volcano is located in an extensional tectonic 

setting where the Basin and Range Province impinges upon 

the arc. Some 600 km3 of lavas were erupted during its halfmillion-year history, ranging in composition from basalt (both 

tholeiitic and calcalkaline) to high-silica rhyolite (DonnellyNolan and others, 2008; Donnelly-Nolan, 2010) and covering 

more than 2,000 km2. Episodic volcanic activity has occurred 

throughout the volcano’s history, none better documented than 

in postglacial time. Since ice melted off the volcano, 17 eruptions have taken place, 8 of them in a brief ~200-yr episode 

of basaltic eruptions ~12,500 years ago (Donnelly-Nolan and 

others, 2007). The 17 eruptions are unusually well understood 

in terms of their distribution in time, space, and composition 

(Donnelly-Nolan and others, 2007)3. 

Late Holocene eruptive activity at MLV produced a 

remarkably diverse array of magmatic compositions (table 1, 

online only) ranging from basalt to rhyolite in nine well-dated 

eruptions representing one of the best-documented late Holocene records in the Cascades. The nine eruptions occurred in 

three episodes at ~1 ka, ~3 ka, and ~5 ka following an eruptive 

hiatus of ~7,000 years. Although not as widely distributed, nor 

as voluminous as the eight earlier eruptions, the late Holocene 

3

Details of the ages and their uncertainties are in Nathenson and others 

(2007), where the ages are listed in table 1 in calibrated years before present 

(B.P.), where present is, by convention, 1950.
2 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Mount Shasta

122°00' 121°30'

41°45'

41°15'

Medicine Lake

 volcano

Cascades axis

Donnelly-Nolan et. al., Figure 1

121°00'

MEDICINE LAKE

0 10

10 20

MILES

0 KILOMETERS

0 10

10 20

MILES

0 KILOMETERS

MEDICINE LAKE

Newberry Volcano

Three Sisters

Mount Jefferson

Mount Hood

Mount Adams

Mount

Mount Rainier

Glacier Peak

Mount Baker

Crater Lake

Mount Shasta

Lassen Peak

Mount McLoughlin

St. Helens

Newberry Volcano

Three Sisters

Mount Jefferson

Mount Hood

Mount Adams

Mount

Mount Rainier

Glacier Peak

Mount Baker

Crater Lake

Mount Shasta

Lassen Peak

Mount McLoughlin

Medicine Lake

volcano

St. Helens

A B

OR

WA

CA

0 5

5 10

MILES

0 KILOMETERS

50

5 10

Figure 1. Shaded relief location maps showing (A) major volcanoes of the Cascades volcanic arc, and (B) a part of northern California 

showing Mount Shasta, the Cascades axis, and Medicine Lake volcano. Red outline of Medicine Lake lavas from Donnelly-Nolan (2010).

lavas nonetheless cover a total of 95 km2 scattered across a 

300 km2 area that extends 33 km north-south by 19 km eastwest (fig. 3) centered on the volcano’s summit caldera. For 

comparison, the area over which the lavas are scattered is 

twice the footprint of the Mt. St. Helens edifice. 

Maps showing the distributions of the late Holocene 

lavas by episode and by composition are shown in figure 4A,B. 

Names of the eruptive units, along with areas, volumes, vent 

locations, and distribution are from Donnelly-Nolan (2010). 

Five of the nine late Holocene eruptions are silicic (rhyolite 

and dacite) and all of their vents are located within or immediately adjacent to the volcano’s central caldera. The ~1-ka 

episode includes five eruptions between 950 and 1,230 years 

ago. In order of increasing age they are the rhyolite of Glass 

Mountain, the rhyolite of Little Glass Mountain, the basaltic 

andesite of Paint Pot Crater, the basaltic andesite of the Callahan Flow, and the rhyolite of the Hoffman flows. The ~3-ka 

episode includes the andesite of the Burnt Lava Flow and the 

basalt of Black Crater and Ross Chimneys. The ~5-ka episode 

includes two small silicic eruptions, the dacite of pit craters 

and the dacite of Medicine Lake Glass Flow.

Evidence for the ages of the postglacial eruptions and for 

identifying the episodes of activity came from both radiocarbon dating and from paleomagnetism (Nathenson and others, 

2007). The role of paleomagnetism in constraining eruption ages was particularly important. Forty-three sites were 

drilled, of which 35 provided good directional information 

even for some very rugged, undissected young lava flows 

that presented significant sampling challenges. In this paper 

we discuss the variety of outcrop types that were drilled and 

describe the successes and failures, along with the laboratory 

approach to solving some paleomagnetic challenges. Of the 43 

sites in late Holocene units that were sampled for paleomagnetism, 33 of the sites were in the ~1-ka units. Most of the effort 

was focused on the Glass Mountain (14 sites) and Little Glass 

Mountain (10 sites) fissure products, in what was ultimately a 

vain attempt to determine whether any significant time elapsed 

during either eruption.

The combination of multiple young erupted lavas exhibiting a wide range of composition, together with the presence of 

a variety of included cognate magmatic materials (table 1), has 

provided an unusual opportunity to understand magmatic processes at the volcano. Quenched magmatic inclusions (QMI) 

and inclusions with cumulate textures were found in more 

than half of the late Holocene lavas. In addition to evidence 

for intrusive events from the physical presence of magmatic 

inclusions, complex multi-stage magmatic histories are 

recorded within the lava flows in less obvious ways, including 

compositional zonation and mineralogical evidence (Grove 

and Donnelly-Nolan, 1986; Grove and others, 1988; Grove 

and others, 1997; Kinzler and others, 2000). Compositional 

data plotted in figure 5 compare late Holocene lavas and their 

cargoes of inclusions to the overall range of compositions at 

Medicine Lake volcano. It is apparent that magmatic materials brought to the surface in the late Holocene both overlap, 

and are distinguishable from, the earlier erupted materials. 
Introduction 3

A

B

Figure 2. Photographs showing (A) view south across basaltic lava in Lava Beds National Monument toward the 

main edifice of Medicine Lake volcano, and (B) aerial view of Medicine Lake in the central caldera of the volcano. This 

namesake lake is about 2 km long. On the distant horizon is the snow-covered peak of Mount Shasta, located about 55 

km to the west-southwest; treeless gray area beyond Medicine Lake is late Holocene Little Glass Mountain.
4 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Lower

Klamath

Lake Tule

Lake

Medicine

Lake

41°52'30''

41°7'30''

 MIR AREDLAC

LBNM

Glass 

Mountain

Burnt 

Lava

Flow 

Giant 

Crater flow

Little Glass 

Mountain

Medicine

Glass Flow

Lake 

Hoffman 

flows 

Callahan 

Flow 

Paint Pot 

Crater flow 

Chimneys 

Ross

Black 

Crater 

pit craters 

Valentine 

Cave 

Visitor

Center 

Devils

Homestead 

flow 

flows 

flow 

Postglacial lavas 

~12,500

Less than 5,200 

years old 

years old 

Volcanic vent 

121°45' 121°15'

0 5

5 10

MILES

0 KILOMETERS

EXPLANATION

Figure 3. Shaded 

relief map of Medicine 

Lake volcano adapted 

from Donnelly-Nolan 

and others (2007). 

Approximate location 

of caldera rim, dashed 

black line; early 

postglacial lavas ~12,500 

years old, light pink; late 

Holocene lavas ~5,200 

years old and younger, 

dark pink; faults, thin 

black lines; Lava Beds 

National Monument 

on north flank of the 

volcano, LBNM. Box is 

area shown in figure 

4A,B.
Introduction 5

121°22'30''

41°45'

41°30'

121°30'

0 1 2 3

1 2 3 4 5

MILES

0 KILOMETERS

Rhyolite

Dacite

Andesite

Basaltic

Andesite

Basalt

EXPLANATION

Caldera Rim

Medicine Lake

Giant Crater

Hoffman

flows

Burnt Lava

Flow

Dacite of the

pit craters

Paint Pot 

Crater flow

Medicine Lake

Glass Flow

& Crater

Glass flows

Little Glass

Mountain

Callahan

Flow

Glass Mountain

Black Crater &

Ross Chimneys

1 9

4

3

5

2

8

6

7

5 2

A

Figure 4. Maps featuring Holocene lavas at Medicine Lake volcano (adapted from Donnelly-Nolan and others, 2007). Area 

of coverage is indicated by box on figure 3. (A) Map showing distribution of late Holocene lavas by composition; in weight 

percent SiO2

, basalt is <53, basaltic andesite 53–56.9, andesite 57–62.9, dacite 63–69.9, and rhyolite >69. Light gray areas 

are ~12.5-ka lavas. Numbers 1 through 9 identify late Holocene eruptive units from youngest (1) through oldest (9); (B) Late 

Holocene lavas shaded by eruptive episode. Crosses are vent locations, red for late Holocene, black for the ~12.5-ka lavas. 

Unit symbols are from Donnelly-Nolan (2010); rhyolite of Glass Mountain, rgm; rhyolite of Little Glass Mountain, rlg; basaltic 

andesite of Paint Pot Crater, mpp; basaltic andesite of Callahan Flow, mcf; rhyolite of Hoffman flows, rh; andesite of Burnt Lava 

Flow, abl; basalt of Black Crater and Ross Chimneys, bbr; dacite of pit craters, dpc; dacite of Medicine Lake Glass Flow, dm.
6 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

41°30'

121°22'30''

41°45'

121°30'

0 1 2 3

1 2 3 4 5

MILES

0 KILOMETERS

Medicine

Lake

bbr

mcf

rgm

rh

abl

rlg

mpp

dpc

dm

rlg

rh

~ 1 ka

~ 3 ka

~ 5 ka

EXPLANATION

B

Figure 4.—Continued
Introduction 7

Magmatic inclusions were absent in the early rhyolitic lavas of 

the volcano (Donnelly-Nolan and others, 2008) and relatively 

scarce until the late Holocene. Chemical analyses of the analyzed inclusions that were found in older lavas are also listed 

in table 1.

Five of the nine late Holocene lava flows also brought 

plutonic inclusions, mostly granitic, but also gabbroic and 

dioritic, to the surface (table 2, online only). Granophyric 

textures in some of the granitic inclusions suggest shallow 

depths of origin. Dating of zircons from selected granitic 

inclusions yielded ages between ~300 ka and late Holocene, 

indicating that they are not fragments of older crust (Lowenstern and others, 2000; Lowenstern and others, 2003). One 

granodiorite sample collected during geothermal drilling in the 

MLV caldera also gave an age of ~300 ka. Petrologic studies 

(for example, Grove and others, 1988; Grove and others 1997) 

indicate that assimilation of granitic material has played a 

significant role in generating evolved compositions at MLV.

The late Holocene lava flows at MLV can all be accessed 

by road and, with a few exceptions, have little or no tephra 

cover and typically only a limited tree cover. The abundance 

of youthful lavas was noted in early geologic work by Peacock 

(1931) and Powers (1932). Anderson (1933) focused initially 

on Glass Mountain and subsequently published an article 

about the volcano as a whole with an accompanying geologic 

map (Anderson, 1941). Additional work was done in the 1970s 

SiO2, in weight percent

50 55 60 65 70 75

2

3

4

5

6

50 55 60 65 70 75

La/Yb

0

4

8

12

MgO, in weight percent

0 2 4 6 8 10

Sr, in ppm

100

300

500

700

A

B

C

D

TH

CA

Glass Mt

Little Glass 

Paint Pot

Callahan

Hoffman

Burnt Lava

Black Crater

Pit craters

Medicine LGF

FeO*/MgO

0

1

2

3

4

5

6

7

8

SiO2, in weight percent

50 55 60 65 70 75

SiO2, in weight percent

Na O, in weight percent

2

~1 ka

~3 ka

~5 ka

EXPLANATION

Figure 5. Plots showing chemical composition of late Holocene lavas and magmatic inclusions compared to the range of all other 

lavas erupted from Medicine Lake volcano (black outline). Symbol colors indicate eruptive episode (see figure 4B and labeled brackets) 

with colored fields indicating the range of inclusion compositions for those episodes. Black line in (A) separates fields designated as 

tholeiitic (TH) and calcalkaline (CA) by Miyashiro (1974).
8 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

and 1980s by Condie and Hayslip (1975), Eichelberger (1975, 

1980, 1981), Mertzman and Williams (1981), and Fink and 

Pollard (1983), among others. A comprehensive list of previous research is available in Donnelly-Nolan (2010). Beginning 

in the early 1980s, T.L. Grove and colleagues began a series 

of detailed petrologic studies, several of which focused on 

the late Holocene lavas (Grove and others, 1982, 1988, 1997; 

Gerlach and Grove, 1982; Grove and Donnelly-Nolan, 1986; 

Kinzler and others, 2000). Updated geologic studies include 

Donnelly-Nolan and others (2007, 2008) and the new detailed 

geologic map (Donnelly-Nolan, 2010). 

This publication is focused on understanding the nature 

and frequency of late Holocene magmatic activity at MLV. 

New data and interpretive studies of individual eruptive events 

will be presented along with summaries of pertinent published 

geologic, geochemical, and petrologic information. Tectonic 

and geophysical constraints on the magma system will also 

be discussed. We begin with the most recent eruption, the 

~950-yr-old Glass Mountain eruption that produced the largest 

Holocene rhyolitic lava flow in the Cascades volcanic arc, 

where rhyolitic volcanism has been relatively rare.

The ~1-ka Episode

In the most recent volcanic episode at Medicine Lake 

volcano, five eruptions took place over an interval of <300 

years from ~950 years ago to ~1,230 years ago (DonnellyNolan and others, 2007). These included the three late 

Holocene rhyolites plus two interspersed mafic eruptions. The 

Glass Mountain and Little Glass Mountain eruptions occurred 

half a century apart in time and 16 km in distance from fissures on opposite sides of the caldera. On trend with Little 

Glass Mountain, but more than a century earlier in time, was 

the basaltic andesite of Paint Pot Crater, so named because of 

the later deposit of Little Glass Mountain white pumice on the 

black and red cone (fig. 6). Just prior to this mafic eruption, 

the compositionally zoned andesite-to-basalt Callahan Flow 

erupted on the north flank of the volcano. The earliest eruption 

in this episode took place when the rhyolitic Hoffman flows 

erupted along a northwest-trending fissure parallel to, and one 

km west of, the subsequent Glass Mountain fissure. The total 

estimated volume produced during this episode was 1.85 km3

, 

and covered an area of ~54 km2.

