Magma chamber
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A magma chamber is the chamber beneath a volcano where the magma is held before an eruption. It is the build up of pressure in this chamber and the sudden release of that pressure by fracturing that causes the eruption.
If the magma is not vented to the surface in a volcanic eruption it will slowly cool and crystalize at depth to form an intrusive igneous body such as granite or gabbro.
Often, a volcano may have a deep magma chamber many kilometres down, which supplies a shallower chamber near the summit. The location of magma chambers can be mapped using seismology: seismic waves from earthquakess move more slowly through liquid rock than solid, allowing measurements to pinpoint the regions of slow movement which identify magma chambers.Template:Geol-stub