Aniakchak is a caldera about 6.2 miles (10 km) across. Large pyroclastic flows surround the mountain. Eruptions inside the caldera have formed many vents, including tuff cones and a 3,280 ft. (1,000 m) high cinder cone. About 10 lava flows have occurred since the caldera's formation 3,400 years ago, none of which were outside the caldera rim.
Aniakchak does not have a symmetrical shape in that it is steeper on its northwest slope than on the its southeast slope that faces its high caldera wall. Aniakchak's most recent activity occurred in 1931.
Sources of Information:
Rowland, Scott K., et al, "Preliminary ERS-1 Observations of Alaskan and Aleutian Volcanoes," Planetary Geosciences Dept. of Geology and Geophysics, School of Ocean and Earth Science and Technology, pp. 4-5, Feb. 22, 1993.
Wood, Charles A. and Kienle, Jurgen, "Volcanoes of North America United States and Canada," Cambridge University Press, 347pp., 1990.