AVO Logo
Site Map | FAQ |
Alaska Volcano Observatory
About Alaska's Volcanoes | Hazards from Alaska's Volcanoes | Map & Alphabetical List | Interactive Map | Eruption Search | Volcano Search | Okmok 
You are here: Home > Volcano Information > Okmok

Mount Okmok description and information



Official Name: Mount Okmok
Type:Central shield complex with nested caldera
Most Recent Activity:July 12, 2008
Seismically Monitored: Yes
Color Code:GREEN
Alert Level:NORMAL
Elevation: 3520 ft (1073 m)
Latitude: 53.419° N
Longitude:168.132° W
CAVW Number:311290
Pronunciation: Sound file
Associated Features:Cone A
Idak, Mt
Jag Peak
Cone B
Cone C
Cone D
Cone E
Cone F
Cone G
Cone H
Cone I
Cone J
Kettle Cape
Waist Tuya 1
Kidney Bean
Idak cindercones
Cinder Point
Waist Tuya 2
Waist Tuya 3
Waist cone/flow 1
Waist cone/flow 2
Waist cone 3
Nearby towns:Nikolski 45 mi (73 km) SW
Unalaska 72 mi (117 km) NE
Akutan 108 mi (174 km) NE
False Pass 215 mi (347 km) NE
Anchorage 863 mi (1389 km) NE
From Miller and others (1998) [1]: "Okmok Volcano occupies most of northeastern Umnak Island. The volcano, built on a base of Tertiary volcanic rocks, consists of three rock series: older flows and pyroclastic beds of a pre-caldera shield complex; pyroclastic deposits of two major caldera-forming eruptions; and a post-caldera field of small cones and lava flows that includes historically active vents within the caldera [2].

"Construction of the pre-caldera volcano began in late Tertiary or early Quaternary time. Two whole-rock K-Ar analyses of a sample of basaltic lava are 1.7 and 2.1 +/- .2 m.y. [3]. Basaltic flows and pyroclastics compose most of the older rocks (unit Qbm); flows are more voluminous than pyroclastic deposits (see Byers, 1959 [2], plate 41). Vent agglomerate is exposed in the walls of the younger caldera and tuffs and tuff-breccias occur further down the flanks. Flows and pyroclastic beds dip radially from the caldera at less than 5 degrees in most sectors, except for local steepening and reversal of dips at former vents. Such radial dips suggest that the older complex was a central volcano with parasitic vents. Domes and plugs of andesite and rhyolitic composition and basaltic lava flows (Byers, 1959 [2], p. 312) mark minor vents that were active before caldera formation; these rocks are only slightly modified by erosion and are presumably early post-glacial in age.

"Catastrophic pyroclastic eruptions resulted in the formation of 2 overlapping calderas (Byers, 1959 [2], p. 274). The deposits, referred to as the Okmok Volcanics (Byers, 1959 [2], p. 314), are mainly thick, non-sorted agglomerate at the caldera rim overlain by ash-flow tuffs, and airfall ash and pumice that cover the flanks of the volcano to the shoreline (unit Qov). Two arcuate ridges located about 1.5 km north and east of the main topographic basin are the remnants of the older caldera, which formed about 8200 years ago [4]. The topographic basin of the younger caldera is about 9.5 km in diameter; the maximum elevation of the rim is about 1070 m and the mean elevation of the floor, exclusive of areas underlain by post-caldera volcanic rocks, is about 370 m. The occurrence of 2 major ash-flow sheets, separated by a basalt flow and erosional unconformity, supports the two caldera-forming eruptions [5]. Miller and Smith (1987) [6] have reported a maximum C14 age of 2400 +/- 200 yr for the second and younger caldera-forming eruption.

