Is the mantle one big pot or double-deckered?
The underlying "flow" of materials 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 just below its melting point, it becomes malleable. Similarly, the high temperatures and pressure within Earth??A?s mantle deform rocks so that they can flow like a slowly moving liquid. Hot materials rise and cold materials sink in circular convection cells. Scientists are pursuing evidence to determine if the entire mantle convects (right side of diagram), or if the mantle is two-tiered (left side of diagram).
Left side of diagram: Older, colder (and denser) oceanic plates collide with other plates and plunge back into the mantle at subduction zones, forming deep ocean trenches and volcanic island arcs.
Middle: At mid-ocean ridges, mantle rock rises, melts, and erupts to form new oceanic crust and volcanic mountain chains. Newly created seafloor crust spreads outward from the ridges.
Right side of diagram: In some seafloor regions, unusually hot areas of the mantle form narrow, isolated plumes that erupt through rigid oceanic plates to form volcanic islands and seamounts.