Glass Mountain

Originally known as Big Glass Mountain (Peacock, 

1931), this spectacular lava flow is the largest Holocene rhyolitic lava flow in the United States. The main vent dome, Glass 

Mountain (fig. 7), rises to an elevation of 2,323 m, less than 

100 m lower than the highest point on the volcano. Draped 

over the steep eastern slope of the volcano (fig. 8), the easternmost tongues reach elevations as low as 1,478 m. The jagged 

surface of the lavas (fig. 9), nearly barren of vegetation, stands 

in marked contrast to the surrounding forested terrane. Viewed 

from the east, halfway across the main lava flow, an 80-m-high 

front of black rhyolitic obsidian glistens in the sunlight, atop 

a stony dacitic lava erupted earlier in the same continuous, 

compositionally zoned eruption. At least 15 vents along the 

Figure 6. Photograph 

taken from rim of late 

Holocene Paint Pot 

Crater showing the 

black and red coloring 

of this youngest 

mafic cinder cone 

at Medicine Lake 

volcano mantled by 

white pumice from the 

adjacent and slightly 

younger Little Glass 

Mountain eruption.
The ~1-ka Episode 9

Glass Mountain

C

dome 8

Mount McLoughlin

Figure 7. Aerial view looking northwest across the upper part of the Glass Mountain flow. The last-erupted 

rhyolite of this youngest eruption at Medicine Lake volcano forms the prominent dome of Glass Mountain 

proper. Toward the viewer, south of the dome is the high margin of the last rhyolite lava flow. Just south of 

that high margin is a set of three north-trending cracks, the uppermost of which (labeled C) provided access 

for paleomagnetic sampling. Beyond the far edge of the main flow are the treeless northern small domes 

along the vent fissure. The distance from the dome of Glass Mountain, to dome 8, is ~2.5 km.
10 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Medicine Lake

Figure 8. Aerial view looking west-southwest across the central part of Medicine Lake volcano toward Mount Shasta dimly 

seen through haze on the horizon. Early dacitic lobes of the Glass Mountain flow are outlined in white for visibility. Beyond Glass 

Mountain is the broad shallow bowl of the 7 by 12-km caldera with Medicine Lake near the center. The distance from Medicine 

Lake to Mount Shasta is ~55 km. Adapted from figure 3 in Donnelly-Nolan and others (2008).

Figure 9. View westward toward the dome of Glass Mountain from eastern zone of mingled and spine-covered rhyodacitic 

lava. Photograph shows typical chaotic and blocky flow surface.
The ~1-ka Episode 11

5-km-long vent fissure produced ~1 km3 total of lava, consisting dominantly of rhyolite with subordinate dacite and 

minor andesite. The flows have an overall compositional span 

of 74.6–61.3 weight percent SiO2. Included within the host 

lava are a variety of quenched magmatic inclusions (QMI), 

cumulate magmatic inclusions, and rare plutonic inclusions. 

About 10 percent of the total volume consists of early rhyolitic 

tephra (Heiken, 1978). Along the fissure to the north and south 

of the main flow, some near-vent tephra has welded to form 

agglutinate deposits adjacent to the small domes. The total 

area covered by the main flow and the adjacent small domes 

(fig. 10) along the north-northwest-trending fissure (domes 1 

through 10 to the north, one to the south) is ~14 km2, the vast 

majority of it in the main flow.

Main flow

1 2 3 45

7 8 9

10 6

A

B

Figure 10. Views east across the northern Glass Mountain vent fissure show (A) the north edge of the main 

flow with domes 1 through 5 labeled; the open area in front of domes 1 and 2 is a former drill pad for a geothermal 

test well; and (B) domes 6 through 10. Note that photo (B) is a closer view than photo (A).
12 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Previous Work

Glass Mountain attracted the early attention of researchers, including Peacock (1931), Powers (1932), and Anderson 

(1933). Both Powers (1932) and Anderson (1933) recognized the presence of rhyolite and dacite in the main Glass 

Mountain flow, along with the presence of inclusions. They 

obtained chemical analyses of the lava and Anderson also 

listed one analysis of an inclusion. Both described the intimate relationship and sharp contacts between the dacite and 

bands of rhyolitic obsidian. Anderson (1933, p. 498) further 

concluded that “...obsidian was being extruded as dacite left 

the vent, for otherwise it is difficult to explain the presence of 

bands of rhyolite obsidian in the front of the dacite tongues.” 

Anderson’s map (1933, his fig. 3) shows the approximate 

distribution of rock types. Subsequently, Eichelberger (1975) 

produced a more detailed map of the main Glass Mountain 

flow and demonstrated that there was overall eruptive continuity from dacite through rhyolite lava. Eichelberger recognized 

the complexity exhibited in the multiple lobes of lava and 

mapped the distribution of subunits that were described as 

“rhyolite with inclusions” and “banded lava”, the latter being 

zones of physically mingled compositions. Eichelberger also 

argued against the idea that separate and internally homogeneous dacite and rhyolite obsidian magma compositions were 

erupted simultaneously. The same year, Condie and Hayslip 

(1975) established the extent of compositional variation within 

the rhyolite and dacite, explaining that the dacites formed 

“...by shallow mixing of a hybrid magma (composed of 

varying amounts of andesite and dacite) with rhyolite prior 

to and during eruption.” Eichelberger (1980, 1981) revised 

his previous interpretation that the mafic inclusions were 

lithic fragments and instead concluded that they represented 

a basaltic foam generated by intrusion of mafic magma into 

a shallow rhyolitic magma chamber. In Eichelberger’s 1981 

model, vesiculation of the basalt was followed by mixing and 

eruption.

Subsequent to Eichelberger’s 1975 study, a limited 

number of chemical analyses of Glass Mountain host lavas 

and inclusions were presented in Grove and Donnelly-Nolan 

(1986) and in Grove and others (1997).

The Glass Mountain Eruption

The Glass Mountain eruption began with explosive 

activity along the length of the vent fissure, generating tephra 

deposits (fig. 11) that have, in part, been exposed by mining activities. Tephra samples collected north and south of 

the main flow contain >72.3 weight percent SiO2, with most 

analyses >73.5 weight percent (table 1). Near the vents, 

adjacent to northern domes 1–10, and adjacent to the southern 

small dome, coarse tephra was hot enough to weld, forming 

agglutinate deposits that are preserved surrounding and adjacent to most of the small domes. The deposits contain bombs 

that have pumiceous interiors surrounded by breadcrust rinds, 

as well as prismatically jointed blocks that were erupted and 

broke apart after cooling. At the main Glass Mountain vent, 

scoriaceous dacitic to andesitic blocks were also ejected and 

preserved in late near-vent agglutinate now buried by the main 

flow. These near-vent blocks are preserved scattered along 

linear bands of dominantly rhyolitic pumice breccia (Anderson, 1941) within the flow (fig. 12). The pumice breccia was 

described by Anderson (1933) as representing the remnants of 

the original pumice cone. The breccia consists of broken and 

jumbled pieces, from sand-size to several meters in length of 

welded agglutinate along with andesitic to rhyolitic pumice 

and prismatically jointed blocks. These narrow breccia zones 

can easily be recognized from a distance because, in contrast 

to the treeless flow rocks, they are host to a sparse growth of 

spindly and undernourished evergreen trees. Scattered andesitic to dacitic scoria samples collected from the dominantly 

rhyolitic pumice breccia contain 61.3 to 65.7 weight percent 

SiO2

. 

Emplacement of lava flows and domes began after the 

explosive eruptions ceased. The first lava to erupt in the 

main flow, as previously recognized by Anderson (1933) and 

Eichelberger (1975), was a phyric mixed dacite mingled with 

mafic blobs of quenched magmatic inclusions (QMI). The 

early dacite also entrained small volumes of aphyric rhyolite. 

As the eruption proceeded, rhyolite and dacite poured nearly 

a kilometer in vertical elevation down the steep eastern slope 

of the volcano. Shear zones developed between major lobes 

apparently traveling at different rates. Eruption of the mingled 

rhyolite-dacite lava was followed by rhyolite containing sparse 

QMI. The most voluminous, and most likely the last, lava to 

be erupted was aphyric, inclusion-free rhyolite that formed 

the bulk of the main flow, including the spine-covered dome 

of Glass Mountain itself that appears to have been the primary 

vent.

Glass Mountain lava flow and tephra compositions range 

in silica content from 61.3 percent to 74.6 weight percent. 

Chemical analyses of 102 lava and tephra samples are presented in table 1. The generalized spatial distribution of lava 

compositions and the locations where samples were collected 

are shown on figure 13. The map was created using the chemical analyses together with field observations and aerial photographs. The new mapping and compositional data strongly 

support the idea (Anderson, 1933) that different compositions 

were erupting from the vent at the same time.

Field criteria for distinguishing rhyolitic lavas from 

dacitic lavas include luster (dull for dacite, glassy for rhyolitic 

obsidian), abundance of phenocrysts (more in dacite; fewer 

or none in rhyolite), and abundance of inclusions (common 

in dacite; few or none in rhyolite). Dacitic lavas (<70 weight 

percent SiO2

) typically exhibit up to 3 percent of 1–3 mm 

phenocrysts in hand specimen, with flow-aligned plagioclase 

more abundant than orthopyroxene and magnetite. Quenched 

magmatic inclusions (QMI) and crystal debris (plagioclase + 

olivine + clinopyroxene) from disaggregated inclusions are 

also commonly present in the dacite. Detailed petrographic 

descriptions of Glass Mountain lavas and inclusions can be 

found in Grove and others (1997). Rhyolitic lavas with more 
The ~1-ka Episode 13

Figure 11. Photo of Glass 

Mountain tephra taken in 1987 

during active quarrying adjacent 

to and east of dome 10 (in 

background).

Figure 12. Photo shows 

elongate pinkish band of pumice 

breccia (Anderson, 1936), easily 

identified by the trees that 

grow in this zone of broken, 

brecciated, and in part oxidized 

pumice and lava. Anderson 

(1933) considered these zones 

to consist of near-vent pumice 

and agglutinate from a pumice 

cone material subsequently 

incorporated into the lava flow 

and dragged eastward.

than 73.5 weight percent SiO2 are aphyric; those with 70–

73.5 percent contain 1–2 percent of 1 mm plagioclase and rare 

1 mm orthopyroxene and magnetite. QMI are present in these 

lower-silica rhyolites, but they are much less abundant than in 

the dacitic lavas. Photomicrographs of selected thin sections 

from a range of lava compositions are shown in figure 14.

Two old bulldozer tracks, one from the south and another 

from the east, provide easily accessible foot traverses across 

parts of the main flow (fig. 13), although many samples were 

also obtained in less accessible areas. Where multiple compositions were obviously present, samples were collected to 

represent the maximum apparent difference in SiO2

 content. A 

broad generalization derived from the mapping is that compositional variation on a scale of tens of meters is commonly 

gradational from dacite to rhyodacite to rhyolite. However, 

the mingling of bands of various compositions also took place 

at the scale of meters to centimeters to millimeters, and the 

definitions of subunits in the geologic mapping are therefore somewhat arbitrary. In some places, flow emplacement 

processes have caused one compositional type to overtop or 

flow across another, juxtaposing lava of very different SiO2

contents. One of the most complexly banded parts of the main 
14 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades 1 km 1 2 3 4 5 6 87 9 10 Glass Mountain

41°35.0'

121°27.5'

121°30.0'

41°37.5'

1 0

1

MILE

KILOMETER 0

South dome

74.6 - 73.8 Weight percent SiO2

73.7 - 70.0 Weight percent SiO2

<70.0 Weight percent SiO2

Sample locationVent location

EXPLANATION

Figure 13. Map showing compositional variation within the Glass Mountain flow and domes. Small domes along the eruptive fissure are numbered 1–10, south to north from the northern edge of the main Glass Mountain flow; “South dome” designates the dome south of the main flow. Chemical analyses of samples are listed in table 1 along with latitudes and longitudes of each sample. Two heavy dashed lines, on south and east sides of main flow, are old bulldozer tracks that provide easy access on foot.
The ~1-ka Episode 15

A B

C D

E F

1 mm

Figure 14. Photomicrographs of Glass Mountain lavas in order of decreasing silica content. (A) 1406M (74.6 weight 

percent SiO2

) aphyric obsidian displays faint foliation defined by tiny microlites; (B) 1135M (71.7 weight percent 

SiO2

) contains small crystal clots of plagioclase and orthopyroxene in glassy groundmass with scattered very small 

plagioclase; (C) 1426M (68.6 weight percent SiO2

) has microphenocrysts of orthopyroxene, magnetite, and plagioclase 

and one clot in a glassy groundmass that contains abundant discernible plagioclase laths; (D,E) plane light and crossedpolarized light views of 1692M (65.6 weight percent SiO2

) display microphenocrysts of plagioclase and orthopyroxene in 

murky groundmass with some vesicles and a limited amount of discernible plagioclase microlites; (F) crossed-polarized 

light image of 1896M (61.3 weight percent SiO2

) shows vesicular groundmass crowded with small plagioclase and a single 

rounded microphenocryst of orthopyroxene. All images are the same scale. Chemical analyses are in table 1.
16 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Figure 15. View looking east along old 

bulldozer road that extends across the 

eastern part of the main Glass Mountain 

flow. The road allows easy pedestrian 

access to some of the most mingled and 

rugged parts of the flow. 

Figure 16. Photograph of a single block 

of Glass Mountain mingled lava, located 

along the east road, shows quenched 

magmatic inclusions, porphyritic (possibly 

mixed) dacite, and a band of aphyric 

rhyolitic obsidian. Hammer head is 

approximately 15 cm long.

flow is the east central portion traversed by the eastern bulldozer track (fig. 15). Mingling occurred in this area on scales 

from microscopic to hundreds of meters, and figure 16 shows 

one example with alternate bands of phyric dacite and aphyric 

rhyolitic obsidian, both of which have flowed around QMI. 

Samples 1683M-A (74.5 weight percent SiO2) and 1683M-B 

(68.5 weight percent SiO2

) were collected in this same area 

of the flow from adjacent bands of dacite and rhyolite within 

a single block only 0.3 m across. Other examples of compositional heterogeneity include samples 1617M (66.4 weight 

percent SiO2) and 1618M (74.5 weight percent SiO2) that were 

collected from different blocks at the end of the shorter of two 

northern distal lobes dominated by dacite. Samples 1541M 

(66.8 weight percent SiO2

) and 1542M (73.5 weight percent 

SiO2) were collected from opposite ends of the same 1-m 

block.

All of the small domes are aphyric rhyolite except dome 

7, which consists of lower silica rhyolite that contains 

~1–2 percent phenocrysts. Four samples of dome 7 yield silica 

contents ranging from 72.3 to 73.4 weight percent (table 1), 

for an average of 73.0 weight percent. One sample from adjacent dome 8 overlaps this range at 73.3 weight percent, but 

the remaining 14 samples of the northern and southern domes 

range from 73.6 to 74.6 weight percent SiO2

 and average 

74.0 weight percent. The total volume of all the small domes 

taken together is very small, <0.02 km3

.
The ~1-ka Episode 17

Inclusions

Three types of inclusions were found in Glass Mountain lavas: quenched magmatic inclusions (QMI), cumulate, 

and a small number of pieces with fully crystallized plutonic 

textures ranging from granitic to gabbroic in composition 

(table 2). The spatial distribution of inclusion types is shown 

in figure 17. The QMI are the most common type (40 of 60 

inclusion analyses listed in table 1). They are fine-grained, 

with <5 percent phenocrysts, and display features typical of 

chilled blobs of mafic magma that have been injected into a 

cooler host magma. As described by Bacon (1986), QMI are 

vesicular, have large vesicle spaces at the ends of the spheroidal blobs, and display crenulate convex-outward margins; 

radial fractures can typically be seen in the surrounding glassy 

lava host. Some fine-grained inclusions lack any phenocrysts 

but most include a scattering of larger crystals, commonly 

aggregates of 1–2 mm plagioclase together with 1 mm clinopyroxene and olivine, in a few cases accompanied by orthopyroxene and Fe-Ti oxides, and rarely by hornblende. Abundant 

sub-millimeter crystals display rapid growth morphologies, for 

example, plagioclase showing hopper or skeletal shapes. Tiny 

pyroxene and plagioclase commonly form sprays of acicular 

crystals, in some cases together with hornblende. Small olivines are almost never present. Fe-Ti oxide grains and vesicular glass are ubiquitous. Photomicrographs of selected QMI 

samples are shown in figure 18.

