National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Aniakchak National Monument and Preserve

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/1033
SW maar

NE maar

New Cone

The Gates

Slag Heap

Half Cone

West Dome

Black

Nose

Windy

Cone

Pumice Dome

Vulcan Dome

Blocky Cone

1931 Main Crater

Bolshoi Dome Surprise

 Lake

Birthday Pass

Vent Mountain

Surprise Cone

Aniakchak

Peak

Doublet Crater

Aniakchak

River

Cobweb lava flow

warm

springs

warm spring

The

Garden

Wall

Plenty

Bear

Creek

Breezy

Cone

158°5'W

158°10'W

56°55'N

56°50'N

0 0.5 1 1.5 2

Miles

0 0.5 1 1.5 2

Kilometers

North

ON THE COVER: Aerial photograph of Aniakchak Crater in early spring. View is to the northeast with Birthday Pass in 

the foreground, Vent Mountain is the highest cone in the caldera on the right, the 1931 main crater is the second largest cone near the caldera wall on the left, and The Gates are in the background and left of the prominent peak known 

as Black Nose. NPS photograph by Roy Wood (10 March 2011).

THIS PAGE: Satellite image mosaic (IKONOS 2004 and 2005) of the Aniakchak Crater with formal and informal names 

of geographic and volcanic features.
Aniakchak National Monument and Preserve

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/1033

Chad Hults 

National Park Service

Alaska Regional Office 

240 W. 5th Ave.

Anchorage, AK 99501

Christina A. Neal 

US Geological Survey

Alaska Volcano Observatory

4210 University Drive

Anchorage, AK 99508

September 2015

U.S. Department of the Interior 

National Park Service 

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, Colorado, publishes a 

range of reports that address natural resource topics. These reports are of interest and applicability to a broad audience 

in the National Park Service and others in natural resource management, including scientists, conservation and 

environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate comprehensive information and analysis about natural 

resources and related topics concerning lands managed by the National Park Service. The series supports the 

advancement of science, informed decision-making, and the achievement of the National Park Service mission. The 

series also provides a forum for presenting more lengthy results that may not be accepted by publications with page 

limitations.

All manuscripts in the series receive the appropriate level of peer review to ensure that the information is scientifically 

credible, technically accurate, appropriately written for the intended audience, and designed and published in a 

professional manner. This report received informal peer review by subject-matter experts who were not directly 

involved in the collection, analysis, or reporting of the data.

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and 

policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products 

does not constitute endorsement or recommendation for use by the U.S. Government. 

This report is available from the Geologic Resources Inventory website (http://go.nps.gov/gripubs), and the Natural 

Resource Publications Management website (http://www.nature.nps.gov/publications/nrpm/). To receive this report in 

a format optimized for screen readers, please email irma@nps.gov.

Please cite this publication as:

Hults, C., and C. A. Neal. 2015. Aniakchak National Monument and Preserve: Geologic resources inventory report. 

Natural Resource Report NPS/NRSS/GRD/NRR—2015/1033. National Park Service, Fort Collins, Colorado.

NPS 181/129749, September 2015
iii

Contents

Page

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Plates (in pocket) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Products and Acknowledgments . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

GRI Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Geologic Setting and Significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Geologic Features and Processes . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Volcanic Eruptions and their Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Outburst Floods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Geothermal Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Nearby Holocene Volcanoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Older Bedrock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Landforms and Surficial Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Geologic History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Jurassic Volcanic Arc and Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Cretaceous Sedimentary Rocks and Unconformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Terrane Translations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Tertiary Sedimentary Rocks and Initiation of a New Volcanic Arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Quaternary Volcanism and Glaciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Geologic Resource Management Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Geohazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Paleontological Resource Inventory, Monitoring, and Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Geothermal Energy Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Petroleum Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Mineral Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Coastal Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Glacier Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Crater Lake Level and Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Potential Future Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Geologic Map Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Geologic Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Source Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

GRI GIS Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

GRI Map Posters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Map Unit Properties Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Use Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Additional References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Geology of Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Geology of National Park Service Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

NPS Resource Management Guidance and Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Climate Change Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Geological Surveys and Societies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

US Geological Survey Reference Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
iv

Contents (continued)

Page

Appendix A: Geologic Exploration History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Appendix B: Administrative History Related to Geology . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 73

1920–1930s—Conservation Efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3 May 1970—Proclaimed as Aniakchak Crater National Natural Landmark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

1 December 1978—Presidential Proclamation 4612 Establishing Aniakchak National Monument . . . . . . . . . . . . . . . . . . . . . . . . 73

2 December 1980—Alaska National Interest Lands Conservation Act (ANILCA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Appendix C: Geologic Resource Laws, Regulations, and Policies . . . . . . . . . . . . . . . . . . . . . . . . . 75

Appendix D: Scoping Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

2005 Scoping Meeting Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

2014 NPS Meeting Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

2014 USGS-AVO Meeting Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Map Unit Properties Tables . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .in pocket
v

Figure 1. Map showing the location of Aniakchak National Monument and Preserve on the Alaska Peninsula. . . . . . . . . . . . . . . . . . . 2

Figure 2. Photograph of Aniakchak Crater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 3. Photograph of Bernard Hubbard with ice ax examining scoriaceous lava blocks in Aniakchak Crater (1931). . . . . . . . . . . . . . . 3

Figure 4. Map showing Holocene volcanoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 5. Diagram showing a cross-section of oceanic crust subducting under continental crust . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 6. Map showing earthquake epicenters greater than magnitude 3.0, colored by depth (1889 to present) . . . . . . . . . . . . . . . . . 5

Figure 7. Stratigraphic chart of rocks making up Aniakchak National Monument and Preserve and surrounding area. . . . . . . . . . . . . . . . 7

Figure 8. Map showing terranes of Alaska and adjacent Canadian regions showing major faults. . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 9. Geologic time scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 10. Diagram showing the formation of the Aniakchak caldera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 11. Diagram showing the explosiveness and eruption column heights of the different types of eruptions . . . . . . . . . . . . . . . . 10

Figure 12. Photographs of the Aniakchak II eruption ignimbrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 13. Diagram showing the inferred effect of a tsunami from a pyroclastic flow entering Bristol Bay . . . . . . . . . . . . . . . . . . . . 13

Figure 14. Detailed geologic map of the Aniakchak Crater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 15. Reconnaissance level eruption history of Aniakchak volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 16. Photograph of Vulcan Dome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 17. Photograph of radial fracture patterns in andesite of Bolshoi Dome (left) and West Dome (right) . . . . . . . . . . . . . . . . . . 16

Figure 18. Aerial photograph of Vent Mountain with Aniakchak Peak on rim in the background . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 19. Photograph showing cross section of Half Cone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 20. Schematic stratigraphic column of intra-caldera stratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 21. Satellite image (IKONOS 2005) of the Cobweb lava flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 22. Photograph of the 1931 main crater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 23. Photograph probably from the Slag Heap cone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 24. Photograph of Black Nose Pumice overlying older lava flows along the caldera rim north of The Gates. . . . . . . . . . . . . . . . 21

Figure 25. Photograph of the Cabin Bluff exposure in Aniakchak Bay showing post-glacial tephra stratigraphy. . . . . . . . . . . . . . . . . 21

Figure 26. Photograph of lake terraces on the east side of Surprise Lake identified by McGimsey et al. (1994). . . . . . . . . . . . . . . . . . 22

Figure 27. Satellite image mosaic (IKONOS 2004 and 2005) showing the proposed maximum lake levels in the caldera. . . . . . . . . . . . . . 23

Figure 28. Photograph of geologist standing on a large boulder deposited by the caldera outburst flood . . . . . . . . . . . . . . . . . . . 23

Figure 29. Photograph of alluvial fan deposits from the caldera breakout flood along a cut bank of the Aniakchak River . . . . . . . . . . . . 23

Figure 30. Photographs comparing the notch in the rim of the NE maar prior to the 2010 outburst flood . . . . . . . . . . . . . . . . . . . . 24

Figure 31. Photographs of the inlet portion of the NE maar lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 32. Comparative photographs showing the 2010 flood deposits along the alluvial fan . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 33. Photographs of geothermal features in Aniakchak Crater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 34. Map showing Tertiary igneous rocks (Eocene to Miocene) along the Alaska Peninsula and into the Alaska Range. . . . . . . . . . . 28

Figure 35. Photographs of hadrosaur dinosaur tracks in the Chignik Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 36. Photograph of the south wall of The Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 37. Map showing the extent of the Naknek Formation along the Alaska Peninsula and southern Talkeetna Mountains. . . . . . . . . . 31

Figure 38. Photograph of Buchia (most likely B. concentrica) fossils, plant detritus, and possible plant impression . . . . . . . . . . . . . . . . 32

Figure 39. Diagram showing the types of glacial deposits created by receding glaciers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 40. Map showing the mapped glacial deposits of the Aniakchak area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 41. Aerial photograph of beach berms at the mouth of the Aniakchak River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 42. Satellite image (IKONOS 2005) of the beach berms of Aniakchak Bay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 43. Wrangellia composite terrane translation history from the Late Triassic to the present . . . . . . . . . . . . . . . . . . . . . . . . 36

Figure 44. Diagram illustration of volcanic hazards. All are potential hazards in and around Aniakchak . . . . . . . . . . . . . . . . . . . . . 39

Figure 45. Map showing volcanic ash and pyroclastic flow deposits, and landslide and rockfall areas. . . . . . . . . . . . . . . . . . . . . . . 40

Figure 46. Photograph of AVO staff installing seismic station “Surprise Lake” (ANSL) within the Aniakchak caldera . . . . . . . . . . . . . . . 41

Figure 47. Map showing the locations of the AVO seismic network as of July 2015. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Figure 48. Map showing the probability of earthquake of magnitude 5.5 over the next 20 years in the Aniakchak area. . . . . . . . . . . . . 42

Figure 49. Photograph looking east at the north side of The Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Page

Figures
vi

Figures (continued)

Page

Figure 50. Map showing the extent of subsurface mineral rights within the boundary of the preserve. . . . . . . . . . . . . . . . . . . . . . 45

Figure 51. Map showing extent of original source geologic maps of the Aniakchak area by Detterman et al. . . . . . . . . . . . . . . . . . . 52

Figure 52. Geologic map of the Alaska Peninsula by Atwood (1911). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Figure 53. Portion of plate XLII from Smith (1925) showing a map of the Aniakchak Crater and vicinity. . . . . . . . . . . . . . . . . . . . . 70

Figure 54. Geologic map of the Aniakchak and Chignik areas by Knappen (1929). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Figure 55. Part of the geologic map of the Alaska Peninsula by Burk (1965), showing the area around Aniakchak. . . . . . . . . . . . . . . . 71

Tables

Page

Table 1. Volcanic explosivity index (VEI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Table 2. Simplified volcanic rock classification and characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 3. Sea level rise projections based on IPCC (Intergovernmental Panel on Climate Change) and USACE (US Army Corps 

of Engineers) scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Table 4. Geology data layers in the Aniakchak National Monument and Preserve GRI GIS data . . . . . . . . . . . . . . . . . . . . . . . . . 53

Plates (in pocket)

Plate 1. Bedrock geologic map of Aniakchak National Monument and Preserve.

Plate 2. Quaternary geologic map of Aniakchak National Monument and Preserve.

Plate 3. Geologic map cross section A–A’.

Plate 4. Panorama map of Aniakchak National Monument and Preserve.
vii

This GRI report was written for resource managers, 

interpretation and educational staff, and park visitors to 

support science-informed decision making and provide 

a detailed reference for understanding the geology of 

Aniakchak National Monument and Preserve. Chapters 

of the report discuss distinctive geologic features 

and processes leading to the present-day landscape, 

describe the geologic history, highlight geologic issues 

facing resource managers, identify topics of possible 

future study, and provide information about the GRI 

geologic map data. Plates 1 and 2 (in pocket) illustrate 

these data. Plate 3 (in pocket) is cross section A-A’ from 

Plate 1. The Map Unit Properties Tables (in pocket) 

summarize report content for each geologic map unit.

Aniakchak National Monument and Preserve, 

containing a historically active volcano, contains 

numerous geohazards including evidence of a 

catastrophic caldera-forming eruption, tsunamis, 

historic and ancient out-burst floods, landslides, and 

rock-falls. Aniakchak lies above the Aleutian megathrust, which causes numerous earthquakes and has the 

potential to produce very large earthquakes. In addition 

to these geohazards, the older bedrock of the park unit 

includes marine and nonmarine sedimentary rocks that 

contain numerous invertebrate fossils, plant fossils, and 

dinosaur tracks. Managing the risk from geohazards 

and protecting the fossils are primary management 

issues. However, the park contains rocks that provide 

data that suggest the formation of Aniakchak began far 

from its present position, was shaped by glacial erosion, 

and is being effected by on-going tectonics. These 

processes have produced the spectacular landforms of 

Aniakchak.

The primary geologic features and processes in 

Aniakchak are those derived from volcanic eruptions 

and their deposits.

● Aniakchak II, The Caldera Forming Eruption. 

The Aniakchak Crater is one of the best exposed 

calderas in the world. Aniakchak Crater formed 

3,660 ± 70 years ago during a voluminous calderaforming eruption that produced an estimated 

bulk volume of approximately 70 km3

 (17 mi3) 

of volcanic material, consisting of extensive 

pyroclastic flows and pumice fall. So much magma 

was erupted from the underlying magma chamber 

that the summit of the mountain collapsed, forming 

the caldera. The eruption, called Aniakchak II, 

was among one of the largest in the world to have 

occurred during the Holocene Epoch (11,700 

years ago to present). It has a volcanic explosivity 

index (VEI) of 6 (“colossal”) on a scale of 0–8. 

Significant ash fall occurred as much as 1,000 km 

(620 mi) to the north on the Seward Peninsula. Ash 

from the eruption traveled around the Northern 

Hemisphere; traces are found in Greenland ice 

cores. The eruption left an extensive rhyodacite 

pumice fall and an andesite ignimbrite that covers 

the flanks of the mountain and fills many of the 

surrounding valleys. Thickness in valley bottoms 

is as much as 100 m (330 ft). The pyroclastic flow 

from the eruption was so great that when it entered 

the water of Bristol Bay it formed a tsunami that 

inundated the north coast of the bay. Ash fall, 

also referred to as “fallout,” from the Aniakchak 

II eruption covered most of western Alaska, 

obliterating vegetation and wildlife. Evidence of 

human habitation in the Aniakchak area does not 

occur until about 1,500 years after the eruption, 

and 100 km (60 mi) to the northeast, in the Ugashik 

region, there was a 1,500 year hiatus in habitation, 

probably as a result of the eruption.

● Post-Caldera Eruption Features. Eruptions 

following the Aniakchak II eruption formed the 

volcanic features on the floor of the caldera, 

Executive Summary

The Geologic Resources Inventory (GRI) is one of 12 inventories funded by the National Park Service 

(NPS) Inventory and Monitoring Program. The Geologic Resources Division of the NPS Natural 

Resource Stewardship and Science Directorate administers the GRI. This report synthesizes discussions 

from a scoping meeting for the parks of the Southwest Alaska Network in February 2005. The meeting 

was facilitated by the Geologic Resources Division to determine geologic resources, the status of geologic 

mapping, and geologic resource management issues and needs. A follow-up meeting was held with park 

and Alaska Regional Office staff in July 2014. This report is a companion document to the previously 

completed GRI digital geologic map data.
viii

including four subaqueous lava domes (Vulcan, 

West, Pumice, and Bolshoi), three tuff cones 

(Surprise, Windy, and Breezy), two composite 

cones (Vent Mountain and Half Cone), a cinder 

cone (Blocky Cone), and two maars (SW and NE). 

Aniakchak volcano most recently erupted from 

May to June 1931, producing pumice and ash 

fall in the area of Port Heiden and forming three 

significant features within the caldera: Main Crater, 

Doublet Crater, and Slag Heap lava flow.

● Pre-Caldera Eruption Deposits. The volcanic 

history of Aniakchak volcano goes back at least 

850,000 years. Two very large Holocene eruptions 

formed deposits found below (older than) the 

Aniakchak II ignimbrite: Aniakchak I ignimbrite 

(from an eruption between approximately 10,700 

and 8,200 years ago) and the Black Nose Pumice 

(from an eruption less than about 7,600 years old 

but older than the Aniakchak II eruption).

In addition to features and processes associated with 

volcanic eruptions and their deposits, Aniakchak 

National Monument and Preserve contains other 

significant geologic features and processes, including 

the following:

● Caldera Outburst Flood and its Deposits. The 

caldera filled with water, forming a large lake 

after the Aniakchak II eruption. Evidence for the 

lake includes subtle wave-cut terraces along the 

northeast wall of the caldera and lava domes that 

formed under water. The lake drained during a 

catastrophic outburst flood. The flood formed 

alluvial terraces containing very large clasts 

downstream of The Gates, and deposited large 

alluvial fans near the Aniakchak River and Albert 

Johnson Creek. Floodwaters scoured channels into 

bedrock in places along the Aniakchak River valley. 

● 2010 Maar Breakout Flood. The NE maar 

contains a lake impounded by loose pumice. In 

2010, the lake released catastrophically and flooded 

downstream into the Aniakchak River. The lake 

level dropped approximately 5 m (16 ft), and the 

flood eroded areas immediately downstream and 

deposited approximately 1.5 m (5 ft) of material on 

the alluvial fan near the Aniakchak River. The flood 

carried material into the Aniakchak River, which 

altered the rapids through The Gates and the river 

channel several miles downstream of The Gates.

● Geothermal Features. Aniakchak National 

Monument and Preserve is one of 16 National Park 

System units designated by the Geothermal Steam 

Act (30 U.S.C. § 1026(a)) as containing significant 

geothermal features. Aniakchak caldera itself is 

considered a significant feature. Within the crater, 

15 warm springs discharge near the northeast side 

of Surprise Lake. Warm ground is present on the 

caldera floor.

● Nearby Holocene Volcanoes. Three Holocene 

volcanoes within 100 km (60 mi) of the monument 

and preserve include Yantarni Volcano on the 

east, the Black Peak caldera to the southwest, and 

Veniaminof Volcano farther southwest. Due to 

the proximity of these volcanoes, they present 

additional volcanic hazards to the park unit.

● Tertiary Igneous and Sedimentary Rocks. 

The Aniakchak area contains extensive Tertiary 

volcanic and volcaniclastic rocks from the Eocene 

and Miocene epochs (55–5 million years ago) and 

some small Tertiary intrusive bodies. These include 

rocks of the Meshik Volcanics. Directly underlying 

the Tertiary volcanic rocks are lower Tertiary 

sedimentary rocks of the Tolstoi Formation, which 

contain plant fossils and some shallow marine 

invertebrate fossils that indicate formation in a 

subtropical environment.

● Upper Cretaceous Nonmarine and Marine 

Sedimentary Rocks. The Tertiary rocks 

unconformably overlie Upper Cretaceous 

(75–66 million years ago) marine and nonmarine 

sedimentary rocks of the Chignik and Hoodoo 

formations, which are nonmarine to marginal 

marine sedimentary rocks. The Chignik Formation 

contains abundant plant fossils, minor marine 

invertebrate fossils, and dinosaur tracks.

● Lower Cretaceous Marine Sedimentary 

Rocks. Just outside of the Aniakchak Preserve 

boundary on the west side of the map area are 

two Lower Cretaceous marine sedimentary units, 

the Herendeen Formation and the Staniukovich 

Formation. These two units contain abundant 

marine invertebrate fossils. A major unconformity 

separates the Lower Cretaceous rocks from the 

Upper Cretaceous rocks.

● Upper Jurassic Marine Sedimentary Rocks. The 

Naknek Formation is the oldest, most extensive, 

and thickest of the Mesozoic marine sedimentary 

rocks in the Aniakchak area. It contains plant debris 

and marine invertebrate fossils.
ix

● Glaciers and Glaciations. Small alpine glaciers 

occur on Aniakchak Peak, on the south wall of the 

caldera, and in the Vent Mountain crater. Many 

of the glaciers, as well as stagnant patches of ice, 

are hard to recognize because they are covered by 

debris. Approximately 12,000 years ago during the 

last glacial maximum in Alaska, glaciers extended to 

the Pacific and Bristol Bay coasts. As these glaciers 

receded, they left glacial deposits in the form of 

moraines, kame topography, outwash plains, and 

pro-glacial lake deposits.

● Raised Marine Terraces and Beach Berms. After 

the last glacial maximum, isostatic rebound on 

the order of 15 m (49 ft) on the Bristol Bay side 

of the mountain crest and 28–30 m (92–98 ft) on 

the Pacific side is marked by raised marine wavecut terraces, wave affected glacial moraines, and 

marine deposits. The amount of uplift attributable 

to isostatic rebound rather compared to tectonic 

forces is not clear, but glacial rebound is probably 

the dominant factor.

Geologic resource management issues identified 

by park managers, NPS Alaska Regional Office, US 

Geological Survey, or the NPS Geologic Resources 

Division include the following:

● Geohazards. The Aniakchak area is susceptible 

to the following geohazards: volcanic eruptions, 

earthquakes, rockfall, landslides, outburst floods, 

and tsunamis. Hazardous events in the area are 

frequent and affect large areas.

● Paleontological Resources Inventory, 

Monitoring, and Protection. Paleontological 

resources have only been studied at the 

reconnaissance level, but the potential for discovery 

is great. Invertebrate marine fossils are common 

in the Mesozoic marine sedimentary rocks, 

dinosaur tracks have been discovered in the Upper 

Cretaceous Chignik Formation, and plant fossils 

are abundant in the Upper Cretaceous and lower 

Tertiary nonmarine sedimentary rocks. 

● Geothermal Energy Development. The Aniakchak 

region is an active volcanic area and thus has 

potential for geothermal energy. The likelihood of 

large geothermal energy development is low due to 

the remoteness of the area, associated challenging 

work environment, and potential hazards from 

volcanic activity.

● Petroleum Development and Mineral 

Extraction. The marine sedimentary rocks of the 

Alaska Peninsula have been the target of petroleum 

exploration since 1869, even though USGS 

reports in the 1920s concluded that the oil and gas 

potential is low. The state has extensive oil and gas 

lease tracts surrounding the north and northwest 

boundary of the monument and preserve, and 

the Koniag Corporation holds subsurface mineral 

rights to much of the western and southern parts of 

the preserve.

● Coastal Issues. Coastal features in Anaikchak 

National Monument and Preserve include 217 km 

(135 mi) of shoreline, raised marine terraces and 

beach berms. It is one of 118 parks servicewide 

that have been identified as potentially vulnerable 

to sea-level change. Sea level rise projections for 

Aniakchak range from 0.04 m (0.13 ft) by 2030 to 

1.42 m (4.66 ft) by 2100 depending on modeling 

scenario. Geohazards may also impact coastal 

features and processes. The few coastal assets 

and infrastructure within Aniakchak National 

Monument and Preserve limits potential impacts to 

human structures.

● Glacier Changes. Within Aniakchak National 

Monument and Preserve, an Alaskawide glacier 

change study counted 16 glaciers overall on recent 

satellite imagery as opposed to 29 counted on the 

1950s USGS topographic map although there is low 

reliability comparing modern sizes and extents to 

those mapped 60 years ago. Nevertheless recent 

Alaskawide glacier trends of negative mass balance, 

diminished ice cover, and reduced ice volume are 

expected to intensify in the coming decades.

● Crater Lake Level and Water Quality. Surprise 

Lake supports spawning and nursing habitat for 

sockeye salmon and arctic char. The water quality 

of the lake varies spatially and with changes in the 

meteorological conditions with strong influence 

from warm springs near their outlets. No salmon 

spawn on the warm spring side of the lake, but 

prefer the side of the lake near the crater wall where 

fresh groundwater flows into the lake.

xi

Products and Acknowledgments

The NPS Geologic Resources Division partners with institutions such as Colorado State University, 

the US Geological Survey, state geological surveys, local museums, and universities to develop GRI 

products. This section describes those products and acknowledges contributors to this report.

GRI Products

The objective of the Geologic Resources Inventory is 

to provide geologic map data and pertinent geologic 

information to support resource management and 

science-informed decision making in more than 

270 natural resource parks throughout the National 

Park System. To realize this objective, the GRI team 

undertakes three tasks for each natural resource park: 

(1) conduct a scoping meeting and provide a summary 

document, (2) provide digital geologic map data in a

geographic information system (GIS) format, and (3)

provide a GRI report (this document). These products

are designed and written for nongeoscientists.

Scoping meetings bring together park staff and geologic 

experts to review and assess available geologic maps, 

develop a geologic mapping plan, and discuss geologic 

features, processes, and resource management issues 

that should be addressed in the GRI report. Following 

the scoping meeting, the GRI map team converts the 

geologic maps identified in the mapping plan to digital 

geologic map data in accordance with the GRI data 

model. After the map is completed, the GRI report team 

uses these data, as well as the scoping summary and 

additional research, to prepare the GRI report. 

The compilation and use of natural resource 

information by park managers is called for in the 1998 

National Parks Omnibus Management Act (section 

204), 2006 National Park Service Management 

Policies, and the Natural Resources Inventory and 

Monitoring Guideline (NPS-75). The “Additional 

References” chapter and Appendix C provide links to 

these and other resource management documents and 

information.

Additional information regarding the GRI, including 

contact information, is available at http://go.nps.gov/gri. 

The current status and projected completion dates of 

products are available at http://go.nps.gov/gri_status.

Acknowledgments

The constructive reviews by Frederic Wilson and 

Game McGimsey improved the report. Thanks to Troy

Hamon (NPS Natural Resources Manager) and Diane 

Chung (Superintendent) for cooperation during the 

initial preparations for the report. We also appreciate 

the time provided by the USGS Alaska Volcano 

Observatory staff (Charlie Bacon, Game McGimsey, 

Tom Miller, and Chris Waythomas) to meet and 

discuss the geology of the area and review an earlier 

draft. Historic materials were provided by Kathryn 

Myers (NPS Alaska Region Museum Curator). Historic 

photographs of Bernard Hubbard were provided by 

the Santa Clara University Library from the Bernard 

R. Hubbard, S.J. Alaskan Photographs, 1927–1961

collection. Additional thanks to Trista ThornberryEhrlich (Colorado State University) for providing some

of the graphics used in this report. Julia Brunner and

Jeremiah Kimbell (NPS Geologic Resources Division)

reviewed the geothermal and petroleum development

sections, respectively. Maria Caffrey (NPS Geologic

Resources Division and University of Colorado)

provided sea level rise projection data.

Authors

Chad Hults (NPS Alaska Regional Office) 

Christina Neal (USGS Alaska Volcano Observatory)

Review 

Frederic Wilson (USGS Alaska Science Center)

Robert (Game) McGimsey (USGS Alaska Volcano 

Observatory) 

Jason Kenworthy (NPS Geologic Resources Division)

Editing

Katie KellerLynn (Colorado State University)

Report Formatting and Distribution

Jason Kenworthy (NPS Geologic Resources Division) 

Rebecca Port (NPS Geologic Resources Division)

Source Maps

Digital data provided by Frederic Wilson (USGS Alaska 

Science Center)
xii

GRI GIS Data Production

James Chappell (Colorado State University)

Chad Hults (NPS Alaska Regional Office)

GRI Geologic Map Poster Design (Plates 1 and 2)

Kari Lanphier (Colorado State University)

Chad Hults (NPS Alaska Regional Office)

Georgia Hybels (Colorado State University)

GRI Geologic Map Poster Editing

Georgia Hybels (Colorado State University)

Rebecca Port (NPS Geologic Resources Division)

Jason Kenworthy (NPS Geologic Resources Division)
1

Aniakchak National Monument and Preserve, located 

in the heart of the Alaska Peninsula (Figure 1; Plate 4), 

encompasses 244,226 ha or 2,442 km2 (603,497 ac or 

943 mi2) of volcanic landscapes, glaciated mountains, 

and coastal landforms. The park unit is approximately 

190 km (120 mi) from NPS headquarters in King 

Salmon, and approximately 650 km (400 mi) from 

Anchorage, the state’s largest city. Aniakchak is often 

considered one of the most remote park units managed 

by the National Park Service and receives the fewest 

visitors of any park (less than 300 visitors per year 

for the last 10 years). Most visitors access the area by 

passenger plane, landing on Surprise Lake in the caldera 

and floating down the Aniakchak River to the Pacific 

Ocean. Packrafts have provided a new, and increasingly 

popular, way to access the caldera by hiking up the 

north flank of the caldera from Port Heiden and floating 

down the Aniakchak River.

The Aniakchak Crater was recognized in 1922 when a 

USGS expedition led by Walter R. Smith hiked into the 

crater (Figure 2; Smith and Baker 1924; see Appendix 

A for a summary of the geologic exploration of the 

Aniakchak area). When the Smith (1925) report was 

published that described the caldera in detail, the 

caldera was instantly recognized as a unique natural 

geologic wonder. Smith (1925, p. 145) presented the 

following colorful description:

The once active scene of terrific earth convulsions is now 

almost oppressively silent. The coloration of the country is 

somber and together with the fretfully driven clouds tends 

to create a rather pleasing weirdness. Aniakchak Crater is 

one of the great natural curiosities of North America and 

is certainly worthy of further investigation.

Attention was focused on Aniakchak Crater in the 

1930s because of popular lectures about the caldera by 

Bernard R. Hubbard, a professor of geology at Santa 

Clara University in California, and a Jesuit priest known 

as the “Glacier Priest” or “Father Hubbard.” Hubbard 

conducted exploration of the caldera in 1930, 1931, and 

1932, spanning a major eruption in 1931 (Figure 3). He 

recorded pre- and post-eruption footage of the caldera, 

and during his 1932 expedition was the first person to 

land on Surprise Lake in an airplane. His lectures about 

This chapter describes the regional geologic setting of Aniakchak National Monument and Preserve.

the caldera helped spur Horace M. Albright, the NPS 

director from 1929 to 1933, to propose that the area be 

preserved as a national monument (Norris 1996). The 

effort was stalled; however, because the acting USGS 

director, Julian D. Sears, stated in a letter to the NPS 

director that the survey “feels a keen interest in having 

as much of a region as possible kept open for free 

development” (Norris 1996, p. 429).

After many more years of effort, the National Park 

Service was successful at conserving the Aniakchak 

Crater and surrounding area when Secretary of the 

Interior Walter Hickel, former governor of Alaska, 

proclaimed it as a national natural landmark in 1970. 

Aniakchak National Monument was established in 

1978 by President Jimmy Carter, who noted that the 

caldera, cinder cones, lava flows, and explosion pits 

are textbook examples of volcanic features. Two years 

later, the area was expanded to include the preserve and 

designate the Aniakchak River as a wild and scenic river 

by the passage of the Alaska National Interest Lands 

Conservation Act (ANILCA), which preserved the 

caldera and associated volcanic features in their natural 

state in order to interpret geological and biological 

processes for visitors. Appendix B includes additional 

information regarding the administrative history of 

monument and preserve.

The dominant landform of Aniakchak National 

Monument and Preserve is the large Aniakchak Crater. 

The crater was formed during an eruption 3,660 ± 70 

cal. yr BP and contains younger volcanic features, the 

latest of which formed during an eruption in 1931. The 

volcanic deposits lie over older bedrock made up of 

Tertiary (66–3 million years ago) volcanic, intrusive, and 

sedimentary rocks; and late Mesozoic (161–66 million 

years ago) sedimentary rocks. The rocks of Aniakchak 

National Monument and Preserve record a complex 

history of volcanism and sedimentary rock deposition 

that spans the last 161 million years. These rocks and 

geomorphic features provide evidence for ancient plate 

tectonic motions of large portions of southern Alaska, 

and recent tectonically driven processes.

