The Engine Beneath Our Feet
Earth's mantle — the 2,900-km-thick layer between crust and core — convects like a pot of extremely viscous syrup heated from below. Despite viscosities a billion billion times that of water, mantle rock flows at centimeters per year over millions of years. This convection is the fundamental engine of plate tectonics: it drives plate motion, creates mid-ocean ridges, powers subduction, and generates the magnetic field through core convection.
Rayleigh-Bénard Instability
When a fluid layer is heated from below, convection begins once the Rayleigh number exceeds a critical value (~1100). Earth's mantle Ra ≈ 10⁷, so convection is vigorous and time-dependent. Hot material rises in plumes and sheets; cold material sinks as slabs. The boundary layers — hot at the base, cold at the top — are where temperature changes most rapidly, analogous to the lithosphere (cold) and D'' layer (hot).
Convection Patterns
Mantle convection exhibits complex, three-dimensional, time-dependent patterns. Upwelling plumes (hotspots like Hawaii) are narrow and cylindrical; downwelling slabs (subduction zones) are planar sheets. Large-scale flow organizes into cells spanning thousands of kilometers. At high Rayleigh numbers, the flow becomes increasingly chaotic with smaller-scale instabilities, time-varying planforms, and complex interactions between plumes and plates.
Heat Budget and Cooling
Earth loses about 44 terawatts of heat through its surface — roughly half from radioactive decay in the mantle and crust, and half from primordial heat of formation. Convection transports this heat far more efficiently than conduction alone, characterized by the Nusselt number Nu ~ Ra^(0.3). Without mantle convection, Earth's surface heat flux would be ~8× lower, the interior far hotter, and plate tectonics as we know it would not exist.