Acehtsunami

On May 18, 1980, the upper 460 meters of Mount St. Helens failed and detached in a huge landslide. This released the pressure on the magma trapped beneath the summit bulge which exploded as a lateral blast, which then released the over-pressure on the magma chamber and resulted in a plinian eruption.

One lobe of the avalanche surged onto Spirit Lake, causing a megatsunami which pushed the lake waters in a series of surges, which reached a maximum height of 260 metres above the summit bulge pre-eruption water level (~975 m asl).

Above the upper limit of the tsunami, trees lie where they were knocked down by the pyroclastic surge; below the limit, the felled trees and the surge deposits were removed by the megatsunami and deposited in Spirit Lake.