Aniakchak volcano is one of many Holocene (the 

past 11,700 years) volcanoes of the Aleutian volcanic 

Geologic Setting and Significance
2

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KATMAI

NATIONAL PARK

& PRESERVE

ANIAKCHAK

NATIONAL MONUMENT

& PRESERVE

Brooks

Lake

Black Peak

Yantarni Volcano

Aniakchak Crater

Mount Veniaminof

Naknek

 Lake

Becharof

 Lake

Bristol Bay

Pacific Ocean

A L A S K A P E N I N S U L A

Naknek

King

Salmon

Hills

Chignik

Ugashik

Sand

Point

Squaw

Port Heiden

Port

Moller

159°W 156°W

58°N

57°N

56°N

0 10 20 30 40

Miles

0 10 20 30 40

Kilometers North

Á

Figure 1. Map showing the location of Aniakchak National Monument and Preserve on the Alaska Peninsula. 
3

Figure 2. Photograph of Aniakchak Crater. The crater is the signature feature of Aniakchak National Monument and 

Preserve. A “colossal” eruption 3,660 ± 70 cal. yr BP created the 10.5 km (6.6 mi) diameter crater. More recent volcanic 

activity created the smaller cones within the crater. NPS photograph by M. Williams taken in 1977 from 4,900 m 

(16,000 ft).

Figure 3. Photograph of Bernard Hubbard with ice ax examining scoriaceous lava blocks in Aniakchak Crater (1931). 

Image provided by Archives & Special Collections, Santa Clara University Library; The Bernard R. Hubbard, S.J. Alaskan 

Photographs, 1927–1961, ACK-00-001.
4

arc, which is part of the greater Pacific 

Ring of Fire (Figure 4). The volcanoes 

of the Aleutian arc are formed by the 

subduction of the Pacific oceanic plate 

under Alaska. Subduction occurs where 

dense oceanic crust moves towards and 

sinks into the asthenosphere below lower 

density continental crust (Figure 5). The 

subducting Pacific oceanic plate under 

Alaska is observed by the earthquakes 

that get progressively deeper under the 

Aleutian volcanic arc as the oceanic plate 

sinks deeper (Figure 6). The boundary 

between the plates is called the Aleutian 

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PACIFIC OCEAN

Bering Sea

Gulf of Alaska

Alaska

Yukon

Territory

British

Columbia

RUSSIA

Kuril Islands

Kamchatka Peninsula

Aleutian Basin

Aleutian Islands

Aleutian megathrust

Sea of Okhotsk

Aniakchak

Crater

Anchorage

Fairbanks

150°E 180° 150°W 120°W

50°N

40°N

0 250 500 750 1,000

Miles

North

l

l

l

l

l

50 mm/yr

(2.0 in/yr)

56 mm/yr

(2.2 in/yr)

64 mm/yr

(2.5 in/yr)

76 mm/yr

(3.0 in/yr)

82 mm/yr

(3.2 in/yr)

0 250 500 750 1,000

Kilometers Á

Figure 4. Map showing Holocene (the past 11,700 years) volcanoes of the Aleutian volcanic arc, Kamchatka Peninsula, 

and Kurile Islands. Arrows show the direction of motions of the Pacific plate relative to the North American and Asian 

continents (Haeussler and Plafker 2004). Alaska National Park System units outlined in brown. Volcano data from the 

Smithsonian Holocene volcano list: http://www.volcano.si.edu/list_volcano_holocene.cfm (accessed 1 January 2015).

Oceanic crust

Continental crust

Back arc

Volcanic arc

Forearc

Trench

Subducting

plate

Partial 

melting

Asthenosphere

100 km

200 km

Figure 5. Diagram showing a crosssection of oceanic crust subducting under 

continental crust. Water in the subducting 

plate reacts with the dry mantle, which 

causes melt to form and rise into the 

overlying plate. Volcanoes form where 

this melt reaches the surface of the earth 

and erupts as lava. Modified from original 

graphic provided by Trista ThornberryEhrlich (Colorado State University).
5

megathrust and is the focus of frequent earthquakes, 

some of which can be very large and cause tsunamis.

The magma that formed the modern Aniakchak volcano 

intruded through and erupted on top of Tertiary 

volcanic and intrusive rocks, and late Mesozoic to early 

Tertiary nonmarine and marine sedimentary rocks 

(Figure 7, Plate 3). The Tertiary igneous rocks formed 

in a similar tectonic setting and relative location as 

the modern Aleutian volcanic arc. However, the late 

Mesozoic sedimentary rocks formed from erosion 

of a magmatic arc sequence called the Talkeetna arc 

and were possibly formed far from the paleo-Alaskan 

margin.

The Alaska Peninsula is cored by rocks of the Peninsular 

terrane (Figure 8). Terranes are groups of rocks that 

have closely related histories and are fault bounded with 

adjacent rocks that have different histories. Terranes 

were displaced from their original locations relative 

to each other and moved along faults into place. The 

terranes of southern Alaska (Peninsular, Wrangellia, 

Alexander, Yakutat, and Chugach) were transported 

from distant locations far south of their present position 

relative to the North American continent. The Talkeetna 

arc, which makes up most of the Peninsular terrane, 

was an island arc that formed in the paleo-Pacific ocean 

and later accreted against the Alaskan margin. The 

Talkeetna arc is made up of plutons and volcanic rocks 

that intruded into and erupted on top of older rocks. 

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##

#### ## # # ######### ###

# ### ####

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# ##

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#

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#

#

PACIFIC OCEAN

Bering Sea

Gulf of Alaska

Alaska

Yukon

Territory

Aleutian Islands

Aleutian megathrust

Bristol Bay

Norton Sound

Aniakchak Crater

DENALI FAULT SYSTEM

TINTINA FAULT

180° 140°W

60°N

50°N

l

l

l

50 mm/yr

(2.0 in/yr)

56 mm/yr

(2.2 in/yr)

64 mm/yr

(2.5 in/yr) 0 125 250 375 500

Miles

North 0 125 250 375 500

Kilometers Á

Depth (km)

0 - 15

16 - 25

26 - 100

101 - 200

201 - 350

# Volcanoes

!

!

!

!

!

Figure 6. Map showing earthquake epicenters greater than magnitude 3.0, colored by depth (1889 to present). The 

Aleutian arc volcanoes (black triangles) form where the subducting Pacific plate reaches 100 km deep (where the 

earthquake epicenters transition from red to purple). At this depth the water retained within the subducting plate 

reacts with the dry mantle to form melt. The Pacific plate motion relative to the North American plate is shown with 

arrows. Earthquake data downloaded from http://www.aeic.alaska.edu (accessed January 1, 2015)
6

Jnn - Northeast Creek Sandstone Mbr

Jns - Snug Harbor Siltstone Mbr

Jni - Indecision Creek Sandstone Mbr

Kst - Staniukovich Formation

Khe - Herendeen Formation

Kc, Kh - Chignik/Hoodoo Formations

Tt - Tolstoi Formation

Tm - Meshik Volcanics

Tvu - Volcanic rocks, undivided

Qv, Qpd - Volcanic rocks and pyroclastic flows

Extensive

Unconformity

Ti - Intrusive rocks

Tiu - Older intrusive rocks

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

160

150

140

130

120

110

100

90

80

70

Late

Early

Late

CRETACEOUS JURASSIC

60

50

40

30

20

10

0 m.y.a.

Paleocene

Eocene

Oligocene

Miocene

Pliocene

Pleistocene Quaternary Holocene TERTIARY Paleogene Neogene

Cross-bedded sandstone and 

conglomerate

Bedded sandstone

Sandstone with shale

Cross-bedded calcarous sandstone

Interbedded sandstone and shale

Tuff

Basalt flows

Rhyolite

Hypabyssal andesite

and dacite

Andesite and dacite

Diorite

Calcarous siltstone

Shale

Cross-bedded sandstone

Inoceramus

(bivalve)

Concretions

Ammonite

Plant fossils

Buchia (bivalve)

Plant debris

Coal

Dinosaur tracks
7

Figure 7 (facing page). Stratigraphic chart of rocks making up Aniakchak National Monument and Preserve and 

surrounding area. Unit symbols (e.g., “Khe”) correspond to the geologic map on Plate 1.

Although the rocks of the Talkeetna arc are not exposed 

in the Aniakchak map area, the late Mesozoic and early 

Tertiary sedimentary rocks in the Aniakchak area were 

derived from erosion of this arc. Figure 9 shows the age 

of the Talkeetna arc in relation to major tectonic events 

of the North American continent and the Northern 

Cordillera.

+

+ + +

+ + + + + + + + + + +

+

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+

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,

!(

!(

!(

!(

!(

!(

!(

!(

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!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

Nome

King

Salmon

Port Alsworth

Bethel

Kodiak

Dillingham

Gustavus

Anchorage

McKinley Park

Seward

McCarthy

Skagway

Juneau

Ketchikan

Sitka

Valdez

Fairbanks

Kotzebue

Barrow

NA

NA

AA

NA

AA AA NA

AA

AM

AM

VN TZ

PC AM

RB

VN

AA

KY

TZ

SD

YK

TZ NA KY

LG WK WC AM

SV

SV IN

FA RB

TZ WK

KY IN YT

CA YT

MK

QN

ML PN

SM

SM

WR

YT

PN FA

YT

WF

SU MY

PE

SM NS

PN

FA

TZ KY

SM

QN CA NS

CG

NY

TG

PW WR YA PW

PE

NS

AX

TU

CG

YA

GD PW

CG KIL TK

CG

AX

PE

TU CG WR

TG

CG AX

CG

ST

NS TU

PE

AX

CGPW

AX

PE

PE CG

CG

CC

MY

170°W 160°W 150°W 140°W 130°W 120°W

65°N

60°N

55°N

Terranes

AX Alexander

AM Angayucham terrane

AA Arctic Alaska

BP Broad Pass

CC Cache Creek

CA Cassiar

CG Chugach

CH Chulitna

FA Farewell

GD Goodnews terrane

IN Innoko terrane

KL Kilbuck

KY Koyukuk

LG Livengood

ML Maclaren

MK McKinley

MN Minook terrane

MY Mystic terrane

NS Nisling

NA North America

NY Nyac

PE Peninsular

PN Pingston

PC Porcupine

PW Prince William

QN Quesnellia

RB Ruby

SV Seventymile

SD Seward

SM Slide Mountain

ST Stikinia

SU Susitna terrane

TU Taku

TK Tikchik terrane

TG Togiak terrane

TZ Tozitna

VN Venetie

WF West Fork

WK Wickersham

WC Woodchopper

WR Wrangellia

YA Yakutat

YK York

YT Yukon-Tanana

0 100 200 300 400

Miles

0 100 200 300 400

Kilometers

North

Á

Figure 8. Map showing terranes of Alaska and adjacent Canadian regions showing major faults and National Park 

System unit outlines (in green). Modified from Silberling et al. (1992).
8

Eon Era Period Epoch m.y.a. Phanerozoic Proterozoic Archean Hadean Cenozoic (CZ) Mesozoic (MZ) Paleozoic (PZ)

Quaternary

(Q)

Neogene

(N)

Cretaceous (K)

Jurassic (J)

Triassic (Tr)

Permian (P)

Pennsylvanian (PN)

Mississippian (M)

Devonian (D)

Silurian (S)

Ordovician (O)

Cambrian (C)

Precambrian (PC, X, Y, Z)

Holocene (H)

Pleistocene (PE)

Pliocene (PL)

Miocene (MI)

Oligocene (OL)

Eocene (E)

Paleocene (EP)

2.6

5.3

23.0

33.9

56.0

145.0

201.3

298.9

323.2

358.9

419.2

443.4

485.4

2500

4000

4600

66.0

541.0

0.01

Paleogene

(PG)

Tertiary (T)

252.2

 Global Life Forms Northern Cordillera Events 

Formation of the Earth

Mass extinction

Mass extinction

Age of Mammals Age of Reptiles Age of Amphibians Fishes Marine Invertebrates

Modern humans 

Spread of grassy

 ecosystems

Extinction of large

 mammals and birds 

Early primates 

Placental mammals 

Early flowering plants 

Dinosaurs diverse and

 abundant

First dinosaurs; first mammals

Flying reptiles

Sharks abundant

First reptiles

Mass extinction

First amphibians

First forests (evergreens)

First land plants

Mass extinction

Primitive fish

Trilobite maximum

Rise of corals

Complex multicelled organisms 

Simple multicelled organisms 

Early bacteria and algae

 (stromatolites)

Origin of life

Mass extinction Breakup of Pangaea begins

Supercontinent Pangaea intact

Ancestral Rocky Mountains

Ice age glaciations;

 glacial outburst floods

Early shelled organisms

Coal-forming swamps

End of the ice ages

Alaska Range uplift

Kanektok Metamorphic Complex

(oldest known rocks in Alaska )

Dextral strike-slip faulting

Early Brookian orogeny

Late Brookian orogeny

Brooks Range uplift

Proto-Aleutian volcanism

Slab-window subduction (SCAK)

Extensive plutonism and volcanism

in the Yukon-Tanana & Brooks Range

Mid-Cretaceous orogeny

Wales orogeny (SEAK)

Kakas orogeny (SEAK)

Sinistral strike-slip faulting

Ellsmerian orogeny

Extensive plutonism

Talkeetna arc

Figure 9. Geologic time scale showing the onset of major global evolutionary events and tectonic events of the North 

American continent and the Northern Cordillera. (SCAK, south-central Alaska; SEAK, southeast Alaska). The divisions of 

the geologic time scale are organized stratigraphically, with the oldest divisions at the bottom and the youngest at the 

top. GRI map abbreviations for each time division are in parentheses. Ages are millions of years ago (m.y.a.). Ages are 

from the International Commission on Stratigraphy (http://www.stratigraphy.org/index.php/ics-chart-timescale; accessed 

7 May 2015). 
9

Geologic Features and Processes

This chapter describes and explains the formation of the distinctive geologic features of Aniakchak 

National Monument and Preserve, and the process of formation. The formation of the volcanic 

features is presented first, starting with the striking Aniakchak Crater and the features found within 

the caldera. Nearby Holocene volcanoes that could impact the park unit are also briefly described. 

The underlying bedrock units are presented from youngest to oldest. Surficial processes that shaped 

the landscape are discussed at the end. Following this chapter, a concise Geologic History chaper is 

provided, where the geology is built up from the oldest to the youngest rocks.

Volcanic Eruptions and their Deposits

Aniakchak II, The Caldera Forming Eruption

Aniakchak Crater is one of the largest and most 

spectacular calderas of the Aleutian volcanic arc. The 

caldera is 10.5 km (6.6 mi) in diameter and averages 

610 m (2,000 ft) deep. The highest point stands 1,340 

m (4,400 ft) above sea level at Aniakchak Peak, and 

the lowest point is 302 m (991 ft) at the bottom of 

Surprise Lake; the lake itself is 19.5 m (64 ft) deep. The 

caldera formed as voluminous magma erupted from 

the underlying magma chamber, creating a void. The 

top of the volcano collapsed into the vacated magma 

chamber (Figure 10). An explosive eruption (volcano 

explosivity index [VEI] 6; Table 1) accompanied 

caldera formation 3,660 ± 70 years ago (cal. yr BP1

). The 

eruption that created the present caldera is referred to 

as the Aniakchak II eruption; it was the second of two 

possible caldera forming eruptions in the last 10,000 

years (Miller and Smith 1987). 

The Aniakchak II eruption formed an extensive 

ignimbrite sheet that extends to the coasts on both 

sides of the Alaska Peninsula (unit Qafd on Plate 2). 

The eruption produced a voluminous amount of 

material, estimated at 70 km3 (17 mi3) bulk volume. 

The ignimbrite sheet covers an area greater than 2,500 

km2

 (965 mi2). The eruption ejected material high into 

the atmosphere where winds carried fine ash 1,000 km 

1 Ages are given in calibrated (calendar) years before present 

(cal. yr BP). “Present” is, by convention, considered to be 

1950 CE and ages are given with an error range of ± one 

standard deviation (1σ). Calibrated ages were calculated 

from reported radiocarbon (carbon-14 or 14C) ages using 

the program OxCal v4.2 with the IntCal13 calibration 

curve (Bronk Ramsey 2009). The IntCal13 calibration 

was developed by correcting 14C ages with known ages of 

plant and animal species with countable annual features 

like tree rings and coral growth rings.

(620 mi) to the north to create tephra deposits on the 

Seward Peninsula (Riehle et al. 1987a, Beget et al. 1992). 

Investigators found minute shards of volcanic glass (ash) 

in Greenland ice cores. This material has a distinctive 

geochemical signature similar to the Aniakchak II 

products, potentially tying it to the eruption (Pearce et 

al. 2004; Coulter et al. 2012). The paucity of snow, ice, 

or water in the caldera, and relatively thin post-caldera 

deposits, makes the Aniakchak Crater one of the best 

exposed calderas in the world (Tom Miller, USGS/AVO, 

emeritus research geologist, personal communication, 

10 October 2014). 

The deposits of the Aniakchak II eruption include an 

initial pumice fall deposit overlain by a thick layered 

ignimbrite (Figure 12). The rhyodacite pumice fall 

deposit (see Table 2 for an explanation of volcanic rock 

type classifications) is 20 cm (8 in) thick 28 km (17 mi) 

from the caldera in sea cliffs on the northern coast, 

and thickens to 75 cm (29 in) thick 5 km (3 mi) from 

the caldera (Bacon et al. 2014). The ignimbrite sheet is 

exposed in sea cliffs along Bristol Bay as far away as 50 

km (31 mi) from the caldera (Bacon et al. 2014). The 

ignimbrite contains a lower rhyodacite layer, a middle 

layer containing a mixture of andesite and rhyodacite 

clasts, and an upper layer of andesite that is a partly 

to nonwelded ignimbrite (Figure 1; Bacon et al. 2014). 

The total thickness of the ignimbrite is variable and 

is as much as 70–100 m (230–330 ft) thick in valley 

bottoms (Bacon et al. 2014). Ash flow from the eruption 

expanded unconstrained on the north side of the 

volcano, but was constricted to valley bottoms on the 

south side of the volcano (Plate 1). The ash flow to the 

south banked around numerous tight corners and rose 

over passes as high as 260 m (850 ft), but still retained 

significant volume to deposit a 10 m (33 ft) thick deposit 

50 km (31 mi) away in Kujulik Bay (Miller and Smith 

1977).
10

Tsunami

Effect on human habitation

Deposits on the north shore of Bristol Bay indicate that 

the pyroclastic flow likely generated a tsunami when 

it entered Bristol Bay. Waythomas and Neal (1998) 

found pumiceous sand interbedded in peat as high as 

12 m (39 ft) above mean high tide at numerous sites 

along the north coast of Bristol Bay. In a couple places, 

the pumiceous sand overlies a tephra layer that was 

deposited from ash fall before the arrival of the tsunami. 

Waythomas and Neal (1998) proposed that the tsunami 

was preceded by pumice and ash fall (Figure 13). Rafted 

pumice was subsequently transported inland by the 

tsunami depositing an anomalous pumiceous sand layer 

in backshore peat deposits.

Fallout from the Aniakchak II eruption covered most 

of western Alaska, obliterating vegetation and wildlife. 

Blackford et al. (2008, 2014) found that approximately 

20% of the wetlands of Alaska were affected by the 

eruption, and caused a 90–200 year hiatus in peat 

deposition as far as 1,100 km (680 mi) north of the 

Eruption of

pumice and

ash.

Caldera collapse after draining

the magma chamber.

Today

Vent

Mt.

Surprise

Lake

Ash &

Pumice

Dikes

Figure 10. Diagram showing the formation of the 

Aniakchak caldera. A large eruption ejects a massive 

amount of pumice and ash over a relatively short 

time frame, forming pyroclastic flows and a column 

of ash. The magma chamber drains and the volcano 

edifice collapses into the void, forming a caldera. Later 

eruptions built the several domes within the caldera. 

Modified from Klimasauskas et al. (2002). 

Height of eruption column

Explosiveness

Hawaiian

<2 km

(1 mi)

Strombolian

<10 km

(6 mi)

Sub-plinian

<30 km

(18 mi)

Vulcanian

<20 km

(12 mi)

Plinian and

ultraplinian

<55 km

(34 mi)

Surtseyan

<20 km

(12 mi)

Phreatoplinian

<40 km

(24 mi)

Figure 11. Diagram showing the explosiveness and 

eruption column heights of the different types of 

eruptions. The Aniakchak I and II eruptions are 

considered “colossal” Plinian and Ultra-Plinian 

eruptions. Graphic by Trista Thornberry-Ehrlich 

(Colorado State University), modified from Cas and 

Wright (1988).
11

volcano. VanderHoek and Myron (2004) found no 

evidence of human habitation in the Aniakchak area 

earlier than about 2,000 cal. yr BP, which they attributed 

to the Aniakchak eruption that may have kept humans 

from inhabiting the area, or that earlier sites were 

obliterated by the Aniakchak II eruption. Evidence that 

the eruption had a significant impact on humans was 

found 100 km (60 mi) to the northeast in the Ugashik 

region, where archeologists found a 1,500 year break 

in human habitation from approximately 3,600 to 

2,100 cal. yr BP (VanderHoek and Myron 2004). The 

Aniakchak II eruption had a significant impact to the 

ecology and resource availability in the area.

Effects on ecology and humans are greater for eruptions 

that occur during the warm season (del Moral and Bliss, 

1993). Vanderhoek (2009) listed four lines of evidence 

that suggest the eruption took place during a warm 

season. First, the tephra fallout was predominately 

to the north-northwest, which is consistent with the 

prevailing wind direction between May and September. 

Second, rip-up clasts were found entrained in the 

ignimbrite, suggesting the ground was not frozen. Third, 

the pyroclastic flow generated a tsunami, suggesting that 

the bay was not frozen that hit the north side of Bristol 

Bay. Fourth, the tsunami deposits also contained rip-up 

clasts, which would only occur if the ground was not 

frozen.

Post-Caldera Eruption Features

Following the Aniakchak II eruption, numerous 

eruptions formed the various domes, cones, maars, and 

lava flows on the caldera floor. Figure 14 is a detailed 

geologic map of the caldera and its features. Figure 15 

presents the last 10,000 years of eruption history for 

Aniakchak volcano. The following is a description of 

some of the distinctive post-caldera eruption features 

visible in the caldera.

Vulcan, West, Pumice, Bolshoi Domes

Vulcan (Figure 16), West, Pumice, and Bolshoi domes, 

(map unit Qd in Figure 14) are formed of dacite lavas 

that have radial fracture patterns (Figure 17) and pillow 

like forms that suggest they were erupted under water 

(Bacon et al. 2014). West Dome is 610 m (2,002 ft) above 

sea level, which is 289 m (947 ft) above the current 

level of Surprise Lake, so a lake must have nearly filled 

the caldera when West Dome was emplaced (see the 

“Outburst Floods” section). Correlation with a dated 

pumice deposit 14–18 km (9–11 mi) north of the caldera 

provides an age for the domes of approximately 2,320 

± 140 cal. yr BP, but more work is necessary to confirm 

this correlation (Bacon et al. 2014).

Surprise, Windy, and Breezy Cones

Three tuff cones in the eastern portion of the caldera 

(Surprise, Windy, and Breezy cones; Figure 14) have 

arcuate ridges of bedded phreatomagmatic tephra 

formed by the interaction of magma and water. The 

lavas from these cones are made of basaltic andesite 

(Bacon et al. 2014). Radiocarbon dating of soil above 

a Surprise Cone–like tephra resulted in a 820 ± 70 cal. 

yr BP age (Bacon et al. 2014), indicating an apparent 

youngest age of these tuff cones. Lacustrine clayey 

Table 1. Volcanic explosivity index (VEI). For eruption classification descriptions see Figure 11.

VEI

Ejecta 

Volume 

(km3

)

Column 

Height 

(km)

Classification Description Examples

0 0.00001 <0.1 Hawaiian Effusive Kilauea (ongoing)

1 0.001 1 Hawaiian/Strombolian Severe Stromboli (numerous)

2 0.01 5 Strombolian/Vulcanian Explosive Cleveland (ongoing), Shishaldin (ongoing)

3 0.1 15 Vulcanian/Sub-Plinian Catastrophic Redoubt (1989, 2009), Augustine (2003)

4 1 25 Sub-Plinian Cataclysmic Aniakchak (1931), Eyjafjallajökull (2010), Crater Peak (1992), 

Kasatochi and Okmok (2008)

5 10 >25 Plinian Paroxysmal Mount St. Helens (1980), Vesuvius (1979)

6 100 Plinian/Ultra-Plinian Colossal Aniakchak I and II (approximately 9,000 and 3,700 years ago), 

Novarupta (1912), Krakatoa (1883), Pinatubo (1991)

7 1,000 Ultra-Plinian Mega-colossal Mount Mazama (7,550 years ago), Tambora (1815)

8 >1,000 Apocalyptic Yellowstone (640,000 years ago), 

Modified from Newhall and Self (1982)
12

Figure 12. Photographs of the Aniakchak II eruption ignimbrite. (A) Coarse-grained ignimbrite overlying a pumice fall 

layer along the caldera rim near The Gates. USGS photograph by Game McGimsey. (B) Rhyodacite ignimbrite layer 

approximately 15 m (50 ft) thick overlies about 20 cm (8 in) of rhyodacite pumice fall east of Port Heiden on the 

Bering Sea coast (geologist for scale). USGS photograph by Game McGimsey. (C) Close-up of the pumice fall deposit 

underlying the ignimbrite. The pumice overlies 1–2 cm (0.5–1 in) of brown soil and well-sorted brown-black sand 

above 15 m (50 ft) of glaciofluvial deposits. USGS photograph by Tina Neal.

Table 2. Simplified volcanic rock classification and characteristics. 

Name Percent Silica (SiO2) Viscosity Typical Explosiveness Aniakchak Lava Range in Red

(example map units)

Rhyolite >72% High Viscosity Very Explosive

Rhyodacite 68%–72% Qpd

Dacite 63%–68% Qda, Qdap, Ti, Tvu

Andesite 57%–63% Qanp, Tvu

Basaltic andesite 53%–57% Qba

Basalt <53% Low Viscosity Less Explosive

From Clynne and Muffler (2010).
13

silt and sand are present atop Surprise Cone deposits, 

suggesting that the eruptions took place in shallow 

water (Bacon et al. 2014), or that shallow water (above 

the current level of Surprise Lake) existed after the 

eruption.

Vent Mountain and Half Cone

Vent Mountain and Half Cone are the most prominent 

features in the caldera (Figure 18, Figure 19). These 

two volcanic edifices produced much of the volcanic 

material on the floor of the caldera during numerous 

eruptions that spanned over hundreds of years starting 

at least 900 years ago (Figure 20; map units Qvm, 

Qhl, Qht, Qhc, in Figure 14). Vent Mountain rises 

approximately 550 m (1,800 ft) above the floor of the 

caldera (Figure 18), and is a spatter and tephra cone 

made up of silicic andesite and dacite scoria and lava 

(Bacon et al. 2014). Numerous blocky lava flows drape 

the cone’s lower flanks, and young fissure vents cut 

the south flank. Black, blocky lavas flowed from these 

fissures and cover much of the southern part of the 

caldera (Figure 14). Half Cone is a distinctive feature 

of the caldera because of its brightly colored layers. 

Half Cone exposes a cross section of an andesite to 

dacite cone that developed on the northeast portion 

of the caldera floor (Figure 19). Deposits from Half 

Cone are interbedded with those from Vent Mountain, 

suggesting that the eruption history is nearly as long 

BRISTOL BAY

ANIAKCHAK

VOLCANO

BRISTOL BAY

ANIAKCHAK

VOLCANO

Pyroclastic flow rapidly enters

Peaty

wetland

NORTH

COAST

Pyroclastic

flow deposit

Tsunami wave train crosses the

bay in less than a few hours. Peaty

wetland

Bristol Bay and initiates a tsunami...

The tsunami breaks over the Nushagak 

Peninsular, depositing floating pumice and 

beach sand over peat.

NORTH

COAST

Ashfall covers

the coast.

Pumice fallout accumulates as pumice rafts.

Figure 13. Diagram showing the inferred effect of a tsunami from a pyroclastic flow entering Bristol Bay during the 

Aniakchak II eruption. Pumice and ash is deposited first in Bristol Bay and over tundra and peat bogs on the northern 

shores. In a few hours, the tsunami reaches the north shore of Bristol Bay where it deposited rafted pumice. Modified 

from Figure 10 of Waythomas and Neal (1998).
14

Figure 14. Detailed geologic map of the Aniakchak Crater. Note that this map is a more detailed map and the geologic 

unit labels do not match those in the GRI Quaternary geologic map on Plate 2. This map is figure 10A in Bacon et al. 

(2014).
15

Ash-producing eruption

Lava-flow or lava-dome eruption

EXPLANATION

Time span represented by events listed

?

70

390

400

560

900

1,000

1,490

2,000

2,300

3,000

3,700

4,000

5,000

10,000

Years

Ago

Volcanic events of the Aniakchak Volcano

A.D. 1931 explosive eruption from western caldera floor

Lava flows and ejecta from Vent Mountain

Strombolian eruption at Blocky Cone

Major explosive eruption and lava flow; destruction of Half Cone

Lava flows and ejecta from Vent Mountain

Explosive eruption from New Cone

Explosive eruption (from Half Cone?)

Explosive eruption and lava dome from Half Cone

Multiple ash-producing eruptions (from Vent Mountain and 

 other sources?)

Draining of high stand of Surprise Lake by this time

Lava-flow eruption from Half Cone

Lava flows from Vent Mountain

Explosive eruptions from three vents wholly or partially

 submerged beneath a much-deeper Surprise Lake; lava flow

Chiefly nonexplosive eruptions of four lava domes on northern

 and western caldera floor

At least one explosive eruption

Explosive eruption

CALDERA FORMATION: Catastrophic eruption (Aniakchak II);

 at least 10 distinct explosive eruptions closely precede climactic event by

 a few decades or centuries

Dacite lava flow (on northwest flank?)

At least eight explosive eruptions

At least 12 explosive eruptions 

(including possible caldera-forming event; 

Aniakchak I)

 

Figure 15. Reconnaissance level eruption history of Aniakchak volcano based on limited tephrachronology and field 

studies. Graphic modified from figure 6 of Neal et al. (2001).
16

Figure 16. Photograph of Vulcan Dome. Vulcan, West, Pumice, and Bolshoi domes may have erupted under water 

approximately 2,300 years ago. Geologist for scale. Photograph by Tom Hunt.

Figure 17. Photograph of radial fracture patterns in andesite of Bolshoi Dome (left) and West Dome (right), which are 

features typical of subaqueous eruption. USGS photographs by Tina Neal (left) and Charlie Bacon (right).
17

Figure 18. Aerial photograph of Vent Mountain with Aniakchak Peak on rim in the background. USGS photograph by 

Game McGimsey.

plinian fall

lava flows

scoria

Cobweb Lava Flow

Figure 19. Photograph showing cross section of Half Cone showing several layers of lava flows (dark cliffs), air-fall 

eruption deposits (lighter deposits on left), and spatter agglutinate deposits (reddish and dark layers near the top). 

The Cobweb lava flow is in the foreground mantled by ash and lapilli from the 1931 eruption. View is approximately 

800 m wide (0.5 mi) at the height of the scoria. USGS photograph by Game McGimsey.
18

and complex (Figure 20; Bacon et al. 2014). The more 

recent explosive deposits from Half Cone cover most 

of the central and northern portions of the caldera 

(Figure 14) and include lava flows, pyroclastic flows, 

and air-fall pumice and ash (Bacon et al. 2014). The 

latest explosive eruption destroyed most of the cone, 

revealing the layers now exposed in the Half Cone wall 

(Neal et al. 2001), and deposited distinctive pink and 

brown pumice-fall layers distributed throughout the 

caldera floor and extensively on the north flank of the 

caldera. This eruption took place about 430 ± 60 cal. yr 

BP (Bacon et al. 2014). The youngest product of Half 

Cone is the radially ridged, crystal-rich, Cobweb dacite 

lava flow that filled a basin at the end of or following 

the explosive eruption about 430 cal. yr BP (Figure 14, 

Figure 21). 

