Question

The upper mantle is also known as .......... and ..........

Answer 1

Answer:

The topmost, thin layer of it is very similar to the Earth's crust. Together with the crust, we call it the lithosphere. Below the lithosphere is a layer of upper mantle called the asthenosphere.

Explanation:

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Answer 2

Answer:

Bullen’s layer B and Bullen’s layer C

Explanation:

Until the discovery of a major transition near 660-km depth, the upper mantle was better known as Bullen’s “layer B,” extending to 400-km depth. A sharp gradient or discontinuity in seismic velocity causes a bend in the travel time curves of P-wave arrivals near 20°, such that the slowness drops from more than 12 s/deg to 10 s/deg. This phenomenon was observed as early as 1931 by Jeffreys, who correctly adopted a suggestion by J.D. Bernal that it represents a high-pressure modification of olivine. The modern notion of the upper mantle actually extends into Bullen’s next layer, “layer C,” which has now been abandoned as a physically meaningful subdivision.

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