Mantle Convection

Rayleigh-Bénard cells in Earth's mantle — hot rises, cold sinks

2.5×10⁶
Rayleigh Number
—
Nusselt Number
—
Active Cells
Temperature ΔT2500 K
Viscosity Contrast10³
Speed1.0×
The mantle convects on geological timescales despite being solid rock — its effective viscosity (~10²¹ Pa·s) allows flow over millions of years. The Rayleigh number Ra = αΔTgd³/(κν) measures convective vigor; Earth's mantle has Ra ~ 10⁶–10⁷. Mantle plumes rise from the core-mantle boundary (D″ layer), creating hotspot volcanism (Hawaii, Iceland). Cold oceanic plates sinking at subduction zones drive slab pull — likely the dominant plate-driving force.