Metamorphic Rocks

Metamorphic rocks are rocks which have undergone some chemical or physical change subsequent to their original formation. This change has been brought about by means of high temperature and pressure aided by the action of water and other chemical agents. The changes involve the formation of new minerals, adding or subtracting of chemical constitutes and a physical  readjustment of the minerals particles to conform to the existing pressure. The original rock from which a metamorphic rock has been derived may be either igneous or sedimentary. As these rocks become involved in movements of the earth’s crust, they are subjected to extreme pressures accompanied usually by high temperatures. The result will be frequently to transform the existing minerals into others more stable under the new conditions. The physical structure of the rock Hill also ordinarily be changed during the process. Because of the pressure to with the rock is subjected the mineral particles will be more or less broken and flattened and rearranged in parallel layers. This banded or laminated character given by the parallel arrangement of its minerals is the most striking peculiarity of a metamorphic rock. Because of this structure a metamorphic rock can be distinguished from a igneous rock. Further, in the great majority of cases a metamorphic rock has a crystalline structure which distinguishes it from a sedimentary rock. There are, of course, all gradations from a typical metamorphic rock into an unaltered sedimentary rock on the one hand and into a unaltered igneous rock on the other. The most common types of metamorphic rocks are brief described beyond.

Gnesiss.
When the word gneiss is used alone it usually refers to a metamorphic rock composed essentially of quartz, feldspar and a mica. The quartz and feldspar occur together in layers which are separated from each other by thin drawn-out bands of mica. A gneiss has usually a light color, although this is not necessarily so. Various verities of gneiss have received distinctive names, most of which are self-explanatory, like banded gneiss, lenticualar-gneiss, hornblende-gneiss, granite gneiss, diarite-gnesiss, etc. Gneiss is a very common rock type, especially in regions in which the oldest rocks, those of the archaic age, are found. Gneiss have been more commonly derived by the metamorphism of igneous rocks, mostly granites, but may have been formed from sedimentary rock as well.

Mica-schist. Mica-shist is a rock composed essentially of quartz and mica, usually either muscovite or biotite. The mica is prominent mineral, occurring in irregular leaves and in foliated masses. The mica plates all lie with their cleavage planes parallel to each other and give to the rock a striking laminated or “schistose” structure. The mica-shists frequently carry characteristic accessory minerals, such as garnet, staurolite, cyanite, epidote, hornblende, etc. the may have been derived from either an igneous or a sedimentary rock. Next to the gneisses, they are the most common metamorphic rocks.

Quartzite. As its name indicates, a quartz is a rock composed essentially of quartz. It is a firm, compact rock which breaks with an uneven, splintery or conchoidal fracture. It is breaks with an uneven, splintery or conchoidal fracture. It is usually light in color. Quartzite has been derived from a sandstone by intense metamorphism. It is a common sn widely distributed rock.

Slate. Shates are exceedingly fine-grained rocks which have a remarkable cleavage which permits them to be split into thin and broad sheeta. Their color is commonly gray to black, but may be green, yellow, brown, red, etc. They have been formed commonly by the metamorphism of shales, Characteristic slate cleavage may or may not be parallel to the bedding planes of the original shales. They are quite common in occurrence.

Various Schists. There are various other kinds of schistose rocks, which are chiefly derived by the metamorphism of the ferromagnesian igneous rocks. The most important types are talc-shist, clorite-schist, amplibolite or hornblende-schist. They each are characterized, as their names indicate, by the preponderance of some metamorphic ferromagnesian mineral.

Marble. A marble is a methamorphosed limestone. It is a crystalline rock composed of grains of calcite, or more rarely dolomite. At times the individual grains are small that they cannot be distinguished by eye, and again the may be quite coarse and show clearly the characteristic cleavage of the mineral. Like limestone, a marble is characterized by is softness and its effervescence with acids. When pure. When pure, marble is white in color, but it may show a wide range of color, due to various impurities that it contains. It is a rocks which is found in many localities and at times in thick and extensive beds.

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