The Rock Cycle
The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle. The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the Sun which powers the hydrological cycle, moving water, wind and air along Earth’s surface.
The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is thick enough to support circulation, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.
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In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma because from magma, new minerals and igneous rock are created. Magma is rock that has been heated to the point of being entirely molten. Magma can either cool slowly within the crust (over centuries to millions of years)—forming intrusive igneous rock, or erupt onto the surface, where it is called lava, and cool quickly (within seconds to years)—forming extrusive igneous rock (volcanic rock). Intrusive igneous rock typically crystallizes at depths of hundreds of meters to tens of kilometers below the surface.
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Plate tectonic processes of mountain building can allow for all types of rock to be uplifted and exposed at the surface. These include rock that is buried deep within the continents and even rock that is part of the crust of the deep ocean basins. Once rock of any type is exposed at the surface, it is attacked by the weathering agents of water, wind, ice and gravity. Rock is weathered, both physically (breaking into smaller pieces) and chemically (by alteration of the minerals). These weathering products—mostly small rock and mineral fragments—are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean. Sediments will eventually become buried by more sediments in some type of an accumulating basin. At depths of hundreds of meters or more, they become compressed and cemented into sedimentary rock.
Any rock type that becomes buried deep within the crust is subjected to increasing heat and confining pressure. Tectonic forces of colliding plates can add additional stress to the buried rock. These changing conditions affect the stability of the pre-existing minerals and the fabric, or texture, of the original rock. This results in the formation of metamorphic rock.
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