The earth is a solid sphere. It is made up of three concentric spheres or layers. These are called the core, the mantle, and the crust. The solid sphere is surrounded by a gaseous sphere, which is called the atmosphere.
We know most about the crust of the Earth which is the outermost sphere. This layer is very thin compared with the diameter of the whole Earth. It is only about 10 km thick under the ocean and about 30 km thick on land. It consists of rock which contains a lot of minerals. These are usually in compounds called oxides, containing oxygen, or sulfides, containing sulfur.
The mantle is much thicker than the crust. It is about 30 km thick. It consists mainly of rocks, but we do not know much about their composition.
The core, which is situated inside the mantle, seems to be divided into two parts. The inner core is about 2800 km in diameter. We believe that it is mainly composed of iron, but it also contains about 10% nickel. The layer surrounding the inner core is called the outer core and is approximately 2000 km thick. It is probably composed of molten iron and nickel. However, the metals in the inner core seem to be rigid, and therefore solid. This is because they are under very high pressure, which causes solidification in spite of the high temperatures at the center of the Earth.
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