U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1810

Postglacial Eruptive History, Geochemistry, and Recent 

Seismicity of Aniakchak Volcano, Alaska Peninsula
Cover photograph. Oblique aerial photograph looking east over Aniakchak volcano and its caldera (National Park Service 

photo by M. Williams, 1977).
Postglacial Eruptive History, Geochemistry, 

and Recent Seismicity of Aniakchak 

Volcano, Alaska Peninsula

By Charles R. Bacon, Christina A. Neal, Thomas P. Miller, Robert G. McGimsey, 

and Christopher J. Nye

Professional Paper 1810

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, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Bacon, C.R., Neal, C.A., Miller, T.P., McGimsey, R.G., and Nye, C.J., 2014, Postglacial eruptive history, geochemistry, 

and recent seismicity of Aniakchak volcano, Alaska Peninsula: U.S. Geological Survey Professional Paper 1810, 74 p., 

http://dx.doi.org/10.3133/pp1810.

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)
iii

Contents

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

Introduction ....................................................................................................................................................2

Eruptive History Prior to 3,400 Years B.P. ...................................................................................................4

Pleistocene Aniakchak ........................................................................................................................4

Products of the Aniakchak I Explosive Andesite Eruption.............................................................6

Precaldera Dacites and Rhyodacites..............................................................................................11

Dacite and Rhyodacite Lava Flows.........................................................................................11

Dacite Pyroclastic Deposits Older Than Black Nose Pumice............................................12

Black Nose Pumice ...................................................................................................................12

Lower Black Nose Pumice Fall.......................................................................................13

Intraplinian Ignimbrite......................................................................................................13

Upper Black Nose Pumice Fall.......................................................................................13

Post-Black Nose Pumice, Pre-Aniakchak II Interval...........................................................14

Products of the Aniakchak II Caldera-Forming Eruption ......................................................................14

Rhyodacite Pumice Fall .....................................................................................................................14

Ignimbrite .............................................................................................................................................14

Rhyodacite Ignimbrite...............................................................................................................14

Mixed Ignimbrite........................................................................................................................15

Andesite Ignimbrite ...................................................................................................................16

Lithic Breccia..............................................................................................................................16

Postcaldera Deposits and History ............................................................................................................16

Early Postcaldera Dacite Pumice Fall .............................................................................................16

Subaqueous Domes............................................................................................................................16

Northwest Flank and Intracaldera Rhyodacite Lava Flows.........................................................22

Ancestral Caldera Lake......................................................................................................................22

Tuff Cones.............................................................................................................................................22

Vent Mountain .....................................................................................................................................24

Half Cone..............................................................................................................................................24

Early Products of Half Cone and Nearby Vents ....................................................................24

Half Cone Pink and Brown Pumice Falls................................................................................27

Proximal Late-Erupted Fall Layers ..........................................................................................27

Cobweb Lava Flow.....................................................................................................................27

Blocky Cone.........................................................................................................................................27

1931 Eruption........................................................................................................................................28

Historical Accounts...................................................................................................................28

Fall Deposit..................................................................................................................................28

Spatter Agglutinate ...................................................................................................................30

Main Crater Lava........................................................................................................................30

Late-Erupted Bombs..................................................................................................................30

Doublet and Slag Heap Lavas..................................................................................................30

Geochemistry................................................................................................................................................30

Pleistocene Aniakchak Edifice.........................................................................................................30
iv

Aniakchak I Ignimbrite.......................................................................................................................34

Precaldera Dacites and Rhyodacites..............................................................................................34

Dacite–Rhyodacite Lava Flows ...............................................................................................34

Black Nose Pumice ...................................................................................................................34

Other Pre-Aniakchak II Dacite Pumice Falls.........................................................................35

Aniakchak II Products........................................................................................................................35

Subaqueous Domes............................................................................................................................35

Northwest Flank and Intracaldera Rhyodacite Lava Flows.........................................................43

Tuff Cones.............................................................................................................................................43

Vent Mountain .....................................................................................................................................43

Half Cone..............................................................................................................................................43

Blocky Cone.........................................................................................................................................44

Products of the 1931 Eruption...........................................................................................................45

Radiogenic Isotopes...........................................................................................................................51

Recent Seismicity ........................................................................................................................................52

Discussion.....................................................................................................................................................52

Eruptive Processes Through Time ...................................................................................................52

Magma Evolution ................................................................................................................................58

Geochemical Features of Arc Magmas at Aniakchak.........................................................58

Precaldera Magma Geochemistry and Subsurface Processes ........................................59

Geochemical Implications of Aniakchak II Magma Compositions....................................60

Rhyodacite Lava Flows and Subaqueous Domes ................................................................61

Basaltic Andesites of Breezy and Blocky Cones .................................................................61

Basaltic Andesites of Windy Cone .........................................................................................61

Andesite of Surprise Cone ......................................................................................................61

Vent Mountain and Half Cone..................................................................................................61

Products of the 1931 Eruption..................................................................................................62

Aniakchak Magmatic System...........................................................................................................63

Magma Transport and Storage Suggested by Earthquakes, Deformation, and Degassing...67

Hazards.................................................................................................................................................68

Conclusions...................................................................................................................................................68

Acknowledgments.......................................................................................................................................70

References Cited..........................................................................................................................................70

Appendix A. Electron Microprobe Analyses...........................................................................................74

Appendix B. Whole-Rock Chemical Analyses ........................................................................................74

Figures

1. Google Earth image of a portion of the Alaska Peninsula showing Aniakchak, 

Black Peak, and Veniaminof volcanoes....................................................................................3

2. Aerial photographs of Aniakchak volcano and caldera ........................................................5

3. Photographs of Aniakchak caldera walls and edifice ...........................................................7

4. Generalized geologic map of Aniakchak volcano and vicinity .............................................9

5. Schematic composite stratigraphic sections..........................................................................9

6. Photographs showing deposits of Aniakchak I.....................................................................10
v

7. Oblique Google Earth view of Birthday Pass and upper Birthday Creek drainage 

showing deposits of Aniakchak I eruption and possible terminal moraine......................11

8. Photographs showing precaldera dacites and rhyodacites north of The Gates.............12

9. Photographs showing deposits of the Aniakchak II eruption.............................................15

10. Maps of Aniakchak caldera .....................................................................................................20

11. Photographs showing surface features of dacite domes inside Aniakchak caldera.....23

12. Photographs of tuff cones and Vent Mountain......................................................................25

13. Photographs of Half Cone and its products ...........................................................................26

14. Photographs of Blocky Cone ....................................................................................................28

15. Photographs showing products of the 1931 eruption...........................................................29

16. Silica variation diagrams for Pleistocene and pre-3,400-yr.-B.P. 

Aniakchak samples ....................................................................................................................31

17. Silica and P2O5 variation diagrams for Pleistocene and pre-3,400-yr.-B.P. 

Aniakchak samples ....................................................................................................................33

18. Chondrite-normalized rare-earth-element diagrams for pre-3,400-yr.-B.P. 

Aniakchak products ...................................................................................................................36

19. Primitive-mantle-normalized multielement diagrams for pre-3,400-yr-B.P. 

Aniakchak products ...................................................................................................................37

20. Silica variation diagrams for Aniakchak II and postcaldera, pre-1931 samples .............38

21. Silica and P2O5 variation diagrams for Aniakchak II and postcaldera, 

pre-1931 samples........................................................................................................................40

22. Chondrite-normalized rare-earth-element diagrams for Aniakchak II and 

postcaldera, pre-1931 samples ................................................................................................41

23. Primitive-mantle-normalized multielement diagrams for Aniakchak II and 

postcaldera, pre-1931 samples ................................................................................................42

24. Silica variation diagrams for Aniakchak II and postcaldera, pre-1931 samples 

with >56 weight percent SiO2....................................................................................................44

25. Silica variation diagrams for products of 1931 eruption of Aniakchak volcano ..............46

26. Silica and P2O5 variation diagrams for products of 1931 eruption of 

Aniakchak volcano .....................................................................................................................48

27. Chondrite-normalized rare earth element diagrams for products of 1931 

eruption of Aniakchak volcano.................................................................................................49

28. Primitive-mantle-normalized multielement diagrams for products of 1931 

eruption of Aniakchak volcano.................................................................................................50

29. Isotopic data for postglacial products of Aniakchak volcano ............................................55

30. Locations of 135 earthquake hypocenters located by the Alaskan Volcano 

Observatory for the period January 1, 2009, through January 3, 2013 ..............................55

31. Schematic west-southwest–east-northeast cross-sections of the Aniakchak 

magmatic system through time ................................................................................................65

Tables

1. Weighted mean and other radiocarbon dates for tephra and ignimbrite from 

Aniakchak volcano .....................................................................................................................17

2. Radiocarbon dates constraining post-caldera tephra fall deposits from 

Aniakchak ....................................................................................................................................18
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Abstract 

Aniakchak is a Pleistocene to Holocene composite 

volcano of the Alaska–Aleutian arc that suffered at least 

one caldera-forming eruption in postglacial time and last 

erupted in 1931. The glacially sculpted Pleistocene edifice 

lies atop gently folded Jurassic to Tertiary sedimentary rocks. 

Aniakchak products have geochemical characteristics typical 

of magmatic arcs. Parental basaltic magmas with a range of 

incompatible-element concentrations are derived from the 

mantle wedge above the Wadati–Benioff zone, drawn from a 

region broader than the edifice footprint, and focused in the 

crust into a conduit and storage system beneath the volcano. 

Over time, a column of crystal–liquid mush and solidified 

intrusions has developed to as shallow as a few kilometers 

below sea level (bsl), allowing differentiated melts to evolve, 

segregate, and accumulate in crystal-poor bodies as large as 

several tens of cubic kilometers in volume. The dominant 

process of magma evolution at Aniakchak is considered to be 

crystallization differentiation.

The oldest recognized postglacial explosive eruption, 

andesitic Aniakchak I, left nonwelded ignimbrite in valleys 

below the edifice and fines-poor welded ignimbrite or agglutinated fall deposits high on its flanks between ca. 9,500 and 

7,000 years ago. A small caldera may have collapsed at the 

source of the ignimbrite, likely just west of the present Vent 

Mountain. Aniakchak I andesite is geochemically distinct 

among the postglacial Aniakchak suite for its high K2O, Zr, 

and other incompatible-element contents. Subsequent to 

Aniakchak I, Plinian eruptions ca. 7,000 14C yr B.P. from a 

vent northeast of the edifice summit produced the Black Nose 

Pumice, consisting of a lower unit of rhyodacite lava, pumice 

fall, and intraplinian welded ignimbrite and an upper unit of 

dacite pumice fall and northeast flank lava flow. At least 20 

additional Holocene eruptions are thought to have occurred 

before the Aniakchak II caldera-forming event.

The spectacular 10-km-diameter Aniakchak Crater is a 

caldera that collapsed during the voluminous Aniakchak II 

eruption ca. 3,590 cal yr B.P. (ca. 3,430 14C yr B.P.). This eruption produced rhyodacite Plinian pumice fall and ignimbrite, 

followed first by ignimbrite with both rhyodacite and andesite 

juvenile clasts, then by andesite ignimbrite. The mixed and 

andesite ignimbrite deposits commonly are partly welded. 

Ignimbrite is present in valleys surrounding the edifice and 

extends to the coast on both northwest and southeast. The 

crystal-poor rhyodacite is compositionally quite uniform 

(69.4–70.4 weight percent SiO2) and is the most evolved and 

only hornblende-phyric magma erupted from Aniakchak. 

The andesite is unusual for Aniakchak in having low V and 

high Na2O, Y, TiO2, and, especially, P2O5 concentrations. 

Because both Aniakchak I and II andesites have close analogs 

at Veniaminof volcano ~100 km down the Alaska Peninsula, 

these comparatively unusual compositions reflect common 

processes and are unlikely to be due to local effects of crustal 

assimilation. With its top as shallow as 1.6–3.7 km bsl, a 

rhyodacite melt lens at least 400 m thick underlay the approximately 6 km by 8 km area described by the foundered cauldron

block. The andesite reservoir apparently was similarly large 

but not contiguous with that of the rhyodacite, and the two 

magmas only mingled during transit to the surface during the 

eruption. The rhyodacite evidently was unrelated to precaldera 

dacite magmas. It and the andesite represent a new cycle of 

recharge, storage, and differentiation.

Products of the earliest recognized postcaldera volcanism 

are small dacite domes extruded from the ring-fracture system 

into a deep caldera lake and a dacite pumice fall dated at 

ca. 2,300 14C yr B.P. The lake probably filled to its maximum 

of ~620 m above sea level (asl) within a few hundred years 

of caldera collapse. Pumice Dome (~67.5 weight percent 

SiO2) is compositionally similar to lava of the northwest 

flank flow and to lava exposed in the west wall of the 1931 

Main Crater within the caldera, suggesting lateral transport 

of magma from a common source. Similarly, Vulcan and 

West Domes (~65.7 weight percent SiO2) are geochemically 

indistinguishable yet were emplaced on opposite sides of the 

caldera floor. Adjacent to Vulcan, Bolshoi Dome is the least 

evolved (64.2 weight percent SiO2). These postcaldera dacite–

rhyodacites are not remnants of Aniakchak II magma but 

resulted from differentiation of recharge andesite or basaltic 

andesite magma.

The three tuff cones on the eastern caldera floor were 

constructed by alternating Surtseyan and hydrovolcanic explosions. Eruption of the basaltic andesite Windy and Breezy 

Cones on the ring-fracture system, and of andesitic Surprise 

Postglacial Eruptive History, Geochemistry, and Recent 

Seismicity of Aniakchak Volcano, Alaska Peninsula

By Charles R. Bacon, Christina A. Neal, Thomas P. Miller, Robert G. McGimsey, and Christopher J. Nye
2 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Cone inboard from them, may be related to unloading of the 

magmatic system by sudden lowering of the intracaldera lake 

by ~200 m ca. 1,860 14C yr B.P. 

Vent Mountain cone was built by strombolian to violentstrombolian eruptions above the southern ring-fracture 

segment. Lava that issued from its base flowed as far as 

~5 km. Alternating with activity of Vent Mountain, Half Cone 

produced vulcanian to Plinian explosive eruptions and effused 

lava above the northwestern ring-fracture segment. Plinian 

eruption at Half Cone ca. 400 14C yr B.P. was responsible for 

the widespread dacitic Pink Pumice and overlying andesitic 

Brown Pumice fall deposits, as well as intracaldera pyroclastic-flow and surge deposits. The most recent Half Cone activity ended with deposition of agglutinated spatter, edifice collapse, and effusion of the dacitic Cobweb lava flow. Although 

both vents erupted broadly similar andesite to dacite products, 

those of Vent Mountain commonly have slightly higher K2O 

and lower Al2O3 and CaO than those of Half Cone. Melt inclusions in plagioclase from Pink Pumice record passive degassing prior to eruption from as shallow as ~2 km bsl.

Strombolian eruption of scoria and cinder constructed 

the small Blocky Cone after Half Cone and most Vent 

Mountain activity. Blocky Cone consists of basaltic andesite 

(~52.4 weight percent SiO2) that is more primitive than any 

other postglacial Aniakchak ejecta save geochemically similar 

scoria of Breezy Cone. Melt inclusions in olivine phenocrysts 

indicate that this magma briefly stalled and partially degassed 

at ~2 km bsl. The relatively primitive magma avoided crystal–

liquid mush by finding a path to the surface at the southwest 

edge of the ring-fracture system. 

The only known historical eruption of Aniakchak took 

place along the outer western segment of the ring-fracture 

system in May and June 1931. In that eruption, nearly continuous explosive activity at Main Crater, as vigorous as 

subplinian, yielded dacite–rhyodacite followed by relatively 

voluminous andesite tephra, ending with minor basaltic 

andesite, in sum spanning the range ~67 to 56 weight percent 

SiO2. Basaltic andesite lava eventually spread across much of 

the Main Crater floor, and small dacite–rhyodacite lava flows 

effused at Doublet Crater and Slag Heap (~66 and 69 weight 

percent SiO2, respectively). Dissolved volatile contents of melt 

inclusions indicate that the main volume of dacite magma was 

stored at a minimum depth of ~4 km bsl. 

Current indications of an active magmatic system at 

Aniakchak include high 3He/4He ratios of CO2-rich gas escaping from a weakly thermal soda spring, a bull’s-eye InSAR 

pattern of ongoing caldera floor subsidence, and episodic seismicity. Low-frequency (long period, LP) events, interpreted to 

reflect movement of magma or fluids in fractures, constitute 

the majority of located earthquakes. Epicenters for 135 events 

during 2009–2012 (maximum ML 2.1) define a bow-tie pattern 

elongated approximately parallel to the Pacific–North America 

plate convergence azimuth, centered between Half Cone 

and Vent Mountain. Hypocenters define a laterally extensive 

region with events 14–25 km bsl that extends to 28 km bsl on 

the south, few events 7–14 km bsl beneath the caldera, and 

events <10 km bsl concentrated beneath Vent Mountain and 

the south caldera rim. Recent seismicity appears to be consistent with a long-lived mush that resides within a seismically 

quiet zone and from which magma escapes upward to be 

stored ephemerally as shallow as within ~2 km bsl. 

Indications of ongoing activity of the Aniakchak magmatic

system suggest that eruptions could be expected following 

seismic unrest, which potentially could be of as short duration 

as hours to days. The most likely locations of new vents would 

appear to be ring-fracture-influenced and west of a northnorthwest—south-southeast line through the caldera center. 

Eruptive scenarios, as described in the preliminary hazard 

assessment, would include hydromagmatic explosions, possibly followed by effusion or strombolian eruption of basaltic 

andesite to Plinian eruption of dacite. Lava and pyroclastic 

flows or surges probably would be limited to within the 

caldera, but ash fall could affect extensive regions downwind. 

Another voluminous eruption, such as Aniakchak II, is considered unlikely in the near future. Although the InSAR study 

indicates subsidence, the possibility that evolved magma may 

be accumulating beneath the central part of the caldera is worthy of consideration when interpreting future monitoring data. 

Introduction 

Aniakchak is a historically active caldera volcano of the 

Alaska–Aleutian arc. Located on the Alaska Peninsula 670 km 

southwest of Anchorage (fig. 1), the volcano and its pyroclastic apron are mainly within Aniakchak National Monument 

and Preserve. Ancestral Aniakchak is a glacially sculpted 

Pleistocene composite volcano that consists of basaltic andesite to dacite lava flows and fragmental rocks. Early in postglacial (Holocene) time, a voluminous eruption (Aniakchak I; 

Miller and Smith, 1987) left geochemically distinctive 

andesite pyroclastic-flow deposits (ignimbrite) in valleys and 

on the flanks of the edifice. This event was followed by dacite 

eruptions, at least one of which produced a widespread pumice 

fall deposit ca. 7,000 yr B.P. (VanderHoek, 2009). Unless 

otherwise noted, ages reported here are given in radiocarbon 

years before present (yr B.P., “present” being 1950 C.E.).
Introduction 3

Aniakchak

Veniaminof

Black 

Peak

Port 

Heiden

M e s h ik

R iv e r

Cin d e r R iv e r

An iak c h a k

R iv e r

BERING SEA 

PACIFIC OCEAN 

Kujulik Bay

Aniakchak 

Bay

BV

0 20 40 60 80 100 KILOMETERS

0 20 40 60 MILES

160° 159° 158° 157°

56°

57°

56.5°

ALASKA

Figure 1

Figure 1. Google Earth image of a portion of the Alaska Peninsula showing Aniakchak, Black Peak, and Veniaminof volcanoes. Blue 

Violet Creek (BV), mentioned in the section on “Products of the Aniakchak II caldera-forming eruption,” drains into Meshik River.
4 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

The striking 10-km-diameter caldera known as Aniakchak Crater formed by collapse of the ancestral Aniakchak 

volcano (fig. 2) during a major explosive eruption ca. 3,430 yr 

B.P. (Miller and Smith, 1987) that is notable for highly mobile 

pyroclastic flows (Miller and Smith, 1977). Those flows 

deposited voluminous compositionally zoned ignimbrite 

composed of buff-colored rhyodacite (~68–70 weight percent SiO2

) mingled with and overlain by dark gray andesite 

(~57–60 weight percent SiO2) (Dreher and others, 2005). The 

precaldera edifice of Aniakchak volcano is unusual among 

eastern Aleutian arc volcanoes with ~10 km diameter calderas 

in being far less voluminous than Veniaminof, Okmok, Fisher, 

or Emmons Lake volcanoes (Miller and Smith, 1987). In fact, 

the Aniakchak caldera walls expose Jurassic and Cretaceous 

sedimentary rocks beneath edifice lavas. Prehistoric postcaldera volcanism has formed four lava domes, a flank lava flow, 

three tuff cones, and a small scoria cone that we refer to with 

informal names (capitalized only for clarity), and the larger 

edifices of Half Cone and Vent Mountain within the caldera 

(Neal and others, 2001). The domes were emplaced within 

a caldera lake that catastrophically drained ca. 1,860 B.P. 

(VanderHoek and Myron, 2004) and thereby rapidly cut The 

Gates notch in the caldera wall (McGimsey and others, 1994; 

Waythomas and others, 1996). The caldera-forming eruption, 

and subsequent eruptive activity, profoundly disrupted human 

occupation of the area for an extended period (VanderHoek 

and Myron, 2004).

The most recent eruption of Aniakchak took place during 

May and June 1931. The 6-week-long eruption, strongest 

during May 1-11, rained “egg-size” scoria 30 km to the west 

in Port Heiden (then Meshik), spread millimeters of ash fall 

as far as Kodiak Island, and repeatedly interfered with radio 

communications in the area. New lava flows were emplaced in 

the Main (1931) Crater and as small flows at Doublet Crater 

and Slag Heap toward the end of the activity (see figure 10 for 

locations of named features within the caldera). Within and 

near the caldera, the 1931 deposit consists of white pumice 

lapilli near the bottom and grades upward through a main 

unit dominated by tan to brown pumice to an upper unit of 

black pumice. Like an eruption of Half Cone ca. 400 yr B.P. 

and eruptions at many other Alaskan volcanoes, the 1931 

Aniakchak eruption began with relatively silica-rich dacite 

magma (as much as 69 weight percent SiO2 in 1931) and 

ended with relatively silica-poor (basaltic) andesite magma (as 

little as 56 weight percent SiO2) (Nicholson and others, 2011). 

Potential volcanic hazards at Aniakchak were assessed by 

Neal and others (2001). Ongoing seismicity, escape of magmatic gas, and subsidence of the caldera floor indicate that the 

magmatic system is active. The Alaska Volcano Observatory 

(AVO), a consortium of the U.S. Geological Survey (USGS), 

University of Alaska Fairbanks (UAF), and Alaska Division 

of Geological and Geophysical Surveys (ADGGS), monitors 

Aniakchak for any signs of reawakening.

Postglacial Aniakchak is of notable significance to 

volcanology for its compositionally zoned and highly mobile 

pyroclastic flows of the caldera-forming eruption, subaqueous domes and catastrophic draining of the caldera lake, and 

the well documented 1931 eruption. Aniakchak is of petrologic and geochemical interest because of the compositional 

diversity of its magmas and the common occurrence of zoned 

eruptions. In this paper we compile geologic information 

from field studies to document the stratigraphic record of 

eruptive history. That record is incomplete and still relatively 

little studied for deposits older than the Aniakchak II 

caldera-forming event. Here, we document what is known 

about the products of the largest eruptions. Because systematic 

petrographic descriptions are not available, we note phenocryst mineralogy only for certain distinctive units. We draw 

upon the AVO database of chemical analyses of whole-rock 

specimens from the authors’ collections and those made during 

University of Alaska Fairbanks (UAF) student research projects. The geochemical data characterize products of specific 

eruptions or volcanic vents, and thereby allow discontinuously 

exposed deposits to be categorized. Although it is beyond the 

scope of this report to quantitatively model the magmatic evolution of Aniakchak, processes responsible for the compositions of erupted magmas may be inferred from the systematics 

of geochemical variations. We plot locations (epicenters) and 

hypocenters of earthquakes located by AVO for 2009–2012 

and suggest how they may be related to magmatic plumbing. 

The stratigraphic and geochemical data we present will serve 

as a framework for future topical studies and refinement of the 

eruptive history of Aniakchak volcano.

Eruptive History Prior to 3,400 Years 

B.P.

This chronicle begins with a brief overview of Pleistocene

ancestral Aniakchak and an account of the state of knowledge 

of postglacial, precaldera volcanism.

Pleistocene Aniakchak

The lava flows that make up the ancestral Aniakchak 

volcano are well exposed in the caldera walls and on the south 

flank of the edifice (fig. 3). Subjacent Mesozoic and Tertiary 

rocks were mapped and described by Detterman and others 

(1981a). Aniakchak volcano is located on the north limb 

and crest of a regional antiform than plunges to the southwest beneath Quaternary alluvial fan and swamp deposits 

(Detterman and others, 1981b) in the Meshik River drainage 

(fig. 1). Eccentric collapse of the caldera has revealed outcrops 

of lava above talus in the north caldera wall and atop Upper 

Jurassic Naknek Formation and Upper Jurassic and Lower 
Introduction 5

Figure 2

A

B

Figure 2. Aerial photographs of Aniakchak volcano and caldera. A, View looking east over caldera (National 

Park Service photo by M. Williams, 1977). B, View south-southwest over flank to caldera. Prominent cone within 

caldera is Vent Mountain.
6 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Cretaceous Staniukovich Formation marine sedimentary rocks 

in the south wall from The Gates to Birthday Pass. Deep erosion of the south flank of Aniakchak volcano on the Pacific 

side of the Aleutian Range exposes the base of an embayed 

cap of Aniakchak lavas on Upper Cretaceous Chignik Formation nearshore and nonmarine sedimentary rocks and Naknek 

and Staniukovich strata. By contrast, the north, west, and 

northeast flanks are almost completely hidden beneath unconsolidated pyroclastic debris (fig. 4). Typical minimum elevation of lava exposure on the north flank and in the north caldera wall is ~1,500 ft (~460 m) above sea level (asl), though 

locally lava crops out as low as 700 ft (~210 m) asl on the 

northwest. A high ridge that radiates northwest from the north 

rim suggests a flank vent system, while a north-northeast ridge 

appears to be a divide between glacial valleys. Gentle slopes 

below 1,500 ft (460 m) asl on the north may be underlain by 

lava flows and (or) fragmental debris from Aniakchak or by 

Eocene and Paleocene Tolstoi Formation sedimentary rocks 

and volcanics in the Eocene and Oligocene Meshik Formation, 

which are well exposed in bluffs and slopes south of Barabara 

Creek west of the edifice and in Birthday Creek valley (fig. 4). 

North-northeast of the caldera, Aniakchak fragmental debris 

onlaps and obscures bedrock, presumably the Tertiary formations, that underlies a ~4 km x 8 km west-southwest–eastnortheast elongated ~1,200 ft (~370 m) asl mesa. In the south 

caldera wall, the base of the Aniakchak lava pile is as high 

as ~3,000 ft (~910 m) asl below Aniakchak Peak, which 

rises to >4,400 ft (>1,340 m) asl. The lowest mapped remnants of Aniakchak lava on the south flank are at ~1,000 ft 

(~300 m) asl. 

The concept of late Pleistocene Aniakchak that emerges 

is of a composite volcano of moderate size whose glacially 

sculpted summit would have been south of the center of the 

present caldera. Bedrock morphology of glacial valleys heading in Aniakchak lavas would have been essentially as now, 

with the exception of that of The Gates and Aniakchak River 

that was extensively modified by draining of the intracaldera 

lake. Birthday Creek glacial valley cut deeply into the southwest flank of the edifice and extended as a significant declivity 

well into the area above the present southwest caldera floor. 

The width and east–west trend of Birthday Creek valley near 

the modern caldera rim suggest that ice of sufficient volume to 

carve this large valley may have been sourced in a small late 

Pleistocene caldera. 

The Pleistocene eruptive history preserved by the edifice 

lavas has yet to be studied in detail. Reconnaissance sampling 

and geochemical analysis of about 30 specimens provides a 

general compositional range for Aniakchak erupted magmas 

over the probable several hundred thousand years during 

which the edifice was constructed (Nye and others, 1993). 

There doubtless exist materials emplaced during glacial 

conditions, so that a record of volcanism during different 

climatic conditions could be obtained through additional field 

study and argon geochronology. For the present report, we 

are limited to employing the geochemistry of edifice lavas 

as context for compositional diversity among products of 

postglacial Aniakchak.

Products of the Aniakchak I Explosive 

Andesite Eruption

Miller and Smith (1987) recognized ignimbrites from two 

major eruptions, Aniakchak I and Aniakchak II. Aniakchak I 

nonwelded andesite ignimbrite is overlain by a sequence of 

tephras that includes ca. 4,600 yr B.P. ash from Black Peak 

(Miller and Smith, 1987; McGimsey and others, 2003) and 

therefore is significantly older than the ca. 3,400 yr B.P. 

Aniakchak II caldera-forming eruption (figs. 5A and 6A). Both 

Aniakchak I and the andesite portion of compositionally zoned 

Aniakchak II occur as dark-gray nonwelded, poorly sorted 

ignimbrite and also as partly welded fines-poor agglutinated 

bomb deposits. The latter material is locally exposed in gullies 

on the north flank (fig. 4, unit Qag). Aniakchak I and andesitic 

Aniakchak II exposures can be indistinguishable in the field. 

Where stratigraphic evidence for a pre-3,400-yr B.P. age is 

lacking, however, these units can be readily identified by their 

chemical compositions. Even though their silica concentrations and crystal contents are similar, Aniakchak I andesite has 

uniquely high K2O, Zr, and many other incompatible-element 

concentrations (see “Geochemistry” section). Systematic 

search for Aniakchak I using geochemical fingerprinting has 

not been done, so the true maximum extent of pyroclastic 

flows from that eruption remains to be determined.

Deposits of unconsolidated and partly welded andesite 

bombs and ash found low in north-northwest flank gully 

exposures, in Birthday Creek drainage and Birthday Pass, 

and below the caldera rim north of The Gates have distinctive 

Aniakchak I chemistry. These are considered to have been 

emplaced by pyroclastic flows during one or a few closely 

spaced eruptions that constitute the Aniakchak I event. The 

deposits in valleys clearly postdate the last major glacial 

advance. Large patches of Aniakchak I partly welded ignimbrite, consisting of flattened bombs as large as ~1 m and lithic 

fragments in an indurated matrix, are present on north and 

south sides of the floor of the Birthday Pass beheaded glacial 

valley at elevations between ~2,100 ft (640 m) and ~2,450 ft 

(750 m) asl (fig. 6B; see also fig. 10A). Vertical cooling joints 

cut the entire ~30-m-thick exposure on the north side of 

the valley, where differences in induration suggest three or 

more flow units within a single cooling unit. Possibly, cliff 

exposures of these remnants of partly welded Aniakchak I 

ignimbrite were quarried by an early Holocene glacier that 

originated higher on ancestral Aniakchak, before formation of 

the caldera that we associate with Aniakchak II. The implication is that the Aniakchak I eruption was not accompanied by 

caldera collapse or, if a caldera formed, it either was too small 

to engulf the high-edifice source of the Birthday Creek glacier 

or it rapidly filled with ice that then spilled out Birthday Pass. 

Alternatively, presence of partly welded Aniakchak I ignimbrite on either side of Birthday Pass but not in the center of the 

valley could reflect deposition of ignimbrite along the margins 

of a glacier that occupied the upper valley at the time of the 

eruption. In that scenario, nonwelded ignimbrite lower in 

Birthday Creek valley was deposited in the valley floor below 
Eruptive History Prior to 3,400 Years B.P. 7

the terminus of the glacier, now marked by a low north-northwest-trending ridge that may be the glacier’s terminal moraine 

1.8 km west of the pass at ~1,850 ft (560 m) elevation (fig. 7). 

In that case, the glacier could have been beheaded by caldera 

collapse associated with the Aniakchak I eruption.

