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MPEG-1  |  2012-12-18  |  18.5 MB  |  320x240  |  29.97fps  |  2 minutes, 18 seconds
Transcription: Volcanoes are formed by magma that leaks out of the mantle through a crack in the lithosphere. Once the magma escapes, it cools, forming rock. That accumulation of rock forms a volcanic mountain. This volcano, of course, is in the middle of erupting. We can see the magma spewing out of it. That makes it an active volcano. Scientists generally consider a volcano to be active if it's erupting, like the one shown here, or if it's shown knowing signs that it might erupt soon. Increased earthquake activity around the volcano or a change in the kinds of gases being emitted by the volcano are typical ...