DAVID HARDY / SCIENCE PHOTO LIBRARY DAVID HARDY / SCIENCE PHOTO LIBRARY
Diagram depicting the evolution and explosive change of a volcano. At top, the volcano is in its 'well-behaved' era: occasional eruptions producing a shallow crater. Eventually, the crater may become very large (centre), with the underlying magma chamber migrating upward. This becomes very unstable. Eventually, either the cone collapses into the magma chamber or very large landslides occur - sometimes both together (bottom). In either case, the sudden drop in pressure causes explosive degassing of the magma and a catastrophic eruption occurs. A large landslide of this type (known as a sector collapse) occurred at Mount St. Helens on 18 May 1980.
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