The unique chemical composition of Aniakchak I andesite

allows the stratigraphy on the volcano’s flanks to be tied to 

deposits on the caldera rim. Stratigraphic relations are well 

displayed north of The Gates (fig. 5B). Here, Aniakchak I 

nonwelded ash, bombs, and ash-coated lithic fragments (as 

large as at least 30 cm), interpreted to be proximal ignimbrite 

or possibly a lahar, make up the lowest exposed postglacial 

deposit (fig. 6C). A minimum of 3 m thick (the base is not 

exposed), this unit is overlain by poorly sorted probable 

fall deposits that contain Aniakchak I bombs to 60-cm 

diameter. A few tens of meters to the east, separated from 

the nonwelded deposit by a covered interval, is a 2m-thick 

outcrop of Aniakchak I agglutinated black vitric cauliform 

(cauliflower-shaped) bombs (fig. 6D). The moderately flattened bombs are as large as 70 cm across. The virtually clastsupported agglutinated bomb deposit, interpreted as proximal 

Aniakchak I ignimbrite, rests on gravel or colluvium and is 

overlain by Black Nose Pumice (described below). Correlation of Black Nose Pumice with a fall deposit described and 

dated at ca. 7,000 yr B.P. by VanderHoek (VanderHoek, 2009; 

VanderHoek and Myron, 2004) constrains the date of the 

Aniakchak I eruption to between ca. 9,500 deglaciation and 

ca. 7,000 yr B.P. 

A B

C D

Upper Jurassic 

and Lower 

Cretaceous 

sedimentary 

rocks

Aniakchak 

edifice lavas

Figure 3. Photographs of Aniakchak caldera walls and edifice. Features shown are labeled in figure 4. A, View over Vent Mountain 

lava east across caldera to The Gates and Black Nose; shoulder of Vent Mountain on right. B, Caldera wall below Black Nose. Dark 

edifice lavas rest on lighter-colored Upper Jurassic Naknek Formation and Upper Jurassic and Lower Cretaceous Staniukovich 

Formation marine sedimentary rocks. C, Edifice lavas in north caldera wall viewed from west end of Surprise Lake. D, Aerial view 

eastward of southeast-flank edifice lavas and breccias of The Garden Wall.
8 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 4

158°30.0’W 158°15.0’W 158°00.0’W

57°07.5’N

57°00.0’N

56°47.5’N

N

Qal

Qac

Qb

Q31

Qhl

Qtc

Qa2

Qvm

Qd

Qht

Qhc

Qag

Qv

TMz

Qbn

Qa1

Qac

Qal

Qal

Qal

Qal

Qd

Qd

Qa2

Qa2

Qa2

Qag

Qag

Qbn

Qbn

TMz

TMz

TMz

TMz

TMz TMz

Qv

Qv

Qv

Qv

NW lava 

flow

NE lava 

flow

G

N

L

A

S

V

B

R

M

GW

P

H

BB

PB

W

GE

0 2 4 6 8 10 KILOMETERS

0 2 4 6 MILES

Qal

Qac

Qtc

Qa2

Qv

Undivided surficial deposits

Ash and colluvium

Tuff cone

Qvm Aniakchak edifice volcanic rocks

Qb

Qd

Qht

Vent Mountain lava and scoria

Blocky cone scoria

1931 lava and tephra Subaqueous domes and related lava

Half Cone tephra

Qhc

Qhl Qag

Half Cone lava and deposits

Half Cone cow flop lava flow

TMz

QUARTERNARY

TERTIARY AND MESOZOIC

Sedimentary rocks

EXPLANATION

Surficial units

Volcanic rocks

Aniakchak II ignimbrite

Agglutinate-like ignimbrite

Qbn Black Nose Pumice and related lava

Qa1 Aniakchak I ignimbrite

Q 31
Half Cone lava

Half Cone upper light pumice

New Cone tephra

Half Cone brown pumice

Half Cone pink pumice

1931 lavas

1931 tephra

Vent Mountain south flank lava

Blocky Cone tephra

Half Cone cobweb lava

Vent Mountain lava

Half Cone lower light pumice

Vent Mountain? tephra

Vent Mountain lava

3 tuff cones

4 subaqueous domes

400±30

380±50

570±40

14C yr B.P.

Schematic composite stratigraphic section, 

Aniakchak caldera 

Schematic stratigraphic section, 

Aniakchak caldera rim north of The 

Gates 14C yr B.P.

Schematic stratigraphic section, 

Aniakchak volcano flanks 

~4,6001

3,430±701 Aniakchak II ignimbrite

Aniakchak I ignimbrite

Aniakchak II fall

Black Peak ash

1

After Miller & Smith (1987)

Half Cone Brown Pumice

Half Cone Pink Pumice

Aniakchak II lithic breccia

Upper Black Nose Pumice fall

Lower Black Nose Pumice fall

Welded ignimbrite

Lower Black Nose Pumice fall

Rhyodacite lava flow

Aniakchak I deposits

Note: Several ash layers and dacitic 

lapilli fall beds are present between 

Aniakchak I and Lower Black Nose 

Pumice fall south of the lava flow 

(Dreher’s AC11b–AV16d; total 

thickness ~1 m); unknown relation 

to lava flow to north

Tephras

Tephras

Note: Unconsolidated deposits above 

Aniakchak II ignimbrite not shown

Correlation with GICC05 ice core 

chronology indicates Aniakchak II 

eruption date of 3,590–3,588±11 cal yr 

B.P. (Coulter and others, 2012) 

~7,0002

14C yr B.P.

2

Correlation with 

VanderHoek (2009) Cabin 

Bluff section

<~9,5002

C

A B

Vent Mountain lava

Half Cone vent-filling lava

Caldera lake drains

840±30

1,860±303

3

After VanderHoek & Myron (2004)

900±80

Eruptive History Prior to 3,400 Years B.P. 9

Figure 4. (facing page) Generalized geologic map of Aniakchak volcano and vicinity (modified after unpublished compilation by 

T.P. Miller, D.H. Richter, R.L. Smith, and C.A. Neal). See figure 10A for more detailed map of caldera geology. Unit Qag consists of 

fines-poor welded andesite ignimbrite that is dominantly Aniakchak II but may include outcrops of Aniakchak I. Unit Qhc consists 

of pyroclastic deposits and lava flows from Half Cone. Unit Qa2 consists of nonwelded to welded ignimbrite that is dominantly 

Aniakchak II but may include outcrops of Aniakchak I; ignimbrite from both eruptions is present in some localities (see text). Tephra from 

1931 eruption shown only within caldera. Geographic names: A, Aniakchak River; B, Birthday Creek; BB, Barabara Creek; G, The Gates; 

GW, The Garden Wall; H, Half Cone; L, Lava Creek; M, Meshik Lake; N, Black Nose; P, Birthday Pass; PB, Plenty Bear Creek; R, Reindeer 

Creek; S, Surprise Lake; V, Vent Mountain; W, Wind Creek. Locality GE on north flank is gully exposure in figure 6A. Hillshade base from 

U.S. Geological Survey 30-m digital elevation data.

Figure 5. Schematic composite stratigraphic sections. 

Ages next to columns are uncalibrated radiocarbon 

ages. A, Aniakchak I and II, with intervening tephras, 

north flank of Aniakchak volcano. B, Caldera rim north 

of The Gates. C, Postcaldera deposits and lava units 

within caldera.
10 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

A B

C D

Aniakchak I

Andesitic Aniakchak II

Rhyodacitic Aniakchak II

Aniakchak II 

lithic breccia

Black 

Peak 

ash

Aniakchak I

Lower Black Nose 

Pumice fall

Upper Black Nose Pumice fall

Welded 

ignimbrite Cover

Cover

Unconsolidated 

deposits

Figure 6. Photographs showing deposits of Aniakchak I. A, Gully exposure 

at locality GE (fig. 4) on north flank of Aniakchak volcano. Nonwelded 

Aniakchak I ignimbrite at lower left is overlain by tephras; Aniakchak II 

rhyodacite pumice fall, welded andesite ignimbrite (dark cliff), and lithic 

breccia; and younger tephras and colluvium. Geologist is standing on 

ash from Black Peak. B, Approximately 30-m-high cliff of partly welded 

Aniakchak I ignimbrite on north side of Birthday Pass ~1.1 km west of 

caldera rim (fig. 7). C, Postglacial pyroclastic section north of The Gates. 

Andesite bombs seen in the lowest part of the exposure are Aniakchak I 

composition. D, Agglutinated Aniakchak I cauliform (cauliflower-shaped) 

bombs underlie Black Nose Pumice a few tens of meters east of the 

exposure in C.
Eruptive History Prior to 3,400 Years B.P. 11

Precaldera Dacites and Rhyodacites

Two lava flows and several pyroclastic deposits are 

younger than the last major glacial advance but preceded the 

Aniakchak II caldera-forming eruption. For convenience, these 

are referred to as precaldera dacites and rhyodacites.

Dacite and Rhyodacite Lava Flows

A blocky flow of dacite lava is present on the northeast 

flank of Aniakchak volcano (fig. 4, included in unit Qbn). 

Compositional affinity of the lava with deposits on the caldera 

rim indicates that the flow likely postdates Aniakchak I. 

Dreher (2002, p. 17) reported Aniakchak II lithic breccia 

resting on the lava. 

A second precaldera silicic lava flow or dome is exposed 

in the cliff at the caldera rim north of The Gates (figs. 5B

and 8A; also included in unit Qbn in map figures). The 

rhyodacite composition of the lava is more differentiated 

than that of the northeast flank lava but is virtually identical 

to that of the overlying lower Black Nose Pumice, which 

suggests that the lava represents an early phase of the Black 

Nose Pumice eruption sequence. The lava appears to overlie 

Aniakchak I deposits.

Figure 7

Birthday 

Pass

Aniakchak I 

partly welded 

ignimbrite

Aniakchak I & II 

nonwelded 

ignimbrite

Terminal moraine?

Blocky 

Cone

Birthday Creek valley

Caldera rim

Caldera rim

Figure 6 B

Figure 7. Oblique Google Earth view of Birthday Pass and upper Birthday Creek drainage showing deposits of Aniakchak I 

eruption and possible terminal moraine. Arrow indicates location of photograph in figure 6B.
12 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Dacite Pyroclastic Deposits Older Than Black 

Nose Pumice

A meter or more thickness of unconsolidated deposits 

overlie Aniakchak I material and underlie the Black Nose 

Pumice in the exposure north of The Gates shown in figure 6C

(“unconsolidated deposits”). The juvenile clasts in these 

deposits are dacite pumice. The sequence consists of ≤1 cm of 

buff ash, 1 cm of gray ash, 15 cm of pumice fall with obsidian 

chips, 5 cm of dark gray ash, 1 m of pumice fall with ≤0.5 cm 

obsidian chips and rare bombs as large as 30 cm, and 1 m 

of ashy probable surge deposit. The two uppermost beds are 

erosionally truncated. Additional work on tephras, below ash 

from Black Peak in flank exposures such as shown in figure 6A

and in distal exposures studied by VanderHoek (2009), may 

establish correlations with the caldera rim deposits.

Black Nose Pumice

Following Dreher’s (2002, p. 17) description of the Black 

Nose Pumice and prior informal usage of this term, we apply 

the name to dacite and rhyodacite Plinian pumice fall deposits 

and intercalated (intraplinian) welded rhyodacite ignimbrite 

north of The Gates and to correlative deposits elsewhere near 

the caldera rim. The unit is named for exposures 2.5 km south 

of The Gates at Black Nose, although a more accessible and 

probably more complete section is present north of The Gates. 

An ~2,000-m2 patch of apparent fall deposit is preserved on 

a northeast-trending spur at ~1,800 ft (550 m) asl south of 

the Aniakchak River immediately east of The Gates, which 

indicates that a deeply incised valley headed just east of the 

present caldera rim at the time of the Black Nose eruptions. 

Similarly, a prominent 1,500 ft (460 m) asl bench ~1–2 km 

east-northeast of The Gates appears to be capped by Black 

Figure 8

A B

C D

Figure 8. Photographs showing precaldera dacites and rhyodacites north of The Gates. A, Aerial view of rhyodacite lava flow 

forming dark cliff at left, overlain by Black Nose Pumice. Buff colored layers are lower Black Nose Pumice fall with intraplinian 

welded ignimbrite showing as dark layer of irregular thickness; overlying dark and orange layers are upper Black Nose Pumice fall. 

Total height of view ~100 m (330 ft). Lowest cliff exposures at lower right are edifice lava. B, Lower Black Nose Pumice fall; upper 

Black Nose Pumice fall at upper right. C, Aerial view of Black Nose Pumice deposits immediately south of and continuous with A; 

intraplinian welded ignimbrite thickens in topographic depressions on underlying lower Black Nose Pumice. Height from top of 

edifice lava to top of view ~75 m (245 ft). D, Detail of intraplinian welded ignimbrite near south end of exposures north of The Gates.
Eruptive History Prior to 3,400 Years B.P. 13

Nose Pumice. Other remnants of Black Nose Pumice may 

be present on the ~400 x 700 m bench at ~2,300-ft (700 m) 

elevation south of The Gates. Black Nose Pumice also may be 

present below younger deposits on the north and west caldera 

rim. North of The Gates, Black Nose Pumice dips gently 

towards the caldera and conformably overlies 1 cm of gray 

ash and 1 cm of brown ash that rest upon the eroded surface 

of the previously described unconsolidated dacite pyroclastic 

deposits. The Black Nose Pumice is overlain by Aniakchak II 

lithic breccia. The Black Nose Pumice is subdivided into 

lower and upper subunits (figs. 5B and 8).

Lower Black Nose Pumice Fall

The well-sorted beds of the lower Black Nose Pumice 

consist of highly inflated crystal-poor buff-colored pumice that 

is rhyodacite in composition. The pumice is chemically less 

evolved and lacks the hornblende phenocrysts that are present 

in Aniakchak II rhyodacite pumice (Dreher, 2002). Maximum 

pumice clast size is ~30 cm. Many clasts have pink interiors 

and are fractured so that they fall apart when removed from 

the outcrop. The unit is ~8–10 m thick north of The Gates. 

The age of the lower Black Nose Pumice is constrained 

by tephrochronology. A 45-cm-thick bed of relatively coarse 

silicic pumice ~40 km southeast of the caldera near Aniakchak 

Bay (fig. 1) was described by VanderHoek and Myron (2004, 

their figures 7, 8; Cabin Bluff section, ANI 99–K) and tentatively correlated by them with Aniakchak I. However, on the 

basis of our electron microprobe analyses of glass, magnetite, 

and ilmenite from sample ANI 99–K and from Black Nose 

Pumice on the caldera rim (appendix A, tables A1 and A2), 

we find that this bed correlates instead with lower Black Nose 

Pumice. A bulk chemical analysis of lapilli from sample ANI 

99–K (appendix B, table B1) bears a resemblance to lower 

Black Nose Pumice but does not exactly match any Aniakchak 

pumice, presumably owing to postdepositional modification or 

another artifact that has resulted in increased MnO and P2O5

and decreased SiO2 concentrations. Fe-Ti oxide thermometry 

and oxygen barometry for lower Black Nose Pumice and sample ANI 99–K yields preeruption temperatures of ~940–950 °C

and oxygen fugacity near that of the Ni-NiO buffer (appendix A,

table A2), similar to conditions calculated in the same way for 

lower Black Nose Pumice oxide analyses in Dreher (2002). 

Peat from beneath the Cabin Bluff pumice bed yielded a radiocarbon age of 7,350±70 yr B.P., whereas soil beneath the same 

bed 600 m distant gave 6,760±60 yr B.P. (table 1; VanderHoek 

and Myron, 2004; VanderHoek, 2009), leading us to suggest 

an age of ca. 7,000 yr B.P. for the lower Black Nose Pumice. 

A comparatively thin tephra bed (VanderHoek and Myron, 

2004, sample ANI 99–L) beneath the principal fall deposit 

at Cabin Bluff (ANI 99–K) contains glass and Fe-Ti oxides 

compositionally similar to those of the principal deposit as 

well as another population of glass that is more evolved and 

oxides that have somewhat different compositions (appendix 

A, tables A1 and A2). We tentatively interpret the lower bed 

as representing one or more eruptions of Aniakchak volcano 

precursory to the Black Nose Pumice eruptions.

Intraplinian Ignimbrite

Welded ignimbrite occurs within the lower Black Nose 

Pumice north of The Gates ~2 m below the top of the lower 

Black Nose Pumice fall (figs. 5B, 8A, 8C, and 8D). About 

1–2 m thick at the locality shown in figure 8D, the ignimbrite 

pinches out south towards The Gates and thickens to the north 

to at least 10 m in topographic lows. The pumice fall deposit 

immediately beneath the ignimbrite is oxidized brick red 

by air trapped and heated by the ignimbrite. Fiamme in the 

welded ignimbrite, which are as large as 30 cm in length, are 

identical in composition to lower Black Nose Pumice clasts. 

Where examined, the ignimbrite consists of a principal thick 

flow unit with reversely graded base, overlain by a possible 

second flow unit ~1 m thick, the entire deposit forming a 

single cooling unit. Although lithic fragments are present, 

no lithic concentration zones were observed. The ignimbrite 

likely represents collapse of the Plinian eruption column, 

probably owing to vent widening and increase in eruption rate 

as deduced for the similar Wineglass Welded Tuff at Crater 

Lake, Oregon (Kamata and others, 1993). 

The ignimbrite is partly to densely welded. Where it is 

relatively thick, welding and crystallization range, from bottom

to top, through (1) black vitrophyric fiamme in a brick-red 

oxidized vitric matrix; (2) a zone in which the larger fiamme 

have devitrified cores; (3) devitrified fiamme in black vitric 

matrix; (4) black vitrophyric fiamme in brick-red vitric matrix; 

and (5) partly welded tuff with rusty tan matrix. The thickest 

sections of ignimbrite, which were not visited, probably are 

devitrified in their interior portions. Thick ignimbrite has gas 

pockets as large as 50 cm by 25 cm high.

Upper Black Nose Pumice Fall

The upper Black Nose Pumice is a Plinian fall deposit 

that conformably overlies the lower Black Nose Pumice. 

As large as 40 cm, clasts of brown pumice are silicic dacite, 

slightly less evolved than the underlying ignimbrite and lower 

Black Nose Pumice. This pumice has greater phenocryst 

content than the lower unit (Dreher, 2002). The well-sorted 

beds are variably oxidized and incipiently welded. Basal 

upper Black Nose Pumice fall is partly welded and forms a 

dark-gray band in caldera rim exposures. This nearly uniformthickness band and the underlying variable-thickness ignimbrite, separated by ~2 m of buff-colored pumice fall, make a 

distinctive pair of dark-gray stripes across exposures near the 

rim of the northeast quadrant of the caldera (figs. 8A and 8C). 

The preserved thickness of upper Black Nose Pumice north 

of The Gates is as much as ~15 m but varies considerably, 

implying modification during a significant period of erosion. 

The deposit is overlain by Aniakchak II lithic breccia.

The distribution of upper Black Nose Pumice and any 

distal correlations have yet to be established, though the thickness, composition, and coarseness of the deposit at the caldera 

rim imply wide dispersal downwind. Analyzed dacite pumice 

from a fall deposit beneath Aniakchak II ignimbrite in lower 

Reindeer Creek (fig. 4, near west edge), sampled by Dreher 

(2002; sample 98AC70D), has unique composition but is 

closest to basal upper Black Nose Pumice. 
14 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Post-Black Nose Pumice, Pre-Aniakchak II 

Interval

The geologic record of Aniakchak eruptive activity in 

this time interval has only begun to be studied. The tephra 

record is complicated by the possible presence of material 

from Black Peak and Veniaminof volcanoes. Some of the 

deposits described by VanderHoek (2009), VanderHoek and 

Myron (2004), and Riehle and others (1999) are thought to 

represent eruptions of Aniakchak. However, few radiocarbon 

ages are associated with these tephras. Neal and others (2001, 

p. 4), referring to Riehle and others (1999), suggest that at 

least 20 explosive eruptions occurred at Aniakchak in postglacial time before the Aniakchak II eruption. It is not surprising 

that deposits from this time interval have not been reported 

from caldera rim exposures, because the irregular thickness of 

upper Black Nose Pumice fall at the caldera rim indicates that 

that unit was eroded before (or possibly by) the Aniakchak II 

eruption. Gully exposures lower on the edifice and in the 

pyroclastic apron beyond (such as in fig. 5A) are likely to be 

fruitful places for future study of Aniakchak tephras that may 

be correlated with distal ash beds.

Products of the Aniakchak II CalderaForming Eruption

Modern Aniakchak caldera formed during a large 

explosive eruption that produced widespread pumice fall and 

an extensive ignimbrite sheet. The ignimbrite is significant 

for the high mobility of the pyroclastic flows that deposited 

it (Miller and Smith, 1977) and for dramatic compositional 

zonation of rhyodacite followed by andesite (Miller and 

Smith, 1977; Dreher, 2002; Dreher and others, 2005). Ash 

from this eruption has been identified over a large part of the 

Alaska Peninsula and on the Alaska mainland as far north as 

the Seward Peninsula (Riehle and others, 1987; Begét and others, 1992). Miller and Smith (1987) reported a weighted mean 

age of 3,430±10 B.P. for three charcoal samples from the 

ignimbrite at widely separated localities. Recalculation of the 

weighted mean age and uncertainty yields 3,430±70 yr B.P. 

with a mean square of weighted deviates (MSWD) of 0.31 

(table 1). Begét and others (1992) reported a similar weighted 

mean age of 3,435±40 yr B.P. (MSWD 1.02) for the three 

charcoal dates plus two on peat beneath ash (from Miller and 

Smith, 1987) and one from beneath distal ash on the Seward 

Peninsula (from Kaufmann and Hopkins, 1985). The calendrical age of the eruption is suggested to be 3,590–3,588±11 cal 

yr B.P. (Coulter and others, 2012) by correlation with chemically analyzed glass shards associated with an acid spike in 

Greenland ice cores (Pearce and others, 2004; Denton and 

Pearce, 2006), which compares favorably with the calibrated 

±2σ age range of 3550–3870 cal yr B.P. and preferred ±1σ age 

of 3,660±70 cal yr B.P. for 3,430±70 14C yr B.P. (table 1).

Rhyodacite Pumice Fall

A buff-colored pumice fall deposit found north and northwest of the caldera marks the first phase of the Aniakchak II 

eruption. In all known exposures it is overlain by ignimbrite. 

The pumice fall is well exposed in sea cliff exposures along 

the Bering Sea coast 28 km northwest of the caldera near the 

mouth of Reindeer Creek (fig. 9A), where an ~20-cm-thick 

bed mantles underlying topography and contains angular 

pumice clasts as large as 5 cm (Dreher, 2002). The deposit is 

~50 cm thick at a locality 10 km north-northwest and ~75 cm 

thick 5 km north of the caldera (Dreher, 2002). Pumice composition is identical to the most evolved rhyodacite pumice in 

the ignimbrite. Begét and others (1992) indicate a northward 

dispersal axis for Aniakchak II tephra, but the total volume of 

the fall deposit is not well constrained.

Ignimbrite

The ignimbrite of the Aniakchak II caldera-forming 

eruption, exposed in glacial valleys around the Aniakchak 

edifice, on the Pacific coast, and in Bering Sea cliffs as far 

as 50 km from the caldera (fig. 4), originally covered an area 

≥2,500 km2 (Miller and Smith, 1977). Ignimbrite in valleys is 

as much as 70–100 m thick. The eruption produced a minimum of 14 km3

 dense rock equivalent (DRE) of rhyodacite 

and 13 km3 DRE of andesite, most apparently deposited as 

ignimbrite (Dreher and others, 2005). This total of 27 km3

does not include the likely substantial volume of ejecta that 

must have accumulated within the caldera (see, for example, 

Lipman, 1997). The following descriptions summarize 

information presented by Dreher (2002), who subdivided the 

ignimbrite deposits into rhyodacite, mixed, and andesite units, 

supplemented by the authors’ field observations.

Rhyodacite Ignimbrite

The first ignimbrite emplaced, the rhyodacite unit is 

largely obscured by overlying mixed and andesite ignimbrite. 

Maximum pumice size in the poorly sorted and seemingly 

structureless deposit typically is ~20 cm. Rhyodacite ignimbrite may have only rhyodacite clasts or may contain several 

percent of andesite clasts, up to a maximum of 25 percent. The 

deposit is as much as 30–40 m thick near the heads of Plenty 

Bear, Wind, and Birthday Creeks (fig. 4). Rhyodacite ignimbrite is exposed in Reindeer Creek (fig. 4) all the way to the 

Bering Sea cliffs. Ignimbrite in most exposures is nonwelded. 

In Reindeer Creek, the deposit is incipiently welded 12 km 
Products of the Aniakchak II Caldera-Forming Eruption 15

from the caldera, where it is at least 23 m thick and overlain 

by andesite ignimbrite; at 16 km, the base is exposed, the 

interior of the deposit is partly welded and forms cliffs as high 

as 3 m, and pumice clast size increases upward. Rhyodacite 

ignimbrite is revealed in gullies 10 km northeast of the caldera,

where it is overlain by fines-poor welded andesite ignimbrite 

(fig. 9B). Nonwelded rhyodacite ignimbrite 2.5 m thick, with 

maximum pumice size 6 cm, overlies the fall deposit in a 

depression in beach deposits visible in the Bering Sea cliffs 

near the mouth of Reindeer Creek (fig. 9A). A small exposure 

26 km south of the caldera in Blue Violet Creek (fig. 1) probably is Aniakchak II rhyodacite ignimbrite or a related surge or 

remobilized deposit.

Mixed Ignimbrite

Dreher (2002) stated that most ignimbrite that contains 

both andesite and rhyodacite juvenile clasts at the same stratigraphic level has at least 25 weight percent andesite, and he 

therefore defined mixed ignimbrite as having 25–75 weight 

percent andesite. The cutoff between mixed and other ignimbrite is arbitrary and, for practical purposes, mixed ignimbrite 

has abundant clasts of both rhyodacite and andesite. Mixed 

ignimbrite is found at ≥10 km from the caldera. The deposit 

occurs between rhyodacite and andesite ignimbrite 15 km 

southwest of the caldera in Barabara Creek (fig. 4). More commonly, rhyodacite ignimbrite, if present, is not exposed and 

A B

C D

Fall deposit

Figure 9. Photographs showing deposits of the Aniakchak II eruption. A, Rhyodacite ignimbrite overlying ~20 cm of rhyodacite 

pumice fall southwest of Reindeer Creek, Bering Sea coast. B, Rhyodacite ignimbrite overlain by thin andesite ignimbrite ~13 km 

west of caldera, Reindeer Creek (photograph by W. Hildreth). C, Mixed ignimbrite overlain by andesite ignimbrite 10 km east of the 

caldera, Lava Creek. D, Conglomerate clast in lithic breccia north of The Gates.
16 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

mixed ignimbrite may be overlain by andesite ignimbrite, as 

in Lava Creek 10 km east of the caldera (fig. 9C). There, Lava 

Creek has cut down to resistant, partly welded mixed ignimbrite, which suggests that mixed ignimbrite may be partly 

welded in other drainages at or below present creek-bed levels. 

In exposures on the Bering Sea coast and in a gravel pit in Port 

Heiden, the mixed deposit is tens of meters thick and comprises three flow units. Mixed ignimbrite occurs on the Pacific 

side of the peninsula, beyond passes as high as 260 m in the 

rugged Aleutian Range and as much as 50 airline km from the 

caldera; total thickness of mixed and andesite ignimbrite at 

Kujulik Bay is 10 m (fig. 1; Miller and Smith, 1977).

Andesite Ignimbrite

Agglutinated cauliform (cauliflower-shaped) andesite 

bombs with little interstitial fine-grained matrix constitute 

partly welded andesite ignimbrite or proximal fall on the 

flanks of the edifice in exposures 2–11 km from the caldera on 

northwest to east-northeast azimuths (figs. 6A and 9C). Comparison with spatter-rich andesitic pyroclastic-flow deposits 

at Santorini, Greece, described by Mellors and Sparks (1991), 

suggests that the Aniakchak II partly welded material probably 

had a similar flow origin, possibly involving access of water 

to the vent(s). At greater distances, the deposit is matrixsupported nonwelded ignimbrite that commonly is lithicrich near its base, such as in Reindeer and Birthday Creeks. 

Banded rhyodacite–andesite pumice clasts are common in 

mixed and andesite ignimbrite.

Lithic Breccia

Clast-supported lithic breccia, comprising volcanic clasts 

from the edifice and cobbles and boulders, including granitic 

rocks, from conglomerates in the underlying Mesozoic sedimentary section, is present on the upper flanks of Aniakchak 

volcano (fig. 9D). Lithic clasts commonly are several tens 

of cm across and may exceed 2 m in maximum dimension 

(figure 8 of Druitt and Bacon, 1986). Juvenile clasts, at least as 

large as 50 cm, are of Aniakchak II andesite. The bulk, if not 

all, of the lithic breccia is considered to be the proximal facies 

of the andesite ignimbrite unit (fig. 6A). 

Postcaldera Deposits and History

As at most volcanoes, the more recent events are represented by deposits accessible in greater detail than earlier 

ones because of burial of the older units. Our understanding of 

postcaldera geologic history is based on unpublished geologic 

mapping summarized in figure 10A. The topographic map 

in figure 10B gives elevations and names of features. The 

sequence of products as presently understood is depicted in the 

composite stratigraphic section in figure 5C. The postcaldera 

record at Aniakchak is complicated by a caldera lake having 

been present for part of the post-Aniakchak II period. Lacking 

detailed chronological information on early post-Aniakchak II 

tephras save one, we have no concrete information about the 

postcaldera eruptive history before extrusion of dactic domes 

on the lake floor. We describe here an early dacite explosive 

eruption, subaqueous lava effusions, catastrophic draining of 

the lake, subsequent hydromagmatic eruptions, later effusive 

and explosive eruptions, and, finally, the 1931 C.E. events 

and products.

Early Postcaldera Dacite Pumice Fall

Cream-colored dacite pumice fall is exposed locally 

in cutbanks beyond the north flank of Aniakchak volcano. 

Soil from beneath this deposit 18 km north-northwest of the 

caldera at ~200 ft (60 m) elevation yields an age of 2,130±40 

yr B.P., whereas organic sediment from beneath the deposit 

14 km north of the caldera at ~300 ft (30 m) elevation gives 

an age of 2,300±80 yr B.P. (table 2). The weighted mean of 

these ages is 2,160±40 yr B.P. (table 1), but the large MSWD 

leads us to prefer the older of the two for a provisional age 

of ca. 2,300 yr B.P. The composition of a sample of pumice 

clasts (NA9413) does not exactly match that of any potentially 

correlative unit within the caldera but nevertheless is rather 

similar to that of West Dome. We tentatively consider the 

ca. 2,300 yr B.P. pumice fall to be the product of an explosive 

eruption within the caldera that preceded or possibly was 

contemporaneous with emplacement of the subaqueous domes 

that are described next.

Subaqueous Domes

Within Aniakchak caldera, the oldest known products of 

postcaldera volcanism are four lava domes that were effused 

in a caldera lake. Relative ages of informally named Pumice, 

West, Vulcan, and Bolshoi Domes (fig. 10A) are unknown. 

Pumice Dome is the most chemically evolved, two analyzed 

samples having 67.2 and 67.7 weight percent SiO2. Single 

samples from Vulcan and West Domes are compositionally 

similar at 65.6 and 65.8 weight percent SiO2, respectively, 

while a sample from Bolshoi Dome has 64.2 weight percent 

SiO2. The virtually identical compositions of the samples from 

Vulcan and West Domes suggest that they were emplaced at 

about the same time.
Postcaldera Deposits and History 17

Table 1. Weighted mean and other radiocarbon dates for tephra and ignimbrite from Aniakchak volcano. 

[Dates calibrated using method of Stuiver and Reimer (1993), Calib 7.0 software, and the calibration of Reimer and others (2013); B.P. (before present) ages are 

reported with respect to 1950 C.E.; age results rounded to the nearest 10 years. S.E., standard error; MSWD, mean square of weighted deviates; n.a., not applicable]

Unit

Reported or 

weighted mean 

14C age ±1 S.E. 

yr B.P.

MSWD Calibrated 2σ age 

range cal yr B.P.

