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A look at Okmok 24 July 2008

Posted by admin in activity reports, Alaska, eruptions, Okmok, United States.
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There are a lot of volcanoes on the Aleutian Islands and they have some great names: Shishaldin, Bogoslof, Chagulak, Adagdak. And there’s Okmok, currently in eruption, situated on Umnak Island in the middle of the Aleutian chain, a large basaltic shield volcano topped by two nested calderas. Being a shield volcano it’s a low, wide thing (35km wide, summit just over 1km) very different in form to most Aleutian volcanoes which tend to resemble the symmetrical crater-topped cones that children come up with when asked to draw a volcano. Recent eruptions at Okmok have come from a cinder cone within the south-western portion of the caldera, ‘Cone A’, marked on the map below.

Jessica Larsen, Alaska Volcano Observatory/Alaska Division of Geological & Geophysical Surveys)

Above: Map of Umnak Island, showing Okmok volcano. Credit: Jessica Larsen, Alaska Volcano Observatory / Alaska Division of Geological & Geophysical Surveys. Click the image to go to the original source.

Okmok is an active volcano, with an eruptive history that includes a VEI=6 (‘colossal’) eruption around 100BC and fairly frequent events of between VEI=1 and VEI=3 over the past two centuries. Its most recent eruption before this month was in 1997.

Despite Okmok’s recent history of eruptions every few years, the current outburst came as a complete surprise because of the lack of precursory signs. Volcanoes usually give warnings of what they are up to, above all through seismic signals, but in this case the upsurge in seismicity preceding the eruption was only about 90 minutes in duration (the SI/USGS Volcanic Activity Report says ‘about an hour’). The eruption began on 12 July 2008 at 1143 local time and threw up an ash cloud to 15 kilometres (50,000 feet), posing a considerable hazard to North Pacific air transport routes. The relatively few inhabitants of Umnak Island were rapidly evacuated by fishing boat. The eruption slowly declined from its initial stages but remained vigorous and continuous for some days.

Phil Walgren and Alaska Airlines)

Above: Aerial view of Okmok from the north, 20 July 2008 (Credit: Phil Walgren and Alaska Airlines). Click on the image to go to the original source.

Volcano and aviation alert levels were lowered on 16 July, and on 17 July the plume had diminished to (still significant) altitudes of 7.5 to 9 kilometres (25,000-30,000 feet). On the following day the Alaska Volcano Observatory described the eruption as ‘episodic, with occasional, ash-producing explosions occurring every 15 to 30 minutes. The plumes from these explosions appear to be limited to about 20,000 feet above sea level’. Since then the eruption continues but has become less explosive – but the AVO warns that ‘Stronger explosive activity could resume at any time with little or no warning’.

Interesting points about the current eruption are its unusually explosive character (for Okmok) and its location at Cone D [this should be ‘near Cone D’ – see comments below], in the eastern part of the caldera, rather than Cone A, which has been the seat of all recent eruptions.

As if to keep Okmok company, Cleveland volcano on Chuginadak Island (about 140 kilometres from Umnak) erupted on 21 July, sending up a 4500-5200 metre (15000-17000 feet) plume. This produced the enjoyable headline ‘Cleveland volcano joins Okmok in Aleutian ash fest’ in the Anchorage Daily News.

News
Okmok volcano forces evacuation of Umnak Island – KTUU.com, 12 July 2008
Volcano erupts, fishing boat rescues 10 people – Associated Press, 13 July 2008
Cleveland volcano joins Okmok in Aleutian ash festAnchorage Daily News, 21 July 2008

Information
Global Volcanism Program: Okmok – summary information for Okmok (1101-29-)
Alaska Volcano Observatory Okmok eruption page – Okmok information and updates from the AVO

The Volcanism Blog

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Comments

1. Ron Schott - 24 July 2008

According the the Friday, July 18 report of the Alaska Volcano Observatory (AVO – https://volcanism.wordpress.com/2008/07/24/a-look-at-okmok/):

“Based on pilot reports, airborne photographs, and radar intensity images, the eruptive vent appears to be new, having formed to the northwest of Cone D (located east of the center of the caldera).”

Have there been more recent reports indicating that this was in error?

2. volcanism - 24 July 2008

Ron – It’s my phraseology that’s in error: the seat of the current eruption (the new vent) is near Cone D, not ‘at’ Cone D. There are no more recent reports I’m aware of that supercede the 18 July AVO bulletin you quote.

3. Christine - 7 August 2008

Is it significant that there has been a recent swarm of earthquakes ranging between 1.8 to 5.9 in the Andreanof Islands, Aleutians in the past two days, literally every few minutes, that seem to be in the vicinity of the great quake of 1957 which is not far from the lat/long of Okmok?


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