The transition zone is the part of Earth's mantle that is located between the lower and the upper mantle, most strictly between the seismic-discontinuity depths of about 410 to 660 kilometres (250 to 410 mi), but more broadly defined as the zone encompassing those discontinuities, i.e., between about 300 and 850 kilometres (190 and 530 mi) depth.
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 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.
It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths (640–720 kilometres or 400–450 miles).