Preferred ±1σ

age cal yr B.P. Notes

Pink Pumice fall 380±50 0.09 310–510 350±20

470±40

Weighted mean of 2 dates

New Cone tephra 400±30 1.45 430–510 480±20 Weighted mean of 3 dates

Half Cone upper light 

pumice

570±40 0.07 520–650 550±10

620±20

Weighted mean of 2 dates

Half Cone lower light 

pumice

840±30 0.03 690–800 760±30 Weighted mean of 3 dates

Soil above Surprise Cone 

(?) tephra

900±80 0.01 680–940 830±80 Weighted mean of 2 dates

Above pebbly sediment, 

Aniakchak River 

mouth; flood?

1,860±30 0.16 1,720–1,870 1,780±50 Weighted mean of 2 dates (VanderHoek 

and Myron, 2004)

Early postcaldera pumice 

fall

2,160±40 3.61 2,040–2,310 2,140±30

2,270±30

Weighted mean of 2 dates

Aniakchak II ignimbrite 3,430±70 0.31 3,550–3,870 3,660±70 Weighted mean of 3 charcoal dates 

(Miller and Smith, 1987)

Lower Black Nose 

Pumice?

6,760±60 n.a. 7,510–7,700 7,620±40 Soil below tephra (VanderHoek, 2009)

Lower Black Nose 

Pumice?

7,350±70 n.a. 8,020–8,330 8,130±80 Peat below tephra (VanderHoek, 

2009)
18 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula Table 2. Radiocarbon dates constraining postcaldera tephra fall deposits from Aniakchak.—Continued [Units dated are preliminary based on field correlations and, where possible, bulk chemistry of juvenile clasts; locations in decimal degrees, NAD83 datum; dates calibrated using method of Stuiver and Reimer (1993), Calib 7.0 software, and the calibration of Reimer and others (2013); B.P. (before present) ages are reported with respect to 1950 C.E.; age results rounded to the nearest 10 years] Field num- ber Laboratory number Locality Latitude (North) Longitude (West) Material dated Unit Reported 14C age ±1

σ yr 

B.P. 

δ13C

Calibrated 

2σ age 

range 

cal yr 

B.P.

Preferred 

calibrated 

±1

σ age 

cal yr B.P.

Notes1

NA02-6C GX-29531 14 km N of caldera 57.0670 158.0900 Wood Half Cone Pink 

Pumice

360±80 -26.7 280–530 410±90 G(STD) full pretreatment

NA94-24A B-76809 Pumice Dome gully 56.9348 158.1104 Wood Half Cone Pink 

Pumice

390±60 -28.8 310–520 470±40 B(AMS)

85AMm28 I-14, 222 Uncertain Uncertain Uncertain Wood Half Cone Pink 

Pumice

500±80 Not determined 430–660 560±80

T

85AMm14 2358 13 km NNE of 

caldera

57.0583 158.0833 Organic 

sediment

Half Cone Pink 

Pumice

590±90 Not determined 480–700 590±60

U

85AMm20 2358 11 km N of caldera 57.0500 158.1250 Organic 

sediment

Half Cone Pink 

Pumice

590±160 Not determined 300–800 600±130

U

NA92-68 WW212

CAMS9851

3 km NE of Vent 

Mtn

56.9018 158.1114 Uncertain New Cone 390±60 Not determined 310–520 470±40 L(AMS)

94-CW104-

C14-1

B-77038 4 km NE of caldera 56.9556 158.0278 Wood New Cone? 340±50 -28.8 310–500 390±70 B(STD) full pretreatment

94-CW105-

C14-1

B-77039 14 km N of caldera 57.0670 158.0901 Organic 

sediment

New Cone? 460±50 -25.1 430–560 510±30 B(AMS) acid 

wash; low C sediment

NA93-104A W-6540 Pumice Dome gully 56.9348 158.1104 Wood Half Cone upper 

light pumice

560±60 Not determined 510–660 540±20

620±20

U(STD) full pretreatment

NA97-12I GX23441 6 km ESE of 

caldera

56.8698 157.9769 Soil; humic 

fraction

Half Cone upper 

light pumice

580±50 -25.8 520–660 550±10

620±30

G(AMS) full 

pretreatment

NA97-6C GX23439 Pumice Dome gully 56.9348 158.1104 Soil; humic 

fraction

Half Cone lower 

light pumice

300±115 -26.5 240–520 390±110 G(STD) full pretreatment

94-CW110-C14-1

B-77041 NE caldera rim 56.9387 158.0899 Plant Half Cone lower 

light pumice

830±60 -28.3 670–910 740±50 B(AMS) full 

pretreatment

NA97-12A GX23440 6 km ESE of 

caldera

56.8698 157.9769 Soil; humic 

fraction

Half Cone lower 

light pumice

840±60 -26.6 680–910 740±60 G(AMS) full 

pretreatment

NA97-13B GX23442 12 km SE of 

caldera

56.7878 157.9566 Soil; humic 

fraction

Half Cone lower 

light pumice

850±55 -26.4 680–910 740±50 G(STD) full pretreatment

NA97-6B GX23438 Pumice Dome gully 56.9348 158.1104 Soil; humic fraction Above Surprise Cone? 890±150 -26.5 620– 1090 810±120 G(STD) full pre- treatment
Postcaldera Deposits and History 19

Table 2. Radiocarbon dates constraining postcaldera tephra fall deposits from Aniakchak.—Continued[Units dated are preliminary based on field correlations and, where possible, bulk chemistry of juvenile clasts; locations in decimal degrees, NAD83 datum; dates calibrated using method of Stuiver and Reimer (1993), Calib 7.0 software, and the calibration of Reimer and others (2013); B.P. (before present) ages are reported with respect to 1950 C.E.; age results rounded to the nearest 10 years] 

Field number Laboratory number Locality Latitude (North) Longitude (West) Material dated Unit

Reported 

14C age 

±1σ yr 

B.P. 

δ13C

Calibrated 

2σ age 

range 

cal yr 

B.P.

Preferred 

calibrated 

±1σ age 

cal yr B.P.

Notes1

NA94-24D B-76810 Pumice Dome gully 56.9348 158.1104 Organic 

sediment

Above Surprise 

Cone?

910±90 -26.2 680–970 840±80 B(STD) acid 

wash; low C sediment

NA02-6B GX29530 14 km N of caldera 57.0670 158.0901 Soil; humic 

fraction

Dacitic pumice; 

source uncertain

2,130±40 -25.1 2,000–

2,180

2,240–

2,300

2,100±60 G(AMS) full 

pretreatment

94-CW106-

C14-1

B-77040 18 km NNW of 

caldera

57.0943 158.2740 Organic 

sediment

Dacitic pumice; 

source uncertain

2,300±80 -25.8 2,110–

2,520

2,320±210 B(STD) acid 

wash; low C sediment

1B, Beta Analytic; U, U.S. Geological Survey Laboratory, Reston (M. Rubin, analyst) and Menlo Park (S. Robinson, analyst); G, Geochron Laboratory; L, Lawrence Livermore, Mass Accelerator date, T, Teledyne; 

STD, Standard 14C processing; AMS, Accelerator mass spectrometry.
20 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 10A

Qal

Qg

Qm

Qt

Qtc

Qbn

Qv

Alluvium and colluvium

Glacier (pumice covered)

Ice-cored moraine

Talus

Tuff cone

Black Nose Pumice

Aniakchak edifice volcanic rocks

Qls

Qvm

Qb

Q3l

Q 3t

Qd

Qht

Landslide

Vent Mountain lava and scoria

Blocky Cone scoria

1931 lava

1931 tephra

Subaqueous domes and related lava

Half Cone tephra

Qhl

Qa1

Half Cone cobweb lava flow

Aniakchak I ignimbrite

Warm spring discharging 

 carbon dioxide

H Warm ground

TMz

QUARTERNARY

TERTIARY AND MESOZOIC

Sedimentary rocks

EXPLANATION

Surficial Units

Volcanic Rocks

Qhc Half Cone lava and deposits

Vent Mountain fissure vents

N

Qal

Qg Qm

Qg

TMz

Qal

SW maar

NE maar

Qv

Qbn

Qtc

Bolshoi 

Dome

Qtc

Qtc

THE

GATES

Qal

TMz

TMz

Aniakchak R

Vulcan 

Dome

Pumice 

Dome Qal

Qbn

Qv

Qbn

Qt

H Qt

H

Qt

Qal

VENT MOUNTAIN

Birthday

Pass

TMz

Qv

QA1

Qal

Qg

Qbn

Qbn

Qal

Qls

Qal

Qal

Qal

Qbn

Qd

Qd

Qd

Qvm

Qvm

Qvm

Qvm

Qht

Qht

Qhc

Qhl

Qb

Q3t

Q3l

Q3l

Qls

Qls Qls

Qls

Qls

Qt

Qls

Qt

Qt

Qt

Qls

Qls

Qal Qt

West 

Dome

1931 

Main 

Crater

Slag 

Heap

Doublet 

Crater

New

Cone

HALF CONE

Surprise 

Cone

Windy 

Cone Breezy 

Cone

Surprise 

Lake

Blocky 

Cone

BLACK 

NOSE

Qd

QA1 Qv?

Qd

Northwest 

flank lava 

flow

Cobweb 

lava 

flow

A

0 1 2 3 KILOMETERS

0 1 2 MILES

56°55’N

158°10’W

158°05’W

56°52.5’N

56°55’N

56°52.5’N

Figure 10. Maps of Aniakchak caldera. A, Simplified geologic map of Aniakchak caldera (modified after figure 5 of Neal 

and others, 2001, and unpublished geologic mapping by C.A. Neal, R.G. McGimsey, and T.P. Miller). Tephra of 1931 eruption 

(unit Q3t) shown by solid color where thick (visible in aerial photographs) and by stipple pattern where thinner or mantling 

northwest flank lava flow. B, Portion of the Chignik D–1 quadrangle 1:63,360-scale topographic map (U.S. Geological Survey, 

1963) with corrected location of Half Cone and addition of informal names.
Postcaldera Deposits and History 21

Figure 10.—Continued.

Figure 10B

56°55’N

56°52.5’N

56°55’N

56°52.5’N

Blocky 

Cone

West 

Dome

Breezy 

Cone Windy 

Cone

Surprise 

Cone 1931 

Main 

Crater

Half 

Cone

Birthday 

Pass

Vulcan Dome

Bolshoi 

Dome

Pumice Dome

NE 

Maar

SW 

Marr New 

Cone

Black 

Nose

Slag 

Heap

Doublet 

Crater

Northwest 

flank lava flow

0 1 2 3 KILOMETERS

0 1 2 MILES

Contour Interval 100 feet

B

158°10’W 158°05’W
22 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Morphologies and surface features of the domes and the 

glassy character of the dacite lavas indicate that they were 

emplaced in water. The surface of Pumice Dome, poorly 

exposed other than in gullies and facing Surprise Lake, 

consists of pumiceous dacite in pillow-like forms ~0.3–2 m 

across (fig. 11A) that are atypical of subaerial silicic lava but 

are consistent with extrusion into standing water. The surfaces 

consist of frothy, friable, light gray to white pumice characterized by cm-scale radial fracturing and irregular jointing (Allen 

and McPhie, 2000). Vugs as large as 20 cm across occur 

within pillows. Interstitial material is also pumiceous, banded, 

and radially fractured. West, Vulcan, and Bolshoi Domes are 

conical mounds of dacite vitrophyre whose flanks are coated 

by talus composed of finely vesicular to dense, polygonal joint 

blocks shed from outcrops on their upper portions. Outcrops 

on these three domes have fracture patterns (figs. 11B–F) that 

are characteristic of chilling of degassed lava effused in water 

(see, for example, Goto and McPhie, 1998). Many structures 

and fracture patterns in lava flows and domes emplaced in 

contact with melt water within glaciers or adjacent to glacial ice (Lescinsky and Fink, 2000; Spörli and Rowland, 

2006) closely resemble those in the domes at Aniakchak. 

Although hyaloclastite or breccia carapaces are not evident, 

2–3-cm-thick patches of gray, sandy, and partially indurated 

tuffaceous sediment are found in pockets on the summit of 

Vulcan Dome. Thick glacial ice probably was not present at 

any time in the post-Aniakchak II caldera, whereas a deep 

intracaldera lake must have existed when the dacite domes 

were emplaced.

Northwest Flank and Intracaldera Rhyodacite 

Lava Flows

Rhyodacite lava high on the northwest flank of Aniakchak

volcano (fig. 4; labeled NW lava flow) is compositionally 

identical to a thin lava flow exposed within the caldera at 

~2,500 ft (760 m) asl 1.5 km to the south, high in the west 

wall of the 1931 Main Crater (fig. 10; see also “Geochemistry” section), above the level of any possible caldera lake. 

Little tephra is present and no Aniakchak I or II bombs have 

been reported on the blocky surface of the northwest lava flow 

where it has been examined, consistent with the flow being 

younger and similar in age to the lava in the caldera. The 

source of the intracaldera lava flow is unknown but appears 

to have been high on the caldera wall just inside the rim. The 

vent for the northwest flank flow must have been at ~2,400 ft 

(730 m) asl, 600 m west-northwest of the caldera rim. On the 

basis of their composition and apparent post-Aniakchak II 

age, these two flows are thought to have been approximately 

contemporary with Pumice Dome.

Ancestral Caldera Lake

Surprise Lake (1,055 ft, 322 m asl) is a remnant of a 

larger caldera lake evidenced by wave-cut benches on the 

northeast caldera walls and patches of lake sediment at several 

locations within the caldera (McGimsey and others, 1994). 

The lake filled until it overtopped a low point on the east 

rim of the caldera and then rapidly drained, possibly aided 

by eruptive activity (McGimsey and others, 1994) or by 

headward erosion of the canyon of the Aniakchak River. The 

draining produced a catastrophic flood as the ensuing torrent 

cut the gorge called The Gates in altered and fractured sedimentary rock (Waythomas and others, 1996). At the time of 

these publications, the ~1,610 ft (490 m) asl wave-cut terrace 

recognized by McGimsey and others (1994) was thought 

to represent the high stand of the caldera lake. Subsequent 

fieldwork indicated that all four dacite domes were emplaced 

subaqueously. Because West Dome has a summit elevation 

of 2,002 ft (610 m), the lake must have reached at least that 

height, although uplift or tilting of the southwestern caldera 

floor since the lake drained cannot be ruled out. McGimsey and 

others (1994) noted that Birthday Pass, elevation ~2,030 ft 

(620 m), evidently was not overtopped by the lake. Two 

additional low points in the caldera rim, northeast and east of 

Surprise Lake, are at ~2,130 ft (650 m) elevation. Both are 

underlain by Aniakchak volcanic rocks. Before downcutting, 

the minimum elevation of the caldera wall at The Gates 

implied by the topographic map (U.S. Geological Survey, 

1963; fig. 10B) could have been between ~1,900 ft (580 m) 

and ~2,100 ft (640 m). Because West Dome was emplaced 

beneath water, its summit elevation and the elevations of 

Birthday Pass and other low points on the caldera rim together 

suggest that the surface of the caldera lake probably reached 

close to 2,030 ft (620 m) asl before it overtopped the wall 

and cut The Gates. When the lake drained is constrained 

by radiocarbon ages of 1,850±40 yr B.P. on soil (silt) and 

1,870±30 yr B.P. on wood from above pebbly sediment near 

the Aniakchak River mouth (VanderHoek and Myron (2004, 

their table 8.1, appendix C, and p. 161–162). The weighted 

mean of these ages is 1,860±30 yr B.P. (table 1), which apparently dates a change from a pebbly beach to a depositional 

environment. The suggested date for the flood falls within 

an ~200 yr gap in human habitation near the mouth of the 

Aniakchak River (VanderHoek and Myron, 2004, p. 190–191). 

Evidence for lake sediments on apparently younger tuff cones 

within the caldera presented in the next section suggests that 

the catastrophic draining that produced the flood did not cut 

The Gates to their entire depth but left a residual lake that was 

larger and higher than present Surprise Lake. 

Tuff Cones

Three breached tuff cones form subdued arcuate ridges 

of bedded phreatomagmatic tephra on the eastern part of 

the caldera floor (fig. 10A, unit Qtc). Embraced and nearly 

surrounded by younger Vent Mountain lava flows, Surprise, 

Windy, and Breezy Cones rise ~120 m above the surrounding terrain to define 500–1,000-m-diameter explosion craters (figs. 12A and B). Two samples of juvenile scoria from 
Postcaldera Deposits and History 23

Figure 11. Photographs showing surface features of dacite domes inside Aniakchak caldera. A, Pillow-like forms of fractured 

pumiceous dacite of Pumice Dome. B, Detail of pillow in Pumice Dome. C, Columns and semi-arcuate structure in vitrophyre of West 

Dome. Note parallel bands made by steps on joint surface that formed by incremental fracture growth. D, Sheet-like sets of columns 

in vitrophyre, surface of Vulcan Dome. E, Poorly developed columns composed of intersecting semiarcuate fractures displaying 

incremental fracture growth in vitrophyre of Bolshoi Dome. F, Pillow forms on West Dome. Note void in center of pillow and radiating, 

curvilinear columns, lens cap (circled) in center of image for scale. (Photos in A, B, and F by C.A. Neal.)

B

Figure 11

A

C

A B

C D

E

F
24 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Surprise Cone have 57.9 and 58.4 weight percent SiO2, while 

single analyses of scoria from Windy and Breezy Cones yield 

55.3 and 52.3 weight percent SiO2, respectively. Based on the 

similar appearances of the tuff cones, we infer that they all 

are roughly similar in age. Radiocarbon dating of a soil above 

lapilli tephra of Surprise Cone-like chemistry (NA97-6A) 

returned a weighted mean age of 900±80 yr B.P. (tables 1 

and 2), giving a minimum age for Surprise Cone. Superposition relationships are unclear, although Windy Cone tephra 

appear to overlap Breezy Cone. The tuff cones have not been 

dated directly but are thought to postdate catastrophic draining 

of the caldera lake to approximately their elevation. Lacustrine 

clayey silt and sand are present in the breach of Surprise Cone 

and on a wave-cut terrace north of Surprise Lake at 1,226 ft 

(374 m) asl (McGimsey and others, 1994). Eruptions responsible for the tuff cones likely took place in shallow water, as 

suggested by the sediments within Surprise Cone. The basaltic 

andesite to andesite eruptions may well have been facilitated 

by unloading that resulted from rapid draining of the ancestral caldera lake to about the ~1,400-ft (430 m) elevation of 

Surprise and Windy Cone craters, although we cannot rule out 

the possibility that the tuff cone eruptions occurred earlier, 

during filling of the lake. Two well-preserved maar craters 

southeast of Windy cone demonstrate that phreatic or phreatomagmatic explosions occurred in the recent past, presumably 

under hydrologic conditions similar to present time. A third 

explosion crater, which also evidently did not produce juvenile 

ejecta and probably is unrelated to ascent of mafic magma, is 

approximately midway between Vent Mountain and Half Cone 

(fig. 10A).

Vent Mountain

Vent Mountain scoria and spatter cone is the most prominent topographic feature within the caldera (figs. 10, 12C

and 12D). The cone rises 440–530 m above the caldera floor 

to an elevation of 3,350 ft (1,021 m). The 210-m-deep summit 

crater has a diameter of ~800 m. Numerous blocky lava flows 

emanate from the cone’s lower flanks and from a prominent 

fissure vent that cuts the southwest flank. Although some outcrops of basaltic andesite and mafic andesite lava west of The 

Gates and in the larger maar crater possibly had source vents 

now hidden beneath Vent Mountain, the majority of products 

attributed to that volcano are silicic andesite and dacite. 

We describe the eruptive history of Vent Mountain 

beginning with lava flows inferred to be some of the earliest. 

Thickly tephra-mantled lava that flowed between Bolshoi 

Dome and Surprise Cone and now fronts onto Surprise Lake 

apparently came from Vent Mountain, as did lava that flowed 

between Surprise and Windy Cones. Early lava flows that 

originated at the west-northwest–east-southeast-trending 

fissure that cuts the south flank of Vent Mountain and from 

the summit crater are exposed in the walls of the fissure, as 

scattered outcrops on the south flank, overlying the east edge 

of New Cone, and in the walls of the two maar craters. The 

most far travelled of these flows terminates ~1 km southwest 

of The Gates (fig. 10A). Tephra compositionally similar to 

Vent Mountain lava is stratigraphically above Half Cone lower 

light pumice (weighted mean age 840±30 yr B.P., table 1) 

and below Half Cone upper light pumice (weighted mean age 

570±40 yr B.P., table 1) in a section 6 km east-southeast of the 

caldera (table 2). On the east-southeast flank of Vent Mountain, New Cone was the source of lava that preceded much of 

that from Vent Mountain itself. Three radiocarbon dates on 

material associated with tephra thought to have a New Cone 

source (table 2) have a weighted mean age of 400±30 yr B.P. 

(table 1), though only the 390±60 yr B.P. date is from within 

the caldera. Tephra considered to predate New Cone is as 

young as Half Cone upper light pumice. 

The youngest Vent Mountain lavas emanate from the 

south fissure vents, draping the south flank between lobes of 

earlier lava and spreading out from the southwest and south 

bases of the Vent Mountain edifice to pond against the caldera 

wall. One of these flows incised a channel into a small glacier 

and descended to the northeast through New Cone crater to 

overlap earlier Vent Mountain lava. Tephra from multiple 

eruptions at the summit, the south fissure, and New Cone blankets the west, south, and east flanks of the Vent Mountain edifice.

The youngest products of Vent Mountain are agglutinated 

spatter plastered on the north side of the cone and a small lava 

dome in the summit crater. The total volume of Vent Mountain 

lava and tephra probably exceeds 1.5 km3. If the radiocarbon 

constraint on New Cone is correct, much of Vent Mountain 

lava was erupted since ca. 400 yr B.P. The youngest flows and 

agglutinate predate only the 1931 eruption.

Half Cone

Half Cone is the crescentic remnant of an andesite–dacite 

composite edifice that abuts the northwest caldera wall, its 

unsupported southeast half having been destroyed during 

explosive eruptions late in this volcano’s life (fig. 13A). The 

most recently active feeding conduit of Half Cone is marked 

by the center of the radially symmetrical Cobweb dacite flow 

(fig. 13B). 

Early Products of Half Cone and Nearby Vents

In the north wall of Half Cone, coarse poorly sorted pumice fall layers form wedges tapering away from both sides of 

the truncated, crater-filling dome (fig. 13C). Beds of the lowest 

approximately one-third of the western wedge, which are rich 

in coarse, relatively dark dense clasts, are crosscut by lighter 

colored pumiceous beds of the middle third. A small remnant 

of a lava dome rests on the latter beds and is overlain by the 

uppermost third of the wedge beds, which are intermediate 

in color. These deposits are overlain by the ca. 400 yr B.P. 

Pink Pumice fall sequence (see next section). Thick exposures 

of lava in the west wall are compositionally identical to the 

crater-filling dome and may be equivalent. Xenoliths of 
Postcaldera Deposits and History 25

Figure 12. Photographs of tuff cones and Vent Mountain. A, Windy Cone in left center, The Gates in distance, and Black Nose on 

right. B, View looking northwest of tephra section on Breezy Cone. Visible tephra section is on the order of 10 m thick. C, Aerial view 

of north flank of Vent Mountain (photograph by R.G. McGimsey). D, Google Earth view of southwest flank of Vent Mountain (summit 

crater diameter ~800 m), showing young lava flows and fissure vent system; Blocky Cone (crater diameter ~300 m) is in lower 

left corner.

Figure 12

C

A B

D
26 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 13. Photographs of Half Cone and its products. A, Aerial view of Half Cone and Cobweb lava flow; flow field is about 2 km 

across and as much as 50 m thick. B, Google Earth view of Half Cone, Cobweb lava flow (right), and 1931 Main Crater (left). C, Steep, 

~300-m-high exposure through Half Cone vent-filling lava (dark cliff) and overlying Half Cone pyroclastic deposits. Cobweb lava flow, 

mantled by 1931 tephra, in foreground. Arrow points to compositionally unique andesite lava. D, Half Cone pyroclastic deposits form 

the lower half of the north wall of the 1931 Main Crater; overlying bedded deposits are from the 1931 eruption (see also fig. 15C and 

Nicholson and others, 2011, their figure 3F). Unit labels and thicknesses after Nicholson and others (2011): VC, 22 m exposed of partly 

welded deposit; CBS, 13 m of cross-bedded surge deposits; PF, 31 m of pumice fall deposits; HCSA, 5 m of spatter agglutinate; T2, 

40 m of 1931 tephra T2; T3, 41 m of 1931 tephra T3. E, Partly welded deposit at base of exposure in D. Photograph by R.G. McGimsey. 

F, Compositionally zoned Pink Pumice fall deposit in gully on Pumice Dome. Pencil for scale. 

Figure 13

T2

T3

HCSA

PF

CBS

VC

D

C

A B

E F

Cobweb 

lava

flow

Cobweb lava flow
Postcaldera Deposits and History 27

distinctive white felsite are common in these dacite lavas. The 

northeasternmost lava in the Half Cone wall, thick dacite that 

has prominent vitrophyre columns above its base, is likely correlative with the crater-filling dome judging from its proximity 

and similar appearance.

The western near-vent fall-deposit wedge of Half Cone 

lies on silicic andesite (63 weight percent SiO2) lava of unique 

composition known only from that locality. Subhorizontal vitrophyre columns ~6–10 cm across, perpendicular to a calderafacing vertical joint surface that strikes east-northeast, suggest 

that this lava may have effused from a buried vent near the 

northwest caldera wall and encountered ice or deep snow in 

the caldera before the eruption of Half Cone. 

Pumice-fall and pyroclastic-flow deposits from Half Cone 

are exposed in the north wall of the 1931 Main Crater 2 km to 

the south (fig. 13D). Here, the cliff at the base of the section 

(fig. 13E) is composed of >22 m of partly welded or indurated coarse pyroclastic-flow material, much like spatter-rich 

pyroclastic-flow deposits at Santorini (Mellors and Sparks, 

1991), containing andesite bombs as large as 0.6 m × 2 m and 

similar in composition to Half Cone pyroclasts (59 weight percent SiO2

). This unit is overlain by 13 m of pumiceous crossbedded surge layers, 31 m of pumice-fall deposits, 5 m of Half 

Cone ca. 400 yr B.P. spatter agglutinate, and an 81-m-thick 

section of 1931 materials (Nicholson and others, 2011, their 

figure 3F). Dacite tephra older than the Pink Pumice and considered to have a Half Cone source, and which therefore may 

be correlative with proximal deposits at Half Cone and with 

the section exposed in the 1931 Main Crater, have weighted 

mean radiocarbon ages of 840±40 and 570±40 yr B.P. (tables 1 

and 2, lower and upper light pumice, respectively).

Half Cone Pink and Brown Pumice Falls

A series of Plinian eruptions ca. 400 yr B.P. produced 

widespread pumice falls and destroyed much of the original 

Half Cone edifice (Neal and others, 2001; Browne, 2006). 

Pyroclastic flows and surges affected much of the caldera 

floor. Canyons to the east of Half Cone expose more than 40 m

of pyroclastic-flow deposits related to this event. Outside 

the caldera, the pumice fall is 10 cm thick at least 50 km to 

the north. Fine ash may extend as far as 330 km (Riehle and 

others, 1999). Total erupted volume is estimated at 0.75 to 

1.0 km3 (Neal and others, 2001). The areally extensive units 

are two volumetrically subequal, successive Plinian fall 

deposits known by the informal unit names of the Pink Pumice 

(fig. 13F) and the overlying Brown Pumice; the contact 

between Pink and Brown Pumice falls is gradational through 

a modest thickness. The dacitic Pink Pumice (63–67 weight 

percent SiO2) is highly vesicular and carries few phenocrysts. 

Two horizons within the Pink Pumice deposit have relatively 

coarse pumice and lithic clasts. The normally graded Brown 

Pumice layer consists of relatively crystal-rich andesite 

pumice (58–62 weight percent SiO2). White felsite (77 weight 

percent SiO2), granitic, and crystal tuff xenoliths are common 

in the fall deposits. Five radiocarbon dates are for materials 

associated with the Pink Pumice (table 2). We consider the 

weighted mean of 380±50 yr B.P. (table 1) for the two youngest dates, which are for wood and for which stratigraphic 

context is clear, to provide the most accurate constraint on the 

Pink Pumice eruption.

Proximal Late-Erupted Fall Layers

Atop the Pink and Brown Pumice layers on Half Cone 

itself are several more light and dark colored fall beds that 

have a total thickness of ~60 m. These vary from nonwelded 

pink-to-buff pumice fall to brick-red oxidized lithic-rich fall 

breccia and dark gray welded agglutinate. The variations 

in lithic content and welding are due to changes in eruption 

intensity at the vent. Although these layers have not been 

studied in detail, their thicknesses decrease substantially over 

short distances from Half Cone. Outcrops of this agglutinate 

are found as short, low ridge segments near the surface on 

the caldera floor northeast of Half Cone, as patches on Vent 

Mountain and West Dome, and in the wall of the 1931 Main 

carter. They are cut by the cliff face of Half Cone and are 

overlain by deposits of the 1931 eruption.

Cobweb Lava Flow

The final product of the Half Cone vent was crystal-rich 

dacite (~65–66 weight percent SiO2) that spread radially to 

form the Cobweb lava flow (figs. 13A and B), now heavily 

mantled with tephra of the 1931 eruption. The flow has arcuate pressure ridges that are approximately concentric about 

a low central cratered cone above the vent, which is located 

somewhat west of center of the 1.5 km (north–south) by 2 km 

(east–west) lava field (fig. 10A). At least six rifts traverse 

the flow surface from the center to the edges like spokes 

of a wheel. The longest of these appears to have channeled 

late-erupted lava northeast all the way to the margin of the 

flow field. Late-moving lava also emerges as short toes from 

the north and south margins of the field. The lava terminates 

on the northwest near the base of the Half Cone cliff. We 

adopt the name Cobweb for this lava flow after B.R. Hubbard 

(1932), who called it the “Avernian Cobweb.” “The poisonous 

vapors of Avernus, Italy, killed many birds, and Hubbard witnessed the same at Aniakchak, which seems why he adopted 

this name” (W. Hildreth, written commun., 2014). 

Blocky Cone

The well-preserved, 60-m-high cinder cone with 

~20-m-deep crater near the southwest caldera wall opposite 

Birthday Pass is informally known as Blocky Cone (figs. 12D

and 14). Although Blocky Cone is mantled by lithic debris 

from the nearby 1931 vent, fresh basaltic andesite scoria is 

exposed on the southeast flank of the cone. This is some of 

the most mafic postcaldera magma (52.2–52.5 weight percent 

SiO2), comparable only to products of Breezy Cone. Blocky 
28 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Cone appears to be younger than the Half Cone Pink and 

Brown Pumice (fig. 5C) and to overlie older lava of Vent 

Mountain. Blocky Cone tephra has not been reported to be 

on the surface of the youngest Vent Mountain lava that nearly 

reaches the cone’s eastern base, which suggests that this lateerupted Vent Mountain lava is younger than Blocky Cone. 

1931 Eruption

The most recent eruption of Aniakchak took place during May and June 1931. The following description of the 

events and deposits is drawn from Neal and others (2001) 

and Nicholson and others (2011). The total volume of erupted 

material was about 0.9 km3 (or 0.3 km3 of dense phenocrystpoor magma; Nicholson and others, 2011), less than 1 percent 

of which was lava. Like the ca. 400 yr B.P. eruption of Half 

Cone and eruptions at many other Alaskan volcanoes, the 

1931 Aniakchak eruption began with relatively silica-rich 

dacite–rhyodacite magma (as much as 69 weight percent SiO2

in 1931) and ended with relatively silica-poor basaltic andesite 

magma (as little as 56 weight percent SiO2) (Nicholson and 

others, 2011). Although Nicholson and others (2011) termed 

the more silicic products trachydacite, the Aniakchak analyses 

straddle the dacite/trachydacite boundary (their figure 7) and 

we choose to refer to these samples as dacite for simplicity. 

