BackgroundKatmai volcano is a large stratovolcano about 10 km in diameter with a central lake-filled caldera whose rim is about 4.2 by 2.5 km in area. The caldera rim has a maximum elevation of 2047 m and in 1975 the lake surface was at an elevation of about 1236 m. The estimated elevation of the caldera floor is about 995 m.
The volcano is one of five stratovolcanoes near the Novarupta dome, which was the source of the large Plinian eruption in 1912. The eruption produced voluminous pyroclastic flows and counts as the largest eruption in the 20th century.
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.
(from: Alaska Volcano Observatory's website).
The June 1912 eruption of Novarupta Volcano altered the Katmai area dramatically. Severe earthquakes rocked the area for a week before Novarupta exploded with cataclysmic force. Enormous quantities of hot, glowing pumice and ash were ejected from Novarupta and nearby fissures. This material flowed over the terrain, destroying all life in its path. Trees up slope were snapped off and carbonized by the blasts of hot wind and gas. For several days ash, pumice, and gas were ejected and a haze darkened the sky over most of the Northern Hemisphere. When it was over, more than 40 square miles of lush green land lay buried beneath volcanic deposits as much as 700 feet deep. At nearby Kodiak, for two days a person could not see a lantern held at arm's length. Acid rain caused clothes to disintegrate on clotheslines in distant Vancouver, Canada. The eruption was ten times more forceful than the 1980 eruption of Mount St. Helens. In the valleys of Knife Creek and the Ukak River, innumerable small holes and cracks developed in the volcanic ash deposits, permitting gas and steam from the heated groundwater to escape. It was an apparently unnamed valley whehn the 20th century's most dramatic volcanic episode took place. Robert Griggs, exploring the volcano's aftermath for the National Geographic Society in 1916, stared awestruck off Katmai Pass across the valley's roaring landscape riddled by thousands of steam vents. The Valley of Ten Thousand Smokes, Griggs named it.
(from: U. S. National Park Service Website, Geology Fieldnotes - Katmai National Park and Preserve, Alaska, April 2000, posted on CVO's website at: vulcan.wr.usgs.gov/Volcanoes/Alaska/description_1912_eruption_novarupta.html)
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