Mount Katmai

Katmai volcano is a large stratovolcano about 10 km in diameter with a central lake-filled caldera about 4.5 by 3 km in area. The caldera rim has a maximum elevation of 2247 m and in 1975 the lake surface was at an elevation of about 1286 m. The estimated elevation of the caldera floor is about 1036 m. The volcano is one of five vents encircling the Novarupta dome, source of the voluminous pyroclastic flows erupted in 1912. It 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 1500 m thick. Much of the volcano is mantled by snow and ice and several valley glaciers radiate out from the flanks and three glaciers originating from the upper caldera walls descend into the crater to the lake. 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 1520 m, as well as north and southeast of the crater. Sedimentary rocks have been reported at an elevation of over 1800 m in the west wall of the caldera and near the bottom of the eastern wall (1036 m).

Volcanic activity

June 6-8, 1912

Little is known about the historical activity of Katmai volcano before the great 1912 eruption. Early Coast and Geodetic Survey maps suggest a pre-caldera summit elevation of about 2286 m and local villagers reported in 1898 that one of the volcanoes in the general area "smoked" occasionally.

In June of 1912, the most spectacular Alaskan eruption in recorded history and the largest eruption in the world in the twentieth century resulted in the formation of a large summit caldera at Katmai volcano. The 60-hour-long eruption actually took place at a vent about 10 km to the west of Mt. Katmai (now marked by Novarupta dome) from which an estimated 30-35 km³ of ash flows and tephra were ejected rather than at Mt. Katmai itself. Based on geochemical and structural relationships, it has been suggested that magma drained from beneath Katmai Volcano to Novarupta via the plumbing system beneath Trident Volcano. The withdrawal of magma beneath Katmai resulted in the collapse of the summit area, forming the caldera. Following the subsidence, a small dacitic cinder cone was emplaced on the floor of the caldera; this is the only juvenile material erupted from Katmai caldera during the historical eruption.

In 1919, Fenner 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 ended about 20 hours after the initial eruption. Approximately 12-15 km3 of magma was vented during the 1912 eruption producing about 35 km³ of tephra. An estimated 11-15 km3 of ash flow tuff traveled 20 km northwest covering an area of about 120 km² in what subsequently came to be known as the Valley of Ten Thousand Smokes. Maximum thickness of the ashflow is estimated to be about 250 m. About 20 km³ of airfall tephra was carried east and southeast with a minor lobe to the north covering 77,000 km² with more than 2.5 cm of ash. Light ash fall was reported as far away as the Puget Sound region (2400 km). Extremely fine ash blown into the stratosphere remained in suspension as aerosols for months and caused spectacular red sunsets in many parts of the world.

Composition

Katmai volcano is composed of rocks ranging in composition from low-silica and low-potassium andesite to dacite but two-pyroxene andesite is the most common rock type. The andesite ranges from gray to purple and is commonly porphyritic with plagioclase and less commonly pyroxene phenocrysts in an aphanitic groundmass of plagioclase, hypersthene, augite, magnetite, and glass. Biotite and hornblende are rare but quartz has been reported in some of the upper flows.