Can sharp seismic discontinuities be caused by non-equilibrium phase transformations?

A long-posed problem of the seismic discontinuities at the depths of 410 and 660 km in Earth's mantle is whether these discontinuities are caused by phase transitions alone or together with chemical layering. Among several tests, the sharpness of the boundary is one of the most crucial. Recent data suggest that the transition region thickness is less than 4 km for both the 410 and the 660 km discontinuities. This may be smaller than predicted by phase equilibria. We suggest that if there is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convective flow, the transformation in rising or descending material does not take place until the metastable overshoot becomes equal to the nucleation barrier. An avalanche-like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity. In this case, the topography of the phase boundary is also substantially modified.