Earth

Internal part of the Earth

The Earth is a spherical body that is made up of several internal layers, with a different thickness (crust, mantle, external nucleus, and internal nucleus). The passage from one layer to another is characterized by discontinuities:

• at 30-40 km of depth, the crust is divided from the mantle by Mohorovicic or Moho discontinuity 
• at around 2900 km of depth the mantle is separated from the external nucleus by the Gutenberg discontinuity 
• at around 5100 km of depth the external nucleus is separated from the internal nucleus by Lehman discontinuity.

The crust
The crust is the most external layer of the Earth. It is divided into earth and ocean crust. Continental crust has an average thickness of 40 km and is made of magmatic rocks (granites) and metamorphic rocks. The superficial layer of continents is mainly covered by sedimentary rocks. The ocean crust is less thick, as it measures around 6-8 km thickness and is made of different types of igneus rocks. In fact the first layer is made of basalts and then, underneath it, there are gabbro and metamorphic rocks. The surface of the ocean crust is also covered by sediments. 

The mantle
The mantle is a layer that mainly consists of magmatic rocks (peridotites), which are denser than the gabbro and basalts that form the crust. The minerals that compose these rocks are fundamentally olivine and pyroxenes. The speed and direction of seismic waves through the mantle experience sudden changes showing that rocks are not always solid. Let us remember that magma originates at a depth of around 100 kilometres.  In fact, between 70 and 200 kilometres of depth (immediately under the crust) there might be melted rocky material.

The nucleus
The behaviour of seismic waves brings up the hypothesis that the external nucleus could be composed of melted rocks, since it has the same characteristics as a liquid material. Researchers think that it is full of iron mixed with silicon and other metals like nickel. The internal nucleus, instead, behaves as a solid material, as it is rigid and elastic: it seems to be made of solid rocks.