Mount Katmai is one of five vents encircling the Novarupta volcano, source of the VEI 6 eruption and associated voluminous pyroclastic flows in 1912. The volcano has caused ten known fatalities due to gas exposure. Katmai consists chiefly of lava flows, pyroclastic rocks, and non-welded to agglutinated air fall. The Quaternary volcanic rocks at Katmai and adjacent cones are less than thick. Much of the volcano is mantled by snow and ice and several valley glaciers radiate out from the flanks. Katmai volcano is built on the sedimentary rocks of the Naknek Formation of Late Jurassic age, which are exposed just west of the caldera rim at an elevation of about, as well as north and southeast of the crater. Sedimentary rocks have been reported at an elevation of over in the west wall of the caldera and near the bottom of the eastern wall near.
Volcanic activity
Little is known about the historical activity of Katmai volcano before the great 1912 eruption. Early U.S. Coast and Geodetic Survey maps suggest a pre-caldera summit elevation of about, and local villagers reported in 1898 that one of the volcanoes in the general area "smoked" occasionally. During June 6 to 9, 1912, the most spectacular Alaskan eruption in recorded history and the twentieth century's largest measured volcanic eruption formed a large summit caldera at Katmai volcano. The eruption happened at a vent about to the west of Mount Katmai. During over 60 hours, the volcano erupted an estimated of ash flows and tephra representing of magma volume. The eruption produced a cloud of suffocating gas and ash that blackened the sky for the inhabitants of the town of Kodiak, who, with ash falling on them were led to the U.S. Coast Guard Cutter Manning where they sheltered in the harbor and were cared for by the crew because Captain Perry determined "an attempt to leave here would be unwise" due to the volcanic conditions degrading navigation capabilities. The withdrawal of magma beneath Katmai resulted in the collapse of the summit area, forming the caldera. Following the subsidence, a small daciticlava dome known as Horseshoe Island was emplaced on the floor of the caldera; this is the only juvenile material erupted from Katmai caldera during the historical eruption. It was visible at the time of the expedition in 1916, but has since been submerged by the crater lake. Still, the eruption from Katmai had a VEI of 3, and possibly involved phreatic eruptions. In 1919, geologists noted a lake covering a large part of the caldera floor, but by 1923 the lake was gone and numerous fumaroles, mud pots, and a large mud geyser had replaced it. The lake has since refilled to a depth of over. Small glaciers have also formed on a bench within the caldera beside the lake. Pumice still floats on Naknek Lake nearby.
