2008 Eruption of Okmok volcano, Alaska
Background
Okmok Volcano, a 10 km diameter Holocene caldera located in the central Aleutian islands, erupted suddenly and violently at 19:43 UTC (11:43 am AKDT) July 12th, 2008. The eruption issued from new vents alongside ~1000 year old Cone D within the caldera. The eruption began several hours after a subtle increase in earthquakes detected by the Alaska Volcano Observatory (AVO). It came at the end of an 11 year repose period, and was a significant departure from the predominantly effusive historical eruptions from Cone A. With assistance from a fishing vessel, the family living at Fort Glenn cattle ranch escaped just following onset of the eruption, and later reported constant ground shaking and flashes of lightning as the eruption began. The eruption lasted 5 weeks, with the last confirmed observations of ash emission August 13-20, 2008. It is the first phreatomagmatic eruption in the United States since Ukinrek Maars in 1977 (Larsen et al., 2009).
Okmok volcano is frequently active. Since about 1900, all eruptions have issued from Cone A, located in the southwestern sector of the caldera (Figure 1). The three eruptions prior to 2008 (1997, 1958, and 1945; Figure 1) were predominantly effusive. All three eruptions were similar, producing ash to 5-6 km, and low level Strombolian explosions. The main features of those eruptions are basaltic lava flows crossing the caldera. Prior to the activity from Cone A, which began about 100 years ago, there was an explosive, phreatomagmatic eruption from Cone B in the northern sector of the caldera. That eruption was violent, and issued along a fissure vent aloong the northern to northwestern margin of the caldera. Eruptive activity included formation of a maar, reports of ash fall in Unalaska, and pyroclastic surges and lahars that swept down Crater Creek, burying the only Aleut settlement in the northeastern sector of Umnak Island at the time (Neal et al., 2005; Beget et al., 2005). The eruption finally produced a cinder cone (Cone B) and associated lava flow. Cone D is the oldest of the post 2050 yBP cones within Okmok caldera. It is an emergent cone, which produced sub-aqueous lavas under a prior caldera lake, eventually erupting enough material to emerge as the tall cinder cone present today. It retains a prominent lava bench from the sub-aqueous to sub-aerial eruptive transition (Almberg, 2003).
Figure 1. Okmok volcano is located in the eastern-central portion of the Aleutian volcanic arc. It is located approximately 100 km southwest of Unalaska/Dutch Harbor. Okmok is frequently active, with approximately 14 confirmed eruptions during historical times. The volcano consists of a basaltic andesite shield, truncated by two nested 10 km diameter calderas that formed during catastrophic eruptions ~12,000 and 2050 years BP. Okmok's eruptive style encompasses extremely violent, large volume caldera-forming events, explosive phreatomagmatic activity noted during historic as well as Holocene (Wong and Larsen, 2009) time, and quiescent, lava flow producing eruptions. Three eruptions prior to 2008 occurred in 1997, 1958, and 1945 from Cone A, located in the southwestern sector of the caldera (Figure 1). The links below provide more information about the eruption through preliminary field, geochemical, and petrological studies.
Please note: The photographs contained within these webpages were taken by Jessica Larsen (UAFGI-AVO). Some of the information contained within is not yet published, some was presented at AGU in December 2008, and some is published in Larsen et al. (2009). Please do not use data, information, or photographs without permission of the author.
