The transition zone is part of the Earth’s mantle, and is located between the lower mantle and the upper mantle, between a depth of 410 and 660 km (250 to 400 miles). The Earth’s mantle, including the transition zone, consists primarily of peridotite, an ultramafic igneous rock.
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 km (250 to 400 miles). This is thought to occur as a result of rearrangement of atoms in olivine (which constitutes a large portion of peridotite) at a depth of 410 km, to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 660 km, evidence suggests that atoms rearrange yet again to form an even denser crystal structure. 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.
A single peak is seen in all seismological data at 410 km which is predicted by the single transition from α- to β- Mg2SiO4 (olivine to wadsleyite). From the Clapeyron slope the Moho discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plumes.
This is the most complex discontinuity seen. It appears in PP precursors (a wave which 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 miles). The Clapeyron slope predicts a deeper discontinuity in cold regions and a shallower discontinuity in hot regions. This discontinuity is generally linked to the transition from ringwoodite to bridgmanite and periclase. This is thermodynamically an exothermic reaction and creates a viscosity jump. Both characteristics cause this phase transition to play an important role in geodynamical models. Cold downwelling material might pond on this transition.
There is another major phase transition predicted at 520 km for the transition of olivine (β to γ) and garnet in the pyrolite mantle. This one is only sporadically been observed in seismological data.