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The rock cycle describes how rocks can change from igneous, sedimentary to metamorphic and back again to any of these types. This cycle was first described by James Hutton in the 1700's. Melting occurs when subduction beneath a continent occurs. One tectonic plate moves under another and sinks into the mantle. This causes melting. Subduction occurs at convergent boundaries. Rocks are in a constant state of change due to weathering by water and the atmosphere, erosion, heat and other forces. If a rock is pushed beneath the Earth's surface, it melts and becomes magma. Magma that cools underground over time solidifies into intrusive igneous rock. Magma that is spewed out of the Earth in a volcano, becomes lava which solidifies into extrusive igneous rock. When igneous or sedimentary rock are exposed to high temperature or pressure conditions, they change to metamorphic rock. This can be regional-over a large area, or it can be via contact metamorphism--a rock near magma gets re-crystallized due to the heat and changes to metamorphic.
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