Historical Accounts

The 6-week-long eruption, strongest during May 1–11, 

rained “egg-size” scoria 30 km to the west in Port Heiden 

(then called Meshik), spread millimeters of ash fall as far 

as Kodiak Island, and repeatedly interfered with radio 

communications in the area. The principal record of the 

events of 1931 comes from the writings of B.R. Hubbard, the 

“Glacier Priest” of the University of Santa Clara, California 

(Hubbard, 1932, and references in Neal and others, 2001, 

and Nicholson and others, 2011; see the latters’ figure 4 for 

summary). The first reported activity was a column of white 

steam seen May 1 at 10 a.m. local time from Meshik, which 

was followed at 12 p.m. by an explosion that sent a black 

mushroom-shaped incandescent plume to an estimated 6 km 

height. On May 11, after nearly continuous explosive activity 

since May 1, a large explosion produced heavy ashfall that 

resulted in total darkness for several hours within 100 km of 

the volcano. Activity then reduced until, on May 20, large 

explosions again occurred. Ash fell through at least May 28, 

and explosive eruption was still in progress on June 10 and 13. 

New lava was emplaced as small flows at Doublet Crater and 

Slag Heap and, towards the end of the activity, in the Main 

(1931) Crater. The Main and Doublet Crater lava flows were 

moving at least as late as July 1 and 3, respectively.

Fall Deposit

Within and near the caldera, the 1931 deposit consists 

of white pumice lapilli near the bottom and grades upward 

through a main unit dominated by tan to brown pumice to an 

upper unit of black pumice (fig. 15A). Nicholson and others 

(2011) divided the fall deposit into corresponding mappable 

tephra units T1, T2, and T3. White to light gray unit T1 

(0.02 km3 DRE) is present to the north of the Main Crater as 

far as 15 km from vent but is not present in the Main Crater 

wall section. Juvenile clasts are dominantly dacite. Unit T1 

may be noticeably darker at its base than its top owing to a 

greater proportion of lithic fragments near the base. Most 

A B

Figure 14. Photographs of Blocky Cone. A, Aerial view southeast over Blocky Cone to southwest flank of Vent Mountain. B, Oblique 

aerial view to north; Blocky Cone on right, Doublet Crater in upper left.
Postcaldera Deposits and History 29

voluminous and lithic-rich of the three units is T2 (0.24 km3

DRE), which consists of alternating beds of fine ash and 

coarser tan, brown, or black andesite lapilli; white pumice is 

rare. Eyewitness accounts of distal ashfall are thought to be 

exclusively unit T2 because reports of white pumice characteristic of T1 are lacking and T2 is the most voluminous 

tephra unit. Unit T3 (0.009 km3 DRE) is seen only in the 

Main Crater wall (fig. 15B), where it consists of lithic-poor 

alternating beds of coarse- and fine-grained relatively dense 

black andesite and basaltic-andesite scoria. Ash grains in 

unit T3 are dense and blocky in comparison to grains in T1 

and T2.

Figure 15. Photographs showing products of the 1931 eruption. A, Basal fall deposit in pit dug above Half Cone. Yellow tape is ~35 cm 

long. Total thickness of fall deposit is ~200 cm here. B, North wall of 1931 Main Crater showing tephra section. Tephra units T2 (40 m 

thick) and T3 (41 m thick) of the 1931 eruption overlie an exposed thickness of 71 m of Half Cone units. Slope is partly mantled by spatter 

agglutinate. (Photograph by C.A. Neal; compare with figure 13D and Nicholson and others, 2011, figure 3F). C, Main Crater with lava on 

crater floor, Half Cone in left distance (photograph by T. Plucinski). D, Aerial view to southeast of Slag Heap and, beyond, Doublet Crater 

lava flows near southwest caldera wall.

C

A B

D

T2

T3

Half Cone 

units
30 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Spatter Agglutinate

A 50–100-cm-thick stratified deposit of agglutinated 

frothy brown and dense black basaltic-andesite lapilli and 

minor scoria bombs partly mantles the north and east wall of 

the Main Crater (fig. 15B). Lapilli and bombs are similar to 

those of unit T3.

Main Crater Lava

Vesicular basaltic andesite lava covers much of the floor 

of the Main Crater (fig. 15C). The flow surface is mantled by 

spatter agglutinate. The likely vent is marked by a depression 

near the center of the flow, while a distinct tongue of lava 

flowed west adjacent to the south base of the crater wall.

Late-Erupted Bombs

Lithic blocks and juvenile bombs fell in two areas at the 

end of the 1931 eruption, a larger bomb field east of the Main 

Crater and a smaller one north of a small phreatic explosion 

pit at the north rim of Doublet Crater. Black to brown highly 

vesicular bombs similar to those of the spatter agglutinate are 

present as much as 500 m east of the Main Crater rim.

Doublet and Slag Heap Lavas

Dacite lava covers the floor of Doublet Crater as a flat 

~100-m-diameter circular flow 2.2 km south of the Main 

Crater (Figures 10A and 15D). Descending from a bench 50 m 

above the caldera floor 2.2 km south of the Main Crater, the 

rhyodacitic Slag Heap flow (fig. 15D) is broken by deep transverse and longitudinal fractures that give its surface a scored 

appearance. The two flows are blanketed with T2 tephra and 

scattered lithic fragments, coarser on the Doublet Crater flow.

Geochemistry

The geochemical database for Aniakchak consists of 

374 analyses of rock samples collected by Alaska Volcano 

Observatory (AVO) geologists and University of Alaska 

Fairbanks graduate students (appendix B, table B1). The 

346 of these analyses that can be assigned unambiguously 

to eruptive units form the basis for figures, descriptions, and 

interpretations in this paper; remaining uncertain samples are 

indicated by “?” in appendix B, table B1. All of the chemical 

analyses were performed by the GeoAnalytical Laboratory 

at Washington State University in Pullman, Washington, by 

wavelength-dispersive X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). Strict 

quality control over many years ensures that the database is 

internally consistent and accurate. See http://soe.wsu.edu/facilities/

geolab/technotes for analytical procedures and precision. 

The compositions of Aniakchak geologic units are 

described here in order of eruption with reference to silica and 

other variation diagrams and chondrite-normalized rare earth 

element (REE) and primitive-mantle-normalized multielement 

(“spider”) plots. Implicit in using the geochemical data to 

help establish stratigraphic relationships is the concept that 

samples from outcrops that are not contiguous but are analytically indistinguishable are likely to represent approximately 

contemporaneous eruption of a single magma or closely 

related magmas. 

Isotopic data for 15 samples from Aniakchak, all 

postglacial in age, were presented by George and others 

(2004). These results are described at the end of the 

“Geochemistry” section.

Pleistocene Aniakchak Edifice

Lava samples from Pleistocene Aniakchak volcano were 

mainly obtained from caldera wall exposures. The 29 analyses 

in the database are presented here without designation of 

relative or absolute age because knowledge of edifice geologic relations is insufficient. Nevertheless, these data give a 

measure of the range of compositions erupted at Aniakchak 

over a likely span of several hundred thousand years (Nye and 

others, 1993) and thus provide context for the chemistry of 

postglacial magmas.

Pleistocene lavas range from basalt to dacite (fig. 16). 

Compatible element concentrations at the mafic end of the 

spectrum are relatively low (for example, MgO<6 weight percent) and indicate that none of the samples represents primary 

magma from the mantle; all have undergone processing and 

chemical fractionation at crustal depths. At any given silica 

content there are wide variations in incompatible element 

concentrations. For example, medium-K and high-K magmas 

(Gill, 1981) are represented, and K2O, TiO2, Zr, and Y vary by 

factors of 2–3 in andesites (fig. 16). Some of the Pleistocene 

lavas have high Al2O3 contents (>18 weight percent), and these 

also have high values of Eu/Eu* (europium anomaly), which 

suggests that they have accumulated plagioclase.

Figure 17 presents diagrams that help to classify the 

Aniakchak rocks and that will be useful for later discussion of 

processes. The Pleistocene lavas fall into two series on the plot 

of silica versus chondrite-normalized La/Yb ratio, a measure 

of the slope of the REE pattern. The high La/Yb series is not 

represented among postglacial products of Aniakchak. Most 

of the Pleistocene lavas plot in the tholeiitic field on the silica 

versus FeO*/MgO diagram (Miyashiro, 1974), though some 

with >59 weight percent SiO2 are calc-alkalic. Nye and others 

(1993) pointed out that early and mid-precaldera basaltic 

andesites are tholeiitic whereas late-precaldera lavas are 

calc-alkalic.
Geochemistry 31

Figure 16. Silica variation diagrams for Pleistocene and pre-3,400-yr.-B.P. Aniakchak samples. Major element oxides 

recalculated to sum to 100 weight percent volatile-free. Fields indicate compositional ranges of postcaldera units (yellow) 

and bimodal Aniakchak II products (orange).

Figure 16, p. 1 of 2

Aniakchak II

All postcaldera

Basaltic 

andesite Andesite Dacite

Rhyodacite

Basalt

U Black Nose Pumice

L Black Nose Pumice

Pumice falls, undivided

Postglacial dacite lava

Aniakchak I ignimbrite

Aniakchak edifice lava

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

TiO2 , in weight percent

14

15

16

17

18

19

Al O2 3 , in weight percent

2

3

4

5

6

7

8

9

10

11

FeO*, in weight percent

0

1

2

3

4

5

6

MgO, in weight percent

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

1

2

3

4

5

6

7

8

9

10

11

CaO, in weight percent

50 52 54 56 58 60 62 64 66 68 70 72

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Na O, in weight percent

2

50 52 54 56 58 60 62 64 66 68 70 72 50 52 54 56 58 60 62 64 66 68 70 72

EXPLANATION
32 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 16.—Continued.

Figure 16, p. 2 of 2

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K O, in weight percent

2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

P O2 5 , in weight percent

50

100

150

200

250

300

350

Zr , in parts per million

10

20

30

40

50

60

Y, in parts per million

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

0

50

100

150

200

250

300

350

400

V, in parts per million

50 52 54 56 58 60 62 64 66 68 70 72

0.6

0.7

0.8

0.9

1.0

1.1

Eu/Eu*

50 52 54 56 58 60 62 64 66 68 70 72 50 52 54 56 58 60 62 64 66 68 70 72

Aniakchak II

All postcaldera

Medium-K

Low-K

High-K

Shoshonitic

U Black Nose Pumice

L Black Nose Pumice

Pumice falls, undivided

Postglacial dacite lava

Aniakchak I ignimbrite

Aniakchak edifice lava

EXPLANATION
Geochemistry 33

Figure 17. Silica and P2O5 variation diagrams for Pleistocene and pre-3,400-yr.-B.P. Aniakchak samples. Major element 

oxides recalculated to sum to 100 weight percent volatile-free. La/Yb ratios normalized to values for CI chondritic meteorites 

(McDonough and Sun, 1995). FeO* is total Fe calculated as FeO. Fields indicate compositional ranges of Aniakchak II 

(orange) and younger units (yellow).

All postcaldera

Aniakchak II

U Black Nose Pumice

L Black Nose Pumice

Pumice falls, undivided

Postglacial dacite lava

Aniakchak I ignimbrite

Aniakchak edifice lava

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

La/Yb, chondrite normalized

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

1

2

3

4

5

6

7

FeO*/MgO

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

P2O5 , in weight percent

0

50

100

150

200

250

300

350

400

V, in parts per million

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

P2O5 , in weight percent

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K O, in weight percent

2

Tholeiitic

 Calc-alkalic

EXPLANATION
34 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Chondrite-normalized REE patterns for edifice lavas 

(fig. 18A) cover ranges of concentrations and slopes that 

encompass those for all other samples and, in fact, are 

considerably greater. The highest light REE (LREE) values 

for edifice lavas exceed, and the lowest heavy REE (HREE) 

values are lower than, those of postglacial samples. Some REE 

pattern slopes are steeper as well, as evident also in figure 17. 

Most edifice lavas have prominent negative Eu anomalies (Eu/

Eu*<1; fig. 16), which also are common among the majority 

of postglacial samples. 

Multielement diagrams in which values for each 

sample are normalized to elemental concentrations deduced 

for Earth’s primitive mantle (McDonough and Sun, 1995), 

arranged in order of increasing incompatibility in mantle minerals from right to left, are useful geochemical fingerprints of 

magmas and highlight differences between eruptive units at a 

glance. Data for edifice lavas plotted in figure 19A delimit the 

range at Aniakchak. The patterns have common features that 

also are characteristic of magmas at convergent plate (subduction) environments such as the Alaska–Aleutian arc: overall 

negative slope, high fluid-mobile U and Pb, and low highfield-strength-elements (HFSE) Nb and Ta. Depressed values 

for Sr, P, and Ti result from sequestration in plagioclase, 

apatite, and titanomagnetite, respectively. These minerals 

were removed from magmas during subsurface crystallization differentiation, or retained in crystal residues of partial 

melting, causing erupted magmas to be depleted in elements 

compatible in them. The few samples with anomalously high 

Sr contents probably have accumulated plagioclase. Scatter 

in Cs relative abundances likely results from minor alteration 

of the affected samples. Although in detail there are real differences in relative abundances of the elements illustrated in 

figure 19A for the edifice samples, the overall similarity of the 

majority of the patterns implies histories involving common 

processes that operated on subduction-related parent magmas.

Aniakchak I Ignimbrite

The distinctive chemical composition of andesite of the 

Aniakchak I ignimbrite allows the deposit to be readily distinguished from the commonly similar appearing Aniakchak II 

andesite ignimbrite simply on the basis of K2O, P2O5, or Zr 

content (fig. 16). Characteristic features that set Aniakchak I 

apart from other postglacial Aniakchak andesites are high 

FeO*, K2O, and incompatible trace elements such as Zr and Y 

coupled with low MgO and Eu/Eu*. The range in SiO2 content 

for 21 clasts is a narrow 59.7–62.4 weight percent. Concentrations of compatible elements decrease, and those of incompatible elements increase, with increasing SiO2

 in a manner 

consistent with crystallization differentiation that involves 

plagioclase, pyroxene, and titanomagnetite. Increases in P2O5

with increasing SiO2 (fig. 16), and with increasing K2O and 

decreasing V (fig. 17), indicate lack of apatite saturation during differentiation. We show the narrow field of Aniakchak I 

andesite compositions on all REE and multielement diagrams 

as an index for illustrating differences among compositions of 

eruptive units.

Rare-earth-element patterns for Aniakchak I andesite 

samples (fig. 18B) are approximately parallel and show greater 

negative Eu anomalies (decreasing Eu/Eu*) with increasing overall REE concentrations. These features result from 

concentration of elements incompatible in the plagioclasedominated crystallizing assemblage and preferential incorporation of Eu2+ into plagioclase. Multielement diagrams (fig. 19B) 

also indicate a narrow compositional range for Aniakchak I 

andesite. These patterns for Aniakchak I ignimbrite are similar 

to patterns for many other postglacial Aniakchak products, 

although they do feature high relative P contents in comparison to postglacial andesites other than Aniakchak II. 

Precaldera Dacites and Rhyodacites

Dacite–Rhyodacite Lava Flows

The dacite lava flow on the northeast flank of the 

Aniakchak volcano is the less evolved of the two units 

considered here. At ~67.4 weight percent SiO2, many features of its chemistry are closely similar to those of upper 

Black Nose Pumice (figs. 16 and 17). However, though SiO2

contents are similar, the lava flow has slightly higher levels 

of incompatible, and lower levels of compatible, elements 

than the pumice (figs. 16 and 17). The composition of the 

rhyodacite lava flow or dome in the caldera wall north of 

The Gates (69.2 weight percent SiO2) is identical to that of 

the lower Black Nose Pumice that rests upon it. As would be 

expected from the above observations, these dacite–rhyodacite 

lavas have REE and multielement patterns that are virtually 

indistinguishable from those of Black Nose Pumice (figs. 18C

and 19C). These lavas logically are effusive equivalents of 

the Black Nose Pumice deposits. The rhyodacite lava was 

extruded before the compositionally similar lower Black 

Nose Pumice and the dacite lava possibly after the upper 

Black Nose Pumice, though that relationship has not been 

investigated specifically by searching for the relevant pumice 

fall deposits on the lava flow. 

Black Nose Pumice

The geochemistry of the lower and upper Black Nose 

Pumice fall deposits, each of distinct composition, is described 

and contrasted in this section. A sample of fiamme (AC-09) 

from the intra-Plinian ignimbrite is compositionally indistinguishable from pumice of the enclosing lower Black Nose 

Pumice fall and therefore is not described separately. 

Lower Black Nose Pumice is rhyodacite, five samples 

having 69.1–69.3 weight percent SiO2. As noted above, the 

composition of a lava flow in the caldera wall north of The 

Gates is identical to that of the overlying lower Black Nose 

Pumice. Incompatible elements, TiO2, and FeO* are present at 

slightly higher concentrations, and Al2O3 lower, in the lower 
Geochemistry 35

Black Nose Pumice than in postcaldera dacites and rhyodacites (fig. 16). Similarly, FeO*/MgO is near the maximum 

value for Aniakchak magmas (fig. 17) and Eu/Eu* is low 

(fig. 16). 

Less evolved than the lower, the upper Black Nose 

Pumice has 67.2–67.6 weight percent SiO2 (four samples) and 

other oxide and elemental concentrations broadly consistent 

with being genetically related to the lower Black Nose Pumice. This also is evident in REE and multielement diagrams 

(figs. 18 and 19), in which lower Black Nose Pumice samples 

have slightly lower compatible element contents, deeper Eu 

anomalies, and higher LREE and HREE contents (compare 

figs. 18E and 18F, 19E and 19F). Patterns for HREE are concave upward for both units. The two units should be readily 

distinguishable (for example, see appendix A) in distal tephra 

sequences and, if both are present, would provide strong evidence of their Aniakchak source; chemical analyses of glass 

in representative pumice clasts from the caldera rim section 

will be necessary for direct comparison with distal tephras. 

Both lower and upper units are measurably less evolved than 

Aniakchak II rhyodacite, which also is distinctive in carrying 

hornblende phenocrysts. The northeast flank dacite lava flow, 

as previously mentioned, is quite similar in composition to 

upper Black Nose Pumice. 

Other Pre-Aniakchak II Dacite Pumice Falls

Three analyses of dacite pumice are not assigned to 

stratigraphic units. These are shown in figures 16, 17, 18D, 

and 19D as “pumice falls, undivided.” In order of increasing 

SiO2 content, they are from beneath Aniakchak II ignimbrite 

in upper Lava Creek (98AC15C) and lower Reindeer Creek 

(98AC70D) (fig. 4), and above the Aniakchak I deposit but 

below the Black Nose Pumice at the caldera rim north of The 

Gates (AC-11f). The last has compositional affinity with Black 

Nose Pumice, but with the upper, not lower, unit. The other 

two samples are compositionally distinct from each other and 

from other analyzed postglacial precaldera eruptive units, 

yet they must have been produced by significant explosive 

eruptions. They serve as reminders that our record of the pre3,400-yr-B.P. eruptive history of Aniakchak is incomplete and 

that further evidence of explosive eruptions there is waiting to 

be discovered through study of promising tephra sections.

Aniakchak II Products

The striking contrast in color between rhyodacite and 

andesite flow units of the Aniakchak II ignimbrite is borne 

out by chemical compositions of clasts. Discounting a few 

analyses of banded or possibly mixed clasts, 20 rhyodacites 

have 69.4–70.4 weight percent SiO2 while 30 andesites have 

57.2–60.4 weight percent SiO2. The two samples from the preignimbrite Plinian pumice fall (AC37A and 92CNA37a) are 

indistinguishable from the most silicic rhyodacite clasts from 

the ignimbrite. The rhyodacite samples, more evolved than 

any other analyzed Aniakchak products, define trends on variation diagrams that are consistent with crystallization differentiation or minor differences in phenocryst contents (fig. 20). 

The trends extend those for postcaldera dacites, though FeO*/

MgO (fig. 21) ranges to higher values. REE concentrations, 

Eu/Eu*, and La/Yb (figs. 22A, 20, and 21, respectively) are 

similar to those of other evolved silicic products, notably 

lower Black Nose Pumice. However, middle REE (MREE) 

abundances are somewhat lower in Aniakchak II rhyodacite, 

probably owing to hornblende fractionation. On a multielement diagram (fig. 23A), in comparison to other silicic units 

Aniakchak II rhyodacite has an unremarkable pattern, except 

that P content is lowest of any sample analyzed.

Andesite of the Aniakchak II eruption is notable among 

postglacial eruptives in having high TiO2 content for its 

silica range (fig. 20). However, it is the exceptionally high 

P2O5 content, emphasized also by Dreher and others (2005), 

that truly sets Aniakchak II andesite apart from all other 

analyzed samples (figs. 20 and 21). The REE in Aniakchak II 

andesite are present at levels similar to in other andesites 

of comparable silica content, though chondrite-normalized 

patterns are subtly steeper than in andesites other than those 

of the Aniakchak I eruption (figs. 21 and 22A). The high P 

of Aniakchak II andesite stands out as a positive anomaly in 

multielement patterns (fig. 23A), which is virtually unique 

among Aniakchak samples. With the exception of other 

compatible elements (for example, Sr, Ti, and to some extent 

MREE), multielement patterns for the andesite are subparallel 

to those for Aniakchak II rhyodacite.

Subaqueous Domes

Compositions of the four subaqueous domes mainly plot 

in a smooth array on silica variation diagrams (fig. 20). Pumice 

Dome is most evolved (two samples at 67.2 and 67.7 weight 

percent SiO2), Bolshoi Dome least evolved (64.2 weight percent SiO2

), and chemically indistinguishable Vulcan and West 

Domes (65.6 and 65.8 weight percent SiO2, respectively) intermediate. The subaqueous dome dacites are compositionally 

similar to the more evolved dacites from Vent Mountain and, 

together, they plot on a trend toward Aniakchak II rhyodacite 

(figs. 20 and 21). Note that in figure 24 Pumice Dome samples 

(the two most silicic orange inverted triangles) plot close to 

northwest flank dacite–rhyodacite lava and to rhyodacite lava 

exposed in the southeast wall of the 1931 Main Crater (cluster 

of magenta inverted triangles).

Abundances of REE (fig. 22B) are lower than in precaldera dacites, subparallel to and lower than those of Aniakchak 

I, but comparable to those in some Vent Mountain and Half 

Cone samples (figs. 18 and 22). On the multielement plot 

(fig. 22B), similarity to Vent Mountain and Half Cone dacites 

is apparent. The order of increasing differentiation from Bolshoi to Vulcan and from West to Pumice Dome is most clearly 

seen in Cs–U and P, the most incompatible trace elements and 

the essential structural constituent of apatite, respectively.
36 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 18. Chondrite-normalized rare-earth-element diagrams (McDonough and Sun, 1995) for pre-3,400-yr.-B.P. 

Aniakchak products. Green field indicating range of patterns for Aniakchak I ignimbrite serves as index for comparison with 

other units. Uptick at Lu evidently results from normalization value for Lu relative to other heavy rare earth elements (HREE) 

in chondrites in comparison with our inductively coupled plasma mass spectrometry (ICPMS) analyses. A, Edifice lavas. 

B, Aniakchak I ignimbrite; red lines indicate data for samples with extreme values of most REE. C, Postglacial dacite lava. 

D, Pumice falls, undivided. E, Lower Black Nose Pumice. F, Upper Black Nose Pumice.

Figure 18

10

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5

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20

30

40

50

60

70

80

 Sample/Chondrite

200

5

Northeast 

flank and 

caldera-rim 

lava flows

 Sample/Chondrite Sample/Chondrite

10

100

20

30

40

50

60

70

80

200

5

10

100

20

30

40

50

60

70

80

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

200

5

10

100

20

30

40

50

60

70

80

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

200

5

E Lower Black Nose Pumice F Upper Black Nose Pumice

C Postglacial dacite lava D Pumice falls, undivided

A Aniakchak edifice lava B Aniakchak I ignimbrite
Geochemistry 37

Figure 19. Primitive-mantle-normalized multielement diagrams (McDonough and Sun, 1995) for pre-3,400-yr-B.P. Aniakchak 

products. Green field indicating range of patterns for Aniakchak I ignimbrite serves as index for comparison with other units. 

A, Edifice lavas. B, Aniakchak I ignimbrite. C, Postglacial dacite lava. D, Pumice falls, undivided. E, Lower Black Nose Pumice. 

F, Upper Black Nose Pumice.

Figure 19

Northeast 

flank and 

caldera-rim 

lava flows

Pb

Sr

Ti

Pb

Sr

Ti

Pb

Sr

Ti

Nb

Nb Nb

Pb

Sr

Ti

Nb

Pb

Sr

Ti

Nb

Pb

Sr

Ti

Nb

Sample/Primitive mantle

1

10

100

2

3

4

5

20

30

40

50

200

Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pb

Pr

Sr

P

Nd

Sm

Zr

Hf

Eu

Ti

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

300

1

10

100

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300

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Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pb

Pr

Sr

P

Nd

Sm

Zr

Hf

Eu

Ti

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

300

Sample/Primitive mantle Sample/Primitive mantle

E. Lower Black Nose Pumice F. Upper Black Nose Pumice

C. Postglacial dacite lava D. Pumice falls, undivided

A. Aniakchak edifice lava B. Aniakchak I ignimbrite
38 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 20, p. 1 of 2

Basaltic 

andesite Andesite Dacite

Rhyodacite

Basalt

Precaldera 

dacites

Aniakchak edifice

Aniakchak I

Blocky Cone

Half Cone

Vent Mountain

Tuff cone

Subaqueous dome

Aniakchak II

Rhyodacite lava

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

TiO2 , in weight percent

14

15

16

17

18

19

Al O2 3 , in weight percent

2

3

4

5

6

7

8

9

10

11

FeO*, in weight percent

0

1

2

3

4

5

6

MgO, in weight percent

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

1

2

3

4

5

6

7

8

9

10

11

CaO, in weight percent

50 52 54 56 58 60 62 64 66 68 70 72

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Na O, in weight percent

2

50 52 54 56 58 60 62 64 66 68 70 72 50 52 54 56 58 60 62 64 66 68 70 72

EXPLANATION

Figure 20. Silica variation diagrams for Aniakchak II and postcaldera, pre-1931 samples. Major element oxides 

recalculated to sum to 100 weight percent volatile-free. Fields indicate compositional ranges of edifice (blue), Aniakchak I 

(green), and precaldera dacite (purple) units.
Geochemistry 39

Figure 20, p. 2 of 2

Medium-K

Low-K

High-K

Aniakchak edifice

Aniakchak I

Precaldera 

dacites

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K O, in weight percent

2 50

100

150

200

250

300

350

Zr , in parts per million

10

15

20

25

30

35

40

45

50

55

60

Y, in parts per million

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

0

50

100

150

200

250

300

350

400

V, in parts per million

50 52 54 56 58 60 62 64 66 68 70 72

0.6

0.7

0.8

0.9

1.0

1.1

Eu/Eu*

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

P O2 5 , in weight percent

50 52 54 56 58 60 62 64 66 68 70 72 50 52 54 56 58 60 62 64 66 68 70 72

Blocky Cone

Half Cone

Vent Mountain

Tuff cone

Subaqueous dome

Aniakchak II

Rhyodacite lava

EXPLANATION

Figure 20.—Continued.
40 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Aniakchak edifice

Aniakchak I

Precaldera 

dacites

Tholeiitic

Calc-alkalic

Blocky Cone

Half Cone

Vent Mountain

Tuff cone

Subaqueous dome

Aniakchak II

Rhyodacite lava

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

La/Yb, chondrite normalized

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

1

2

3

4

5

6

7

FeO*/MgO

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

P2O5 , in weight percent

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K O, in weight percent

2

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

P2O5 , in weight percent

0

50

100

150

200

250

300

350

400

V, in parts per million

EXPLANATION

Figure 21. Silica and P2O5 variation diagrams for Aniakchak II and postcaldera, pre-1931 samples. Major element oxides 

recalculated to sum to 100 weight percent volatile-free. La/Yb ratios normalized to values for CI chondritic meteorites 

(McDonough and Sun, 1995). FeO* is total Fe calculated as FeO. Fields indicate compositional ranges of edifice (blue), 

Aniakchak I (green), and precaldera dacite units (purple).
Geochemistry 41

Figure 22

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5

Rhyodacite

Andesite

Aniakchak I 

ignimbrite

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 Sample/Chondrite

Northwest 

flank & 1931 

Main Crater 

lava flows (red)

 Sample/Chondrite

10

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80

 Sample/Chondrite

200

5

Surprise 

Cone

Windy 

Cone

Breezy 

Cone

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Breezy 

Cone

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La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

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La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

200

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Breezy 

Cone (red)

Breezy 

Cone × 2

E. Half cone F. Blocky Cone

C. Tuff cones D. Vent Mountain

A. Aniakchak II B. Subaqueous domes

Figure 22. Chondrite-normalized rare-earth-element diagrams (McDonough and Sun, 1995) for Aniakchak II and 

postcaldera, pre-1931 samples. Green field indicating range of patterns for Aniakchak I ignimbrite serves as index for 

comparison with other units. Uptick at Lu evidently results from normalization value for Lu relative to other heavy rare earth 

elements (HREE) in chondrites in comparison with our inductively coupled plasma mass spectrometry (ICPMS) analyses. 

A, Aniakchak II. Dashed pattern is for Plinian fall. B, Subaqueous domes. C, Tuff cones. D, Vent Mountain. E, Half Cone. 

Dashed pattern is for sample ANI-02-10 from agglutinate. F, Blocky Cone.
42 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 23

P

Zr

Eu

Northwest 

flank & 

1931 Main 

Crater lava 

flows (red)

Surprise 

Cone

Windy 

Cone

Breezy 

Cone

Breezy 

Cone

Breezy 

Cone (red)

Breezy 

Cone × 2

Rhyodacite

Andesite

Aniakchak I 

ignimbrite

Pb

Sr

Ti

Pb

Sr

Ti

Pb

Sr

Ti

Nb

Nb

Nb

Pb

Sr

Ti

Nb

Pb

Sr

Ti

Nb

Pb

Sr

Ti

Nb

Sample/Primitive mantle

1

10

100

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3

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Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pb

Pr

Sr

P

Nd

Sm

Zr

Hf

Eu

Ti

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

300

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Er

Tm

Yb

Lu

300

Sample/Primitive mantle Sample/Primitive mantle

E. Half cone F. Blocky Cone

C. Tuff cones D. Vent Mountain

A. Aniakchak II B. Subaqueous domes

Figure 23. Primitive-mantle-normalized multielement diagrams (McDonough and Sun, 1995) for Aniakchak II and 

postcaldera, pre-1931 samples. Green field indicating range of patterns for Aniakchak I ignimbrite serves as index for 

comparison with other units. A, Aniakchak II. Dashed pattern is for Plinian fall. B, Subaqueous domes. C, Tuff cones. 

D, Vent Mountain. E, Half Cone. Dashed pattern is for sample ANI-02-10 from agglutinate. F, Blocky Cone.
Geochemistry 43

Northwest Flank and Intracaldera Rhyodacite 

Lava Flows

As already noted, the northwest flank rhyodacite lava 

flow (67.9–68.4 weight percent SiO2), which stratigraphic 

evidence suggests postdates the Aniakchak II caldera-forming 

eruption, is compositionally indistinguishable from rhyodacite

lava exposed in the west wall of the 1931 Main Crater. The 

geochemical similarity is so complete that we are confident 

that these two lava flows represent a single magma and, 

therefore, probably were extruded at about the same time. 

These rhyodacite lavas are only marginally more evolved than 

Pumice Dome, so probably are closely related in origin. 

