Earth's Structure, Upper Mantle
Résumé
The upper mantle is defined as that part of the mantle between the crust and the phase transition of γ-olivine to perovskite. The total mass of the upper mantle is 1.06 × 1024 kg, about a quarter of the total mass of the mantle. Its volume, 2.95 × 1011 km3, is a third of the total volume of the mantle. 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...