Roughly 3000 km below us, almost halfway to the center of Earth, lies the boundary between the solid rock mantle and the predominantly liquid-iron outer core. Grade-school textbooks typically depict Earth’s interior as basic, brightly colored concentric shells of the crust, mantle, outer core, and inner core. But recent research dramatically alters that simplistic view to include two enormous anomalous structures sitting at the base of the mantle, on top of the core–mantle boundary. Positioned on nearly opposite sides of the globe, one anomaly is underneath the Pacific Ocean, and the other sits beneath western Africa and parts of the Atlantic Ocean.
The two structures are the size of large continents and extend in some places more than 1000 km vertically into the mantle. Those massive features, called large low-velocity provinces (LLVPs), are characterized by significant reductions in seismic wave velocities for both shear and compressional waves (commonly known as...