The macroscopic features typical of the fine-grained 

chilled blobs of mafic magma (QMI) also characterize the 

cumulate inclusions, with the important exception that these 

latter inclusions exhibit 30–75 percent accumulated crystals. 

Nineteen such inclusions were analyzed and are listed in table 

1. The accumulated crystals range in size from 1–5 mm and 

include phenocrysts of plagioclase, clinopyroxene, and olivine 

in all samples, orthopyroxene and Fe-Ti oxides in six samples, 

and hornblende in just three of the samples. The larger crystals 

are set in a glassy vesicular matrix exhibiting abundant tiny 

plagioclase and pyroxene crystals, Fe-Ti oxides, and in 11 

cases, hornblende. The groundmass crystals display rapid 

growth morphologies, commonly occur in sprays of acicular 

crystals, and mimic the appearance of the fine-grained QMI 

inclusions. The abundance of groundmass glass and quench 

crystals like those of the fine-grained QMI indicate that the 

cumulate inclusions are co-magmatic. Photomicrographs of 

selected cumulate samples are shown in figure 19.

Inclusions range in size from microscopic crystal clots 

to nearly a meter across. Most are 15 cm or less in the longest dimension. QMI are common in the dacitic portions of 

the main Glass Mountain flow, where they may constitute 1 

percent or more of the volume. They are present, but less common, in rhyolitic host lavas. Of 27 inclusions collected from 

the main flow, all are QMI with the single exception of one 

cumulate (1579M-a, table 1, fig. 17). One sample of included 

material (1543M-a, table 1) is characterized as neither QMI 

nor cumulate. It occurs as an aphyric rind on a partially melted 

granitic inclusion and is higher in SiO2

 (62.8 weight percent) 

than the bulk of the QMI (54.5 to 58.8 weight percent SiO2

). 

This inclusion and sample 1679M (with SiO2 of 60.7 weight 

percent) are similar to quenched andesitic inclusions found in 

Little Glass Mountain (Grove and Donnelly-Nolan, 1986).

The granitic inclusions found in Glass Mountain lavas 

are similar to those found in Little Glass Mountain and in 

the Medicine Lake Glass Flow. The latter flow also contains 

gabbroic inclusions similar to those found in Glass Mountain 

dome 7. Photomicrographs of a selected subset of the inclusions are shown in figure 20; chemical analyses are listed in 

table 2.

The most diverse suites of inclusions were found at the 

distal ends of the Glass Mountain eruptive fissure. Inclusions 

have been found in the five northernmost domes (domes 6–10) 

and the southernmost dome, although no inclusions large 

enough for analysis were found in dome 6, the smallest of the 

Glass Mountain domes. One small granitoid inclusion was 

seen in dome 6. The six analyzed inclusions from the northernmost dome (dome 10) are all either plutonic or cumulate in 

texture, although thin sections of small, unanalyzed inclusions 

indicate that QMI are also present. Four QMI and four cumulate-textured inclusions were analyzed from the southernmost 

dome, where two granitic inclusions were also found. The 12 

QMI in the small domes along the fissure range in composition 

from 54.2 to 60.7 weight percent SiO2

, a somewhat greater 

compositional range than QMI in the main flow (54.5–58.8 

weight percent SiO2), but the average silica content is nearly 

the same: 56.4 weight percent for the domes, 56.6 weight 

percent for the main flow. Table 1 includes 60 analyses of 

magmatic inclusions, which range in silica content from 50.8 

weight percent to 62.2 weight percent, with the most mafic 

inclusions having cumulate textures. Inclusions were found 

only in lavas containing <73.5 weight percent SiO2. Available 

exposures of the initial tephra deposit along the length of the 

fissure do not appear to contain inclusions, but the deposits 

were not examined in detail.

The Glass Mountain eruption produced an unusually 

wide range of compositions from basalt to rhyolite. Welldefined arrays exist for most samples on many variation 

diagrams (fig. 21), although dacitic samples commonly show 

more scatter than the rhyolites and some inclusions plot off 

the trends. Compositions of all the erupted magmatic components at Glass Mountain form continuous arrays from basalt to 

rhyolite on major- and trace-element plots.

Storage and Extrusion of Magma

The Glass Mountain eruption was clearly complex, 

involving multiple compositions erupted along the length of 

the 5-km fissure. The fissure eruption was dike-fed near the 

surface, although the shape of the deeper magma reservoir is 

unknown. The distribution of cumulate and plutonic inclusions, which are nearly all found at the ends of the fissure, 

suggests that the ends of the dike may have sampled reservoir 

margins. Eichelberger (1975) proposed a model of the magma 

system in which dacitic magma was postulated to overlie 
18 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades 1 km 1 2 3 4 5 6 87 9 10 Glass Mountain

41°35.0'

121°27.5'

121°30.0'

41°37.5'

South dome

1 0

1

MILE

KILOMETER 0

Distribution of inclusions

Quenched magmatic inclusionsCumulate inclusionsPlutonic inclusions

Vent location

EXPLANATION

Figure 17. Map showing the distribution of the variety of inclusions found in Glass Mountain lavas. Old bulldozer tracks shown as two dashed lines, in south and east parts of flow. Chemical analyses and locations of individual samples are in table 1.
The ~1-ka Episode 19

A B

C D

E F 1mm

Figure 18. Photomicrographs of quenched magmatic inclusions in Glass Mountain lavas. Images on left taken in plane 

light; those on right are equivalent images under crossed-polarized light. (A,B) ML-4 (54.2 weight percent SiO2) contains 

abundant elongate plagioclase together with clinopyroxene in part as sprays of elongate crystals in dark vesicular 

groundmass; (C,D) 1140M-f (56.7 weight percent SiO2

) has intergrown pyroxene and skeletal plagioclase in a vesicular 

matrix of brownish glass together with small brown needles of amphibole; (E,F) 1715M (58.7 weight percent SiO2

) shows 

small prisms of brown amphibole together with larger pyroxene and plagioclase, plus oxide grains in a vesicular, brownish 

glassy groundmass that contains tiny needles of plagioclase and pyroxene(?). All images are the same scale. Chemical 

analyses are in table 1.
20 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

A B

C D

E F

1 mm

Figure 19. Photomicrographs of cumulate-textured inclusions in Glass Mountain lavas. Images on left of A and B, C and 

D taken in plane light; those on right are equivalent images under crossed-polarized light. (A,B) 1141M (52.1 weight percent 

SiO2

) exhibits phenocrysts of olivine, clinopyroxene, and plagioclase in a fine-grained matrix with plagioclase, amphibole, 

and pyroxene needles, and oxide grains in vesicular glass; (C,D) 1312M (53.0 weight percent SiO2

) contains plagioclase, 

clinopyroxene, and olivine phenocrysts in fine-grained matrix of pyroxene needles studded with small opaque grains, 

small amphibole and plagioclase laths, and glass; (E) cross-polarized light image of 1691M (54.9 weight percent SiO2

) 

has intergrown plagioclase, brown amphibole, clinopyroxene, and orthopyroxene crystals with interstitial glass matrix 

containing plagioclase laths, oxide grains, and elongate skeletal amphibole; (F) plane light image of 1139M-a (54.1 weight 

percent SiO2

) displays a large crystal of brown amphibole and adjacent clinopyroxene and orthopyroxene in a glassy 

vesicular matrix containing small skeletal plagioclase, pyroxene needles, and oxide cubes. All images are the same scale. 

Chemical analyses are in table 1.
The ~1-ka Episode 21

A

B

C

D

E

F

Figure 20. Photomicrographs of plutonic inclusions in late Holocene silicic lavas. Samples shown in images A, B, and C

on the left are granitic; samples D, E and F on the right are gabbroic. (A) Sample 2028M (73.7 weight percent SiO2

) from the 

Medicine Lake Glass Flow, in crossed-polarized light view shows plagioclase, quartz, and oxidized biotite, in a matrix of 

graphic-textured intergrowth of quartz and potassium feldspar. (B) Sample 2029M (73.7 weight percent SiO2

) from Little Glass 

Mountain flow shows partial melting in this crossed-polarized light view; note that a thin zone of melt surrounds crystals of 

feldspar, quartz, biotite, and small orthopyroxene and oxide grains. (C) Sample 663M-c (73.6 weight percent SiO2

) from Little 

Glass Mountain; shows intergrown feldspar and quartz plus long needle of biotite near the center of the crossed-polarized 

light image. (D) Sample 1045M-a (50.0 weight percent SiO2

) from Glass Mountain dome 7; the crossed-polarized light view 

shows intergrown plagioclase, pyroxene, olivine, biotite, and amphibole. (E, F) Two views of sample 2027M from Medicine 

Lake Glass Flow (50.3 weight percent SiO2

) which show intergrown plagioclase, two pyroxenes, and minor olivine together 

with small oxide grains and vesicular interstitial glass; E is plane light image; F is crossed-polarized light image. Scale bars 

are each 1 mm in length. Chemical analyses are in table 2.
22 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

0

1

2

3

4

5

6

7

50 55 60 65 70 75

GM lava & tephra

GM quenched inclusions

GM cumulate inclusions

LGM lava & tephra

FeO*/MgO

wt.% SiO2

TH

CA

2

4

6

8

10

50 55 60 65 70 75

CaO

SiO2

0

1

2

3

4

5

0 2 4 6 8 10

wt.% K O2

wt.% MgO

0

2

4

6

8

10

12

14

16

50 55 60 65 70 75

ppm Th

wt.% SiO2

5

10

15

20

25

0 1 2 3 4 5

ppm La

wt.% K2O

EXPLANATION

14

15

16

17

18

19

20

2 4 6 8 10

wt.% Al O2 3

wt.% FeO*

SiO2, in weight percent

SiO2

, in weight percent

SiO2

, in weight percent

MgO, in weight percent

FeO, in weight percent

CaO, in weight percent Th, in ppm

K2O, in weight percent

Al O2 3, in weight percent K O, in weight percent

2 La, in ppm

FeO*/MgO

Figure 21. Plots showing the full range of compositions found in Glass Mountain (GM) including lavas, tephra, and inclusions). 

Little Glass Mountain (LGM) rhyolite samples are also shown for comparison with the Glass Mountain rhyolite.
The ~1-ka Episode 23

4.25

4.3

4.35

4.4

4.45

4.5

4.55

73.6 73.8 74 74.2 74.4 74.6

wt.% K O2

wt.% SiO2

1.55

1.6

1.65

1.7

1.75

1.8

1.85

1.9

73.6 73.8 74 74.2 74.4 74.6

Dome 1

Domes 2-5

Domes 6-10+So.

In early dacite

Main rhyolite flow

Tephra

wt.% FeO*

EXPLANATION

SiO2, in weight percent

SiO2

, in weight percent

K O, in weight percent

2 FeO, in weight percent

Figure 22. Plots show the highest-silica compositions of the 

Glass Mountain eruption. The 5 samples with the highest SiO2

(>74.4 weight percent) are from dome 1 and from 4 rhyolite “pods” 

within the early dacitic lobes of lava.

rhyolite magma prior to eruption. Eichelberger and others 

(2000) proposed a general model in which silicic magma 

can underlie and intrude overlying, more mafic magma; 

Glass Mountain is among the examples proposed. We reject 

this model because the early rhyolitic tephra came from the 

entire length of the fissure, clearly implying that rhyolite was 

the magma closest to the surface when the eruption began. 

We suggest instead that a layered Glass Mountain magma 

reservoir had aphyric rhyolite at the top, overlying mixed 

dacite that was itself a product of the mixing of rhyolite with 

intruded mafic magma. When mafic magma again intruded, 

setting off the eruption (as proposed by Eichelberger, 1981), 

some was incorporated as QMI. Following the initial rhyolitic 

explosive activity, underlying lower-viscosity dacitic magma 

was drawn up into the centrally located vents for the main 

flow. This may have occurred, in part, because the explosive 

emptying of the near-surface rhyolite magma allowed the 

hotter, less viscous dacite to rise more quickly than the higher 

viscosity rhyolite magma could be replenished laterally, resulting in a transition to effusive rather than explosive activity.

Petrologic considerations also support the intrusion of underlying mafic magma into a shallower rhyolitic magma reservoir. 

Amphibole-bearing QMI record deeper crystallization depths 

(~6 km) than andesitic QMI lacking amphibole (~3 km), 

suggesting that fractional crystallization and crustal melting 

processes occurred over a range of depths that culminated in 

a shallow storage reservoir of aphyric rhyolite magma (Grove 

and others, 1997).

At times during the eruption, more than one magmatic 

composition entered the vent such that two of the early dacite 

lobes of the main flow dragged along large pod-like bodies, 

as much as 50 m in size, of aphyric rhyolite to the ends of the 

flow lobes. Compositional plots (fig. 22) of Glass Mountain 

lava and tephra samples that contain the highest 1 percent of 

SiO2

 content (73.6 to 74.6 weight percent SiO2) indicate that 

neither the initial rhyolitic tephra nor the final Glass Mountain 

rhyolite lobe has the highest silica content. Rather, four samples of aphyric rhyolite caught up in the early dacite lava lobes 

as well as the single analysis of dome 1 (1406M, table 1) have 

the highest analyzed SiO2 and K2O contents. This suggests 

that a small, localized cap of highest-silica viscous rhyolite 

existed locally at the center of the vent fissure. It is possible 

that some of this material may be present as individual pumice 

fragments in the initial tephra, but only a few single pumices 

were analyzed for this study. Intimately mingled rhyolite and 

dacite (for example, fig. 16) were probably also produced by 

juxtaposition in the vent. As the eruption progressed, either the 

QMI-bearing dacitic magma was depleted in the reservoir, or 

it was excluded from the vent conduit as the dominant upper 

rhyolitic magma forced its way back into the vent.

Recent rhyolite eruptions in Chile, at Chaiten (Castro 

and Dingwell, 2009; Pallister and others, 2013) and at Cordon 

Caulle (Tuffen and others, 2013), show that significant volumes of high-viscosity silicic lava can erupt in relatively short 

periods of time, from months to a year. The Glass Mountain 

eruption, larger and more complex than either of these, may 

have taken somewhat longer to erupt, but the new evidence 

from actual eruptions suggests that an elapsed time of only 

months to a few years would be adequate for 1 km3

 of silicic 

magma to be emplaced. Thus, our failure to find a time signal 

using paleomagnetic sampling in Glass Mountain eruptive 

products is easily explained.

Little Glass Mountain

Erupted only half a century earlier than Glass Mountain, Little Glass Mountain (fig. 23) is located ~16 km west 

of Glass Mountain on the opposite side of the caldera. This 

obsidian flow is the largest of 12 rhyolite extrusions, including 
24 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

the Crater Glass flows (figs. 24 and 25), that erupted ~1,000 

years ago from a 10-km-long north-northeast-trending fissure. 

The eruption was preceded and accompanied by significant 

fracturing (fig. 26), creating an assemblage of north-northeast-trending open ground cracks (Fink and Pollard, 1983) 

shown in figure 27. Following an initial rhyolitic tephra eruption of ~0.05 km3

 (Fisher, 1964; Heiken, 1978), effusion of 

lava covered a total area of 6.3 km2. The total volume erupted 

was ~0.4 km3. By contrast with Glass Mountain, Little Glass 

Mountain lavas and tephra are only rhyolitic and are limited 

in compositional range (fig. 21, table 1). However, none of 

the Little Glass Mountain lava is aphyric. Instead, noticeable 

phenocrysts are present throughout, typically 1–2 percent of 

1–2 mm plagioclase, orthopyroxene, and magnetite. A photomicrograph of the rhyolite lava is shown in figure 28A.

