METAMORPHISM. When rocks of either igneous or sedimentary origin undergo sufficient change so that they largely develop new minerals, and become other types of rocks, they are said to be metamorphosed, and the process is called metamorphism. Such changes may be brought about in many ways. One of the most familiar is the process of weather ing (q.v.) by which the minerals of a granite, for example, undergo alteration, the quartz remaining behind as quartz, the feldspar alter ing chiefly to clay, and the soluble lime, mag nesia, potash and soda going to the sea, the first two to form limestone, the latter to produce the salt of ocean water. Thus the complete weathering of a granite and the sorting of the products finally results in the formation of shale, sandstone and limestone. This phase of metamorphism is called katamorphism. It is produced by the atmospheric agencies and results in simpler mineral compounds and rocks.
If on the other hand, a sedimentary rock be subjected to intense heat or pressure, or both, the minerals present recombine to form new minerals, usually more complex, which are bet ter suited to conditions of heat and pressure. This phase of metamorphism is usually called anamorphism. In general usage, the term metamorphism is commonly restricted to what is here termed anamorphism, that is, to those changes due to heat and pressure. The term metamorphic rocks (q.v.) is wholly restricted to rocks which have undergone changes of anamorphic nature. Thus, for example, a shale which has resulted from the weathering of a granite, though it has undergone metamorphism in the broadest sense, is not considered a meta morphic rock.
The heat and pressure which cause meta morphism may he produced by compression and folding over large areas, in which case there is said to be dynamic or regional metamorphic; or they may he due to the intrusion of large masses of highly heated igneous rock (magma). in which case there is said to be contact meta morphism.
Regional metamorphism results largely from pressure, heat is only incidental. The most common minerals to result are those which are flat like mica (q.v.), or elongated like horn blende (q.v.). These naturally develop in paral
lel arrangement with their smallest dimension opposed to the pressure, that is their larger dimension is at right angles to the compres sion. This seems to he the easiest way in which such minerals can develop. As a result of this arrangement, such rocks split more easily in certain directions, just as wood splits parallel to the grain. This is called cleavage, and is best manifested in slates ((IA.) and schists (q.v.).
Contact metamorphism, on the other hand, is due largely to heat ; pressure plays a less im portant part. The changes at a contact may be of several types. The rocks may be simply baked or they may be actually fused. The minerals present may recombine to form new minerals. There may be much new mineral matter actually added from the igneous mass, as in the case'of contact ore deposits, where ore minerals make up a large part of the con tact rock. Or the heat of the igneous mass may. drive off substances already present, espe cially water and carbon dioxide. At most con tacts these processes have probably all played a part. The intensity of the metamorphism and the width of the altered zone depends on sev eral factors,,such as the size and temperature of the igneous body, the amount of the water or steam present, and the resistance of the enclos ing wall rock. A quartzite (q.v.) wall rock may be little changed, while a limestone (q.v.), because of its greater solubility, may be pro foundly altered. An abundance of steam aids in producing a marked effect, since it readily works its way out through pores in the rock, carrying much mineral matter. Alteration pro duced largely by steam or hot water is termed hydrothermal metamorphism. Changes pro duced by ordinary circulating waters are some times termed hydrometamorphism. See CLEAV AGE; GEOLOGY; METAMORPHIC ROCKS; ORE DEPOSITS. Consult Van Hise, C. R., 'A Treat ise on Metamorphism,' (U. S. Geological Sur vey, Mon. XLVII, Washington 1904); Leith, C. K., and Mead, W.J., Geol ogy' (New York 1915) ; Leith, C. K., Cleavage' (U. S. Geological Survey, Bull. 239 Washington 1905); Grubenmann, U., 'Die Crystallinen Schiefer.'