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Also evident in Figure 3 is an elaborate dendritic pattern of rills and gullies on the sides of the pit that resembles badlands topography.
Virtually all of these gullies and rills formed within the first five days after May 18 by the retrogressive slumping of the rim, not by water erosion.
As the jet of steam continued to ream the 125-foot-deep pit, mass wasting enlarged it to a length of 2300 feet and a width of 1000 feet. Largest steam explosion pit in the process of formation on May 23, 1980, looking east. The southwest shore of Spirit Lake is in the upper left corner.Most of the rockslide debris from the volcano's north face filled the upper 16 miles of the North Fork of Toutle River across its entire width, diverting tributary inflow to the river and blocking the outflow of Spirit Lake (Voight et al. Two-thirds of a cubic mile of rockslide and eruption debris occupies approximately 23 square miles of the valley to an average depth of 150 feet and a maximum depth of 600 feet (Voight et al. The hummocky surface of the rockslide and eruption deposits has been highly susceptible to the ravages of erosion.An enormous plain of hot pumice was deposited over the headwaters of the North Fork of the Toutle River and the south shore of Spirit Lake.As the pressure was released by the departure of the rockslide, superheated water inside the volcano flashed to steam creating a twenty megaton, northward-directed blast of hot gas and rock fragments.In six minutes the blast leveled 3.2 billion board feet of prime forest (enough lumber to build 640,000 houses) over an area in excess of 150 square miles.