"Numerous small flows, plugs, and cinder cones on the flanks of Okmok Volcano are interbedded with or overlie the upper pars of the Okmok Volcanics (unit Qyv). Within the caldera, the oldest post-caldera deposits are brecciated pillow lavas and pyroclastic rocks that were deposited in a caldera lake. The lake attained a maximum depth of about 150 m and the upper surface reached an elevation of about 475 m, at which point it overtopped the low point of the caldera rim. A small shallow lake located near the outlet of the caldera is all that remains today. Three dissected tuff cones may have been produced by eruptions beneath the former caldera lake. Other cinder cones occur atop pillow lavas; such cones apparently breached the surface of the former lake. Cinder cones and associated lava flows that are younger than the caldera lake are identified by structures and textures characteristic of subaerial eruption. The documented eruption of 1945 occurred at a cinder cone near the southwest caldera wall [2] [7]; this cone may have been the site of all historical activity of Okmok volcano. Hot springs and fumaroles occur both within Okmok caldera and at Hot Springs cove, 20 km to the southwest."

Catalog of the historically active volcanoes of Alaska, 1998
citation imageMiller, T. P., McGimsey, R. G., Richter, D. H., Riehle, J. R., Nye, C. J., Yount, M. E., and Dumoulin, J. A., 1998, Catalog of the historically active volcanoes of Alaska: U.S. Geological Survey Open-File Report 98-0582, 104 p.
Download PDF title page PDF : 52
Download PDF intro and TOC PDF : 268 KB
Download PDF eastern part - Wrangell to Ukinrek Maars PDF : 972 KB
Download PDF central part - Chiginagak to Cleveland PDF : 2,463 KB
Download PDF western part - Carlisle to Kiska PDF : 956 KB
Download PDF references PDF : 43 KB

Geology of Umnak and Bogoslof Islands, Aleutian Islands, Alaska, 1959
citation imageByers, F. M. Jr., 1959, Geology of Umnak and Bogoslof Islands, Aleutian Islands, Alaska: in Investigations of Alaskan volcanoes, U.S. Geological Survey Bulletin 1028-L, p. 267-369, 5 sheets, scale 1 at 1:63,360, 1 at 1:96,000, and 1 at 1:300,000.
Download PDF full-text PDF : 3.5 MB
Download PDF plate 39 PDF : 2.2 MB
Download PDF plate 40 PDF : 3.9 MB
Download PDF plate 41 PDF : 5.6 MB
Download PDF plate 48 PDF : 85 KB
Download PDF table 3 PDF : 149 KB

Palaeosecular variation of the geomagnetic field in the Aleutian Islands, Alaska, 1972
citation imageBingham, D. K., and Stone, D. B., 1972, Palaeosecular variation of the geomagnetic field in the Aleutian Islands, Alaska: The Geophysical Journal of the Royal Astronomical Society, v. 28, n. 4, p. 317-335.

Late-Quaternary geomorphic processes: effects on the ancient Aleuts of Umnak Island in the Aleutians, 1975
Black, R. F., 1975, Late-Quaternary geomorphic processes: effects on the ancient Aleuts of Umnak Island in the Aleutians: Arctic, v. 28, n. 3, p. 159-169.

Two caldera-forming eruptions on Umnak Island, eastern Aleutian Islands, 1975
Miller, T.P., and Smith, R.L., 1975, Two caldera-forming eruptions on Umnak Island, eastern Aleutian Islands: The United States Geological Survey in Alaska: Accomplishments during 1975: U.S. Geological Survey Circular 733, p 45.

Late Quaternary caldera-forming eruptions in the eastern Aleutian arc, Alaska, 1987
citation imageMiller, T. P., and Smith, R. L., 1987, Late Quaternary caldera-forming eruptions in the eastern Aleutian arc, Alaska: Geology, v. 15, n. 5, p. 434-438.
Download PDF full-text PDF : 2.5 MB

Exploring Aleutian volcanoes, 1948
citation imageRobinson, G. D., 1948, Exploring Aleutian volcanoes: National Geographic Magazine, v. 94, n. 4, p. 509-528.

Contact AVO Privacy Accessibility Information Quality FOIA
URL: avo.alaska.edu/volcanoes/volcinfo.php
Page modified: September 29, 2020 12:19
Contact Information: AVO Web Team

twitter @alaska_avo
facebook alaska.avo
email Receive volcano updates by email: USGS VNS

This website is supported by the U.S. Geological Survey under Cooperative Agreement Grant G22AC00137

Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey.