The Alaska Volcano Observatory detected elevated surface temperatures Jan. 24 at Cleveland's summit. Satellite data obtained last week indicated that a growing lava dome on the floor of the summit crater had reached about 328 feet, or 100 meters, in diameter.
The summit crater itself is nearly 10 times that size, said Chris Waythomas of the U.S. Geological Survey.
"It does this from time to time and it's a fairly small lava dome," he said Wednesday. "It's not gigantic."
Lava domes form a lid on a volcano chamber that holds magma. When they grow big enough, lava domes can become unstable and collapse. Decompression of the magma chamber can lead to an explosion as the conduit inside the volcano suddenly becomes unsealed and gasses escape.
The new lava dome led the observatory to change the volcano's alert level to "orange," indicating heightened or escalating unrest. Development of a lava dome indicates sudden explosions of ash higher than 20,000 feet -- a threshold for concern for international air carriers -- could be produced with little warning.
The volcano in the past has sent ash clouds much higher. Cleveland Volcano's last major eruption was 12 years ago. It began in February 2001 and produced three explosive events that produced ash clouds as high as 39,000 feet.
Cleveland has had burst of smaller plumes nearly every year since then. In December 2011, Cleveland sent up an ash cloud to 15,000 feet that did not disrupt international air carriers. Scientists last detected minor ash emissions in November.
Alaska's Redoubt Volcano blew on Dec. 15, 1989, and sent ash 150 miles away into the path of a KLM jet carrying 231 passengers. Its four engines flamed out and the jet dropped more than 2 miles, from 27,900 feet to 13,300 feet, before the crew was able to restart all engines and land the plane safely at Anchorage.
Cleveland Mountain is a 5,675-foot peak on uninhabited Chuginadak Island about 940 miles southwest of Anchorage. The nearest village is tiny Nikolski on another island about 50 miles east. Previous eruptions of Cleveland were not considered a threat to Nikolski's 16 permanent residents.
The observatory is a joint program between the USGS, the University of Alaska Geophysical Institute and the state Division of Geological and Geophysical Surveys.
The observatory has no real-time seismic equipment on Cleveland. Pressure sensors on Okmok Volcano -- 74 miles away-- have in the past detected a pressure wave from an explosion, Waythomas said.
"We have a couple people who are watching for that," he said.
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