Situated at the centre of the Earth, about 3,500km from the surface, the core is made up of the internal core and the external core. It is said that its components are heavy steel and nickel, and that the internal core is solid, while the external core is liquid.
(ii) The theory of plate tectonics
This theory says that the surface of the Earth is covered with dozens of hard plates, moving over a layer of less density, called the Athenosphere, and the main movements on the surface are generated by the frictioon between the plates. The theory also explains the generation of earthquakes.
Figure 2: The generation of plates, their movement and their subduction
2 Figure 1 Fomation, movement and subduction of plates
3 Island arc
4 Trench
5 Plate
6 Mantle convection
7 Volcanic island
8 Continent
9 x indicates the Seismic epicentre
10 How the plates are formed and disappear
11 Model of the occurrence of a major earthquake
12 Continental plate
13 Oceanic plate
14 1 The oceanic plate subducts below the continental plate (S. Ueda's theory).
15 2 The tip of the continental plate is dragged down, and tension accumulates.
16 3 The tip of the continental plate springs back and creates a reverse fault.
17 4 The tension is released and the plates return to their original state.
