Lithosphere or Crust

The uppermost layer of the earth is called litho-sphere or the crust. Its average thickness is 30 km to 40 km under the continents, 5 km to 10 km under the ocean the ocean floor and up to 60 km under the mountains. At certain places below the pacific floor, its thickness is very little. The earth’s crust consists of two layers- the upper layer and the lower layer. The upper layer is called SIAL (silicate + aluminum). It is made up of granite-like rocks which mainly consist of silica and alumina. Below the upper layer lies the lower layer called SIMA (silicate + magnesium) consisting of denser rocky material. Higher temperature prevails in this region. It is the same material of which the rocks on the surface of the earth are made.

There are three kinds of rock structure in the upper continents crust of the earth. Of them, the most abundant are the Pre-Cambrian crystalline shields. In the second place are the coastal areas formed of sedimentary rocks. The structure of the third part is folded mountains.

The crust of the earth is made up of a number of plates- large in size and of varied thickness, called the crustal plates. Some of them are in the form of ocean floor while others are in the form of continents. Earthquakes occur due to the upheavals of these plates.

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