Tuff Cones

Scoria from Surprise Cone, the most evolved of the three 

tuff cones (57.9 and 58.4 weight percent SiO2), is andesite, 

(fig. 20). Scoria from Windy Cone (55.3 weight percent SiO2) 

is less evolved, but more so than tephra from Breezy Cone 

(NA93-102, 52.3 weight percent SiO2). Although tephra 

from Blocky Cone is compositionally similar, the Breezy 

Cone sample is the most magnesian (NA93-101, 5.27 weight 

percent MgO) of any postglacial Aniakchak sample. Windy 

Cone scoria is compositionally similar to “purple scoria” from 

above The Gates (NA947), lava (92CNA14) exposed in the 

wall of the northeast young maar, and lava (NA94-5) from 

300 m north of the maar, attributed to Vent Mountain but more 

mafic than any other materials from that source, which suggests that the latter three samples in fact may have been from 

Windy Cone or a nearby hidden vent. Surprise Cone scoria is 

similar to other mafic postcaldera andesites in most geochemical respects, though it is slightly higher in TiO2

, FeO*, and V, 

as well as in K2O, P2O5, and Zr; Y content and La/Yb (fig. 21) 

are relatively high, similar to Aniakchak II andesite.

On chondrite-normalized REE plots (fig. 22C), patterns 

for tuff cone samples are approximately parallel, but those for 

Surprise Cone show modest negative Eu anomalies whereas 

those for less-differentiated Breezy and Windy Cone typically 

do not. The same parallelism applies to multielement patterns 

(fig. 23C), except for elements compatible in plagioclase (Sr 

and Eu).

Vent Mountain

Most lava and tephra samples that are considered to 

have a Vent Mountain source are silicic andesites and dacites 

(61.4–66.0 weight percent SiO2; fig. 20). Lava from west of 

The Gates (97ANB59), which may have been effused early in 

the history of Vent Mountain, is markedly less differentiated at 

58.2 weight percent SiO2. The silicic andesites and, especially, 

dacites of Vent Mountain origin typically have higher K2O 

and lower Al2O3 and CaO at a given silica concentration than 

do products of Half Cone. The subtle geochemical difference 

between magmas from the two vents is perhaps most apparent on the P2

O5 versus K2O plot in fig. 21. Compositions of 

the Vent Mountain silicic andesites and dacites form relatively smooth trends on silica variation diagrams. There does 

not appear to be a consistent relationship between inferred 

stratigraphic order and composition, except that the oldest 

lava thought to have come from Vent Mountain is the least 

differentiated one.

Rare-earth-element patterns for Vent Mountain silicic 

andesites and dacites show a restricted concentration range, 

moderate negative Eu anomalies, and a relatively shallow 

slope for HREE (fig. 22D). The above characteristics also are 

evident in multielement plots (fig. 23D). The mafic andesite sample has REE and multielement (fig. 23D) patterns 

like those for silicic andesites but with lower incompatible 

element abundances. 

Half Cone

The 58.1–68.1 range in silica (figs. 20 and 24) of tephra 

and lava attributed to Half Cone is greater than that of the 

Vent Mountain samples and extends to more silicic dacite. 

There are few gaps in the array of Half Cone data on variation diagrams. However, in the ~64–66 weight percent SiO2

range, there is a distinct second group of high-Ti dacites (using 

TiO2 as a convenient index) with higher TiO2, K2O, P2O5, 

and incompatible trace element contents as are more typical 

of Vent Mountain dacites (figs. 20, 21, and 24). A few silicic 

andesite lava and tephra samples continue this trend to as low 

as ~62 weight percent SiO2. Half Cone andesites cross from 

the tholeiitic to the calc-alkalic fields on the SiO2 versus FeO*/

MgO diagram at ~61 weight percent SiO2 (fig. 21), and most 

of the more silicic Half Cone samples are calc-alkalic. 

When eruption order is considered, we find that the 

highTi dacites are predominantly early tephras and lavas, 

such as are exposed in the wall of Half Cone (fig. 24). Andesites and the other dacites are mainly from the ca. 400 yr 

B.P. Brown Pumice fall, Pink Pumice fall, and the Cobweb 

lava flow. Pink Pumice fall samples cluster between 66 and 

67 weight percent SiO2 and Brown Pumice samples between 

58 and 61 weight percent SiO2, though intermediate compositions are present as well. Partly welded or indurated coarse 

pyroclastic-flow material exposed at the base of the north wall 

of the 1931 Main Crater contains andesite bombs (~59 weight 

percent SiO2) that are compositionally similar to Brown 

Pumice fall clasts (fig. 24), which strongly implicates a Half 

Cone source.

Andesites and dacites from Half Cone have parallel REE 

patterns (fig. 22E), except that Eu anomalies are less negative in andesites (see also fig. 20, Eu/Eu*). Maximum REE 

concentrations are similar to those in dacites of Vent Mountain. A single pattern, for a sample from Half Cone agglutinate (ANI-02-10), has low REE abundances and a positive 

Eu anomaly. Multielement patterns (fig. 23E) are similar to 

those for Vent Mountain products, but andesites are heavily 
44 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

represented by detailed sampling of the Brown Pumice fall. 

The outlier (ANI-02-10) has low incompatible element abundances but high Pb, consistent with the feldspar accumulation 

implication of its positive Eu anomaly.

Description of Half Cone geochemistry would not be 

complete without mention of the white felsite xenoliths that 

are common in the ca. 400 yr B.P. deposits. The analyzed 

felsite xenolith (97ANB03; appendix B, table B1; not plotted 

on figures) has 77.0 weight percent SiO2 and is therefore 

high-silica rhyolite. The abundance of such xenoliths implies 

that they were derived from shallow intrusive rock, possibly 

related to the Quaternary magmatic system. Regardless, 

the analyzed quartz + plagioclase-phyric xenolith has high 

concentrations of elements that are incompatible in feldspar 

+ mafic silicates + Fe–Ti oxides + apatite and low concentrations of elements strongly partitioned into these phases. 

Blocky Cone

Blocky Cone is the youngest basaltic vent at Aniakchak 

volcano. Four analyzed tephra samples from Blocky Cone 

have 52.2–52.5 weight percent SiO2. The only other postglacial eruptive material with such low silica content is from 

Breezy tuff cone, which has somewhat lower TiO2 and Al2O3

and ~1 weight percent higher MgO (fig. 20). Blocky Cone 

thus is the best known candidate for most primitive recent 

Aniakchak magma. The Blocky Cone tephra FeO*/MgO ratio 

plots unequivocally in the tholeiitic field (fig. 21). Few precaldera edifice samples are as mafic. 

Blocky Cone tephra REE patterns are parallel, span a 

narrow range, and lack significant Eu anomalies (figs. 20 

and 22F). They are similar to the pattern for Breezy Cone, 

though the latter has a slightly more negative slope. Blocky Figure 24

Aniakchak II

Subaqueous dome

Vent Mountain

HC in 1931 Main Crater wall

Surprise Cone

HC 3 Pink and Brown Pumices

Half Cone 0

Half Cone 1

HC 4 Cobweb lava flow

Half Cone 2

Rhyodacite lava

56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

0.1

0.2

0.3

0.4

0.5

P O2 5 , in weight percent

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

TiO2 , in weight percent

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

K O, in weight percent

2

56 58 60 62 64 66 68 70 72

100

125

150

175

200

225

250

275

Zr, in parts per million

56 58 60 62 64 66 68 70 72 56 58 60 62 64 66 68 70 72

EXPLANATION

Figure 24. Silica variation diagrams for Aniakchak II and postcaldera, pre-1931 samples with >56 weight percent SiO2. Major 

element oxides recalculated to sum to 100 weight percent volatile-free. Symbols for products of Half Cone (HC) indicate order 

of eruption, progressively warmer colors with decreasing age. “Half Cone 0, 1, 2” refer to informal geologic map units of C.A. 

Neal (unpublished geologic map, 2014) that refer to order of eruption.
Geochemistry 45

Cone and Breezy Cone REE patterns have the shallowest 

slopes of any postglacial Aniakchak samples (fig. 21). Blocky 

Cone tephra multielement patterns likewise are similar to 

that for Breezy Cone, though the latter has slightly lower 

abundances of several incompatible elements (fig. 23F). The 

subduction-related geochemical signature is strongly displayed 

by these relatively primitive samples, which makes a strong 

case for its inheritance from subcrustal processes, though 

doubtless these magmas experienced considerable crystallization differentiation after they left the mantle.

Products of the 1931 Eruption

The 1931 eruption of Aniakchak vented juvenile magma 

ranging in composition from basaltic andesite to rhyodacite 

(56.0–67.3 weight percent SiO2; late bombs and lava reached 

68.9). Among 47 analyzed samples, the compositional variation is nearly continuous, though a discontinuity at approximately 59.5 and a gap at 63–65 weight percent SiO2

 suggest 

that there are at least three groups: basaltic and mafic andesite, 

silicic andesite, and dacite–rhyodacite (fig. 25). The most 

silicic 1931 sample, from the north end of the Slag Heap flow, 

is compositionally similar to least-evolved Aniakchak II rhyodacite pumice, though the latter has somewhat lower compatible element abundances. At the mafic end of the spectrum, 

1931 basaltic andesite has relatively high TiO2, Na2O, and 

P2O5 in comparison with earlier postcaldera basaltic andesites. 

In a manner qualitatively similar to Half Cone eruptives, 

1931 andesites cross from tholeiitic to calc-alkalic fields on the 

SiO2 versus FeO*/MgO diagram at ~60 weight percent SiO2

(fig. 26). Dacite–rhyodacite samples have distinctly higher 

FeO*/MgO than less evolved 1931 products. Most of the 1931 

samples define smooth trends on variation diagrams, but there 

are exceptions. Three andesite bombs and a scoria clast have 

anomalously low TiO2, Na2O, and, especially, P2O5 (figs. 25 

and 26). It is possible that these four andesite samples, which 

were collected from Blocky Cone and the Doublet Crater rim, 

are not juvenile 1931 magma. Dacite–rhyodacite samples fall 

into two groups, one consisting of four basal tephra samples 

that plot on the projected trend defined by andesites and a 

second that comprises lava, bombs, and other tephra and that 

has higher incompatible element (for example, K2O, Zr, Y) 

and lower compatible element (for example, V) concentrations 

and Eu/Eu*. 

Rare earth elements display systematic variation with 

major element composition of 1931 products. Chondritenormalized patterns for basaltic andesites and andesites are 

approximately parallel (fig. 27A–D) to each other and to the 

Aniakchak I reference field. Europium anomalies are lacking 

in the most mafic samples to slightly negative in andesites 

(figs. 25 and 27). For comparison, Aniakchak II andesite has 

roughly similar LREE and MREE, lower HREE, and Eu/Eu* 

~1 (fig. 27A). Lava from the Main Crater floor, representative of 1931 mafic andesites, has smooth REE patterns that 

are elevated and slightly steeper relative to the base level 

suggested by Blocky Cone basaltic andesite tephra (fig. 27C). 

Overall REE abundances are higher in dacite–rhyodacite, 

Eu anomalies become increasingly negative, and patterns 

develop a concave-upward shape with increasing differentiation (figs. 27D–F and 25), though the most evolved 1931 lava 

does not attain the REE concentrations present in Aniakchak II 

rhyodacite (compare with fig. 22A).

The primitive-mantle-normalized multielement diagrams 

in figure 28 illustrate the geochemical characteristics of the 

1931 magmas. Basaltic andesites through rhyodacites have 

subparallel patterns that are in most respects, other than Sr, P, 

and Ti, simply elevated versions of the Blocky Cone tephra 

patterns (fig. 28C). Incompatible element concentrations systematically increase with differentiation, and compatible element concentrations decrease as plagioclase, titanomagnetite, 

and, in turn, apatite, join the fractionating assemblage. 
46 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 25, p. 1 of 2

Agglutinate

Bomb

Main Crater lava

Pumice and scoria

Doublet Crater lava

Slag Heap lava

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.5

TiO2 , in weight percent

14

15

16

17

18

19

Al O2 3 , in weight percent

2

3

4

5

6

7

8

9

10

11

FeO*, in weight percent

0

1

2

3

4

5

6

MgO, in weight percent

50 52 54 56 58 60 62 64 66 68 70 72

1

2

3

4

5

6

7

8

9

10

11

CaO, in weight percent

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Na O, in weight percent

2

1.4

50 52 54 56 58 60 62 64 66 68 70 72 50 52 54 56 58 60 62 64 66 68 70 72

Basaltic 

andesite Andesite Dacite

Rhyodacite

Basalt

Precaldera 

dacites

Postcaldera, pre-1931

Aniakchak I

Aniakchak II

EXPLANATION

Figure 25. Silica variation diagrams for products of 1931 eruption of Aniakchak volcano. Major element oxides recalculated 

to sum to 100 weight percent volatile-free. Fields indicate compositional ranges of precaldera dacites (purple), Aniakchak I 

(green), Aniakchak II (orange), and pre-1931, postcaldera units (pink).
Geochemistry 47

Figure 25, p. 2 of 2

Medium-K

Low-K

High-K

Aniakchak I

Precaldera 

dacites

Aniakchak II

Postcaldera,

pre-1931

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K O, in weight percent

2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

P O2 5 , in weight percent

100

150

200

250

300

350

Zr , in parts per million

20

30

40

50

60

Y, in parts per million

50 52 54 56 58 60 62 64 66 68 70 72

0

50

100

150

200

250

300

400

V, in parts per million

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

0.7

0.8

0.9

1.0

1.1

Eu/Eu*

0.0

50 10

350

0.6

50 52 54 56 58 60 62 64 66 68 70 72 50 52 54 56 58 60 62 64 66 68 70 72

Agglutinate

Bomb

Main Crater lava

Pumice and scoria

Doublet Crater lava

Slag Heap lava

EXPLANATION

Figure 25.—Continued.
48 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Precaldera 

dacites Aniakchak I

Aniakchak II

Postcaldera, pre-1931 Tholeiitic

Calc-alkalic

Agglutinate

Bomb

Main Crater lava

Pumice and scoria

Doublet Crater lava

Slag Heap lava

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

La/Yb, chondrite normalized

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

1

2

3

4

5

6

7

FeO*/MgO

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

P2O5 , in weight percent

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K O, in weight percent

2

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

P2O5 , in weight percent

0

50

100

150

200

250

300

350

400

V, in parts per million

EXPLANATION

Figure 26. Silica and P2O5 variation diagrams for products of 1931 eruption of Aniakchak volcano. Major element oxides 

recalculated to sum to 100 weight percent volatile-free. La/Yb ratios normalized to values for CI chondritic meteorites 

(McDonough and Sun, 1995). FeO* is total Fe calculated as FeO. Fields indicate compositional ranges of precaldera dacites 

(purple), Aniakchak I (green), Aniakchak II (orange), and pre-1931, postcaldera units (pink).
Geochemistry 49

Figure 27

10

100

20

30

40

50

60

70

80

200

5

Representative 

Aniakchak II 

andesite (orange)

10

100

20

30

40

50

60

70

80

200

5

10

100

20

30

40

50

60

70

80

 Sample/Chondrite

200

5

Blocky 

Cone 

(blue)

 Sample/Chondrite Sample/Chondrite

10

100

20

30

40

50

60

70

80

200

5

Dacitic 

pumice

Andesitic 

scoria

10

100

20

30

40

50

60

70

80

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

200

5

10

100

20

30

40

50

60

70

80

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

200

5

E. Doublet Crater lava F. Slag Heap lava

C. Main Crater lava D. Pumice and scoria

A. Agglutinate B. Bombs

Figure 27. Chondrite-normalized rare earth element diagrams (McDonough and Sun, 1995) for products of 1931 eruption of 

Aniakchak volcano. Green field indicating range of patterns for Aniakchak I ignimbrite serves as index for comparison with 

other units. Uptick at Lu evidently results from normalization value for Lu relative to other heavy rare earth elements (HREE) 

in chondrites in comparison with our inductively coupled plasma mass spectrometry (ICPMS) analyses. A, Agglutinate. 

Orange pattern is for representative Aniakchak II andesite. B, Bombs. C, Main Crater lava. Blue patterns are for Blocky Cone 

tephra. D, Pumice and scoria; dacite pumice in yellow, andesite scoria in purple. E, Doublet Crater lava. F, Slag Heap lava. 
50 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 28

Representative 

Aniakchak II 

andesite (orange)

Blocky 

Cone 

(blue)

Dacitic 

pumice

Andesitic 

scoria

Pb

Sr

Ti

Pb

Sr

Ti

Pb

Sr

Ti

Nb

Nb Nb

Pb

Ti Nb

Pb

Ti

Nb

Pb

Sr

Ti

Nb

Sample/Primitive mantle

1

10

100

2

3

4

5

20

30

40

50

200

Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pb

Pr

Sr

P

Nd

Sm

Zr

Hf

Eu

Ti

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

300

1

10

100

2

3

4

5

20

30

40

50

200

300

1

10

100

2

3

4

5

20

30

40

50

200

300

1

10

100

2

3

4

5

20

30

40

50

200

300

1

10

100

2

3

4

5

20

30

40

50

200

300

1

10

100

2

3

4

5

20

30

40

50

200

Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pb

Pr

Sr

P

Nd

Sm

Zr

Hf

Eu

Ti

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

300

P

P

Sr

Sr

Sample/Primitive mantle Sample/Primitive mantle

E. Doublet Crater lava F. Slag Heap lava

C. Main Crater lava D. Pumice and scoria

A. Agglutinate B. Bombs

Figure 28. Primitive-mantle-normalized multielement diagrams (McDonough and Sun, 1995) for products of 1931 eruption of 

Aniakchak volcano. Green field indicating range of patterns for Aniakchak I ignimbrite serves as index for comparison with 

other units. A, Agglutinate. Orange pattern is for representative Aniakchak II andesite. B, Bombs. C, Main Crater lava. Blue 

patterns are for Blocky Cone tephra. D, Pumice and scoria; dacite pumice in yellow, andesite scoria in purple. E, Doublet 

Crater lava. F, Slag Heap lava. 
Geochemistry 51

Radiogenic Isotopes

The Sr, Nd, Pb, Th, and U isotopic compositions of 

15 samples from postglacial Aniakchak eruptive units were 

determined by George and others (2004). In addition, Ra isotope data were given for the eight samples of products of the 

1931 eruption. The principal focus of the George and others 

(2004) study was on U-series disequilibria, and these authors 

did not subdivide data on diagrams beyond separating 1931 

from prehistoric eruptions. We present the data of George and 

others (2004) in figure 29, in which analyses are plotted by 

eruptive unit as in the other geochemical figures in this paper. 

One sample (“94CNA11”) in table 2 of George and others 

(2004) has been omitted from figure 29 because its reported 

major element chemical analysis does not match any in the 

AVO database and, in that database, 94CNA11 is a Pleistocene 

basalt. We have plotted sample NA94–7 as being from Windy 

Cone on the basis of its elemental composition reported by 

George and others (2004, table 2), though the source vent for 

this tephra deposit is uncertain. It is not a goal of this paper to 

exhaustively interpret the limited isotopic data but to present 

them for the sake of completeness. We revisit implications of 

the isotopic analyses in the “Discussion” section, where they 

apply to specific eruptive units.

Radiogenic isotope data for Aniakchak volcanic rocks 

can be expected to show effects of assimilation of Mesozoic 

or younger continental crust of the Alaska Peninsula superimposed on some variability in mantle contributions owing to 

incorporation of subduction-related components into the mantle

wedge. In Sr isotope data (fig. 29A), the basaltic andesites of 

Blocky Cone, lava southwest of The Gates (plotted here as a 

tuff cone product), and Windy Cone have 87Sr/86Sr ratios of 

0.70325–0.70331. The Aniakchak II andesite sample similarly 

has the relatively low 87Sr/86Sr ratio of 0.70333, whereas Sr 

in dacite from Vent Mountain is marginally more radiogenic 

and Aniakchak II rhyodacite is defensibly higher at 0.70340. 

The 1931 lavas of Doublet Crater and Slag Heap overlap with 

Aniakchak II rhyodacite, as does one 1931 andesite bomb. 

However, the rest of the 1931 samples have Sr that is notably 

more radiogenic, including Main Crater andesite lava, the 

highest 87Sr/86Sr ratio being for basal dacite tephra at 0.70351.

The Sr–Nd diagram (fig. 29B) separates Blocky Cone, 

lava southwest of The Gates, and dacitic Vent Mountain 

samples from a larger group, forming two arrays that have 

decreasing 143Nd/144Nd with increasing 87Sr/86Sr. Two 1931 

samples plot slightly above the larger group. That both high 

143Nd/144Nd and low 143Nd/144Nd arrays have basaltic andesite 

samples at their high 143Nd/144Nd, low 87Sr/86Sr ends suggests at 

least two different parent magmas, or possibly an unreported 

Nd analytical problem. Assuming that the difference in 

143Nd/144Nd ratios is real, each of the two arrays is interpreted 

as a mixing trend. Because 87Sr/86Sr increases with SiO2

(fig. 29A), it is reasonable to suggest that the principal influence that raised 87Sr/86Sr ratios was incorporation of crustal 

material. Among the 1931 samples, basal dacite tephra has 

the highest 87Sr/86Sr and lowest 143Nd/144Nd, consistent with it 

containing the largest crustal contribution. The 87Sr/86Sr and 143Nd/144Nd data indicate that Aniakchak II rhyodacite cannot be solely a product of closed-system differentiation of 

Aniakchak II andesite. Either the rhyodacite includes assimilated crustal rock or melt, or it differentiated from a parent 

different from Aniakchak II andesite.

The Pb isotope data define a single array of wellcorrelated Pb isotope ratios. Figure 29C plots 206Pb/204Pb 

versus 207Pb/204Pb. Although not shown, a plot of 206Pb/204Pb 

versus 208Pb/204Pb appears similar. The reported 206Pb/204Pb for 

outlier (97ANB-44, Main Crater lava) evidently is incorrect, 

because this sample lies at the radiogenic end of a linear array 

on a plot of 207Pb/204Pb versus 208Pb/204Pb. The stated analytical uncertainties are large relative to the range in ratios, but 

nevertheless the data are consistent with the Sr and Nd results 

in their indication of mixing between a mantle component that 

has Pb isotopic composition near the Northern Hemisphere 

Reference Line (Hart, 1984) and a more radiogenic component. The 206Pb/204Pb versus 207Pb/204Pb Aniakchak array has 

the positive correlation that is typical of magmatic arcs and 

commonly is attributed to addition of Pb to the mantle from 

subducted sediment. However, for Aniakchak postglacial 

samples, this trend does not project to the measured composition of marine sediment suggested by George and others 

(2004) to be represented by Aleutian sediment (Plank and 

Langmuir, 1998, their table 2). Alternatively, the radiogenic 

end member at Aniakchak could be close to the composition of 

Alaska trench sediment given by Plank and Langmuir (1998, 

their table 2) or of Alaska Peninsula crust.

The U-series diagram (fig. 29D) plots activity ratios 

of long-lived parent isotope 238U to 232Th versus short-lived 

daughter 230Th (half-life ~76,000 yr) to 232Th. Where a point 

lies on this plot is sensitive to magmatic processes and to time 

elapsed since the Th/U ratio of the magma was last modified. 

Through in-growth of 230Th on the right or decay on the left, 

points will move counterclockwise back to the equiline after a 

few half lives. In young rocks, such as postglacial Aniakchak 

samples, the 230Th/232Th ratio also is an isotopic tracer. Here 

(230Th/232Th) ratios for 1931 products are broadly similar but 

do suggest differences, for instance, between Main Crater 

andesite and Doublet Crater lava that are consistent with Sr 

and Nd isotopic compositions. The greatest difference among 

contemporaneous eruptives, well in excess of analytical uncertainty, is shown by Aniakchak II andesite and rhyodacite.

A useful property of U-series isotopic composition is 

its implication for processes that contribute to a magma’s 

composition (Gill and Williams, 1990). Points for relatively 

primitive samples in figure 29D that plot with U excess, that 

is, to the right of the equiline, such as Windy Cone and lava 

southwest of The Gates, are thought to reflect dominance 

of U-enrichment relative to Th in the mantle source by 

subduction-related fluids. On the contrary, points for mafic 

samples that plot to the left of the equiline, such as Blocky 

Cone, may have been affected more by interactions with deep 

crustal rocks than by subduction-related geochemical modification of the mantle source. In addition to source-related 
52 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

isotopic characteristics, intermediate and silicic volcanics may 

present evidence of comparatively recent events that modified 

their Th/U ratios, such as crystallization differentiation or 

partial melting. That 1931 samples plot on both sides of the 

equiline (for example, Slag Heap and basal tephra on the left, 

Doublet on the right) suggests mixing of at least two components with different recent histories, those 1931 samples that 

straddle the equiline being mixtures (Main Crater lava) that 

may plot there coincidentally. 

George and others (2004) reported (226Ra/230Th) ratios 

for the eight 1931 samples (their table 2; uncertainties are 

given as ±1.3 percent). Because the half-life of 226Ra is a mere 

1,600 years, variation in (226Ra/230Th) provides information 

on the time scale of recently active magmatic processes. The 

(226Ra/230Th) ratios of 1931 samples range from 1.167 for an 

andesite bomb to 1.005 for Slag Heap lava. The latter value 

is within uncertainty of secular equilibrium, implying dominance of material in the silicic end member of 1931 products 

that was either magma stored for several thousand years or 

remelted rock. Curves calculated for two-component mixing

between average 1931 andesite and average 1931 silicic 

samples approximately reproduce variation of (226Ra/230Th) 

with Th concentration and with Eu/Eu* (figs. 29E and 29F). 

Recent Seismicity

The distribution of earthquakes beneath volcanoes may 

illuminate the subsurface plumbing system for transport and 

storage of magma. The AVO seismic network at Aniakchak 

has detected a substantial number of earthquakes since its 

installation in 1997. Earthquake hypocenters located by 

AVO between 1 January 2009, and 3 January 2013, a total 

of 135 events with a maximum ML of 2.1, are plotted on a 

shaded-relief map in figure 30A. Earthquakes clearly associated with Aniakchak volcano and with hypocentral depths 

<10 km are concentrated within the caldera. Deeper events 

occur in a bow-tie pattern centered northwest of Vent Mountain, broadening northwest of Half Cone and southeast of 

Vent Mountain. 

In order to better visualize the distribution of earthquakes 

in the third dimension we plot hypocenters projected onto a 

vertical cross section through Vent Mountain oriented parallel 

to Pacific–North America Plate convergence (fig. 30B). This 

would be the plane of maximum horizontal tectonic compressive stress that is expected to be the preferred orientation of 

dikes (Nakamura and others, 1977). On this projection, as 

well as on the map, the events at >25 km below sea level 

(bsl) occur to the south of Vent Mountain, there is a laterally 

extensive region with numerous events at depths of 14–25 km, 

few events 7–14 km in depth occur beneath the caldera, and 

the shallowest events are beneath Vent Mountain and the south 

caldera wall. There is an overall focusing of seismicity upward 

beneath the southern part of the caldera. 

The vast majority of located earthquakes are described 

as low frequency (long period, LP) events. Power and others 

(2004) found a similar pattern for the period 1997–2002 and 

interpreted deep long-period (DLP) events to reflect relatively 

steady-state magma ascent over a broad area in the lower to 

middle crust. Aniakchak has one of the highest rates of lower 

to midcrustal seismicity yet lacks the upper mantle events that 

are common at most of the other monitored Alaska–Aleutian 

arc volcanoes (Power and others, 2013; Buurman and 

others, 2014).

The plot of earthquake hypocentral depths in temporal 

sequence (fig. 30C) reveals that LP events commonly occur in 

clusters that span a substantial depth range. In several cases, 

the DLP events are closely followed, or even preceded, by 

shallow earthquakes. 

Discussion

Eruptive Processes Through Time

In the Pleistocene Aniakchak volcano built a composite 

edifice of lava flows and subordinate fragmental strata. This 

material presumably issued from a central vent in Vulcanian 

and Strombolian eruptions, as well as by lava effusion at various times over a period of perhaps several hundred thousand 

years (Nye and others, 1993). The prominent hill of ~2,800 ft 

(~850 m) elevation on the volcano’s flank ~3 km northnorthwest of Half Cone is underlain by edifice lava (fig. 4) that 

may have vented from a hypothetical lateral dike system that 

would be approximately parallel to the maximum horizontal 

compressive stress, here the direction of plate convergence as 

suggested for Aleutian arc volcanoes by Nakamura and others 

(1977). Our suggestion of a possible small caldera east of 

Birthday Pass invites speculation on late Pleistocene explosive 

eruptions, but deposits from such eruptions, should they be 

preserved, have not yet been identified.

The earliest postglacial volcanism for which deposits 

are recognized is the Aniakchak I eruption, which occurred 

sometime between ~9,500 and ~7,000 yr B.P. This violent 

explosive andesitic eruption was sufficiently energetic and 

voluminous to deposit bombs on the slopes of the edifice and 

feed pyroclastic flows that descended Birthday Creek valley 

and the northwest flank valleys to elevations at least as low 

as 500–800 ft (150–240 m). In Birthday Pass, Aniakchak I 

ignimbrite is partly welded, and elsewhere proximal bomb 

accumulations may be agglutinated. Some exposures of fartraveled nonwelded ignimbrite in southern valleys may be 

Aniakchak I deposits. An eruption of this magnitude surely left 

distal tephra that can be identified by its distinctive geochemistry. Proximal lithic breccia has not been identified, and it is 

unclear if a caldera formed during this eruption. Judging from 

the known distribution of Aniakchak I ignimbrite, the vent(s), 

and possibly small caldera, likely were located just west of 

present Vent Mountain. 
Discussion 53

Black Nose Pumice and related lava were vented in a 

series of closely spaced eruptions, which included a Plinian 

event that was responsible for the pumice bed near Aniakchak 

Bay that rests on carbonaceous material dated at ca. 7,000 

yr B.P. by VanderHoek and Myron (2004) and VanderHoek 

(2009). Although other eruptions may have occurred in the 

interval between Aniakchak I and the Black Nose Pumice 

events, we lack sufficient information to comment further on 

these. The vent for the Black Nose Pumice and lava must have 

been located northeast of the summit of ancestral Aniakchak 

volcano, west-northwest of what is now The Gates, because 

the precursory lava flow and the intra-Plinian welded ignimbrite have been found only in a paleotopographic low near 

the top of the caldera wall north of The Gates. Vent-opening 

explosive eruptions likely produced dacite pumice, possibly 

represented by deposits stratigraphically between Aniakchak I 

scoria and the lower Black Nose Pumice north of The Gates. 

Degassed dacite lava subsequently flowed a short distance 

to the east-northeast, much as the Cleetwood lava flow at 

Crater Lake, Oregon, was effused shortly before the climactic 

eruption of Mount Mazama (Bacon, 1983). After a pause of 

unknown length, a Plinian eruption resulted in deposition of 

the lower Black Nose Pumice. Bedding defined by changes 

in clast size reflects changing conditions between Plinian 

eruption and less vigorous explosions or lulls in activity. 

Changes in wind direction are considered unlikely to have 

affected deposition so close to the Plinian vent. The intraplinian

welded ignimbrite near the top of the lower Black Nose Pumice

was deposited by a pyroclastic flow that occurred when the 

eruption column collapsed, presumably owing to high mass 

eruption rate. After a brief return to Plinian venting of the 

lower Black Nose Pumice magma, the eruption paused for an 

unknown but probably brief period before resuming, this time 

fed by the upper Black Nose Pumice magma. 

The fall deposit of upper Black Nose Pumice is bedded, 

probably because of changes in eruption parameters, as also 

inferred for the lower unit. Welding of basal upper Black Nose 

Pumice probably reflects a combination of high mass-eruption 

rate and relatively low column height at the onset of eruption 

that led to heat retention by pyroclasts sufficient for welding. 

Incipient welding of much of the upper Black Nose Pumice at 

the caldera rim is a consequence of these factors and possibly 

the slightly less evolved composition and therefore higher 

preeruption temperature of the magma. Further investigation 

of Black Nose Pumice exposures doubtless would result in a 

refined eruption reconstruction, and the distal tephra record 

of Aniakchak eruptions between Black Nose Pumice and 

Aniakchak II time is yet to be studied in detail.

The Aniakchak II caldera-forming eruption began with 

a Plinian column that was the source of pumice fall deposits 

locally exposed north of the volcano. The location of this vent 

on the edifice is unknown. The eruption character apparently 

transformed rapidly to feed pyroclastic flows, initially of 

rhyodacite exclusively. The rhyodacite ignimbrite shows little 

welding and may have been sourced by a comparatively high 

eruption column that allowed admixing of air and cooling of 

falling pyroclasts. The subsequent mixed rhyodacite–andesite 

ignimbrite commonly is partly welded, which suggests it may 

have been emplaced by pyroclastic flows fed by collapsed 

eruption column(s) issuing at high mass-eruption rate from 

vents around an incipiently foundering caldera-floor block. 

