Is the mantle one big pot or is it double-decked?

The underlying “flow” of rocks in the mantle drives geological phenomena at Earth’s surface, ranging from earthquakes and volcanoes to the creation of mountains and oceans. As any blacksmith knows, when a hard, brittle material like iron is heated to temperatures and pressure within Earth’s mantle deform rocks so that they can flow like a slowly moving liquid. Hot materials rise and cold materials sink in circular convection celss. Scientists are pursuing evidence to determin if the entire mantle convects (right side of the diagram), or if the mantle is two-tiered (left side of diagram). (Illustration by Jayne Doucette, © Woods Hole Oceanographic Institution)