2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 7-9
Presentation Time: 10:25 AM


WAITT, Richard, B., US Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., #100, Vancouver, WA 98683, MOSBRUCKER, Adam, U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Court, Suite 100, Vancouver, WA 98683 and CRISAFULLI, Charles M., USDA Forest Service, PNW Research Station, Olympia Forestry Sciences Laboratory, Mount St. Helens National Volcanic Monument, 42218 NE Yale Bridge Road, Amboy, WA 98601

The 18 May 1980 eruption of Mount St. Helens visited several catastrophes upon nearby Spirit Lake (old level 975 m). Geomorphic and stratigraphic relations reveal a sequence. (1) Front of gigantic hot surge sweeping 500 km/hr northeast across lake and landscape levels mature coniferous forest and deposits gravel. (2) Great landslide enters lake about 250 km/hr and plows north along both arms. (3) Water ejected by the landslide runs as a giant wave north up the narrowing arms as high as 265 m above old lake level. (4) The elevated water cascades back to the basin that the landslide has dammed to 1038 m—63 m above its old level. (5) The waning hot surge deposits sand, including on areas just swept by giant water wave.

All this excitement lasts three minutes. Above a sharp line of trim and log jams at altitudes 265 m above old lake level, trees lie where felled by the surge. Below the line the giant wave washed off trees, surge deposit, and soil. Most of the eroded wood returned to the basin with the water.

The lake’s surface area expanded but depth shoaled. With no outlet the new lake rose 12 m until an engineered outlet stabilized it. Recent bathymetry shows the lake’s east arm as well as the west arm deeply filled. Maximum lake depth in 1974 had been 60 m, the mean 40 m. Maximum depth in 2011 is 40 m, the mean 30 m. The landslide below lake level averages 86 m thick (maximum 122 m), its volume more than 425 million cubic meters. The deepest hollow in new Spirit Lake lies 37 m above the surface of old Spirit Lake. Just before eruption surface water had been clear and about 6 degrees C. Just after eruption it was murky, nearly 33 degrees C, and reeked of sulfur. Before eruption Spirit Lake had frozen over each winter. After the eruption it didn’t freeze again until February 2008. It took that long to cool the filling debris.

Just after eruption floating tree trunks and splintered wood covered more than half the lake’s surface. By 2014 much wood had waterlogged and sunk, and the raft now covers only a fifth of the lake’s surface. Waterlogging has been by species. True firs and hemlocks have sunk, but much Douglas fir and western redcedar still floats after 34 years. The lake’s 1980 shoaling greatly changed its ecology. Rooted aquatic plants and animal life abound now far more than they could in deep, steep-sided old Spirit Lake.