Ever wondered what our earth is made of?  Think of it as an apple.  An apple constitutes the skin, the pulp and the core in the middle.  Similarly, the earth is made up of the thin outermost layer called the crust, the innermost part called the core, and the part in between them called the mantle.

(Fig 1.1) The apple and the structure of the Earth.

     At the beginning, the early earth seemed to have heated up, the center became molten, and convection currents developed as the lighter compounds tended to rise towards the surface forming the brittle crust.  Together with the top part of the mantle, it formed the hard slabs known as lithosphere.  The continents are embedded in these slabs.  The lithosphere is divided into oceanic and continental crusts.

     Oceanic crust (sima), the floor of the deep oceans, is thin, about 7km thick, and made of relatively dense rocks like basalt.  Continental crust (sial) is much thicker, averaging 33km, and is composed of relatively light material such as granite.

(Fig 1.2) The Crust (Oceanic and Continental).

     The denser materials such as iron sank to form the core.  It is partly solid.  Temperatures are extremely high, at about 3000^oC.

(Fig 1.3) The Core.

     Between the core and the crust, the intermediate zones form the mantle, which is mainly solid rocks but there is also a layer of molten rock called magma nearer the core. Temperatures are high, at about 2000^oC.

(Fig 1.4) The Mantle.