Metamorphism

METAMORPHISM, in petrology, denotes the sum of the processes effecting fundamental alterations in composition, mineral or chemical, structural or textural in solid rock masses, the altera tions determining completely the character of the rock mass.

The term (from Gr. j.tera, change of, and pop4i7, shape) is now very generally used to exclude those alterations of decomposition, disintegration and cementation taking place in the upper parts of the lithosphere under the influence of the atmosphere and surface waters (weathering, cementation). See METASOMATISM and PNEUMATOLYSIS. Metamorphic rock types treated separately include QUARTZITE, SLATE, PHYLLITE, SCHISTS (including MICA SCHIST) and GNEISS (in part)—also AMPHIBOLITE, ECLOGITE (in part), EPIDIORITE, EPIDOSITE, GRANULITE, HORNFELS, MARBLE, MYLONITE and the SCAPOLITE rocks. The original materials upon which metamorphic processes have operated are either sediments, igneous rocks or mixtures of these (as tuffs and other pyroclastic aggregates). Rocks may, however, be subjected to more than one epoch of metamorphism. Such rocks have been termed poly metamorphosed rocks, and the process polymetamorpliism. Meta morphic rocks are thus to be regarded as a group of rocks of co ordinate importance with sedimentary and with igneous rocks; they are formed in the wide temperature and pressure region ex isting between the region of formation of sedimentary rocks (low temperature, low pressure), and that of igneous rocks (high tem perature sometimes accompanied by high pressures).

According as metamorphism takes place without appreciable change in chemical composition, or with addition or exchange of material, two types of metamorphic rock may be broadly dis tinguished, viz., (a) products of normal metamorphism, in which the chemical composition remains unchanged except for loss of such volatile constituents as water or carbon dioxide, and (b) products of metasomatic metamorphism, where fundamental changes are brought about by the introduction of new materials. In normal metamorphism, the chemical analysis of the rock is usually sufficient to determine its origin.

Sandstones yield quartzites and quartz schists, limestones are converted into marbles, shales into mica-schists, without their bulk composition being greatly modified. The simple loss of water or carbon dioxide, as in the conversion of a calcareous sandstone into a rock composed of wollastonite is not usually con sidered a case of metasomatic metamorphism. Metasomatism, in

its relation to metamorphism, implies addition of new material, which reacts with the original components of the rock. The f or mation of skarn rocks from carbonate sediments, whereby iron compounds from magmatic sources react with carbonate to form andradite- and hedenbergite-bearing rocks, is a case of metaso matic metamorphism. Except in the vicinity of igneous intrusions, metamorphism is accomplished without significant change in com position.

Differences of metamorphism can be distinguished, viz., (a) contact or thermal metamorphism, in which the chief factor is temperature. It embraces all those processes which operate on solid rock under the influence of magmatic heat, and from its localized occurrence around igneous intrusions is sometimes re ferred to as local metamorphism; (b) load metamorphism (Ger. Belastungsmetamorphose), affecting rock masses buried deep within the crust. The operating factors are pressure due to the weight of superincumbent material and temperature appropriate to the depth. This type is frequently referred to as geothermal metamorphism; and (c) dynamo-thermal metamorphism, in which the operating factors are stress, uniform pressure and temperature in varying degree.

Contact or Thermal Metamorphism.

Any kind of rock— igneous or sedimentary—which has come into contact with molten magma, is likely to show alteration of this type. Heat is the prin cipal agent of metamorphism and is conveyed from the magma principally by the process of conduction. Other factors, however, are important. Volatile materials, principally water, pass into the surrounding rocks from the magma and assist in the processes of solution and reaction. The medium in which the operative proc esses take place is the interstitial liquid occupying the capillary pores and fissures in the rocks. A rise in temperature facilitates diffusion and reaction. Chemical reactions which proceed at an infinitesimal rate at low temperatures are enormously accelerated by rise of temperature, a rise of oo° C being sufficient in many cases to increase the rate of reaction a thousandfold. The net result of these processes is eventually a recrystallization of the whole rock, which is now built up of a mineral assemblage ad justed to, or approaching adjustment to, the new temperature conditions. The extent and intensity of the alteration effected de pends chiefly on two factors—(a) the nature of the rock, and (b) the magnitude of the igneous mass.