The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about 10 km (6.2 mi) under the oceans, and about 35 km (22 mi) under the continents) and ends at the top of the lower mantle, at about 670 km (420 mi). Temperatures range from around 900 K (627 °C; 1,160 °F) at the upper boundary with the crust to around 1,200 K (930 °C; 1,700 °F) at the boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55% olivine, 35% pyroxene, and 5–10% of calcium oxide and aluminum oxide minerals such as plagioclase, spinel, or garnet, depending on depth.
Seismic structure
The density profile through Earth is determined by the velocity of seismic waves. Density increases progressively in each layer, largely due to compression of the rock at increased depths. Abrupt changes in density occur where the material composition changes. The upper mantle begins just beneath the crust and ends at the top of the lower mantle. The upper mantle causes the tectonic plates to move. Crust and mantle are distinguished by composition, while the lithosphere and asthenosphere are defined by a change in mechanical properties. The top of the mantle is defined by a sudden increase in the speed of seismic waves, which Andrija Mohorovičić first noted in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho." The Moho defines the base of the crust and varies from 10 km (6.2 mi) to 70 km (43 mi) below the surface of the Earth. Oceanic crust is thinner than continental crust and is generally less than 10 km (6.2 mi) thick. Continental crust is about 35 km (22 mi) thick, but the large crustal root under the Tibetan Plateau is approximately 70 km (43 mi) thick. The thickness of the upper mantle is about 640 km (400 mi). The entire mantle is about 2,900 km (1,800 mi) thick, which means the upper mantle is only about 20% of the total mantle thickness.
The boundary between the upper and lower mantle is a 670 km (420 mi) discontinuity. Earthquakes at shallow depths result from strike-slip faulting; however, below about 50 km (31 mi), the hot, high-pressure conditions inhibit further seismicity. The mantle is viscous and incapable of faulting. However, in subduction zones, earthquakes are observed down to 670 km (420 mi).
Lehmann discontinuity The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at a depth of 220 km (140 mi) (Note that this is a different "Lehmann discontinuity" than the one between the Earth's inner and outer cores labeled in the image on the right.)
Transition zone The transition zone is located between the upper mantle and the lower mantle between a depth of 410 km (250 mi) and 670 km (420 mi). This is thought to occur as a result of the rearrangement of grains in olivine to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 670 km (420 mi), due to pressure changes, ringwoodite minerals change into two new denser phases, bridgmanite and periclase. This can be seen using body waves from earthquakes, which are converted, reflected, or refracted at the boundary, and predicted from mineral physics, as the phase changes are temperature and density-dependent and hence depth-dependent.
410 km discontinuity A single peak is seen in all seismological data at 410 km (250 mi), which is predicted by the single transition from α- to β- Mg2SiO4 (olivine to wadsleyite). From the Clapeyron slope this discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plumes.
670 km discontinuity This is the most complex discontinuity and marks the boundary between the upper and lower mantle. It appears in PP precursors (a wave that reflects off the discontinuity once) only in certain regions but is always apparent in SS precursors. It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths (640–720 km, or 397–447 mi). The Clapeyron slope predicts a deeper discontinuity in colder regions and a shallower discontinuity in hotter regions. This discontinuity is generally linked to the transition from ringwoodite to bridgmanite and periclase. This is thermodynamically an endothermic reaction and creates a viscosity jump. Both characteristics cause this phase transition to play an important role in geodynamical models.
Other discontinuities There is another major phase transition predicted at 520 km (320 mi) for the transition of olivine (β to γ) and garnet in the pyrolite mantle. This one has only sporadically been observed in seismological data. Other non-global phase transitions have been suggested at a range of depths.
Temperature and pressure Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 4,200 K (3,930 °C; 7,100 °F) at the core-mantle boundary. The highest temperature of the upper mantle is 1,200 K (930 °C; 1,700 °F). Although the high temperature far exceeds the melting points of the mantle rocks at the surface, the mantle is almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents melting because the temperature at which melting begins (the solidus) increases with pressure. Pressure increases as depth increases since the material beneath has to support the weight of all the material above it. The entire mantle is thought to deform like a fluid on long timescales, with permanent plastic deformation. The highest pressure of the upper mantle is 24.0 GPa (237,000 atm) compared to the bottom of the mantle, which is 136 GPa (1,340,000 atm). Estimates for the viscosity of the upper mantle range between 1019 and 1024 Pa·s, depending on depth, temperature, composition, state of stress, and numerous other factors. The upper mantle can only flow very slowly. However, when large forces are applied to the uppermost mantle, it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries. Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core.
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