Geophysical events

Authors

  • Anonymous


Abstract

Mount St. Helens Volcano, Cascade Range, southern Washington, U.S.A. (46.20°N, 122.18° W). All times are local (GMT − 8 h through April 26; GMT − 7 beginning April 27). An eruption of Mount St. Helens began on March 27, after a week of local seismicity. During the first 10 days of the eruption, pulses of steam and ash rose as much as 3 km above the summit. Most of the ash fell on a thinly populated zone ranging from northeast to southeast of Mount St. Helens. Slight ashfalls occurred as far away as Spokane (about 500 km to the northeast) and Bend, Oregon (250 km to the south). On March 31, an unusual east wind deposited a thin layer of ash on the Kelso-Longview area, about 65 km west of the volcano. Numerous debris flows descended the northern and eastern flanks early in the eruption, reaching nearly to timberline in some areas. These were composed of ice blocks and sparse blocks of rock in a matrix of dark-colored mixed ash and snow. Nonincandescent ash avalanches occasionally flowed down the flanks during the early part of the eruption.

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