General Knowledge for SSC Exams (Physical Geography)

Structure of the Earth

The Earth is an oblate spheroid. It is composed of a number of different layers as determined by deep drilling and seismic evidence. These layers are:

•  The core which is approximately 7000 kilometer in diameter (3500 kilometer in radius) and is loc<.ted at the Earth’s center.

•  The mantle which surrounds the core and has a thickness of 2900 kilometer.

•  The crust floats on top of the mantle. It is composed of basalt rich oceanic crust and granitic rich continental crust.

The core is a layer rich in iron and nickel that is composed of two layers: The inner and outer cores. The inner core is theorized to be solid with a density of about 13 gram per cubic centimeter and a radius of about 1220 kilometer. The outer core is liquid and has a density of about 11 gram per cubic centimeter. It surrounds the inner core and has an average thickness of about 2250 kilometer.

The mantle is almost 2900 kilometer thick and comprises about 83% of the Earth’s volume. It is composed of several different layers. The upper mantle exists from the base of the crust downward to a depth of about 670 kilometer. The top layer of the upper mantle, 100 to 200 kilometer below surface, is called the asthenosphere. Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle.

The rocks in this upper portion of the mantle are more rigid and brittle because of cooler temperature and lower pressures. Below the upper mantle is the lower mantle that extends from 670 to 2900 kilometer below the Earth’s surface. This layer is hot and plastic. The higher pressure in this layer causes the formation of minerals that are different from those of the upper mantle.

The lithosphere is a layer that includes the crust and the upper most portion of the asthenosphere. This layer is about 100 kilometer thick and has the ability to glide over the rest of the upper mantle. Because of increasing temperature and pressure, deeper portions of the lithosphere are capable of plastic flow over geologic time. The lithosphere is also the zone of earthquakes, mountain building, volcanoes, and continental drift. The topmost part of the lithosphere consists of crust. This material is cool, rigid, and brittle. Two types of crust can be identified: Oceanic crust and continental crust. Both of these types of crust are less dense than the rock found in the underlying upper mantle layer. Ocean crust is thin and measures between 5 to 10 kilometer thick.

It is also composed of basalt and has a density of about 3.0 gram per cubic centimeter. The continental crust is 20 to 70 kilometer thick and composed mainly of lighter granite. The density of continen-tal crust it about 2.7 gram per cubic centimeter.

It is thinnest in areas like the Rift Valleys o1 East Africa and in an area known as the Basin and Range Province in the westerr United States (centered in Nevada this area is about 1500 kilometer wide and runs about 4000 kilometer North/South). Continental crust is thickest beneath mountain range: and extends into the mantle.

Both of these crust types are composed of numerous tectonic plates that float on top of the mantle. Convection currents within the mantle cause these plates to move slowly across the asthenosphere.