Explosive eruption

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An explosive eruption is a volcanic term to describe a violent, explosive type of eruption. Mount St. Helens in 1980 was a good example of an explosive eruption. Such an eruption is driven by gas including water vapour accumulating under great pressure. Driven by the hot rising magma as it interacts with the ground water the pressure increases until it bursts violently through the overmantle of rock. This is merely the beginning. In many cases the rising magma will have vast quantities of gas dispersed through it, partially dissolved; held only by the enormous pressure. With the sudden release of pressure following the initial explosion this gas resumes its gaseous form, violently and explosively. This secondary explosion is often far more violent than the first one; the rocks, dust, gas and pyroclastic material may be blown 20 km into the atmosphere at rate of up to 100,000 tonnes per second, travelling at several hundred metres per second.

Sooner or later this cloud collapses, almost as violently, creating a pyroclastic flow, the killer cloud of hot volcanic matter.

See also effusive eruptions, the gentler kind of volcano.