Presence of banded rhyodacite–andesite clasts demonstrates 

that both magmas vented simultaneously and mingled during 

the transition from venting of one magma composition to the 

other. The abundance of lithic fragments in the base of the 

andesite ignimbrite of the final phase of the eruption reported 

by Dreher (2002) suggests that caldera collapse was underway 

when the first andesite ignimbrite was deposited. Welding 

of andesite agglutinate on the flanks of the edifice implies 

relatively low eruption columns towards the conclusion of the 

eruption. A key observation yet to be made and documented 

would be the composition of any juvenile clasts in the proximal lithic breccia found on and near the caldera rim (Druitt 

and Bacon, 1986). This could indicate when in the eruption 

scenario the caldera began to collapse. Regardless, the diameter of Aniakchak caldera and the circumferential distribution of postcaldera vents indicates subsidence of a piston-like 

caldera-floor block and outward expansion of the caldera by 

inward landsliding of the oversteepened walls during and 

following the eruption.

Following caldera collapse, a lake developed within the 

closed basin. By analogy with other caldera lakes, groundwater

draining from the walls and precipitation took at most a few 

hundred years to fill the caldera to its rim. For example, Crater 

Lake, Oregon, though differing from Aniakchak in being 

limited in maximum level by leakage through a permeable 

layer in the caldera wall, reached a depth of nearly 600 m 

in a little as ~400 years (Nathenson and others, 2007). The 

ancestral caldera lake at Aniakchak was at most ~300 m deep. 

The minimum depth is constrained by the summit elevations 

of subaqueous dacite domes, West Dome being the highest at 

610 m asl, and the maximum depth by low points on the caldera

rim. Joint patterns indicate that Pumice, Vulcan, Bolshoi, and 

West Domes were emplaced under lake water. Pyroclastic 

deposits that may have been produced before or during dome 

extrusion have not been recognized. The four domes apparently effused slowly from vents located within a ring-fracture 

system surrounding the foundered caldera block. Sometime 

after the Aniakchak II eruption, rhyodacite lava whose composition is closely comparable to that of Pumice Dome, was 

extruded high on the northwest flank of the edifice and near 

the west caldera wall.

Breezy, Windy, and Surprise tuff cones vented basaltic 

to andesite magma following draining of the caldera lake. 

Although the height-to-width aspect ratios of these U-shaped 

features could imply the name tuff ring (Wohletz and 

Sheridan, 1983; Cas and Wright, 1987), historical usage at 

Aniakchak and the ~100 m height of the partial rings leads us 

to retain the term tuff cone. The Aniakchak tuff cones appear 

to be transitional between the extremes of tuff rings and tuff 

cones, though their forms have been modified by erosion 

and infilling of craters by pyroclastic and alluvial material. 
54 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 29

50 52 54 56 58 60 62 64 66 68 70 72

SiO2 , in weight percent

0.70320

0.70325

0.70330

0.70335

0.70340

0.70345

0.70350

0.70355

87Sr/86Sr

Aniakchak II

Blocky Cone

Vent Mountain

Tuff cone

1931 Main Crater lava

1931 bomb

1931 pumice and scoria

1931 Doublet Crater lava

1931 Slag Heap lava

0.7032 0.7033 0.7034 0.7035 0.7036 87Sr/86Sr

0.51290

0.51295

0.51300

0.51305

0.51310

0.51315

0.51320

143Nd/ 144Nd

18.7 18.8 18.9 19.0 19.1 206Pb/ 204Pb

15.45

15.50

15.55

15.60

15.65

15.70

207Pb/ 204Pb

1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65

(

238U/ 232Th)

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

(230Th/ 232Th)

2 3 4 5 6 7

Th, in parts per million

0.95

1.00

1.05

1.10

1.15

1.20

1.25

(226Ra/230Th)

1.00 0.95 0.90 0.85 0.80 0.75 0.70

Eu/Eu*

0.95

1.00

1.05

1.10

1.15

1.20

1.25

(226Ra/230Th)

D

E F

A B

C

Aleutian 

sediment

Alaska 

sediment

NHRL

Equiline

Equilibrium Equilibrium

Mixing

Mixing

NE Pacific 

MORB

To NE Pacific 

MORB

To Alaska 

& Aleutian 

sediment

* *

EXPLANATION
Discussion 55

Figure 29 (facing page). Isotopic data for postglacial products of Aniakchak volcano (George and others, 2004, table 2). Error bars 

on isotopic ratios are ±2σ; symbols as in earlier figures (see legend in D). Where shown in B and C, following George and others, 

field of Northeast Pacific mid-ocean ridge basalt (MORB) is from Langmuir and others (1992), and ocean floor sediment averages 

off the Aleutian and Alaska arc are from Plank and Langmuir (1998). A, SiO2 versus 87Sr/86Sr. B, 87Sr/86Sr versus 143Nd/144Nd. Arrows 

indicate vectors to Northeast Pacific MORB and Alaska and Aleutian sediment, off scale at 87Sr/86Sr = 0.7026 and ~0.706, respectively. 

C, 206Pb/204Pb versus 207Pb/204Pb. Northern Hemisphere Reference Line (NHRL) is from Hart (1984). Outlier is sample 97ANB44, 1931 

Main Crater lava, for which reported 206Pb/204Pb is suspect. Alaska and Aleutian trench sediment average compositions from Plank 

and Langmuir (1998, their table 2). D, (

238U/232Th) versus (230Th/232Th) activity ratios. Secular equilibrium is represented by the equiline. 

E, Th concentration versus (226Ra/230Th) activity ratio. Horizontal line at (226Ra/230Th) = 1 represents secular equilibrium. Mixing curve 

(Langmuir and others, 1978) was calculated for simple two-component mixtures of average andesite and average silicic lava. 

Uncertainty in Th concentration by inductively coupled plasma mass spectrometry (ICPMS) assumed to be ±0.1 ppm, ~2 percent. F, Eu/

Eu* versus (226Ra/230Th) activity ratio. Line and curve as in E. Uncertainty in Eu/Eu* from ICPMS data assumed to be ±0.025, ~3 percent. 

Uncertainties (±2σ) inferred from George and others (2004) are as follows: 87Sr/86Sr, 28 ppm (0.000020); 143Nd/144Nd, 25 ppm (0.000014); 

Pb isotope ratios, 0.2 percent (0.03–0.08); (230Th/232Th) and (238U/232Th), 1 percent (~0.015) and 0.7 percent (~0.010), respectively; Th 

concentration by ICPMS, ~2 percent (0.1 ppm); Eu/Eu* from ICPMS data ~3 percent (0.025).

Figure 30. Locations of 135 earthquake hypocenters located by the Alaskan Volcano Observatory (AVO) for the period January 1, 2009, 

through January 3, 2013 (Dixon and others, 2010, 2011, 2012, 2013). A, Map of earthquake locations with magnitude and depth indicated. 

Northing and Easting in Universal Transverse Mercator (utm) grid in Aniakchak region are rotated ~1 degree clockwise relative to lines 

of longitude and latitude. Map location for Mogi model point-source for caldera deflation detected by InSAR is from Kwoun and others 

(2006). Hillshade base from U.S. Geological Survey 30-m digital elevation data. B, Hypocenters projected to line of section in A plotted as 

distance north-northwest from arbitrary origin (558.8 km E, 6290.0 km N) versus depth below sea level. Hypocenters ≥8 km from line of 

section excluded from plot. Line of section is Pacific–North America Plate convergence azimuth (N24°W) from DeMets and others (2010) 

interpolated from Ryan and others (2012, their figure 3). Minimum magma storage depths below ground surface were calculated from 

dissolved volatile concentrations in melt inclusions (Bacon, 2002) for vapor-saturated magma and overburden density of 2,200 kg/m3. 

C, Hypocentral depths through time. Hypocenters ≥8 km from line of section in A, excluded from plot. 

540 545 550 555 560 565

Easting, in kilometers

6,290

6,295

6,300

6,305

6,310

6,315

Northing, in kilometers

A

2

1

0

–1

0 to 5

5 to 10

10 to 15

15 to 20

20 to 25

25 to 30

Depth in

kilometers

below sea level 

Earthquake 

magnitude

InSAR Mogi 

model point 

source

ANPB

AVO seismic 

station

EXPLANATION

Line of section 

in Figure 30

B

ANNE

ANPK

ANPB

AZAC

ANON

ANNW
56 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 30B

2

1

0

–1

0 to 5

5 to 10

10 to 15

15 to 20

20 to 25

25 to 30

Earthquake 

magnitude

InSAR Mogi 

model point 

source

Minimum 

storage 

depth from 

melt 

inclusions

EXPLANATION

30 25 20 15 10 5 0

Distance, in kilometers from origin

-5

0

5

10

15

20

25

30

Depth, in kilometers below sea level

B

NNW SSE

Vent 

Mtn

Aniakchak 

caldera

Pink 

Pumice 

dacite

1931 

dacite

Blocky Cone 

basaltic andesite

Mogi 

source

Depth in

kilometers

below sea level 

Figure 30.—Continued.
Discussion 57

Figure 30.—Continued.

Figure 30C

0 200 400 600 800 1,000 1,200 1,400

Days after January 1, 2009

-5

0

5

10

15

20

25

30

Depth, in kilometers below sea level

C

2

1

0

–1

0 to 5

5 to 10

10 to 15

15 to 20

20 to 25

25 to 30

Earthquake 

magnitude

EXPLANATION

Depth in

kilometers

below sea level 
58 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

This implies eruption in shallow standing water or saturated 

ground very near lake level at an elevation of ≤400 m asl, the 

approximate elevation of their crater floors. This elevation corresponds to the middle terrace, 82 m above modern Surprise 

Lake, identified by McGimsey and others (1994). The deposits 

in the cones themselves suggest variation between Surtseyan 

explosion and hydrovolcanic blasts that resulted in fall and 

surge deposition, respectively. We speculate that the eruptions 

of mafic to intermediate magma that built the three tuff cones 

were triggered by unloading (decompression) of the magmatic 

system owing to rapid draining of the caldera lake.

Our current understanding of stratigraphic relations 

between tephra deposits and lava flows indicates that activity 

of Vent Mountain and Half Cone followed that of the tuff 

cones and alternated for possibly ~1,000 years between these 

two andesite–dacite vents situated on the ring-fracture system 

(fig. 5C). Accessible deposits suggest that Half Cone erupted 

more explosively than Vent Mountain, at least late in their 

histories. The Vent Mountain cone was built by accumulation of scoria bombs and lapilli from Strombolian to violent 

Strombolian explosions forming unconsolidated to agglutinated layers. Lava flows issued from the base of the cone and 

also from fissure eruptions on the southwest flank. New Cone, 

on the southeast flank of Vent Mountain, was constructed by 

violent Strombolian eruptions from the same magma plumbing system. Half Cone produced a smaller volume of lava but 

more voluminous pyroclastic deposits than did Vent Mountain. 

Early Half Cone explosive eruptions may have been Vulcanian 

to Plinian, sending pyroclastic flows across the caldera floor 

to leave partly welded deposits such as are exposed at the base 

of the north wall of the 1931 Main Crater. These eruptions 

alternated with effusion of degassed lava as domes and local 

flows. Towards the end of its life, ca. 400 yr B.P., Plinian 

eruption of Half Cone resulted in deposition of the widespread 

pink and brown pumice falls and intracaldera pyroclastic 

flows and surges. This event was followed by Strombolian 

or violent Strombolian eruptions that laid down agglutinated 

spatter on the surface of Half Cone. This activity ended with 

destruction of the Half Cone edifice and subsequent effusion 

of the degassed Cobweb lava flow that spread radially about 

the central conduit, now marked by a ring of tephra.

Blocky Cone is a cinder and scoria cone formed by 

Strombolian eruption of one of the most primitive of magmas 

erupted at Aniakchak. Like the majority of postcaldera vents, 

Blocky Cone is located above the ring-fracture system. Its 

feeder conduit somehow missed intersecting any resident 

evolved magma stored beneath the southwestern edge of the 

caldera at the time of its <400 yr B.P. prehistoric eruption.

In their detailed reconstruction of events of the 1931 

eruption, Nicholson and others (2011) concluded that, after 

initial vent-clearing steam explosions, the principal tephra fall 

originated from a subplinian column. This phase of the eruption was succeeded by intermittent phreatomagmatic explosions as magma supply diminished and magma interacted with 

groundwater. While the latter activity was ongoing, effusion of 

degassed silicic lava began at Doublet Crater and Slag Heap 

vents. Change to more mafic andesite and basaltic andesite 

at the Main Crater vent resulted in transition from phreatomagmatic to Strombolian eruption of spatter and effusion of 

degassed lava on the crater floor. The three 1931 vents are 

sited on the outer edge of the inferred ring-fracture system. We 

will return to a conceptual model of the Aniakchak magmatic 

system after providing interpretation of the geochemistry of 

erupted magmas.

Magma Evolution

Having described the geochemical characteristics of 

postglacial eruptive units in earlier sections, we now interpret 

Aniakchak magma geochemistry in terms of processes likely 

responsible for the diversity of erupted compositions. This 

qualitative analysis is intended to highlight notable geochemical features and to guide future research and quantitative 

modeling of Aniakchak petrogenesis.

Geochemical Features of Arc Magmas at 

Aniakchak

A framework for understanding magma evolution at 

Aniakchak is provided by widely accepted common features 

of magmatism above subduction zones manifested by arc 

volcanoes. The magma transport and storage systems of arc 

volcanoes receive input, or parent, magma that originates as 

basaltic partial melt of the mantle and, with few exceptions, 

carries with it the geochemical fingerprint of hydrous fluids 

or melts produced in subduction-related processes. This 

fingerprint is characterized by high abundances of large-ion 

lithophile elements (LILE) K, Rb, Cs, Sr, and Ba, as well as 

Pb, Th, U, and LREE, relative to high field strength elements 

(HFSE) Nb and Ta (see, for example, fig. 19F). En route 

upward to crustal depths, melts separate from precipitated 

crystals and some interaction of magma with mantle and deep 

crustal rocks doubtless occurs, so that parental melts that reach 

the upper 5–20 km of the crust typically are fractionated relative to primary mantle melts. The most MgO-rich basalts and 

basaltic andesites at Aniakchak, such as some Pleistocene lavas 

(fig. 16) and postglacial Breezy and Blocky Cones (fig. 20), 

we take as representative of the parents of magmas erupted 

by Aniakchak. Because even these magmas have experienced 

olivine fractionation, and thus have low contents of olivinecompatible Ni, we plot concentrations of V (figs. 16, 17, 20, 

21, 25, and 26) as a moderately compatible trace element that 

is useful to illustrate fractionation of a plagioclase-dominated, 

pyroxene- and titanomagnetite-bearing assemblage. 

Note that Pleistocene mafic magmas display a range of 

mantle-incompatible element concentrations (for example, P, 

K, Y, and Zr in fig. 16) that would be very difficult to explain 

solely by crystallization differentiation of a single primary 

magma. It is important to recognize this incompatible-element 

concentration range because Breezy and Blocky Cones, the 

most primitive postglacial erupted magmas, may not represent 
Discussion 59

the entire spectrum of mafic inputs into Aniakchak in post‐

glacial time. Aspects of that spectrum may result from different degrees of partial melting of a uniform source, which 

would affect all incompatible elements, or from variation in 

the amount of added subduction component in local domains 

of the mantle wedge above the Alaska–Aleutian subduction 

zone, which would affect LILE preferentially. Further variation in certain elemental concentrations in primitive magmas 

may be attributed to local mantle fertility, the capacity to yield 

silicate melt at a given temperature and pressure. 

Most of the erupted magmas at Aniakchak and the 

majority of arc volcanoes worldwide show the geochemical 

effects of processing in the crust. As SiO2 content increases, 

concentrations of elements compatible in minerals stable at 

magmatic temperatures and crustal pressures (olivine, pyroxenes, hornblende, plagioclase, Fe–Ti oxides, apatite, and 

other accessory minerals) decrease and those of incompatible 

elements increase. These effects result from crystallization 

differentiation, that is, separation of melt from crystals, with or 

without addition of wall rock or partial melts thereof (assimilation–fractional crystallization, or AFC; DePaolo, 1981). 

Some occurrences of silicic magmas are thought to represent 

partial melts of crustal rocks with little or no component of 

mantle-derived magma. Further complications are the possibility of magma mixing (Anderson, 1976) and common admixing 

of crystal cumulate material from the Quaternary magmatic 

system (for example, Nakada and others, 1994; Dungan and 

Davidson, 2004). We use graphical patterns characteristic of 

these processes to identify those that apparently contributed to 

produce the compositions of eruptive units at Aniakchak.

Precaldera Magma Geochemistry and 

Subsurface Processes

The relatively small number of analyses of edifice lavas 

and the lack of strict temporal control presently limit use of 

these data to merely providing a measure of the compositional 

range exhibited at Aniakchak over a relatively long period of 

time. The range of edifice lava compositions typically encompasses that of all younger analyzed products of Aniakchak 

volcano, save the most silicic postglacial lavas and pyroclasts. 

In fact, for a given SiO2 content, the range in many elemental 

abundances is far greater among edifice lavas than among later 

products (figs. 16 and 17).

Aniakchak I ignimbrite data form a coherent trend or 

grouping on geochemical diagrams. These trends are consistent with crystallization differentiation involving plagioclase 

and titanomagnetite but not apatite. For example, because 

of strong preference of plagioclase for Eu2+ and of titanomagnetite for Ti and V, Eu/Eu*, TiO2

, and V decrease with 

increasing SiO2 (figs. 16 and 17). In contrast, P2O5 steadily 

increases with other incompatible element concentrations 

and with SiO2 because Aniakchak I andesite was not crystallizing apatite. The trend for Aniakchak I compositions is not 

an effect of mixing with silicic magma, because the latter 

would have a far lower P2O5 content that would be inconsistent with Aniakchak I compositional trends (figs. 16 and 17). 

The narrow range of subparallel patterns in REE (fig. 18B) 

and multielement (fig. 19B) diagrams for Aniakchak I also is 

consistent with crystallization differentiation and is shown as 

a convenient reference on all other such figures. The parent 

magma for Aniakchak I ignimbrite had incompatible element 

concentrations substantially higher than other postglacial 

eruptives but within the span defined by edifice lavas. These 

geochemical properties are exemplified by TiO2, K2O, Zr, 

and Y in figure 16. Note that Aniakchak I has a higher FeO*/

MgO ratio than other postglacial andesites (fig. 17), which 

suggests tholeiitic affinity of its parent magma. Aniakchak I 

andesite also has relatively low Al2O3. The AVO database of 

~740 analyses of samples from Veniaminof volcano, located 

~100 km down the Alaska Peninsula from Aniakchak (fig. 1), 

contains several low-silica dacite analyses (63.4–64.0 weight 

percent SiO2) that are similar to those of Aniakchak I andesites (appendix B, table B2). Relative to Aniakchak I andesite 

(59.7–62.4 weight percent SiO2), the Veniaminof dacites have 

slightly diminished Ti, Fe, Mg, Ca, Sc, V, K, Rb, Cs, Nb, 

Ta, Th, and U, and slightly elevated Si, Na, and Pb. Overall, 

however, the similarity of incompatible element abundances 

in this subset of silicic andesite–low-silica dacite samples 

from the two volcanoes points to a common process. Because 

all incompatible elements have high concentrations in these 

magmas, a credible explanation may be that a relatively low 

degree of mantle melting was responsible for generating their 

parent basalts in comparison to parents of the majority of 

intermediate-composition magmas at each volcano.

Postglacial, precaldera rhyodacites and dacites share 

diagnostic geochemical features with Aniakchak I andesites. In 

comparison with other postglacial silicic products, these have 

low Al2O3 and high concentrations of mantle-incompatible 

elements, such as K, Zr, Y, and Ti. Eruption of the precaldera 

rhyodacites and dacites shortly after Aniakchak I andesite, 

coupled with their geochemistry, lead us to conclude that 

these silicic magmas were generated dominantly by crystallization differentiation of Aniakchak I andesite parent magma 

and not by partial melting of crustal rocks. The differentiation 

processes involved separation of residual melt from a plagioclase + pyroxenes + titanomagnetite + apatite assemblage, 

probably in the mid to upper crust, in which apatite began to 

crystallize when the residual liquid reached a SiO2 concentration of ~64 weight percent (fig. 16). Light and heavy REE 

contents in precaldera rhyodacites and dacites are similar to 

those of the most REE-rich, evolved Aniakchak I samples, but 

middle REEs Gd, Tb, and Dy are less elevated (fig. 18). Apatite/

melt partition coefficients of REEs are large—substantially 

greater for trivalent MREE than for LREE or HREE—and 

their magnitude increases (to ~100) with melt evolution and 

decreasing temperature (Watson and Green, 1981; Fujimaki, 

1986). Similarly, clinopyroxene favors MREE, and all 

trivalent REE save possibly La are compatible in this phase 

in the most silicic magmas (Sisson, 1991). The combination 

of trivalent REE incompatibility in plagioclase and strong 
60 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

partitioning of MREE into apatite, plus MREE partitioning 

into modally more abundant clinopyroxene, appears to explain 

REE behavior in the postglacial, precaldera magmas. The 

increased effects of fractionation, particularly of plagioclase 

(Sr), titanomagnetite (Ti), and apatite (P), from Aniakchak I 

andesite through undivided pumice falls and upper Black Nose 

Pumice to lower Black Nose Pumice also are evident in the 

multielement diagrams in figure 19.

Geochemical Implications of Aniakchak II 

Magma Compositions

The compositionally zoned Aniakchak II eruption raises 

questions of how the andesite and rhyodacite magmas were 

formed and how they may have been related to one another. 

The andesite is unusual for an Aniakchak magma of its silica 

content in having low V and high Na2O, Y, TiO2, and, especially, P2

O5 (fig. 20). In addition, the andesite Eu/Eu* and La/

YbN ratios are relatively high (figs. 20 and 21), as are concentrations of Sr and REE La–Tm (appendix B, table B1). Dreher 

and others (2005) reported many of these observations and 

called upon addition of ~1 weight percent apatite to account 

for the anomalous P2O5 and MREE in Aniakchak II andesite 

versus other analyzed andesites. They pointed out, however, 

that apatite alone would not account for the Sr content of 

Aniakchak II andesite. Moreover, massive amounts of unusual 

apatite-rich material (~350 million metric tons) would have 

to have been assimilated in order to affect the entire 13 km3

(DRE) of andesite reported by Dreher and others (2005) to 

have been erupted. 

In addition to the high P2O5 content, the elevated Na2O 

and TiO2 coupled with low V concentrations are particularly 

significant to understanding the origin of Aniakchak II 

andesite. Vanadium in magmas is strongly partitioned into 

titanomagnetite. Addition of titanomagnetite is not responsible 

for the high TiO2 in Aniakchak II andesite samples, because 

V concentrations in them are low relative to most other Aniakchak andesites. Search of the AVO database of ~740 analyses 

of samples from Veniaminof volcano returned two andesites 

whose compositions are similar to the Aniakchak II andesites 

(appendix B, table B2). Relative to Aniakchak II andesites, the 

Veniaminof samples have similar geochemistry but slightly 

less Mg, Ca, Sr, V, Sc, Rb, Cs, Nb, Ta, Th, and U. The 

observation that Alaska–Aleutian arc volcanoes can produce 

andesite magmas of similar but rather extreme compositions 

at different locations argues strongly against assimilation 

of local crustal material that is exceptionally rich in apatite 

or other accessory minerals. We suggest that the high P2O5, 

Na2O, and TiO2 contents of Aniakchak II andesite are inherited 

from the mantle source of the andesites’ parental basalt. The 

high P2O5, Na2O, and TiO2 could reflect partial melting of a 

relatively fertile mantle domain, that is, one comparatively 

rich in fusible material. Because incompatible trace elements 

are not exceptionally concentrated in Aniakchak II andesites, 

we attribute the high P2O5, Na2O, and TiO2 contents to source 

bulk composition rather than enhancement by a low degree 

of partial melting of an otherwise uniform mantle wedge. The 

crystal-poor andesite contains plagioclase, augite, hyperthene, 

and magnetite (Larsen, 2006). It is not lacking in evidence for 

mantle and crustal contributions, because Dreher and others 

(2005) also reported traces of olivine, ilmenite, and quartz 

among a total of ~8 volume percent crystals. The single Aniakchak II andesite sample analyzed by George and others (2004) 

has Sr, Nd, Pb, and Th isotopic compositions rather similar to 

their basaltic andesite sample thought to have been vented at 

Windy Cone (figs. 29A–D), which suggests that geochemical 

effects of upper crustal assimilation in the andesite are minor.

The Aniakchak II rhyodacite (69.4–70.4 weight percent 

SiO2) is the most evolved magma known to have erupted 

from Aniakchak volcano. In figures 20, 21, 25, and 26, the 

observation that the rhyodacite analyses plot more closely 

on a trend with postcaldera eruptives than with precaldera 

silicic rocks suggests that the rhyodacite is not simply a more 

evolved version of, for instance, lower Black Nose Pumice 

magma. Moreover, as pointed out earlier, REE patterns for 

Aniakchak II rhyodacite show depletion in MREE relative 

to other Aniakchak silicic magmas that would be expected to 

result from hornblende fractionation (Sisson, 1994). Indeed, 

hornblende is a common but not abundant phenocryst in the 

rhyodacite, in which Larsen (2006) reported ~9 volume percent

total phenocrysts of plagioclase, hypersthene, magnetite, 

ilmenite, amphibole, and rare augite. Dreher and others (2005, 

p. 1,763) suggested that the rhyodacite represents the interstitial liquid in a mafic crystal mush, in other words, the product 

of crystallization differentiation. The same authors noted the 

different Sr isotopic compositions (George and others, 2004) 

of Aniakchak II andesite and rhyodacite (fig. 29A) and inferred 

that the rhyodacite was not derived by crystallization of the 

coerupted andesite. This inference is supported by differences 

in Nd–Sr and U–Th isotopic systematics between the two 

magmas (figs. 29B and 29D). The Sr and Nd isotopic ratios for 

the rhyodacite sample would be consistent either with derivation from a parent different from Aniakchak II andesite and 

or with assimilation of crustal material. The U–Th isotopic 

results indicate that the rhyodacite magma was not dominated 

by pre-late-Pleistocene crust; else the analysis would plot 

near the equiline (fig. 29D). The ≥14 km3 (DRE) of rhyodacite magma vented (Dreher and others, 2005) in the calderaforming eruption must have separated from the crystal mush 

and accumulated into a contiguous body, carrying with it or 

crystallizing in situ its phenocrysts, before the eruption began. 

Dreher and others (2005) suggested that the rhyodacite magma 

formed at greater depth and intruded stored andesite magma, 

triggering the caldera-forming eruption. However, Larsen 

(2006) concluded that both rhyodacite and andesite magmas 

resided at similar depth before eruption. The preponderance 

of evidence indicates that the two magmas were not genetically related and that either Aniakchak II andesite invaded 

the rhyodacite magma and instigated the eruption or, alternatively, the andesite was drawn up during the evacuation of the 
Discussion 61

rhyodacite from its holding reservoir. Syn-eruptive mixing and 

mingling of the two magmas produced hybrid compositions 

and “banded” (layered) pumice. 

Rhyodacite Lava Flows and Subaqueous Domes

The northwest flank rhyodacite, compositionally similar 

lava exposed high in the 1931 Main Crater west wall, and the 

subaqueous domes on the caldera floor are the earliest recognized postcaldera eruptive products. Compositions of the four 

small domes foreshadow dacite magmas of more voluminous 

eruptions from Vent Mountain and Half Cone (figs. 20–23). At 

similar SiO2 contents, the domes have K2O and P2O5 contents 

like Vent Mountain products but CaO, TiO2, and Y contents 

and LREE/HREE more like Half Cone samples. In detail, 

however, dome compositions are not strictly identical to Vent 

Mountain or Half Cone dacites. The Bolshoi Dome analysis 

has no close match, but Vulcan and West Dome analyses are 

comparable to some Half Cone and, especially for Vulcan, also 

Vent Mountain and late-erupted 1931 pumice data, the two 

domes having slightly lower HREE and higher Rb contents. 

Analyses of the two Pumice Dome samples are much like 

those of the northwest flank lava and the early lava in the 1931 

Main Crater southeast wall. Although dome and rhyodacite 

lava compositions trend towards Aniakchak II rhyodacite in 

most silica variation diagrams (excepting CaO, Al2O3, and Y; 

figs. 20 and 21), the most silicic is significantly less evolved 

and does not appear to be remnant Aniakchak II magma backmixed with andesite. We interpret the subaqueous dome lavas 

to be differentiates of new andesite magma that was emplaced 

in the upper crust, or evolved there from basaltic andesite, and 

partially crystallized in the mushy zone beneath the caldera 

during the ~2,000 years subsequent to caldera collapse.

Basaltic Andesites of Breezy and Blocky Cones

The basaltic andesites of Breezy and Blocky Cones are 

the most mafic postglacial Aniakchak eruptives known. They 

are far from primitive, however, because at 52.2–52.5 weight 

percent SiO2 they have only 4.3–5.3 weight percent MgO, 

15–17 ppm Ni, and Mg’ ≤0.51 (molar Mg/(Mg+Fe)). Fractionation of magnesian olivine readily extracts Ni from silicate 

melt and lowers Mg’. It is unclear at what depth olivine 

separation occurs and therefore whether more primitive 

magmas enter the mid to upper crustal magma storage zone 

beneath Aniakchak. Regardless, melts at least as primitive as 

the Breezy and Blocky Cone ejecta definitely get through or 

around the upper mush column and, therefore, we consider 

them as representative of input or replenishment magma least 

modified since it left the mantle. Of the Aniakchak samples 

analyzed for their isotopic compositions by George and others 

(2004), the one from Blocky Cone has nearly the least radiogenic Sr and Pb (fig. 29). 

Basaltic Andesites of Windy Cone

The four basaltic andesite samples with 53.6–55.3 weight 

percent SiO2 plotted in figures 20 and 21 are attributed here to 

Windy Cone or a nearby hidden vent for similar lava. Two of 

these had been considered early Vent Mountain lava (NA94-5 

and 92CNA14). One (NA94-7) is scoria from above The Gates 

that had been thought to have had a Vent Mountain source, and 

one (NA93-102) is scoria from Windy Cone itself. Lacking 

direct ties in the field to Vent Mountain, the compositions of 

the isolated scoria deposit and the two lava outcrops, along 

with their proximity to Windy Cone, argue for grouping them 

with unequivocal Windy Cone scoria (figs. 22C and 23C). 

Like the Blocky Cone sample, the two of these samples 

analyzed by George and others (2004) have Sr and Pb isotope 

ratios that are among the least radiogenic. However, reported 

143Nd/144Nd ratios are similar to that for Blocky Cone for one 

but higher for the other. In keeping with their higher SiO2

contents, the geochemistry of the Windy Cone samples indicates that they are more differentiated than Blocky or Breezy 

Cone ejecta. The observation that REE and multielement 

patterns (figs. 22C and 23C) are parallel to those for Breezy 

and Blocky Cones, but elevated, is consistent with the interpretation that the four Windy Cone samples are simply more 

differentiated versions of the same general basaltic andesite 

magma type. If the reported difference in 143Nd/144Nd ratios 

between the two Windy Cone samples plotted in figure 29 is 

real, it would reflect a difference in mantle source because 

assimilation of crustal material, or addition of a subduction 

component, should also raise 87Sr/86Sr and 207Pb/204Pb.

Andesite of Surprise Cone 

Closest of known vents to the caldera center, Surprise 

Cone ejected the earliest post-Aniakchak II andesite yet 

recognized. The two scoria clasts analyzed have compositions 

that are unremarkable, but nonetheless subtle differences make 

them unique in the database (figs. 20 and 21). The most similar 

material is andesite lava from west of The Gates (97ANB59), 

which is thought to have a Vent Mountain source. We interpret 

andesite of Surprise Cone to be the product of differentiation 

of magma similar to basaltic andesite of Windy Cone.

