# Global distribution of the continent and ocean 8 343

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Global distribution of the continent and ocean -----------------------------------------8 3.4.3 Significant surface features of the oceans.-----------------------------------------------8 3.5 Sequence of the solid earth’s surface ----------------------------------------------------9 3.6 Self assessment exercise 2-----------------------------------------------------------------9 4.0 Conclusion----------------------------------------------------------------------------------10 5.0 Summary -----------------------------------------------------------------------------------10 6.0 Tutor marked assignment ----------------------------------------------------------------10 7.0 Further readings and other resources. --------------------------------------------------11 1.0 Introduction

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- 32 - The internal structures of the solid earth are probed using some geophysical methods. The information obtained are utilised to draw model of the pictures about the interiors of the solid earth. Further, most of the features existing on the earth surface are consequences of the events which either took place around the time of formation of the earth or after mat effects of other event taking place with time. This unit deals with the models of the interior structure of the earth based on the scientific facts. The causes and structure of the external features of the solid earth are treated. 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

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- 34 - (i) Stable region (cratons)
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• Winter '12
• JOHN
• Equator, earth surface, solid earth, geomagnetism

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