Earth's Internal Structure: Crust, Mantle, and Core

The Continental Crust

The continental crust forms the continents and the continental shelves submerged under the ocean. Its structure is categorized as follows:

• Horizontal Structure: This consists of two types of areas. The first are cratons, which are extensive regions with mild or no topography. The second are mountain ranges, which are formed by the deformation and elevation of rock masses.
• Vertical Structure: Its thickness averages about 30 km but can reach up to 70 km under mountain ranges. The structure varies greatly from region to region, and its surface is heavily altered by exogenous processes.
• Age: As an ancient crust, the cratons may be more than 500 million years old.

The Earth's Core

The innermost layer of the Earth extends from the Gutenberg discontinuity to the center. This consists mainly of iron, along with certain amounts of nickel, sulfur, and oxygen. The behavior of seismic waves allows us to distinguish two regions separated by the Lehmann discontinuity:

• The Outer Core: Situated between 2,900 and 5,100 km in depth, it exists in liquid form. Material flows within it due to variations in temperature and density.
• The Inner Core: Occupying the center of the Earth from a depth of 5,100 km, this region is composed of solid metal.

The Earth's Mantle

The mantle is the most voluminous layer and contains most of the Earth's mass. It lies between the Mohorovicic and Gutenberg discontinuities, consisting of rocks with a high content of iron and olivine silicates.

Zones of the Mantle

• The Upper Mantle: This extends from the Mohorovicic discontinuity to a depth of 670 km. Its outermost zone behaves much like the crust; together, they form the lithosphere, which consists of fragmented, rigid lithospheric plates. The upper mantle beneath this is not rigid due to high pressures and temperatures. A significant reduction in the speed of seismic waves in this area indicates the likely presence of partially melted rocks.
• The Lower Mantle: This begins at a depth of 670 km, where the minerals making up the rocks have internal structures that are more compact and denser. High pressures and temperatures cause the rocks to behave plastically enough to allow for material flow.
• The Core-Mantle Boundary: This occupies the last 200 km of the lower mantle, with a varying thickness of up to 20 km. In some regions of this layer, decreases in the rate of seismic wave propagation indicate the presence of partially melted rocks.