SW and NE Maars

The SW and NE maars on the east side of the crater 

(Figure 14) are attributed to later Vent Mountain 

eruptions (Bacon et al. 2014). The NE maar is about 

450 m (1,476 ft) by 515 m (1,690 ft). Maars are formed 

when hot magma contacts shallow groundwater causing 

a steam explosion. The steam explosion breaks the 

overlying rock and excavates a pit. The rock gets ejected 

into the air and falls back to earth, forming a tephra ring 

around the explosion pit.

1931 Main Crater

The informally named 1931 main crater (map units Q3l

and Q3t in Figure 14) on the west side of the caldera 

is the first feature that hikers encounter when they 

descend into the caldera over the west rim from Port 

Heiden (Figure 22). Hubbard called the 1931 crater 

the “Pit of Hades.” The crater was formed during an 

eruption that lasted six weeks in May and June 1931 

(Neal et al. 2001; Nicholson et al. 2011). On 1 May 

1931, residents of Meshik (now Port Heiden, 27 km [16 

mi] west of the caldera) observed a white steam cloud 

followed by a black mushroom cloud that dropped 

ash and pea- to egg-sized black pumice clasts (Neal 

et al. 2001). The eruption waxed and waned with 

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

Three tuff cones

(Surprise, Windy, Breezy)

Four subaqueous domes

(Vulcan, Bolshoi, Pumice, West)

Vent Mountain lava

Half Cone vent-filling lava

Caldera lake drains

1After VanderHoek & Myron (2004)

470±60

430±60

600±40

750±40

1,800

±401

820±70

2,320

±1402

2Correlated pumice deposit outside caldera

 (Bacon et al. 2014)

age

cal yr BC

Figure 20. Schematic stratigraphic column of intracaldera stratigraphy showing the recurrent and 

complex post-Aniakchak II eruption history. Ages 

are approximate and inferred from stratigraphic 

relationships and dating of soils beneath tephra layers. 

Colors match geologic map units shown on Figure 14. 

Modified from figure 5 of Bacon et al. (2014).

Half Cone

Cobweb lava flow

0 250 500 750 1,000

Yards

0 250 500 750 1,000

Meters

North

Á

Figure 21. Satellite image (IKONOS 2005) of the Cobweb 

lava flow. Hubbard (1931a, p. 327) described the lava 

flow as “the cinder cone in the midst of radiating lava 

flows, all linked together, resembles a giant spider lying 

in wait for prey.” Map unit Qhl in Figure 14.
19

episodic explosive and effusive eruptions. An account 

of the eruption from Meshik resident Frank Wilson, as 

told to Bernard Hubbard, was reported in Mush You 

Malemutes! (Hubbard 1932, p. 56-58):

A closer sound soon aroused the already terrified people of 

Meshik, when cinders, first the size of peas, then as large as 

eggs, beat a tattoo on their houses. 

The eruption lasted uninterruptedly until the eleventh of 

May, when a final terrific explosion shook the surrounding 

country and sent into the air rocks and ashes which 

descended in such great masses as to make it pitch dark 

for several hours at distances more than sixty miles from 

the volcano. Wilson left Meshik after the explosion of May 

1, and on the way to Bristol Bay, where he met the author 

and narrated the story of the eruption, had the paint 

scraped off his boat and engine ruined, pushing his way 

through more than five miles of floating pumice the size of 

water buckets.

The volcano was quiet only for a few days, breathing 

dense clouds of gas and smoke. Then explosive activity 

heard 200 miles away culminated in another major 

eruption on May 20. For several more days the 

detonations of the volcano sounded like the beatings of 

distant surf from Ugashik, more than sixty miles away. 

Lava then welled up into the new vents and another phase 

of activity began...

The most distant sufferers were the reindeer in the 

interior back of Nushagak. It was fawning time, and 

when volcanic ash covered the feed, the reindeer started 

to migrate, leaving the helpless young to perish. Reindeer 

and caribou ground their soft teeth down to the gums 

from the grit in their food and died. Dead swans and 

geese floated down the rivers from lakes on the tundra 

of the Alaska Peninsula, and cutting them open revealed 

the cause of their destruction—entrails full of volcanic 

ash. Hibernating bears, squirrels, and small game living 

inside the crater probably were consumed in the first great 

explosion...

So much heat suddenly liberated, as well as millions of 

tons of powdered ash filling the air, caused an interesting 

phenomenon shortly after the eruption. Clouds condensed 

rapidly all over the region and rain drops, which had 

formed about the tiny ash bits, started falling. Turning 

Figure 22. Photograph of the 1931 main crater. The crater, floored in lava flows, is approximately 800 m (0.5 mi) wide. 

Part of Half Cone and Cobweb lava flow are visible on the left and Surprise Lake is visible in the background. USGS 

photograph by Cyrus Read.
20

the ashes to mud, it literally rained mud balls the size of 

walnuts for hours at a time, making the snowfields and 

glaciers black as ink and causing the surrounding country 

to look as though covered by a huge funeral pall. The ash 

mud was sticky, too, and insisted on getting into every 

crack and corner and penetrating the closest-woven 

fabric.

The total volume of erupted material was approximately 

0.9 km3 (0.2 mi2) and consisted mostly of daciticrhyodacitic lava that ended with basaltic andesite 

lava (Bacon et al. 2014). Most of the products of the 

eruption consisted of scoria, pumice, lapilli, and ash, 

but near the end of the eruption small lava flows formed 

at the bottom of the 1931 main crater, in Doublet 

Crater, on the caldera floor just north of Doublet 

Crater, and a feature known as Slag Heap (Figure 23; 

Figure 23. Photograph probably from the Slag Heap 

cone taken during exploration of the eruption by 

Bernard Hubbard and assistant in 1931. Archives & 

Special Collections, Santa Clara University Library; The 

Bernard R. Hubbard, S.J. Alaskan Photographs, 1927–

1961, (ACK-00-196).

Bacon et al. 2014). Venting from the Cobweb lava flow 

was observed by Bernard Hubbard during the 1932 

exploration of the caldera (Neal et al. 2001). The 1931 

eruption is one of the largest recent historic eruptions in 

Alaska and had a volcanic explosively index (VEI) rating 

of 4 (“Cataclysmic”; Table 1).

Pre-Caldera Eruption Features

Aniakchak volcano eruption history goes back at least 

850,000 years, as indicated by dating of the lava flows 

that make up the caldera walls (Nye et al. 1997), but 

little has been published about these older features. 

However, two significant pre-caldera forming eruptions 

produced distinct deposits: a thick ignimbrite unit 

near the caldera rim (Aniakchak I), and an overlying 

distinctive orange pumice (Black Nose Pumice). 

Aniakchak I

One of the pre-caldera eruptions is a possible earlier 

caldera-forming event, referred to as the Aniakchak I

eruption (Miller and Smith 1987). The Aniakchak I

eruption deposited a thick, nonwelded, andesite 

ignimbrite that crops out in isolated outcrops near the 

caldera rim and in a few of the valleys on the flanks of 

the volcano (map unit QA1 in Figure 14; Bacon et al. 

2014). In Birthday Pass, the ignimbrite is approximately 

30 m (98 ft) thick and locally contains flattened bombs 

up to 1 m (3.3 ft) in diameter and lithic clasts (Bacon 

et al. 2014). Bacon et al. (2014) suggested that the 

Aniakchak I eruption occurred after deglaciation about 

10,700 cal. yr BP (age of peat above glacial deposits in 

Figure 25), and is older than the overlying Black Nose 

Pumice (8,165 ± 89 cal. yr BP).

Black Nose Pumice

Above Aniakchak I deposits and below the Aniakchak 

II deposits is an orange- to buff-colored dacite and 

rhyodacite pumice and intercalated welded rhyodacite 

ignimbrite called the Black Nose Pumice (map unit Qbn

in Figure 14). The Black Nose Pumice deposit forms a 

distinct package seen high on the walls of the caldera 

(Figure 24). The unit is as much as 15 m (49 ft) thick, 

but the thickness is variable due to erosion (Bacon 

et al. 2014). It consists of a lower pumice layer that is 

approximately 8–45 cm (3–18 in) thick and contains 

bombs as much as 30 cm (12 in) in diameter, many with 

pink interiors (Bacon et al. 2014). The pumice layer is 

overlain by an ignimbrite layer, which is 1–10 m

(3–30 ft) thick (Bacon et al. 2014). This ignimbrite 
21

caused oxidation of the lower pumice layer, turning it 

brick red (Bacon et al. 2014). Fiamme (flattened pumice 

clasts) in the ignimbrite are as large as 30 cm (12 in) 

and are identical to the lower pumice clasts (Bacon et 

al. 2014). Above the ignimbrite is an upper pumice fall 

deposit containing brown clasts of silicic dacite up to 40 

cm (16 in) in diameter (Bacon et al. 2014).

An orange-colored pumice deposit along Cabin Bluff 

in Aniakchak Bay (Figure 24) was recently correlated 

with the Black Nose Pumice by Bacon et al. (2014). This 

deposit was originally correlated with the Aniakchak I 

deposit by VanderHoek and Myron (2004), but Bacon 

et al. (2014) suggested that the pumice at Cabin Bluff 

is instead an air-fall deposit with an orange color and 

geochemical signature that more accurately matches the 

Figure 24. Photograph of Black Nose Pumice overlying older lava flows along the caldera rim north of The Gates. The 

lower Black Nose Pumice forms the buff colored lowest layer, which is oxidized red by the overlying dark-colored 

welded ignimbrite. The layered buff/black/orange unit is the upper Black Nose Pumice. USGS photograph by Charlie 

Bacon (figure 6A from Bacon et al. 2014).

Aniakchak II

Peat

Tephra

Black Nose pumice

Interbedded peat

and tephra

Glacial till

8,165 ±89/

7,617 ±47* cal BP

9,506 ±21 cal BP

10,029 ±92 cal BP

10,712 ±116 cal BP Figure 25. Photograph of the Cabin Bluff exposure in 

Aniakchak Bay showing post-glacial tephra stratigraphy. 

Figure modified from VanderHoek and Myron (2004, 

figure 7.7) with calibrated ages calculated from the 

radiocarbon ages presented in their figure 7-8, and a 

date reported in VanderHoek (2009) indicated with an 

asterisk (*).
22

Figure 26 (above). Photograph of lake terraces on the east side of Surprise Lake identified by McGimsey et al. (1994). 

USGS photograph by Game McGimsey.

West Dome

Birthday

Pass Vent Mountain

Surprise

Lake

The Gates

610 m (2,002 ft)

(Bacon et al., 2014)

490 m (1,610 ft)

(McGimsey et al., 1994)

158°10'W 158°5'W

56°56'N

56°54'N

56°52'N

0 0.5 1 1.5 2

Miles

0 0.55 1.1 1.65 2.2

Kilometers

North

Á
23

Figure 27 (facing page). Satellite image mosaic (IKONOS 

2004 and 2005) with outlines showing the proposed 

maximum lake levels in the caldera. The 490 m

(1,610 ft) level was derived from the highest wave-cut 

terraces reported in McGimsey et al. (1994). The 610 m 

(2,002 ft) level as estimated by Bacon et al. (2014) from 

the elevation of the highest of the four subaqueous 

domes, West Dome (see Figure 14). Outlines of the lake 

levels were derived from the National Elevation Dataset 

30 m resolution digital elevation model. Note that Vent 

Mountain did not exist during the time of the lake.

Black Nose Pumice. This correlation suggests that the 

Black Nose eruption occurred after 7,617 ± 47 cal. yr 

BP, which is the youngest date acquired from a sample 

of peat underlying the pumice (Figure 25).

Outburst Floods

Caldera Lake 

After the Aniakchak II eruption, the caldera filled with 

a large lake that eventually drained by a catastrophic 

outburst flood. Evidence for the lake includes subtle 

wave-cut terraces along the northeast wall of the caldera 

(Figure 26). During times of stable lake stands (levels), 

waves cut into the sides of the caldera, creating these 

terraces (McGimsey et al. 1994). The highest wave-cut 

terrace identified is 490 m (1,610 ft) above sea level, or 

169 m (555 ft) above the current level of Surprise Lake. 

However, based on features such as pillow lava, Bacon 

et al. (2014) suggested that four of the volcanic domes 

within the caldera erupted under water (map unit Qd in 

Figure 14). The highest of these subaqueous domes is 

West Dome, which is 610 m (2,002 ft) above sea level, or 

289 m (947 ft) above the current level of Surprise Lake 

(Bacon et al. 2014). The subaqueous domes of Bacon et 

al. (2014) are higher than the highest wave-cut terrace of 

McGimsey et al. (1994), thereby requiring further study 

and reinterpretation (see “Potential Future Studies” 

section). These two lake levels are depicted on a satellite 

image of the caldera in Figure 27.

Caldera Outburst Flood and its Deposits

During field studies in 1993 and 1994, G. McGimsey 

and co-workers recognized that the caldera lake 

drained in a catastrophic flood down the Aniakchak 

River. Downstream of The Gates, the Aniakchak 

River cuts through high alluvial terraces (unit Qat on 

Plate 2) that contain very coarse clasts and boulders 

indicative of large flood deposits (Figure 28). Further 

downstream, scabland-like scoured bedrock channels 

in the vicinity of Hidden Creek (Waythomas et al. 1996), 

and large alluvial fans along Albert Johnson Creek 

and the Aniakchak River provide further evidence of a 

catastrophic flood (Figure 29; unit Qaf on Plate 2). 

Near the mouth of the Aniakchak River in Aniakchak 

Bay, an ancient flood channel (unit Qac on Plate 2) 

contains pebbly sediment that VanderHoek and Myron 

Figure 28. Photograph of geologist standing on a large 

boulder deposited by the caldera outburst flood on the 

north side of the Aniakchak River downstream from The 

Gates. USGS photograph by Game McGimsey. 

Figure 29. Photograph of alluvial fan deposits from 

the caldera breakout flood along a cut bank of the 

Aniakchak River near Albert Johnson Creek (unit Qaf 

on Plate 2). The deposit is made of horizontally bedded, 

moderately sorted gravel and sand. NPS photograph by 

Tahzay Jones.
24

Figure 30. Photographs comparing the notch in the rim of the NE maar prior to the 2010 outburst flood (in 1992; Neal 

et al. 2001), and after the flood (2014) with water flowing out the deeply eroded notch. The shape and depth of the 

notch prior to the flood is depicted by the white line. Left: USGS photograph by Tina Neal. Right: photograph by Tom 

Hunt.

Figure 31. Photographs of the inlet portion of the NE maar lake comparing the lake level from July 2010 (top) and 

September 2011 (bottom). Compare the lake level to the bottom of the waterfall in both photos. Top: photograph by 

Tom O’Keefe. Bottom: USGS photograph by Game McGimsey.
25

(2004) suggested may have been the result of the 

outburst flood. VanderHoek and Myron (2004) cored 

the flood channel near the mouth of the Aniakchak 

River and dated soil and wood above a pebbly layer 

that resulted in an age of 1,870 ± 30 cal. yr BP, which 

may date the flood. The catastrophic flood probably 

did not completely drain the lake down to the current 

level of Surprise Lake because lake deposits were found 

overlying Surprise Cone (see “Surprise, Windy, and 

Breezy Cones” section).

2010 Maar Breakout Flood

In late summer of 2010, the NE maar lake (Figure 14) 

drained in what appears to have been a sudden outburst 

flood that reconfigured the alluvial fan downstream of 

the maar and moved debris into the Aniakchak River. 

Before the flood, water from the lake seeped through 

gravels of the northern rim of the maar and issued from 

a spring 240 m (790 ft) from the edge of the lake, and 

flowed north about 1.7 km (1 mi) to the Aniakchak 

River. Neal et al. (2001) recognized the flood potential 

of the maar and noted numerous strand lines just below 

the level of a notch on the north end of the maar (Figure 

30).

During the 2010 flood, the level of the lake dropped 

approximately 5 m (15 ft) (Figures 31 and 32) and 

drained approximately 645,000 m3 (844,000 yds3) of 

water (McGimsey et al. 2014). The flood wiped out 

vegetation, excavated and widened the stream banks, 

and deposited approximately 1.5 m (4.9 ft) of material 

on the alluvial fan downstream of the maar (Figure 

32). Debris was pushed into the Aniakchak River 

and significantly changed the character of the rapids 

through The Gates making them more challenging 

and abrasive to boats. Using analysis of photographs, 

weather data, and observations from a local guide, 

McGimsey et al. (2014) estimated that the flood likely 

occurred between late July and late September 2010.

Figure 32. Comparative photographs showing the 2010 flood deposits along the alluvial fan downstream (north) 

of the NE maar. The photograph on the left is from 1993 and shows abundant vegetation on the alluvial fan that 

was subsequently scoured and overlain by deposits from the 2010 maar outburst flood. USGS photographs by Game 

McGimsey.

Geothermal Features

The Aniakchak Crater contains 15 warm springs 

(Figure 33) that range from 18° to 22° C (64° to 72°F), 

and warm ground (labeled H in Figure 14) with a 
26

measured temperature of 85°C (185°F) at depths of 

25 cm (10 in) near Half Cone (Lyle and Dobey 1973; 

Miller 1990; Cameron and Larson 1992; Motyka et 

al. 1993). The mineral rich (iron-soda) warm springs 

enter Surprise Lake along the northwestern shore and 

have low dissolved oxygen (5%–10%), high alkalinity 

(400–500 mg/l), and moderately low pH (5.0–6.0) 

levels (Cameron and Larson 1992). The largest spring 

near Surprise Lake (Figure 33) emits carbon dioxide 

and helium gasses of magmatic origin, but not at toxic 

levels (Neal et al. 2001). These features are subject to 

monitoring as required by the Geothermal Steam Act 

of 1970 (amended in 1988; see the “Geologic Resource 

Management Issues” chapter).

Figure 33. Photographs of geothermal features in Aniakchak Crater. Top: Warm springs flowing into Surprise Lake. 

USGS photograph by Cyrus Read. Bottom: Waters emitting from the warm spring near Bolshoi Dome. Orange color is 

from iron-tolerant microbes. Note tents for scale. Photograph by Tom Hunt.
27

Nearby Holocene Volcanoes

Three Holocene volcanoes are within 100 km (62 mi) 

of Aniakchak Monument and Preserve (Figure 1) and 

one, Yantarni Volcano, lies within the geologic map 

boundary (Plate 2). Deposits from eruptions emanating 

from these volcanoes are present within the park unit 

boundaries, so they have a high potential of affecting the 

area and are a potential hazard to visitors to the area.

Yantarni Volcano

Yantarni Volcano is a small stratovolcano located 9 km 

(6 mi) east of the preserve boundary, but within the 

geologic map extent (Plate 2). Deposits are andesitic to 

dacitic lava flows, breccia, and pyroclastic deposits that 

range in age from 620,000 ± 230 years ago to as young 

as 2,000 years ago (Riehle et al. 1987b). An eruption 

3,500 to 2,000 years ago produced debris-avalanche 

deposits, pyroclastic flows, and a lava dome (Riehle et 

al. 1987b). The debris-avalanche deposits (unit Qdf on 

Plate 2) were formed during a catastrophic collapse of 

the east side of the cone, much like the initiation of the 

1980 Mount St. Helens eruption (Riehle et al. 1987b). 

Following the debris avalanche, an explosive eruption 

produced a pyroclastic flow that covered much of the 

northeast side of the volcano and deposited ash flows 

and ash falls (part of unit Qafd on Plate 2 near Yantarni 

Volcano). Closely following the pyroclastic flow, a lava 

dome formed that is about 1 km (0.6 mi) in diameter 

and located inside the cone (unit Qd on Plate 2).

Black Peak and Veniaminof Volcanoes

Black Peak and Mount Veniaminof, located 50 km 

and 100 km (31 mi and 62 mi) to the southwest of 

Aniakchak volcano, respectively, are two other nearby 

Holocene volcanoes (Figure 1). Black Peak consists of 

a caldera that is 4 km (2 mi) across and has walls made 

of andesite lava domes, lava flows, and volcaniclastic 

rocks (McGimsey et al. 2003). Pyroclastic deposits from 

a large eruption dated approximately 5,300 cal. yr BP 

are as much as 100 m (330 ft) thick in valleys on the 

west side of the Black Peak caldera (Miller and Smith 

1987, McGimsey et al. 2003). Air fall deposits from this 

eruption have a distinct color and texture referred to as 

“salt and pepper ash” (McGimsey et al. 2003). Tephra 

from this eruption is seen in exposures on the rim of the 

Aniakchak Crater (Bacon et al. 2014). The bulk volume 

from the eruption is estimated at 10–20 km3

 (2–5 

mi3) (Miller and Smith 1987; McGimsey et al. 2003). 

No historic eruptions have been documented. The 

Veniaminof volcano is a large andesitic stratovolcano 

comprising an ice-filled caldera that is 10 km (6.2 mi) 

across. The volcano stands at 2,507 m (8,225 ft) above 

sea level. Small eruptions from an intracaldera cinder 

cone occur frequently making Mount Veniaminof one 

of the most active volcanoes in Alaska. A post-glaciation 

ash flow covers the north flank of the volcano and 

reaches the Bering Sea; it was the result of the volcano’s 

caldera-forming eruption. The age of the flow is 

approximately 4,000 cal. yr BP (Miller and Smith 1987).

Older Bedrock

The Quaternary volcanic rocks overlie older bedrock 

(Plate 1, Plate 3). These older rocks are grouped into 

three packages separated by unconformities (Figure 

7, Plate 3): (1) Tertiary igneous and sedimentary 

rocks, (2) Upper Cretaceous marine and nonmarine 

sedimentary rocks, and (3) Upper Jurassic to Lower 

Cretaceous marine sedimentary rocks. After extensive 

marine deposition through most of the Jurassic 

Period, a major uplift began in the Late Jurassic that 

eventually resulted in an unconformity between the 

Lower Cretaceous and Upper Cretaceous sedimentary 

rocks. Uplift exposed the Lower Cretaceous rocks to 

erosion, whereby Middle Cretaceous sedimentary rocks 

were not deposited or were removed prior to the Late 

Cretaceous Period. Early Cretaceous predominately 

marine deposition was followed by fluvial, nearshore, 

and marine deposition after uplift and erosion of the 

older rocks (Figure 7). Volcanism along the Alaska 

Peninsula started in the early Tertiary and continued 

intermittently into the present day. Paleontological 

resources (fossils) are important resources preserved 

in many of the sedimentary rock units described below 

(see also “Geologic Resource Management Issues”).

Tertiary Igneous and Sedimentary Rocks

Tertiary Igneous Rocks

The map area contains extensive Tertiary intrusive 

(plutonic), volcanic, and volcaniclastic rocks that range 

in age from Late Eocene to Pliocene (40–3 million 

years ago; Plate 1, Figure 7). The intrusive rocks consist 

of Pliocene to Oligocene (34–3 million years ago) 

dacitic and andesitic hypabyssal (shallowly intruded) 

rocks and a small outcrop of quartz diorite (unit Ti on 

Plate 1). These intrusive bodies fed small dacitic and 

andesitic lava flows (unit Tvu on Plate 1). The Meshik 

Volcanics (unit Tm on Plate 1) are slightly older and 

range from Early Oligocene to Late Eocene (Wilson 
28

1985; Detterman et al. 1996). They consist of basalt to 

dacite flows, agglomerate, ash, and minor volcaniclastic 

rocks (Wilson 1985; Detterman et al. 1996). Small 

dioritic and undivided intrusive stocks and dikes (unit 

Tiu on Plate 1) intrude the Meshik volcanic rocks. The 

Tertiary igneous rocks are part of a larger belt of similar 

age igneous rocks that span the length of the Alaska 

Peninsula and continue up to the Alaska Range (Figure 

34).

+

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# #

####

#

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#

#

#

#

#

Aniakchak

NM&P Al a ska

Peninsula

Alaska Range

160°W 156°W 152°W

62°N

58°N

# Volcanoes

Plutonic

Volcanic

0 30 60 90 120

Miles

0 50 100 150 200

Kilometers Á

North

Figure 34. Map showing Tertiary igneous rocks (Eocene to 

Miocene) along the Alaska Peninsula and into the Alaska 

Range. Plutonic rocks are intrusive (cooled beneath Earth’s 

surface) igneous rocks. Volcanic rocks are extrusive (erupted 

onto Earth’s surface). Data derived from digital geologic 

map compilations by Wilson et al. (1998, 1999, 2006, 2012, 

2013).

Tertiary Sedimentary Rocks of the Tolstoi Formation

The Tolstoi Formation (unit Tt on Plate 1) is an Upper 

Paleocene to Middle Eocene (59–40 million years ago) 

nonmarine and shallow marine sandstone, siltstone, 

and conglomerate (Detterman et al. 1996). Burk (1965) 

and Detterman et al. (1981a) originally described 

this unit as containing abundant volcanic grains and 

interbedded volcanic flows, but Detterman et al. (1996) 

reassigned the volcanic rocks and volcanic clast rich 

conglomerate to the Meshik Volcanics. They noted that 

volcanic grains in the Tolstoi Formation make up less 

than 35% of the clasts, and the volcanic grains that were 

present were altered, suggesting an older Mesozoic 

source, not coeval volcanism. Clasts in conglomerate 

are mostly chert, quartz, quartzite, metalimestone, and 

volcanic rock fragments. The sandstone is arkosic (rich 

in feldspar) with abundant granitic detritus (Detterman 

et al. 1996). The Tolstoi Formation was deposited 

in nonmarine braided river and nearshore deltaic 

environments (Detterman et al. 1996). 

The Tolstoi Formation contains abundant plant fossils 

such as Acerarcticum Heer, Alnus sp., Cocculus flobella

(Newberry) Wolfe, Grewiopsis curiculaecrodatus

(Hollick) Wolfe, Liquidambar sp., Metasequoia

occidentalis (Newberry) Chaney, Protophyllum

semotum Hickey, and Thuires interruptus (Newberry) 

Bell, which indicate a Paleocene age (Detterman et al. 

1996). The megaflora indicates a subtropical rainforest 

environment with a mean annual temperature of 22°C 

(72°F) (Detterman et al. 1996). For comparison, the 

recent mean annual temperature (1981–2010) in King 

Salmon is 1.78°C (35.2°F) (Wendler et al. no date). It 

also contains some Eocene marine mollusks, including 

Turritella uvasana stewarti Merriam and Microcallista

(Costacallista) conradiana (Gabb), Nuculana sp., 

Tellina sp., Ostrea sp., Solena sp., Corbicula sp., Volsella

sp., and Glycymeris sp.; and dinoflagellates such 

as Impagidinium californiense, Adnatosphaeridium

multispinosom, Turbiosphaera filosa, Glaphyrocysta 

retiintexta, and Rhombodinium sp. (Detterman et 

al. 1996). The marine invertebrate species indicate 

deposition in a very shallow marine environment in 

temperate to subtropical waters (Detterman et al. 1996).

Upper Cretaceous Marine and Nonmarine 

Sedimentary Rocks

Chignik and Hoodoo Formations

The Upper Cretaceous (upper Campanian to lower 

Maastrichtian, 80–70 million years ago; Figure 7) 

Chignik Formation is a cyclic nearshore marine, tidal 

flat and nonmarine floodplain and fluvial deposit. The 

Hoodoo Formation is a lateral marine facies deposited 

on a marine shelf and lower slope of a submarine fan; it 

is coeval with the Chignik Formation (Detterman et al. 

1996). Both units crop out on the southeast portion of 
29

the map area (units Kc and Kh on Plate 1). The Chignik 

Formation comprises pebble-cobble conglomerate 

and coarse-grained sandstone interbedded with 

shale, coal, and siltstone (Detterman et al. 1996). 

The unit is primarily nonmarine and contains woody 

debris and coal beds. The marine portion contains 

abundant bivalves Inoceramus balticus var. kunimiensis 

and I. schmidti, and the ammonite Canadoceras

newberryanum (Detterman et al. 1996). Hadrosaur 

dinosaur tracks also occur in the Chignik Formation 

(Figure 35; Fiorillo et al. 2004; Fiorillo and Parrish 

2004). The Hoodoo Formation is a dark marine siltstone 

with thin sandstone beds that have clasts consisting 

of volcanic and plutonic rocks and minor chert and 

quartz (Detterman et al. 1996). Beds commonly have 

sedimentary structures typical of turbidites (beds 

formed by underwater density flows), including graded 

sandstone, rip-up clasts, convolute bedding, and flame 

structures (Detterman et al. 1981a). Marine fossils are 

sparse and include Inoceramus schmidti, I. balticus, 

and I. vancouverensis, and ammonites Diplomoceras 

notabile, Neophylloceras hetonaise, and Canadoceras 

newberryanum (Detterman et al. 1996).

Upper Jurassic to Lower Cretaceous Marine 

Sedimentary Rocks

Herendeen Formation

The Herendeen Formation crops out in a small area in 

the southwest part of the map area outside the preserve 

boundary (unit Khe on Plate 1). It is a thin-bedded 

calcareous sandstone and siltstone that contains tabular 

cross-bedding and has a strong petroliferous odor 

when broken (Detterman et al. 1996). The sandstone 

is made up of quartz, feldspar, and volcanic grains, 

but also contains abundant Inoceramus prism (shell 

cross sections) fragments (Detterman et al. 1996). 

Where siltstone beds occur, they contain intact Lower 

Cretaceous (Hauterivian and Barremian, 146–135 

million years ago; Figure 7) fossils Inoceramus ovatoides, 

ammonites Acrioceras and Hoplocrioceras, and the 

belemnite Acroteuthis (Detterman et al. 1996).

Staniukovich Formation

Similar to the Herendeen Formation, the Staniukovich 

Formation crops out in a small area in the southwest 

part of the map area outside the preserve boundary 

(unit Kst on Plate 1). It consists of marine siltstone 

and shale with minor sandstone. It contains Lower 

Cretaceous (Valanginian to Berriasian, 145–136 million 

years ago) fossil bivalves Buchia uncitoides and Buchia 

crassicolis solida, Pleuromya, and Inoceramus prisms 

(Detterman et al. 1996). Geology maps prior to the 

Wilson et al. (1999) Alaska Peninsula geologic map 

show the Staniukovich Formation within the boundary 

of the monument and preserve; however, the age and 

definition of the Staniukovich Formation was later 

revised by Detterman et al. (1996). In the Aniakchak 

area, much of what was previously mapped as the 

Figure 35. Photographs of hadrosaur dinosaur tracks in 

the Chignik Formation. Tracks in both photographs are 

approximately 50 cm (20 in) wide. Top: NPS photograph 

by Tahzay Jones. Bottom: photograph by Tom Hunt.
30

Staniukovich Formation was revised to the Indecision 

Creek Member of the Naknek Formation.

Naknek Formation

The oldest rocks within the map area, and the most 

extensive Mesozoic rock unit, are the marine and minor 

nonmarine sedimentary rocks of the Upper Jurassic 

(161–145 million years ago) Naknek Formation (Figure 

7). The Naknek Formation crops out along the south 

wall of the Aniakchak Crater and in the mountains to 

the south and west of the caldera (units Jn, Jni, Jns, and 

Jnn on Plate 1). The Naknek forms the whitish cliffs 

underlying dark Quaternary volcanic rocks in The 

Gates area (Figure 36).

