Scientists are using a combination of seismic and GPS instruments on the ground and weather and radar satellites in space to track the progress of the eruption. Human visual observations are limited because airborne ash obscures a view of what is happening inside the volcano's 6-mile-diameter caldera and the area is too hazardous to enter.

"We are dealing with a scientific challenge because the volcano went from very quiet to a large eruption, putting ash to high altitudes with almost no warning," said John Power, Acting Scientist in Charge of the Alaska Volcano Observatory (AVO).

The powerful eruption in the Eastern Aleutian Islands began unexpectedly on July 12, sending up a wet, ash and gas-rich plume that reached an altitude of 50,000 ft above sea level. Heavy ash fall occurred on eastern Umnak Island. A dusting of ash fell in the busy fishing community of Unalaska, 65 miles northeast of Okmok volcano. The ash plume soon spanned several hundred miles across the North Pacific, causing many trans-Pacific flights to be diverted and cancellation of flights to the Dutch Harbor airport. The gas cloud from the eruption is now over Montana.

The eruption also destroyed or damaged seismic and deformation sensing equipment at two monitoring stations. A third station has lost its communication pathway due to destruction at the other two. Seismic equipment relays earthquake information and GPS equipment is used in monitoring the deformation of the ground surface in response to magma movement. Seven seismic stations are still operational and seismicity has gradually decreased in intensity since the initial eruption.

At a minimum, activity at Okmok is likely to continue for days or weeks. Strong gas-driven explosions can produce rock ballistics or larger volcanic debris that can be hurled beyond the crater rim of the volcanic caldera, potentially landing in surrounding areas several miles away. Fast moving clouds of ash, larger debris, and hot gas can form and flow across the caldera floor, rise up over the caldera wall and continue to flow down Okmok's flanks. Rain mixed with ash could create mudflows and rapid flooding along island drainages. As soon as conditions allow, AVO scientists will travel to the volcano in order to document and understand the sudden onset of explosive activity and repair damage to monitoring equipment.

The Okmok caldera formed during catastrophic eruptions 12,000 and 2,000 years ago. There are about a dozen cones within the modern caldera that formed in the last 2000 years, and the most recent eruptive activity occurred in 1945, 1958 and 1997. One violent eruption of Okmok in 1817 produced many feet of ash and "scoria" rock debris on the northeastern caldera rim, as well as ash fall on Unalaska Island and floods that buried an Aleut village at Cape Tanak on the northeast Bering Sea Coast of Umnak Island.

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