Alaska, United States
Mount Okmok is the highest point on the rim of Okmok Caldera (Unmagim Anatuu in Aleut) on the northeastern part of Umnak Island in the eastern Aleutian Islands of Alaska, USA. This 5.8 mile (9.3 km) wide circular caldera truncates the top of a large shield volcano. A crater lake once filled much of the caldera to a depth of over 500 feet, but the lake ultimately drained through a notch eroded in the northeast rim. Only a small remnant lake about 1000 feet across exists in a depression in the western area of the largely flat caldera floor.
The last major eruptions of Okmok—with an estimated VEI strength of 6—occurred 8,300 and 2,400 years Before Present.
Following the formation of the caldera, numerous satellite cones and lava domes have formed on the flanks of the volcano. They include Mount Tulik (1253 meters), Mount Idak (585 meters), and Jag Peak.
The volcano is currently rated by the Alaska Volcano Observatory as Aviation Alert Level Green and Volcanic-alert Level Normal.
July 2008 eruption
On Saturday, July 12, 2008, Okmok Caldera exploded without warning, sending a plume of ash over 33000 feet into the air. No lava flow has been reported, though the volcano has issued flows in the past. Mt. Okmok is roughly 860 miles southwest of Anchorage and 60 miles from the airport at Dutch Harbor, and is to be monitored closely. The volcano last erupted in 1997 and produced a lava flow. This time it erupted with no initial warnings. Residents of Fort Glenn, a nearby ranch, fled from falling ash and volcanic rock. The United States Coast Guard has been carrying out rescue operations. The Alaska Volcano Observatory issued an advisory stating ash in the Dutch Harbor area poses a threat to aircraft. Two planned flights from the area have already been canceled.
The eruption ended on July 12, 2008. The eruption had a VEI of 4, thus giving it a rating of 'cataclysmic'.