FLOW CATASTROPHES AT SPIRIT LAKE NEW MOUNT ST. HELENS 18 MAY 1980—PYROCLASTIC SURGE, LANDSLIDE, GIANT WAVE

An updated 1980s legacy report and new retrospective report revise details of Mount St. Helens’ 18 May 1980 eruption. In the first 3 minutes the eruption visited several catastrophes upon adjacent Spirit Lake. Geomorphic and stratigraphic patterns reveal a sequence. (1) Front of gigantic hot surge sweeps 500 km/hr northeast across lake and landscape and levels mature conifer forest and deposits gravel. (2) Huge landslide enters lake about 250 km/hr and plows north through both arms. (3) Water propelled by the landslide runs as a giant wave over spurs and up the lake’s arms as high as 265 m above lake level. (4) The elevated water cascades back to the basin that the landslide has dammed, the new lake level thus 63 m above the old level. (5) The waning hot surge deposits sand on areas just swept by water wave.

The excitement lasts three minutes. Above a sharp trimline and log jams about 265 m above old lake level, trees lie where the surge had felled them. Below the trimline and jams the giant wave washed off trees, surge deposit, and soil, floating most of the wood down to new Spirit Lake. The lake’s surface area expanded but depth shoaled. With its former outlet landslide-blocked, the new lake gradually rose another 12 m before an engineered outlet stabilized it.

Bathymetry shows the lake’s east arm as well as the west arm deeply filled by the great avalanche. Preeruption lake depth had been 60 m, mean depth 40 m. Maximum posteruption depth is 40 m, mean depth 30 m. Landslide debris below lake level averages 86 m thick, maximum 122 m, its volume above 425 million cubic meters. The deepest hollow in the floor of new Spirit Lake lies 37 m above the surface of old Spirit Lake. Before the eruption, surface water had been clear and 6̊ C. Just afterward it was murky, 33̊ C, and reeked of sulfur. Before the eruption Spirit Lake froze over each winter. Afterward it didn’t freeze again till February 2008.