Atka Island description and information

From Wood and Kienle (1990) ^[1]: "Atka is the largest (~200 cubic km) volcanic center in the central Aleutians. There are no larger centers westward, and the closest larger center is Umnak, some 300 km east. Seguam lies directly to the east, and Great Sitkin to the west. The overall structure of the center is that of a broad central shield which once supported a large (~2,200 m) center cone (Atka volcano) ringed by as many as 7 or 8 smaller satellite volcanoes. The central cone was lost to caldera formation, shutting down the whole system, and the satellite vents still remain at various stages of erosional decay. Sarichef is perhaps the youngest satellite vent and has survived erosion largely unscathed. More often, summit ice buildup has breached the crater walls, forming active cirques, which have deeply incised the satellite vents. Tangential to these vents are U-shaped valleys, formed by moving ice.

"Kliuchef volcano grew on the north rim of the now ice-filled Atka caldera and formed a series of five vents striking northeast. The two main summit vents and the easternmost vent are fresh; the latter is most likely the source of the 1812 eruption attributed to Sarichef. Double-coned Korovin volcano next appeared, although overlapping considerably in time with Kliuchef. Six km north of Kliuchef, Konia volcano occupies the middle ground between Korovin and Kliuchef and is as old as much of Korovin itself. Korovin has been and remains the principal active volcano on Atka. It is unusual in that its summit crater marks an open, cylindrical vent reaching nearly to sea level; it has been observed by pilots flying over to sometimes contain a crater lake and at other times to be brimming with magma. This vent is the source of most recent eruptions.

"All the volcanoes consist principally of crystal-rich, thin, (<~3m) basaltic lavas with interbedded scoria yielding increasing amounts of pyroclastic debris, autobreccias, and lahars. The summit of Kliuchef is mostly glassy dacite as are some late flows of Korovin and Konia. A thick (~400 m) pink dacite with pumiceous and glassy cooling units was erupted upon formation of Atka caldera, but no ash flows have been found. Thick, expansive lahar aprons fill many early Pleistocene glacial valleys. These have been cut by parse dikes spanning the volcanic center, whose emplacement apparently attended caldera collapse. The lavas themselves are overwhelmingly (>or= 90% by volume) high-alumina basalt, strikingly free of xenoliths. The basalt contains plagioclase, orthopyroxene, magnetite, and clinopyroxene. The andesite and dacite contain orthopyroxene at the expense of olivine; trace amounts of biotite are also found in the dacite. No hydrous phases are found in any basalt or andesite lava. Three hot spring areas are found apparently associated with Kliuchef and Atka caldera, and a fourth spring occurs some 7.5 km west of Kliuchef."