The Rock Types and Cycles

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A rock is naturally formed, consolidated material usually composed of grains of one or more minerals. Earth changes because of its internal and external heat engines. Earth has a highly varied and ever – changing surface. And minerals and rocks change as well. A useful aid in visualizing these changing relationships is the rock cycle shown in figure 1. The three major rock types – igneous, metamorphic, and sedimentary – are shown. Magma is molten rock. Igneous rocks form when magma solidifies. If the magma brought to the surface by volcanic eruption, it may solidify into an extrusive igneous rock. Magma also may solidify very slowly beneath the surface. The resulting intrusive igneous rock may be exposed later after uplift and erosion remove the overlying rock. The igneous rock, being out of equilibrium, may then undergo weathering and erosion, and debris produced is transported and ultimately deposited (usually in sea floor) as sediment. If the unconsolidated sediment becomes lithified (cemented or otherwise consolidated into a rock), it becomes sedimentary rock. As the rock is buried by additional layers of sediment and sedimentary rock, heat and pressure increase. Tectonic forces may also increase the temperature and pressure. If the temperature and pressure become high enough, usually at depth greater than several kilometers below the surface, the original sedimentary rock is no longer in equilibrium and recrystallizes. The new rock that forms is called a metamorphic rock. If the temperature gets very high, the rock partially melts, producing magma and completing the circle.