The Naknek Formation is an extensive unit found 

on the Alaska Peninsula and the southern Talkeetna 

Mountains (Figure 37). Maximum thickness is 3,205 

m (10,500 ft), with an average thickness of 1,700–2,000 

m (5,600–6,600 ft) (Detterman et al. 1996). Of the 

formation’s five members, three crop out in the 

Aniakchak area (Figure 7). The uppermost Indecision 

Creek Sandstone Member (Jni) is arkosic sandstone and 

siltstone. The next lower member in the map area is the 

Snug Harbor Siltstone (Jns) that consists of dark-gray 

sandy siltstone and shale with a few sandstone interbeds 

and moderately abundant calcareous concretions 

(Detterman et al. 1981a). The lowest member is the 

Northeast Creek Sandstone (Jnn), which is nonmarine, 

fine to medium grained sandstone that is laminated 

and contains both aeolian and fluvial cross-bedding 

(Detterman et al. 1981a). This member also contains 

conglomerate comprising plutonic clasts, but also clasts 

of chert and white quartz (Detterman et al. 1981a).

The arkosic nature of the sandstones and the abundant 

granitic clasts in the conglomerates suggest that the 

Naknek Formation was derived predominantly from 

Jurassic plutonic rocks of the Peninsular terrane (Burk 

1965; Detterman et al. 1996; Trop et al. 2005). Facies 

changes in the Naknek occur parallel and transverse 

to the Alaska Peninsula. Coarser-grained nonmarine 

sedimentary rocks are located on the northwestern 

margins of the Naknek Formation, and deeper 

Quaternary

volcanic rocks

Naknek Formation

Figure 36. Photograph of the south wall of The Gates showing the dark Quaternary volcanic rocks overlying the lightcolored cliffs of the Naknek Formation. Note the slight angular unconformity (delineated by a hashed line) at the 

contact with the overlying volcanic rocks. Photograph by Tom Hunt.
31

marine facies occur on the southeast (Detterman et al. 

1996). The Naknek Formation is present only on the 

southeastern side of the Bruin Bay fault (Figure 37), 

which was an active reverse fault during the time of 

deposition (Burk 1965; Trop et al. 2005). The rocks of 

the Peninsular terrane were uplifted on the northwest 

side of the fault where high-energy streams eroded 

older rocks and deposited the sediment on a shallow 

marine shelf on the southeastern flanks of the terrane. 

The rapid deposition of the Naknek Formation records 

a major uplift and unroofing (extensive erosion) event 

affecting the Peninsular terrane (Trop et al. 2005).

The Naknek Formation contains abundant marine 

invertebrate fossils; the most dominant is the bivalve 

Buchia (Figure 38). Buchia fossils identified within 

the map area include B. concentrica (Sowerby), B. 

mosquensis (von Buch), and B. rugosa (Fischer). 

Other bivalves include Meleagrinella sp., Oxytoma sp., 

Pholadomya sp., Pleuromya sp., Entolium sp., Leda

sp., Lima sp., Melagrinella sp., Modiolus sp., Tancredia

sp. (Detterman et al. 1981c). Also present, but in 

much lower density, are the ammonites Discophinctes

sp., Lytoceras sp., Phylloceras sp., the belemnite 

Cylindroteuthis sp., gastropods, and occasional plant 

detritus. Southwest of the map area, near Black Peak, 

theropod dinosaur tracks were found within beds 

containing Buchia mosquensis fossils (Druckenmiller et 

al. 2011).

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!(

!(

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!(

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Aniakchak

NM&P Ala s ka

Peninsula

Alaska Range

Talkeetna

Mountains

King

Salmon

Port Alsworth

Bethel

Kodiak

Chignik

Dillingham

Anchorage

Seward

Valdez

RB

WR

PE

PN

FA

KY

TZ

NY

TG

PW

PE

GD

CG

TK

KIL PW

PE CG

TG

CG

CG

PE

PW

PE CG

PE

MY

B

R

U I N

B A Y

F A U L T

62°N 162°W 156°W 150°W

60°N

58°N

56°N

Naknek Formation

Other Bedrock

Terranes

CG Chugach

CH Chulitna

FA Farewell

GD Goodnews terrane

KL Kilbuck

KY Koyukuk

MY Mystic terrane

NY Nyac

PE Peninsular

PN Pingston

PW Prince William

RB Ruby

TK Tikchik terrane

TG Togiak terrane

TZ Tozitna

WR Wrangellia

0 25 50 75 100

Miles

0 40 80 120 160

Kilometers

North

Figure 37. Map showing the extent of the Naknek Formation along the Alaska Peninsula and southern Talkeetna 

Mountains (Wilson et al. 1998, 1999, 2006, 2012, 2013). It is part of the Peninsular (PE) terrane. Modified from 

Silberling et al. (1992).
32

Figure 38. Photograph of Buchia (most likely B. 

concentrica) fossils, plant detritus, and possible plant 

impression in the Naknek Formation near The Gates. 

Inset shows an ammonite impression at approximately 

the same scale. Photographs by Tom Hunt.

Landforms and Surficial Deposits

The interplay between geologic, tectonic, and climatic 

forces formed the present-day landforms at Aniakchak 

National Monument and Preserve. The volcanic 

deposits and bedrock of the area have been modified 

and eroded by surface geomorphic processes, leaving 

scoured mountains, incised valleys, beach berms, 

alluvial deposits, and glacial 

deposits. 

Glaciers and Glaciations

During the Pleistocene Epoch 

(2.6 million–11,700 years ago), 

glaciers covered the entire map 

area except for the highest ridges. 

The current glacial extent is 

minimal, approximately 5 km2

(2 mi2) and includes a few alpine 

glaciers on Yantarni Volcano, a 

few small hanging glaciers clinging 

to Aniakchak Peak and buried 

beneath thick volcanic debris at 

the base of the south caldera wall 

(Figure 14), and a small alpine 

glacier on the south wall inside the 

Vent Mountain crater (Loso et al. 

2014).

As the Pleistocene glaciers receded, they left numerous 

deposits, which are shown on Plate 2. The types of 

glacier deposits are also listed on the Quaternary 

geologic map unit properties table and shown in Figure 

39. Terminal and recessional moraines formed at the 

maximum extent of glaciation, and during pauses 

or readvances in the overall recession. The glacier 

moraines at Aniakchak are linear ridges consisting 

of coarse, angular, poorly sorted debris deposited 

directly from a glacier. The unit descriptions from 

the source maps (Detterman et al. 1981b, 1987) also 

describe knob and kettle topography, which refers to 

the lumpy sub-glacial deposits left behind as a glacier 

recedes (also referred to as kame topography). Kettle 

lakes formed where ice blocks that were incorporated 

in the unconsolidated glacier drift (any deposit left by 

a glacier) or alluvial deposits and later melted, leaving 

enclosed depressions. Pro-glacial lakes formed in front 

of receding glaciers where glacial deposits, typically 

moraines, created dams that blocked surface water. 

Glacial lake deposits (unit Qgl on Plate 2) were left 

behind when these lakes drained after the dams eroded. 

Outwash deposits accumulated where meltwater 

streams or rivers deposited moderately-sorted gravel 

and sand, forming relatively flat areas downstream of 

terminal moraines (units Qblo and Qmho on Plate 2).

Pleistocene glacial deposits of the Aniakchak area are 

divisible into those of the Brooks Lake glaciation, and 

the Mak Hill glaciation (Figure 40; Detterman 1986, 

Retreating glacier

Braided stream

Drumlins

Terminal

moraine

Outwash 

plain

Ground Bedrock

moraine

Recessional

moraine

Future

kettle lake

Esker

Glacial

erratics

Kames

Kettle

lakes

Meltwater

terrace Outwash 

delta

Outwash Pro-glacial

fan lake

Figure 39. Diagram showing the types of glacial deposits created by receding 

glaciers. Graphic by Trista Thornberry-Ehrlich (Colorado State University).
33

Detterman et al. 1987). Names for the glaciations 

and stades (intermediate advances) in the Aniakchak 

area are after geographic features in and near Katmai 

National Park and Preserve. Deposits of the Mak Hill 

Glaciation, which is the older of the two glaciations, 

are mapped in a small area on the northwest margin of 

the map area (Figure 40 and Plate 2). These deposits 

extend to the Bristol Bay coast, and elsewhere extend 

beyond the coast (Detterman 1986). A minimum age 

for the Mak Hill glaciation of approximately 43,000 or 

47,000 cal. yr BP was determined from radiocarbon 

ages of organic matter in glacial or post-glacial deposits 

(Detterman 1986; Stilwell and Kaufman 1996). 

Deposits of the Brooks Lake glaciation are better 

preserved than the Mak Hill glaciation; they have 

sharper moraine crests and are less eroded than the 

older glacial deposits (Detterman et al. 1981b, 1987; 

Detterman 1986). In the northern part of the map area 

Mother

 Goose

 Lake

Amber

 Bay

Aniakchak

Caldera

Kujulik Bay

Aniakchak River

Cinder River

Painter Creek

Yantarni

Volcano

Yantarni

Meshik Bay

Lake

Aniakchak Bay

Map Boundary

Qblil

Qblil

Qblil

Qblil Qblil

Qblil

Qblil

Qblk

Qblk

Qblk

Qblk

Qblk

Qblk

Qblk Qblk

Qgl

Qblu

Qblu

Qblu

Qblu

Qblu

Qblu

Qblu

Qblu Qblu

Qblu

Qblu

Qbln

Qbln

Qbln

Qbln

Qblk

Qblk Qblk Qblk

Qblo

Qblo

Qblo

Qblo

Qblo

Qblo

Qblo

Qblo Qblo

Qblo

Qmhd

Qmho

158°30'W 158°W 157°30'W 157°W

57°15'N

57°N

56°45'N

56°30'N

Moraine crest

Water

Glacier (Holocene)

Qgl - Glacial lake deposits (Quaternary)

Qblo - Brooks Lake glaciation outwash deposits, (Late Wisconsin)

Qblu - Brooks Lake glaciation drift undivided (Late Wisconsin)

 Qbln - Newhalen stade drift

 Qblil - Iliamna stade drift

 Qblk - Kvichak stade drift

Qmho - Mak Hill glaciation outwash (Early Wisconsin)

Qmhd - Mak Hill glaciation drift (Early Wisconsin)

0 2 4 6 8

Miles

0 2 4 6 8

Kilometers Á

North

Figure 40. Map showing the mapped glacial deposits of the Aniakchak area (for more detail see Plate 2). 
34

the Brooks Lake glaciation is divided into three stades: 

Kvichak, Iliamna, and Newhalen (Figure 40). Deposits 

of the Kvichak stade, the oldest stade of the Brooks 

Lake glaciation, extend onto the Bristol Bay plain. 

Plant material underlying outwash associated with the 

Kvichak moraine to the north of the map area provided 

a maximum age of 30,400 ± 340 cal. yr BP (Stilwell and 

Kaufman 1996). Moraines of the Iliamna stade extend 

slightly beyond the mouth of the major river valleys. 

Radiocarbon ages from the Iliamna Lake area indicate 

the stade is older than 16,500–20,000 cal. yr BP (Stilwell 

and Kaufman 1996; Stevens 2012). In the map area, the 

Iliamna stade is constrained to the upper portion of the 

Painter Creek valley (Figure 40 and Plate 2). Detterman 

(1986) suggested that age constraints on the younger 

Newhalen stade indicate that it is approximately 11,000 

cal. yr BP. Undivided Brooks Lake glaciation deposits 

are present on the western and southeastern portions 

of the map area (Figure 40). In the Cabin Bluff exposure 

(Figure 25) studied by VanderHoek and Myron (2004), 

peat directly above glacial drift had an age of 10,712 ± 

116 cal. yr BP, which provides a minimum age for the 

timing that glaciers extended to the Pacific coast in the 

Aniakchak Bay area.

Raised Marine Terraces

On the Bristol Bay side of the Alaska Peninsula, marine 

deposits and wave-cut scarps (steep banks) are present 

about 38 km (24 mi) inland, up the Meshik River valley, 

at about the 15 m (50 ft) above sea level. On the Pacific 

side, marine terraces and deposits are present near the 

coast, 28–30 m (92–98 ft) above sea level (unit Qmt on 

Plate 2; Detterman et al. 1981b). Possible causes of these 

raised marine features include isostatic rebound as a 

result of glacier loss and regional tectonic uplift. The 

weight of glacial ice can depress Earth’s crust and when 

it melts, the crust gradually rebounds. Detterman (1986) 

provided a brief overview of the tectonic and isostatic 

evidence for the Alaska Peninsula, suggesting that these 

marine terraces may be the result of differential glacial 

isostatic rebound. The peninsula is located over the 

Aleutian megathrust; however, so the relative effects 

of ongoing deformation, movement along faults that 

create earthquakes, and isostatic rebound are hard to 

decipher.

Beach Berms

Beach berms are present in Aniakchak, Amber, and 

Kujulik bays. In the Aniakchak area, abundant loosely 

consolidated and easily erodible volcanic sediments are 

carried by rivers to beaches where wave action causes 

longshore currents that transport sediment down the 

beaches. Aniakchak Bay has great examples of multiple 

generations of beach berms, because the input of 

sediment outpaces erosion (Figure 41, Figure 42). As a 

result, the beach progresses oceanward. Winter wave 

action carries sediment and logs up and over the top of 

the front berms; the logs reinforce the berms. Aeolian 

deposits cap many of the berms. These beach berms 

are 10–50 m (30–160 ft) wide and up to 20 m (65 ft) 

tall (VanderHoek and Myron 2004). The beach berms 

are mapped as part of unit Qb on Plate 2, which also 

consists of modern beach deposits.

Figure 41. Aerial photograph of beach berms at the 

mouth of the Aniakchak River. NPS photograph by 

Tahzay Jones.

0 1,000 2,000

Yards

0 1,000 2,000

Meters Á

North

beach berms

Aniakchak

Bay

Aniakchak River

Aniakchak

Lagoon

abandoned

channel

Figure 42. Satellite image (IKONOS 2005) of the beach 

berms of Aniakchak Bay.
35

Geologic History

This chapter describes the chronology of geologic events that formed the geologic features discussed in 

the Geology Features and Processes chapter. 

Jurassic Volcanic Arc and Sedimentary Rocks

The oldest rock unit at Aniakchak National Monument 

and Preserve is the Upper Jurassic (161–145 million 

years old) Naknek Formation, which consists of arkosic 

sandstone, siltstone, and conglomerate. The arkosic 

nature and granitic clasts in conglomerate suggests that 

the Naknek Formation was formed by erosion of the 

plutonic roots of the Lower Jurassic (202–176 million 

years ago) Talkeetna arc, which makes up most of the 

Peninsular terrane (Figure 8). The Talkeetna arc formed 

as an island arc in the paleo-Pacific ocean. Erosion of 

the deep roots of the arc and presence of nonmarine 

portions of the Naknek Formation attest to the uplift of 

the Talkeetna arc during the Late Jurassic.

Cretaceous Sedimentary Rocks and 

Unconformity

The Naknek Formation is overlain by the Lower 

Cretaceous (145–135 million years ago) marine 

sedimentary rocks of the Staniukovich and Herendeen 

formations. After these units were deposited, a period 

of erosion took place which corresponds with the uplift 

of the Peninsular terrane (Figure 7). The first units 

deposited after this hiatus were the Upper Cretaceous 

(about 80–70 million years ago) Chignik and Hoodoo 

formations, which lie unconformably on the older 

rocks. Fluvial and nearshore sedimentary rocks of the 

Chignik Formation are the proximal facies of the coeval 

finer grained rocks of the Hoodoo Formation that were 

deposited farther offshore on a marine shelf, slope, and 

submarine fan.

Terrane Translations

Paleomagnetic evidence and fossil assemblages suggest 

that the Talkeetna arc most likely formed offshore 

and south of its current position relative to the North 

American continent (Figure 43). The paleomagnetic 

inclination of the Earth, which changes with latitude, is 

preserved in sedimentary rocks by magnetite mineral 

grains. During deposition, some of the magnetite 

grains rotate and align to Earth’s magnetic field. When 

the sediment is cemented together to form rock, the 

aligned magnetite grains are locked in place. The earth’s 

inclination is steepest at the poles and shallowest near 

the equator. Reconnaissance paleomagnetic studies 

of the Chignik and Hoodoo formations suggested 

that the units were located about 25° ± 17° south of 

their present latitude, relative to the North American 

craton, when they were deposited (Stone and Packer 

1977, 1979). These results indicate that the Peninsular 

terrane formed somewhere off of Baja California, 

so Stone and Packer (1977) coined the term “Baja 

Alaska” to represent the translation history of these 

rocks. These original results have high error bars, but 

are similar to paleomagnetic measurements in other 

Upper Cretaceous rocks of southern Alaska and British 

Columbia (Hillhouse and Coe 1994; Stamatakos et al. 

2001; Kent and Irving 2010). These data initiated a longstanding debate over the amount of translation these 

rocks have undergone since the Cretaceous. The debate 

is now referred to as the “Baja-BC hypothesis” (see 

summary in Cowan et al. 1997).

The position of the Peninsular terrane through time 

(paleogeography) is important for understanding the 

past environments and habitats of extinct species found 

in the rock record of the Aniakchak area. For example, 

because dinosaurs lived on land, dinosaur tracks in 

the Naknek and Hoodoo formations indicate that the 

Peninsular terrane was near a continent during the 

Late Jurassic and Cretaceous periods. However, the 

Lower Jurassic rocks of the Peninsular terrane were 

formed as an island arc, so an understanding of the 

terrane translations would shed light on the possible 

pathways dinosaurs followed to roam onto the terrane. 

A recent discovery of marine glacial deposits in the 

Upper Jurassic Naknek Formation (Wartes and Decker 

2015) suggests that the Peninsular terrane experienced 

a very cold climate during an extended period of 

greenhouse conditions globally. Translation of the 

Peninsular terrane would affect the paleoenvironment 

of deposition of the geologic units and the biodiversity 

of fossils found in those units. Investigators have studied 

these ideas for other terranes elsewhere along the west 

coast of North America, but very little follow up of the 

rocks that started the Baja-BC (and Alaska) debate has 

ensued.
36

Paleolatitude

60°N

30°N

Equator

Recent

eE

Eocene

50Ma

mid-K

midCretaceous

90Ma

Early

Jurassic

190Ma

Late Triassic

225Ma

Wrangellia

composite terrane

and

Stikinia terrane

Figure 43. Schematic map showing Wrangellia composite terrane translation history from the Late Triassic (235–202 

million years ago) to the present. Evidence from fossils suggests that the terrane was far offshore of North America 

during the Triassic and Jurassic periods (235–146 million years ago), but how far offshore is not known. Ma = “million 

years ago.” Modified from Kent and Irving (2010) to include the Peninsular terrane attached to the rest of the 

Wrangellia composite terrane (see Wrangellia-Peninsular connections in Nokleberg et al. 1994 and Rioux et al. 2010).

Tertiary Sedimentary Rocks and Initiation of a 

New Volcanic Arc

Deposition of the nonmarine lower Tertiary Tolstoi 

Formation followed a short period of erosion, marked 

by a regional unconformity (Figure 7). Braided rivers 

brought sediment eroding from older rocks of the 

Peninsular terrane to nearshore deltas. Analysis of the 

plant fossils and marine mollusks indicates that the 

climate during deposition was subtropical.

Volcanism on the Alaska Peninsula and Aleutian Islands 

began approximately 46 million years ago (Jicha et al. 

2006). Arc volcanism shifted from the Bering sea coast 

to the Aleutian arc possibly as a result of the accretion 

of a piece of oceanic crust or migration of the arc 

westward (Scholl et al. 1986, 2015). Subduction shifted 

outboard and volcanism started forming the middle 

Tertiary Meshik Volcanics and associated intrusive 

rocks (Figure 34). Volcanism was intermittent, but 

continues today along generally the same trend as the 

modern Aleutian volcanic arc.

Quaternary Volcanism and Glaciation

Volcanic History

The Aleutian volcanic arc is the result of subduction of 

the Pacific plate under Alaska. Aniakchak volcano is an 

example of the potential explosiveness of Aleutian arc 

volcanoes. Deposits of Aniakchak eruptions go back at 

least 850,000 years, but these older volcanic deposits 

exposed in the caldera walls are not well studied. A 

couple large post-glacial eruptions formed deposits 

referred to as Aniakchak I (approximately 10,000–8,000 

cal. yr BP) and the Black Nose Pumice (8,165 ± 89 cal. 

yr BP; Figure 24). A few outcrops of these deposits are 

preserved around the crater rim (units Qa1 and Qbn in 

Figure 14). The Black Nose Pumice is correlated with an 

orange pumice layer in the sea bluff exposures along the 

Pacific coast in Aniakchak Bay (Figure 25).
37

The Aniakchak Crater formed during a massive caldera 

forming eruption 3,660 ± 70 cal. yr BP, referred to as the 

Aniakchak II event, which formed a thick ignimbrite 

sheet surrounding the volcano (unit Qafd on Plate 2) 

and ejected ash into the atmosphere that was carried by 

winds and deposited in peat bogs up to 1,000 km (620 

mi) north on the Seward Peninsula. Traces of the ash 

were also found in Greenland ice cores. The massive 

pyroclastic flow from the eruption reached both 

coastlines, and upon entering Bristol Bay, it generated 

a tsunami that inundated the northern mainland coast 

(Figure 13). Fallout from the Aniakchak eruption 

obliterated vegetation and wildlife and probably 

resulted in a 1,500 year hiatus in human habitation in 

the area.

After the Aniakchak II eruption, water filled the caldera 

with a lake (Figure 27). Subaqueous eruptions emplaced 

four domes (Vulcan, West, Pumice, Bolshoi domes) 

under the lake. Possibly around 1,800 cal. yr BP, a great 

flood rushed out of the caldera through The Gates, 

scouring the valley of the Aniakchak River, depositing 

large alluvial terraces (unit Qat on Plate 2), and 

depositing large alluvial fans (unit Qaf on Plate 2). The 

floodwaters created a second channel at Aniakchak Bay 

(unit Qac on Plate 2).

Numerous eruptions after the flood formed many 

more volcanic domes, cones, maars, and dikes. 

Vent Mountain and Half Cone, which have erupted 

numerous times over the last 820 years, are the most 

prominent (Figure 20, Figure 14). The youngest feature 

is the 1931 crater and related vents (Figure 22) resulting 

from a six-week-long eruption, which was documented 

by Bernard Hubbard. This eruption was one of the 

largest historic eruptions in Alaska.

Glacial History

The oldest glacial deposits are from the Mak Hill 

glaciation, which took place more than 44,000 cal. yr 

BP. These deposits are exposed near the western edge 

of the map area (Figure 40; unit Qmhd on Plate 2). Most 

glacier deposits are younger and part of the Brooks 

Lake glaciation, which occurred 10,000 cal. yr BP or 

earlier. 

Post-Glacial History

After retreat of the Pleistocene (2.6 million–11,700 

years ago) glaciers, the land underneath rebounded 

after the weight of the ice was removed. The Pacific 

side, where precipitation was greatest and the glaciers 

were thickest, the land uplifted 28–30 m (92–98 ft), 

compared to the 15 m (50 ft) of uplift on the Bristol 

Bay side of the Alaska Peninsula where the glaciers 

were thinner. However, tectonic forces could also have 

caused some of the uplift. Tsunami deposits along the 

coast provide evidence of large earthquakes along 

the Pacific coast. Glaciers had a profound impact on 

the shape of the landscape, but since they receded, 

rivers and other erosive geomorphic processes have 

altered the landscape. After formation of the Aniakchak 

Crater, the caldera filled with water (Figure 27), which 

eventually breached catastrophically. The outburst 

flood dumped a massive amount of sediment along the 

modern Aniakchak River valley (Figure 28, Figure 29) 

and scoured what are now abandoned river channels 

(Qac on Plate 2). A much smaller outburst flood from 

the NE maar lake (Figure 30, Figure 31) scoured the 

alluvial fan (Figure 32) and modified the Aniakchak 

River for kilometers downstream of The Gates. Rivers 

have carried the easily eroded loose volcanic material 

to the coast, and where they reach bays, beach ridges 

have formed (Figure 41, Figure 42). Landslides mapped 

throughout the area (Qls on Plate 2) indicate a changing 

landscape. The Aniakchak area is a dynamic place with 

active geomorphic processes and volcanism, which 

created a dramatic but potentially dangerous landscape. 

Geohazards are described in the “Geologic Resource 

Management Issues” chapter.

39

Geologic Resource Management Issues

This chapter describes geologic features, processes, or human activities that may require management 

for visitor safety, and preservation of natural and cultural resources in Aniakchak National 

Monument and Preserve. 

Geologic resource management issues were discussed 

during an October 2005 meeting and during a follow-up 

meeting held in 2014 (see Appendix D and summary by 

Graham 2005). The following geology related resource 

management issues were identified:

● Geohazards

● Paleontological Resource Inventory, Monitoring, 

and Protection

● Geothermal Energy Development

● Petroleum Development

● Mineral Extraction

● Coastal Issues

● Glacier Changes

● Crater Lake Level and Water Quality

Resource managers may find Geological Monitoring

(Young and Norby 2009; http://go.nps.gov/

geomonitoring) useful for addressing these geologic 

resource management issues. The manual provides 

guidance for monitoring vital signs—measurable 

parameters of the overall condition of natural resources. 

Each chapter covers a different geologic resource 

and includes detailed recommendations for resource 

managers, suggested methods of monitoring, and case 

studies.

Geohazards

The Aniakchak area has a very high potential for natural 

hazards. Aniakchak volcano is one of many active 

volcanoes of the Aleutian volcanic arc (Figure 4). The 

area also lies above the tectonically active Aleutian 

megathrust (Figure 6), and movement on this thrust 

causes frequent and large earthquakes.

The following is a list of potential geohazards in the 

Aniakchak area along with a brief description of each 

hazard and areas susceptible to them. A thorough 

explanation of the volcanic hazards of the Aniakchak 

area can be found in Neal et al. (2001).

Volcanic Eruptions

The USGS Alaska Volcano Observatory (AVO) is the 

primary source for evaluating volcanic hazards. Their 

website, https://www.avo.alaska.edu/ (accessed 2 

September 2015), provides real time volcanic activity 

status, warnings, and information about volcanoes 

across Alaska. According to the USGS volcanic threat 

assessment (Ewert et al. 2005), the Aniakchak area 

contains two historically active volcanoes (Aniakchak 

and Veniaminof) that are rated as a HIGH threat, and 

two volcanoes (Black Peak and Yantarni; Figure 1) rated 

as a MODERATE threat. Types of volcanic hazards 

expected in the Aniakchak area are illustrated in Figure 

44. Much of the Aniakchak area is blanketed by volcanic 

Figure 44. Diagram illustration of volcanic hazards. 

All are potential hazards in and around Aniakchak. 

USGS graphic by Myers and Driedger (2008), available 

online: http://pubs.usgs.gov/gip/64/gip64.pdf (accessed 2 

September 2015).
40

Aniakchak

Crater

The Gates

Aniakchak River

Amber

Bay

158°W 157°30'W

57°N

56°45'N

Landslides and rockfall

Slopes >40°

Volcanic ash and pyroclastic flows 0 2 4 6 8

Miles

0 2 4 6 8 10

Kilometers Á

North

Figure 45. Map showing volcanic ash and pyroclastic flow deposits, and landslide and rockfall areas (units Qc – 

colluvium, Qls – landslides, Qmf – mud flows, and Qdf – debris flows on Plate 2), and slopes greater than 40o, which 

have a high potential for producing rockfall. Slope map derived from the National Elevation Dataset 30 m grid digital 

elevation model.

ash, pyroclastic material, lahar deposits, and lava flows 

(Figure 45). All of Aniakchak National Monument 

and Preserve is at risk from volcanic hazards from 

Aniakchak volcano or any of the nearby volcanoes; 

however, most post-caldera eruptions have been within 

the confines of the caldera walls, so the greatest risk is 

within the caldera. Generally, volcanic eruptions are 

preceded by earthquakes that can be detected by the 

AVO seismic network on Aniakchak volcano.

Volcanic eruptions also involve the release of gas, 

including water vapor, carbon dioxide, sulfur dioxide, 

and hydrogen sulfide. Bernard Hubbard noted many 

dead birds in the caldera during the 1931 eruption; 

high initial concentrations of carbon dioxide gas likely 

killed these birds. No harmful concentrations of gas are 

currently being emitted, but the onset of an eruption 

may increase gas emission. One of the warm springs 

near Surprise Lake emits carbon dioxide and helium 

gases, but not at toxic levels (Neal et al. 2001).

Volcanic Monitoring

The historic eruption of 1931 and geological studies at 

Aniakchak volcano in the early 1990s underscore the 

potential for violent explosive eruptions and the need 

to monitor the volcano for signs of reawakening. In the 

mid-1990s, AVO installed a single seismometer on the 

west rim of Aniakchak volcano (Figure 46). Data from 

this station were sent to Port Heiden where a resident 

changed helicorder paper daily and sent them to AVO 

periodically. Data indicated a low background rate of 
41

Figure 46. Photograph of AVO staff installing seismic 

station “Surprise Lake” (ANSL) within the Aniakchak 

caldera. This station is typical of the 1990s style seismic 

stations and was installed in 1997 and decommissioned 

in 2000. USGS photograph by Game McGimsey.

small earthquakes occurring at the volcano. Experience 

early on with this station foretold of very challenging 

environmental conditions. For example, in 1997, AVO 

staff discovered that the steel antenna support pipe had 

suffered significant wind erosion after just one year of 

deployment.

A modern, six-station seismic network was deployed 

around and in the caldera in summer 1997 (Figure 

47). Each station consisted of a buried seismometer, 

a power system (deep-cycle batteries and solar panel 

array), electronic processor, digitizer, and a radio and 

antenna. Data from the network were telemetered to 

the Federal Aviation Administration facility at the Port 

Heiden airport and sent to AVO facilities via telephone. 

Data were examined daily by AVO analysts; a subset was 

shared with the public via the AVO website 

(https://www.avo.alaska.edu/webicorders/Aniakchak/; 

accessed 30 July 2015).

"

"

"

"

"

"

ANNE

ANNW

ANON

ANPB

ANPK

AZAC

158°10'W 158°W 57°N

56°50'N

0 1 2 3 4

Miles

0 1 2 3 4

Kilometers Á

North

Vent

Mountain

The Gates

Surprise

Lake

Figure 47. Map showing the locations of the AVO 

seismic network as of July 2015.

This network received maintenance visits by AVO 

personnel on many occasions following installation 

to replace batteries and failed components and to 

make modifications to mitigate the damaging effects 

of snowdrifts, icing, wind abrasion, and bears. A 

combination of these factors over the years made the 

Aniakchak network one of the least reliable of the AVO 

volcano networks, despite persistent efforts of field 

engineers to reconfigure and strengthen components. 

Despite technical problems, the network established 

a background rate/base level of seismicity, as well as 

detected earthquakes consistent with that of a young 

and potentially active volcanic system.

On 21 January 2014, AVO announced that the 

Aniakchak network was down and would not be 

repaired due to budget constraints and the necessity of 

focusing on the highest threat volcanoes in Alaska. In 

July 2015, AVO staff repaired five of the six stations in 

the network.

In addition to the seismic network, the following 

methods are used to detect volcanic unrest at Alaska 
42

volcanoes: (1) daily satellite analysis of images from 

space to look for signs of elevated surface temperatures 

or airborne volcanic ash; (2) regional infrasound 

capability that may be able to detect, with potentially 

hours of delay, an explosion from the vicinity of 

Aniakchak; (3) space-based radar interferometry 

(InSAR) is used to detect uplift or subsidence that may 

occur prior to eruptions, though this is not a realtime technique; and (4) pilot reports and other direct 

observations of changes at the volcano. Additional 

information about volcano monitoring in the National 

Park Service is available in the Geological Monitoring

chapter about volcanoes (Smith et al. 2009).

