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2.0 objectives by the end of this unit you would

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Unformatted text preview: 2.0 Objectives By the end of this unit you would learn the following: (a) Earth models based on the chemical composition and ability of the material to flow. (b) Evidence of in homogeneity of the earth crust (c) Formation of three states of the earth. (d) Global distribution and surface sequences of the ocean and continent. 3.1 Evidences of the Inhomogeneity of the Solid Earth The crust is the layer closest to the surface of the solid earth. It is the layer that shows high level of structural variations within the rock layers. The inhomogeneity of the earth is shown in the following. (1) Density variation: The surface density of the solid earth has been found to be about 2.67g/cm 3, but the average density of the earth is 5.5g/cm 3. This implies that deep beneath the earth’s surface is denser, i.e. density increases with depth. - 33 - (2) Rotational Analogous of mass is Moment of inertia (I). That is, as I is a measure of resistance of a rotating body to changing its angular velocity ( ) mass is the measure of resistance of a body to change its initial states. So, I depends on mass and the distribution of mass in a body. So, for a sphere with uniform density, I = bma 2 , where b = 2 / 5, Now, for earth model I = b = ma 2 , If b is greater than 2 / 5, density decreases with depth and if b is less than 2 / 5, density increases with depth. But for planet earth b has been found to be 0.3308 which is less than 2/5; hence density increases with depth in the earth. 3.2 Internal Models of the Solid Earth The nature of the solid earth’s interior is obtained using various criteria, based on the available scientific information. Two models are discussed below. 3.2.1 Earth’s Model based on chemical composition The solid earth is oblate with equatorial and polar radii approximately equal to 637.8388 km and 6356.912 km respectively. Based on the chemical compositions seismological evidence gave the following subdivisions (a) Crust: There is continental and oceanic crust and it is a solid layer. Continental crust is basically solid, 30 – 40 km thick, mostly granite rocks and Gabbro with seismic primary wave velocity of 6 – 7 km/s. It is basically in three forms based on their geologic history of formation over the last 100 million years - 34 - (i) Stable region (cratons) - Little evidences of vertical or horizontal movement but with evidences of close warping of few minor faults (ii) Semi-mobile region - Characterised by differential vertical movement and formation of Sedimentary basin (iii) Mobile belts - Characterised by young mountain ranges - Strong deformation - Strong vertical and horizontal movements Oceanic crusts are mostly basalts and Gabbro, 5 – 11 km thick and they mostly exist in three layers namely: (i) Layer 1 - It is a non consolidated sediment - It has seismic velocity of between 1.5 – 1.8 km/s - It is 0.3 to 0.8 km thick (ii) Layer 2 - It is 1 – 2 km thick - It has seismic velocity of 2.1 – 5.5 km/s (iii) Layer 3 - It is a basaltic layer - It has seismic velocity of between 6.5 and 7.0 km/s - 35 -...
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2.0 Objectives By the end of this unit you would learn the...

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