Only two months after the explosive eruption of Chaiten Volcano produced ash plumes that were visible from space, an Alaskan volcano has followed with an explosive eruption of its own. The eruption cloud was detected by satellite sensors and provided valuable information to volcanologists who couldn’t get close to the action.
Okmok Volcano, located within Alaska’s remote Aleutian Island Chain, erupted July 12, 2008 at approximately 11:43 a.m. local time, according to the Alaska Volcano Observatory. The eruption cloud top was reported at altitudes ranging from 30,000-35,000 feet initially and moved in a direction that was east to southeast of the volcano.
Ash and Gas from Aleutian Island Volcano
The volcanic cloud contained heavy loads of ash and sulfur dioxide that were detected using satellites sensors in space. NASA’s Ozone Monitoring Instrument (OMI), which can distinguish aerosol types such as ash, smoke and sulfates, detected high amounts of aerosols and sulfur dioxide within the cloud. Initial estimates of the amount of sulfur dioxide within the cloud calculated from satellite images were about 0.1 Teragrams, or 100,000 metric tons – a very large amount, indicating that this eruption was quite large.
Ash was also detected by NOAA’s Advanced Very High Resolution Radiometer (AVHRR) sensor and the Geostationary satellite (GOES). Ash acts like a wavelength filter in the atmosphere and can be detected by satellite sensors because of its absorption effects at 11 and 12 microns. The 11 micron wavelength is absorbed more strongly by ash than the 12 micron wavelength.
Volcanologists simply subtract the amount of energy detected at the 12 micron wavelength from the amount of energy detected at the 11 micron wavelength and if the value is negative, then ash is present. This is called the “split-window” technique. For the Okmok volcanic cloud, satellite sensors detected ash until about July 16, when ash was no longer visible in satellite imagery using the “split-window” technique.
Far Reaching Plume
Even after ash was no longer visible in the satellite imagery, a sulfur dioxide plume was still visible. This plume continued to move east from the volcano and satellite imagery days after the eruption showed a plume extending from the northern West Coast to Lake Superior. The extent of this plume is evidence that even if you are nowhere near a volcano, your environment can still be impacted by an eruption.
Dangerous Lahars
The Okmok eruption also produced lahars that wiped out roads and flooded farms in the nearby town of Ft. Glenn. Alaskan volcanoes can commonly produce lahars because their summits are usually covered with snow, even in the summer. Furthermore, these volcanoes erupt large amounts of loose ash and pyroclasts, which, when mixed with melting snow, produce a thick, sludgy mud flow. Combined with the volcano’s steep slopes, these flows can quickly rush down the mountain and cause devastation in low-lying areas, such as surrounding farms.
Okmok Volcano last erupted in 1997.
Sources:
Alaska Volcano Observatory Okmok activity page
NASA Spacecraft and Instruments
Personal Communication, Simon Carn, Joint Center of Earth Systems Technology, University of Maryland, Baltimore Campus
Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
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