A description of the federal–state interagency response 

plan for volcanic unrest episodes in Alaska can be found 

in the Alaska Interagency Operating Plan for Volcanic 

Ash Episodes at https://www.avo.alaska.edu/pdfs/

cit3996_2014.pdf (accessed 1 January 2015).

Earthquakes

Earthquakes are common in the Aniakchak area 

because it lies over the Aleutian megathrust (Figure 

6). Subduction megathrusts generate the largest 

earthquakes of any type of plate boundary, thus very 

large earthquakes can occur in the Aniakchak area. 

According to the USGS 2009 Probabilistic Seismic 

Hazard Analysis Model (Figure 48), the probability for 

a moderate earthquake, greater than magnitude 5.5, in 

the next 20 years is between 0.20 and 0.50 or a 20% to 

50% “chance.” The direct effect of an earthquake may 

be large enough to make a person fall down, but it is the 

indirect effects of an earthquake, such as rockfall and 

landslides, that pose a higher risk. The lack of major 

infrastructure within the monument and preserve 

limits the risk to human development. The AVO seismic 

stations (mentioned in the “Volcanic Monitoring” 

section) provide seismic monitoring capability for the 

Aniakchak area. Additional information is available in 

Aniakchak Crater

Figure 48. Map showing the probability of earthquake of magnitude 5.5 over the next 20 years in the Aniakchak area. 

Map generated using the US Geological Survey earthquake probability mapping program (http://geohazards.usgs.gov/

eqprob/2009/index.php; accessed 9 January 2015).
43

the Geological Monitoring chapter regarding seismic 

activity (Braile 2009) which also described the following 

methods and vital signs for understanding earthquakes 

and monitoring seismic activity: (1) monitoring 

earthquakes, (2) analysis and statistics of earthquake 

activity, (3) analysis of historical and prehistoric 

earthquake activity, (4) earthquake risk estimation, (5) 

geodetic monitoring and ground deformation, and (6) 

geomorphic and geologic indications of active tectonics.

Rockfall and Landslides

Areas under steep cliffs have a high potential for 

rockfall (Figure 45). The Aniakchak Crater and flanks 

of the volcano consist of loose volcanic material that 

sheds constantly, and will increase during earthquakes 

and strong weather events. Talus and scree mapped 

as colluvium (unit Qc on Plate 2) represent areas of 

considerable rockfall. Steep slopes and shear walls are 

probable sources for rockfall. Such areas are shown in 

Figure 45.

Landslides (unit Qls on Plate 2) are present nearly 

everywhere steep slopes occur in the map area (Figure 

45). A large landslide is visible at The Gates (Figure 49), 

and may have formed during formation of the caldera 

(Game McGimsey, USGS/AVO, research geologist, 

personal communication, 27 March 2014). Landslides 

are associated with the poorly consolidated Tertiary 

sedimentary rocks of the Tolstoi Formation and Meshik 

Volcanics (units Tt and Tm on Plate 1). Landslides can 

occur at any time, but are typically triggered by volcanic 

eruptions, earthquakes, and heavy prolonged rain 

events.

For more information about assessing slope movements, 

hazards, and risks, the Geological Monitoring chapter 

about slope movements (Wieczorek and Snyder 

2009) described five vital signs for understanding and 

monitoring slope movements: (1) types of landslide, 

(2) landslide causes and triggers, (3) geologic materials 

in landslides, (4) measurement of landslide movement, 

and (5) assessment of landslide hazards and risks.

Figure 49. Photograph looking east at the north side of The Gates showing a scarp face and a jumbled landslide block 

with disrupted and tilted bedding. Black lines show the orientation of bedding within the in-place bedrock and in the 

landslide block. The two proposed lake maximum levels are shown as dashed lines on the caldera wall colored light 

blue and dark blue lines corresponding to the levels shown in Figure 27. Point A marks the location of lake terraces 

identified by McGimsey et al. (1994). USGS photograph by Game McGimsey (figure 4B of McGimsey et al. 1994).
44

Outburst Floods

The 2010 outburst flood quickly lowered the level of 

the water level in the NE maar lake by several meters 

(Figure 30). The flood wiped out vegetation on a large 

portion of the alluvial fan downstream of the maar 

(Figure 32), excavated the channel banks, and pushed 

debris into the Aniakchak River, which changed the 

character of the rapids through The Gates. If the 

outburst flood occurred during the summer visitor 

season, people camping on the alluvial fan near The 

Gates or along the Aniakchak River immediately below 

The Gates likely would have been harmed. Although 

the water level in the maar has dropped and a creek is 

currently flowing from the lake, high water levels during 

spring runoff or a heavy rainfall event may again cause a 

flood.

Tsunamis

Large earthquakes anywhere in the North Pacific can 

generate tsunamis that can affect the Aniakchak area. 

Movement along the Aleutian megathrust can cause 

the sea floor to rise or fall significantly, so the area has 

a high potential for earthquake generated tsunamis, 

as illustrated by the 1964 Alaska “Good Friday” 

earthquake and tsunami (more information about this 

earthquake is available at http://earthquake.usgs.gov/

earthquakes/events/alaska1964/, accessed 1 January 

2015), the 2004 Indian Ocean earthquake and tsunami, 

and the 2011 Tōhoku earthquake and tsunami in Japan. 

An earthquake generated tsunami in 1937 filled the 

Aniakchak lagoon to the extreme high tide line. A 

year later, fishermen were struck by a tsunami in Main 

Creek (in Amber Bay) (VanderHoek and Myron 2004). 

The NOAA National Tsunami Warning Center (based 

in Palmer, Alaska) monitors global earthquakes and 

tsunami potential for the coast of North America and 

publishes real-time watches, warnings, and advisories 

on its website (http://wcatwc.arh.noaa.gov/, accessed 8 

September 2015).

Tsunami deposits were recognized in the Aniakchak 

area by VanderHoek and Myron (2004), who noted 

a massive sand and silt bed approximately 38–50 

cm (15–20 in) below the surface in the Kujulik Bay 

area. Radiocarbon ages from above and below the 

bed constrain the age from 1,200 to 600 cal. yr. BP. 

A core in Aniakchak lagoon had two graded sand 

layers approximately 52–58 cm (20–23 in) below the 

surface. Radiocarbon ages from above and below 

this layer constrains the age between 660 and 650 cal. 

yr BP. VanderHoek and Myron (2004) also dated a 

log above an obvious unconformity in a beach berm 

near Main Creek at 660 cal. yr BP. They suggested 

that this evidence indicates a major tsunami affected 

the Aniakchak coast between 700 and 600 cal. yr BP, 

which they correlated with the age of a major Cascadia 

subduction zone earthquake.

Paleontological Resource Inventory, 

Monitoring, and Protection

Aniakchak National Monument and Preserve contains 

abundant and scientifically significant paleontological 

resources. The Jurassic and Cretaceous sedimentary 

units contain many marine invertebrate fossils, dinosaur 

tracks, and minor plant debris (Figure 7). The Mesozoic 

rocks have not yielded any well-preserved plant fossils 

to date, though minor petrified wood with preserved 

tree rings was found in the Hoodoo Formation and coal 

and plant debris are present in the Chignik Formation. 

The Naknek Formation contains abundant Buchia

fossils and some ammonite fossils (Figure 38). Three 

hadrosaur dinosaur tracks have been found in the 

Chignik Formation (Figure 35). Thus this formation 

warrants a more intense search and inventory of 

dinosaur tracks and a study of its paleoenvironment. At 

a minimum, an effort to inventory dinosaur tracks for a 

baseline would be useful for monitoring their status.

The Tertiary sedimentary rocks, and interbedded 

sedimentary rocks in the volcanic units, contain plant 

fossils that are well-preserved. Fossils from the Tolstoi 

Formation indicate the environment of deposition was 

subtropical, so additional study of these fossils and their 

paleogeography is an intriguing research opportunity.

Fossils in NPS areas occur in rocks or unconsolidated 

deposits, museum collections, and cultural contexts 

such as building stones or archeological resources. 

As of August 2015, 260 parks, including Aniakchak, 

had documented paleontological resources in at least 

one of these contexts. The NPS Geologic Resources 

Division Paleontology website, http://go.nps.gov/grd_

paleo, provides more information. All paleontological 

resources are non-renewable and subject to scienceinformed inventory, monitoring, protection, and 

interpretation as outlined by the 2009 Paleontological 

Resources Preservation Act (see Appendix D). As of 

August 2015, Department of the Interior regulations 

associated with the Act were being developed.
45

A field-based paleontological resource survey can 

provide detailed, site-specific descriptions and resource 

management recommendations that are beyond the 

scope of this report. Although a park-specific survey 

has not yet been completed for Aniakchak National 

Monument and Preserve, a preliminary paleontological 

inventory was completed for the Southwest Alaska 

Network by Kenworthy and Santucci (2003). 

In the Geological Monitoring chapter about 

paleontological resources, Santucci et al. (2009) 

described five methods and vital signs for monitoring 

in situ paleontological resources: (1) erosion (geologic 

factors), (2) erosion (climatic factors), (3) catastrophic 

geohazards, (4) hydrology/bathymetry, and (5) human 

access/public use. Fossils along NPS coastlines present 

additional management challenges and considerations 

as outlined by Brunner et al. (2009). 

Geothermal Energy Development

Having active volcanoes, the Aniakchak area has 

geothermal resources. The warm springs in the 

Aniakchak Crater were listed on the geothermal 

resources map of Motyka et al. (1993). Aniakchak 

is an area of historically active volcanism; however, 

developing infrastructure for a long-term energy project 

has significant risks. Notwithstanding the eruption 

risk, the remoteness of the Aniakchak area precludes 

it as a viable energy source because the current cost of 

transmission would be too great to make it economical. 

The prospects of geothermal energy exploration or 

development in the Aniakchak area are low.

The Geothermal Steam Act of 1970, as amended in 1988 

(see Appendix C), prohibits geothermal leasing in parks, 

and authorizes the Secretary of the Interior to mitigate 

or not issue geothermal leases outside parks that 

would have a significant adverse impact on significant 

thermal features in Aniakchak National Monument and 

Preserve and the other 15 parks listed in the act. The Act 

also requires monitoring of these significant features. 

According to the Federal Register, the “Aniakchak 

caldera” is a significant feature. As described in the 

“Geologic Features and Processes” chapter, there are 

15 warm springs within the caldera. In the Geological 

Monitoring chapter about geothermal systems and 

hydrothermal features, Heasler et al. (2009) described 

the following methods and vital signs for understanding 

geothermal systems and monitoring hydrothermal 

features:

(1) thermal feature location, (2) thermal feature extent, 

(3) temperature and heat flow, (4) thermal water 

discharge, and (5) fluid chemistry.

Petroleum Development

The marine sedimentary rocks of the Alaska Peninsula 

have been the target of petroleum exploration since 

1869. The first oil leases in the Aniakchak area, near 

Cold Bay, were granted in the 1890s. The first oil 

well was drilled in 1903 (Smith and Baker 1924). Oil 

seeps reported near Aniakchak Bay led to the granting 

of many leases in the Aniakchak area in the 1920s 

after the passage of an oil-leasing act (Norris 1996). 

Interest in the area waned after the USGS sent a field 

party to evaluate the oil potential of the area in 1922. 

The reported oil seeps could not be found and after 

evaluating the geology of the area, the group concluded 

that the Aniakchak district “is covered by large areas 

of igneous rocks, in which oil does not occur. The 

sedimentary rocks are mostly Tertiary and of a character 

that makes the possibility of the occurrence of oil in 

commercial quantities in them very slight” (Smith and 

Baker 1924, p. 211).

Although the Aniakchak area has a low potential 

for petroleum development, new technologies are 

changing the equation for economical deposits, so 

future exploration and development are possible. 

Presently, the Koniag Corporation holds subsurface 

mineral rights on significant tracts, known as “selected” 

158°W 157°30'W

57°N

56°30'N

State Leases

Selected

Patented

0 5 10

Miles

0 5 10

Kilometers

North

Figure 50. Map showing the extent of subsurface 

mineral rights within the boundary of the preserve. 
46

and “patented” areas within the preserve boundary 

(Figure 50). Also, many state oil leases are held outside 

the preserve boundary, mostly to the north (Figure 50). 

Although leases within the preserve can be developed, 

NPS regulations limit the impact that development 

would have on the park resources. Potential impacts 

include groundwater and surface water contamination, 

erosion and siltation, introduction of exotic plant 

species, reduction of wildlife habitat, impairment 

of viewsheds and night skies, excessive noise, and 

diminished air quality. Visitor safety and overall 

degradation of the visitor experience are particular 

concerns. The NPS Geologic Resources Division 

Energy and Minerals website, http://go.nps.gov/grd_

energyminerals, provides additional information.

Mineral Extraction

Studies of the mineral resources in the Aniakchak area 

found a low to moderate mineral potential (Knappen 

1929; Lyle and Dobey 1973; Cox et al. 1981). Five 

mineral occurrences are in the map area and are listed 

in the Alaska Resource Data File (records UG001, 

SW001, SW005, SW006, and SW008) at http://mrdata.

usgs.gov/ardf/ (accessed 30 July 2015). They are also 

included in the GRI GIS data (“Mine Point Features” 

layer). Four are hydrothermally altered areas associated 

with Tertiary intrusive igneous bodies, and one is a 

placer claim area along the Aniakchak River. None of 

the sites have been developed and only one of the sites 

in the park unit boundary, on Cape Kumlik (SW001), 

was explored in the 1970s by a mining company that 

found only minor copper values. In addition, the Mike 

Prospect (UG001) outside the boundaries of the park 

unit was drilled in the 1970s but never developed. 

Outside the map area, the beaches on the Bristol Bay 

side of the peninsula have claims for potential heavy 

mineral deposits (see ARDF record BB001).

Some of the Koniag subsurface mineral rights tracts 

(Figure 50) allow for the extraction of materials for 

supporting oil development. Exploration for pumice 

deposits in the 1950s found that the Aniakchak area 

was a potential source of pumice, particularly along the 

beaches near the mouth of the Aniakchak River and in 

deposits on Bear Creek (Moxham 1952). However, the 

distance from Anchorage or other markets presently 

precludes economically viable extraction of pumice 

from the Aniakchak area.

Unlike most Alaska NPS units, there are no abandoned 

mineral lands (AML) features or sites documented 

within Aniakchak National Monument and Preserve 

(Burghardt et al. 2014).

Coastal Issues

The raised marine terraces and beach berms are 

significant coastal features within Aniakchak (see 

“Geologic Features and Processes” chapter). In 

addition, Aniakchak National Monument and Preserve 

contains 217 km (135 mi) of shoreline (Curdts 2011). 

It is one of 118 parks servicewide that have been 

identified as potentially vulnerable to sea-level change. 

Geohazards (see “Geohazards” section), particularly 

tsunamis, may impact coastal features and processes. 

The NPS has developed a variety of databases and 

guidance for managing coastal resources and planning 

for the impacts of climate change. Refer to Appendix 

C for laws, regulations, and NPS policies pertaining 

to coastal resources. The few coastal assets and 

infrastructure within Aniakchak National Monument 

and Preserve limits potential impacts to human 

structures although those issues are pronounced 

elsewhere in the NPS. The 2006 monitoring plan for the 

Southwest Alaska Network (Bennett et al. 2006) did not 

establish a geomorphic coastal change monitoring plan 

for Aniakchak, although other parks in the network are 

monitored. Refer to the Marine Nearshore Monitoring 

website for additional information (http://science.

nature.nps.gov/im/units/swan/monitor/nearshore.

cfm?tab=0; accessed 4 September 2015).

Coastal Resource Management and Planning

The NPS Coastal Adaptation Handbook (RM 39-3; 

in review, expected 2016) will provide climate change 

adaptation guidance to coastal park managers in the 

118 parks that have been identified by their regional 

offices as potentially vulnerable to sea-level change. 

Focus topics will include NPS policies relevant to 

climate change, guidance on evaluating appropriate 

adaptation actions, and adaptation opportunities for 

planning, incident response, cultural resources, natural 

resources, facilities and assets, and infrastructure. The 

handbook will also provide guidance on developing 

communication and education materials about climate 

change impacts, and it will detail case studies of the 

many ways that individual parks are implementing 

adaptation strategies for threatened resources.
47

Additional Reference Manuals that guide coastal 

resource management include NPS Reference Manual 

#39-1: Ocean and Coastal Park Jurisdiction, which can 

provide insight for parks with boundaries that may 

shift with changing shorelines (available at http://www.

nps.gov/applications/npspolicy/DOrders.cfm); and 

NPS Reference Manual #39-2: Beach Nourishment 

Guidance (Dallas et al. 2012) for planning and managing 

nourishment projects.

The NPS is also developing a cultural resources 

climate change response strategy that connects climate 

science with historic preservation planning. The 

summary report from the Preserving Coastal Heritage 

workshop in 2014 identified and described six climate 

change adaptation options for cultural resources and 

cultural landscapes (no active intervention; offset 

stressors; improve resilience; manage change; relocate 

or facilitiate movement; document and release). 

Additional information about the workshop, and 

associated presentations and reports, are available at 

https://sites.google.com/site/democlimcult/ (accessed 4 

September 2015).

Coastal Resource Datasets

The NPS Geologic Resources Division (GRD) and 

Climate Change Response Program (CCRP) are 

developing sea-level rise and storm surge data that 

parks can use for planning purposes over multiple 

time horizons. The project, led by Maria Caffrey of 

GRD, should be completed by 2016 and will analyze 

rates of sea level coupled with potential storm surge 

in 105 of the vulnerable parks in order to project, for 

each park, the combined elevations of storms surge 

and sea level by 2030, 2050, and 2100. For Aniakchak 

National Monument and Preserve, the projections 

(Table 3) were created using various scenarios from the 

IPCC (Intergovernmental Panel on Climate Change) 

and USACE (US Army Corps of Engineers). There is a 

wide spread in projections particularly for the USACE 

data (calculated for Sand Point, Alaska) but the IPCC 

numbers do not account for land movement (e.g., 

see “Raised Marine Terraces” section; M. Caffrey, 

Research Associate, University of Colorado, written 

communication, 4 September 2015).

The NPS developed a report entitled Adapting To 

Climate Change in Coastal Parks: Estimating the 

Exposure of FMSS-Listed Park Assets to 1 m of SeaLevel Rise (Peek et al. 2015). This report includes 

the geospatial location and approximate elevation 

of over 10,000 assets in 40 coastal parks, based on 

information within the NPS Facilities Management 

Software System (FMSS) and supplemented with other 

datasets, collaboration with park staff, and field visits to 

locate assets. Assets were characterized based on their 

overall exposure to long-term (1 m) sea-level rise and 

associated storm vulnerability, and were categorized 

as having either high exposure or limited exposure to 

sea-level rise impacts. There are no coastal assets in 

Aniakchak National Monument and Preserve.

Parks can also consult suggested protocols such as the 

Geological Monitoring chapter about coastal features 

and processes defined in Bush and Young (2009), which 

described methods and vital signs for monitoring the 

following coastal features and processes:

(1) shoreline change, (2) coastal dune geomorphology, 

(3) coastal vegetation cover, (4) topography/elevation, 

(5) composition of beach material, (6) wetland position/

acreage, and (7) coastal wetland accretion.

Table 3. Sea level rise projections based on IPCC (Intergovernmental Panel on Climate Change) and USACE (US Army Corps 

of Engineers) scenarios. 

Scenario 2030 (m) 2030 (ft) 2050 (m) 2050 (ft) 2100 (m) 2100 (ft)

IPCC RCP 2.6 0.06 0.20 0.12 0.39 0.34 1.12

IPCC RCP 4.5 0.09 0.30 0.16 0.52 0.38 1.25

IPCC RCP 6.0 0.09 0.30 0.17 0.56 0.4 1.31

IPCC RCP 8.5 0.1 0.33 0.17 0.56 0.5 1.64

USACE Low (based on Sand Point, AK) 0.04 0.13 0.05 0.16 0.1 0.33

USACE Intermediate (based on Sand Point, AK) 0.07 0.23 0.14 0.46 0.42 1.38

USACE High (based on Sand Point, AK) 0.2 0.66 0.43 1.41 1.42 4.66

Table provided by Maria Caffrey, NPS GRD and University of Colorado, Research Associate, written communication, 4 September 2015). RCPs are 

“Representative Concentration Pathways” and are described online: http://sedac.ipcc-data.org/ddc/ar5_scenario_process/RCPs.html (accessed 4 

September 2015).
48

The NPS Water Resources Division, Ocean and Coastal 

Resources Branch website (http://www.nature.nps.gov/

water/oceancoastal/; accessed 4 September 2015) has 

additional information about servicewide programs and 

the resources and management programs at the ocean, 

coastal, and Great Lakes parks. Shoreline maps of each 

park, along with shoreline and water acreage statistics 

from Curdts (2011), are available at http://nature.nps.

gov/water/oceancoastal/shorelinemaps.cfm (accessed

4 September 2015).

Glacier Changes

Loso et al. (2014) conducted an Alaskawide glacier 

change study that included Aniakchak National 

Monument and Preserve. They counted 16 glaciers 

overall in recent satellite imagery as opposed to 29 

counted on the 1950s USGS topographic maps. 

However, the glacier area changed from 4.1 to 4.6 

km2. Small glaciers on the eastern boundary of the 

park shrunk or disappeared, while glaciers in the 

caldera grew or were mapped for the first time in the 

satellite imagery. The changes measured have a low 

reliability, and in fact Loso et al. (2014, p. 44) state that 

“implied changes over time should, in [Aniakchak], 

be disregarded” because the air photos used for the 

topographic maps were taken early in the season when 

there were many snow fields that might have been 

interpreted as glaciers. Also, the caldera glaciers are 

covered with debris, so their extent is difficult to map.

Alaskawide, Loso et al. (2014) noted that the trend 

of warmer summers and wetter winters will continue 

for at least the next 50 years and warming will 

accelerate. Recent glacier trends of negative mass 

balance, diminished ice cover, and reduced ice volume 

will intensify (Loso et al. 2014). The Southwest 

Alaska Network does not include glacial extent 

as a monitorable vital sign for Aniakchak National 

Monument and Preserve (Bennett et al. 2006). More 

information about glaciers in other Southwest Alaska 

Network parks is available at the network website 

http://science.nature.nps.gov/im/units/swan/monitor/

landscape.cfm?tab=1 (accessed 4 September 2015). 

Servicewide glacier monitoring techniques and vital 

signs were suggested by Karpilo (2009). Information 

about climate change in National Park Service and/

or Alaska is available at the resources listed in the 

“Additional References” chapter.

Crater Lake Level and Water Quality

Surprise Lake supports spawning and nursing habitat 

for sockeye salmon and arctic char (Cameron and 

Larson 1992). Cameron and Larson (1992) conducted 

a thorough lake and warm spring water quality and 

hydrologic study of the Aniakchak area. They measured 

the Surprise Lake height changes during one summer 

season and recorded a maximum variation of 15 cm 

after the largest rainstorm. They marked a rock along 

the lake as a reference point for future measurements. 

Cameron and Larson (1992) made a variety of water 

quality measurements that are only briefly summarized 

herein. The water quality of the lake varies spatially and 

with changes in the meteorological conditions. During 

the winter the lake is frozen over, so that precipitates 

(iron-oxide) from the warm springs are deposited 

near the spring mouth. These precipitates are quickly 

suspended after breakup when strong winds create 

waves that batter the shoreline. The warm springs have 

the strongest influence on the water quality near their 

outlets, but strong windstorms are effective at mixing 

and homogenizing the water chemistry. Water clarity 

is normally good until August when phytoplankton 

bloom. Temperature, conductance, alkalinity, and 

hardness are highest near the warm springs. Dissolved 

oxygen and pH are lowest near the warm springs. No 

salmon spawn on the warm spring side of the lake, 

but prefer the side of the lake near the crater wall 

where fresh groundwater flows into the lake (Hamon 

and Pavey 2012). Surface hydrology and freshwater 

chemistry are vital signs monitored for Aniakchak 

National Monument and Preserve as part of the 

Southwest Alaska Network monitoring plan (Bennett 

et al. 2006; see also the network’s Freshwater Flow 

Systems monitoring website at http://science.nature.

nps.gov/im/units/swan/monitor/freshwater.cfm?tab=0

(accessed 4 September 2015). The NPS Water 

Resources Division is the servicewide contact for water 

quality issues (http://nature.nps.gov/water; accessed 4 

September 2015).

Potential Future Studies

Aniakchak II Eruption

The Aniakchak II deposits have been studied only 

on a reconnaissance level. Little is known about the 

distribution, changing character, stratigraphy, and 

sedimentology of this world class ignimbrite. Previous 

studies have focused on petrology rather than physical 

processes, so a research opportunity is to investigate 
49

how this deposit came to rest on the landscape. Also, 

studies on how this cataclysmic eruption impacted the 

regional ecosystem may yield insights into how large 

volcanic events affect the biosphere, building on the 

work of VanderHoek and Myron (2004) and Bacon et 

al. (2014).

Aniakchak I Eruption Age and Chemistry

Aniakchak I distal eruption deposits have not yet been 

identified. Tephra layers around the Aniakchak area and 

elsewhere on the Alaska Peninsula, notably in exposures 

along the coast, could yield information about the 

Aniakchak I eruption. For example, comparing the 

geochemistry of the tephra layers in the Cabin Bluff 

exposure (Figure 25) to the geochemistry of proximal 

deposits reported in Bacon et al. (2014) may reveal 

which layer (if any) are correlative with the Aniakchak I 

eruption.

Source, Significance, and Timing of Proximal and 

Distal Deposits

The Holocene eruption history of Aniakchak volcano 

deserves refinement. Potential studies include a detailed 

tephra chronology, radiocarbon dating, and correlation 

of deposits. Tephra samples collected by VanderHoek 

and Myron (2004) from Cabin Bluff (Figure 25) were 

recently tested for geochemical comparison with rim 

samples. Analysis confirmed the correlation between 

the thick, orange tephra layer with that of the Black 

Peak Pumice (Bacon et al. 2014). This reconnaissance 

effort highlights the importance for more detailed 

work correlating distal deposits with proximal deposits 

near the caldera. A detailed chronology of Holocene 

eruptions would provide insights to the frequency of 

eruptions for the volcano.

Older Volcanic Flows

Older volcanic flows making up the caldera walls have 

been little studied. Only abstracts about the age and 

chemistry of the lava flows have been published (Nye 

et al. 1995, 1997), but no thorough study has been 

completed for these older lava flows. Based on Nye 

et al. (1997), the lava flows are at least 850,000 yr BP. 

Also, different ages of lavas show some geochemical 

differences, hinting at an interesting geologic story 

and leading to the following questions: What is the 

chronology of the lava flows? How far back does the 

eruption history of Aniakchak go? As magma erupted 

from the volcano, how did the composition change over 

time? Detailed mapping, geochemistry, petrology, and 

radiometric dating of the rim and wall rock and older 

deposits exposed outside the caldera are needed to 

answer these questions.

Outburst Flood Deposits

Questions also remain about the extent of flood 

deposits. The outburst flood deposits were studied by 

McGimsey et al. (1994) and Waythomas et al. (1996) 

primarily near The Gates and along the Aniakchak 

River, but more distal areas contain thick deposits 

from the flood and were apparently affected by it. Thus 

future studies could address the path of the flood: Did 

floodwaters flow down the Aniakchak River, Cinder 

Creek, over Birthday Pass, or all three? The flood debris 

may have created a temporary dam along the preAniakchak River valley at the constriction 10 km (6 mi) 

downstream. The flow direction appears to have taken a 

turn to the north down Cinder Creek. 

The timing of the flood is only tentatively constrained 

by the age of peat deposits above a sandy layer in a 

hypothesized outburst flood channel near Aniakchak 

Bay. Other sites could be studied that may help 

constrain the age of the flood. For example, if the base 

of the alluvial fan deposits (Figure 29) can be identified, 

then a test of the age of the outburst flood could be 

made. The ponds overlying the scoured bedrock 

channels may hold peat that could help constrain the 

minimum age of the flood deposits.

Also needed is a good model for what may have caused 

the outburst flood. This has implications for volcano 

hazards worldwide from similar lake filled calderas (see 

review by Manville 2010).

Paleontology

Dinosaur tracks in the Chignik Formation were found 

by paleontologist Anthony Fiorillo and party in 2001 

during a short reconnaissance field study (Fiorillo et 

al. 2004; Fiorillo and Parrish 2004). More tracks have 

been found by NPS staff and visitors (Figure 35). These 

dinosaur tracks hint at a greater potential. Further 

exploration is necessary to understand the extent and 

diversity of these dinosaurs, their biostratigraphic 

significance, and the environment in which they lived. 

Reconnaissance paleomagnetic studies in the 1970s 

of the Chignik and Hoodoo formations suggested 

that the Peninsular terrane was situated far south of 

its present position relative to the North American 

continent (Stone and Packer 1977). Since publication 
50

of these studies, better techniques have been 

developed to extract more reliable paleomagnetic 

data from sedimentary rocks. New investigation of 

the Chignik Formation is critical to understanding its 

paleogeography, latitude, and environment where these 

dinosaurs inhabited and left tracks. 

The Naknek Formation has also produced dinosaur 

tracks near Black Lake, southwest of the Aniakchak 

area (Druckenmiller et al. 2011). The occurrence of 

plant fossils in the Naknek Formation in the map area 

suggests a potential for other types of fossils, possibly 

dinosaur tracks, in nearshore deposits.

The plant fossils and marine mollusks in the Tolstoi 

Formation indicate the unit was deposited in a warm 

climate (Detterman et al. 1996), and paleomagnetic 

data show the unit was far south of its present position 

relative to the North American continent (Stone and 

Packer 1977; Coe et al. 1985; Hillhouse and Coe 1994). 

The paleoenvironment and paleogeography of this unit 

warrants detailed study.

Detailed Surficial Geologic Map

New interferometric synthetic aperture radar (InSAR) 

digital elevation models are being collected for Alaska 

that have a 5 m (16 ft) pixel resolution, which is much 

better than the available 30 m (100 ft) pixel resolution 

DEM, and is good for 1:24,000 scale surficial mapping. 

Currently available surficial geologic maps were 

published at a 1:250,000 scale (Plate 2; Detterman et al. 

1981b, 1987), so mapping using the new detailed DEM 

will be more detailed. A detailed surficial geologic map 

would be useful for evaluation of landslide and rockfall 

hazards; evaluation of parent material for soils mapping; 

mapping of outburst flood deposits, mapping volcanic 

features and deposits; coastal geomorphology studies; 

engineering properties for a potential ATV trail near the 

northern boundary of the preserve; and interpretation 

of marine terrace uplift and glacial histories.
51

Geologic Maps

Geologic maps facilitate an understanding of an area’s 

geologic framework and the evolution of its present 

landscape. Using designated colors and symbols, 

these maps portray the spatial distribution and 

temporal relationships of rocks and unconsolidated 

deposits. Geologic maps can be divided into two 

primary types: surficial and bedrock. Surficial 

geologic maps typically encompass deposits that are 

unconsolidated and formed during the past 2.6 million 

years (the Quaternary Period). Surficial map units 

are differentiated by geologic process or depositional 

environment. Bedrock geologic maps encompass 

older, typically more consolidated sedimentary, 

metamorphic, and/or igneous rocks. Bedrock map units 

are differentiated based on age and/or rock type.

Geologic maps often depict geomorphic features, 

structural interpretations (such as faults or folds), 

and locations of past geologic hazards that may be 

susceptible to future activity. Anthropogenic features 

such as mines or quarries, as well as observation or 

collection locations, may be indicated on geologic 

maps. The American Geosciences Institute website, 

http://www.americangeosciences.org/environment/

publications/mapping, provides more information 

about geologic maps and their uses. 

