Abstract
The 08:32 PDT 18 May 1980 Mount St. Helens eruption began as an explosion (blast) that produced a laterally flowing pyro-clastic current (blast surge). It moved outward in a 180° arc at supersonic speeds and then slowed to subsonic speeds. Supersonic flow severely eroded the ground and produced a ground layer (formerly, layer A0). Above the ground layer lies the deposit derived from the blast surge.
The blast surge deposits formed in three continuous and concurrent stages. The first stage was movement of the transport system (that is, the blast surge), carrying fragments most of the distance from the vent. The next two stages involved the depositional system. This system developed from rapidly accumulating sediment by gravity segregation at the base of the blast surge. Two stages of sediment gravity flows with independent flow regimes formed sequentially in rapid succession from the moving blast surge. Each moved downhill due to gravity, independently of the generating blast surge that was driven by explosive expansion. Units repeated in some sections are deposits derived from divergent and crossing surge lobes caused by irregular topography. Locally ponded massive deposits in valleys and depressions formed from gas-rich pyroclastic flows and originated in two ways: (1) topographic blocking by steep, volcano-facing slopes. Blocking includes diversion or damming of the lower-elevation, high-density part of the blast surge that could not surmount high ridges. (2) Also occurring to form valley ponds was drainage of just-deposited (or "almost deposited") material from steep slopes (>∼30°-35°), moving downhill in all slope directions into separate valleys or depressions.
Distances traveled by the blast surge, and rates at which coarsest fragments decrease laterally, are related to the topographic "grain." Surge runout was farther where flow directions paralleled topographic trends, and least where trends were at right angles to the flow. Obstacle-interference depositional patterns (thinning-thickening; rapid fine-to-coarse facies changes) caused a spread in median diameter values that are as great in local areas near the volcano as they are over the width of the devastated region.
