What do you think when I say magnet?
Maybe something like this:
So what do you think If I tell you that this is a magnet?
Our Earth behaves as a big magnet. It is like there is a magnetic bar inside, with two poles.
The hypothesis that there is a magnetic bar inside the Earth is inconceivable because at the Curie temperature (768 °C for the iron) the magnetic field destroys, at the centre of the Earth the temperature is much bigger! (around 5000 °C). So the magnetic field could not conserve.It was discovered that the magnetic field is generated by the convection of the outer nucleus. So why the convection of the mantle doesn’t generate the magnetic field? First of all because the material in the mantle is viscous while in the nucleus it is very fluid, secondly because in the nucleus there is the presence of iron that is a good electrical conductor. Rapid movements in the outer nucleus can produce electrical currents in the iron, generating a geodynamo with a big magnetic field.
The rocks record the field magnetic orientation, in fact studying the ocean floor it was discovered that the magnetic field was reversed many times! When the lava comes out from the oceanic ridges some of her crystals orient according to the current magnetic field and when the lava cools and forms rocks they maintain the magnetic field orientation.
The chrons (normal of inverse epochs) last about a half million years. The chrons are interrupted by some subchrons that are short.
The black bars indicates normal polarity; the white bars indicates inverse polarity.
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.