Structure of the Earth
Information about the internal structure of the earth is derived through the studies of earthquake waves. The shock waves produced at the time of an earthquake are recorded by a seismograph and an analysis of the time of approaching waves at a particular point helps in finding out the epicenter (the point on the surface of the earth where the earthquake is felt first of all) and the seismic focus (the point of origin of the earthquake in the interior of the earth. This point is immediately below the epicenter). The behavior of the earthquake waves as they pass through different layers of the earth , indicates the interior structure of the earth. Such a study of the earthquake waves indicated that the earth is not made up of homogeneous material. The density of the material increases as we go inwards in the earth and the composition of the rocks also changes inwards. Generally the lighter and less dense rocks are found on the surface of the earth while the rocks in the interior are denser and heavier.
The topmost solid crustal layer of the earth is called the lithosphere. The upper part of the lithosphere is made of lighter rocks formed of substances such as silica and aluminum. This part is called the sial layer. Below this layer the rocks are denser and they are composed of materials such as silica and magnesium and this layer of denser rocks is called the sima layer. The continents are made up primarily of the sial type of material while the ocean bottoms have a very thin layer of such material and sima is the dominant material forming the ocean bottoms.
The layer below the crust is called the mantle. Most of the material constituting this layer is of simatic type. This second layer of the earth extends up to a depth of about 2900 km. This layer is also called the asthenosphere due to its greater plasticity and elasticity. The material forming this layer is in a molten state. The third part of the earth is the metallic core. The rocks in this part have a high density and they are composed of materials like nickel and iron. This material is called the Nife zone. Although the temperature in this layer is too high to allow the rocks to remain in a solid state, due to the excessive pressure of the overburden, the rocks in this part do not flow freely like liquid. This part of the earth is called the centrosphere. A transitional zone between the crust and the mantle layers is called Mohorovicic Discontinuity or the Moho layer while an analogous layer or transitional zone between the core and the mantle is called the Gutenberg Discontinuity.
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