Source Maps

The source of digital geologic map data for National 

Park System units in Alaska is the Alaska digital geologic 

map database, which is compiled and maintained under 

the direction of Frederic H. Wilson of the USGS Alaska 

Science Center. Wilson has been compiling geologic 

data for the State of Alaska for more than two decades 

and has produced numerous regional geologic maps as 

part of this project (see http://mrdata.usgs.gov/geology/

state/state.php?state=AK).

The source of the digital geologic map data for 

Aniakchak National Monument and Preserve is an 

updated version of this statewide database. The most 

recent digital data set for the Aniakchak area was 

published as a geologic map of the Alaska Peninsula by 

Wilson et al. (1999):

Wilson, F. H., R. L. Detterman, and G. D. Dubois. 1999. 

Preliminary geologic framework of the Alaska Peninsula, 

Southwest Alaska, and the Alaska Peninsula terrane. OpenFile Report 99-317:41 (1:500,000). US Geological Survey, 

Washington, DC. http://geopubs.wr.usgs.gov/open-file/

of99-317/ (accessed 30 July 2015).

For the Aniakchak area, the Wilson et al. (1999) 

compilation was based on the following geologic maps 

(Figure 51):

Geologic Map

Detterman, R. L., J. E. Case, F. H. Wilson, and M. E. Yount. 

1987. Geologic map of the Ugashik, Bristol Bay, and western 

part of Karluk quadrangles, Alaska (scale 1:250,000). 

Miscellaneous Investigations Series Map 1685. US 

Geological Survey, Reston Virginia. 

Detterman, R. L., T. P. Miller, M. E. Yount, and F. H. Wilson. 

1981. Geologic map of the Chignik and Sutwik Island 

quadrangles, Alaska (scale 1:250,000). Miscellaneous 

Investigations Series Map I-1229. US Geological Survey, 

Reston Virginia.

Riehle, J. R., Yount, M. E., and Miller, T. P. 1987. Petrography, 

chemistry, and geologic history of Yantarni Volcano, 

Aleutian volcanic arc, Alaska (scale 1:63,360). Bulletin 1761. 

US Geological Survey, Reston Virginia.

Quaternary Geologic Map

Detterman, R. L., T. P. Miller, M. E. Yount, and F. H. Wilson. 

1981. Quaternary geologic map of the Chignik and Sutwik 

Island quadrangles, Alaska. Miscellaneous Investigations 

Series Map 1292 (1:250,000). US Geological Survey, Reston 

Virginia.

Detterman, R. L., F. H. Wilson, M. E. Yount, and T. P. Miller. 

1987. Quaternary geologic map of the Ugashik, Bristol 

Bay, and western part of Karluk quadrangles, Alaska. 

Miscellaneous Investigations Series Map 1801(1:250,000). 

US Geological Survey, Reston Virginia.

GRI GIS Data

The GRI team implements a GIS data model that 

standardizes map deliverables. The data model is 

available at: http://go.nps.gov/gridatamodel. This data 

model dictates GIS data structure, including layer 

Geologic Map Data

This chapter summarizes the geologic map data available for Aniakchak National Monument and 

Preserve used for the bedrock and Quaternary geologic maps presented on Plate 1 and Plate 2. The 

map unit properties tables (in pocket) summarize this report’s content for each geologic map unit. 

Complete GIS data are available at the GRI publications website: http://go.nps.gov/gripubs.
52

architecture, feature attribution, and relationships 

within ESRI ArcGIS software. The GRI team digitized 

the data for Aniakchak National Monument and 

Preserve using data model version 2.2. The GRI 

Geologic Maps website, http://go.nps.gov/geomaps, 

provides more information about GRI map products. 

GRI digital geologic data are available through the NPS 

Integrated Resource Management Applications (IRMA) 

portal (https://irma.nps.gov/App/Portal/Home). Enter 

“GRI” as the search text and select a park.

There are two map products for Aniakchak (bedrock 

and surficial). The following components are part of the 

data set:

● A GIS readme file (ania_gis_readme.pdf) 

that describes the GRI data formats, naming 

conventions, extraction instructions, use 

constraints, and contact information;

● Data in ESRI geodatabase GIS format;

● Layer files with feature symbology (Table 4);

● Federal Geographic Data Committee (FGDC)–

compliant metadata;

● Ancillary map information documents (ania_

geology.pdf) that contain information captured 

from source maps such as map unit descriptions, 

geologic unit correlation tables, legends, crosssections, and figures;

● ESRI map documents (ania_geology.mxd [bedrock] 

and asur_geology.mxd [Quaternary]) that display 

the digital bedrock and surficial data; and

● A KML/KMZ version of the data viewable in 

Google Earth (Table 4)

GRI Map Posters

Folded posters of the GRI digital geologic data 

draped over a shaded relief image of the park unit 

and surrounding area are included with this report as 

Plates 1 and 2. Not all data layers are included on the 

posters (Table 4). Geographic information and selected 

park features have been added to the posters. Digital 

elevation data and added geographic information are 

not included in the GRI GIS data set, but are available 

online from a variety of sources. Contact the GRI team 

for assistance locating these data.

Map Unit Properties Tables

The map unit properties tables, located in the pocket 

at the end of the report, list the geologic time division, 

symbol, and a simplified description for each of the 

bedrock and surficial geologic map units in the GRI GIS 

data. Following the structure of the report, the tables 

summarize the geologic features, processes, resource 

management issues, and geologic history associated 

with each map unit. 

Use Constraints

Graphic and written information provided in this 

report is not a substitute for site-specific investigations. 

Ground-disturbing activities should neither be 

permitted nor denied based upon the information 

provided here. Please contact GRI with any questions.

Aniakchak

NM&P

Detterman et al. (1981a,b)

Detterman et al. (1987a,b)

Figure 51. Map showing extent of original source 

geologic maps of the Aniakchak area by Detterman et 

al. Black line indicates the extent of the GRI geologic 

map data. 
53

Table 4. Geology data layers in the Aniakchak National Monument and Preserve GRI GIS data

Map Data Layer On Poster? Google Earth Layer?

Bedrock (Plate 1) Geologic Cross Section Lines Yes No

Bedrock (Plate 1) Geologic Attitude Observation Localities (strike and dip) No No

Bedrock (Plate 1) Radiometric Sample Localities No No

Bedrock (Plate 1) Radiocarbon Sample Localities No No

Bedrock (Plate 1) Mine Point Features Yes No

Bedrock (Plate 1) Map Symbology Yes No

Bedrock (Plate 1) Folds No Yes

Bedrock (Plate 1) Faults Yes Yes

Bedrock (Plate 1) Linear Dikes No Yes

Bedrock (Plate 1) Volcanic Line Features (caldera boundary) Yes Yes

Bedrock (Plate 1) Alteration and Metamorphic Area Boundaries No No

Bedrock (Plate 1) Alteration and Metamorphic Areas No Yes

Bedrock (Plate 1) Geologic Contacts No Yes

Bedrock (Plate 1) Geologic Units Yes Yes

Quaternary (Plate 2) Glacial Feature Lines (moraine crest) Yes Yes

Quaternary (Plate 2) Volcanic Line Feature (caldera boundary) Yes Yes

Quaternary (Plate 2) Geologic Contacts No Yes

Quaternary (Plate 2) Geologic Units Yes Yes

55

Glossary

These are brief definitions of selected geologic terms relevant to this report. Definitions are based on 

those in the American Geosciences Institute Glossary of Geology (5th edition; 2005). Additional terms 

are defined at http://geomaps.wr.usgs.gov/parks/misc/glossarya.html.

accretion (structural geology). The addition of island-arc or 

continental material to a continent via collision, welding, 

or suturing at a convergent plate boundary. 

aeolian. Describes materials formed, eroded, or deposited 

by or related to the action of wind. 

agglomerate. A consolidated pyroclastic rock made 

primarily of bombs. Roughly synonymous with spatter.

agglutinate. A welded pyroclastic deposit characterized by 

vitric material binding the individual clasts that commonly 

became fused while hot and viscous.

alluvial fan. A low, relatively flat to gently sloping, fanshaped mass of loose rock material deposited by a stream, 

especially in a semiarid region, where a stream issues from 

a canyon onto a plain or broad valley floor.

alluvial terrace. A stream terrace composed of 

unconsolidated alluvium produced by a rejuvenated 

stream via renewed downcutting of the floodplain or 

valley floor, or by the covering of a terrace with alluvium.

alpine glacier. A small glacier in a mountain range (i.e., not 

an ice cap or ice sheet) that usually originates in a cirque 

and may flow down into a valley previously carved by a 

stream. Synonymous with “mountain glacier” and “valley 

glacier.”

ammonite. Any ammonoid belonging to the suborder 

Ammonitina, characterized by a thick, ornamental shell 

with sutures having finely divided lobes and saddles. 

Range: Jurassic to Cretaceous.

andesite. A volcanic rock characteristically medium dark in 

color and containing approximately 57%–63% silica and 

moderate amounts of iron and magnesium.

angular unconformity. An unconformity between two 

groups of rocks whose bedding planes are not parallel 

or in which the older, underlying rocks dip at a different 

angle (usually steeper) than the younger, overlying strata.

ash. Fine-grained material, less than 2 mm (0.08 in) across, 

ejected from a volcano.

asthenosphere. Earth’s relatively weak layer below the 

rigid lithosphere where isostatic adjustments take place, 

magmas may be generated, and seismic waves are strongly 

attenuated; part of the upper mantle.

avalanche. A large mass of snow, ice, soil, or rock, or a 

mixture of these materials, falling, sliding, or flowing very 

rapidly under the force of gravity; velocities may exceed 

500 kph (300 mph). Commonly, in geologic writing, an 

avalanche other than a “snow avalanche” includes a 

descriptive term such as “debris avalanche.”

backshore. The upper or inner, usually dry, zone of a shore 

or beach. Also, the area lying immediately at the base of a 

sea cliff. Synonymous with “backbeach.”

bank. A submerged ridge of sand in the sea, a lake, or a river, 

usually exposed during low tide or low water.

basalt. A volcanic rock that is characteristically dark in color 

(gray to black), contains approximately 53% silica or less, 

and is rich in iron and magnesium.

basaltic andesite. A volcanic rock that is commonly dark 

gray to black and contains approximately 53%–57% silica.

basin (sedimentary). Any depression, from continental to 

local scale, into which sediments are deposited.

bathymetry. The measurement of ocean or lake depths and 

the charting of the topography of the ocean or lake floor. 

beach. The unconsolidated material at the shoreline that 

covers a gently sloping zone, typically with a concave 

profile, extending landward from the low-water line to 

the place where there is a definite change in material 

or physiographic form (e.g., a cliff), or to the line of 

permanent vegetation (usually the effective limit of the 

highest 

bed. The smallest sedimentary stratigraphic unit, commonly 

ranging in thickness from about 1 cm (0.4 in) to 1 to 2 m 

(40 to 80 in) and distinguishable from beds above and 

below.

bedding. Depositional layering or stratification of 

sediments.

bedrock. Solid rock that underlies unconsolidated 

sedimentary deposits and soil.

berm. A low, impermanent, nearly horizontal or landwardsloping bench, shelf, or ledge above the high-water line on 

a beach.

bivalve. Having a shell composed of two distinct, but equal 

or nearly equal, movable valves, which open and shut.

block. A pyroclast ejected in a solid state with a diameter 

greater than 64 mm (2.5 in).

block (fault). A segment of Earth’s crust bounded 

completely or partially by faults.

breccia. A coarse-grained, generally unsorted sedimentary 

rock consisting of cemented angular clasts more than 2 

mm (0.08 in) across. 

breccia (volcanic). A coarse-grained, generally unsorted 

volcanic rock consisting of partially welded angular 

fragments of ejected material.

calcareous. Describes a substance that contains calcium 

carbonate. When applied to a rock name it implies that as 

much as 50% of the rock is calcium carbonate.

caldera. A large, basin-shaped volcanic depression formed 

by collapse during an eruption.

cape. An extensive, somewhat rounded irregularity of land 

jutting out from the coast into a large body of water, either 

as a peninsula (e.g., Cape Cod, Massachusetts) or as a 

projecting point (e.g., Cape Hatteras, North Carolina). 

Also, the part of the projection extending farthest into the 

water.
56

channel. The bed of a stream or river. Also, a natural 

passageway or depression of perceptible extent 

containing continuously or periodically flowing water, or 

forming a connecting link between two bodies of water.

chert. An extremely hard sedimentary rock with conchoidal 

fracturing, consisting mostly of interlocking crystals of 

quartz. 

chronology. The arrangement of events in their proper 

sequence in time.

cinder cone. A conical hill, commonly steep, ranging 

from tens to hundreds of meters tall, formed by the 

accumulation of solidified fragments of lava that fell 

around the vent during a basaltic or andesitic eruption. 

clast. An individual constituent, grain, or fragment of a rock 

or unconsolidated deposit, produced by the mechanical 

or chemical disintegration of a larger rock mass.

coal. An organic sedimentary rock consisting of carbon, 

hydrogen, and oxygen, with some sulfur and nitrogen. 

Formed by the destructive distillation of plant remains 

under anaerobic and progressively rising pressures and 

temperatures.

coarse-grained. Describes a crystalline rock and texture 

in which the individual minerals are relatively large, 

specifically an igneous rock whose particles have an 

average diameter greater than 5 mm (0.2 in). Also, 

describes sediment or sedimentary rock and texture in 

which the individual constituents are easily seen with the 

unaided eye, specifically sediment or rock whose particles 

have an average diameter greater than 2 mm (0.08 in).

colluvium. A loose, heterogeneous, and incoherent mass 

of rock fragments and soil material deposited via surface 

runoff or slow continuous downslope creep; usually 

collects at the base of a slope or hillside, but includes 

loose material covering hillsides.

conglomerate. A coarse-grained, generally unsorted, 

sedimentary rock consisting of cemented, rounded clasts 

larger than 2 mm (0.08 in) in diameter.

contact. The surface between two types or ages of rocks.

continental. Formed on land rather than in the sea. 

Continental deposits may be of lake, swamp, wind, 

stream, or volcanic origin.

continental crust. Earth’s crust that is rich in silica 

and aluminum and underlies the continents and the 

continental shelves; ranges in thickness from about 25 

km (15 mi) to more than 70 km (40 mi) under mountain 

ranges, averaging about 40 km (25 km) thick.

continental shelf. The shallowly submerged—covered 

by water depths of less than 200 m (660 ft)—part of a 

continental margin that extends from the shoreline to the 

continental slope.

continental slope. The relatively steep slope from the outer 

edge of the continental shelf down to the more gently 

sloping ocean depths of the continental rise or abyssal 

plain.

cordillera. An extensive assemblage of more or less parallel 

chains of mountains (together with their associated 

valleys, basins, plains, plateaus, rivers, and lakes), 

especially the main mountain axis of a continent.

craton. The relatively old and geologically stable interior of 

a continent. 

cross-bedding. Uniform to highly varied sets of inclined 

beds deposited by wind or water that indicate flow 

conditions such as direction and depth. 

crust. Earth’s outermost layer or shell. 

dacite. A volcanic rock that is characteristically light in 

color and contains approximately 63%–68% silica and 

moderate amounts of sodium and potassium. 

debris flow. A moving mass of rock fragments, soil, and 

mud, with more than half of the particles larger than sand 

size. Slow debris flows may move less that 1 m (3 ft) per 

year; rapid ones reach 160 kph (100 mph). 

deformation. The process of folding, faulting, shearing, or 

fabric development in rocks as a result of Earth stresses.

detritus. Loose rock and mineral material that is worn off or 

removed by mechanical processes.

diorite. A coarse-grained, intrusive igneous rock 

characteristically containing plagioclase, as well as darkcolored amphibole (especially hornblende), pyroxene, 

and sometimes a small amount of quartz; diorite grades 

into monzodiorite with the addition of alkali feldspar.

discharge. The rate of flow of surface water or groundwater 

at a given moment, expressed as volume per unit of time.

dome. Any smoothly rounded landform or rock mass; more 

specifically, an elliptical uplift in which rocks dip gently 

away in all directions. 

drainage. The manner in which the waters of an area flow 

off in surface streams or subsurface conduits; also, the 

processes of surface drainage of water from an area by 

streamflow and sheet flow, and the removal of excess 

water from soil by downward flow.

drainage basin. A region or area bounded by a drainage 

divide and occupied by a drainage system, specifically 

the tract of country that gathers water originating as 

precipitation and contributes it to a particular stream 

channel or system of channels, or to a lake, reservoir, or 

other body of water.

drift. All rock material (clay, silt, sand, gravel, and boulders) 

transported and deposited by a glacier, or by running 

water emanating from a glacier. 

edifice. The constructional mass of a volcano.

effusive eruption. An eruption that produces mainly lava 

flows and domes. 

erosion. The general process or group of processes that 

loosen, dissolve, wear away, and simultaneously move 

from one place to another, the materials of Earth’s crust; 

includes weathering, solution, abrasive actions, and 

transportation, but usually excludes slope movements.

explosive eruption. An energetic eruption that produces 

mainly ash, pumice, and fragmental ballistic debris. 

fabric. The complete spatial and geometrical configuration 

of all components that make up a deformed rock, 

including texture, structure, and preferred orientation.

facies (sedimentary). The depositional or environmental 

conditions reflected in the sedimentary structures, 

textures, mineralogy, fossils, and other components of a 

sedimentary rock.

fault. A break in rock characterized by displacement of one 

side relative to the other.
57

feldspar. A group of abundant silicate (silicon + oxygen) 

minerals, comprising more than 60% of Earth’s crust and 

occurring in all types of rocks. 

felsic. Derived from feldspar +silica to describe an igneous 

rock having abundant light-colored minerals such as 

quartz, feldspars, or muscovite; also, describes those 

minerals.

fiamme. From “fiamma,” meaning “flame” in Italian. Dark, 

vitric lenses in welded tuffs, averaging a few centimeters 

in length, perhaps formed by the collapse of fragments of 

pumice. 

fissure vent. A volcanic conduit having the form of a crack 

or fissure at Earth’s surface.

flame structure. A wave- or flame-shaped plume of mud 

that squeezed irregularly upward into an overlying 

sedimentary layer.

floodplain. The surface or strip of relatively smooth land 

composed of alluvium and adjacent to a river channel, 

constructed by the present river in its existing regimen 

and covered with water when the river overflows its 

banks. A river has one floodplain and may have one or 

more terraces representing abandoned floodplains.

fluvial. Of or pertaining to a river or rivers.

fluvial channel. A natural passageway or depression 

produced by the action of a stream or river. 

formation. Fundamental rock-stratigraphic unit that is 

mappable, lithologically distinct from adjoining strata, 

and has definable upper and lower contacts.

fossil. A remain, trace, or imprint of a plant or animal that 

has been preserved in the Earth’s crust since some past 

geologic time; loosely, any evidence of past life.

fracture. The breaking of a mineral other than along planes 

of cleavage. Also, any break in a rock such as a crack, joint, 

or fault.

geology. The study of Earth, including its origin, history, 

physical processes, components, and morphology.

geomorphology. The study of the general configuration of 

surface landforms and their relationships to underlying 

structures, and of the history of geologic changes as 

recorded by these surface features.

geothermal. Pertaining to the heat of the interior of the 

Earth.

gravel. An unconsolidated, natural accumulation of typically 

rounded rock fragments resulting from erosion; consists 

predominantly of particles larger than sand; that is, 

greater than 2 mm (1/12 in) across.

groundwater. That part of subsurface water that is in the 

zone of saturation, including underground streams.

heavy mineral. A detrital mineral from a sedimentary rock, 

having a specific gravity higher than a standard (usually 

2.85). It is commonly a minor constituent or accessory 

mineral of the rock and less than 1% in most sands.

hoodoo. A bizarrely shaped column, pinnacle, or pillar of 

rock, commonly produced in a region of sporadic heavy 

rainfall by differential weathering or erosion of horizontal 

strata, facilitated by layers of varying hardness and joints.

hydrology. The study of liquid and solid water properties, 

circulation, and distribution, on and under the Earth’s 

surface and in the atmosphere.

hypabyssal. Describes an igneous rock or intrusive body 

formed at shallow depth. 

igneous. Describes a rock or mineral that solidified 

from molten or partly molten material; also, describes 

processes leading to, related to, or resulting from the 

formation of such rocks. One of the three main classes of 

rocks—igneous, metamorphic, and sedimentary.

ignimbrite. A pyroclastic flow deposit.

inlet. A small, narrow opening, recess, indentation, or other 

entrance into a shoreline through which water penetrates 

into the land; or a waterway entering a sea, lake, or river. 

Also, a short, narrow waterway between islands, or 

connecting a bay, lagoon, or similar body of water with a 

larger body of water.

intercalated. Describes the presence or insertion of a layer 

of material between layers of a different type.

intrusive. Pertaining to intrusion, both the process and the 

rock body.

island arc. A offshore, generally curved belt of volcanoes 

above a subduction zone.

kame. A mound, knob, or short irregular ridge, composed of 

stratified sand and gravel deposited by a subglacial stream 

as a fan or delta at the margin of a melting glacier.

karst. A type of topography that is formed on limestone, 

gypsum, and other soluble rocks, primarily by dissolution. 

It is characterized by sinkholes, caves, and underground 

drainage.

lacustrine. Describes a process, feature, or organism 

pertaining to, produced by, or inhabiting a lake.

lagoon. A narrow body of water that is parallel to the 

shore and between the mainland and a barrier island; 

characterized by minimal or no freshwater influx and 

limited tidal flux, which cause elevated salinities. Also, a 

shallow body of water enclosed or nearly enclosed within 

an atoll.

lahar. A mixture of water and volcanic debris that moves 

rapidly down the slope of a volcano, characterized by a 

substantial component (>50%) of fine-grained material 

that acts as a matrix to give the deposit the strength it 

needs to carry the bigger clasts. 

landslide. A collective term covering a wide variety of 

slope-movement landforms and processes that involve the 

downslope transport of soil and rock material en masse 

under the influence of gravity.

lapilli. Pyroclastic materials ranging between 2 and 64 mm 

(0.08 and 2.5 in) across with no characteristic shape; 

may be either solidified or still viscous upon landing. An 

individual fragment is called a lapillus.

lava. Molten or solidified magma that has been extruded 

though a vent onto Earth’s surface.

lava dome. A steep-sided mass of viscous, commonly 

blocky, lava extruded from a vent; typically has a rounded 

top and covers a roughly circular area; may be isolated or 

associated with lobes or flows of lava from the same vent; 

typically silicic (rhyolite or dacite) in composition.

lithic. Described a medium-grained sedimentary rock or 

pyroclastic deposit that contains abundant fragments of 

previously formed rocks.
58

lithic tuff. A dense deposit of volcanic ash that includes 

fragments of previously formed rocks that solidified in a 

vent and were then ejected.

maar. A low-relief, broad volcanic crater formed by multiple 

shallow explosive eruptions. It is surrounded by a lowrelief rim of fragmental material, and may be filled by 

water.

magma. Molten rock beneath Earth’s surface capable of 

intrusion and extrusion. 

magmatic arc. An arcuate line of plutons, volcanic rocks, or 

active volcanoes formed at a convergent plate boundary.

magnetite. Iron oxide. An oxide mineral composed of 

oxygen and iron, Fe3

O4; commonly contains manganese, 

nickel, chromium, and titanium. A very common and 

widely distributed accessory mineral in rocks of all kinds 

and as a “heavy mineral” in sand.

mantle. The zone of the Earth below the crust and above the 

core.

marine terrace. A relatively flat-topped, horizontal or 

gently inclined, surface of marine origin along a coast, 

commonly veneered by a marine deposit (typically silt, 

sand, or fine gravel).

member. A lithostratigraphic unit with definable contacts; a 

subdivision of a formation.

metamorphic. Pertaining to the process of metamorphism 

or to its results. 

metamorphic rock. Any rock derived from preexisting 

rocks that was altered in response to marked changes 

in temperature, pressure, shearing stress, and chemical 

environment. One of the three main classes of rock—

igneous, metamorphic, and sedimentary.

mineral. A naturally occurring inorganic crystalline solid 

with a definite chemical composition or compositional 

range.

moraine. A mound, ridge, or other distinct accumulation 

of unsorted, unstratified glacial drift, predominantly till, 

deposited mostly by direct action of a glacier.

mush. Partially crystallized magma.

oceanic crust. Earth’s crust that underlies the ocean basins 

and is rich in iron and magnesium; ranges in thickness 

from about 5 to 10 km (3 to 6 mi).

outcrop. Any part of a rock mass or formation that is 

exposed or “crops out” at Earth’s surface. 

outwash. Glacial sediment transported and deposited by 

meltwater streams.

outwash plain. A broad, gently sloping sheet of outwash 

deposited by meltwater streams flowing from the front of 

or beyond a glacier, and formed by coalescing outwash 

fans.

oxidation. The process of combining with oxygen.

paleogeography. The study, description, and reconstruction 

of the physical landscape in past geologic periods.

paleontology. The study of the life and chronology of 

Earth’s geologic past based on the fossil record.

parent material. The unconsolidated organic and mineral 

material from which soil forms.

peat. An accumulation of partly decomposed plant remains 

in swampy lowlands. It is an early stage or rank in the 

development of coal.

pebble. A small rounded rock, especially a waterworn stone, 

between 4 and 64 mm (0.16 and 2.5 in) across.

period. The fundamental unit of the worldwide geologic 

time scale. It is lower in rank than era and higher than 

epoch. The geochronologic unit during which the rocks of 

the corresponding system were formed.

phreatomagmatic. See “hydrovolcanic.”

placer. A concentrated deposit of minerals, usually heavy, 

such as gold, cassiterite, or rutile, in a beach or stream 

deposit. 

placer mining. The extraction of metals or minerals from 

placers, usually involves running water.

plate boundary. A zone of seismic and tectonic activity 

along the edges of lithospheric plates, resulting from the 

relative motion among plates. 

plate tectonics. A theory of global tectonics in which the 

lithosphere is divided into about 20 rigid plates that 

interact with one another at their boundaries, causing 

seismic and tectonic activity along these boundaries.

plutonic. Describes an igneous rock or intrusive body 

formed at great depth beneath Earth’s surface.

provenance. A place of origin, specifically the area from 

which the constituent materials of a sedimentary rock 

were derived.

pumice. A highly vesicular pyroclast with very low bulk 

density and thin vesicle walls.

pumiceous. Describes a texture of volcanic rock consisting 

of tiny gas holes such as in pumice; finer than scoriaceous.

pyroclastic. Describes clastic rock material formed by 

volcanic explosion or aerial expulsion from a vent; also, 

describes a rock texture of explosive origin. It is not 

synonymous with “volcanic.”

pyroclastic flow. A hot, typically >800°C (1,500°F), chaotic 

mixture of rock fragments, gas, and ash that travels rapidly 

(tens of meters per second) away from a volcanic vent or 

collapsing flow front.

quartz. Silicon dioxide, SiO2. The only silicate (silicon + 

oxygen) mineral consisting entirely of silicon and oxygen. 

Synonymous with “crystalline silica.”

quartzite. Metamorphosed quartz sandstone. A mediumgrained, nonfoliated metamorphic rock composed mostly 

of quartz.

radiocarbon age. An isotopic age expressed in years and 

calculated from the quantitative determination of the 

amount of carbon-14 remaining in an organic material. 

Synonymous with “carbon-14 age.”

rebound. Upward flexing of Earth’s crust. Synonymous with 

“upwarping.”

reverse fault. A contractional high-angle (greater than 45°) 

dip-slip fault in which the hanging wall moves up relative 

to the footwall. 

rhyodacite. A volcanic rock that contains approximately 

68%–72% silica and is intermediate in composition 

between rhyolite and dacite.

rhyolite. A volcanic rock that is characteristically light in 
59

color, contains approximately 72% or more of silica, and 

is rich in potassium and sodium. 

rip-up clast. A mud clast (usually flat) that has been “ripped 

up” by currents from a semiconsolidated mud deposit, 

transported, and deposited elsewhere; commonly 

associated with a storm or other high-energy event.

rock. An aggregate of one or more minerals (e.g., granite), a 

body of undifferentiated mineral matter (e.g., obsidian), 

or a body of solid organic material (e.g., coal).

rockfall. The most rapid type of slope movement in which a 

newly detached fragment of bedrock of any size falls from 

a cliff or other very steep slope, traveling straight down or 

in a series of leaps and bounds down a slope.

sand. A clastic particle smaller than a granule and larger than 

a silt grain, with a diameter ranging from 1/16 to 2 mm 

(0.0025 to 0.08 in).

sandstone. Clastic sedimentary rock composed of 

predominantly sand-sized grains.

scarp. A steep cliff or topographic step resulting from 

displacement on a fault or as a result of slope movement 

or erosion. Synonymous with “escarpment.”

scoria. A bomb-size pyroclast that is irregular in form and 

generally very vesicular.

scoriaceous. Describes a texture of volcanic rock consisting 

of relatively large gas holes such as in vesicular basalt; 

coarser than pumiceous.

sediment. An eroded and deposited, unconsolidated 

accumulation of rock and mineral fragments.

sedimentary. Pertaining to or containing sediment. 

sedimentary rock. A rock resulting from the consolidation 

of loose sediment that has accumulated in layers; it may 

be “clastic,” consisting of mechanically formed fragments 

of older rock; “chemical,” formed by precipitation from 

solution; or “organic,” consisting of the remains of plants 

and animals. One of the three main classes of rock—

igneous, metamorphic, and sedimentary. 

sedimentation. The process of forming or accumulating 

sediment into layers, including the separation of rock 

particles from parent rock, the transportation of these 

particles to the site of deposition, the actual deposition or 

settling of the particles, the chemical and other changes 

occurring in the sediment, and the ultimate consolidation 

of the sediment into solid rock. 

seismic. Pertaining to an earthquake or Earth vibration, 

including those that are artificially induced.

seismicity. The phenomenon of movements in the Earth’s 

crust. Synonymous with “seismic activity.”

sequence. A succession of geologic events, processes, or 

rocks, arranged in chronologic order to show their relative 

position and age with respect to geologic history as a 

whole. Also, a rock-stratigraphic unit that is traceable 

over large areas and defined by sediment associated with a 

major sea level transgression–regression.

series. A chronostratigraphic unit next in rank below system 

and above stage; the rocks formed during an “epoch” of 

geologic time.

shale. A clastic sedimentary rock made of clay-sized 

particles and characterized by fissility.

shear. Deformation resulting from stresses that cause 

contiguous parts of a body to slide relatively to each other 

in a direction parallel to their plane of contact. 

silica. Silicon dioxide, SiO2, an essential constituent of many 

minerals, occurring as crystalline quartz, cryptocrystalline 

chalcedony, and amorphous opal.

silicic. Describes a silica-rich igneous rock or magma.

silicic magma. Describes magma that contains more than 

65% silica; generally viscous, gas-rich, and tends to erupt 

explosively.

silt. Clastic sedimentary material intermediate in size 

between fine-grained sand and coarse clay, 0.0039 to 

0.063 mm (0.00015 to 0.0025 in) across.

siltstone. A clastic sedimentary rock composed of silt-sized 

grains.

slope. The inclined surface of any part of Earth’s surface, 

such as a hillslope. Also, a broad part of a continent 

descending into an ocean.

soil. The unconsolidated portion of the Earth’s crust 

modified through physical, chemical, and biotic processes 

into a medium capable of supporting plant growth.

sorted. Describes an unconsolidated sediment consisting of 

particles of essentially uniform size.

spatter. An accumulation of initially very fluid pyroclasts, 

usually stuck together, coating the surface around a vent.

spring. A place where groundwater flows naturally from a 

rock or the soil onto the land surface or into a body of 

surface water. 

stratigraphic. Of or pertaining to strata.

stratigraphy. The geologic study of the origin, occurrence, 

distribution, classification, correlation, and age of rock 

layers, especially sedimentary rocks.

stratovolcano. A volcano that is constructed of alternating 

layers of lava and pyroclastic deposits, along with 

abundant dikes and sills. Viscous, high-silica lava may 

flow from fissures radiating from a central vent, from 

which pyroclastic material is ejected. Synonymous with 

“composite volcano.”

stream. Any body of water moving under gravity flow in a 

clearly confined channel.

stream channel. A long, narrow depression shaped by the 

concentrated flow of stream water.

stream terrace. A planar surface alongside a stream valley 

representing the remnants of an abandoned floodplain, 

stream bed, or valley floor produced during a former stage 

of erosion or deposition.

structure. The attitudes and relative positions of the rock 

masses of an area resulting from such processes as 

faulting, folding, and igneous intrusion.

subaqueous. Describes conditions and processes, or 

features and deposits, that exist or are situated in or under 

water.

subduction. The process of one lithospheric plate 

descending beneath another.

subduction zone. A long, narrow belt in which subduction 

takes place.
60

submarine. Something situated or living under the surface of 

the sea.

subsidence. The sudden sinking or gradual downward 

settling of part of Earth’s surface.

sulfide. A mineral group composed of sulfur plus an element 

or elements, for example, lead in galena, PbS, and iron in 

pyrite, FeS2

.

tectonic. Describes a feature or process related to large-scale 

movement and deformation of Earth’s crust.

tectonics. The geologic study of the broad structural 

architecture and deformational processes of the 

lithosphere and asthenosphere.

tephra. A collective term used for all pyroclastic material, 

regardless of size, shape, or origin, ejected into the air 

during a volcanic eruption.

terrace. Any long, narrow, relatively level or gently inclined 

surface (i.e., a bench or steplike ledge) that is bounded 

along one edge by a steeper descending slope and along 

the other edge by a steeper ascending slope, thus breaking 

the continuity of the slope; commonly occurs along the 

margin and above the level of a body of water, marking a 

former water level.

terrane. A fault-bounded body of rock of regional extent, 

characterized by a geologic history different from that of 

contiguous terranes or bounding continents.

thermal. Pertaining to or caused by heat.

till. Unstratified drift deposited directly by a glacier without 

reworking by meltwater and consisting of a mixture of 

clay, silt, sand, gravel, and boulders ranging widely in size 

and shape.

topography. The general morphology of Earth’s surface, 

including relief and locations of natural and human-made 

features.

trend. The direction or bearing of an outcrop of a geologic 

feature such as an ore body, fold, or orogenic belt.

tuff. Consolidated or cemented volcanic ash and lapilli.

turbidite. Sediment or rock deposited from a turbidity 

current (underwater flow of sediment) and characterized 

by graded bedding, moderate sorting, and well-developed 

primary structures in the sequence noted by the Bouma 

cycle.

unconformity. A substantial break or gap in the geologic 

record where a rock unit is overlain by another that is not 

next in stratigraphic succession, resulting from either a 

change that caused deposition to cease for a considerable 

span of time or erosion with loss of the previously formed 

record.

uplift. A structurally high area in Earth’s crust produced by 

movement that raises the rocks.

vent. Any opening at Earth’s surface through which magma 

erupts or volcanic gases are emitted.

volcanic. Pertaining to the activities, structures, or rock types 

of a volcano. A synonym of extrusive.

volcanic arc. A large-scale (hundreds of kilometers) 

generally curved belt of volcanoes above a subduction 

zone. 

volcaniclastic. Pertaining to all clastic volcanic materials 

formed by any process of fragmentation, dispersed by any 

kind of transporting agent, deposited in any environment, 

or mixed in any significant portion with nonvolcanic 

fragments.

volcanism. The processes by which magma and its 

associated gases rise into Earth’s crust and are extruded 

onto the surface and into the atmosphere.

warm spring. A thermal spring whose temperature is 

appreciably above the local mean annual atmospheric 

temperature, but below that of the human body.

welded tuff. A glass-rich pyroclastic rock that has been 

indurated by the welding together of glass shards under 

the combined action of the heat retained by the particles, 

the weight of overlying material, and hot gases. Cinders 

and bombs may also be welded together to form solid 

outcrops of rock. 
61

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Additional References

This chapter lists additional references, resources, and websites that may be of use to resource 

managers. Web addresses are valid as of August 2015. Refer to Appendix B for laws, regulations, and 

policies that apply to NPS geologic resources.

