Do subducting slabs stop at the base of the upper mantle?
Seismic tomography is a method for studying the Earth’s interior. Using many earthquakes throughout the globe whose waves pass through a given point in the Earth’s interior, the seismic velocity of the rocks at that point can be determined. Any differences in seismic velocity are the result of small differences in density of the rocks. Since the composition of the mantle is all ultramafic, density differences are the result of temperature differences from place to place. So seismic tomography measures temperature variations in the mantle. Images of the mantle produced in this way show subducting slabs as zones of high velocity (lower temperature rocks) angling downward from their respective trenches. In some subduction zones, these high velocity zones pass through the 670 km seismic discontinuity and continue downward through the lower mantle, in some cases nearly to the core mantle boundary. In other cases the high velocity zone flattens out just above 670 km. Clearly, some slabs pass
