Phase Change Instability in the Mantle

In the presence of a temperature gradient, phase changes of the type believed to exist in the upper mantle, in which the less dense phase lies above the dense phase, may be unstable. Approximate calculations show such phase change instabilities are possible for both the 400-kilometer olivine-spinel phase transition and also for partial melting at shallower depths. The resulting flow patterns may provide a driving mechanism for the new global tectonics.