For the past two decades, seismological studies have identified anomalous low-velocity zones above the CMB, such as the large low-shear-velocity provinces (LLSVPs) beneath Africa and the Pacific.

These "Large-Low-Velocity-Provinces" (LLVPs ... made up of oceanic crust that was pushed into the mantle at subduction zones. This crustal material was then stirred through the mantle over ...