The eruption brought a cargo of QMI and cumulate 

inclusions to the surface. The inclusions were previously 

studied by Mertzman and Williams (1981), Grove and 

Donnelly-Nolan (1986), and Brophy and others (1996). In 

contrast with Glass Mountain, no preferential spatial distribution of inclusion types is displayed in Little Glass Mountain lavas (fig. 27). In addition to the magmatic inclusions, 

granitic fragments (figs. 20 and 29) were also brought to the 

surface. A few inclusions were also found near-vent in the 

initial tephra, but primarily as scattered constituents in the 

lava flows and domes that make up the bulk of the eruptive 

volume. Inclusions range in size from microscopic crystal 

clots to 70 cm in diameter (fig. 30). Inclusions large enough 

for analysis (typically >7 cm) are relatively rare. Several types 

of magmatic inclusions are present, including a variety of 

QMI ranging in silica content from 52.2 to 62.2 weight percent SiO2

, and cumulate inclusions that span a compositional 

range from 51.8 to 57.3 weight percent SiO2, most of which 

are hornblende gabbro cumulates containing ~53 to 56 weight 

percent SiO2. Table 1 includes 9 analyses of the host rhyolite 

and 49 total analyses of magmatic inclusions, 27 of which are 

hornblende-bearing and show cumulate textures. Among the 

22 other inclusions, 20 are QMI, of which 10 contain hornblende. Two other inclusions are possibly cumulate, one with 

and one without hornblende. Photomicrographs of a variety 

of inclusions are shown as figure 28B-F. Some analyses were 

previously published in Grove and Donnelly-Nolan (1986) and 

Brophy and others (1996).

Detailed petrographic descriptions of the inclusions 

can be found in both Grove and Donnelly-Nolan (1986) and 

Brophy and others (1996), so only a general description will 

be included here. Among the QMI, the basaltic andesite to 

lower-silica andesite inclusions are typically aphyric, but 

contain microphenocrysts of olivine, plagioclase, and clinopyroxene in a vesicular glassy groundmass. Andesitic inclusions 

with >57 weight percent SiO2 contain orthopyroxene, plagioclase, and Fe-Ti oxide phenocrysts ± hornblende ± clinopyroxene phenocrysts in a vesicular rhyolitic glass matrix. The 

QMI with the highest silica contents are aphyric and contain 

Figure 23. Aerial view of Little Glass Mountain, looking west, with snow-covered Mount Shasta on the horizon. 

Prominent white cone in center just beyond the Little Glass Mountain rhyolite flow is Pumice Stone Mountain, 

which is mantled by early tephra of the Little Glass Mountain eruption. On left, just beyond the rhyolite flow is late 

Holocene Paint Pot Crater, also mantled by younger tephra of Little Glass Mountain.
The ~1-ka Episode 25

Figure 24. Aerial view 

looking northeast across 

the rocky and unvegetated 

Crater Glass flows. The 

small explosion crater to 

the left of the nearest flow 

gives this part of the Little 

Glass Mountain rhyolite 

fissure its name.

Little Glass Mountain

Figure 25. Aerial view looking southwest over the Crater Glass 

flows toward Little Glass Mountain along the entire length of the 

~1,000-yr-old rhyolite eruptive fissure.

Figure 26. The Little Glass Mountain eruption was accompanied 

by the opening of cracks along, and parallel to, the vent fissure 

(Fink and Pollard, 1983). The walls of the crack fit back together in 

east-west direction. Red ellipse encompasses person for scale. 

This photograph was previously published as figure 7 in DonnellyNolan and others (2007).
26 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Little Glass Mountain

Crater

Glass

flows Rhyolite of Little Glass Mountain

Vents

Host lava samples

Inclusions

Quenched magmatic 

Cumulate

Granitoid

121°37.5' 

41°35.0' 

121°42.5' 

41°37.5' 

EXPLANATION

0 1

1 2

MILE

0 KILOMETERS

Figure 27. Map showing lavas erupted from the Little Glass Mountain fissure (gray) and vents (black crosses). Also shown 

are the locations where samples of lava and the different types of inclusions were collected. Two of the cumulate inclusions 

were collected in the tephra deposits and plot away from the lava flows. Open ground cracks shown as black lines are from 

Donnelly-Nolan (2010). More detailed mapping of the cracks is shown in Fink and Pollard (1983).
The ~1-ka Episode 27

A B

C D

E F

Figure 28. Photomicrographs of the Little Glass Mountain host lava and a variety of inclusions. (A) The host rhyolite lava 

(sample 35M, 73.7 weight percent SiO2

) showing flow foliation defined by aligned microlites wrapping crystals of plagioclase and 

orthopyroxene; plane light. (B) Sample 1235M (a quenched magmatic inclusion containing 59.8 weight percent SiO2

), in plane light 

shows rosettes dominated by dark hornblende needles; needles of pyroxene and plagioclase are less visible; round white areas 

are vesicles surrounded by clear glass. (C,D) Plane and crossed-polarized light views of sample 1025M-i (a quenched magmatic 

inclusion containing 55 weight percent SiO2

) show larger euhedral plagioclase crystals set among smaller plagioclase, pyroxene, 

hornblende, and Fe-Ti oxides; some of the smaller crystals are arrayed in rosettes best seen in (D); glass and vesicles are present 

in interstices. (E) Plane light view of sample 1746M, a hornblende gabbro cumulate inclusion containing 55.3 weight percent SiO2

, 

that shows brown hornblende together with plagioclase, two pyroxenes, and Fe-Ti oxide grains in a vesicular glassy groundmass; 

see also figure 30 for a photograph of the inclusion as discovered in the field; other hornblende gabbro inclusions are depicted and 

described in Grove and Donnelly-Nolan (1986). (F) Plane light view of sample 1025M-f, a cumulate hornblende gabbro inclusion with 

53.6 weight percent SiO2

, that displays the same mineralogy as 1746M except that it is coarser and olivine is also present (high-relief 

irregularly fractured mineral in upper right of view). Scale bars are each 1 mm in length. Chemical analyses are in table 1.
28 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Figure 29. Photograph showing typical occurrence of granitic inclusions found in Little Glass Mountain lava. 

Hammer handle is 47 cm long.

Figure 30. Largest sampled magmatic cumulate inclusion (1746M) in Little Glass Mountain sits among blocks 

of rhyolite at the edge of the flow. Hammer handle is 33 cm long. See table 1 for chemical analysis and location 

information, figure 28 for photomicrographs.
The ~1-ka Episode 29

2

3

4

5

6

7

8

9

10

50 52 54 56 58 60 62

GM quenched inclusions

GM cumulate inclusions

LGM quenched inclusions

LGM cumulate inclusions

wt.% MgO

wt.% SiO 2

15

16

17

18

19

20

50 52 54 56 58 60 62

wt.% Al O2 3

wt.% SiO2

2.5

3

3.5

4

4.5

2 3 4 5 6 7 8 9 10

wt.% Na O2

wt.% MgO

200

250

300

350

400

450

500

50 52 54 56 58 60 62

ppm Sr

wt.% SiO2

EXPLANATION

SiO2 SiO , in weight percent 2

, in weight percent

SiO2 MgO, in weight percent , in weight percent

MgO, in weight percent

Al O2 3, in weight percent

Na O, in weight percent

2

Sr, in ppm

Figure 31. Plots comparing mafic magmatic inclusion suites in the Glass Mountain (GM, red symbols) and Little Glass Mountain 

(LGM, blue symbols) lavas. Solid symbols are cumulate inclusions; open symbols represent quenched magmatic inclusions.

quench groundmass amphibole in a vesicular glass matrix. The 

cumulate inclusions typically contain plagioclase, clinopyroxene, olivine, orthopyroxene, hornblende, and Fe-Ti oxides that 

form a network of crystals set in a vesicular matrix of rhyolite 

glass.

Silica variation diagrams that compare compositions 

of QMI and cumulate inclusions in Little Glass Mountain 

and Glass Mountain are shown in figure 31. The inclusions 

brought to the surface in the two eruptions overlap in composition. Only a few of the Glass Mountain inclusions contain 

hornblende and closely resemble the hornblende gabbro 

cumulates found at Little Glass Mountain. The Little Glass 

Mountain QMI show a broader range of compositions.

Basaltic Andesite of Paint Pot Crater

The Paint Pot Crater eruption produced the youngest 

mafic lava flow at MLV. This small flow, known locally as 

“tilted rocks” for its rugged surface, erupted from a cinder cone 

and spatter vents located ~5 km southwest of the caldera rim 

(fig. 4A). The eruption took place on the same northeast-trending alignment as the subsequent Little Glass Mountain rhyolite. 

The unit covers ~2.7 km2

 and has a volume of ~0.04 km3. Its 

name is derived from the multi-colored red and black cinder 

cone, which is heavily mantled by white pumice (figs. 6 and 

32) from the nearby Little Glass Mountain eruption. The Paint 

Pot Crater flow was not directly dated, but its age is closely 
30 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Little Glass Mountain

Paint Pot Crater

Figure 32. Aerial view looking northeast across the late Holocene Paint Pot Crater flow and cone, which displays a mantle of white 

pumice from the slightly younger (by a few decades) Little Glass Mountain eruption. The Little Glass Mountain rhyolite flow fills the 

middle ground just above and beyond Paint Pot Crater, with the rim of Medicine Lake caldera on the horizon.

constrained because its early tephra is enclosed between the 

underlying Callahan Flow tephra and the overlying Little Glass 

Mountain tephra (Donnelly-Nolan and others, 1990). The 

age is further constrained to ~1,170 years ago by the nearly 

identical orientation of its paleomagnetic direction with that of 

the radiocarbon-dated Callahan Flow (Nathenson and others, 

2007).

Kinzler and others (2000) found no evidence of significant compositional zonation. Four chemical analyses of 

the lava ranging from 52.9 to 53.2 weight percent SiO2 are 

presented in table 1. Fine-grained basaltic crystal clots from 

<1 mm to cm-sized are present in the lava flow although 

none were large enough for chemical analysis. A few granitoid inclusions were also found (table 2). The flow contains 

noticeable 2-mm-wide olivine phenocrysts, among a total of 

approximately 5 percent of 1–2 mm plagioclase and olivine 

phenocrysts, with plagioclase more abundant than olivine. A 

photomicrograph of the lava is shown in figure 33A.

The Paint Pot Crater flow, the youngest mafic lava flow 

at Medicine Lake volcano, represents a wet calcalkaline 

basalt with geochemical characteristics typical of arc basalts. 

However, it is not the most recent mafic magma to be erupted 

because the younger Little Glass Mountain and Glass Mountain eruptions brought up numerous quenched magmatic inclusions (QMI) of calcalkaline basaltic andesite and andesite.

Callahan Flow

The andesite-to-basalt compositionally zoned Callahan 

Flow (Kinzler and others, 2000) erupted on the north flank 

of MLV ~1,180 years ago. The primary vent is a large cinder 

cone named Cinder Butte (fig. 34), but lava also escaped to 
The ~1-ka Episode 31

A B

C D

E F

Figure 33. Photomicrographs of the late Holocene mafic host lavas. (A) The basaltic andesite of Paint Pot Crater flow 

(sample 1662M, 53.2 weight percent SiO2

), in crossed-polarized light, shows phenocrysts of olivine and plagioclase in a 

glassy to microcrystalline groundmass containing smaller plagioclase and olivine. (B-D) Three samples of the Callahan 

Flow showing lava samples with a range of silica contents and a variety of plagioclase-dominated textures: (B) sample 

146M (crossed-polarized light) contains 57.6 weight percent SiO2

; (C) sample 1103M (plane light) has 54.7 weight percent 

SiO2

; and (D) sample 1399M (crossed-polarized light) has 51.8 weight percent SiO2. (E) A plane-light view of sample 695M, 

the andesite of the Burnt Lava Flow (57.5 weight percent SiO2

), shows a few larger microphenocrysts of plagioclase in a 

felted matrix dominated by small plagioclase. (F) crossed-polarized light image of sample 1675M (50.3 weight percent SiO2

) 

from the Black Craters flow of the basalt of Black Crater and Ross Chimneys; the unit is typically porphyritic with as much 

as 25 percent of plagioclase and olivine phenocrysts. Scale bars are each 1 mm in length. Chemical analyses are in table 1.
32 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Figure 34. View northeast across the southern Callahan Flow and its main vent, Cinder Butte. Cone on left in 

middle distance is Schonchin Butte in Lava Beds National Monument.

the surface by way of adjacent north- and northwest-trending 

spatter vents. The flow covers an area of 23.7 km2, part of 

which lies within Lava Beds National Monument (fig. 3). 

The estimated volume of the flow is 0.33 km3. Silica content 

ranges from 51.8 percent to 58.0 percent (table 1), with the 

first-erupted andesite making up 50 percent of the volume, 

whereas the last-erupted basalt amounts to just 1 percent. This 

mostly treeless, dominantly ʻaʻā lava flow was subdivided and 

mapped in five groups using petrographic and compositional 

criteria. A map showing the aerial distribution of the five different composition groups is shown as figure 1 in Kinzler and 

others (2000). Their figure 3 is a plot of log volume versus 

MgO content for the five compositional groups showing a 

nearly linear trend, indicating that both volume and silica 

content decreased as the eruption proceeded. Heights of flow 

fronts also decreased as silica content and viscosity decreased.

The eruption also produced tephra, some of which was 

found to the southwest underlying tephra from the Paint Pot 

Crater eruption (Donnelly-Nolan and others, 1990). Additional 

tephra was deposited on top of early flow lobes, indicating that 

explosive activity persisted during the eruption (Kinzler and 

others, 2000).

Photomicrographs of Callahan Flow lavas are shown in 

figure 33B-D. The early andesite facies is noticeably phyric 

and has 2–5 percent of 1–3 mm plagioclase and <1 percent 

of 1 mm orthopyroxene, along with rare 1 mm olivine and 

a glassy interstitial groundmass. The most mafic rocks are 

nearly aphyric, with rare 1–3 mm plagioclase set in a crystalline, sugary groundmass. Compositional variations in the 

Callahan lavas resulted from the combination of fractional 

crystallization, crustal assimilation, and magma mixing prior 

to eruption. The mineral assemblages preserve evidence of 

these processes in populations of phenocrysts of widely varying composition (Kinzler and others, 2000).

No quenched magmatic inclusions were found in the flow 

although one cumulate inclusion was collected (Kinzler and 

others, 2000). Numerous melted silicic inclusions present in 

the Cinder Butte vent cone match a nearby older rhyolite flow 

in composition and petrography. No granitoid fragments were 

found.

Hoffman Flows

The earliest of the three late Holocene rhyolite eruptions 

at Medicine Lake volcano, the informally named Hoffman 

flows erupted ~1,230 years ago from a north-northwest-trending 2-km-long fissure tangential to the northeast caldera rim 

and 1 km west of the parallel Glass Mountain fissure (fig. 35). 

