Description: From Miller and others (1998): "Capps (1929) suggested that a summit caldera, largely buried by ice, is associated with Mount Spurr.
Later, Juhle and Coulter (1955) disagreed with the caldera interpretation suggesting that the peaks around Mount Spurr only coincidentally resemble the rim of a large subsidence structure. Most recent studies, however, suggest that ancestral Mt. Spurr, constructed during late Pleistocene time (Turner and Nye, 1986), was partially destroyed by a major Bezymianny-type eruption possibly as late as early Holocene time (Riehle, 1985; Nye and Turner, 1990). The eruption produced a voluminous volcanic debris avalanche and subsequent pyroclastic flows that resulted in the formation of a 5- to 6-km-diameter explosion caldera (fig. 7). The volcanic debris avalanche contains blocks as much as 100 m in diameter and traveled a minimum of 25 km. The overlying pyroclastic flows are partially welded and are composed chiefly of high silica andesite."