Earth's Mantle and Core: Structure and Composition

Earth's Mantle: Structure and Composition

The middle layer is currently not able to be pierced. The survey in the Kola peninsula reaches 13 km deep. The Bavaria project in another poll is 14 km, where the inconvenience can be saved because the temperature does not exceed 300ºC, otherwise, the drill bit would melt. Mantle rocks were put on the surface through favorable tectonic structures. In the mantle, we must distinguish two layers:

• The upper mantle from 30 to 670 km of depth.
• The lower mantle from 670 to 2,900 km.

The Repetti discontinuity separates them. The predominant rocks are peridotites, which have the same density in the upper mantle. They outcrop in the mountain ranges in the form of ophiolite complexes. Their typical minerals are olivine, pyroxene, spinel, and garnet. The existence of peridotites is based on the partial fusion of garnet peridotite and pyroxene, affecting the consolidated basalts again.

Mantle Anisotropy

Seismic waves propagate faster in some directions than in others. Their velocity is greater in the direction of elongation of the peridotite minerals. Measured anisotropy in the mantle is almost the same as in peridotites. The composition is similar to carbonaceous chondrites if we remove the volatiles accordingly. For the whole mantle, differentiation is supported by changes in the physical properties of minerals due to the gradual increase in pressure. In the upper mantle, olivine is transformed into spinel, which is 10% denser. In the lower mantle, spinel becomes perovskite. Convection currents in the mantle silicates produce the motion of lithospheric plates. The current model assumes the existence of convection currents confined to the mantle and a third type, plumes, which can move from the core region and may form surface ridges or hot spots.

Earth's Core

The existence of a solid inner core and a liquid outer core is evidenced by the propagation of waves. The waves do not propagate, and the P-waves increase their velocity, crossing the Lehman discontinuity. The temperature ranges between 4,000 and 5,000ºC, and pressures are between 1.3 to 3.6 million atmospheres. Calculated densities are lower than those determined for iron. The existence of convection currents in the outer core enables the creation of the Earth's magnetic field, acting as a dynamo. Such a field has existed for 6,500 million years. The geodynamo model is based on the existence of coil currents of free electrons in the external core. The convective motion might be generated by the gradual cooling of the core. The solidification of molten material increases density, reflecting a fall of materials towards the background and fluids moving outwards. These movements generate the power needed by the geodynamo to maintain the magnetic field.