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Space Shuttle photo STS047-077-034 looking north-northwest across southern Unimak Island to Fisher caldera. Photo taken in September 1992.
Fisher is an andesite stratovolcano capped by a caldera. The volcanic center has a volume of about 300 cubic km and a diameter of about 21 km. The stratovolcano has been deeply eroded and glaciated. The caldera is 11 by 18 km and contains two lakes. There are several cinder and spatter cones in the caldera, some up to 2 km is diameter and 400 m in height. Ash-flow tuffs that blanket parts of the volcano are compositionally-zoned from basal dacite (64% silica) up to basaltic andesite (53% silica). The ashflows were very mobile, traversing 15 km and climbing 500 m up nearby mountains. Fisher had a caldera-forming eruption ca. 9100 years ago. Fisher had a poorly documented ash-producing eruption in 1826.
Unimak Island, pictured here in a radar image taken by the Seasat satellite, has many different volcanoes, both young and old. At the top of the image is Westdahl, a large shield volcano that is covered by a glacier; eruptions occurred in 1964-5 and 1978. On the northwest (left) side of Westdahl is Pogromni, a steep-sided stratovolcano. Little is known about Pogromni, and it hasn't erupted in 160 years.
In the middle of the radar image is a broad and low ridge that makes the elliptical rim of Fisher Caldera. This 11 x 18 km wide caldera formed about 9,100 years ago by the eruption of ash flows and the sinking of the original volcanic mountain that was previously there.
At the bottom of the image is a beautiful stratovolcano named Shishaldin. The Aleut Indians who live in the area call the mountain Sisquk, which means, "mountain which points the way when I am lost." Shishaldin is such a tall (2,857 m high) and recognizable volcano that it is a landmark. Shishaldin almost always has a puff of steam coming from its summit crater.
Sources of Information:
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.
Charles A. Wood & Jurgen Kienle, 1990, Volcanoes of North America: United States and Canada, Cambridge University Press, Cambridge, p. 45.; 354