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USGS CMG InfoBank: Magma

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Comment: 18:02 - 18:59 (00:57)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: magma, density, "convergent plate", crust, stoping, xenolith

Our transcription: Magma rises from the Earth's interior because it is bouyant, that is, less dense than the rock materials surrounding it.
It ascends in several ways.
At divergent plate boundaries, magma simply fills up the spaces opened as the crust is pulled part.
By contrast, at convergent plate boundaries, rock enclosing a magma body may be pushed aside as the magma passes through it; or the magma may fracture the overlying country rock through a process called "stoping".
Bits of fractured country rock sink into the magma chamber where they may be preserved upon cooling in the form of fragments called "xenoliths".
In part, magma may even melt its way through the crust increasing in volume and changing in composition.

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