An early tephra deposit was exposed under the Glass Mountain tephra by quarrying adjacent to the north side of the Glass 

Mountain flow. Two separate areas of lava flow were produced 

from multiple vents (fig. 4A,B). The southern lobes flowed 

west into the caldera and east an unknown distance where they 

underlie the younger rhyolite of Glass Mountain. An estimate 

of the original areal extent of the unit is ~8 km2

; the estimated 

volume is ~0.2 km3. Seven chemical analyses indicate a range 

of compositions with two analyses at 68.4 weight percent SiO2

and the other five between 71.3 and 72.4 weight percent. Two 

chemical analyses were presented in Grove and DonnellyNolan (1986); these, plus additional data are included in table 
The ~1-ka Episode 33

1. No attempt was made to map out areas of lower-silica and 

higher-silica lava in the flows because only the westernmost 

intracaldera part of the southern flow area is sufficiently 

exposed. Closer to Glass Mountain, tephra from that eruption 

mantles the Hoffman flows and a thick tree cover is present, 

limiting exposures.

The Hoffman flows contain ubiquitous QMI ranging in 

size from microscopic to 30 cm in length. The larger QMI 

are disc-shaped, unlike the typical spheroidal shape of Glass 

Mountain and Little Glass Mountain inclusions and those 

described by Bacon (1986). Lower-silica host samples show 

abundant microscopic crystal clots containing plagioclase, 

olivine, and clinopyroxene, whereas the higher-silica samples 

display far fewer tiny inclusions, suggesting that the compositional variation is most likely caused by disaggregation of 

mafic magmatic inclusions. The rhyolite of the Hoffman flows 

is the most porphyritic of the three late Holocene rhyolites. 

Photomicrographs of thin sections are shown in figure 36. The 

highest-silica (and thus least mixed) samples contain 

5–10 percent of 1–2 mm plagioclase phenocrysts, plus 

~1 percent of 1 mm orthopyroxene and <1 mm magnetite.

A variety of inclusions were collected from the two areas 

of lava flow, among them one cumulate inclusion and eight 

QMI. No plutonic-textured fragments were seen. Among 

the eight analyzed QMI, two distinct types are present: (1) 

two andesitic inclusions containing plagioclase phenocrysts 

have 55.9 and 57.9 weight percent SiO2

 and resemble the wet 

calcalkaline QMI that are present in Glass Mountain and Little 

Glass Mountain lavas; (2) five of the much more abundant 

fine-grained, nearly aphyric basaltic inclusions have 49.6 to 

51.6 weight percent SiO2. QMI with basaltic compositions are 

rare (Bacon, 1986) and QMI with such low silica contents are 

absent from the other late Holocene rhyolites, although they 

are present in the ~3-ka andesitic Burnt Lava Flow. These 

low-SiO2 inclusions resemble dry tholeiitic magmas like those 

described by Hart and others (1984) who labeled them HAOT 

(high-alumina olivine tholeiite). Thus both wet calcalkaline 

and dry tholeiitic magmas were present beneath the volcano. 

One cumulate inclusion (1044M-a, table 1) was analyzed 

and is shown in figure 36E,F along with photomicrographs of 

host lavas and one fine-grained basaltic QMI.

Figure 35. Aerial view west across the northern domes of the Glass Mountain fissure in the foreground to the northern part of the 

Hoffman flows in the center of view. The rhyolitic Hoffman flows erupted about three centuries prior to the Glass Mountain eruption 

along a parallel north-northwest-trending fissure. Just beyond the northern Hoffman flow is Mount Hoffman, the highest point on the rim 

of Medicine Lake volcano at ~8,000 ft. (2438 m) and the dome of a late Pleistocene rhyolite flow. To the left of Mount Hoffman, near the 

edge of the photo and surrounded by trees is Medicine Lake. Snow-covered Mount Shasta is seen on the horizon.
34 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

A B

C D

E F

Figure 36. Photomicrographs of the host lava and inclusions from the Hoffman flows. Views A-D are of host lava sample 

26M (71.8 weight percent SiO2

). (A,B) Plane and crossed-polarized light images showing plagioclase phenocrysts along 

with microphenocrysts of plagioclase and pyroxene in a glassy matrix that displays zones with flow-aligned microlites 

that wrap around crystals. (C,D) Plane and crossed-polarized light images showing a wider view and apparent mingling 

of different glass compositions. (E) crossed-polarized light image of cumulate inclusion sample 1044M-a (47.8 weight 

percent SiO2

) with intergrown olivine and plagioclase phenocrysts set in a matrix displaying rapid growth morphologies 

in small plagioclase; (F) A fine-grained basaltic inclusion (sample 1652M, 51.6 weight percent SiO2

), in plane light, displays 

some microphenocrysts of plagioclase in a groundmass dominated by plagioclase microlites. Scale bars are each 1 mm in 

length. Chemical analyses are in table 1.
The ~3-ka Episode 35

Figure 37. Photograph of 

High Hole Crater, vent for 

the ~3-ka Burnt Lava Flow, 

as seen across a part of this 

blocky andesite flow.

wt.% SiO2

56.5 57.0 57.5 58.0 58.5

wt.% FeO*

5.9

6.1

6.3

6.5

wt.% K

O2

1.4

1.5

1.6

1.7

SiO2, in weight percent

FeO, in weight percent K O, in weight percent

2

Figure 38. Plots showing compositional variability in the Burnt 

Lava Flow. Data available in table 1 and in Grove and others (1988).

The ~3-ka Episode

Nearly two millenia prior to the ~1 ka eruptive episode, 

two eruptions took place within a brief time span of ~130 years 

(Nathenson and others, 2007). The andesite of the Burnt Lava 

Flow erupted on the south flank and was preceded by the 

basalt of Black Crater and Ross Chimneys on the far north 

flank (fig. 4A). The total erupted volume for this episode was 

just over 0.5 km3, nearly all of it in the Burnt Lava Flow. The 

area covered by eruptions during this episode was 34.75 km2, 

including 0.45 km2 for the basalt of Black Crater and Ross 

Chimneys. Both of the eruptions generated spatter vents that 

were readily sampled for their paleomagnetic directions. These 

units also display compositional zonation, with the most mafic 

lava erupting early in both cases.

Burnt Lava Flow

This rugged, mostly unvegetated lava flow has an estimated volume of 0.5 km3

. Lava erupted from a north-south 

alignment of vents including High Hole Crater (fig. 37) and 

covered 34.3 km2. Early explosive activity built the large cinder cone, but no evidence of a tephra has been found beyond 

the limits of the lava flow. Because of its youthful appearance, 

this flow was originally thought to have erupted as recently 

as 300 years ago (Finch, 1933), but charcoal retrieved from a 

large tree mold in a distal flow lobe indicated that the eruption 

occurred about 3,010 years ago. Silica content of the lava flow 

ranges from 56.8 to 58.3 weight percent based on 20 analyzed 

samples (Grove and others, 1988). Plots of SiO2

 versus FeO 

and K2O (fig. 38) show that cryptic compositional variation is 

present, although no attempt was made to map out the areal 

distribution of the compositional range because of limited 

petrographic variability in the field. However, examination of 
36 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

A B

C D

E F

Figure 39. Photomicrographs of quenched basaltic inclusions in the Burnt Lava Flow. Plane light (left) and crossedpolarized light (right) views of three quenched magmatic inclusions. (A,B) sample 659M (48.4 weight percent SiO2

); 

(C,D) sample 695Mb (47.9 weight percent SiO2

), and (E,F) sample 1156M (49.4 weight percent SiO2). Elongate euhedral 

plagioclase and smaller olivine crystals are accompanied by interstitial clinopyroxene in all samples. The chilled texture 

and rapid growth morphologies are best seen in (C,D). Additional images and petrographic descriptions are in Grove and 

others (1988). Scale bars are each 1 mm in length. Chemical analyses are in table 1.

that record fractional crystallization, assimilation, and magma 

mixing processes that gave rise to the andesite of Burnt Lava 

Flow. The inclusions are typically fine-grained and aphyric 

with a crystalline groundmass (fig. 39). Partly melted granitoid 

inclusions were also found in this flow (table 2). Additional 

aerial photos together with the known locations of the analyzed samples in Grove and others (1988) indicates that the 

lava erupted earlier is more mafic than later lobes.

A photomicrograph of one sample (fig. 33E) shows the 

compositionally and texturally diverse population of crystals 
The ~3-ka Episode 37

Figure 40. Photograph of the uppermost Ross Chimneys spatter vents in northern Lava Beds National Monument. This view looking 

south shows numerous flank cinder cones and the top of Medicine Lake volcano on the horizon.

photomicrographs and more detailed petrographic descriptions 

can be found in Grove and others (1988).

Five analyses of host rocks, along with seven analyses of 

inclusions are included in table 1. Six analyzed basaltic inclusions range in composition from 48.4 to 49.6 weight percent 

SiO2. One inclusion with 51.7 weight percent SiO2 shows 

physical evidence of contamination by andesite or melted 

granite (Grove and others, 1988). These QMI are similar in 

composition to the basaltic QMI in the later Hoffman flows 

and to high-alumina olivine tholeiites (HAOT of Hart and 

others, 1984) that have erupted intermittently throughout the 

history of MLV.

Basalt of Black Crater and Ross Chimneys

Numerous spatter vents (fig. 40) scattered along a 2.5-kmlong north-northeast-trending fissure produced two very small 

areas of basalt flows (Donnelly-Nolan and Champion, 1987; 

Donnelly-Nolan, 2010) that together cover 0.45 km2

 on the 

lower north flank of MLV (figs. 4A and 41). The lavas are 

compositionally similar to basaltic QMI in the Burnt Lava 

Flow and in the Hoffman flows. The total volume of the two 

thin flows that erupted ~3,140 years ago is ~0.001 km3. This is 

the only late Holocene eruption to have taken place within the 

boundary of Lava Beds National Monument (see fig. 3).

Despite the small volume of the eruption and the limited 

compositional span (48.3 to 50.6 weight percent SiO2), compositional variation as a function of latitude is clearly recorded 

(fig. 41). Lava from the upper (southern) Black Crater vents 

overlies lava from the lower (northern) Black Crater vents 

suggesting that higher-silica lava erupted after lower-silica 

lava. No stratigraphic relationship was recognized within the 

separate Ross Chimneys lava to the north, but if the relationship holds that higher-silica lavas erupted later than lowersilica lavas, then the eruption apparently propagated from 
38 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

.

. . .

.

.

.

.

.

. .

. .

.

.

.

.

.

121° 33.0' 121° 32.5'

41°

46.0'

41°

46.5'

41°

47.0'

Black Crater

Ross Chimneys

Vents

Samples

y

o

EXPLANATION

Black Cr. upper

Black Cr. lower

Ross Ch. upper

Ross Ch. lower

Latitude, N 41°

46.0'

46.4'

46.6'

46.8'

46.2'

47.0'

45.8'

wt.% SiO2

48.0 49.0 50.0 51.0

EXPLANATION

Latitude, N 41°

46.0'

46.4'

46.6'

46.8'

46.2'

47.0'

45.8'

wt.% K2O

0.20 0.30 0.40 0.50

ppm Ba

Latitude, N 41°

46.0'

46.4'

46.6'

46.8'

46.2'

47.0'

45.8'

60 80 100 120 140

Latitude, N 41°

46.0'

46.4'

46.6'

46.8'

46.2'

47.0'

45.8'

ppm Th

0.40 0.60 0.80 1.00 1.20 1.40

0 0.25 MILE

0 0.25 0.5 KILOMETER

SiO2, in weight percent

K2

O, in weight percent

Th, in ppm

Ba, in ppm

Figure 41. Outline map of the ~3-ka basalt of Black Crater and Ross Chimneys. Vents for this small fissure eruption are shown 

with red crosses; sample locations with green dots, and younger (y) - older (o) relationship shown for southern part of the eruption. 

Accompanying the map are four plots showing chemical components versus latitude demonstrating a correlation between spatial 

position along the fissure and composition. Chemical analyses and location coordinates are in table 1.
The ~5-ka Episode 39

Figure 42. Paleomagnetic drilling in a crease structure on the surface of the Medicine Lake Glass Flow 

(site 4B306).

north to south. North-northeast-trending faults and large open 

ground cracks north of the lavas and on trend with the vents 

suggest that an extensional episode likely tapped the magma, 

allowing it to rise to the surface. The compositional variability recorded in the eruption likely resulted from mixing of 

dry tholeiitic parent basalt with more silicic evolved magma. 

Work by Sisson and Layne (1993) showed that glass inclusions within Black Crater olivines have very low H2

O content 

(<0.3 weight percent).

Variation diagrams in figure 41 show that enrichments of 

some elements are pronounced when comparing the lowestto highest-silica lavas, for example, K2

O more than doubled 

(0.18 to 0.51 weight percent), as did Ba (61 ppm to 141 ppm) 

and Th (0.24 to 1.4 ppm) (table 1). A small, zoned magma 

reservoir or contaminated dike-like body is implied by the 

data. The porphyritic nature of the basalt (~25 percent of 

1–4 mm phenocrysts with plagioclase much more abundant 

than olivine) suggests a protracted residence time for crystallization in the crust before eruption. The location of this unit 

is more than 20 km from the center of MLV, indicating the 

broad impact of the Medicine Lake magmatic system in late 

Holocene time.

The ~5-ka Episode

Two small caldera-focused dacite eruptions (fig. 4) were 

the first lavas to reach the surface at MLV following the widespread ~12.5-ka basaltic episode (fig. 3). The Medicine Lake 

Glass Flow erupted nonexplosively near the northwest wall of 

the caldera. The lower-silica dacite of pit craters, by contrast, 

erupted explosively from a fissure at the southeastern caldera 

margin. The estimated total volume for the two eruptions is 

~0.1 km3

 and the area covered was ~4.8 km2. No stratigraphic 

constraints exist to establish the relative ages of the two 

dacites. Radiocarbon ages suggest that the dacite of pit craters 

is the younger and that less than ~100 years elapsed between 

the two eruptions (Nathenson and others, 2007). Very similar 

paleomagnetic directions also support the short time interval. 

Welded spatter in the wall of one pit crater and in a roadcut 

near another pit crater provided opportunities for paleomagnetic sampling. However, paleomagnetic sampling of Medicine Lake Glass Flow with its steep marginal talus-covered 

slopes was more challenging. Crease structures (Anderson and 

Fink, 1992) on the surface (fig. 42) eventually yielded a successful magnetic direction.

