40. Aniachak Volcano and Surprise Lake, Alaska, USA
The summit cone of Aniachak stands 1353 meters (4450 feet) high. Surprise Lake is ponded within the great caldera, which is some 10 kilometers (6 miles) in diameter. The volcano was only discovered in 1923 and was believed extinct until it erupted on May 1, 1931. In many respects, Aniachak closely resembles the much better known Crater Lake caldera, which was produced by a massive eruption some 6000 years ago. It is not certain when Aniachak’s caldera formed, but it was undoubtedly produced by the eruption of tens of cubic kilometers of pumiceous pyroclastic flow and fall ashes, and the subsequent foundering of part of the ancestral volcano into the magma chamber. The Crater Lake caldera is now filled with water; at Aniachak we can see what the caldera might look like if it were drained.
The similarity is strengthened by the small young cone in the caldera; Crater Lake has an almost identical (slightly smaller) feature that now forms Wizard Island.
STS-41G. October 1984. Picture #17-33-043.