Stones

Definition: A rock is a naturally occurring aggregate of minerals and mineraloids. The Earth's crust and mantle are formed of rock. However the word “Stone” is used to indicate an individual piece of rock. Stones are broadly categorized into three types: •Igneous •Sedimentary •Metamorphic

IGNEOUS STONES are formed when hot magma cools and solidifies, with crystallization either below the surface as intrusive (plutonic) stones or on the surface as extrusive (volcanic) stones. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes: Increase in temperature. Decrease in pressure. Change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust.

SEDIMENTARY STONES are usually created by the breakdown of igneous rocks. They tend to form in layers as small particles of other rocks. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. A Sedimentary rock is formed in four main ways: by weathering (erosion) caused mainly by friction of waves. by the accumulation and the consolidation of sediments. by the deposition of the results of biogenic activity. by precipitation from solution. Sedimentary stones include common types such as chalk, limestone, sandstone and shale. Sedimentary stones cover

METAMORPHIC ROCKS form when sedimentary or igneous rocks are altered by heat and pressure deep underground. Metamorphic rock is the result of the transformation of a pre-existing rock type, in process called metamorphism, which means "change in form". The existing rocks are subjected to immense heat and extreme pressure causing profound physical and chemical change. The existing rocks may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage. Some examples of metamorphic rocks are gneiss, slate, marble, and schist.

Prepared REFERENCES: by: Manjeet Singh Microsoft Encarta. S.Sc Master Internet. Govt. Secondary School, Gandiwind, Tarn Taran PUNJAB.