Convection Inside the Mantle

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Most of what we know about Earth’s interior comes from observing how seismic waves–mostly from earthquakes–bounce around. As our observations have gotten better–more seisometers, better imaging techniques–scientists have identified two large anomalies sitting near the bottom of the mantle. Known as large low-velocity provinces, or LLVPs, these zones take up continent-sized areas beneath parts of Africa and the Pacific.

Seismic waves show lower speeds in large, continent-sized zones that sit beneath Africa and the Pacific. These large low-velocity provinces (LLVPs) are outlined in red.

The LLVPs are hot, which would normally make them buoyant, but their stationary nature suggests they are made up of extremely dense material. Narrow plumes of hot material make their way up from the LLVPs to form volcanic hotspots like those that made the Hawaiian and Galapagos Islands. Balancing that upward convection is the downward convection of former tectonic material carried into the mantle at subduction zones.

What the LLVPs are made of remains an active research question. One suggestion is that they contain remnants of Theia, the planet thought to have impacted the proto-Earth to form our Moon. (Image credits: E. Garnero and C. Richardson; see also Physics Today)

Illustration showing convection in the Earth’s inner mantle. Former tectonic material subducts downward in dark blue zones. The large low-velocity provinces (LLVPs) are shown in orange. Hot-spot volcanic activity is seen at the surface (black triangles) above narrow plumes that lift material from the LLVPs toward the surface.