US GEOLOGICAL SURVEY / SCIENCE PHOTO LIBRARY US GEOLOGICAL SURVEY / SCIENCE PHOTO LIBRARY
Mount St Helens with an intact summit. Photograph of the northeastern side of Mount St Helens on 30th March 1980, prior to its devastating eruption on 18th May 1980. The large bulge on the right side is caused by pressure building up inside the volcano. This pressure led to one of the largest eruptions in the USA, which destroyed the summit (peak) of the volcano and left 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 volcano, mixed with glacier ice, snow and meltwater from the eruption.
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