The mantle

What it is

The mantle is a dense, hot layer of rock roughly 2,900 kilometres thick, containing more iron, magnesium and calcium than the crust, and hotter and denser with depth as temperature and pressure rise.^[1,2]

It is solid. Subjected to heat and pressure over millions of years, solid rock softens and changes shape, exactly as a solid metal such as steel does when worked. Flow and melting are not the same thing.^[1,2]

Where melt actually is

Melt exists in the mantle, but locally and in small fractions. Below the lithospheric plates, at some depth, the mantle is partially molten and can flow slowly under sustained stress — a small proportion of liquid distributed through solid rock, not a liquid layer.^[1]

Magma forms where conditions allow some rock to melt. It rises because it is lighter than the solid rock around it, and collects before some of it reaches the surface. Most magma originates near active plate boundaries or at hot spots, not everywhere beneath the crust.^[1,2]

Circulation, and what is not known about it

Heat from radioactive decay and from the planet’s formation drives convection through this solid material, and that circulation is coupled to the motion of the plates above.^[1,2]

Its geometry is another matter. How many convection cells exist, where and how they originate, and what structure they take are unresolved. The tidy cells drawn in textbook cross-sections are illustrations of a principle, not maps of the mantle.^[1]

Tomographic imaging has resolved broad, quasi-vertical conduits beneath many hotspots, reaching from the core–mantle boundary up to about 1,000 kilometres depth. Their vertical orientation suggests that background circulation below that depth is very sluggish — an inference from an image, offered as one.^[1,2]

Commonly misunderstood