Structure of the Earth and internal envelopes: the PREM model
A sequence to better understand how the use of seismic waves has enabled the study of the different layers that make up the interior of the Earth. The Earth is made up of three major layers: the crust, the mantle, and the core. The transition from one layer to the other is marked by seismic discontinuities. The Moho, or Mohorovicic discontinuity, separates the crust from the mantle. The Moho is located at a depth of 30 km beneath the continents and 10 km beneath the oceans. Two types of crust are therefore distinguished: the thicker continental crust and the thinner oceanic crust, thus explaining the bimodal distribution of altitudes on Earth. Deeper down, the Gutenberg discontinuity, located at a depth of 2,900 km, separates the mantle from the core.
The mantle can be subdivided. A distinction is made between the upper and lower mantle. The boundary between the two is located at a depth of 700 km. The Earth's core also comprises two parts, corresponding to the liquid outer core and the solid inner core, or core. The boundary between the two, or Lehman discontinuity, is located at a depth of 5,400 km. The upper mantle is made up of the lithospheric upper mantle and the asthenospheric upper mantle. The boundary separating them is located at a depth of 150 km and corresponds to the LVZ, or Low Velocity Zone, i.e., an area of lower seismic wave speed. The importance of the LVZ lies in the fact that it allows us to distinguish two major geological entities. Above the LVZ, we can distinguish the lithosphere, which is a cold, rigid, and brittle geological entity, and at the LVZ is the asthenosphere, a hotter, ductile, and deformable geological entity.
We can conclude this sequence by specifying the nature of the rocks that make up the envelopes: the continental crust is mainly made up of granitoids, rocks related to granite; The oceanic crust is composed of basalts on the surface and gabbros deeper down; the mantle is composed of peridotite and the core contains iron and nickel.