Dacite of Pit Craters

The low-silica dacite of the pit craters (63.1weight 

percent SiO2) erupted explosively ~5,100 years ago as 

nearly aphyric spatter from a northeast-trending fissure at 

the southeastern margin of the caldera. The fissure is marked 

by a nearly 3-km-long set of explosion craters rimmed by 

welded, phenocryst-poor (fig. 43A) dacite spatter. The thickness, degree of welding, and size of individual fragments in 

the deposit decreases rapidly away from the craters. The area 

mapped as blanketed by the eruption (Donnelly-Nolan, 2010) 
40 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

A B

C D

E F

Figure 43. Photomicrographs of the ~5-ka dacitic lavas and inclusions. (A) Sample 110M, the dacite of pit craters (63.4 weight percent 

SiO2

) displays microphenocrysts of plagioclase set in a groundmass showing abundant small crystals (mostly plagioclase?) in plane 

light. (B-F) Samples from Medicine Lake Glass Flow; (B) Host lava sample 45M (68.5 weight percent SiO2

) that shows phenocrysts 

of plagioclase and smaller pyroxene in glassy groundmass displaying incipient crystallization. (C-E) Views of quenched magmatic 

inclusions. (C) Plane light view of sample 1151M-c (61.6 weight percent SiO2

) that shows needles of hornblende together with less 

obvious needles of colorless plagioclase and pyroxene and small equant Fe-Ti oxides in a clear, glassy vesicular groundmass. (D) 

Plane light view of sample 1117M (59.0 weight percent SiO2

) that shows abundant white areas of vesicles; crystals are plagioclase and 

pyroxene, many elongate, and are set in a matrix of brownish glass. (E) Plane light view of sample 1631M (57.9 weight percent SiO2

) 

dominated by elongate mottled plagioclase crystals together with smaller and less abundant orthopyroxene crystals in a matrix of 

brownish vesicular glass. (F) Cumulate inclusion sample 1151M-a (56.3 weight percent SiO2

) in plane light view displays a coarse texture 

of intergrown mottled plagioclase, pyroxene, and olivine; elongate brown needles of hornblende can be seen in the interstitial finergrained matrix. Scale bars are each 1 mm in length. Chemical analyses are in table 1.
The ~5-ka Episode 41

covers 2.4 km2 although a thin tephra deposit extends beyond 

the mapped area. The erupted volume is ~0.02 km3. 

The erupted lava is considered homogeneous based on 

two chemical analyses (table 1) and the lack of petrographic 

variability. Except for small and rare angular lithic fragments, 

the unit apparently lacks inclusions.

Medicine Lake Glass Flow

The age of the Medicine Lake Glass Flow was previously 

estimated at ~ 2,000 yrs B.P. (Donnelly-Nolan and others, 

1990). However, more recent studies (as discussed in Nathenson and others, 2007) of lake sediments in nearby Medicine 

Lake indicate that a thin layer found in the sediments matches 

the flow in composition. No mappable tephra deposit from this 

eruption was identified, suggesting that the eruption was characterized dominantly by quiescent flow emplacement. Its age 

is constrained by radiocarbon dating and an apparently linear 

sedimentation rate in the Medicine Lake sediments at 

~5,200 years ago. The pancake-like dacite flow (fig. 44) 

erupted from two vents that trend N. 45° W. near the northwest 

wall of the caldera. It covers ~2.4 km2 and has an estimated 

volume of 0.08 km3. No compositional zonation is evident 

based on three very similar chemical analyses 

(68.2–68.8 weight percent SiO2, table 1) and homogeneous 

petrographic appearances. Glassy flow rocks are noticeably 

porphyritic with ~5 percent of 1–3 mm plagioclase and <1 percent of 1–2 mm orthopyroxene. A photomicrograph is shown 

as figure 43B.

The flow hosts a sparse and interesting suite of magmatic 

inclusions including QMI ranging in silica content from 57.9 

to 61.6 weight percent as well as cumulate-textured inclusions 

(Mertzman and Williams, 1981; Grove and Donnelly-Nolan, 

1986). Gabbroic and granitic inclusions similar to those 

in Glass Mountain lavas are also present (table 2, fig. 20). 

Chemical analyses of host rocks, four QMI, and one cumulate 

magmatic inclusion are presented in table 1. Photomicrographs of three amphibole-bearing fine-grained andesitic QMI 

and one cumulate inclusion are shown as figure 43C-F. These 

inclusions record fractional crystallization, assimilation, and 

magma mixing processes similar to those documented in the 

Glass Mountain and Little Glass Mountain lavas. The presence 

of amphibole provides evidence of wet calcalkaline parentage.

Figure 44. View west-southwest from Mount Hoffman, the highest point on the rim of Medicine Lake caldera, toward Mount Shasta. 

Medicine Lake Glass Flow is the unforested brown area in center of view. Part of Medicine Lake can be seen to the left of the flow.
42 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Figure 45. Photograph showing the use of a sun compass to orient core samples collected for paleomagnetic study.

Identifying the Timing of the Eruptions 

Using Paleomagnetism

Most late Holocene units preserved material that could be 

dated by radiocarbon, but large measured errors in some ages, 

combined with the difficulty of assessing multiple disparate 

age samples, and the lack of datable material for some units 

led to uncertainties in the ages (Nathenson and others, 2007). 

In order to better constrain the eruption ages of the late Holocene lavas and the time intervals between them, we utilized 

paleomagnetism to determine and compare their magnetic 

directions. Our goals included (1) using the paleomagnetic 

directions to constrain the ages of otherwise undated eruptive 

events using geomagnetic secular variation and then comparing measured directions to the magnetic directions of dated 

lava flows at nearby volcanic centers, (2) identifying brief episodes of activity in cases where lava flows have overlapping 

magnetic directions, and (3) investigating possible extended 

time spans of individual eruptions (for example, Champion 

and Donnelly-Nolan, 1994; Turrin and others, 1998; Muffler 

and others, 1994). We then used records of secular variation in 

the Western U.S. (Champion, 1980; Hagstrum and Champion, 

2002) to clarify the ages of the late Holocene eruptive products, some of which were imprecisely determined or undated 

by radiocarbon (Nathenson and others, 2007).

We sought rock outcrop environments where the collection of oriented core samples would provide information about 

the direction of the magnetic field at the time the lava cooled 

below its Curie temperature. One-inch-diameter cores were 

oriented in the field using a sun compass (fig. 45), followed 

by laboratory analysis at the U.S. Geological Survey in Menlo 

Park, Calif. Obtaining highly precise directions of magnetization from viscous, high-aspect-ratio lava flows, such as the 

young rhyolite flows that are surrounded by steep talus-dominated slopes and mostly covered with spines and loose blocks 

was a significant challenge (Champion and Donnelly-Nolan, 

2008). More fluid lava flows of mafic composition provide 

better opportunities for sampling relatively quickly chilled 

and unrotated magmatic products. Identifying fluid or plastic 

behavior in the dominantly brittle environment of silicic flows, 

however, can be problematic. In many cases, the viscous 

silicic lava flows continue to move after the exterior carapace 

has cooled below its Curie temperature. In the absence of any 

erosion or artificial cuts that expose the interior of rhyolite 

lava flows under a typical mantle of blocky chaos, a variety 

of possible outcrops and materials were tested to identify 

environments that preserved a stable direction and could be 

sampled effectively. 

Sample locations are shown in figure 46; successful 

and unsuccessful sampling attempts are also listed in table 3 

(online only). Unsuccessful sites are included in the table for 

completeness and are shown both in the table and on the map 

with parentheses. Forty-three sites were attempted, of which 

eight were considered to be failed sites. Well-grouped directional information was obtained from two or more sites in each 

late Holocene unit with the exception of the Medicine Lake 

Glass Flow, where just one site was successful.
Identifying the Timing of the Eruptions Using Paleomagnetism 43

0 1 2 3

1 2 3 4 5

MILES

0 KILOMETERS

121°22'30'' 121°30'

41°45'

41°30'

3A155

BkCC

(7A104)

5B260

2B266 BkFC

2B254

2B350

4B567

(5B396)

4B322

4B334

3B869

2B512

2B520

4B346

(4B368)

4B306

4B444

2A350

4B531

2B494

2B506

BrLC

4B456

4B472

4B377 (2B379)

(2B387) 3B854

4B406

4B394

7A140

7A177

7A165

7A153

4B431

5B345 (B8490)

4B519

B8478

Medicine Lake

Caldera Rim

(B8671)

(5B370)

mcf

bbr

rgm

rh

dpc

dm

rh

rlg

rlg

mpp

abl

Rhyolite

Dacite

Andesite

Basaltic

Andesite

Basalt

EXPLANATION

Figure 46. Map showing paleomagnetic site locations listed in table 3. Paleomagnetic sites in parentheses were attempted, but 

considered to be failed sites. Unit symbols are from Donnelly-Nolan (2010); rhyolite of Glass Mountain, rgm; rhyolite of Little Glass 

Mountain, rlg; basaltic andesite of Paint Pot Crater, mpp; basaltic andesite of Callahan Flow, mcf; rhyolite of Hoffman flows, rh; andesite of 

Burnt Lava Flow, abl; basalt of Black Crater and Ross Chimneys, bbr; dacite of pit craters, dpc; dacite of Medicine Lake Glass Flow, dm.
44 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Figure 47. Paleomagnetic drilling in crease structures on the 

southern of the two Hoffman flows (site 4B456).

Crease Structures

Crease structures (Anderson and Fink, 1992) are fractured diapiric structures that form when lateral flow advance 

is restricted and hot, ductile, internal lava is forced upward 

and outward in a dominantly plastic manner, akin to the more 

brittle tumuli on basaltic lava flows. An axial crack in the center of the structure is flanked by outwardly curving walls that 

progressively rotate and separate. These walls can be rough in 

detail, but smooth compared to the surrounding blocky lava 

surface (figs. 42 and 47). The angle separating the walls can be 

as small as 20 degrees (Anderson and Fink, 1992) or as much 

as 180 degrees where the walls have flattened by continued 

outward movement.

Crease structures provided opportunities to sample 

plastic flow morphologies on the tops of silicic lava flows. 

When available, we sampled crease structures with subparallel 

walls within 45 degrees of vertical. Sites were selected, when 

possible, in crease structures with differing orientations, and 

cores were drilled on both sides of the axial cracks. Sequences 

of magnetic directions derived through progressive thermal 

demagnetization of individual samples from opposite sides 

of the crack were fitted with small circle planes to solve the 

nature of rotation of each crack. Contrasting crack orientations 

allowed us to fit mean directions of remanent magnetization 

to the intersections of planes from different cracks using the 

method of Kirschvink (1980). In several cases, by way of line 

fits on vector component diagrams, we were able to derive 

low temperature PTRM (partial thermal remanent magnetization) acquired after the crack ceased moving, but while it was 

still hot. In at least two cases (B8490 and 4B368) the crease 

structure had moved appreciably after cooling, and no reliable 

plane nor low temperature PTRM line-fit solutions could be 

derived through thermal demagnetization. 

Results with statistically good values (high kappa) were 

obtained from crease structures on rhyolites of Little Glass 

Mountain, the Hoffman flows, and the Medicine Lake Glass 

Flow. Only a few, small crease structures were seen on Glass 

Mountain, where thermal demagnetization on samples from 

a single site (B8490) produced only a poorly defined normal 

polarity. Crease structures where the angle between the walls 

exceeded 90 degrees are apparently too deformed and were 

likely too cold by the time of final deformation to appreciably 

and accurately record the magnetic field.

Near-vent Welded Tephra

Good results were also obtained from variably welded 

agglutinate deposits generated by early near-vent tephra 

falls along both the Glass Mountain fissure (where Anderson 

[1933] referred to them as pumice cones) and the Little Glass 

Mountain fissure. Initial tephra that fell close to the vents was 

hotter and coarser than more distal tephra, allowing variably inflated fragments to stick together (figs. 48 and 49) and 

sometimes flatten. Most such deposits were found adjacent to 

the small domes along the Glass Mountain (fig. 49) and Little 

Glass Mountain rhyolite fissures. All of these sites produced 

well-grouped magnetic directions. 

Quenched Magmatic Inclusions (QMI)

In one isolated case, quenched magmatic inclusions 

(QMI) in Glass Mountain obsidian were drilled where they 

were exposed in the vertical walls of a deep crack within the 

rhyolite flow (fig. 7 and 50). This 20-m-deep crack appears to 

be a crevasse that formed when the front of the rhyolite flow 

descended down the steep eastern slope of the volcano. The 

crack apparently opened without producing rotations of the 

walls when the interior of the flow was still very hot, with hot 

gases traveling up the crack and oxidizing the exterior surfaces 

of the broken rhyolitic glass and the QMI. The inclusions were 

quite crystalline and easily drilled, whereas attempts to drill 

cores in the surrounding rhyolitic obsidian failed. Results of 

two sampling trips to this locality, one when much of the crack 

was filled with snow and enabled drilling high on the walls 

(fig. 50A, site 4B519), and a second much deeper when most 
Identifying the Timing of the Eruptions Using Paleomagnetism 45

Figure 48. Photograph of welded near-vent agglutinate located adjacent to Glass Mountain dome 3 (site 

3B854). A 1-inch-diameter drill core located to left of hammer and not yet removed is a typical example of the 

dozen cores drilled at each sample site.

Figure 49. Photograph of drilling in a crease structure on dome 6, the smallest of the Glass Mountain domes. 

The dome was emplaced into a crater formed by explosion through the initial near-vent welded tephra, which 

can be seen rimming the crater. Both the dome and the welded tephra were sampled (sites 4B406 and 4B394 in 

table 3). On the right is dome 7, which has in part flowed south into the dome 6 crater.
46 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

A B

Figure 50. Photographs of paleomagnetic sampling at big crack in the center of the main Glass Mountain 

flow. This is crack labeled “C” in the aerial view in figure 7. Photo (A) shows the location of the first 

sampling trip (site 4B519) when the crack was mostly filled with snow; (B) shows the crack on the second 

trip later in summer when most of snow had melted and samples could be collected low in the crack (site 

B8478) (photograph by C.D. Miller). Both photos look north. The red arrow pointing to the left wall of the 

crack in (B) shows the approximate location of sampling site in (A).

Figure 51. Paleomagnetic drilling at Ross Chimneys in mafic lava showing evidence of fluid morphology. 

Near-vent spatter flows offer readily sampled material that typically provides well-grouped results.
Identifying the Timing of the Eruptions Using Paleomagnetism 47

of the snow had melted (fig. 50B, site B8478), are listed in 

table 3.

Prismatically Jointed Peripheral Blocks

Blocks displaying radial joints that form intersecting 

surfaces having the appearance of jigsaw-puzzle pieces can 

be found at the edges of many of the silicic flows immediately 

adjacent to steep flow margins. The joints are interpreted as 

cooling cracks analogous to columnar joints in basalt flows. 

Intact, but significantly cracked blocks as much as several 

meters across were interpreted to be hot when they fell off 

the side of the adjacent flow or dome, with the joints having 

formed after the block fell. Had the joints existed and blocks 

been cold when they fell, they would presumably have broken 

apart along the joints. We identified potential candidate blocks 

adjacent to both the Little Glass Mountain flow and the Medicine Lake Glass Flow and drilled them following successful 

application of the method used for Chaos Crags rhyodacite 

domes at the Lassen Volcanic Center farther south in the 

Cascades arc (D.E. Champion and M.A. Clynne, unpub. data). 

However, our results (table 3) were dispersed, suggesting that 

the sampled blocks had substantially cooled below their Curie 

temperature before tumbling off the side of the flow.

Lava Levee Environments Including Channel 

Walls and Shear Planes

Examples of surfaces exhibiting plastic behavior were 

found on marginal walls of lava channels that transported 

dacite of the compositionally zoned Glass Mountain flow. 

Lava shear zones and adjacent surfaces identified locally 

within the andesitic Callahan and Burnt Lava Flows also 

exhibited the morphology of chilled, but plastically deformed 

surfaces.

