The earth is the planet we live on, but only 29% of the earth’s surface is and, the other 71% is ocean. It is shaped like a large ball; about 12700km in diameter and an atmosphere surrounds it.
Note that the atmosphere is divided into: Troposphere (bottom), Stratosphere (middle), Lonosphere and Exosphere.
The earth consists of three layers namely the core, the mantle and the crust.
The Core (Baryshpere)
This is the innermost zone of the lithosphere usually found at the centre of the earth and has a diameter of about 7,000km. ii is very hot and under great pressure. Iron, Nickel and possibly, some silicon make up most of the core.
The Mantle (Mesosphere)
This layer lies beneath the lithosphere and it is about 2,800km thick, It is probably composed of denser forms of certain minerals (olivine) that also occur in the top layer, which is called the crust. The boundary between the mantle and the earth is called Moho.
This is the out most layer of the earth and has an average thickness of 30km. The crust forms the surface of the continents, which are in the upper part of the crust, with less dense rocks such as granite. The earth crust is composed of two layers sial (known as upper layer) and sima (known as lower layer).