Discover

'Mount Katmai' is a large stratovolcano on the Alaska Peninsula in southern Alaska, located within Katmai National Park and Preserve. It is about in diameter with a central lake-filled caldera about 3 by 2 mi (4.5 by 3 km) in area. The caldera rim reaches a maximum elevation of . In 1975 the surface of the crater lake was at an elevation of about , and the estimated elevation of the caldera floor is about .

Contents

Geology

Volcanic activity

References

Geology

The volcano is one of five vents encircling the Novarupta dome, source of the voluminous pyroclastic flows erupted 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 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 , 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.
On June 6-8, 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 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.
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 northwest covering an area of about 120 km² in what subsequently came to be known as the Valley of Ten Thousand Smokes. The ash flow tuff produced in the 1912 eruption is made up of a silica-rich volcanic rock called rhyolite. In fact, this is the only major Quaternary eruption of rhyolite to have occurred in Alaska. Maximum thickness of the ashflow is estimated to be about . 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 1 in (2.5 cm) of ash. Light ash fall was reported as far away as the Puget Sound region away. 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.
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.

References

1. There are no credible reports of eruptions at Katmai since 1912


★

★ Alaska Volcano Observatory: Katmai

★ Volcanoes of North America, , Charles A., Wood, Cambridge University Press, ,