Mount St Helens erupting. Mount St Helens underwent one of the largest eruptions in the USA on 18th May 1980. This devastating eruption destroyed the summit (peak) of the volcano, leaving a large caldera (crater). The lahars (volcanic mud flows) covered distances of nearly 30 kilometres at speeds of several kilometres per hour and flowed down valleys in the surrounding area, demolishing trees, bridges and houses, and then solidifying like concrete. They consisted of material from the collapse of the northern flank of the volcano, mixed with glacier ice, snow and meltwater from the eruption.