Some subducted slabs seem not to be able to penetrate the major discontinuity in the mantle that lies at a depth of about 670 kilometers whereas other subducted oceanic plates can penetrate all the way to the core-mantle boundary.
Some subducted slabs seem to have difficulty to penetrate the major discontinuity in the mantle, marking the boundary between the upper mantle and lower mantle, that lies at a depth of about 670 kilometers. Other subducted oceanic plates can penetrate all the way to the core-mantle boundary.