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USGS CMG InfoBank: Volcanoes and Magma

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Comment: 02:19 - 03:46 (01:27)
Source: Annenberg/CPB Resources - Earth Revealed - 13. Volcanism
Keywords: "James Sadd", volcano, magma, "interior of the Earth", eruption, "volcanic gas", "volcanic ash", atmosphere, climate, Sun, "sea floor", "mid-oceanic ridge", crust

Our transcription: Volcanoes are proof that the interior of the Earth is very hot, hot enough, in fact, to melt rock.
This molten rock called "magma" is less dense than the surrounding unmelted rock. It rises bouyantly, just as currents of hot air rise from the surface of the Earth.
Where the magma reaches the Earth's surface, volcanoes erupt. Different types of volcanoes can erupt in very different ways.
In some eruptions magma pours out onto the surface of the Earth in rivers of molten rock called "lava flows".
In others, high gas pressures underground cause the volcano to erupt explosively.
The magma is sprayed into the air as a cloud of very small hot particles called "volcanic ash".
This fine rock powder can remain aloft in the atmosphere for days and travel thousands of kilometers before finally settling to the surface of the Earth.
Sometimes volcanic ash can remain suspended in the atmosphere for months, blocking out sunlight and affecting global weather.
Volcanoes not only erupt in different ways, but they're also found in a wide variety of geologic environments.
There are about 500 known active volcanoes on dry land, but most volcanic activity occurs in places hidden from view.
There are probably several thousand active volcanoes on the floor of the open ocean, and most of these lie astride mid-ocean ridges, where Earth is forming new crust.

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