Tuesday, October 27, 2009

ROCKS

ROCKS:
Any aggregate of mineral particles that forms part of the earth's crust is called a rock. Composed of mineral matter in solid state, it is present in a wide range of compositions, physical characteristics and ages. While a few rocks are constituted almost by a single mineral, there are others composed of two or more minerals. Rock formation is a continuous process. Though most rocks of the earth's crust are very old, the times of formation range back to millions of years. Rocks are classified into three major groups-igneous, metamorphic and sedimentary; each group is further subdivided into different types each with its own composition, character and age.

IGNEOUS ROCKS
Igneous rocks are rocks that have solidified from molten magma at considerable depth in the earth under conditions of very high temperatures and pressure. They do not occur in layers. Most of them are crystalline and do not contain fossils. Igneous rocks are classified on the basis of chemical composition as well as on grades of sizes of the component crystals:
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(i) Extrusive rocks These are rocks formed out onto the surface of the earth as magma before cooling and are generally glassy or fine-crystalled. These are also called volcanic rocks, or lava, e.g., basalt.
(ii) Intrusive rocks These cool and solidify within the earth's crust and reach the earth's surface only by being exposed by erosion. These may be either (a) plutonic which cool deep within the crust and have large crystals, e.g., granite, or (b) hypabyssal which cool at intermediate depths and contain moderate-sized crystals.
Igneous rocks are also classified on the basis of chemical composition. For example, one classification is based on silica content and has the following divisions: acid rocks (over 66 per cent silica); intermediate rocks (55-66 per cent silica); basic rocks (45-55 per cent silica; e.g., basalt); ultra basic rocks (less than 45 per cent silica).

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