Earth is divided by its composition in crust, mantle and nucleus and by its physical characteristics in lithosphere, asthenosphere and mesosphere. The crust is the most exterior part of the Earth, it is formed mostly of rocks (70% basalts), thus it has a solid behaviour.
We can distinguish two kind of crust: oceanic crust and continental crust. The oceanic crust has a thickness between 3 and 8 km, it is dense 2,9 mg/cc and his composition is mainly granitic. The continental crus has an average thickness of 30 km (but it can get to 70 km!), it is dense 2,7 mg/cc and his composed mostly by basalts and gabbros.
Between the crust and the mantle there is the Mohorovičić discontinuity (called Moho), discovered thanks to the waves study.
The mantle is divided in: superior mantle and inferior mantle. The superior mantle has a solid behaviour, while the inferior has a plastic behaviour. The convection occurs in the mantle and it regulates the theory of plates. According to some geologists there is a different convection in the superior and in the inferior mantle. According to someone else geologists there is an only convection that interests both superior and inferior mantle.
Between the inferior mantle and the nucleus there is the Gutenberg discontinuity (2.900 km).
The nucleus is the inner part of the Earth and it is divided in: exterior nucleus and interior nucleus. The first one has a plastic behaviour and the second one has a solid behaviour. The convection occurs also in the exterior nucleus and this convection generates the Earth’s magnetic field. The Earth behaves as a dynamo that feeds thanks to the presence of iron that has a high electrical conductivity. In fact, thanks to the rapid movements of the exterior mantle electrical currents are generated.
Between the exterior and the interior nucleus there is the Lehmann discontinuity (5.100 km).
The lithosphere is formed by the crust and the superior mantle and it has a solid behaviour; the asthenosphere is formed by the rest of the superior mantle and it has a plastic behaviour; the mesosphere is formed by the intermediate and the inferior mantle and it has a solid behaviour.
The interior of the Earth has been studied thanks to the waves. The primary waves, in fact, after the Mohorovičić discontinuity accelerate from 7 km/sec to 8 km/sec probably because the mantle is composed of peridotites and ultramafic rocks. After that the primary waves slow down and the secondary waves don’t propagate anymore, probably because of the plastic behaviour. So that there is a shadow from 105° to 142° where primary waves don’t propagate and a shadow from to 105° onwards where the secondary waves don’t propagate.