The Mantle The second major part of the earth is the mantle which is the source-region of most of the earth’s internal energy and of forces responsible for ocean-floor spreading, continental drift, orogeny and major earthquakes.
The mantle extends from below the Mohrovicic discontinuity (which separates it from the overlying crust) up to a depth of 2900 kms. Thus, its thickness is about 2865 kms. It forms about 83 per cent of the earth by volume and 68 per-cent by mass.
Since the P and S waves record a definite increase in their velocities with depth, it is logically assumed that the material of the mantle is more dense than that of the overlying crustal rocks.
The material is olivine-pyroxene complex , which exists in a solid state. It is believed that the upper mantle has amix of 3 parts of ultramafic rix. ks and one part of basalt. This mix is known as Pyrolite.
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The upper-mantle extends upto a depth of 1000 kms. The lower mantle extends from 1000 kms to the core boundary (Gutenberg- Weichert discontinuity).
The upper mantle is consisting of two layers which are distinguished on the basis of velocity of propagation of seismic waves. The upper layer of the upper mantle lying between Mohorovicic discontinuity and a boundary at a depth of 410 km is characterized by a decrease in the seismic velocity.
This layer is called the Gutenberg layer. The crust and the upper part of the Gutenberg layer together constitute what is known as lithosphere. The lithosphere is underlain
by ‘asthenosphere’ which is a layer of virtually of no strength to resist deformation and it is the low seismic velocity layer. The asthenosphere is situated somewhat between 70 to 220 km depth. To be more precise, the lithosphere is separated from the rest of the mantle by the asthenosphere.
The lower part of the upper mantle is known as Golitsyn’s layer in which the velocity of the seismic waves sharply rise, reaching about 11.3 to 11.4 km/second at the depths of 900-1000 km.
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The lower-mantle is about 1900 kms thick, and consists of two parts: (i) 1000 kms to 2700 kms and (ii) 2700 kms to 2900 kms. The upper layer is characterized by a further increases in the seismic-velocity. The velocity of P-wave reaches its maximum i.e 13.7 km/sec at this layer. At a depth of 2700 to 2900 kms, the velocity of propagation decreases to 12.6 km/second for the primary-waves. This may
(i) Density break
At a depth of 80 km, density changes from 3.36 to 3.87.
(ii) Gravity break
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At a depth of 150 km, gravity changes from 984 cm/sec2 to 974 cm/sec2 till it reaches a depth of 1200 km.
(iii) Seismic-discontinuity
At 410 km depth, there is a decrease in the seismic velocity.
(iv) At 700 km. depth, there changes the capability of the materials in storing the elastic-strain energy. Up to 700 kms the capability is more.
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(v) Repetti discontinuity
At 950 to 1000 km depth there is a rapid rise in the velocity of seismic waves.
(vi) Gravity break
At 1200 km depth, gravity attains its minimum value i.e. 974 cm/sec2, thereafter it rises up to 1068cm/sec2 at the core-boundary.
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(vii) Seismic-discontinuity
At a depth of 2700 km. the seismic velocity reaches its maximum i.e. 13.7 km/sec and gradually it decreases.