Introduction. A rock is defined as a naturally-occurring association of consolidated solid or semisolid matter of organic and/or inorganic (mineralogic) origin. Rocks constitute the broadest category of Earth materials and therefore account for nearly all of Earth's mass. A rock may consist of a single mineral or organic compound. on the other hand a rock may consist of complex mixture of many minerals and/or organic compounds and even some liquids (both "frozen" and fluid). Despite their remarkable compositional diversity, the classification of all rock is based on their mode of origin, that is, the pressure and temperature conditions which characterized the environment in which they were formed as well as the availability of certain types of materials. Heating and cooling (temperature control factors) together with burial and exhumation (pressure control factors) primarily influence a rock's character although other factors are also at work to create a final rock product. Since the Earth and all of its dynamic processes are thermally-driven (that is, powered by the loss of internal heat created by the spontaneous breakdown of unstable radiogenic elements in the core), it is reasonable to conclude that both the temperature and pressure changes that lead to the creation of the different rock classes are also thermally-driven.
Igneous Rocks. As the name suggests, igneous rocks are those which descended directly from a hot, molten parent. Since the Earth has been losing heat since its birth, material of an igneous or once molten nature can be thought of as the original parent of all rocks. That is, all solid Earth surface materials at one time must have been molten and cooled to their present state. Igneous rocks are by far the dominant rock type in the Earth's crust, both within continents as well as in the oceans. Igneous rocks contain little (if any) organic matter. They are comprised principally of silicate minerals, with subordinate amounts of oxides, sulfides, and others. Igneous rocks, as the other classes of rocks, are classified on the basis of texture and composition. Texture refers to the presence, absence, sizes, orientations and development of individual mineral-crystal grains in a rock. Composition refers to the percent proportions of minerals in a rock (a monomineralic rock is one which possesses only one type of mineral whereas a polymineralic rock is one which contains two or more different types of minerals). Igneous rocks are either coarse-grained ("phaneritic", with grain sizes >1mm), fine-grained ("aphanitic", with grain sizes <1mm), or glassy (with no grains at all). Textures are a direct outgrowth of rates of cooling and therefore give information as to the depth of burial during which time the rock cooled. As far as igneous rock textures is concerned, the slower the rate of cooling, the deeper its formation depth in the Earth, the larger the grain size in the rock. Compositionally, an igneous rock belongs in either one of five categories. Each of the five categories is based on the percentage of dark-colored silicate minerals which is present in the rock: granitic/rhyolitic (0-20%), dioritic/andesitic (20-40%), gabbroic/basaltic (40-60%), peridotitic (60-90%) and ultramafic (90-100%). Since dark-colored silicate minerals possess a higher melting point than light-colored silicate minerals, rocks with a higher percentage of dark minerals were formed at a higher temperature than those with lighter minerals. Similarly, dark-colored silicate minerals possess a greater density than light-colored silicate minerals, also indicating an origin at significantly greater depth. As such, igneous rocks are those which have experienced extreme pressures and temperatures which in turn have resulted in melting. Such extreme pressures and temperatures usually indicate great depth of origin in the Earth's crust. The molten parent from which all igneous rocks arise is magma, whereas molten igneous rock erupted onto Earth's surface is referred to as lava.
Sedimentary Rocks. As the name suggests, sedimentary rocks are those which are produced by the process of sedimentation. Sedimentation is the process whereby fragments of earth surface materials, both organic and inorganic, are deposited in a particular location after being picked up and transported by a moving surface fluid such as wind, water, ice or the direct influence of gravity. Sediments are deposited usually in a basin or area of depressed elevation and buried as a series of layers called beds or strata. Even solid materials originally dissolved in water that settle out and accumulate on the bottom of a depositional basin constitute sediments. Sedimentary rocks and the processes that produce them are entirely confined to within the uppermost 1 km of the Earth's surface. Sedimentary rocks belong to either of three classes: clastic, chemical, or organic. Clastic sediments consist entirely of fragments of pre-existing rocks, chemical sediments are the solute portion of a body of water that have settled out on the bottom, and organic sediments consist of the remains of organisms. Clastic sediments are converted into clastic sedimentary rock though the processes of deposition, burial, compaction, dewatering, and cementation. Chemical sedimentary rocks are converted into chemical sedimentary rocks primarily though evaporation, supersaturation, deposition and crystallization. Organic sediments are converted into organic sedimentary rocks though a variety of processes which may or may not involve burial, compaction, dewatering, heating, cementation, crystallization, and others. Although only 5% of the rocks of the crust are sedimentary by volume, sediments and sedimentary rocks occupy 85% of the Earth's surface. As such, sedimentary rock formation processes are directly attributable to Earth surface processes, features, and environments.
Metamorphic Rocks. As the name suggests, metamorphic rocks are those which possess a composition and texture which suggest that it has most recently lived in an environment transitional between that of the igneous rock realm and the sedimentary rock realm. That is, the component minerals reveal a texture and a composition that indicate a significant temperature and/or pressure increase (or a temperature and/or pressure decrease) has been experienced by the rock. However, such pressure and temperature changes were well-below that which produces melting and much greater than those which occurs at Earth's surface. As with the other rock classes, metamorphic rocks are classified and interpreted on the basis of texture and composition. Metamorphic rocks are produced from either the heating, burial, or heating and burial of any pre-existing rock. The parent rock from which a metamorphic rock was created is called the protolith. Foliated metamorphic rocks are those which possess a planar fabric or layering of the component minerals. This layering is frequently distorted or folded and is usually accompanied by the presence of mica minerals. When foliation is present and mica minerals dominate, the protolith was probably rich in water or water-bearing minerals such as clays. The size of the mineral grains in a metamorphic rock is directly proportional to the depth of burial: large grains indicate deep depths, small grains indicate shallow depths. Nonfoliated metamorphic rocks, such as marble and quartzite possess a crystalline texture in which the boundaries between grains are polygonal. Metamorphic rocks are extremely dense and are usually exposed in the cores of eroded mountain belts, in the ancient uplifted cores of the continents, and at the margins (contacts) of igneous rock bodies.