The channel walls of Glass Mountain dacite (2B379 and 

2B387) yielded initially scattered results, and thermal demagnetization failed to reveal a coherent low temperature PTRM 

with a single normal polarity direction. Evidently both sites, 

sampled on opposing levees, were deformed by brittle deformation of the levees during inflation and deflation of the block 

lava flow. Sites in the more mafic parts of the compositionally 

zoned Callahan Flow were successfully sampled in deep, railroad cut outcrops (BkFC and 2B266) and at one surface outcrop of late-erupted, less viscous basalt (5B260). Two attempts 

(7A104 and B8671) to sample the higher silica, more viscous 

early-eruption parts of that flow produced coherent directional 

groups, but the directions were not similar to each other, nor 

to the three closely grouped magnetic directions obtained from 

sites in more fluid lava. In the latter cases, thermal demagnetization did not reveal an interpretable low temperature PTRM 

component. Apparently, post-magnetization deformation of 

lava blocks the size of railroad cars was possible in this flow 

field.

Spatter Vents and Flow Surfaces in Mafic Lava 

Flows

Mafic spatter vents provide environments where fluid 

magma chills rapidly. Reliable magnetic directions can be 

recorded even in vesiculated low-density lava materials. Such 

outcrops were sampled (fig. 51) at Black Crater and Ross 

Chimneys (sites BkCC and 3A155), at the fissure vent at the 

northern margin of the Burnt Lava Flow (BrLC), and at the 

rim of a spatter vent for the Paint Pot Crater flow (2B254). 

Two large horizontal flow surfaces at Burnt Lava Flow (2B494 

and 2B506) also produced well-grouped results. In general 

these types of outcrops provide the most coherent and reliable 

sites for field paleomagnetic study of mafic volcanic terranes.

Results of Paleomagnetic Sampling

Plots showing the paleomagnetic results for each of the 

nine late Holocene lava flows are shown in figure 52. The 

locations of sample sites are on figure 46. Because it is more 

difficult to sample the silicic lava flows, more sites were 

necessary to obtain a reliable magnetic direction. Multiple 

approaches and different kinds of sites were valuable as 

checks on the accuracy of the results. Paleomagnetic field 

studies at other volcanoes in the Western U.S. document 

similar directions of magnetization at these times (Hagstrum 

and Champion, 2002). Identification of potential sample sites 

typically involved both examination of aerial photographs, 

preferably of 1:24,000 scale or better, and field exploration.

A total of eight sites were rejected from consideration 

because of their scattered or deviant directions of magnetization compared to other sites in the same volcanic units. Of 

these, six were in rhyolite and dacite block flows, where the 

stability of outcrops can be easily misjudged. Thermal demagnetization, which can usually detect some low temperature 

vestige of coherent PTRM magnetizations, failed to reveal a 

reliable signal from these sites, which are shown in parentheses in table 3.

In a few cases, radiocarbon ages suggested that some 

of the Holocene volcanic units were of similar age. In other 

cases, the spatial contiguity, the on-strike or en echelon aspect 

of vents, chemical compositional similarity, or morphologic 

appearance also suggested units that might share the same 

general age. For example, sites in the dacite of Medicine Lake 

Glass Flow and in the dacite of pit craters, both radiocarbondated at ~5 ka, share very similar magnetizations. The same 

situation exists for the basalt of Black Crater and Ross Chimneys and for the andesite of Burnt Lava Flow, both dated at 

~3 ka. The similar remanent directions for each of these two 

cases suggest two episodes of volcanism each enduring only 

decades or 100–200 years at most (Champion and Shoemaker, 

1977). Based on both radiocarbon (Nathenson and others, 

2007) and paleomagnetic evidence (fig. 52, table 3), two short 

periods of activity occurred during the ~1-ka episode. First, 

the Hoffman flows erupted, soon followed by the Callahan 
48 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

80° 70° 60° 50° 40°

N

pit craters

Medicine Lake

Glass Flow

~5 ka

80° 70° 60° 50° 40°

N

Burnt Lava Flow

Black Craters

Ross Chimney

~3 ka

80° 70° 60° 50° 40°

N

Hoffman flows Callahan Flow

Paint Pot Crater

Little Glass Mtn. Glass Mountain

~1.2-0.9 ka

Rhyolite

Dacite

Andesite

Basaltic

Andesite

Basalt

EXPLANATION

Figure 52. Plots of paleomagnetic directions for late Holocene 

units grouped by age.

Flow and then the Paint Pot Crater flow between ~1,230 and 

1,170 years ago. After a hiatus of <200 years, the Little Glass 

Mountain and Glass Mountain eruptions occurred ~50 years 

apart.

Geophysical Data and Implications

Geodetic data obtained from leveling, global positioning 

system surveys, and satellite interferometry at MLV (Dzurisin 

and others, 1991, 2002; Poland and others, 2006; Parker and 

others, 2013) indicate relatively rapid subsidence centered on 

the caldera, with nearly 1 cm/yr maximum subsidence since 

1954 decaying symmetrically outward over the flanks of the 

volcano. Ongoing subsidence focused on the central caldera, 

along with fumarolic activity near Glass Mountain, the presence of an explored geothermal system, and long-period (LP) 

seismic activity all combine to suggest that the volcano is 

likely to erupt again.

 Two long-period volcanic earthquakes have been 

recorded at MLV (Pitt and others, 2002). The location of the 

1986 event was poorly constrained by the single seismometer 

in place on the volcano. However, the 1989 event was located 

at a depth of 15 km below the western part of the caldera by a 

temporary array of seismometers installed to monitor a swarm 

of shallow, brittle failure earthquakes that began under the 

caldera in 1988. Pitt and others (2002) interpret the LP events 

as likely related to movement of basaltic magma at depth. 

It is possible that additional LP events have occurred at the 

volcano, but were not recorded during lengthy gaps in seismic 

monitoring.

A low-velocity anomaly was identified under the caldera 

by a high-resolution active source seismic tomography experiment with 1–2 km resolution (Evans and Zucca, 1998). The 

authors interpret the anomaly as a small silicic magma chamber no larger than a few tens of cubic kilometers in volume. 

Using the tomographic data together with seismic refraction 

data, Chiarabba and others (1995) identified a low-velocity 

zone at a depth of 3–7 km beneath a cone-shaped highvelocity body in the first 3 km of the crust. They interpret the 

high-velocity body as a shallow, subsolidus, intrusive complex 

that was previously identified by Finn and Williams (1982) 

using gravity data. The central zone of low velocity is as small 

as 10 km3

 and the authors suggest that it may be a small silicic 

magma chamber containing partially molten material. Further 

analysis of teleseismic tomography data (Ritter and Evans, 

1997) concluded that no large (>100 km3) silicic magma 

chamber is present under MLV.

Geothermal drilling at the volcano encountered temperatures in excess of 250 °C and altered granitoid intrusive 

rocks 2.5 km below the eastern caldera floor (Lutz and others, 

2000). The study of drill cuttings from another deep drillhole 

near the center of the caldera indicates that granitoid rocks 

are present as shallow as 2 km (Donnelly-Nolan and others, 

2008). According to Lutz and others (2000), the geothermal 

system “... is perhaps the most promising, currently undeveloped, electrical-grade geothermal resource in the contiguous 

United States.” This is despite the near absence of hydrothermal phenomena at the surface. The single active fumarole at 

the surface is the Hot Spot, located ~2 km west-northwest of 

Glass Mountain between the Glass Mountain and Hoffman 

flows vent fissures.

Magma Sources and Processes

The large size of MLV, both in area and volume, indicates a major magmatic focus. The broad distribution of vents 

across the edifice allows for multiple independent pathways to the surface, in contrast to more focally constrained 
Magma Sources and Processes 49

stratovolcanoes where homogenization is more likely. Late 

Holocene eruptions are spread out physically over a large area 

(fig. 4A,B). Chemically, they include a broad range of compositions from basalt to rhyolite. The opportunity to understand 

magmatic processes at the volcano is enhanced by the presence of magmatic inclusions captured by individual magma 

reservoirs prior to eruption.

Comprehensive sampling (table 1) of the compositionally zoned lavas, especially Glass Mountain and the Callahan 

Flow, as well as collection of numerous magmatic inclusions, 

combined with detailed petrologic, geochemical, and experimental studies of several units provide information critical 

to assessing the overall sequence of magmatic and intrusive 

events and processes at MLV. Compositions of ~5 ka, ~3 ka, 

and ~1 ka lavas and their included QMI are shown on variation diagrams in figure 5. Some late Holocene lavas and 

inclusions fall outside the range of earlier MLV lavas (fig. 

5). Also, compositions of QMI in Glass Mountain and Little 

Glass Mountain lavas do not match the compositions found 

in the late Holocene erupted mafic lavas (fig. 53). The variety 

of magmatic materials brought to the surface in late Holocene 

time at MLV is summarized in table 4. Detailed petrologic and 

geochemical study of the range of lava compositions at MLV 

(for example, Grove and others, 1988; Grove and others, 1997; 

Kinzler and others, 2000) has led to a general understanding 

of parental magma compositions and the processes that have 

generated the more silicic magmas. 

Two types of parental basalt have erupted in late Holocene time, both tholeiitic (dry) and calcalkaline (wet). Among 

the eight postglacial eruptions that took place at ~12.5 ka, all 

were tholeiitic, notably including the basalt of Giant Crater 

(Donnelly-Nolan and others, 1991), the most voluminous of 

the postglacial lavas. Petrologic work (Baker and others, 1991) 

showed that the more silicic basaltic andesite components of 

this very large compositionally zoned lava flow were derived 

from primitive high-alumina olivine tholeiite (HAOT of Hart 

and others, 1984) by processes of fractionation, assimilation, 

and mixing. Similar HAOT basalts have intermittently erupted 

on the flanks of the volcano throughout the history of MLV, 

but petrologic study of the ~3 ka Burnt Lava Flow (Grove 

and others, 1988) showed that combined assimilation and 

crystallization processes involving dry parental HAOT basalt 

magmas lead to the production of melts that are only as silicic 

as andesite.

Late Holocene Mafic Lavas 

In late Holocene time, a variety of erupted and included 

mafic compositions reached the surface in the ~3 ka and ~1 ka 

episodes (fig. 53). Basalt similar to HAOT erupted ~3 ka at 

Black Crater and Ross Chimneys. Analysis of glass inclusions within olivines from Black Crater lava (Sisson and 

Layne, 1993) yielded very low water content (0.1 to 0.3 

weight percent) similar to mid-ocean ridge basalts. However, 

elevated K2

O, TiO2, and P2O5 contents in a subset of the glass 

inclusions led the authors to suggest a qualitative similarity to 

the combined effects of shallow tholeiitic differentiation and 

assimilation of granite, akin to the explanation (Grove and 

others, 1988) for the chemical characteristics of the andesite of Burnt Lava Flow. The Burnt Lava Flow is distinctive 

in preserving both primitive HAOT-type QMI and partially 

melted granitic inclusions. The model proposed by Grove 

and others (1988) for the origin of this andesite flow involves 

initial intrusions of HAOT melt into the crust, followed by 

multiple replenishments of the same type of primitive basalt 

along with dry (nearly anhydrous) fractional crystallization 

of the basalt and melting of crust with the heat of crystallization. The assimilation process occurs when replenishment of 

primitive HAOT magma triggers mixing of the melted crust 

with residual iron-rich liquids produced by fractional crystallization. The preservation of the primitive basalt QMI in the 

andesite host indicates that a final intrusion of basalt mingled 

with the andesitic magma just prior to or during eruption. 

Oxygen isotope data for the basaltic QMI in the ~3 ka Burnt 

Lava Flow and in the Hoffman flows have d18O values from 

+5.8 to +6.1 per mil, compatible with an origin as mantlederived basalt (Donnelly-Nolan, 1998).

By contrast with the dry tholeiitic inputs recorded in 

the ~12.5 ka and ~3 ka eruptions, mafic lavas that erupted at 

the surface during the ~1 ka episode show a wet calcalkaline 

signature. Kinzler and others (2000) identified a hydrous 

signature within the Paint Pot Crater flow when they found 

amphibole-bearing melt inclusions within olivine phenocrysts, 

together with calcic plagioclase indicative of 3 to 6 weight 

percent H2O based on plagioclase-melt equilibria. In addition 

to this evidence for water, Kinzler and others (2000) used the 

results of experimental petrology combined with geochemistry 

and isotopic data to show that both the Paint Pot and Callahan 

flows have calcalkaline arc-type signatures indicating a significant role for water in the generation of the basaltic parental 

magmas. The estimated water content for parent melts of Paint 

Pot and Callahan flows is 3 to 6 weight percent.

Late Holocene Silicic Lavas

The earliest of the late Holocene silicic lavas, the ~5-ka 

dacites that erupted in the caldera, show evidence of calcalkaline basalt parentage. The presence of amphibole-bearing 

andesitic inclusions in the Medicine Lake Glass Flow indicates that high water content was involved in production 

of the andesitic melt (Grove and others, 1997; Kinzler and 

others, 2000). The abundance of wet basaltic melt erupted 

and intruded at MLV in late Holocene time, and apparently 

intruded pre-5-ka, likely facilitated melting of geophysically 

identified subjacent older granitic crust beneath the volcano. 

This process helped to generate significant amounts of silicic 

lava that erupted in larger volumes and with significantly 

higher silica contents than at any time at MLV since ~300 ka 

(Donnelly-Nolan and others, 2008). Petrologic work by 

Kinzler and others (2000) showed that when combined with 
50 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

4

5

6

7

8

9

10

11

48 50 52 54 56

wt.% MgO

wt.% SiO2

2

2.5

3

3.5

48 50 52 54 56

wt.% Na O2

wt.% SiO2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

48 50 52 54 56

wt.% K O2

wt.% SiO2

0.6

0.8

1

1.2

1.4

1.6

1.8

2

48 50 52 54 56

FeO*/MgO

wt.% SiO2

TH

CA

rgm QMI

rlg QMI

mpp

mcf 

rh QMI

abl

abl QMI

bbr

~1 ka

~3 ka

EXPLANATION

SiO2 SiO , in weight percent 2, in weight percent SiO2, in weight percent

SiO2

, in weight percent

Na O, in weight percent

2

MgO, in weight percent

FeO*/MgO

K O, in weight percent

2

SiO2, in weight percent

Figure 53. Silica variation diagrams showing compositions of mafic lavas and quenched 

magmatic inclusions (QMI) erupted in late Holocene time. Symbols in purple and red for ~1-ka 

eruptions; symbols in black for ~3-ka eruptions. Unit symbols are from Donnelly-Nolan (2010); 

rhyolite of Glass Mountain, rgm; rhyolite of Little Glass Mountain, rlg; basaltic andesite of Paint 

Pot Crater, mpp; basaltic andesite of Callahan Flow, mcf; rhyolite of Hoffman flows, rh; andesite of 

Burnt Lava Flow, abl; basalt of Black Crater and Ross Chimneys, bbr. No analyzed ~5-ka quenched 

magmatic inclusions are this mafic. Black line separates fields designated as tholeiitic (TH) and 

calcalkaline (CA) by Miyashiro (1974).
Magma Sources and Processes 51

7

8

9

10

11

12

48 50 52 54 56

wt.% CaO

wt.% SiO2

150

200

250

300

350

400

450

500

48 50 52 54 56

ppm Sr

wt.% SIO2

15

16

17

18

19

20

48 50 52 54 56

wt.% Al O3

2

wt.% SiO2

1

2

3

4

5

6

7

8

48 50 52 54 56

La/Yb

wt.% SiO2

SiO2 SiO , in weight percent 2, in weight percent

SiO2 SiO , in weight percent 2

, in weight percent

CaO, in weight percent Al O2 3, in weight percent

Sr, in ppm La/Yb

Figure 53.—Continued
52 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades Table 4. Summary of late Holocene erupted materials at Medicine Lake volcano. [Map units from Donnelly-Nolan, 2010; Age, in calendar years before present (Nathenson and others, 2007); km3, cubic kilometers; wt. %, weight percent; plutonic inclusion types shown in italics; *, compositional zonation sequence documented; QMI, quenched magmatic incusions; CAB, calc-alkaline basalt; HAOT, high-alumina olivine tholeiite] Eruptive episode Name Map unit Age (cal. yr BP) Volume (km3

