As Earth’s tectonic plates shift and collide, slabs of cold, dense oceanic crust get pushed down into the mantle. The subduction process carries volatile compounds and water into the mantle along with crustal material that has a different isotopic signature from primitive mantle material. Heat and pressure in Earth’s interior can transform the subducted crust into different minerals and may eventually return it to the surface in the magma that upwells and forms new crust. However, the depth to which crust material descends during that cycling is still a subject of debate among geophysicists and is key to understanding heterogeneities in the mantle structure.
Knowledge of Earth’s interior structure is based on inferences of how seismic waves travel at different depths. The boundary between the upper and lower mantle is marked by a sharp change in density, and therefore of seismic-wave velocities, at a depth of 660 km. Toward the...
