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Okmok update, 2 August 2008 2 August 2008

Posted by admin in activity reports, Alaska, eruptions, Okmok, United States.
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Okmok's eruption plume 30 July 2008 (Image courtesy of Lt William Mowitt, National Oceanic and Atmospheric Administration)

No dramatic developments at Okmok, but the eruption continues. The latest weekly report from the Alaska Volcano Observatory comments that over the last week an ash and steam plume reaching altitudes of between 3000 and 10600 metres (10000-35000 feet) have been continuously erupted and light ash fall has taken place on Umnak and Unimak islands. From 28 to 30 July Okmok went to aviation colour code red because of a particularly vigorous eruptive episode: it is currently at colour code orange. Seismic activity is elevated but steady. During this eruption there have been periods of continuous and pulsating tremor strong enough to register on the seismic networks of neighbouring Alaskan volcanoes.

Image: Photo of Okmok’s eruption plume on July 30, 2008, as seen from the NOAA ship Oscar Dyson, located about 80 nautical miles to the north of Okmok. Photograph taken by Lt William Mowitt, National Oceanic and Atmospheric Administration. [source]

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

Comments

1. Boris Behncke - 7 August 2008

There are some interesting details emerging about this unusually explosive eruption of Okmok, which are revealed by some of the most recent aerial photographs of the eruption posted on the AVO web site.
It is evident that the source of the eruption is different from that of all previous eruptions that were visually documented (i.e. 1945, 1958, 1997 plus a few smaller explosive events), which was a large scoria cone named Cone A. The current eruption is taking place from an area further east in the caldera, apparently next to a cone named Cone D, of which no observed eruptions are known. Next to this cone is a lake, which is well represented in a photo at http://www.avo.alaska.edu/image.php?id=14728
and which seems to have strongly influenced the style of the current eruption. The activity is a fine example of a phreatomagmatic eruption; numerous photographs posted at AVO show evidence for dilute pyroclastic density currents named base surges (see, for example, the photo at http://www.avo.alaska.edu/image.php?id=14441). These are vapor-rich, hurricane-like currents, less heavily charged with rock debris than pyroclastic flows, which typically result from explosive magma-water interaction. However, other images show evidence for more typical, “dry” pyroclastic flows (http://www.avo.alaska.edu/image.php?id=14436), and some of the most recently posted photographs are particularly intriguing. If you look at the full size version of the image at http://www.avo.alaska.edu/image.php?id=14783 and focus at the area just in front of the base of the eruption column, you will see pyroclastic flow deposits (partly containing quite large chunks of rock debris) pockmarked with pits of varying size. Such pits have been seen also at Mount St. Helens in 1980 where pyroclastic flows covered, and interacted with, chunks of glacial ice and lake water trapped in the hot rock; a good photograph of these pits (partly still exploding) is at http://libraryphoto.cr.usgs.gov/htmllib/btch137/btch137j/btch137z/btch137/cvoa0065.jpg

Finally, this seems to be the largest eruption at Okmok at least since 1817, when an Aleut village on the coast of Umnak island (which also hosts the volcano) is said to have been destroyed by “a flow of ashes”. Okmok has a history of two cataclysmic, caldera-forming eruptions in the past 12,000 years, the younger occurring about 2000 years ago. Excellent information is available in a report “Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska”, which can be downloaded at http://www.dggs.dnr.state.ak.us/webpubs/dggs/ri/text/ri2004_003.PDF (yes, the final letters “PDF” are in capital letters!)
Cheers from Mount Etna, Sicily


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