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Okmok volcano
One of the most active volcanoes in the Aleutian island chain (Alaska)
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Volcano type |
Central shield complex with nested caldera |
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Location |
Aleutian islands, 1383 km (859 mi) from Anchorage, 53.397° N / 168.166° W |
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Summit elevation |
1073 m (3520 ft) |
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Last eruptions |
1817, 1824, 1878(?), 1899, 1931, 1935, 1936, 1938, 1943, 1945, 1958, 1960, 1981, 1983, 1986, 1997, 2008 |
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Typical eruption style |
Explosive and effusive |
Background:Okmok Volcano is one of the most active volcanoes in the Aleutian chain. It consists of a complex of shields truncated by a caldera and several active vents inside the caldera, which have built small cones and lava flows.
Okmok's caldera was formed by at least two collapses following catastrophic pyroclastic eruptions, at around 8200 y B.P. and 2400 y. B.P.
Within the caldera, the oldest volcanic deposits are brecciated pillow lavas and pyroclastic rocks once 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 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; 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.
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Source: abbreviated from AVO (http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Okmok)
Okmok volcano: news
Jul, 13 2008 - Mar, 10 2009
Tuesday, Mar 10, 2009
The AVO raised the level of danger to the color code Yellow, because of increased seismicity. A eruption in near future is possible.
Thursday, Aug 21, 2008
AVO reported that on 13 August low-level steam-and- ash plumes from Okmok were visible on satellite imagery drifting SE at altitudes of 3-4.6 km (10,000-15,000 ft) a.s.l. During 14-17 August satellite observations were hindered due to cloud cover; seismic levels fluctuated possibly indicating that steam-and- ash emissions continued. During 18-19 August, ash plumes were seen on satellite imagery at altitudes of 3-4.6 km (5,000-15,000 ft) a.s.l. The Volcano Alert Level remained at Watch and the Aviation Color Code at Orange.
Tuesday, Aug 05, 2008
Thursday, Jul 24, 2008
On 19 July, activity at Okmok increased as interpreted from seismicity. Possibly, ash plumes rose to altitudes of 7.6-9.1 km (25,000-30,000 ft) a.s.l. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red.
On 20 July, seismicity decreased. Satellite imagery revealed that an ash plume about 20 km from Okmok drifted SE at an altitude of 3.7 km (12,000 ft) a.s.l. Additional ash plumes observed on satellite imagery and spotted by pilots rose to altitudes of 4.6-6.1 km (15,000-20,000 ft) a.s.l.
During 21-22 July, ash plumes rose to altitudes of 6.1-9.1 km (20,000-30,000 ft) a.s.l. and drifted SE.
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(adapted from SI/USGS Weekly Volcanic Activity Report 16-22 July 2008)
Wednesday, Jul 16, 2008
Monday, Jul 14, 2008
The strong eruption at Okmok continues based on ongoing seismic activity and satellite observations. ash emissions continue reaching altitudes of 30,000 - 35,000 ft asl, and are posing a large hazard to all aircraft in the area.
Sunday, Jul 13, 2008
A large explosive eruption at Okmok volcano began at approximately 11:43 AM local time on July 12 and continues. An ash cloud with a top of at least 30,000-35,000 ft. height above sea level is present.
The main mass of the ash cloud is moving southeast from the volcano, with lesser amounts of ash moving eastward. ash fall has been reported on eastern Umnak Island and in Unalaska/Dutch Harbor.
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