The mantle, about 2900 km thick, occupies most of the Earth’s interior. Geophysics also gives us indirect information about its structure and composition, which, as we have seen, is mainly peridotidic. By studying the properties of the seismic waves that we know have crossed the mantle we can derive crucial information about its characteristics. Its upper limit is identified by Mohorovicic (Moho) boundary. Two changes in the speed of seismic waves define two discontinuities within it at 410 km and 660 km depth. We therefore have a 3-layer mantle, the upper mantle, a transition zone and the lower mantle.
The upper part of the mantle contains an area of low seismic velocity that we call the Astenosphere. The low seismic velocity (and an almost cancellation of the S waves) must be due to areas partially melted in the upper mantle that would allow isostatic movements and as we will see later, those of the plates. Basically these are materials in a state similar to that of a toothpaste that reacts rigidly to rapid impulses such as seismic waves, and fluidly to slow impulses such as tectonic movements.
The transition zone (410-660 km) is due to the abrupt thickening of the minerals that compose it. The crystalline structures adapt to the enormous pressures that occur at these depths, generating structures typical of the spinel. Its rheological properties (of reaction to stress) should prevent large exchanges of materials with the upper mantle. A peculiar characteristic of the transition zone is the abundant presence of water (in the form of OH- ions). It is thought that water is released by the transformation of crystals into phases that are in equilibrium with the pressures and temperatures of the base of the transition zone.
The lower, denser and warmer mantle of the transition zone extends to a depth of about 2700 km. Rheologically, the lower mantle is much stiffer than the rest of the mantle, showing characteristics similar to perovskite-like crystalline structures. Although the temperature is very high, the pressures are such that the rocks are not very ductile. Obviously it is the least known and most debated layer of the mantle by geologists and geophysicists.
Below the lower mantle a thin layer (up to 2900 km deep) designated as D” has been found. This is the boundary with the core and is an area where it is believed that there are processes of fusion of rocks whose material is pushed upwards into the more rigid rocks of the lower mantle. It is a silicic material with a high iron content and very dense structures called ‘post-perovskite’. Some believe that they can generate plumes that also reach the transition zone.
Below layer D” we have the boundary with the core (CMB = Core mantle Boundary).