Transition zone (Earth)
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.
410 km (250 mile) discontinuity
A single peak is seen in all seismological data at 410 km (250 miles) which is predicted by the single transition from α- to β- Mg2SiO4 (olivine to wadsleyite). From the Clapeyron slope the discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plumes.
660 km (400 mile) discontinuity
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.
There is another major phase transition at 520 km which is possibly split into two transitions at 500 km (310 miles) and 560 km (348 miles). Multiple phase transitions are needed to explain this, probably transitions of olivine (β to γ) and garnet in the pyrolite mantle.
Other non-global phase transitions have been suggested at a range of depths. 
- C.M.R. Fowler, The Solid Earth (2nd Edition), Cambridge University Press 2005.