Volcanic eruptions

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Volcanoes occur at techtonic plate boundariesor where there are weaknesses in like earth's crust.

Rocks in contact with molten magma are baked and under goes metamorphysis: mud becomes slate; limestone becomes marble.

If molten lava cools above the earths surface it becomes granite and has small crystals.

If molten lava cools below the earths surface it has large crystals and is called basalt.