Transition zone (Earth)

The mantle was divided into the upper mantle, transition zone, and lower mantle as a result of sudden seismic-velocity discontinuities at depths of 410 and 660 kilometres (250 and 410 mi). This is thought to occur as a result of rearrangement of grains in olivine (which constitutes a large portion of peridotite) at a depth of 410 kilometres (250 mi), to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 660 kilometres (410 mi), evidence suggests 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 – phase transition

A peak is seen in seismological data at about 410 kilometres (250 mi) as is predicted by the transition from α- to β-Mg₂SiO₄ (olivine to wadsleyite). From the Clapeyron slope, this change is predicted to occur at shallower depths in cold regions, such as where subducting slabs penetrate into the transition zone, and at greater depths in warmer regions, such as where mantle plumes pass through the transition zone. Therefore, the exact depth of the "410 km discontinuity" can vary.