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Geology of Interactions of Volcanoes, Snow, and Water:
Mount St. Helens, Washington

Tsunami on Spirit Lake early during 18 May 1980 eruption

On 18 May 1980 the upper 1500 ft (460 m) including the former summit of Mount St. Helens suddenly detached as a gigantic landslide (debris avalanche). The great avalanche slipped off the volcano and slammed into Spirit Lake, raising the lake surface by 63 m (207 ft) and sending a cataclysmic tsunami surging around the lake basin as high as 250 m (820 ft) above the old lake level.

Hummocky debris avalanche deposit at the head of the west arm of Spirit Lake arrived by plowing through the lake. Displaced water ran up the valley head more than 200 m above the preruption surface of Spirit Lake. This displaced, momentarily elevated water then swept back to the lake, rinsing the valley sides clean of timber and sediment, jamming logs and boulders against the debris avalanche hummocks.

The tsunami also swept up the east arm of the lake. Its upper limit along the north side of Harmony Falls basin lies an amazing 225 m (738 ft) above the old level of Spirit Lake. Above the limit trees lie where felled by a cataclysmic hot pyroclastic surge (see previous summary); below the limit the downed trees and the surge deposit were removed by the tsunami as it swept up the basin. This amazing area can be visited is hiking Harmony Falls trail in Mount St. Helens National Volcanic Monument.

Waitt, R.B., and Pierson, T.C., 1994, The 1980 (mostly) and earlier explosive eruptions of Mount St. Helens volcano [guide for fieldtrip of 1994 Annual Meeting of Geological Society of America], in Swanson, D.A., and Haugerud, R.A., eds., Geologic field trips in the Pacific Northwest: Department of Geological Sciences, University of Washington, v. 2, Chapter 21, 37 p.

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11/03/97, Lyn Topinka