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Metamorphic Rocks
The problem discussed concerns metamorphic rocks which compose the third large family of rocks. "Metamorphic" means "changed from". It shows that the original rock has been changed from its primary form to a new one. Being subjected to pressure, heat and chemically active fluids beneath the Earth's surface, various rocks in the Earth's crust undergo changes in texture, in mineral composition and structure and are transformed into metamorphic rocks. The process described is called metamorphism.
As is known, metamorphic rocks have been developed from earlier igneous and sedimentary rocks by the action of heat and pressure.
Gneisses, mica schists, phyllites, marbles, slate, quartz, etc. belong to the same group of rocks. Having the same mineral composition as granite, gneisses consist chiefly of quartz, orthoclase and mica. However unlike granite, they have a schistose structure. It means that their constituents are distributed in bands or layers and run parallel to each other in one direction. If disturbed the rock cleaves easily into separate plates.
The role of water in metamorphism is determined by at least four variable geologically related parameters: rock pressure, temperature, water pressure, and the amount of water present.
During a normal progressive metamorphism rock pressure and temperature are interdependent, and the amount of water and the pressure of water are related to the sediments and to the degree of metamorphism in such a way that, generally speaking, the low-grade metamorphic rocks are characterized by the excess of water. The medium-grade rocks defined by some deficiency of water and the high-grade metamorphic rocks are characterized by the absence of water.
Many of the metamorphic rocks mentioned above consist of flaky materials such as mica and chlorite. These minerals cause the rock to split into thin sheets, and rocks become foliated.
Slate, phyllite, schist and gneiss belong to the group of foliated metamorphic rocks. Marble and quartzite are non-foliated metamorphic rocks.
The structure of metamorphic rocks is of importance because it shows the nature of pre-existing rocks and the mechanism of metamorphic deformation. Every trace of original structure is of great importance to geologists. It gives an opportunity of analysing the causes of its metamorphism.
Being often called crystalline schists, metamorphic rocks such as gneisses and mica have a schistose structure. Metamorphic rocks represent the oldest portion of the Earth's crust. They are mostly found in the regions of mountain belts where great dislocations on the Earth once took place.