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The Lithosphere

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The Lithosphere

The term lithosphere refers to the solid crust of the earth The materials constituting the crust of the earth are called rocks. The tocks can be defined as aggregates of minerals. This solid and rigid layer of the earth is made up of lighter materials. It is believed that the density of the rocks increases in the interior of the earth and the innermost layers of the earth are made up of the highest density’ or heaviest rocks.

Interior Structure of the Earth

The interior structure of the earth is usually a subject of debate and even the most logical of theories in this connection arc not free from the criticism, However, all theories concede that the earth has a layered or zonal structure Most conclusions regarding the interior structure of the earth are based upon indirect evidence. ITiough there are several indirect methods of knowing about the internal structure of the earth, such as the increasing emperature with increasing depth, the most reliable and detailed information is provided by a study of the earthquake waves and their behaviour as they pass trough the central part of the earth This and other evidences lead to the conclusion that both density and temperature increase with increasing depth in the earth.

Layers of the Earth Though there is the difference of opinion among scholars about the number and nomenclature of different layers of the earth, most accept a division of the earth into three major layers and two transitional zones. The basis of differentiation between these layers is the density of the rocks in different parts. A brief account of different layers of the earth from the surface to the interior is provided here.

The Crust The crust is the topmost and the thinnest layer of the earth. This layer has the least density and its thickness vanes from about eight to 40 kilometers. A transitional zone called Mohorovicic Discontinuity or Moho marks the lower limit of the crust and this discontinuity is identified on the basis of rock density. Thickness of the emst is more under the continents and lesser under the oceans. Under mountains the thickness of the crust may be as much as70 km The rocks forming the crust of the earth are rich in lighter minerals like silica and aluminium. Hence this layer is called sial (silica and aluminium) layer also. It is customary to divide it into the upper silicate zone and the lower silicate zone. The average density’ of this layer is 2.7, which is much lower than the overall density of the earth.

The crust is the most important part of the earth from the human point of view. The continents and oceans are situated on this part only. The most important constituents of this layer are oxygen (46.6%), silicon (27 7%), aluminum (8 1%), iron (5%). calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (2.1%). Other constituents include titanium, hydrogen, phosphorus, barium, and strontium, etc.

The Mantle This layer is the intermediate layer of the earth in terms of both its location and density’. Mantle is a layer about 2895 km thick, composed of minerals in a semisolid state. The lower part of the mantle is believed to be molten. The rocks in this layer may be in a glassy state. Due to the presence of minerals like silica and magnesium in the rocks forming this layer of the earth, the mantle layer is also known as sirna (silica and magnesium). The rocks in this part exhibit rigidity and high density. The average density of this layer is about 5.68 The transitional zone separating the mantle from the core is called the Gutenberg Discontinuity.

The Core The core is the innermost part of the earth and occupies its center. It is about 3475 km in radius. The outer part of the core is believed to have the properties of a liquid and the innermost part of the core (about 1255 km in radius) may be called solid or crystalline. The core is the densest part of the earth and the rock material forming this zone of the earth is rich in metallic minerals like iron and nickel. It is due to the large concentration of such metallic minerals in the core of the earth that this zone is known as nife (nickel and iron). Temperature of the earth ‘s core is between 2200° and 2750°C. Density of this part of the earth (17.2) is many times greater than the average density of the earth (5.53).

According to another popular system, the earth is divided into lithosphere, asthenosphere, and centrosphere. The lithosphere is the solid upper part. In terms of the terminology of crust, mantle, and core, lithosphere includes entire crust and upper solid part of the mantle. The asthenosphere is the glass or plastic layer below the lithosphere and includes the lower molten part of the mantle. This is also called the zone of weakness due to its liquid nature. According to the theory of plate tectonics, the solid lithosphere floats over the liquid asthenosphere. The centrosphere is the central part of the earth which in spite of having a very’ high temperature, behaves like solid due to the immense pressure of the overlying rocks.

Materials of the Earth’s Crust-Rocks and Minerals

The crust is the part of the earth that influences human life most profoundly. As mentioned earlier it is the uppermost part of the earth and it is solid. The material of which the crust is formed is called the rocks. A rock can be defined as an aggregate of minerals. For being called a proper rock, the matter should be in a consolidated form. A mere collection of particles of several minerals cannot be called a rock unless they occur in the form of a consolidated mass. Sand particles, for example, cannot be called a rock unless they are cemented together. The process of rock formation, consolidation of mineral particles is called lithification Rocks can be soft, such as clay and sandstone, and hard, such as granite and marble A rock containing large the proportion of a single mineral so that the said mineral can be economically extracted from it is called an ore of that mineral A mineral is a natural substance with characteristic properties and more or less specific chemical composition The commonly found minerals arc called rock-forming minerals and feldspar, quartz and calcite are found in largest amounts in the rocks of the crust.

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