Geology of Alaska

● USGS Publications: http://pubs.er.usgs.gov/

● Alaska Division of Geological and Geophysical 

Surveys (and Alaska USGS) publications:

http://dggs.alaska.gov/pubs/pubs.jsp

● Alaska Digital Geologic Maps:

http://mrdata.usgs.gov/geology/state/alaska.html

● Alaska (Minerals) Resource Data File:

http://ardf.wr.usgs.gov/

● Alaska Paleontology Database:

http://www.alaskafossil.org/

Geology of National Park Service Areas

● NPS Geologic Resources Division

Energy and Minerals; Active Processes and Hazards; 

Geologic Heritage:

http://nature.nps.gov/geology/

● NPS Geologic Resources Inventory: http://www.

nature.nps.gov/geology/inventory/index.cfm.

● NPS Geoscientist-In-the-Parks (GIP) internship 

and guest scientist program:

http://www.nature.nps.gov/geology/gip/index.cfm

● NPS Views program (geology-themed modules are 

available for Geologic Time, Paleontology, Glaciers, 

Caves and Karst, Coastal Geology, Volcanoes, and a 

variety of geologic parks):

http://www.nature.nps.gov/views/

NPS Resource Management Guidance and 

Documents

● NPS Alaska Regional Office: http://www.nps.gov/

akso/index.cfm

● Southwest Alaska Inventory and Monitoring 

Network: http://science.nature.nps.gov/im/units/

swan/index.cfm

● Alaska Park Science Journal: http://www.nps.gov/

akso/nature/science/ak_park_science/index.cfm

● Management Policies 2006 (Chapter 4: Natural 

resource management):

http://www.nps.gov/policy/mp/policies.html

● 1998 National parks omnibus management act: 

http://www.gpo.gov/fdsys/pkg/PLAW-105publ391/

pdf/PLAW-105publ391.pdf

● NPS-75: Natural resource inventory and 

monitoring guideline:

http://www.nature.nps.gov/nps75/nps75.pdf

● NPS Natural resource management reference 

manual #77: http://www.nature.nps.gov/Rm77/

● Geologic monitoring manual (Young, R., and 

L. Norby, editors. 2009. Geological monitoring. 

Geological Society of America, Boulder, Colorado): 

http://nature.nps.gov/geology/monitoring/index.

cfm

● NPS Technical Information Center (TIC) (Denver, 

Colorado; repository for technical documents): 

http://www.nps.gov/dsc/technicalinfocenter.htm

Climate Change Resources

● NPS Climate Change Response Program Resources: 

http://www.nps.gov/subjects/climatechange/

resources.htm

● US Global Change Research Program:

http://globalchange.gov/home

● Intergovernmental Panel on Climate Change:

http://www.ipcc.ch/

Geological Surveys and Societies

● Alaska Division of Geological and Geophysical 

Surveys: http://dggs.alaska.gov/

● US Geological Survey: http://www.usgs.gov/

● Geological Society of America:

http://www.geosociety.org/

● American Geophysical Union: http://sites.agu.org/

● American Geosciences Institute:

http://www.americangeosciences.org/

● Association of American State Geologists:

http://www.stategeologists.org/
68

US Geological Survey Reference Tools

● National geologic map database (NGMDB):

http://ngmdb.usgs.gov/

● Geologic names lexicon (GEOLEX; geologic unit 

nomenclature and summary):

http://ngmdb.usgs.gov/Geolex/geolex_home.html

● Geographic names information system (GNIS; 

official listing of place names and geographic 

features): http://gnis.usgs.gov/

● GeoPDFs (download searchable PDFs of any 

topographic map in the United States):

http://store.usgs.gov (click on “Map Locator”)

● Publications warehouse (many publications 

available online): http://pubs.er.usgs.gov

● Tapestry of time and terrain (descriptions of 

physiographic provinces):

http://tapestry.usgs.gov/Default.html
69

Appendix A: Geologic Exploration History

1898: USGS party led by J. E. Spurr (1900) explored the 

northern portions of the Alaska Peninsula and Katmai 

area. Although the Aniakchak Crater was not explored, 

they were the first to describe and name some of the 

geologic formations that crop out in the Aniakchak area.

1908: USGS party led by W. W. Atwood (1911), 

explored coal fields of the Alaska Peninsula with a 

topographer, H.M. Eakin, and produced the first 

geologic map of the Alaska Peninsula (Figure 52) and 

the first detailed map of the Chignik Bay area.

1921: Oil leasing in the Aniakchak area by the federal 

government led to explorations by W.W. French, 

a private oil engineer. French was head of the first 

geologic party reported to set foot into the Aniakchak 

Crater, but they did not document the discovery (Norris 

1996).

1922: USGS party led by W. R. Smith and A. A. Baker 

(1924) was the first government-led group that explored 

the Aniakchak area. The private sector speculated 

high oil potential of the area, so the US Geological 

Survey dispatched field parties to study this potential. 

Smith (1925) published the first report and map of the 

Aniakchak Crater (Figure 53).

1925: USGS party led by R. S. Knappen (1929) was 

the second party of government scientists to explore 

the Chignik and Aniakchak areas for oil potential. A 

detailed map of the areas was produced and is shown in 

Figure 54. Both the Smith and Knappen parties found 

that the area was unfavorable for oil occurrence.

1930/1931: B. Hubbard (1931b) was a popular explorer 

of Alaska who visited the Aniakchak Crater the year 

before the 1931. He undertook a field expedition the 

Figure 52. Geologic map of the Alaska Peninsula by Atwood (1911). 
70

next year during the eruption. His party captured many 

photographs during the eruption (Figure 22). 

1950: USGS geologist R. M Moxham (1952) explored 

the pumice potential of the Alaska Peninsula. Industry 

and the economy in Alaska were growing, and pumice 

was being used as a light-weight additive to concrete. 

The Kukak Bay area in Katmai National Monument 

(redesignated a national park and preserve in 1980) was 

recognized as a good pumice source, but the National 

Park Service was reluctant to open up the area to 

commercial mining, so alternatives along the peninsula 

were explored.

1957: Oil speculation led to a new round of leasing and 

private exploration, but these leases fared no better than 

those of the 1920s (Norris 1996).

1958, 1959, and 1961: C. A. Burk (1965), who was a 

PhD candidate at Princeton University and funded 

by the Richland Oil Company, was the first to use 

a helicopter for geologic exploration of the Alaska 

Peninsula. He produced the first detailed geologic map 

of the peninsula (Figure 55). Unfortunately, his field 

notes from a season’s work were lost in transit from the 

field. His was a monumental effort.

1973: Geologists W. M. Lyle and 

P. L. Dobey (1973) of the Alaska 

Department of Natural Resources, 

Division of Geological & Geophysical 

Surveys, evaluated the mineral 

potential of the Aniakchak area, after 

the Aniakchak River was proposed 

as a wild and scenic river. They had a 

rough time rafting the river and lost 

most of their equipment, and found 

that the monument and Aniakchak 

River corridor had little mineral 

potential.

1976: USGS volcanologists T. P. 

Miller and R. L. Smith (1977) were 

the first geologists to formally study 

the eruption history of Aniakchak 

Figure 53. Portion of plate XLII from Smith (1925) 

showing a map of the Aniakchak Crater and vicinity.

Figure 54. Geologic map of the 

Aniakchak and Chignik areas by 

Knappen (1929).
71

volcano. They recognized the “spectacular mobility 

of ash flow” from the Aniakchak II eruption. While 

cataloging caldera forming eruptions along the Aleutian 

volcanic arc, they also identified the Aniakchak I 

eruption deposits (Miller and Smith 1987).

1979–1982: R. L. Detterman and other USGS geologists 

mapped the Aniakchak area in detail as part of the 

USGS effort to evaluate the mineral potential of public 

land (Detterman et al. 1981a, 1981b, 1981c, 1987, 1990), 

a consequence of the Alaska Native Claims Settlement 

Act (ANCSA). After the ANILCA legislation, this 

effort was formally referred to as the Alaska Mineral 

Resources Assessment Project (AMRAP). These 

geologic maps were part of the compilation by Wilson 

et al. (1999) and the GRI geologic maps on Plate 1 and 

Plate 2. These mapping projects led to the keystone 

Alaska Peninsula stratigraphic framework USGS 

Bulletin by Detterman et al. (1996).

1990s–present day: USGS volcanologists studied 

Aniakchak volcano eruption history and produced 

reports on the volcanic history and potential of the 

volcano (Neal et al. 2001, Bacon et al. 2014), the 

resulting tsunami (Waythomas and Neal 1998; Watts 

and Waythomas 2003), and subsequent caldera outburst 

flood (McGimsey et al. 1994; Waythomas et al. 1996).

1997–2001: Archeologists R. VanderHoek and R. 

Myron (2004) conducted a comprehensive study of the 

human habitation of the Aniakchak coastal areas. Their 

detailed soil profile pits, which contain abundant tephra 

layers, and radiocarbon dates have helped constrain the 

ages of the major post-glacial eruptions from Aniakchak 

volcano (Figure 25).

Figure 55. Part of the geologic map of the Alaska Peninsula by Burk (1965), showing the area around Aniakchak.

73

Appendix B: Administrative History Related to Geology

1920–1930s—Conservation Efforts

After publishing the discovery of the Aniakchak 

Crater by Smith and Baker (1924), efforts were made 

to establish the area as a national monument (Norris 

1996). In the 1930s, NPS Director Horace M. Albright 

proposed establishing a national monument (Norris 

1996). However, that effort was stalled, because Acting 

USGS Director Julian D. Sears stated in a letter to the 

NPS director that the survey “feels a keen interest in 

having as much of a region as possible kept open for 

free development” (Norris 1996, p. 429). 

3 May 1970—Proclaimed as Aniakchak Crater 

National Natural Landmark

The National Park Service was finally successful at 

protecting the Aniakchak Crater when Secretary of 

the Interior Walter Hickel, who was a former Alaska 

governor, proclaimed the crater as a national natural 

landmark (NNL) in 1970. The following are excerpts 

from Norris (1996, p. 436):

On May 3, 1967, Assistant NPS Director Theodor Swem 

made $20,000 available to the University of Alaska to 

study potential NNL sites in the state. The Aniakchak and 

Veniaminof areas were among ten sites examined that 

year. The Alaskasearch, company evaluated the various 

sites and felt that Aniakchak Crater was fully eligible as a 

NNL site. Ellis Taylor, the author of the report, noted that

“Aniakchak, the real estate agent would say, has 

everything. Scientific importance, popular appeal, 

dramatic, heroic proportions, a mysterious past and an 

unpredictable, possibly spectacular geologic future, even 

hot and cold running water.”

The consultant found that “From no viewpoint is 

Aniakchak unimpressive,” and further stated that the 

crater “is recognized by geologists as an outstanding 

feature, unmatched in many ways, by volcanic 

phenomena of far greater renown.” The report’s 

recommendations were quickly embraced by Interior 

Department officials. In November 1967 the NPS, which 

was responsible for carrying out the NNL program, 

approved the designation of Aniakchak Crater as a 

National Natural Landmark. The 20,000-acre site, 

however, could not receive official designation until it 

received the Interior Secretary’s approval. In 1969, 

former Alaska governor Walter Hickel became the 

Interior Secretary, and in August 1970 he gave final 

approval to the NNL designation. The landmark was 

later renamed Aniakchak Crater.

1 December 1978—Presidential Proclamation 

4612 Establishing Aniakchak National 

Monument

Language Relative to the Geology of the 

Monument

“The Aniakchak Crater is located in the heart of the 

Alaska Peninsula. It is so unexpected a feature that 

it remained unknown to all but the Natives of the 

region until about 1920. With its average diameter of 

approximately six miles, Aniakchak is one of the world’s 

largest calderas.

In the interior of the caldera are textbook examples 

of certain volcanic features such as lava flows, cinder 

cones, and explosion pits. Also lying within the caldera 

is Surprise Lake which is fed by warm springs and is 

uniquely charged with chemicals. Surprise Lake is the 

source of the Aniakchak River, which cascades through 

a 1,500 foot gash in the caldera wall and downward for 

27 miles to the Pacific Ocean.

The flanks of the caldera provide a geological and 

biological continuum by which to make a comparative 

study of the formation of the caldera and the significant 

process of biological succession of both plant and 

animal species occurring in the vicinity of the caldera, 

an area that was rendered virtually devoid of life forms 

by a major eruption of the volcano in 1931.

The caldera is also climatologically unique in that, 

because of its topography and setting, it appears 

to be able to generate its own weather. A striking 

phenomenon known as cloud “niagaras” occurs 

frequently as strong downdrafts form over the caldera 

walls.”

2 December 1980—Alaska National Interest 

Lands Conservation Act (ANILCA)

Language Relative to Geology of Parks in Alaska 

and Research

“SEC. 101.(a) In order to preserve for the benefit, 

use, education, and inspiration of present and future 

generations certain lands and waters in the State of 

Alaska that contain nationally significant natural, scenic, 
74

historic, archeological, geological, scientific, wilderness, 

cultural, recreational, and wildlife values, the units 

described in the following titles are hereby established.

SEC. 101. (b) It is the intent of Congress in this Act 

to preserve unrivaled scenic and geological values 

associated with natural landscapes... and to maintain 

opportunities for scientific research and undisturbed 

ecosystems.”

Language Specific to Aniakchak

“SEC. 201. The following areas are hereby established 

as units of the National Park System and shall be 

administered by the Secretary under the laws governing 

the administration of such lands and under the 

provisions of this Act:

(1) Aniakchak National Monument, containing 

approximated one hundred and thirty-eight thousand 

acres of public lands, and Aniakchak National Preserve, 

containing approximately three hundred and seventysix thousand acres of public lands, as generally depicted 

on map numbered ANIA-90,005, and dated October 

1978. The monument and preserve shall be managed 

for the following purposes, among others: To maintain 

the caldera and its associated volcanic features and 

landscape, including the Aniakchak River and other 

lakes and streams, in their natural state... to interpret 

geological and biological processes for visitors.”
75

Appendix C: Geologic Resource Laws, Regulations, and Policies

The NPS Geologic Resources Division developed this table to summarize laws, regulations, and policies 

that specifically apply to National Park Service minerals and geologic resources. The table does not 

include laws of general application (e.g., Endangered Species Act, Clean Water Act, Wilderness Act, 

National Environmental Policy Act, or National Historic Preservation Act). The table does include the 

NPS Organic Act when it serves as the main authority for protection of a particular resource or when 

other, more specific laws are not available. Information is current as of August 2015. Contact the NPS 

Geologic Resources Division for detailed guidance.

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Paleontology

National Parks Omnibus Management 

Act of 1998, 16 USC § 5937 protects the 

confidentiality of the nature and specific 

location of paleontological resources and 

objects.

Paleontological Resources Preservation 

Act of 2009, 16 USC § 470aaa et 

seq. provides for the management and 

protection of paleontological resources on 

federal lands.

36 CFR § 2.1(a)(1)(iii) prohibits destroying, 

injuring, defacing, removing, digging or 

disturbing paleontological specimens or 

parts thereof.

Prohibition in 36 CFR § 13.35 applies 

even in Alaska parks, where the surface 

collection of other geologic resources is 

permitted.

Regulations in association with 2009 PRPA 

are being finalized (August 2015).

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Section 4.8.2.1 emphasizes Inventory and 

Monitoring, encourages scientific research, 

directs parks to maintain confidentiality of 

paleontological information, and allows 

parks to buy fossils only in accordance with 

certain criteria.

Rocks and Minerals

NPS Organic Act, 16 USC § 1 et seq.

directs the NPS to conserve all resources 

in parks (including rock and mineral 

resources), unless otherwise authorized by 

law.

36 CFR § 2.1 prohibits possessing, 

destroying, disturbing mineral resources...

in park units.

Exception: 36 CFR § 13.35 allows some 

surface collection of rocks and minerals 

in some Alaska parks (not Klondike 

Gold Rush, Sitka, Denali, Glacier Bay, 

or Katmai) by non-disturbing methods 

(e.g., no pickaxes), which can be stopped 

by superintendent if collection causes 

significant adverse effects on park 

resources and visitor enjoyment.

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Park Use of Sand and Gravel

Materials Act of 1947, 30 USC § 601

does not authorize the NPS to dispose of 

mineral materials outside of park units.

None applicable.

Section 9.1.3.3 clarifies that only the 

NPS or its agent can extract park-owned 

common variety minerals (e.g., sand and 

gravel), and:

-only for park administrative uses;

-after compliance with NEPA and other 

federal, state, and local laws, and a 

finding of non-impairment;

-after finding the use is park’s most 

reasonable alternative based on 

environment and economics;

-parks should use existing pits and create 

new pits only in accordance with parkwide borrow management plan;

-spoil areas must comply with Part 6 

standards; and

-NPS must evaluate use of external 

quarries.

Any deviation from this policy requires a 

written waiver from the Secretary, Assistant 

Secretary, or Director.
76

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Upland and Fluvial Processes

Rivers and Harbors Appropriation 

Act of 1899, 33 USC § 403 prohibits the 

construction of any obstruction on the 

waters of the United States not authorized 

by congress or approved by the USACE.

Clean Water Act 33 USC § 1342 requires 

a permit from the USACE prior to any 

discharge of dredged or fill material 

into navigable waters (waters of the US 

[including streams]). 

Executive Order 11988 requires federal 

agencies to avoid adverse impacts to 

floodplains. (see also D.O. 77-2)

Executive Order 11990 requires plans for 

potentially affected wetlands (including 

riparian wetlands). (see also D.O. 77-1)

None applicable.

Section 4.1 requires NPS to manage 

natural resources to preserve fundamental 

physical and biological processes, as well 

as individual species, features, and plant 

and animal communities; maintain all 

components and processes of naturally 

evolving park ecosystems.

Section 4.1.5 directs the NPS to reestablish natural functions and processes 

in human-disturbed components of natural 

systems in parks, unless directed otherwise 

by Congress.

Section 4.4.2.4 directs the NPS to allow 

natural recovery of landscapes disturbed by 

natural phenomena, unless manipulation of 

the landscape is necessary to protect park 

development or human safety. 

Section 4.6.4 directs the NPS to (1) 

manage for the preservation of floodplain 

values; [and] (2) minimize potentially 

hazardous conditions associated with 

flooding.

Section 4.6.6 directs the NPS to manage 

watersheds as complete hydrologic systems 

and minimize human-caused disturbance 

to the natural upland processes that deliver 

water, sediment, and woody debris to 

streams.

Section 4.8.1 directs the NPS to allow 

natural geologic processes to proceed 

unimpeded. Geologic processes...include...

erosion and sedimentation...processes. 

Section 4.8.2 directs the NPS to protect 

geologic features from the unacceptable 

impacts of human activity while allowing 

natural processes to continue.

Caves and Karst Systems

Federal Cave Resources Protection Act 

of 1988, 16 USC §§ 4301 – 4309 requires 

Interior/Agriculture to identify “significant 

caves” on Federal lands, regulate/

restrict use of those caves as appropriate, 

and include significant caves in land 

management planning efforts. Imposes 

civil and criminal penalties for harming 

a cave or cave resources. Authorizes 

Secretaries to withhold information about 

specific location of a significant cave from 

a Freedom of Information Act (FOIA) 

requester. 

National Parks Omnibus Management 

Act of 1998, 16 USC § 5937 protects the 

confidentiality of the nature and specific 

location of cave and karst resources.

36 CFR § 2.1 prohibits possessing/ 

destroying/disturbing...cave resources...in 

park units.

43 CFR Part 37 states that all NPS caves 

are “significant” and sets forth procedures 

for determining/releasing confidential 

information about specific cave locations to 

a FOIA requester.

Section 4.8.1.2 requires NPS to maintain 

karst integrity, minimize impacts.

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Section 4.8.2.2 requires NPS to protect 

caves, allow new development in or on 

caves if it will not impact cave environment, 

and to remove existing developments if 

they impair caves.

Section 6.3.11.2 explains how to manage 

caves in/adjacent to wilderness.
77

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Mining Claims

Mining in the Parks Act of 1976, 16 

USC § 1901 et seq. authorizes NPS 

to regulate all activities resulting from 

exercise of mineral rights, on patented and 

unpatented mining claims in all areas of the 

System, in order to preserve and manage 

those areas.

General Mining Law of 1872, 30 USC 

§ 21 et seq. allows US citizens to locate 

mining claims on Federal lands. Imposes 

administrative and economic validity 

requirements for “unpatented” claims (the 

right to extract Federally-owned locatable 

minerals). Imposes additional requirements 

for the processing of “patenting” claims 

(claimant owns surface and subsurface). 

Use of patented mining claims may be 

limited in Wild and Scenic Rivers and 

OLYM, GLBA, CORO, ORPI, and DEVA. 

Surface Uses Resources Act of 1955, 

30 USC § 612 restricts surface use of 

unpatented mining claims to mineral 

activities.

36 CFR § 5.14 prohibits prospecting, 

mining, and the location of mining claims 

under the general mining laws in park 

areas except as authorized by law.

36 CFR Part 6 regulates solid waste 

disposal sites in park units.

36 CFR Part 9, Subpart A requires the 

owners/operators of mining claims to 

demonstrate bona fide title to mining 

claim; submit a plan of operations to 

NPS describing where, when, and how; 

prepare/submit a reclamation plan; and 

submit a bond to cover reclamation and 

potential liability.

Section 6.4.9 requires NPS to seek to 

remove or extinguish valid mining claims in 

wilderness through authorized processes, 

including purchasing valid rights. Where 

rights are left outstanding, NPS policy is 

to manage mineral-related activities in 

NPS wilderness in accordance with the 

regulations at 36 CFR Parts 6 and 9A.

Section 8.7.1 prohibits location of new 

mining claims in parks; requires validity 

examination prior to operations on 

unpatented claims; and confines operations 

to claim boundaries.

Nonfederal Oil and Gas

NPS Organic Act, 16 USC § 1 et seq.

authorizes the NPS to promulgate 

regulations to protect park resources and 

values (from, for example, the exercise of 

mining and mineral rights).

36 CFR Part 6 regulates solid waste 

disposal sites in park units.

36 CFR Part 9, Subpart B requires the 

owners/operators of nonfederally owned 

oil and gas rights to

-demonstrate bona fide title to mineral 

rights;

-submit a plan of operations to NPS 

describing where, when, how they intend 

to conduct operations;

-prepare/submit a reclamation plan; and 

-submit a bond to cover reclamation and 

potential liability.

Section 8.7.3 requires operators to comply 

with 9B regulations.

Nonfederal minerals other than oil and gas

NPS Organic Act, 16 USC §§ 1 and 3

Surface Mining Control and 

Reclamation Act of 1977, 30 USC § 

1201 et. seq. prohibits surface coal 

mining operations on any lands within the 

boundaries of a NPS unit, subject to valid 

existing rights.

NPS regulations at 36 CFR Parts 1, 5, 

and 6 require the owners/operators of 

other types of mineral rights to obtain 

a special use permit from the NPS as a 

§ 5.3 business operation, and § 5.7 – 

Construction of buildings or other facilities, 

and to comply with the solid waste 

regulations at Part 6.

SMCRA Regulations at 30 CFR Chapter 

VII govern surface mining operations on 

Federal lands and Indian lands by requiring 

permits, bonding, insurance, reclamation 

, and employee protection. Part 7 of the 

regulations states that National Park System 

lands are unsuitable for surface mining.

Section 8.7.3 states that operators 

exercising rights in a park unit must comply 

with 36 CFR Parts 1 and 5.
78

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Geothermal

Geothermal Steam Act of 1970, 30 USC 

§ 1001 et seq. as amended in 1988, 

states that

-no geothermal leasing is allowed in parks;

-“significant” thermal features exist in 16 

park units (features listed by the NPS at 52 

Fed. Reg. 28793-28800 [August 3, 1987], 

and thermal features in Crater Lake, Big 

Bend, and Lake Mead);

-NPS is required to monitor those features; 

and

-based on scientific evidence, Secretary 

of Interior must protect significant NPS 

thermal features from leasing effects.

Geothermal Steam Act Amendments 

of 1988, Public Law 100--443 prohibits 

geothermal leasing in the Island Park 

known geothermal resource area near 

Yellowstone and outside 16 designated 

NPS units if subsequent geothermal 

development would significantly adversely 

affect identified thermal features. 

None applicable

Section 4.8.2.3 requires NPS to

-preserve/maintain integrity of all thermal 

resources in parks.

-work closely with outside agencies, and

-monitor significant thermal features.

Coastal Features and Processes

NPS Organic Act, 16 USC § 1 et. 

seq. authorizes the NPS to promulgate 

regulations to protect park resources and 

values (from, for example, the exercise of 

mining and mineral rights).

Coastal Zone Management Act, 16 USC 

§ 1451 et. seq. requires Federal agencies 

to prepare a consistency determination for 

every Federal agency activity in or outside 

of the coastal zone that affects land or 

water use of the coastal zone.

Clean Water Act, 33 USC § 1342/Rivers 

and Harbors Act, 33 USC 403 require that 

dredge and fill actions comply with a Corps 

of Engineers Section 404 permit. 

Executive Order 13158 (marine 

protected areas) (2000) requires every 

federal agency, to the extent permitted by 

law and the maximum extent practicable, 

to avoid harming marine protected areas.

36 CFR § 1.2(a)(3) applies NPS regulations 

to activities occurring within waters subject 

to the jurisdiction of the US located 

within the boundaries of a unit, including 

navigable water and areas within their 

ordinary reach, below the mean high water 

mark (or OHW line) without regard to 

ownership of submerged lands, tidelands, 

or lowlands.

36 CFR § 5.7 requires NPS authorization 

prior to constructing a building or other 

structure (including boat docks) upon, 

across, over, through, or under any park 

area.

Section 4.1.5 directs the NPS to reestablish natural functions and processes 

in human-disturbed components of natural 

systems in parks unless directed otherwise 

by Congress.

Section 4.4.2.4 directs the NPS to allow 

natural recovery of landscapes disturbed by 

natural phenomena, unless manipulation of 

the landscape is necessary to protect park 

development or human safety.

Section 4.8.1 requires NPS to allow natural 

geologic processes to proceed unimpeded. 

NPS can intervene in these processes 

only when required by Congress, when 

necessary for saving human lives, or when 

there is no other feasible way to protect 

other natural resources/ park facilities/

historic properties.

Section 4.8.1.1 requires NPS to:

-Allow natural processes to continue 

without interference, 

-Investigate alternatives for mitigating the 

effects of human alterations of natural 

processes and restoring natural conditions, 

-Study impacts of cultural resource 

protection proposals on natural resources, 

-Use the most effective and natural-looking 

erosion control methods available, and 

-Avoid putting new developments in areas 

subject to natural shoreline processes 

unless certain factors are present.
79

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Federal Mineral Leasing (Oil and Gas, Salable Minerals, and Non-locatable Minerals)

The Mineral Leasing Act, 30 USC § 181 

et seq., and the Mineral Leasing Act for 

Acquired Lands, 30 USC § 351 et seq. 

do not authorize the BLM to lease federally 

owned minerals in NPS units. 

Exceptions: Native American Lands Within 

NPS Boundaries Under the Indian Allottee 

Leasing Act of 1909, (25 USC § 396), 

and the Indian Leasing Act of 1938 (25 

USC §§ 396a, 398 and 399) and Indian 

Mineral Development Act of 1982 

(25 USC §§ 2101-2108), all minerals 

are subject to lease and apply to Native 

American trust lands within NPS units.

Federal Coal Leasing Amendments Act 

of 1975, 30 USC § 201 does not authorize 

the BLM to issue leases for coal mining on 

any area of the national park system.

