Mount St Helens with a newly formed caldera (crater). Photograph of the northeastern side of Mount St Helen with a steaming caldera, on 31st May 1980. This was after its devastating eruption on 18th May 1980, one of the largest eruptions in the USA. 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.