)

Lava wt.% SiO2

(see table 1)

Inclusions (wt.% SiO2

of QMI; see table 1)

Inclusion 

Type(s) Magma type Reference

~1 ka Glass Mountain rgm 890 1 61.3–74.6 * 54.2–60.7 QMI CAB inclusions Grove and others, 1997 

silicic-intermediatesilicic Cumulate

Granitic, dioritic, 

gabbroic (table 2)

Plutonic

Little Glass 

Mountain

rlg 940 0.4 72.6–74.3 52.2–62.2 QMI CAB inclusions Grove and DonnellyNolan, 1986

Cumulate

Granitic (table 2) Plutonic

Paint Pot Crater 

flow

mpp 1,110 0.04 52.9–53.2 small, not analyzed basaltic clots CAB Kinzler and others, 2000

Granitic (table 2) Plutonic

Callahan Flow mcf 1,120 0.33 51.8–58.0 most 

silicic early

Single 49.0 Cumulate CAB Kinzler and others, 2000

Hoffman flows rh 1,170 0.2 68.4, 71.3–72.4 49.6–51.6 QMI HAOT inclusions Grove and DonnellyNolan, 1986

55.9, 57.9 QMI with plag CAB inclusions

Cumulate

~3 ka Burnt Lava Flow abl 2,950 0.5 56.8–58.3 * 48.4–49.6, 51.7 QMI HAOT inclusions Grove and others, 1988

 most mafic early Granitic (table 2) Plutonic

Black Crater & bbr 3,080 0.001 48.3–50.6 * HAOT Sisson and Layne, 1993

Ross Chimneys most mafic early

~5 ka Pit craters dpc 5,040 0.02 63.1

Medicine Lake 

Glass Flow

dm 5,140 0.08 68.2–68.8 57.9–61.6

Granitic and gabbroic 

(table 2)
Magma Sources and Processes 53

100 MPa

200 MPa

Mixing 

2.5

3

3.5

4

4.5

5

50 55 60 65 70 75

Lava + tephra

Quenched inclusions

Cumulate inclusions

wt.% Na

O2

wt.% SiO2

EXPLANATION

SiO2, in weight percent

Na O, in weight percent

2

Figure 54. Plot showing experimentally determined compositional paths (gray lines) 

compared to the actual Glass Mountain compositions. Experimental pressures indicate 

depths of 3–6 km for the fractionation and assimilation that occurred to create the Glass 

Mountain magmatic products. Details of the study are in Grove and others (1997).

melting and assimilation of granitic crust, fractional crystallization processes involving wet calcalkaline basalt can lead to 

the production of significant amounts of rhyolite. The presence 

of hydrous calcalkaline mafic lavas at MLV indicates that subduction-related processes (for example, slab fluids) operated to 

generate magmas like those erupted in late Holocene time.

High d18O values (+7.5 to +8.5) in the two ~5-ka calderacentric dacites indicate large contributions of crustal material 

such as the granitic inclusions found in the Medicine Lake 

Glass Flow (Donnelly-Nolan, 1998). The latest Holocene 

rhyolites, those erupted during the ~1 ka episode, all have 

elevated d18O values, also indicating incorporation of crustal 

material. No granitic inclusions were found in the oldest of 

these flows, the Hoffman flows, although they were found in 

the younger Little Glass Mountain and Glass Mountain lavas. 

The Hoffman flows are distinctive in being the only rhyolitic 

lava flows containing basaltic QMI with compositions and low 

d18O values similar to the HAOT erupted at Giant Crater, the 

~3-ka Black Crater and Ross Chimneys, and as QMI in the 

~3-ka Burnt Lava Flow. The Glass Mountain eruption took 

place less than 300 years after the Hoffman flows on a nearby 

parallel fissure, but contains instead wet calcalkaline andesitic 

QMI, many of which crystallized hornblende. Little Glass 

Mountain, slightly older than Glass Mountain and 16 km to 

the west, also has hornblende-bearing inclusions, although 

none of the host rhyolite lavas have crystallized this mineral. 

The variety of included materials produced by the Glass 

Mountain and Little Glass Mountain eruptions indicate complex magmatic histories involving several episodes of mafic 

magma injection. Petrologic study of the Glass Mountain 

system (Grove and others, 1997) included experimental work 

indicating fractional crystallization of an H2

O-rich andesitic 

parent magma at pressures of 100 to 200 MPa. Experimentally 

determined liquid lines of descent are shown at two pressures 

on an SiO2-Na2O plot (fig. 54). The experimental petrology 

together with geochemistry and isotopic data indicate that 

assimilation of significant volumes of granitic crustal material 

was also important in generating the rhyolitic magma. The 

Glass Mountain dacite is a mixture of variously evolved rhyolitic melts produced by fractionation of intruded parental mafic 

andesite, combined with a granitic crustal melt, and mixed 

with a more mafic andesitic melt (Grove and others, 1997). 

The dacite is charged with andesitic QMI, some of which may 

have been incorporated immediately prior to eruption.

The three late Holocene rhyolite eruptions that occurred 

during the ~1-ka episode took place on fissures tangent to the 

west and east sides of the caldera. Heiken (1978) suggested 

that the Glass Mountain and Little Glass Mountain rhyolites 
54 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Intrusive Activity

Age, in calendar years

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0 Eruptions rgm

rlg

mpp

mcf

rh

abl

bbr

dpc

dm

8 basaltic eruptions

(includes Giant Crater

HAOT)

Granitoid

HAOT

(dry) (wet)

CAB

Andesite

HAOT

HAOT

CAB

BAndesite

CAB

inferred

Rhyolite

Dacite

Andesite

BAndesite

CAB

EXPLANATION

HAOT

Figure 55. Illustration showing Holocene eruptive episodes color-coded by composition as 

in figure 4A for late Holocene time. The dense cluster of lines at about 12.5 ka represents the 

eight basaltic eruptions that occurred in a short span of ~200 years in immediate postglacial 

time. On the right side of the figure is a diagrammatic representation of intrusive events, some 

documented by the presence of quenched magmatic inclusions of a range of compositions (solid 

horizontal lines), some by petrologic evidence (dashed horizontal lines; see references in table 

4). HAOT, high-alumina olivine tholeiite basalt; CAB, calcalkaline basalt; BAndesite, basaltic 

andesite. Evidence for granitoid intrusion is from the dating of zircons in granitoid inclusions 

found in the rhyolite of Little Glass Mountain and in the dacite of Medicine Lake Glass Flow 

(Lowenstern and others, 2000 and 2003). The CAB on lower right of diagram is inferred based on 

the petrologic and geochemical arguments of Kinzler and others (2000) that wet (calcalkaline) 

magmas differentiate to produce silicic melts, whereas dry (HAOT) magmas can differentiate 

only to andesite.
Magma Sources and Processes 55

were fed by cone sheets (arcuate dikes) from a small magma 

body under the caldera. Eichelberger (1981) suggested instead 

that a single large magma body that stretched beyond the 

12-km east-west extent of the caldera fed both eruptions. 

Donnelly-Nolan (1988) argued for a model of multiple small 

magma bodies under the volcano. Geophysical data (Evans 

and Zucca, 1998) support the current presence of a small 

magma body, but the configuration of magma reservoirs 

1,000 years ago is unknown. Petrography and inclusions 

indicate some differences between the two youngest rhyolitic 

eruptions. We suggest that rather than both Glass Mountain 

and Little Glass Mountain rhyolites being fed by a single 

magma body, a more likely explanation of the similarity in 

rhyolite composition reflects the similarity of the calcalkaline 

parental magmas from which they were derived, and the magmatic processes that operated on both the west and east sides 

of the caldera.

Episodic Eruptive Activity and Tectonics

The abundance and variety of erupted and included 

magmatic materials combined with petrologic evidence for 

multiple wet and dry parental magmas provides a rich record 

documenting an active magmatic system that is likely to erupt 

again. In figure 55 we depict the nine eruptive events and the 

apparent timing and types of intrusive events. At least seven 

intrusive events are directly indicated by the variety of QMI in 

the erupted lavas. Intrusions are also inferred based on petrologic evidence for multiple inputs of mafic magma, notably in 

the generation of the Burnt Lava, Callahan, Little Glass Mountain, and Glass Mountain magmas. Because the host lavas 

for the QMI are poorly porphyritic and the inclusions commonly display textures indicating rapid cooling, we conclude 

that lengthy subjacent storage prior to eruption is unlikely. A 

plausible combined total of at least 25 intrusive and extrusive 

events is indicated, or approximately one every 170 years on 

average between 5,200 years ago (eruption of Medicine Lake 

Glass Flow) and 950 years ago (eruption of Glass Mountain). 

This is likely a minimum estimate of the total number of magmatic events. There is no evidence for intrusive activity since 

the Glass Mountain eruption, but neither can it be ruled out.

A fundamental characteristic of MLV is the interaction 

of tectonism and volcanism (Donnelly-Nolan and others, 

2008) and it is likely that the episodic eruptive behavior of the 

volcano is controlled by tectonic forces. All of the late Holocene lavas were erupted from multiple vents (figs. 3 and 4) 

and eight of the nine were clearly fissure-fed. The linear vent 

arrays also imply that emplacement of dikes is a major process 

at MLV. Vents for the ~3-ka mafic eruptions are separated by 

a north-south distance of 27 km, but because both involved 

similar basaltic magma, the timing suggests that they could be 

related to a single diking event. All late Holocene units except 

the caldera-bound Medicine Lake Glass Flow erupted from 

vents aligned within 40° of north. More than half have alignments within 25° of north, strongly reflecting tectonic control 

A

B

Figure 56. Photographs showing examples of numerous open 

cracks at Medicine Lake volcano. (A) Crack on the main edifice at 

the northeastern extent of the dacite of pit craters fissure (child 

for scale); (B) on lower east flank in Pleistocene basalt (graygreen sagebrush plants ~0.5 m high for scale), both reflecting the 

overall east-west tectonic regime.

by the east-west regional extensional environment (DonnellyNolan and others, 2008), which is also shown by faults on and 

adjacent to the volcano that display northwest to northeast 

trends (fig. 3). Faulting includes vertical offset of the ~12.5-ka 

basalt of Giant Crater by ~10 m (fig. 6 in Donnelly-Nolan 

and others, 2007), multiple open cracks (fig. 26) that formed 

during the Little Glass Mountain eruption (Fink and Pollard, 

1983), and the many north-to northeast-trending open cracks 

(fig. 56) found scattered around the north and east flanks of the 

volcano.
56 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Conclusions

The nine compositionally diverse eruptions that took 

place at the >2,000-km2 Medicine Lake volcano since ~5 ka 

constitute one of the most frequent late Holocene eruptive 

records at a Cascades arc volcano. The eruptions are spread 

over an area of ~300 km2, an area about twice the footprint of 

the Mt. St. Helens edifice. The diversity of erupted compositions from basalt to rhyolite in such a short interval of time 

is remarkable. Application of a variety of methods including 

geologic mapping and stratigraphy, geochemistry, paleomagnetism, radiocarbon dating, and experimental petrology has 

illuminated one of the best-understood records of late Holocene magmatic activity in the Cascades arc. Evidence from 

paleomagnetism and radiocarbon dating strongly indicate that 

the eruptive activity actually occurred in episodes. An episode 

of two caldera-centric dacitic eruptions separated by ~100 

years took place at the volcano’s summit caldera at ~5 ka.

At ~3 ka, an eruption of basalt and another of andesite took 

place on the north and south sides of MLV, respectively, 

within a period of <200 years. After another two millenia of 

quiescence, five eruptions took place within a period of <300 

years beginning at ~1.2 ka, including the largest and youngest Holocene rhyolite flow in the Cascades volcanic arc, the 

spectacular 950-yr-old Glass Mountain lava flow. 

In addition to the documented eruptive record, evidence 

for intrusive events is preserved as quenched magmatic inclusions (QMI) in five of the nine erupted lavas and as compositional zoning in four eruptions. Petrologic work provides 

critical evidence that multiple magmatic recharge events are 

required to generate the erupted lavas. A total of at least 20 

intrusive and extrusive events occurred, giving an overall 

average rate of one magmatic event every 250 years. 

Medicine Lake volcano lies in an extensional environment east of the main arc axis where the Basin and Range 

Province impinges on the Cascades arc. Well-defined northeast- to northwest-trending vent alignments and documented 

fissure eruptions indicate the strong interaction of tectonism 

and volcanism. Both dry tholeiitic basalt typical of the westernmost Basin and Range (HAOT of Hart and others, 1984) 

and wet arc-type calcalkaline basalt have erupted at MLV in 

late Holocene time as documented by experimental, mineralogical, and geochemical data from several petrologic studies. 

The QMI can also be characterized as being of dry or wet 

origin based on composition and mineralogy. The chronology 

of the erupted host lavas indicates that wet parental magma 

was present under the caldera at ~5 ka, as represented by 

amphibole-bearing andesitic inclusions in the Medicine Lake 

Glass Flow. At ~3 ka, dry HAOT-type melts both intruded and 

erupted on the north and south sides of the volcano. Basalt 

similar to HAOT is also present as inclusions in the ~1.2-ka 

rhyolite eruption on the eastern side of the caldera, but was 

soon followed by the eruption to the west and north of the 

caldera of two mafic lavas petrologically shown to have 

calcalkaline arc-type basalt parents. Hornblende-bearing mafic 

magmatic inclusions in the two youngest eruptions, the caldera-bracketing fissure rhyolites of ~1,000-yr-old Little Glass 

Mountain and the ~950-yr-old Glass Mountain were also 

derived from calcalkaline parental basalts. The QMI in these 

rhyolites are geochemically distinct from any of the erupted 

mafic lavas. The abundance of wet basaltic melt erupted and 

intruded at MLV in late Holocene time likely facilitated melting of granitic crust beneath the volcano, helping to generate the significant amounts of silicic lava that erupted in late 

Holocene time.

Evidence including the frequency of eruptive and 

intrusive magmatic activity in late Holocene time, geophysical documentation for active deformation, the presence of a 

high-temperature geothermal system, and the occurrence of 

intermittent long-period earthquakes all combine to indicate 

that MLV is still restless and will likely erupt again.

Acknowledgments

We are grateful to the many field assistants and students 

who helped in the field. Reviews by Michelle Coombs and 

Heather Wright are gratefully acknowledged. National Science 

Foundation Grants EAR-0073766, EAR-0440172, EAR0538179, and EAR-118598 supported the participation of TLG 

in this work.

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Menlo Park Publishing Service Center, California

Manuscript approved February 23, 2016

Edited by Katherine Jacques

Design and layout by Vivian Nguyen
60 Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades

Donnelly-Nolan, Champion, and Grove—Late Holocene Volcanism at Medicine Lake Volcano, Northern California Cascades—Professional Paper 1822

ISSN 2330-7102 (online)

ISSN 1044-9612 (print)

http://dx.doi.org/10.3133/pp1822