Vent Mountain and Half Cone

Activity at Vent Mountain and at Half Cone overlapped 

in time and in composition of their products. Compositional 

variation appears more continuous for Half Cone, whose interior is exposed and which produced the zoned Pink and Brown 

Pumice, whereas for Vent Mountain only surface lava and 

some tephras are available for sampling. For example, the only 

andesite with <61 weight percent SiO2 from Vent Mountain is 

the lava flow southwest of The Gates, but Half Cone Brown 

Pumice compositions fill in the gap between 59 and 61 weight 
62 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

percent (figs. 20 and 21). Modest geochemical differences 

described earlier separate some Half Cone and Vent Mountain 

ejecta with similar SiO2 contents. However, the data appear 

to be consistent with crystallization differentiation of andesite 

magma in the magma storage and stagnation zone beneath 

the caldera being the dominant process responsible for the 

andesite-to-dacite range from Vent Mountain and Half Cone. 

The one Vent Mountain dacite analyzed by George and others 

(2004) has low 143Nd/144Nd similar to Blocky and Windy Cone 

basaltic andesites but moderately higher 87Sr/86Sr (figs. 29A

and 29B), which suggests that additional crustal material was 

assimilated during its differentiation. The same dacite has 

notably high (238U/232Th) relative to (230Th/232Th), so that it 

plots the farthest to the right of the equiline of any analyzed 

sample (fig. 29D), which suggests that recent differentiation 

was dominant over assimilation of older material, though the 

time scale of differentiation is not constrained by these data.

A small number of Half Cone silicic andesites have 

anomalously high P2O5 contents at ~62–63 weight percent 

SiO2. On the P2O5 versus K2O and V plots in figure 21 these 

samples trend toward the field of Aniakchak II andesite, 

whereas Vent Mountain and other Half Cone samples form 

vertical arrays that evolve into decreasing P2O5 with increasing 

differentiation owing to apatite fractionation. These relations 

suggest admixing of andesite magma that had at least some of 

the geochemical characteristics of Aniakchak II andesite.

Early Half Cone and most Vent Mountain dacites define a 

group with relatively high TiO2, K2O, P2O5, and incompatible 

trace element contents on silica variation diagrams (figs. 20, 

21, and 24), as noted earlier. Using TiO2 as a convenient index, 

this highTi trend projects back toward Aniakchak II andesites 

as potential parent magmas. Overall, the more silicic of these 

products were erupted later (“Half Cone 2” in fig. 24). The 

final Half Cone products, the Pink and Brown Pumices and 

the Cobweb flow, as well as a few earlier Half Cone and Vent 

Mountain samples, present a fairly continuous array back 

to ~58 weight percent SiO2 at lower incompatible element 

concentrations than the highTi magmas. The Pink Pumice 

represents the most evolved magma to be definitively attributed to Half Cone. Dacite of the Cobweb flow, last erupted 

and rich in crystals and crystal clusters with disequilibrium 

textures, appears to be a mixture of dacite of the Pink Pumice, 

lesser amounts of coeval andesite of the Brown Pumice, and 

crystal mush.

Products of the 1931 Eruption

Grouping of data points on the silica variation diagrams 

in figure 25 indicates that the 1931 eruption tapped several

magmas that had histories more complex than can be 

explained solely by two-component mixing or a single liquid 

line of descent. The Sr, Nd, and Pb isotopic data of George 

and others (2004) indicate that analyzed 1931 eruptives contain either more subduction component than Blocky and tuff 

cone basaltic andesites or, more likely, more crustal material 

(fig. 29). The four andesite bombs and scoria clasts that have 

anomalously low TiO2, Na2O, and P2O5 must have tapped an 

isolated pocket of magma in the storage system or have been 

recycled from earlier activity; they evidently do not represent 

a significant volume of erupted material, cannot be simple 

mixtures of other erupted 1931 magmas (fig. 26, P2O5 versus 

K2O and V), and will not be discussed further. 

Basaltic andesite and mafic andesite erupted in 1931 

(56.0–58.2 weight percent SiO2), which represent the smallvolume late-erupted unit T3, Main Crater lava, and spatter 

agglutinate of Nicolson and others (2011), differ from other 

postcaldera eruptives with ≤58 weight percent SiO2 by their 

higher TiO2, Na2O, and, especially, P2O5, as well as low V, 

contents that are a feature of their parent magma (fig. 25). 

As in some Half Cone samples, these characteristics are 

reminiscent of Aniakchak II andesite, though TiO2 and P2O5

concentrations do not attain the levels found in the latter. 

Compositional variation among the basaltic andesites and 

mafic andesites appears to be consistent with crystallization 

differentiation of melts that were not apatite saturated (figs. 25 

and 26). Significant 226Ra/230Th disequilibrium (fig. 29) in the 

mafic andesite bomb and Main Crater lava analyzed by George 

and others (2004) indicates that crystallization differentiation 

took place in the recent past, though determining the absolute 

time scale depends on the initial 226Ra/230Th ratio assumed for 

the parent magma. 

Dacite and rhyodacite (~65–69 weight percent SiO2) 

vented in 1931 form two trends or clusters of points on variation diagrams. The trends could be consistent with crystallization differentiation, in this case involving apatite in addition to 

plagioclase, pyroxenes, and titanomagnetite, or with mixing of 

silicic and intermediate-composition end member magmas. We 

note that four analyzed dacite pumice samples from the base 

of the 1931 tephra section, the basal part of unit T1 of Nicholson

and others (2011), that have relatively low K2O, Zr, and Y 

and high compatible element concentrations plot apart from 

other 1931 silicic samples in figures 25 and 26 (for example, 

fig. 25, K2O and Zr versus SiO2) and more in line with data for 

andesites. The volume of magma erupted as dacite pumice in 

unit T1 amounts to at least two orders of magnitude more than 

that extruded as lava (Nicholson and others, 2011, their figures 

4 and 5 and table 4). Dacite–rhyodacite of the Slag Heap lava, 

the most evolved 1931 product, does not have a counterpart 

among analyzed 1931 pumices. A dense bomb from the rim 

of Blocky Cone nearly matches Slag Heap lava chemistry, 

and that bomb likely was sourced by a part of the Slag Heap 

feeder system. The less evolved Slag Heap samples are close 

in composition to the northwest flank rhyodacite flow and lava 

in the 1931 Main Crater wall. Doublet Crater lava compositions overlap those of the most evolved 1931 pumices. Slag 

Heap lava has (226Ra/230Th) in secular equilibrium, Doublet 

lava and other dacites and silicic andesites nearly so (fig. 29). 

The (226Ra/230Th) data imply one or more silicic end member 

components in 1931 magmas that either (1) differentiated 

several thousand years ago and survived the Aniakchak II 

caldera-forming eruption or (2) contain a dominant fraction 

of middle Holocene or older material. Residual Aniakchak II 
Discussion 63

rhyodacite may be a candidate that meets most criteria but 

has some characteristics, such as elevated LREE and HREE 

contents and lower (230Th/232Th), that are not a perfect match 

for the evolved 1931 magmas.

Analyses of bombs, pumice, and scoria with 

~59.5–64 weight percent SiO2 ejected in 1931, including 

unit T2 of Nicholson and others (2011), form smooth trends 

on variation diagrams that project towards the group of four 

basal dacite pumice samples. At the low-silica end of this 

andesite trend, however, is a discontinuity in some elemental 

concentrations when compared with more mafic andesites (for 

example, V in fig. 25). This demands a cryptic mafic andesite 

magma either as the parent for crystallization differentiation 

or the end member for mixing with silicic magma. The mafic 

end member was apatite saturated, as evidenced by decreasing 

P2O5 with increasing SiO2 (fig. 25) and K2O and decreasing V 

(fig. 26). Isotopic variability among 1931 andesites (figs. 29B

and 29D) suggests more complexity than simple crystallization differentiation or two-component mixing. 

In summary, the 1931 eruption tapped a variety of magmas. In the order and units presented by Nicholson and others 

(2011, their figures 4 and 5 and table 4), these were: (1) silicic 

dacite tephra (T1) from magma derived by crystallization differentiation of andesite and containing a substantial component derived from precaldera materials; (2) relatively voluminous andesite tephra and bombs of a series of differentiated 

melts, or hybrid mixtures with T1 dacite, of a cryptic mafic 

andesite; (3) a tiny volume of resident or remelted dacite–

rhyodacite magma of Doublet Crater and Slag Heap, possibly 

related to nonerupted Aniakchak II rhyodacite; and (4) also a 

relatively small volume of mafic andesite to borderline basaltic 

andesite of unit T3, spatter agglutinate, and the Main Crater 

lava flow that form a differentiation series in their own right, 

likely evolved recently from a basaltic andesite parent with 

some characteristics in common with Aniakchak II andesite. 

Significantly, the geologic record that will remain accessible 

for perhaps some millennia will consist of the voluminous and 

widespread tephra units T1 and T2 whose magmas clearly are 

related to one another, whereas the extreme mafic and silicic 

eruptives may not be detectable.

Aniakchak Magmatic System

By postglacial time, Aniakchak volcano had a wellestablished feeder system that brought magma from the mantle 

wedge into and through the crust along repeatedly used pathways. A portion of this magma erupted, but much of it likely 

crystallized at depth. Magma was drawn from a region of the 

mantle broader than the footprint of the Aniakchak edifice and 

focused, probably in the deep crust, into the magma conduit 

and storage system beneath the volcano (fig. 31A). Heating of 

wall rocks through time conditioned the mid to upper crust for 

magma storage and differentiation. The eruptive history and 

magma geochemistry of postglacial Aniakchak, along with 

other studies of active and extinct arc volcanoes (for example, 

Mangan and others, 2009), provide a basis for our conceptual model of what took place in the upper ~15 km beneath 

Aniakchak and what might be anticipated for the future.

The distinctive incompatible element abundance pattern 

of the crystal-poor andesite magma of the voluminous 

Aniakchak I eruption implies that this differentiation product 

of atypical parent magma had accumulated in an anomalously 

large crustal reservoir (fig. 31B). There, or earlier at a deeper 

level, crystallization differentiation produced andesite melt 

that separated from accumulating crystals. Explosive eruption vented the andesite melt, but enough must have been 

left behind to impart aspects of its geochemical fingerprint to 

subsequently erupted precaldera dacites (for example, Black 

Nose Pumice). The precaldera dacite magmas are thought to 

have formed by crystallization differentiation of andesite melt 

left behind in the magma storage region for the Aniakchak I 

eruption, perhaps as intergranular melt in crystal mush or as a 

crystal-poor melt lens. Magma that produced the Upper Black 

Nose Pumice likely came from a region or layer in the storage 

system that was somewhat hotter and possibly deeper than 

that of the more-evolved lower Black Nose Pumice magma; 

both erupted from a vent that may have been northeast of the 

Aniakchak I feeder (fig. 31B). Because precise geochronological control is lacking for the deposits and hybrid or physically 

mingled andesite–dacite ejecta, it remains unknown whether 

the dacite magma was stored in a region not tapped by the 

Aniakchak I eruption or whether the dacite evolved subsequent to that event. Our new electron microprobe analyses 

and those reported by Dreher (2002) indicate preeruptive 

temperatures of ~940–950 °C for lower Black Nose Pumice 

from Fe–Ti oxide thermometry, notably higher than those 

reported by Larsen (2006) for the more evolved, hornblende 

phyric Aniakchak II rhyodacite. The northeast flank dacite 

flow probably also was fed from the general vicinity of Black 

Nose magma storage. The association of Aniakchak I and the 

precaldera dacite eruptions indicates that a mature storage and 

differentiation system had been established by Black Nose 

Pumice time and the mid to upper crust had been thermally 

conditioned to provide a nursery for accumulation of silicic 

magma. By this time, if not before, a composite pluton had 

begun to grow in the upper crust beneath Aniakchak. The 

relative proportions of crystal mush with intergranular melt 

and solidified intrusions cycled as thermal conditions fluctuated owing to the interplay of magmatic input, eruption, and 

hydrothermal cooling.
64 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Figure 31

0 5 10 15 20

Distance, in kilometers from origin

1015202530

0

5

Depth, in kilometers below sea level

3540

0 5 10 15

05100510

0 5 10 15 20

Distance, in kilometers from origin

0510

Depth, in kilometers below sea level

Ice

DCHZ

M

P

oho

leistocene 

intrusive 

complex

Dikes

Mush 

column 

with 

solidified 

intrusions

A

Aniakchak I

Black Nose 

Pumice

Pleistocene 

intrusive 

complex

Northeast 

lava flow

Crystal mush 

& solidified 

intrusions

B

Aniakchak II 

rhyodacite

Pleistocene 

intrusive 

complex

Aniakchak II 

andesite

Crystal mush 

& solidified 

intrusions

C

Pleistocene 

intrusive 

complex

North

west 

lava flow Pumice 

Dome

Bolshoi 

Dome

Vulcan 

Dome

West 

Dome

Dacite 

lava Lake

Caldera fill

Crystal mush 

& solidified 

intrusions

D

Pleistocene 

intrusive 

complex

Crystal mush 

& solidified 

intrusions

Surprise 

Cone

Breezy 

Cone

Windy

Cone

Maar

E

Pleistocene 

intrusive 

complex

Crystal mush 

& solidified 

intrusions

Half 

Cone Vent 

Mtn

Blocky 

Cone

Dacite Basaltic 

andesite

Andesite

F

Pleistocene 

intrusive 

complex

Crystal mush 

& solidified 

intrusions

Main 

Crater

Dacite

Basaltic 

andesite

Andesite

G

WSW ENE
Discussion 65

Figure 31. (facing page) Schematic west-southwest–east-northeast cross-sections of the Aniakchak magmatic system through time. Surface features projected to N78°E line with arbitrary origin at 541.8 km E, 6,304.8 km N; some features translated slightly or exaggerated for clarity. Thicknesses of dikes and sills and sizes of some magma bodies enhanced for clarity. Dikes shown as cutting intrusive rock and crystal mush with low fraction of intergranular melt, but not mush with high melt fraction. No attempt has been made to depict variable crystallinity within mass of crystal mush and solidified intrusions. A, Late Pleistocene. Crustal thickness inferred after Fliedner and Klemperer (2000); seismic Moho may represent transition to ultramafic cumulates rather than to mantle peridotite (Calvert and McGeary, 2013). Deep crustal hot zone (DCHZ) modified after Annen and others (2006) and Solano and others (2012). DCHZ is equivalent to zone of melting, assimilation, storage, and homogenization (MASH) of Hildreth and Moorbath (1988). Patterns within DCHZ indicate sills of varying age and composition. Aniakchak volcano edifice reconstructed without allowing for glacial erosion. Several hundred meters of ice were present during glacial intervals. Pleistocene intrusive complex patterns indicate range of magmas released from mush column and frozen in stagnation region beneath edifice. B, Precaldera postglacial system depicting Aniakchak I and Black Nose magma bodies. Pleistocene intrusive complex in B–G generalized from A; some features, such as magma reservoirs and intracaldera volcanoes, not included in successive diagrams for simplicity. Black Nose Pumice magma shown as developed after Aniakchak I eruption and crystallization of remaining Aniakchak I magma. Note common source of Black Nose Pumice and northeast dacite lava flow. C, Aniakchak II reservoirs before the caldera-forming eruption. D, Dacite volcanism within the caldera lake. Caldera fill consists of juvenile pyroclastic material (lighter shading) and landslide megabreccia (darker shading). Note common sources of Pumice Dome and northwest lava flow, and of West and Vulcan Domes. Appearance of crossing conduits is an artifact of projection to cross section. E, After draining of the caldera lake. Surprise, Breezy, and Windy tuff cones vented where andesite–basaltic andesite dikes could penetrate mush east of active core zone. Source depth of andesite of Surprise Cone may have been shallower than implied by feeder dike. Although the large maar crater is shown here, it may be younger. F, Ca. 400 yr B.P. For simplicity, Half Cone and Vent Mountain are shown as supplied by a common reservoir, though geochemistry indicates subtle differences between erupted magmas. Basaltic andesite of Blocky Cone avoided mush near its southwestern edge and erupted from within the ring-fracture zone. G, 1931 C.E., showing Main Crater and principal reservoir only. Sources of minute volumes of evolved dacite–rhyodacite erupted at Doublet Crater and Slag Heap are thought to have been stagnant residual melt pockets within mush. See Nicholson and others (2011, their figure 9) for a more detailed series of cross sections that depict the plumbing system for the 1931 eruptions. Modern seismicity (see fig. 30) suggests that the Vent Mountain plumbing system, not shown in G, remains active. 
66 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Andesite and rhyodacite of the Aniakchak II calderaforming eruption have geochemical characteristics suggesting that these magmas represent fresh recharge and a new 

cycle of storage and differentiation. The two magma types 

do not appear to be genetically related, despite comingling 

during the eruption. Instead, a body of crystal-poor rhyodacite magma segregated from crystal mush to be stored at a 

minimum pressure between 95 to 150 MPa, as constrained 

by phase equilibria for H2O-saturated magma (Larsen, 2006). 

Assuming an average upper crustal density of 2,620 kg/m3

from measurements on Naknek Formation in the Katmai area 

(Kienle, 1970), this corresponds to a depth range of 3.7 to 

5.8 km (though the Jurassic–Cretaceous sedimentary section 

near Aniakchak is perhaps no thicker than ~2 km according 

to Detterman and others, 1981a, and it is uncertain what lies 

beneath it). Taking into account a lower density of the volcanic 

edifice above Naknek rocks leads to a depth range of perhaps 

3.9 to 6.0 km. Note that this is depth below the ground surface, 

which of course depends on the precaldera height of the volcanic edifice, perhaps as much as 2,300 m above sea level. Minimum depth of the stored rhyodacite magma then would have 

been 1.6–3.7 km below sea level. Ring-fracture-controlled 

collapse of Aniakchak caldera suggests that the rhyodacite 

melt lens underlay approximately the area of the foundered 

cauldron block. Presence of andesite and mixed ignimbrite 

on all sides of the edifice implies that, at the time of eruption, 

the andesite magma either underlay the rhyodacite in a sheet 

or was drawn into the feeder vents from deeper in the system 

as a consequence of eruption of the rhyodacite (fig. 31C). 

The distribution of postcaldera vents suggests the maximum 

dimensions of the cauldron block are 5.8 km northwest–

southeast by 7.7 km northeast–southwest, outlining an area of 

35 km2. If this constrains the lateral dimensions of rhyodacite 

and andesite magma reservoirs, erupted volumes require that 

each was equivalent to an elliptical disk at least ~400 m thick. 

Rapid evisceration of part or all of the liquid-rich portion of 

the magmatic system, foundering of the cauldron block along 

with infall of pyroclasts and landslide megabreccia, and subsequent filling of the caldera lake must have affected physical 

conditions in the upper crustal magmatic system. However, the 

disturbance was not so profound or lasting as to stifle production

of evolved magmas. 

By the time the subaqueous domes were emplaced, 

ongoing recharge with basaltic to andesitic magma had led 

to renewed segregation of silicic magma within the intrusive 

complex beneath Aniakchak volcano and its caldera lake 

(fig. 31D). These dacites and rhyodacites do not appear to 

be remnant Aniakchak II magma. Vulcan and West Domes 

consist of identical magma, distinct from the less evolved 

Bolshoi Dome and more evolved Pumice Dome, yet Vulcan 

was effused in the cluster that includes Bolshoi and Pumice 

Domes and West Dome vented 5.5 km to the southwest, on the 

opposite side of the inferred cauldron block. The Vulcan and 

West Dome compositional uniformity implies either a laterally 

extensive dacite melt lens or, considered more plausible, lateral transport of magma from a smaller reservoir in dikes subparallel to the arc axis. Relatively low-density evolved magma 

within the intrusive complex, and possibly the added load 

imposed by the lake, inhibited throughgoing transport of mafic 

magma, which instead lodged within or beneath the complex.

The dacite–rhyodacite lava of the northwest flank lava 

flow and the compositionally similar lava high in the 1931 

Main Crater west wall apparently were emplaced sometime 

before the Half Cone vent became active. These lavas are virtually identical chemically to Pumice Dome, though slightly 

more evolved. Conceivably, Pumice Dome bears a similar 

relationship to the northwestern flank–1931 Main Crater wall 

dacite–rhyodacites as Vulcan Dome does to West Dome, that 

is, likely fed by lateral dikes emanating from a more centrally 

located reservoir rather than from different pockets of evolved 

magma located directly beneath the vents. A conundrum 

remains as to why a vent on the northwest flank at ~1,800 ft 

(550 m) would produce relatively voluminous lava identical to 

a flow that apparently emanated from a vent at lower elevation 

nearby within the caldera and similar to Pumice Dome. The 

mechanical difficulty of venting lava on the northwest flank 

in preference to within the caldera would be decreased but not 

eliminated by the load imposed by a deep caldera lake. 

Catastrophic draining of the caldera lake and the resulting 

unloading of the magmatic system is thought to have immediately preceded eruption of the three tuff cones. We speculate 

that eruption of basaltic andesite at Breezy and Windy Cones 

on the ring-fracture system and andesite magma within the 

cauldron block at Surprise Cone was enabled by decompression of the magmatic system consequent on draining of the 

lake. Decompression could have caused degassing-induced 

crystallization of locally resident dacite magma, which would 

have allowed mafic–intermediate magma to fracture through 

the intrusive complex, and (or) transient decompressionmelting boosted recharge with mafic magma that fed the 

eruptions (fig. 31E).

The only postcaldera vents that have persisted long 

enough to build composite edifices are Half Cone and Vent 

Mountain, both located on the ring-fracture system. From 

that time forward, magmatic eruptions have been confined to 

the western half of the caldera (fig. 10). Even accounting for 

Half Cone’s ca. 400 yr B.P. Pink and Brown Pumice eruption, 

Vent Mountain appears to have erupted a larger volume of 

magma (ejecta volumes of ≤1.0 km3 versus ≥1.5 km3, respectively). Activity at the two volcanoes has overlapped in time 

and produced similar silicic andesite and dacite, though Half 

Cone may have ejected a greater amount of mafic andesite. Both have erupted dacite of the highTi group, but only 

Half Cone is known to have also produced highTi andesite 

(fig. 24). Still, most Half Cone andesite belongs to the lowTi 

group. The highTi andesite and dacite magmas are thought to 

have evolved by crystallization differentiation from andesite magma with Aniakchak II geochemical characteristics, 

whether stored within the mush column from that time or 

newly arrived. The lowTi andesite and dacite, including the 

Pink Pumice, evolved by a similar process within the mush 

column but from a more typical Aniakchak basalt or basaltic 

andesite parent. The ca. 400 yr B.P. eruption was sufficiently 

intense that andesite of the Brown Pumice was drawn into the 
Discussion 67

conduit and vented, along with hybrid mixtures of Pink and 

Brown Pumice magmas, or, alternatively, recharge with Brown 

Pumice magma triggered eruption of the Pink Pumice magma. 

The Cobweb flow, erupted last from Half Cone, is a mixture 

of these end member magmas. Preeruption magmatic temperatures from Fe–Ti oxide thermometry (Bacon and others, 1997) 

and measurement of dissolved volatiles in melt inclusions 

in phenocrysts in Pink Pumice dacite (Bacon, 2000, 2002) 

indicate storage at shallow depth (fig. 31F). Dacite magma 

partially degassed, losing any CO2 and some sulfur but retaining ~3 weight percent H2

O, before erupting. Assuming vapor 

saturation, as implied by evidence for passive degassing, the 

H2O concentration implies a confining pressure of 60 MPa. 

Depending on choice of roof rock density, this corresponds 

to a depth of 2.3 km (ρ=2,620 kg/m3; Naknek Formation, 

Kienle, 1970) to 2.8 km (ρ=2,200 kg/m3; minimum estimate 

for heterogeneous caldera fill) below the modern caldera 

floor. The zoned ca. 400 yr B.P. Half Cone eruption, along 

with compositional similarity of Half Cone and Vent Mountain products, provides compelling evidence for the existence 

of a range of magma compositions in the intrusive complex 

beneath Aniakchak and for repeated crystallization differentiation of recharge magma. This view is supported by the one 

U-series isotopic analysis of Vent Mountain lava presented by 

George and others (2004), the U excess of which (fig. 29) is 

consistent with recent differentiation. 

Prehistoric eruption of Blocky Cone sometime after the 

ca. 400 yr B.P. Half Cone eruption demonstrates that basaltic 

andesite, among the most primitive of postglacial erupted 

magmas, was supplied to the subcaldera intrusive complex 

recently. Additionally, Blocky Cone shows that recharge 

magma was not trapped by lower density magma or mush 

beneath its feeder conduit at the southwest edge of the ringfracture system (fig. 31F). Dissolved volatile concentrations 

in melt inclusions suggest that olivine phenocrysts crystallized in basaltic andesite of Blocky Cone at a pressure of at 

least 60 MPa, meaning at about the same depth as preeruption storage of Pink Pumice dacite of the ca. 400 yr B.P. Half 

Cone eruption (Bacon, 2002). The basaltic andesite magma 

had passively degassed any CO2 before melt was trapped in 

growing olivine. 

Detailed knowledge of the eruption products and chronology presented by Nicholson and others (2011), supplemented 

by isotopic data from George and others (2004) and chemical 

analyses (appendix B, table B1), provides the basis for a conceptual model of the magmatic system during Aniakchak’s latest eruption in 1931 (fig. 31G). Schematic cross sections that 

illustrate the series of events in 1931 are shown by Nicholson 

and others (2011, their figure 9). Dissolved volatile concentrations in melt inclusions from basal dacite tephra indicate 

passive degassing during preeruptive storage at a pressure of 

110 MPa (Bacon, 2000), or a depth of 4.3 to 5.1 km using the 

same range of densities of overburden as employed above for 

the dacite of Half Cone. The voluminous dacite and andesite 

tephra, units T1 and T2 of Nicholson and others (2011), represent genetically related magmas that had evolved by crystallization differentiation of cryptic basaltic andesite combined 

with assimilation of older material near the western margin of 

the subcaldera intrusive complex. The two magmas mixed to 

form intermediate andesite hybrids during the transition from 

eruption of dacite to andesite. Pressurization and (or) changes 

in the local stress field during the Plinian eruption caused miniscule volumes of resident, largely degassed dacite–rhyodacite 

magma to extrude at Doublet Crater and Slag Heap, perhaps 

thawed at depth by heat given off by accumulating T1–T2 

magma. Sufficient magma was vented as units T1 and T2 

that recharge basaltic andesite–mafic andesite subsequently 

erupted as the small-volume unit T3. Unrelated to the more 

voluminous andesite of unit T2, the recharge magma was 

derived by recent crystallization differentiation of magma with 

geochemical characteristics similar to the parent for Aniakchak II andesite.

Magma Transport and Storage Suggested by 

Earthquakes, Deformation, and Degassing

With the 1931 eruption as a reminder of upper crustal 

storage of evolved magma and recharge with basaltic andesite magma from greater depth, what is the current state of 

the magmatic system beneath Aniakchak volcano? Clues are 

provided by seismic, geodetic, and gas-emission monitoring. 

Earthquake hypocenters plotted in figure 30 give an indication 

of the magmatic plumbing system beneath Aniakchak volcano 

that can be compared with petrologic and geodetic inferences. 

The bow-tie distribution of hypocenters in figure 30A presumably reflects movement of magma, particularly in the mid to 

deep crust, in dikes oriented parallel to the maximum horizontal compressive stress, here tending to be parallel to plate 

convergence (Nakamura and others, 1977). Magma is drawn 

from a substantial region at depth and focused on the established conduit system and mush column beneath the volcano. 

Also shown in figure 30B, in addition to hypocenters, are 

(1) the Mogi point source from a model by Kwoun and others 

(2006) for caldera floor subsidence documented by InSAR 

and (2) minimum storage depths from melt inclusion data for 

vapor-saturated dacite magma of the Pink Pumice and 1931 

eruptions and basaltic andesite of Blocky Cone (Bacon, 2002). 

Implied storage depths below sea level for these magmas are 

in the same range as determined by Larsen (2006) for Aniakchak II rhyodacite (accounting for the load of the precaldera 

edifice). The principal body of long-lived crystal-rich mush 

could be within the seismically quiet zone roughly 7–14 km 

bsl beneath the caldera, from which evolved magma escapes 

upward to be stored ephemerally within as little as ~3 km of 

the surface. It is important to note that Blocky Cone is 2.6 km 

west of the cross section, near the caldera margin, where 

basaltic andesite magma clearly found its way to the surface 

without being trapped in the mush or by more buoyant evolved 

magma. The deep long-period (DLP) events, ~14–28 km bsl, 

may result from input of basalt to basaltic andesite magma, or 

other fluid flow, in the cumulate, largely solid part of the mush 

column and in its surroundings where fractures can be created 

and sustained long enough to produce LP signals. Presence of 

evolved melt, probably as a small fraction within crystal mush, 

in the seismically quiet depth range (~7–14 km bsl) may limit 

fracturing and instead promote intergranular percolation and 

accumulation of magma. 
68 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

The LP event clusters (fig. 30C) may reflect pulses of 

intrusion of magma from greater depth that produce DLP 

events and transient pressure increases within the mush column. Pressure increases in mush could induce fluid pressure 

pulses within the overlying brittle upper crust, resulting in 

fracture propagation or slip on faults implied by the shallow 

earthquakes. The shallowest earthquakes likely are associated with the magma and (or) hydrothermal plumbing system 

beneath Vent Mountain and the caldera ring-fracture system to 

its south.

An interferometric synthetic aperture radar (InSAR) 

geodetic study of Aniakchak for 1992–2002 netted a bull’s-eye 

pattern of subsidence within the caldera with maximum mean 

annual subsidence rate of ~13 mm/yr (Kwoun and others, 

2006). No substantial surface deformation was detected outside of the caldera within a 40-km radius of the caldera center. 

The InSAR results are fit well by the simple Mogi model of 

a point source at 4.2±0.8 km depth and volume change of 

–0.0012±0.000375 km3/yr. Kwoun and others (2006) suggest 

that the cause of the subsidence is related to (1) cooling, crystallization, and (or) degassing of residual magma from recent 

eruptions or magma intruded since 1931 and (or) (2) reduction 

of pore-fluid pressure within a hydrothermal system. Alternatively, perhaps as a corollary to (2), we suggest that subsidence 

also might result from ongoing compaction of Aniakchak II 

intracaldera tuff, which could be most readily compacted in 

the center of the caldera because relatively incompressible 

megabreccia would be concentrated near the caldera margins 

(Lipman, 1997).

The sole gas analysis reported from Aniakchak is for 

CO2-rich gas bubbling through shallow water in what is 

described as a soda spring (25 °C) at the southwestern shore of 

Surprise Lake (Symonds and others, 2003a). This gas sample 

has a 3He/4He ratio (R/RA ≈ 8) and δ13C (–6.1 per mil) value 

that indicate shallow degassing of magma. The anhydrous gas 

composition is consistent with dominance of mantle-derived 

magmatic gas. Understandably, this gas sample has been 

affected by contact with air-saturated water. It is interpreted 

as high-temperature magmatic gas that has been scrubbed in 

the upper crustal hydrothermal system at low gas/water ratio 

with simultaneous water-rock reaction (Symonds and others, 

2003b). Regardless of caveats, the composition of the Surprise 

Lake gas sample provides compelling evidence for ongoing 

degassing of subsurface magma.

Hazards

The data and analysis presented here augment the existing volcano hazard assessment for Aniakchak (Neal and 

others, 2001) by providing a more detailed, geochemically 

informed conceptual framework for the magmatic system and 

the nature of its likely future activity. Aniakchak will erupt 

again and could do so on short notice at any time. Because of 

the locations of recent vents, eruption would be more likely 

in the southwestern half of the caldera than in the northeastern half, and eruption is very unlikely outside of the caldera. 

Eruptions could be explosive, either because of interaction 

with groundwater or because they involved gas-rich magma. 

