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Rugged pinnacles form the summit of the deeply eroded Isanotski stratovolcano, locally known as Ragged Jack, at the center of an E-W-trending group of three volcanoes on eastern Unimak Island. Four poorly documented historical eruptions were noted in the 18th and 19th centuries. However, Miller et al. (1998) considered that some or all of these eruptions could have been from neighboring Shishaldin volcano and that historical eruptions of Isanotski must be considered unlikely, given the extreme degree of erosional dissection.
Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).
|Start Date||Stop Date||Eruption Certainty||VEI||Evidence||Activity Area or Unit|
|[ 1845 ]||[ Unknown ]||Uncertain|
|[ 1831 Mar ]||[ 1831 May 6 ± 1 days ]||Uncertain|
|[ 1830 Nov ]||[ Unknown ]||Uncertain||2|
|[ 1825 Mar 10 ]||[ Unknown ]||Uncertain||4|
|[ 1795 ]||[ Unknown ]||Uncertain||3|
|Issanakski | Sannak | Isannach | Ragged Jack | Isannachotski|
|Three prominent volcanoes are constructed along an E-W line on the eastern half of Unimak Island. The aptly named Roundtop volcano in the lower foreground is glacially eroded and has had no historical eruptive activity. In the 1930s warm springs were found on its slopes. The recent discovery of Holocene pyroclastic-flow deposits and a group of lava domes south of Roundtop indicate it is still an active volcano. The glacially dissected Isanotski volcano and the beautifully symmetrical Shishaldin volcano are the prominent peaks behind Roundtop.
Photo by Clayton and Marcia Brown, 1986 (courtesy of John Reeder, Alaska Div. Geology Geophysical Surveys).
|Three prominent stratovolcanoes are constructed along an E-W line on Unimak Island. Steaming Shishaldin, the highest of the three, rises to 2857 m in the foreground and is one of the most active volcanoes in the Aleutians. The dissected Isanotski volcano (right) has had reported eruptions of uncertain validity, and Roundtop volcano (center distance) has had Holocene eruptions, but no historical activity.
Photo by Clayton and Marcia Brown, 1987 (courtesy of John Reeder, Alaska Div. Geology & Geophysical Surveys).
|Glaciers drape the ruggedly dissected upper flanks of Isanotski volcano and its dramatic summit pinnacles. Isanotski, locally known as Ragged Jack, is at the center of an E-W-trending group of three volcanoes on Unimak Island. Four poorly documented historical eruptions were noted in the 18th and 19th centuries. Some or all of these eruptions, however, could have been from neighboring Shishaldin volcano, and historical eruptions of Isanotski must be considered unlikely, given the extreme degree of erosional dissection.
Photo courtesy of Alaska Volcano Observatory, 1994.
Coats R R, 1950. Volcanic activity in the Aleutian Arc. U S Geol Surv Bull, 974-B: 35-47.
IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..
Miller T P, McGimsey R G, Richter D H, Riehle J R, Nye C J, Yount M E, Dumoulin J A, 1998. Catalogue of the historically active volcanoes of Alaska. U S Geol Surv Open-File Rpt, 98-582: 1-104.
Motyka R J, Liss S A, Nye C J, Moorman M A, 1993. Geothermal resources of the Aleutian arc. Alaska Div Geol Geophys Surv, Prof Rpt, no 114, 17 p and 4 map sheets.
Smith R L, Shaw H R, Luedke R G, Russell S L, 1978. Comprehensive tables giving physical data and thermal energy estimates for young igneous systems of the United States. U S Geol Surv Open-File Rpt, 78-925: 1-25.
Wood C A, Kienle J (eds), 1990. Volcanoes of North America. Cambridge, England: Cambridge Univ Press, 354 p.