and the lower mantle between a depth of and . 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 , 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 , 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 playing an important role in geodynamical models. Other discontinuities There is another major phase transition predicted at 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 at the upper boundary with the crust to approximately at the core-mantle boundary. The highest temperature of the upper mantle is . 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