Mantle Convection and Plate-Driving Forces

Plate motion is sustained by the slow convective overturn of the mantle, with the sinking of dense subducted slabs and the gravitational sliding of plates away from ridges providing the dominant driving forces.

Definition

Plate-driving forces are the set of gravitational and viscous forces — chiefly slab pull, ridge push, and mantle drag — that arise from thermal convection in the mantle and govern the speed and direction of lithospheric plate motion.

Scope

This topic covers the dynamics behind plate tectonics: thermal convection in the mantle, the balance among slab pull, ridge push, and basal drag, and the role of mantle plumes. It treats the physical drivers of plate motion, complementing the kinematic description of boundaries and adjoining the deeper concerns of geophysics.

Core questions

• Is the mantle's convection whole-mantle or layered?
• Which forces dominate plate motion, and how do they scale with slab length and plate area?
• How do mantle plumes relate to plate-scale convection?

Key theories

Slab pull as the dominant driving force

Forsyth and Uyeda showed by force-balance analysis that plates attached to long subducting slabs move fastest, implying that slab pull is the primary driver of plate motion while ridge push and basal drag are secondary.

Mantle plume hypothesis

Morgan proposed that narrow upwellings of hot material from the deep mantle, fixed relative to plate motion, produce hotspot volcanic tracks and represent a convective component distinct from plate-scale flow.

Mechanisms

Heat from the core and from radioactive decay drives slow creep of the solid mantle. Cold, dense oceanic lithosphere sinks at subduction zones, pulling the trailing plate (slab pull); the elevated ridge exerts a gravitational push on the plate (ridge push); and viscous coupling between the plate base and underlying mantle flow can either drive or resist motion. Localized plumes add a separate, largely vertical convective flux.

Clinical relevance

Understanding driving forces explains why some plates move faster than others, constrains models of long-term sea-level and climate change driven by changing mantle flow, and informs interpretations of intraplate volcanism and uplift.

History

Arthur Holmes suggested mantle convection as a driving mechanism in the 1930s. After plate tectonics was established, quantitative analyses in the 1970s — notably Forsyth and Uyeda's force-balance study and Morgan's plume hypothesis — clarified the relative contributions of the candidate driving forces.

Debates

Whole-mantle versus layered convection

Whether the mantle convects as a single layer or as two separately circulating layers has long been contested; seismic tomography showing slabs penetrating the lower mantle favors whole-mantle convection, but geochemical reservoirs suggest some persistent layering.

Key figures

• Donald Forsyth
• Seiya Uyeda
• W. Jason Morgan
• Arthur Holmes

Seminal works

• forsythuyeda1975
• morgan1971

Frequently asked questions

What is the main force that moves tectonic plates?

Most evidence points to slab pull — the weight of cold, dense lithosphere sinking at subduction zones — as the dominant driving force, with ridge push and mantle drag playing smaller roles.