The pit is 2300 feet long (east-west), 1000 feet wide (north-south), and extends 125 feet deep into the pumice deposit of May 18. (Photograph courtesy of the Washington State Department of Natural Resources).The pumice is composed of fine dacite ash and coarse rock fragments.The nine-hour eruption which followed was one of the most photographed and studied volcanic events in history.Not as well understood are the erosional and depositional events associated with and following the eruption of May 18.
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.Part of the rockslide debris catastrophically displaced the water of Spirit Lake, producing waves up to 850 feet high at the north shore of the lake (Coffin 1983, Voight et al. As the water returned to its basin, it scoured slopes of trees and soil, and, together with material from the initial eruption, produced a 320-foot-thick deposit on the bottom of the lake (Meyer and Carpenter 1983).The new level of Spirit Lake has been stabilized since August 1982 by the Army Corps of Engineers at an elevation of 3460 feet, 262 feet higher than its pre-eruption level.A miniature "Grand Canyon" over 100 feet deep divides into several tributary drainages.The upland flat between the tributary canyons has already been severely altered by rill and gully erosion. Most of the major excavation of the new canyons of the North Toutle River appears to have occurred during the mudflow erosion on March 19, 1982.