The Mantle
The mantle is the second major part of the earth, it is the source region of most of the earth’s internal energy and is responsible for ocean floor spreading, continental drift, orogeny, and major earthquakes.Â
The mantle extends from below the Mohorovicic discontinuity to a depth of 2900 km.Â
It is 2865 km thick and constitutes about 83% of the earth by volume and 68% by mass.
Since the P and S waves record a definite increase in their velocities with depth, it is assumed that the material of the mantle is denser than that of the overlying crustal rocks.
The Olivine-pyroxene complex material, known as Pyrolite, exists in a solid state in the mantle.
The upper mantle extends to a depth of 1000 km, and the lower mantle extends from 1000 km up to the core boundary.
The upper mantle has two layers, which are distinguished based on the velocity of propagation of seismic waves.
The upper layer of the upper mantle between Mohorovicic discontinuity and a boundary at a depth of 410km, is characterized by a decrease in the seismic velocity and is called the Gutenberg layer. This along with the entire crust together constitute the lithosphere.
The lithosphere is underlain by asthenosphere (in between 80-200 km) and is a highly viscous, mechanically weak, and ductile, deforming region of the upper mantle. It has a low seismic velocity. It is separated from the rest of the mantle by the asthenosphere.
The asthenosphere is the main source of magma and the lithospheric plates/ continental plates move over this layer only.
The lower part of the upper mantle is known as Golitsyn’s layer in which the velocity of the seismic waves sharply rises, reaching about 11.3 to 11.4 km/sec at depths of 900-1000 km.
The discontinuity between the upper mantle and the lower mantle is known as Repetti Discontinuity (950 to 1000 km), here a rapid rise in the velocity of seismic waves has been observed.
The lower mantle is about 1900 kms thick and consists of two parts: (i) 1000kms to 2700 kms and (ii) 2700 kms to 2900 kms.
A further increase in the seismic velocity characterizes the upper layer. The velocity of P-waves reaches its maximum i.e. 13.7 km/sec at this layer. At a depth of 2700 to 2900 km, the propagation velocity of P-waves decreases to 12.6 km/sec. This may be due to the existence of a transitional layer
The mesosphere is the portion of the mantle that is just below the lithosphere and asthenosphere, but above the core.
