Mount Katmai - Volcanic Activity

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 7,500 ft (2,300 m), and local villagers reported in 1898 that one of the volcanoes in the general area "smoked" occasionally.

On June 6-9th, 1912, the most spectacular Alaskan eruption in recorded history and the largest volcanic eruption in the world of the twentieth century resulted in the formation of a large summit caldera at Katmai volcano. The eruption actually took place at a vent about 6 mi (10 km) to the west of Mount Katmai (at the Novarupta Volcano). During over 60 hours, the volcano erupted an estimated 28 km³ of ash flows and tephra representing 13 km³ of magma volume. The withdrawal of magma beneath Katmai resulted in the collapse of the summit area, forming the caldera. Following the subsidence, a small dacitic lava 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 800 ft (240 m). Small glaciers have also formed on a bench within the caldera beside the lake. Pumice still floats on Naknek Lake nearby.