Mantle Convection and Rheology

Driven by internal heat, the solid mantle creeps and convects over millions of years, and the way mantle rock deforms, its rheology, sets the vigor, style, and pattern of this planet-scale flow.

Definition

Mantle convection is the slow, buoyancy-driven flow of the solid mantle that transports the Earth's internal heat to the surface; rheology is the description of how mantle rock deforms under stress, principally by thermally activated creep, which determines the viscosity governing that flow.

Scope

This topic covers thermal convection in the mantle and the rheology that controls it: the governing equations of buoyancy-driven creeping flow, the Rayleigh number and the onset and vigor of convection, the role of internal heating and basal heating, and the styles of convection from layered to whole-mantle. It treats the deformation mechanisms of mantle minerals, diffusion and dislocation creep, the strong temperature and pressure dependence of viscosity, and the influence of phase transitions on flow. The emphasis is on how the rheology of solid rock governs convective heat transport.

Core questions

• What equations govern buoyancy-driven creeping flow in the mantle?
• How does the Rayleigh number control the onset and vigor of convection?
• By what creep mechanisms does solid mantle rock deform?
• How do temperature, pressure, and phase transitions shape convective style?

Key concepts

• Buoyancy-driven creeping (Stokes) flow
• Rayleigh number and convective vigor
• Diffusion and dislocation creep
• Temperature- and pressure-dependent viscosity
• Internal heating and convective style

Key theories

Thermal convection and the Rayleigh number

Whether the mantle convects, and how vigorously, is governed by the Rayleigh number balancing buoyancy against viscous and thermal diffusion; above a critical value convection sets in, and the high mantle Rayleigh number implies vigorous, time-dependent flow.

Solid-state creep rheology

Mantle rock flows by the thermally activated migration of point defects and dislocations, giving a strongly temperature- and stress-dependent effective viscosity that controls the rates and patterns of convection and the strength of the lithosphere.

Mechanisms

Heat from radioactive decay and from the core warms the deep mantle, lowering its density and making it buoyant; because mantle rock deforms by solid-state creep, it rises in hot plumes and sinks in cold downwellings, transporting heat far more efficiently than conduction, while the strong dependence of viscosity on temperature and the effects of mineral phase transitions modulate whether flow spans the whole mantle or is partly layered.

Clinical relevance

Mantle convection is the engine of plate tectonics and the source of intraplate volcanism and dynamic topography; understanding it constrains the Earth's thermal history and links surface tectonics to the deep interior imaged by seismology.

History

Holmes proposed mantle convection as the driver of continental drift in the 1930s; quantitative treatment followed plate tectonics, with the 1974 McKenzie, Roberts, and Weiss study pioneering numerical mantle convection, and laboratory rock-deformation studies establishing the creep laws that set mantle rheology.

Debates

Layered versus whole-mantle convection

Geophysicists long debated whether the upper and lower mantle convect as separate layers, suggested by some geochemical reservoirs, or as a single whole-mantle system, supported by seismic images of slabs penetrating the lower mantle; current views favor largely whole-mantle flow with complications near the transition zone.

Key figures

• Arthur Holmes
• Dan McKenzie
• Shun-ichiro Karato
• Gerald Schubert

Seminal works

• schubert2001
• mckenzie1974
• karato2008

Frequently asked questions

How fast does the mantle convect?

Extremely slowly by everyday standards, with flow speeds of a few centimeters per year, comparable to the rate plates move and to how fast fingernails grow; convection is significant only because it operates over millions to billions of years.

What is the Rayleigh number?

It is a dimensionless measure of how strongly a fluid tends to convect, comparing the buoyancy that drives flow against the viscosity and thermal diffusion that resist it; the mantle's very high Rayleigh number means it convects vigorously despite being solid rock.