36 CFR § 5.14 states prospecting, mining, 

and...leasing under the mineral leasing 

laws [is] prohibited in park areas except as 

authorized by law.

BLM regulations at 43 CFR Parts 3100, 

3400, and 3500 govern Federal mineral 

leasing.

Regulations re: Native American Lands 

within NPS Units:

25 CFR Part 211 governs leasing of tribal 

lands for mineral development. 

25 CFR Part 212 governs leasing of 

allotted lands for mineral development. 

25 CFR Part 216 governs surface 

exploration, mining, and reclamation of 

lands during mineral development. 

25 CFR Part 224 governs tribal energy 

resource agreements.

25 CFR Part 225 governs mineral 

agreements for the development of Indianowned minerals entered into pursuant to 

the Indian Mineral Development Act of 

1982, Pub. L. No. 97-382, 96 Stat. 1938 

(codified at 25 USC §§ 2101-2108).

30 CFR §§ 1202.100-1202.101 governs 

royalties on oil produced from Indian 

leases. 

30 CFR §§ 1202.550-1202.558 governs 

royalties on gas production from Indian 

leases. 

30 CFR §§ 1206.50-1206.62 and §§ 

1206.170-1206.176 governs product 

valuation for mineral resources produced 

from Indian oil and gas leases. 

30 CFR § 1206.450 governs the valuation 

coal from Indian Tribal and Allotted leases.

43 CFR Part 3160 governs onshore oil and 

gas operations, which are overseen by the 

BLM.

Section 8.7.2 states that all NPS units 

are closed to new federal mineral leasing 

except Glen Canyon, Lake Mead and 

Whiskeytown-Shasta-Trinity NRAs.
80

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Soils

Soil and Water Resources Conservation 

Act, 16 USC §§ 2011–2009 provides for 

the collection and analysis of soil and 

related resource data and the appraisal of 

the status, condition, and trends for these 

resources.

Farmland Protection Policy Act, 7 USC 

§ 4201 et. seq. requires NPS to identify 

and take into account the adverse effects 

of Federal programs on the preservation 

of farmland; consider alternative actions, 

and assure that such Federal programs 

are compatible with State, unit of local 

government, and private programs 

and policies to protect farmland. NPS 

actions are subject to the FPPA if they 

may irreversibly convert farmland (directly 

or indirectly) to nonagricultural use and 

are completed by a Federal agency or 

with assistance from a Federal agency. 

Applicable projects require coordination 

with the Department of Agriculture’s 

Natural Resources Conservation Service 

(NRCS).

7 CFR Parts 610 and 611 are the US 

Department of Agriculture regulations 

for the Natural Resources Conservation 

Service. Part 610 governs the NRCS 

technical assistance program, soil erosion 

predictions, and the conservation of private 

grazing land. Part 611 governs soil surveys 

and cartographic operations. The NRCS 

works with the NPS through cooperative 

arrangements.

Section 4.8.2.4 requires NPS to

-prevent unnatural erosion, removal, and 

contamination;

-conduct soil surveys;

-minimize unavoidable excavation; and

-develop/follow written prescriptions 

(instructions).
81

Appendix D: Scoping Participants

The following people attended the GRI scoping meeting for Aniakchak National Monument and 

Preserve, held on 14–18 February 2005, a follow-up report writing kick-off meeting held on 21 July 

2014 at Katmai National Park and Preserve, and a meeting with USGS geologists held on 9 October 

2014 at the Alaska Volcano Observatory. Discussions during these meetings supplied a foundation for 

this GRI report. The scoping summary document is available on the GRI publications website:

http://go.nps.gov/gripubs.

2005 Scoping Meeting Participants

Name Affiliation Position

Beavers, Rebecca NPS-Geologic Resources Division Coastal geologist

Bennett, Alan NPS-Southwest Alaska Network Network coordinator

Bundtzen, Tom Pacific Rim Geological Consulting Geologist

Connors, Tim NPS-Geologic Resources Division Geologist

Covington, Sid NPS-Geologic Resources Division Geologist

Cusick, Joel NPS-Alaska Regional Office GIS specialist

Dickson, George NPS-Alaska Regional Office GIS team manager

Fiorillo, Tony Dallas Museum of Natural History Curator

Giffen, Bruce NPS-Alaska Regional Office Geologist

Graham, John Colorado State University Geologist

Griffiths, Lynn NPS-Alaska Regional Office Geological engineer

Haeussler, Peter USGS-Alaska Science Center Geologist

Hall, Shelley Kenai Fjords National Park Resource management chief

Halloran, Jim NPS-Alaska Regional Office Geologist

Heiser, Patricia University of Alaska, Anchorage Assistant professor - Geology

Kozlowski, Janis Alaska Regional Office Resource management specialist

Matt, Colleen Lake Clark National Park and Preserve Natural Resources chief

Miller, Amy NPS-Southwest Alaska Network Ecologist

Miller, Joe NPS-Southwest Alaska Network Fishery biologist

Mortenson, Dorothy NPS-Southwest Alaska Network Data manager

Mow, Jeff Kenai Fjords National Park Superintendent

Neal, Tina USGS-AVO Geologist

Piercy, Joni NPS-Alaska Regional Office GIS specialist

Pinamont, John NPS-Alaska Regional Office GIS specialist

Rice, Bud NPS-Alaska Regional Office Environmental protection. specialist

Schaefer, Janet Alaska Division of Geological & Geophysical Surveys/AVO Geologist

Stromquist, Linda NPS-Alaska Regional Office Geologist

Tetreau, Mike Kenai Fjords National Park Resource Management Specialist

VanderHoek, Richard DNR/Parks/Office of History & Archaeology/U. of Illinois Archaeologist

Wesser, Sara NPS-Alaska Regional Office Inventory and Monitoring coordinator

Wilson, Frederic (Ric) USGS Geologist
82

2014 NPS Meeting Participants

Name Affiliation Position

Chad Hults NPS Alaska Regional Office, Geologic Resources Division Geologist

Sarah Venator NPS Alaska Regional Office Geologist

Troy Hamon NPS Aniakchak NM & Pres, Katmai NP & Pres Chief of natural resources

Diane Chung NPS Aniakchak NM & Pres, Katmai NP & Pres Superintendent

Sherri Anderson NPS Katmai National Park and Preserve Wildlife biologist

2014 USGS-AVO Meeting Participants

Name Affiliation Position

Chad Hults NPS Alaska Regional Office, Geologic Resources Division Geologist

Sarah Venator NPS Alaska Regional Office Geologist

Tina Neal USGS Alaska Volcano Observatory Research geologist

Game McGimsey USGS Alaska Volcano Observatory Research geologist

Chris Waythomas USGS Alaska Volcano Observatory Research geologist

Tom Miller USGS Alaska Volcano Observatory Research geologist emeritus

Charlie Bacon USGS Volcano Observatory Research geologist emeritus
The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and 

other information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated 

Island Communities.

NPS 181/129749, September 2015
Natural Resource Stewardship and Science 

1201 Oak Ridge Drive, Suite 150 

Fort Collins, Colorado 80525

www.nature.nps.gov

National Park Service

U.S. Department of the Interior

EXPERIENCE YOUR AMERICATM

Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 1 of 3

Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve (see Plate 1) 

Unit descriptions were derived primarily from Detterman et al. (1981, 1987), Detterman et al. (1996), and Wilson et al. (1999). Gray shaded map units are not mapped within Aniakchak National Monument and Preserve. Bold text refers to report sections.

Age Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

QUATERNARY

Surficial deposits

(Qs) 

Undifferentiated, unconsolidated deposits.

See Quaternary Geologic Map Unit Properties Table and Plate 

2 for individual units. 

See Quaternary Geologic Map Unit Properties Table. See Quaternary Geologic Map Unit Properties Table. See Quaternary Geologic Map Unit Properties Table.

QUATERNARY

(Holocene and 

late Pleistocene)

Moraines and other 

glacial deposits

(Qm) 

Unconsolidated, poorly sorted, and nonstratified glacial drift 

forming moraines and kame topography. Glacial drift divided 

into glacial advances in Plate 2 and the Quaternary Geologic 

Map Unit Properties Table.

Glaciers and Glaciations—End, lateral, and ground 

moraines; glaciolacustrine deposits; knobs, kettles, and 

ice-contact deposits 

Geohazards—Unconsolidated units are easily eroded 

and susceptible to landsliding when exposed on slopes or 

undercut by streams.

Quaternary Volcanism and Glaciation—The oldest 

glacial deposits are from the Mak Hill glaciation, which 

took place more than 44,000 years ago. Most glacier 

deposits are younger and part of the Brooks Lake 

glaciation, which occurred 10,000 years ago or earlier.

QUATERNARY

(Holocene and late 

Pleistocene)

Pyroclastic and 

debris-flow deposits

(Qpd) 

Consolidated and unconsolidated, poorly sorted volcanic ash 

and debris flows. Consists of basaltic andesitic to rhyolitic 

pumice, ash, ash-flow tuffs, scoria bombs, and lithic 

fragments. 

Aniakchak II, The Caldera Forming Eruption—Forms 

broad fields of loose materials in the caldera and along its 

flanks. Where dissected by rivers or streams, the welded 

ignimbrites can form steep cliffs.

Quaternary Volcanism and Glaciation—Most of the 

pyroclastic and debris flow materials were formed during 

the caldera forming eruption (Aniakchak II). The 

Aniakchak II eruption deposits in the caldera are overlain 

by the numerous eruptions from Half Cone and Vent 

Mountain, and locally by the 1931 eruption.

QUATERNARY

Younger volcanic 

rocks, undivided

(Qv) 

Dacite, andesite, and basalt lava flows, breccia, pyroclastic 

flows, and air-fall deposits.

Pre-Caldera Eruption Features—The caldera walls are 

formed of pre-Aniakchak II eruption volcanic rocks that 

are as yet undifferentiated.

None reported

Quaternary Volcanism and Glaciation—The caldera 

wall forming volcanic rocks are as old as 850,000 years.

TERTIARY

Older volcanic 

rocks, undivided

(Tvu) 

Andesite, dacite, and basalt flows, domes, and stocks. 

Tertiary Igneous Rocks—Tertiary igneous rocks are part 

of a larger belt of similar age igneous rocks that span the 

length of the Alaska Peninsula and continue up to the 

Alaska Range 

Tertiary Sedimentary Rocks and Initiation of a New 

Volcanic Arc—The Meshik Volcanics (Tm) and 

associated intrusive rocks represent volcanic activity 

along the Aleutian arc after the subduction zone moved. 

TERTIARY

(Pliocene and 

Miocene)

Younger Intrusive 

rocks

(Ti) 

Hypabyssal rhyolite porphyry, dacitic felsite, and hornblende 

andesite that form dikes, plugs, and stocks. Tertiary Igneous Rocks—Dikes, plugs, and stocks.

Mineral Extraction—The Mike prospect (Mo, Cu), 

located 8 km (5 mi) northeast of the preserve boundary, is 

associated with rhyolitic porphyry dikes of this unit.

TERTIARY

(early Oligocene 

and late Eocene)

Older intrusive rocks

(Tiu) Primarily small stocks and plugs of diorite. Tertiary Igneous Rocks—Small stocks and plugs None reported
Unit descriptions were derived primarily from Detterman et al. (1981, 1987), Detterman et al. (1996), and Wilson et al. (1999). Gray shaded map units are not mapped within Aniakchak National Monument and Preserve. Bold text refers to report sections.

Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 2 of 3

Age Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

TERTIARY

(early Oligocene 

and late Eocene)

Meshik Volcanics

(Tm) 

Leucobasaltic to dacitic flows, tuff, agglomerate, and lahar 

flows. Associated hypabyssal basalt and andesite plugs. 

Minor amounts of volcaniclastic sedimentary rocks. 

Tertiary Igneous Rocks—Flows, tuff, plugs, and 

sedimentary rocks.

Geohazards—Poorly consolidated Tertiary units are 

associated with many large landslides (geologic map unit 

Qls; see Plate 2 and Quaternary Geologic Map Unit 

Properties Table).

Tertiary Sedimentary Rocks and Initiation of a New 

Volcanic Arc—The Meshik Volcanics (Tm) and 

associated intrusive rocks represent volcanic activity 

along the Aleutian arc after the subduction zone moved.

TERTIARY

(middle Eocene to late 

Paleocene)

Tolstoi Formation

(Tt) 

Nonmarine and shallow marine sandstone, siltstone, and 

conglomerate.

Tertiary Sedimentary Rocks of the Tolstoi 

Formation— Abundant plant fossils, including petrified 

logs, and marine mollusks and dinoflagelates.. 

Geohazards—Poorly consolidated Tertiary units are 

associated with many large landslides (geologic map unit 

Qls; see Plate 2 and Quaternary Geologic Map Unit 

Properties Table).

Paleontological Resource Inventory, Monitoring, and 

Protection—possible theft of petrified wood and plant 

fossils. A field-based paleontological resource survey can 

provide detailed, site-specific descriptions and resource 

management recommendations

Tertiary Sedimentary Rocks and Initiation of a New 

Volcanic Arc—Megaflora and marine invertebrate 

species indicate the unit was deposited when the climate

was warmer than at present. 

Unconformity

LATE 

CRETACEOUS

Hoodoo Formation

(Kh) 

Marine deep water siltstone and sandstone turbidites

deposited along a marine shelf and slope in submarine fans.

Upper Cretaceous Marine and Nonmarine 

Sedimentary Rocks—Inoceramus and ammonite fossils.

Paleontological Resource Inventory, Monitoring, and 

Protection—Possible theft or vandalism of invertebrate

fossils or dinosaur tracks. A field-based paleontological 

resource survey can provide detailed, site-specific 

descriptions and resource management recommendations

Cretaceous Sedimentary Rocks and Unconformity—

Kh is a lateral marine facies deposited on a marine shelf 

and lower slope of a submarine fan; it is coeval with the 

Chignik Formation (Kc) 

LATE 

CRETACEOUS

Chignik Formation

(Kc) 

Nonmarine and marine pebble-cobble conglomerate and 

coarse-grained sandstone that is interbedded with shale, 

coal, and siltstone. Deposited as a cyclic nearshore marine, 

tidal flat, and nonmarine flood plain and fluvial deposit

Upper Cretaceous Marine and Nonmarine

Sedimentary Rocks—Hadrosaur tracks were first record 

of Cretaceous dinosaurs in western Alaska. Minor petrified 

wood in the nonmarine sections. Inoceramus and 

ammonite fossils in the marine portions of the unit.

Cretaceous Sedimentary Rocks and Unconformity—

Lower contact of Kc is unconformable with lower units, 

representing a major erosion period followed by 

nonmarine deposition.

Unconformity

EARLY CRETACEOUS

Herendeen 

Formation 

(Khe) 

Marine thin-bedded calcareous sandstone and siltstone.

Upper Jurassic to Lower Cretaceous Marine 

Sedimentary Rocks—Sandstone is made up of quartz, 

feldspar, and volcanic grains. Inoceramus, ammonite, and 

belemnite fossils. Paleontological Resource Inventory, Monitoring, and 

Protection—A field-based paleontological resource 

survey can provide detailed, site-specific descriptions and 

resource management recommendations although these 

units are not mapped within the monument and preserve.

Cretaceous Sedimentary Rocks and Unconformity—

Khe and Kst deposited in marine setting between 

approximately 145 and 135 million years ago.

EARLY 

CRETACEOUS

Staniukovich 

Formation 

(Kst) 

Marine siltstone and shale with minor sandstone.

Upper Jurassic to Lower Cretaceous Marine 

Sedimentary Rocks—Buchia, Inoceramus, and other 

bivalve fossils.
Unit descriptions were derived primarily from Detterman et al. (1981, 1987), Detterman et al. (1996), and Wilson et al. (1999). Gray shaded map units are not mapped within Aniakchak National Monument and Preserve. Bold text refers to report sections.

Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 3 of 3

Age Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

LATE JURASSIC

Naknek Formation, 

undivided

(Jn) 

Marine and locally nonmarine arkosic conglomerate and 

sandstone with abundant feldspar grains and plutonic clasts. Upper Jurassic to Lower Cretaceous Marine 

Sedimentary Rocks—The oldest rocks within the map 

area, and the most extensive Mesozoic rock unit. The 

Naknek Formation was derived predominantly from 

Jurassic plutonic rocks of the Peninsular terrane. Buchia, 

bivalve, ammonite, belemnite, and gastropod fossils. 

Potential for dinosaur tracks in lowest nonmarine 

member.

Jni forms the lower white cliffs at The Gates and The 

Garden Wall and contains Buchia, bivalve, ammonite, 

belemnite, and gastropod fossils.

Jns contains Buchia, bivalve, ammonite, belemnite, and 

gastropod fossils.

Jnn contains aeolian and fluvial cross-bedding. Potential 

for dinosaur tracks in lowest nonmarine member.

Paleontological Resource Inventory, Monitoring, and 

Protection—Possible theft or vandalism of invertebrate 

fossils or dinosaur tracks. A field-based paleontological 

resource survey can provide detailed, site-specific 

descriptions and resource management recommendations

Jurassic Volcanic Arc and Sedimentary Rocks—

Records initiation of major uplift of the parts of the 

Peninsular terrane west of the Bruin Bay fault.

LATE

JURASSIC

Naknek Formation, 

Indecision Creek 

Sandstone Member

(Jni) 

Arkosic sandstone.

LATE 

JURASSIC

Naknek Formation, 

Snug Harbor 

Siltstone Member

(Jns) 

Marine siltstone and shale containing calcareous concretions.

LATE 

JURASSIC

Naknek Formation, 

Northeast Creek 

Sandstone Member

(Jnn) 

Nonmarine and shallow marine sandstone and conglomerate.

Quaternary Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 1 of 4

Quaternary Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve (see Plate 2) 

Unit descriptions were derived from Detterman et al. (1981, 1987). Glaciation ages are from Detterman (1986). When units are assigned an age more specific than Quaternary, it is indicated. Colors correspond to Plate 2.

Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

LANDFORMS AND SURFICIAL DEPOSITS

Holocene

Dune deposits

(Qdu) 

Fine to medium sand and pumice.

Includes beach dunes along the heads of bays on the Pacific 

side, and dunes formed near the end of loose pyroclastic 

materials on the north side of Aniakchak volcano.

None reported

None reported

Holocene

Lacustrine deposits

(Ql) 

Silt and fine sand deposited in lake basins and 

locally sandy beach deposits.

Glaciers and Glaciations—Includes modern glacier moraine 

dammed lakes along the Meshik River, and lake deposits 

around Meshik Lake after the lake shrank.

Holocene

Swamp deposits

(Qsw) 

Highly organic silt and clay. Formed in bogs, locally forming quaking bogs, or bogs 

where floating vegetation forms along the perimeter.

Largely formed between glacial moraines of the Brooks Lake 

and Mak Hill drift deposits.

Holocene

Estuarine deposits

(Qes) 

Laminated silt and clay, with thin interbeds of fine 

sand; locally rich in organic material. Estuaries along the Bristol Bay coast.

Geohazards—Tsunamis may inundate these deposits 

(and other low-lying coastal areas) None reported

Holocene

Beach deposits

(Qb) 

Well-sorted and stratified sand and gravel.

Beach Berms—Beaches on the Bristol Bay side of the Alaska 

Peninsula are commonly made up of volcanic rocks and 

pumice; whereas, beaches on the Pacific side are made up of 

locally eroded bedrock, except where major rivers have 

carried volcanic material to the coast.

Holocene

Alluvial deposits

(Qal) 

Poorly to moderately well sorted, non-stratified to 

well-stratified gravel and sand along streams and 

rivers. 

Point bars, lateral bars, mid-channel bars, junction bars, and 

braidplains.

Geohazards—Flooding.

Volcanic and Post-Glacial History—Rivers have carried 

easily eroded loose volcanic material to the coast. Large 

alluvial fan deposits along the Aniakchak River and Albert 

Johnson Creek were formed during the caldera outburst 

Alluvial fan deposits flood.

(Qaf) 

Poorly sorted and poorly stratified gravel and 

cobbles near the heads of fans, to well-sorted and 

well-stratified sandy gravel near the toes of fans.

Caldera Outburst Flood and its Deposits—Large alluvial 

fans were formed near Albert Johnson Creek during the 

caldera outburst flood.

Alluvial terrace deposits

(Qt) 

Moderately sorted and stratified sandy gravel, 

locally containing cobbles and boulders. Terraces 

along modern and abandoned channels.

Caldera Outburst Flood and its Deposits—Downstream 

of The Gates, the Aniakchak River cuts through terraces that 

contain very coarse clasts and boulders indicative of large 

flood deposits

Petroleum Development and Mineral Extraction—

Some of the subsurface rights owned by the Koniag 

Corporation allow extraction of construction materials. 

Alluvial terraces are good sources of gravel for roads 

and platforms.

Volcanic and Post-Glacial history—Terraces formed along 

modern and abandoned channels. Terraces downstream of 

The Gates contain large boulders and were formed during 

the caldera outburst flood. 

Abandoned channel deposits

(Qac) 

Moderately well-sorted stratified sand and gravel in 

channels fronting end moraines.

Glaciers and Glaciations—Abandoned channels left dry 

after glaciers receded.

Caldera Outburst Flood and its Deposits—Ancient flood 

channel may have been the result of the outburst flood.

None reported

Glacial History—Associated with the end moraines of the 

Brooks Lake glaciation.

Volcanic and Post-Glacial History—Caldera outburst flood 

created a second channel at Aniakchak Bay.

Colluvium

(Qc) Coarse angular rubble. Talus slopes and cones. Geohazards—Rockfall, landslides, debris flows.

Post-Glacial History—Active geomorphic processes and 

volcanism created a dramatic but potentially dangerous 

landscape. 
Unit descriptions were derived from Detterman et al. (1981, 1987). Glaciation ages are from Detterman (1986). When units are assigned an age more specific than Quaternary, it is indicated. Colors correspond to Plate 2.

Quaternary Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 2 of 4

Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

LANDFORMS AND 

SURFICIAL DEPOSITS

Landslide deposits

(Qls) 

Large blocks and slides consisting of large angular 

unsorted, unstratified rubble. 

Commonly derived from weakly consolidated Tertiary and 

Quaternary units.

Geohazards—Landslides occur when unconsolidated 

deposits or weakly consolidated bedrock form steep 

slopes, undercut by streams, or loosened by weathering 

or earthquakes.

Volcanic and Post-Glacial History—A large landslide at 

The Gates may have been the cause of the caldera outburst 

flood approximately1,800 years ago.

Marine terrace deposits

(Qmt) 

Well-sorted sand and gravel on raised marine 

terraces. 

Raised Marine Terraces—Range from 28 to 30 m (92 to 

98 ft) above sea level on the Pacific side and 15 m (49 ft) on 

the Bristol Bay side. On the Bristol Bay side, wave cut 

terraces extend as far as 38 km (24 mi) inland. 

None reported

Glacial and Post-Glacial History—Raised marine terraces 

were formed primarily by isostatic rebound following the last 

glacial maximum; however, some of the uplift was probably 

due to tectonic flexing of the crust caused by subduction of 

the Pacific Plate.

VOLCANIC DEPOSITS

Volcanic rocks, undivided

(Qv) Flows and volcaniclastic rocks. Forms the caldera walls.

Geohazards—Steep cliffs in the caldera and long its 

flanks are susceptible to landslides and rockfall during 

extended periods of precipitation or earthquakes.

Formed during eruptions of Aniakchak and Yantarni 

volcanos.

Holocene

Cinder and spatter cones

(Qcs) 

Cones formed from scoriaceous to vitrophyric 

cinder to bomb-sized volcanic rock.

Post-Caldera Eruption Features—Cinder cones in the 

Aniakchak Crater. Geohazards—Represents past volcanic eruptions. Formed after the approximately 3,600 cal. yr BP caldera 

forming Aniakchak II eruption.

Holocene

Ash-flow and ash-fall deposits

(Qafd) 

Stratified, welded and non-welded ash, pumice, 

and scoria. High-silica andesite and dacite.

Aniakchak II, The Caldera Forming Eruption—Extensive 

ash-flow and ash-fall deposits surrounding the Aniakchak 

and Yantarni volcanoes.

Nearby Holocene Volcanoes—Yantarni 

Geohazards—Ash-flow and ash-fall deposits are the 

most common and most extensive materials erupted 

from Aleutian volcanos. They can sterilize very large 

areas.

Volcanic History—Primarily Aniakchak II eruption deposits 

formed approximately 3,600 years ago. The Aniakchak II 

eruption formed an extensive ignimbrite sheet that extends 

to the coasts on both sides of the Alaska Peninsula. Also 

includes Yantarni Volcano deposits that are as young as 

2,000 years old.

Holocene

Volcanic avalanche deposits

(Qva) 

Unconsolidated volcanic rock debris. Nearby Holocene Volcanoes—Formed on the south rim of 

Yantarni Volcano and flowed south.

Geohazards—During volcanic eruptions, flanks of 

volcanos can collapse forming large avalanches (Qva), 

ice and snow can melt to form lahars (Qmf), and 

masses of material can flow from steep slopes (Qdf). Volcanic and Post-Glacial History—Active geomorphic 

processes and volcanism created a dramatic but potentially 

dangerous landscape. Qva is coeval with the youngest 

(2,000 years ago) eruption. Yantarni Volcano dome (Qd) 

formed 2,000 to 3,500 years ago.

Holocene

Volcanic mudflow deposits

(Qmf) 

Volcanic mudflows, in part lahars. Unsorted, 

nonstratified ash and large angular debris. Wellindurated to poorly consolidated.

Mudflows in and on the south flanks of the Aniakchak 

Crater 

Holocene

Volcanic debris flow deposits

(Qdf) 

Coarse angular volcanic rock fragments in a matrix 

of ash, mud, and lava.

Debris flows on the south flanks of Aniakchak and Yantarni 

volcanoes.

Holocene

Volcanic explosion debris

(Qed) 

Proximal deposits of angular to rounded volcanic 

bombs and scoria blown out during volcanic 

eruptions. 

Volcanic debris on the flanks of Aniakchak and Yantarni 

volcanoes.

Volcanic domes

(Qd) High-silica andesite, dacite, and rhyolite domes. Yantarni Volcano dome and a small dome on the northeast 

flank of Aniakchak volcano.

Geohazards—Volcanic eruptions.

Andesite plug domes

(Qanp) 

Dark-gray, fine- to coarse-grained hypabyssal 

andesite plugs and shallow intrusions. Small dome on the south flank of Aniakchak volcano.
Unit descriptions were derived from Detterman et al. (1981, 1987). Glaciation ages are from Detterman (1986). When units are assigned an age more specific than Quaternary, it is indicated. Colors correspond to Plate 2.

Quaternary Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 3 of 4

Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

VOLCANIC DEPOSITS

Basalt and basaltic andesite 

flows

(Qba) 

Medium- to dark-gray basalt and basaltic andesite 

flows. Flows on the west side of Yantarni Volcano.

Geohazards—Volcanic eruptions.

Volcanic and Post-Glacial History—Active geomorphic 

processes and volcanism created a dramatic but potentially 

dangerous landscape. Qva is coeval with the youngest 

(2,000 years ago) eruption. Yantarni Volcano dome (Qd) 

formed 2,000 to 3,500 years ago.

Dacite flows

(Qda) Light- to medium-gray dacite flows. Flows in and on the flanks of Aniakchak volcano.

Dacite plugs

(Qdap) 

Light- to dark-gray, medium- to coarse-grained 

hypabyssal dacite. Includes dikes and sills.

Dikes are harder to erode than softer marine sedimentary 

rocks, so they form prominent spires along the Pacific coast.

GLACIAL DEPOSITS

Glacial lake deposits

(Qgl) 

Highly organic silt and clay, locally with stratified 

sand and pebbles near moraine fronts.

Not mapped in Aniakchak National Monument and 

Preserve.

Left behind when lakes drained after moraines (acting as 

dams) eroded. Lake terraces extend 58 km (36 mi) along 

Dog Salmon River valley where the area is poorly drained 

and contains numerous small lakes, ponds, and quaking 

bogs.

Geohazards—Unconsolidated units are easily eroded 

and susceptible to landsliding when exposed on slopes 

or undercut by streams.

Glacial History—Brooks Lake glaciation deposits are Late 

Pleistocene in age.

Qbln is approximately 11,000 years old.

Qblil has a minimum age of between 20,000 and 16,500 

years old.

Qblk has a maximum age of approximately 30,400 years old.

Pleistocene

Brooks Lake glaciation

undivided drift

(Qblu) 

Unsorted, unstratified, coarse to fine till. Freshly to moderately weathered irregular knob and kettle 

topography.

Pleistocene

Brooks Lake glaciation

Newhalen Stade drift

(Qbln) 

Unsorted, unstratified, coarse to fine till; locally 

contains sorted and stratified ice-contact deposits.

Not mapped in Aniakchak National Monument and 

Preserve.

Lightly eroded, well-developed arcuate end and lateral 

moraines with prominent knob and kettle topography.

Pleistocene

Brooks Lake glaciation

Iliamna Stade drift

(Qblil) 

Unsorted, unstratified, coarse to fine till; locally 

contains sorted and stratified ice-contact deposits.

Eroded and rounded large arcuate, lobate, end and lateral 

moraines. Knob and kettle topography considerably 

modified by mass wasting with many kettles partly filled.

Pleistocene

Brooks Lake glaciation

Kvichak Stade drift 

(Qblk) 

Unsorted, unstratified, coarse to fine till; locally 

contains sorted and stratified ice-contact deposits.

Considerably weathered knob and kettle topography; many 

kettles are filled with sediment. Large arcuate, lobate, and 

rounded end moraines.

Pleistocene

Brooks Lake glaciation 

outwash deposits

(Qblo) 

Moderately well-sorted sand and gravel.

Slightly eroded, flat and gently sloping plains in front of 

Brooks Lake glaciation end moraines. Outwash deposits 

accumulated where meltwater streams or rivers deposited 

moderately-sorted gravel and sand

Pleistocene

Mak Hill glaciation drift

(Qmhd) 

Unsorted, unstratified, coarse to fine till. Sorted 

and stratified ice-contact deposits present in some 

beach-cliff exposures along Bristol Bay.

Not mapped in Aniakchak National Monument and 

Preserve.

Highly weathered, rounded and flattened knobs. Kettle 

ponds mostly filled with sediment. Covered by marine 

terrace deposits and outwash from the Brooks Lake 

glaciation. End moraine is highly segmented. Geohazards—Unconsolidated units are easily eroded 

and susceptible to landsliding when exposed on slopes 

or undercut by streams.

Glacial History—oldest glacial deposits are from the Mak 

Hill glaciation. Minimum age 43,000 to >47,000 years old. 

Pleistocene

Mak Hill glaciation outwash

deposits

(Qmho) 

Moderately well-sorted, stratified, consolidated 

sandy gravel.

Eroded and segmented deposits of flat and sloping plains in 

front of the Mak Hill end moraine. Outwash deposits 

accumulated where meltwater streams or rivers deposited 

moderately-sorted gravel and sand
Unit descriptions were derived from Detterman et al. (1981, 1987). Glaciation ages are from Detterman (1986). When units are assigned an age more specific than Quaternary, it is indicated. Colors correspond to Plate 2.

Quaternary Geologic Map Unit Properties Table: Aniakchak National Monument and Preserve. Page 4 of 4

Map Unit

(Symbol) Description Features and Processes Resource Management Issues History

TERTIARY to JURASSIC

Bedrock, undivided

(TJbu)

Undifferentiated bedrock. 

See Geologic Map Units Properties Table and Plate 

1 for individual units.

See Geologic Map Units Properties Table and Plate 1. See Geologic Map Units Properties Table and Plate 1. See Geologic Map Units Properties Table and Plate 1.