Apparent quiescence of Half Cone and possibly Vent Mountain since ca. 400 yr B.P. and the venting of recently differentiated andesite followed by a different basaltic andesite in 

1931 suggest that future eruptions will feature new batches of 

magma. Seismicity, the 3He/4He ratio of gas, presence of weak 

thermal areas, and deformation revealed by InSAR all suggest 

that the magmatic system—the mush column—is active. 

Because Half Cone and Vent Mountain have been quiet for a 

century or more and the 1931 eruption apparently exhausted 

its own stored dacite magma, a future silicic eruption likely 

would involve newly differentiated magma or resident magma 

rejuvenated by mafic-to-intermediate recharge. An event 

comparable to that of 1931 would not be a surprise, nor would 

a Strombolian to Vulcanian eruption of basaltic andesite to 

mafic andesite from a ring-fracture vent, such as occurred 

at Blocky Cone. An explosive hydrovolcanic eruption could 

eject lithic debris, with or without juvenile magma, virtually 

anywhere within the caldera if a propagating dike ascended 

close to the surface. Eruption of partially degassed andesite to 

dacite magma, such as occurred at Vent Mountain, is possible, 

especially if encouraged by mafic recharge. Another calderaforming eruption of dacite–rhyodacite magma is highly 

unlikely because of the varied geochemical nature and small 

volume of 400-yr-B.P. and 1931 dacites and the fact that they 

were followed by andesite. However, should a large volume 

of volatile-rich andesite magma accumulate in the mush, a 

voluminous explosive eruption, such as enlarged or formed 

the caldera at Veniaminof volcano (Miller and Smith, 1987; 

Bacon and others, 2007), might be possible. The subsidence 

documented by InSAR appears inconsistent with this possibility for present-day Aniakchak. Determining the probabilities 

of specific types of eruption awaits more detailed chronology 

of postcaldera eruptions through application of direct methods 

for dating features within the caldera and through further study 

of tephra from Aniakchak.

Conclusions

Pleistocene Aniakchak was a composite volcano that 

grew over a period of several hundred thousand years atop 

Jurassic–Tertiary sedimentary rocks by effusion of basaltic 

andesite to dacite lava. The Aniakchak edifice was smaller 

than those of other volcanoes with ~10 km diameter calderas 

in the eastern Aleutian arc, and it was repeatedly sculpted by 

ice during glacial intervals so that deep valleys were present 

at the end of the last glaciation. Basaltic magmas, with a range 

of incompatible element concentrations characteristic of arcs, 

were drawn from the mantle wedge into the deep crust beneath 

Aniakchak, where they were modified through crystallization differentiation and interaction with their surroundings in 

a deep crustal hot zone. Fractionated magmas that escaped 

upward further differentiated and partially to completely 

solidified to gradually build an intrusive complex, or mush 

column, in the mid to upper crust that was well developed by 

the end of the last glaciation. Postglacial Aniakchak magmas 

have traversed or been extracted from the mush column, and 
Conclusions 69

all show some degree of fractionation in comparison with 

primary melts of the mantle.

The earliest recognized products of postglacial Aniakchak 

are andesite ignimbrite, Aniakchak I, present near the modern 

caldera rim and beneath younger deposits in peripheral valleys. Aniakchak I andesite has distinctly high concentrations 

of incompatible elements, notably Zr, and differentiated 

from a parent that originated as a low-degree partial melt of 

the mantle. This ignimbrite may be sufficiently voluminous 

that a small caldera could have formed during the eruption 

ca. 9,500–7,000 yr B.P., when glacial ice evidently reached as 

low as 560 m elevation in Birthday Creek valley. The dacitic 

Black Nose Pumice, which followed Aniakchak I, comprises 

extensive Plinian fall deposits and minor lava flows and 

welded ignimbrite that are present along the eastern half of the 

modern caldera rim. Correlation with a distal pumice bed that 

lies on carbonaceous material indicates an age of ca. 7,000 yr B.P.

for the Plinian eruption. The dacite probably was derived by 

crystallization differentiation of Aniakchak I andesite magma 

lodged within the upper mush column. 

The 3,590–3,588±11 cal yr B.P. Aniakchak II catastrophic 

eruption of rhyodacite followed by andesite left rhyodacite 

Plinian fall deposits dispersed along a northerly axis and 

compositionally zoned ignimbrite in valleys surrounding the 

edifice. The modern caldera collapsed during the ignimbriteproducing phase of this eruption. Aniakchak II rhyodacite 

accumulated in the upper reaches of the mush column and 

evolved by crystallization differentiation, but not from the coerupted andesite or the precaldera Black Nose Pumice dacite. 

The distinctive composition of Aniakchak II andesite, such as 

its exceptionally high P2O5 content, suggests that it evolved 

from a parent derived from relatively fertile mantle. Both 

Aniakchak I and II andesites have close analogs at Veniaminof 

volcano 100 km down the Alaska Peninsula. Aniakchak II 

rhyodacite and andesite indicate a vigorous new cycle of 

recharge, storage, and differentiation in a mush column that 

had become well established through thermal conditioning 

owing to subsurface crystallization of magma during the prior 

history of the volcano.

Subsequent to the Aniakchak II eruption, a lake accumulated in the caldera and four dacite–rhyodacite domes were 

extruded on its floor above the ring-fracture system. Apparently during the same period, dacite–rhyodacite lava issued 

from vents near the base of the west caldera wall and immediately beyond the caldera rim, the latter feeding a 3-km-long 

flank flow. Similarity of dome and lava compositions on opposite sides of the caldera suggests lateral transport of magma 

approximately parallel to the arc axis. The dacites–rhyodacites 

reflect differentiation of post-Aniakchak II recharge magma. 

After the lake drained catastrophically ca. 1,860 yr B.P., 

cutting The Gates through the caldera wall, the character of 

volcanism changed to hydromagmatic eruptions that built 

three tuff cones of basaltic andesite and andesite on the east 

caldera floor, including at Breezy Cone the least differentiated 

of any postglacial magma. This escape of basaltic andesite 

along the ring fracture system and of andesite inboard is 

thought to be linked to sudden unloading of the magmatic 

system by ~200 m lowering of the lake level.

After the eruption of the tuff cones, Half Cone on the 

northwest and Vent Mountain on the south ring-fracture system erupted broadly similar andesite–dacite, alternating their 

activity. Minor geochemical differences distinguish products 

of the two volcanoes, though there also is compositional overlap. Crystallization differentiation yielded nearby or contiguous andesite–dacite reservoirs in the mush column, as shallow 

as 1.9–2.4 km bsl (2.3–2.8 km below the caldera floor). In the 

ca. 400 yr B.P. Plinian eruption from Half Cone, evolved Pink 

Pumice dacite was followed by Brown Pumice andesite in a 

recent prehistoric example of a zoned eruption. The final Half 

Cone eruptive, the Cobweb flow, is a mixture of those magmas 

and crystal mush. Younger than Half Cone and most Vent 

Mountain lava, late-prehistoric Blocky Cone vented basaltic 

andesite nearly as primitive as that of Breezy Cone. Blocky 

Cone magma skirted the mush by ascending at the southwest 

edge of the ring-fracture system, though partially degassing 

and crystallizing olivine at ~2 km bsl. 

The May–June 1931 subplinian eruption, like 400 yr B.P. 

Half Cone, produced dacite and andesite tephra whose compositions can be related by crystallization differentiation. Mafic 

andesite and basaltic andesite ejected and effused later in the 

sequence are related to each other, but not to the principal volume of zoned tephra. Magma of the initial dacite tephra was 

stored at a minimum depth of 3.8–4.6 km bsl. Uranium-series 

isotopic data suggest that the small volumes of dacite and 

rhyodacite lava effused from Doublet crater and at Slag Heap 

are older material remobilized in 1931. These isotopic data 

also suggest that basaltic andesite–mafic andesite differentiation was recent and that dacite magma contains a substantial 

component that may be precaldera in age. Postcaldera silicic 

magmas, except possibly Slag Heap, evidently did not include 

large fractions of nonerupted Aniakchak II rhyodacite. In contrast, some mafic–intermediate magmas, such as late-erupted 

1931 basaltic andesite, have elemental relative-concentration 

patterns that suggest derivation from parents similar to that for 

Aniakchak II andesite.

The Aniakchak magmatic system continues to be active, 

as evidenced by escaping magmatic gas, caldera subsidence 

revealed by InSAR, and episodic bursts of low-frequency 

earthquakes. Seismicity is consistent with the concept of a 

deep crustal hot zone of basaltic magma injection that focuses 

upward as melt-bearing and solidified intrusions—the mush 

column—beneath the caldera. Events at 14–28 km bsl may 

result from injection of magma into the cumulate, largely solid 

part of the mush column and its surroundings. A seismically 

quiet zone at 7–14 km bsl may correspond to a region in which 

a fraction of melt limits fracturing and promotes percolation 

and accumulation of magma in long-lived crystal–liquid 

mush, from which magma batches escape upward to be stored 

ephemerally at depths as shallow as ~2 km bsl. These observations indicate that Aniakchak is likely to erupt again, most 

likely within the caldera and after a period of unrest of uncertain duration. Small explosive or effusive eruptions and hydrovolcanic explosions, to subplinian or Plinian eruptions like 

the 1931 and 400 yr B.P. events are possible, whereas another 

caldera-forming eruption is considered highly unlikely.
70 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Acknowledgments

The information and data synthesized in this paper were 

collected by many scientists, technical experts, and students 

associated with the Alaska Volcano Observatory (AVO) and 

supported by the Volcano Hazards Program of the U.S. Geological Survey. We particularly appreciate the theses and publications that resulted from graduate research at the University 

of Alaska Fairbanks by Scott Dreher, Robert Nicholson, 

and Brandon Browne. The commitment of the GeoAnalytical Laboratory at Washington State University to delivering 

precise, accurate chemical analyses of rocks over many years 

of service to AVO enabled the geochemical correlations and 

process inferences that we have made. Kristi Wallace, Chad 

Hults, and Rich VanderHoek provided samples, and Leslie 

Hayden performed electron microprobe analyses, for correlation of Black Nose Pumice with distal tephra. We thank Wes 

Hildreth and Michelle Coombs for technical reviews that led 

to clearer presentation of data and interpretations. Our work at 

Aniakchak started with the reconnaissance of the Alaska Peninsula volcanoes funded by the Geothermal Research Program 

of the U.S. Geological Survey and conducted by Thomas P. 

Miller and Robert L. Smith in the 1970s. We are particularly 

indebted to Smith for guidance therein and for emphasizing the 

geologic significance and natural heritage of Aniakchak.

References Cited

Allen, S.R., and McPhie, J., 2000, Water-settling and resedimentation of submarine rhyolitic pumice at Yali, eastern 

Aegean, Greece: Journal of Volcanology and Geothermal 

Research, v. 95, p. 285–307.

Anderson, A.T., 1976, Magma mixing—Petrological process 

and volcanological tool: Journal of Volcanology and Geothermal Research, v. 1, p. 3–33.

Annen, C., Blundy, J.D., and Sparks, R.S.J., 2006, The genesis 

of intermediate and silicic magmas in deep crustal hot 

zones: Journal of Petrology, v. 47, p. 505–539.

Bacon, C.R., 1983, Eruptive history of Mount Mazama and 

Crater Lake caldera, Cascade Range, U.S.A.: Journal of 

Volcanology and Geothermal Research, v. 18, p. 57–115.

Bacon, C.R., 2000, Preeruptive volatiles in the most recent 

eruptions of Aniakchak volcano, Alaska [abs.]: Eos 

(American Geophysical Union Transactions), v. 81, no. 48, 

p. 1376–1377.

Bacon, C.R., 2002, Depths of magma reservoirs inferred from 

preeruptive dissolved volatiles in the most recent postcaldera eruptions of Aniakchak volcano, Alaska [abs.]: Eos 

(American Geophysical Union Transactions), v. 83, no. 19, 

p. S378.

Bacon, C.R., and Hirschmann, M.M., 1988, Mg/Mn partitioning as a test for equilibrium between coexisting Fe-Ti 

oxides: American Mineralogist, v. 73, p. 57–61.

Bacon, C.R., Neal, C.A., Nye, C.J., and McGimsey, R.G., 

1997, Pre-eruptive temperatures for postcaldera magmas of 

Aniakchak Volcano, Alaska [abs.]: Eos (American Geophysical Union Transactions), v. 78, no. 46, p. 792–793.

Bacon, C.R., Sisson, T.W., and Mazdab, F.K., 2007, Young 

cumulate complex beneath Veniaminof caldera, Aleutian 

arc, dated by zircon in erupted cumulate blocks: Geology, 

v. 35, p. 491–494.

Begét, J., Mason, O., and Anderson, P., 1992, Age, extent, and 

climatic significance of the ca. 3400 BP Aniakchak tephra, 

western Alaska, U.S.A.: The Holocene, v. 2, no. 1, p. 51–56.

Browne, B., 2006, Investigating magma withdrawal dynamics 

during plinian eruptions through mineral and eruptive stratigraphies; Examples from the chemically zoned 400 yr BP 

eruption of Half Cone volcano, Aniakchak National Park, 

Alaska [abs]: Eos (American Geophysical Union Transactions), v. 87, no. 52, Fall Meeting Supplement, Abstract 

V53E-05.

Buurman, H., Nye, C.J., West, M.E., and Cameron, C., 2014, 

Regional controls on volcano seismicity along the Aleutian 

arc: Geochemistry, Geophysics, Geosystems, v. 15, 

p. 1147–1163.

Calvert, A.J., and McGeary, S.E., 2013, Seismic reflection 

imaging of ultradeep roots beneath the eastern Aleutian 

island arc: Geology, v. 41, p. 203–206.

Cas, R.A.F., and Wright, J.V., 1987, Volcanic successions: 

London, Allen and Unwin, 528 p.

Coulter, S.E., Pilcher, J.R., Plunkett, G., Baillie, M., Hall, 

V.A., Steffensen, J.P., Vinther, B.M., Clausen, H.B., and 

Johnson, S.J., 2012, Holocene tephras highlight complexity 

of volcanic signals in Greenland ice cores: Journal of Geophysical Research, v. 117, doi:10.1029/2012JD017698.

DeMets, C., Gordon, R.G., and Argus, D.F., 2010, Geologically current plate motions: Geophysical Journal International, v. 181, p. 1–80.
References Cited 71

Denton, J.S., and Pearce, N.J., 2006, Confirming the 1645±4 

BC date for the 2nd millennium BC caldera forming eruption 

of Aniakchak [abs.]: Eos (American Geophysical Union 

Transactions), v. 87, no. 52, Abstract V33B-0669.

DePaolo, D.J., 1981, Trace element and isotopic effects of 

combined wallrock assimilation and fractional crystallization: Earth and Planetary Science Letters, v. 53, p. 189–202.

Detterman, R.L., Miller, T.P., Yount, M.E., and Wilson, F.H., 

1981a, Geologic map of the Chignik and Sutwik Island 

Quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1229, scale 1:250,000.

Detterman, R.L., Miller, T.P., Yount, M.E., and Wilson, F.H., 

1981b, Quaternary geologic map of the Chignik and 

Sutwik Island Quadrangles, Alaska: U.S. Geological 

Survey Miscellaneous Investigations Series Map I-1292, 

scale 1:250,000.

Dixon, J.P., Stihler, S.D., Power, J.A., and Searcy, C.K., 2010, 

Catalog of earthquake hypocenters at Alaskan volcanoes—

January 1 through December 31, 2009: U.S. Geological 

Survey Data Series 531, 84 p., available at http://pubs.usgs.

gov/ds/531/.

Dixon, J.P., Stihler, S.D., Power, J.A., and Searcy, C.K., 2011, 

Catalog of earthquake hypocenters at Alaskan volcanoes—

January 1 through December 31, 2010: U.S. Geological 

Survey Data Series 645, 82 p., available at http://pubs.usgs.

gov/ds/645/.

Dixon, J.P., Stihler, S.D., Power, J.A., and Searcy, C.K., 2012, 

Catalog of earthquake hypocenters at Alaskan volcanoes—

January 1 through December 31, 2011: U.S. Geological 

Survey Data Series 730, 82 p., available at http://pubs.usgs.

gov/ds/730/.

Dixon, J.P., Stihler, S.D, Power, J.A., Haney, M., Parker, T., 

Searcy, C.K., and Prejean, S., 2013, Catalog of earthquake 

hypocenters at Alaskan volcanoes—January 1 through 

December 31, 2012: U.S. Geological Survey Data Series 

789, 84 p., available at http://pubs.usgs.gov/ds/789/.

Dreher, S.T., 2002, The physical volcanology and petrology 

of the 3400 yBP caldera-forming eruption of Aniakchak 

volcano, Alaska: Fairbanks, University of Alaska Fairbanks, 

Ph.D. dissertation, 174 p.

Dreher, S.T., Eichelberger, J.C., and Larsen, J.F., 2005, The 

petrology and geochemistry of the Aniakchak calderaforming ignimbrite, Aleutian arc, Alaska: Journal of 

Petrology, v. 46, p. 1747–1768.

Druitt, T.H., and Bacon, C.R., 1986, Lithic breccia and ignimbrite erupted during the collapse of Crater Lake caldera, 

Oregon: Journal of Volcanology and Geothermal Research, 

v. 29, p. 1–32.

Dungan, M.A., and Davidson, J., 2004, Partial assimilative 

recycling of the mafic plutonic roots of arc volcanoes—

An example from the Chilean Andes: Geology, v. 32, 

p. 773–776.

Fliedner, M.M., and Klemperer, S.L., 2000, Crustal structure transition from oceanic arc to continental arc, eastern 

Aleutian Islands and Alaska Peninsula: Earth and Planetary 

Science Letters, v. 179, p. 567–579.

Fujimaki, H., 1986, Partition coefficients of Hf, Zr, and REE 

between zircon, apatite, and liquid: Contributions to Mineralogy and Petrology, v. 94, p. 42–45.

George, R., Turner, S., Hawkesworth, C., Bacon, C.R., Nye, 

C., Stelling, P., and Dreher, S., 2004, Chemical versus 

temporal controls on the evolution of tholeiitic and calcalkaline magmas at two volcanoes in the Alaska—Aleutian 

arc: Journal of Petrology, v. 45, p. 203–219.

Ghiorso, M.S., and Evans, B.W., 2008, Thermodynamics of 

rhombohedral oxide solid solutions and a revision of the 

Fe-Ti two-oxide geothermometer and oxygen-barometer: 

American Journal of Science, v. 308, p. 957–1039.

Gill, J.B., 1981, Orogenic andesites and plate tectonics: Berlin, 

Springer-Verlag, 390 p.

Gill, J.B., and Williams, R.W., 1990, Th isotope and U-series 

studies of subduction-related volcanic rocks: Geochimica et 

Cosmochimica Acta, v. 54, p. 1427–1442.

Goto, Y., and McPhie, J., 1998, Endogenous growth of a 

Miocene submarine dacite cryptodome, Rebun Island, 

Hokkaido, Japan: Journal of Volcanology and Geothermal 

Research, v. 84, p. 273–286.

Hart, S.R., 1984, A large-scale isotope anomaly in the Southern Hemisphere mantle: Nature, v. 309, p. 753–757.

Hildreth, W., and Moorbath, S., 1988, Crustal contributions to 

arc magmatism in the Andes of central Chile: Contributions 

to Mineralogy and Petrology, v. 98, p. 455–489.

Hubbard, B.R., 1932, Mush, you malemutes!: New York, 

America Press, 179 p.

Kamata, H., Suzuki-Kamata, K., and Bacon, C.R., 1993, 

Deformation of the Wineglass Welded Tuff and the timing

of caldera collapse at Crater Lake, Oregon: Journal of 

Volcanology and Geothermal Research, v. 56, p. 253–265.
72 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Kaufmann, D., and Hopkins, D., 1985, Late Cenozoic radiometric dates, Seward and Baldwin Peninsulas, and adjacent 

continental shelf, Alaska: U.S. Geological Survey Open-File 

Report 85–374, 27 p.

Kienle, J., 1970, Gravity traverses in the Valley of Ten Thousand Smokes, Katmai National Monument, Alaska: Journal 

of Geophysical Research, v. 75, p. 6641–6649.

Kwoun, O.-I., Lu, Z., Neal, C., and Wicks, C., Jr., 2006, 

Quiescent deformation of the Aniakchak Caldera, Alaska, 

mapped by InSAR: Geology, v. 34, p. 5–8.

Langmuir, C.H., Vocke, R.D., Jr., Hanson, G.N., and Hart, 

S.R., 1978, A general mixing equation with applications to 

Icelandic basalts: Earth and Planetary Science Letters, v. 37, 

p. 380–392.

Langmuir, C.H., Klein, E.M., and Plank, T., 1992, Petrological systematics of mid-ocean ridge basalts; constraints on 

melt generation beneath ocean ridges, in Phipps Morgan, 

J., Blackman, D.K., and Sinton, J.M., eds., Mantle flow and 

melt generation at mid-ocean ridges: American Geophysical 

Union Geophysical Monograph 71, p. 183-280.

Larsen, J.F., 2006, Rhyodacite magma storage conditions prior 

to the 3430 yBP caldera-forming eruption of Aniakchak 

volcano, Alaska: Contributions to Mineralogy and Petrology, v. 152, p. 523–540.

Lescinsky, D.T., and Fink, J.H., 2000, Lava and ice interaction at stratovolcanoes—Use of characteristic features to 

determine past glacial extents and future volcanic hazards: 

Journal of Geophysical Research, v. 105, p. 23711–23726.

Lipman, P.W., 1997, Subsidence of ash-flow calderas—Relation to caldera size and magma-chamber geometry: Bulletin 

of Volcanology, v. 59, p. 198–218.

Mangan, M., Miller, T., Waythomas, C., Trusdell, F., Calvert, 

A., and Layer, P., 2009, Diverse lavas from closely spaced 

volcanoes drawing from a common parent—Emmons Lake 

volcanic center, Eastern Aleutian arc: Earth and Planetary 

Science Letters, v. 287, p. 363–372.

McDonough, W.F., and Sun, S.-s., 1995, The composition of 

the Earth: Chemical Geology, v. 120, p. 223–253.

McGimsey, R.G., Waythomas, C.F., and Neal, C.A., 1994, 

High stand and catastrophic draining of intracaldera 

Surprise Lake, Aniakchak Volcano, Alaska, in Till, A.B., 

and Moore, T.E., eds., Geologic studies in Alaska by the 

U.S. Geological Survey, 1993: U.S. Geological Survey 

Bulletin 2107, p. 59–71.

McGimsey, R.G., Neal, C.A., Adleman, J.N., Larsen, J.F., 

and Ramsey, M.S., 2003, Black Peak caldera, Alaska—

Preliminary investigations of the ~4600 BP caldera-forming 

eruption and subsequent post-caldera activity [abs.]: Eos 

(American Geophysical Union Transactions), v. 84, no. 46, 

Fall Meeting Supplement, Abstract V42B-0342.

Mellors, R.A., and Sparks, R.S.J., 1991, Spatter-rich 

pyroclastic flow deposits on Santorini, Greece: Bulletin of 

Volcanology, v. 53, p. 327–342.

Miller, T.P., and Smith, R.L., 1977, Spectacular mobility of 

ash flows around Aniakchak and Fisher calderas, Alaska: 

Geology, v. 5, p. 173–178.

Miller, T.P., and Smith, R.L., 1987, Late Quaternary calderaforming eruptions in the eastern Aleutian arc, Alaska: 

Geology, v. 15, p. 434–438.

Miyashiro, A., 1974, Volcanic rock series in island arcs and 

active continental margins: American Journal of Science, 

v. 274, p. 321–355.

Nakada, S., Bacon, C.R., and Gartner, A.E., 1994, Origin of 

phenocrysts and compositional diversity in pre-Mazama 

rhyodacite lavas, Crater Lake, Oregon: Journal of Petrology, 

v. 35, p. 127–162.

Nakamura, K., Jacob, K.H., and Davies, J.N., 1977, Volcanoes 

as possible indicators of tectonic stress orientation—Aleutians and Alaska: Pure and Applied Geophysics, v. 115, 

p. 87–112.

Nathenson, M., Bacon, C.R., and Ramsey, D.W., 2007, 

Subaqueous geology and a filling model for Crater Lake, 

Oregon: Hydrobiologia, v. 574, p. 12–27.

Neal, C.A., McGimsey, R.G., Miller, T.P., Riehle, J.R., and 

Waythomas, C.F., 2001, Preliminary volcano-hazard assessment for Aniakchak volcano, Alaska: U.S. Geological 

Survey Open-File Report 00-519, 35 p.

Nicholson, R.S., Gardner, J.E., and Neal, C.A., 2011, Variations in eruption style during the 1931 A.D. eruption of 

Aniakchak volcano, Alaska: Journal of Volcanology and 

Geothermal Research, v. 207, p. 69–82.

Nye, C.J., Neal, C.A., and McGimsey, R.G., 1993, Extreme 

and abrupt transition from tholeiitic to calcalkaline volcanism at Aniakchak volcano, eastern Aleutian arc [abs.]: Eos 

(American Geophysical Union Transactions), v. 74, no. 43, 

p. 674.
References Cited 73

Pearce, N.J.G., Westgate, J.A., Preece, S.J., Eastwood, 

W.J., and Perkins, W.T., 2004, Identification of Aniakchak (Alaska) tephra in Greenland ice core challenges 

the 1645 BC date for Minoan eruption of Santorini: 

Geochemistry, Geophysics, Geosystems, v. 5, no. 3, 

doi:10.1029/2003GC000672.

Plank, T., and Langmuir, C.H., 1998, The chemical composition of subducting sediment and its consequences for the 

crust and mantle: Chemical Geology, v. 145, p. 325–394.

Power, J.A., Stihler, S.D., White, R.A., and Moran, S.C., 2004, 

Observations of deep long-period (DLP) seismic events 

beneath Aleutian arc volcanoes; 1989–2002: Journal of Volcanology and Geothermal Research, v. 138, p. 243–266.

Power, J.A., Stihler, S.D., Ketner, D.M., Haney, M.M., 

Prejean, S.G., and Parker, T.J., 2013, Lower-crustal and 

upper-mantle seismicity beneath Aleutian arc volcanoes—A 

temporal link for magmatic processes between the lowercrust and the surface [abs.]: Abstract V13B-2615 presented 

at 2013 Fall Meeting, American Geophysical Union, San 

Francisco, California, 9–13 December.

Reimer, P.J., Bard, E., Bayliss, A., and 27 others, 2013, 

IntCall13 and Marine13 radiocarbon age calibration 

curves 0–50000 years cal BP: Radiocarbon, v. 55, no. 4, 

p. 1869–1887.

Riehle, J.R., Meyer, C.E., Ager, T.A., Kaufman, D.S., and 

Ackerman, R.E., 1987, The Aniakchak tephra deposit, a late 

Holocene marker horizon in western Alaska, in Hamilton, 

T.D., and Galloway, J.P., eds., Geologic studies in Alaska by 

the U.S. Geological Survey during 1986: U.S. Geological 

Survey Circular 998, p. 19–22.

Riehle, J.R., Meyer, C.E., and Miyoka, R.T., 1999, Data on 

Holocene tephra (volcanic ash) deposits in the Alaska Peninsula and Lower Cook inlet of the Aleutian volcanic arc, 

Alaska: U.S. Geological Survey Open-File Report 99–135, 

unpaginated.

Ryan, H.F., von Huene, R., Wells, R.E., Scholl, D.W., Kirby, 

S., and Draut, A.E., 2012, History of earthquakes and 

tsunamis along the eastern Aleutian-Alaska megathrust, 

with implications for tsunami hazards in the California Continental Borderland, in Dumoulin, J.A., and Dusel-Bacon, 

C., eds., Studies by the U.S. Geological Survey in Alaska, 

2011: U.S. Geological Survey Professional Paper 1795–A, 

31 p., http://pubs.usgs.gov/pp/1795/a.

Sisson, T.W., 1991, Pyroxene-high silica rhyolite trace element 

partition coefficients measured by ion microprobe: Geochimica et Cosmochimica Acta, v. 55, p. 1575–1585.

Sisson, T.W., 1994, Hornblende-melt trace element partition 

coefficients measured by ion-microprobe: Chemical 

Geology, v. 117, p. 331–344.

Solano, J.M.S., Jackson, M.D., Sparks, R.S.J., Blundy, J.D., 

and Annen, C., 2012, Melt segregation in deep crustal hot 

zones—A mechanism for chemical differentiation, crustal 

assimilation and the formation of evolved magmas: Journal 

of Petrology, v. 53, p. 1999–2026.

Spörli, K.B., and Rowland, J.V., 2006, ‘Column on column’ 

structures as indicators of lava/ice interaction, Ruapehu 

andesite volcano, New Zealand: Journal of Volcanology and 

Geothermal Research, v. 157, p. 294–310.

Stormer, J.C., Jr., 1983, The effects of recalculation on estimates of temperature and oxygen fugacity from analyses of 

multicomponent iron-titanium oxides: American Mineralogist, v. 68, p. 586–594. 

Stuiver, M., and Reimer, P.J., 1993, Extended 14C data base 

and revised CALIB 3.0 14C age calibration program: Radiocarbon, v. 35, p. 215–230.

Symonds, R.B., Poreda, R.J., Evans, W.C., Janik, C.J., and 

Ritchie, B.E., 2003a, Mantle and crustal sources of carbon, 

nitrogen, and noble gases in Cascade-Range and AleutianArc volcanic gases: U.S. Geological Survey Open-File 

Report 03-436, 26 p.

Symonds, R.B., Janik, C.J., Evans, W.C., Ritchie, B.E., 

Counce, D., Poreda, R.J., and Iven, M., 2003b, Scrubbing 

masks magmatic degassing during repose at Cascade-Range 

and Aleutian-Arc volcanoes: U.S. Geological Survey OpenFile Report 03-435, 22 p.

U.S. Geological Survey, 1963, Chignik (D–1), Alaska: U.S. 

Geological Survey topographic map, scale 1:63,360.

VanderHoek, R., 2009, The role of ecological barriers in 

the development of cultural boundaries during the later 

Holocene of the central Alaska Peninsula: Urbana, Illinois, 

University of Illinois at Urbana-Champaign, Ph.D. dissertation, 413 p.

VanderHoek, R., and Myron, R., 2004, An archaeological 

overview and assessment of Aniakchak National Monument and Preserve: U.S. National Park Service Research/

Resources Management Report AR/CRR-2004-47.

Watson, E.B., and Green, T.H., 1981, Apatite/liquid partition 

coefficients for the rare earth elements and strontium: Earth 

and Planetary Science Letters, v. 56, p. 405–421.

Waythomas, C.F., Walder, J.S., McGimsey, R.G., and Neal, 

C.A., 1996, A catastrophic flood caused by drainage of a 

caldera lake at Aniakchak Volcano, Alaska, and implications 

for volcanic hazards assessment: Geological Society of 

America Bulletin, v. 108, p. 861–871.

Wohletz, K.H., and Sheridan, M.F., 1983, Hydrovolcanic 

explosions II—Evolution of basaltic tuff rings and tuff 

cones: American Journal of Science, v. 283, p. 385–413.
74 Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula

Menlo Park, California, and Raleigh, North Carolina, 

Publishing Service Centers

Manuscript approved for publication November 29, 2014

Edited by Peter H. Stauffer

Layout and design by James E. Banton

Appendix A. Electron Microprobe Analyses

Appendix B. Whole-Rock Chemical Analyses

This appendix consists of two Excel tables available at http://pubs.usgs.gov/pp/1810.

Table A1. Electron microprobe analyses of glass from lower Black Nose Pumice and from tephra at Cabin Bluff.

Table A2. Electron microprobe analyses of iron-titanium oxide crystals from lower Black Nose Pumice and from 

 tephra at Cabin Bluff.

This appendix consists of two Excel tables available at http://pubs.usgs.gov/pp/1810.

Table B1. Chemical analyses of rock specimens from Aniakchak volcano and vicinity.

Table B2. Chemical analyses of rock specimens from Aniakchak and Veniaminof volcanoes.

Bacon and others—Postglacial Eruptive History, Geochemistry, and Recent Seismicity of Aniakchak Volcano, Alaska Peninsula—Professional Paper 1810

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

http://dx.doi.org/10.3133/pp1810