Today most knowledge about the earth is indirectly obtained from the study of earthquake waves. For this reason there are a variety of possible explanations, but in general all of them consider the earth to have three main components, namely a crust, which is relatively very thin, a mantle, and a core.
The crust is known from direct observation and consists of thin oceanic shell on which the continents of the world rest. The continental crust is therefore limited to the continental and nearby areas.
The ocean crust on which it rests is broken up into a number of plates, and these plates are moved by convection currents in the mantle, and in turn carru the continents with them. This is what is meant by ‘plate tectonics’.
The mantle form tge largest part of the earth. Its upper part is somewhat complex, but divided quite sharply from the crust by the Mohorovicic discontinuity (Moho), a boundary zone between rocks of different density. Although it is not liquid, the mantle is subject to plastic flow, as heat rises towards the crust, and these slow currents are the driving force for the movement of the crustal plates. At the Moho the mantle rocks can melt if pressure is reduced by fracturing, and magma is ejected into and onto the crust in the form of lava flows and volcanic eruption.
The core is the least known zone, but earthquakes waves pass through it quite differently from the mantle, indicating that it is solid at the centre and molten on the outside. Temperature increase towards the centre of the earth, reaching 1200°C at the moho 3500°C at the Gutenberg Discontinuity and 5500°C at the centre of the earth.