Rock_Environments

Igneous Environments. Although the birth of the parent material of igneous rocks is primarily confined to the lower levels of the crust and the upper levels of the mantle, igneous rocks can be formed at any depth ranging from lower crustal levels up to the Earth's surface. Since confining temperatures (and hence pressures) increase with increasing depth, the rate of heat loss from a body of initially molten rock therefore decreases with increasing depth because rocks are not thermally conductive. As such, the environment permitting the development of crystals in a body of cooling igneous material is more favorable the deeper within the crust the body exists. Therefore, aphanitic igneous rocks are interpreted to have been formed at great depth in what is referred to as the plutonic environment, whereas aphanitic igneous rocks are interpreted to have been formed at shallow depth in the hypabyssal environment. Glassy, vesicular, and scoriaceous rocks are diagnostic and indicative of the volcanic environment which occurs at the Earth's surface where heat loss rates are comparatively instantaneous and crystals have little opportunity to form. The composition of an igneous rock also yields important information about the source rock (and hence crustal region) from which it was derived. Compositionally, igneous rocks of a felsic composition are believed to have been derived primarily from a continental source (that is, the melting of a portion of continental crust) due to the similarity in composition, density, texture, and close physical association between old and young continental rocks. Granite, therefore, is a rock which is interpreted to have been derived from the melting and crystallization of an original continental parent rock at great depth. Rhyolite, the aphanitic equivalent of granite on the other hand, is interpreted to have crystallized at much shallower depths, very near Earth's surface in the hypabyssal and volcanic environment. Great exposures of mafic igneous rocks on the other hand are interpreted to have been formed at great depths within the crust from an originally oceanic or perhaps an upper mantle source. Peridotitic and ultramafic rocks are derived entirely from upper mantle sources, and are similarly confined almost entirely to the oceanic setting, although rare occurrences of such rocks, as minor associates to more felsic rock masses, have been found within the continental realm.

Sedimentary Environments. Since sedimentary rocks, their sources, and the processes that produce them are confined to the uppermost 1 kilometer of the Earth, the environments of formation of sedimentary rocks are directly attributable to the interplay between the geosphere, the biosphere, the hydrosphere, and the atmosphere. The weathering (breakdown) of pre-existing rocks masses by physical and/or chemical means in response to the interplay between the atmosphere and the geosphere provides the raw materials of all sedimentary rocks. These raw materials are transported across Earth's surface during the process of erosion by one (or more) agents which are either fluids (namely, wind, water, or ice) acting under the influence of forces (namely, gravity and thermal gradients). The deposition of these raw materials into basins no longer under the influence of the erosional process results in the settling and accumulation of these materials into bottom layers. Clastic sedimentary rocks are formed when the deposited material consist of non-dissolved fragments of pre-existing rocks that have been buried, compacted, dewatered (if present), and cemented under the weight of subsequent layers deposited on top. Crystalline sedimentary rocks, (a.k.a. evaporites) form as originally dissolved materials in a body of water precipitate (settle) to the bottom of a basin in response to supersaturation via evaporation of water or overaddition of solute. Organic sedimentary rocks are formed in similar fashion to either clastic or crystalline sedimentary rocks, depending upon the source material and the environment of deposition. It is the presence of fossils, however, that provides clear evidence of a sedimentary rock's environment of formation. The presence of a fresh-water fish remains in a siltstone, for example, suggest deposition in a lake, in the terrestrial (continental) environment, whereas the presence of shark teeth in a sandstone however indicate the influence of a shallow-water marine setting. Mixed marine and terrestrial organisms in a sedimentary rock suggest the influence of both environments in the estuarine (near-shore) environment.

Metamorphic Environments. Metamorphism and accompanying processes occur over a great range of depth within the crust, from the near surface to near the base of the crust. Metamorphism occurs when a rock, stable in its original environment of formation, is placed into a new environment in response to uplift or subsidence and the concomitant changes in temperature (heating or cooling) and/or pressure (exhumation or burial). A metamorphic rock's composition is based on the composition of the original rock, called the protolith, which may be igneous sedimentary or metamorphic in character. A metamorphic rock which contains silicate minerals that are rich in iron and magnesium suggest a mafic igneous protolith, for example. The texture of a metamorphic rock is attributable to the environment of formation, that is, the prevailing pressure, temperature, and/or presence of fluids. The progressive development of foliation in a metamorphic rock suggests increasing pressure and temperature influences and hence increasing depth of formation. There are six main types of metamorphism based upon pressure/temperature/composition considerations. These are (a) burial metamorphism (a.k.a. anchimetamorphism), confined to near Earth's surface which is low temperature and low pressure, (b) contact metamorphism, confined to the margin of igneous rocks which is low pressure/high temperature, (c) regional metamorphism, confined to the cores of mountain belts and the centers of continents which is medium temperature and pressure, (d) island-arc metamorphism, confined to volcanic chains, which is high temperature, medium pressure, (e) subduction zone metamorphism, confined to the deep-ocean trenches and below, which is low temperature and high pressure, (f) oceanic metamorphism, which occurs at the base of the ocean floors, which is low temperature and high water pressure, and (g) granulite metamorphism, confined to the deepest levels of the continental crust (but not to the point of melting), which is high pressure and high temperature. The presence of fluids, most notably water and carbon dioxide, usually accelerate the rate and efficiency of metamorphic processes. These fluids may be a by-product produced from the rocks themselves, or introduced from an outside source. The protoliths of the vast majority of metamorphic rocks (in volumetric terms) are either granites or sedimentary rocks derived from the weathering of granitic (continental) rocks. Mica is a principal mineral constituent of metamorphic rocks in which water-bearing minerals such as clays were a major part of the protolith, such as shale. Quartz and feldspar are common constituents in all metamorphic rocks. The foliated metamorphic rocks are classified on the basis of increasing grain (crystal) size and development of foliation (layering). The progressive sequence slate-phyllite-schist-gneiss represents the expected sequence of rock development as a shale is progressively buried to deeper and deeper depth. The original composition remains the same, but the texture changes in response to greater temperature and pressure. Gneisses therefore, represent the upper limit of metamorphism. They are further subdivided on the basis of protolith into orthogneisses (originally granites and other igneous rocks) and paragneisses (originally shales, sandstones, and other clastic sedimentary rocks). Marble, a nonfoliated metamorphic rock consisting entirely of calcite, is formed from the metamorphism of limestone (a chemical/organic sedimentary rock). Migmatite is a transitional rock, in which metamorphism is extreme, and a rock is very near the melting point. Migmatites reveal a wavy texture, in which the foliation appears to have flowed like a paste, suggesting a very low viscosity and hence, very high temperature. Metamorphism that is characterized by a general increase in temperature and pressure (from its original state) is referred to as prograde and that which is characterized by a general decrease in temperature and pressure (from its original state) is referred to as retrograde.