Earth's thin crust softens considerably when it dives down into the Earth attached to a tectonic plate. That is demonstrated by X-ray studies carried out on a mineral which occurs in large quantities in basaltic crust. This softening can even cause the crust to peel away from the underlying plate. The delaminated crust has different physical properties from the rest of the mantle, which may explain anomalies in the speed with which seismic waves propagate through the mantle.
