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The earth is composed of three internal concentric layers of increasing densities. These concentric layers are: the crust, mantle and core. They are made up of increasing densities towards centre of the Earth;-

1. The Crust (Mesosphere):This is the outermost part of the earth. It consists of silica and aluminium (sial). They form the upper layer of continent and most of them are composed by granite rocks. The layer below SIAL is called SIMA is composed of silica and manganese. It is composed of basaltic rocks which are denser and underlies continental block to form ocean floor.

2. The Mantle (Mesosphere): This is the layer below the crust. It is composed of iron and manganese. It lies between the crust and the core. The layer which separates crust and mantle is called Mohorovic discontinuity. Mantle is made up of very dense and hot igneous rocks, found in semi liquid states. It extends downwards 2900 kms and the temperature ranges at 50000C 70000C. The density of mantle is 3 3.3gm/cm3. Mantle is divided into two parts; the upper and lower mantle. The upper mantle is the rigid and combines with crust to form a layer called lithosphere. Below the upper mantle there is a layer called asthenosphere.

3. The Core (Barysphere): This is the inner most layer of the Earth. It is also referred to as Barysphere. It is composed of nickel and iron. Its diameter is approximately 2500 2700 kms and its temperature is about 55000 C. The average density of the Barysphere is about 5.2gm/cm3. Most geographers believe that the core is divided into solid and liquid core. The total mass of the earth is about 5.976 x 1021 tones.

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