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USGS CMG InfoBank: Volcanic Eruptions

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Comment: 00:40 - 02:04 (01:24)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: volcano, eruption, "lava flow", magma, crust, mantle, crystallization, intrusion, "igneous rock", "Mount Saint Helens", Kilauea, "James Sadd"

Our transcription: A volcanic eruption is one of the most awe inspiring sights in all of nature; but whether an eruption is sudden and explosive such as Mt. St. Helens or more subdued like the eruptions of Kilauea in Hawaii, volcanic activity is not an unusual event.
Volcanic eruptions are the pulse of geologic activity in the Earth's interior.
They graphically demonstrate the geologic processes inside the Earth can have a direct impact here on the surface.
The lava that's produced during a volcanic eruption is, of course, rock that melted somewhere beneath the volcano.
Rock melts in a variety of geologic settings in the crust and upper mantle of the Earth depending on the temperature and pressure conditions and on the composition and water content of the rock.
But lava represents only a tiny proportion of the magma that forms within the Earth.
Most of this magma either seeps into voids and fractures within rocks of the Earth's crust, or it rises and intrudes into the cooler rocks above as enormous globs that require thousands or even millions of years to cool.
In fact, most of the rock of this planet was formed from the slow cooling and crystallization of magma deep underground.
This is known as intrusive igneous rock.

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