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The Rock Cycle

     A rock is an aggregate of minerals.   Minerals are naturally occurring, inorganic, crystalline solids that have definite physical and chemical properties.  Minerals are composed of elements such as oxygen, silicon, and aluminum, and elements are made up of atoms, the smallest particles of matter that still retain the characteristics of an element.   More than 3,500 minerals have been identified and described, but only about a dozen make up the bulk of the rocks in the Earth's crust.

     Geologists recognize three major groups of rocks-igneous, sedimentary, and metamorphic- each of which is characterized by its mode of formation.  Each group contains a variety of individual rock types that differ from one another on the basis of composition or texture (the size, shape, and arrangement of mineral grains).

     The rock cycle is a way of viewing the interrelationships between the Earth's internal and external processes.  It relates the three rock groups to each other; to surficial processes such as weathering, transportation, and deposition; and to internal processes such as magma generation and metamorphism.  Plate movement is the mechanism responsible for recycling rock materials and therefore drives the rock cycle.

     Igneous rock results from the crystallization of magma or from the accumulation and consolidation of volcanic ejecta such as ash.  As a magma cools, minerals crystallize, and the resulting rock is characterized by interlocking mineral grains.  Magma that cools slowly beneath the Earth's surface produces intrusive igneous rocks, while magma that cools at the Earth's surface produces extrusive igneous rocks.

     Rocks exposed at the Earth's surface are broken into particles and dissolved by various weathering processes.  The particles and dissolved material may be transported by wind, water, or ice and eventually deposited as sediment.   This sediment may then be compacted or cemented into sedimentary rock.

     Sedimentary rocks originate by consolidation of rock fragments, precipitation of mineral matter from solution, or compaction of plant or animal remains.  Because sedimentary rocks form at or near the Earth's surface, geologists can make inferences about the environment in which they were deposited, the type of transporting agent, and perhaps even something about the source from which the sediments were derived.  Accordingly, sedimentary rocks are very useful for interpreting Earth history.

     Metamorphic rocks result from the alteration of other rocks, usually beneath the Earth's surface, by heat, pressure, and the chemical activity of fluids.  For example, marble, a rock preferred by many sculptors and builders, is a metamorphic rock produced when the agents of metamorphism are applied to the sedimentary rock limestone or dolestone.  Metamorphic rocks are either foliated or nonfoliated.  Foliation is the parallel alignment of minerals due to pressure.  It gives the rock a layered or banded appearance.