Plate Tectonics and Mantle Dynamics

The Earth's rigid outer shell is broken into plates that move over the convecting mantle, interacting at ridges, trenches, and transform faults to produce most of the planet's earthquakes, volcanism, and mountain building.

Definition

Plate tectonics is the theory that the Earth's lithosphere is divided into rigid plates moving relative to one another over the asthenosphere, with deformation concentrated at their boundaries, driven by forces arising from mantle convection.

Scope

This topic covers the theory of plate tectonics and its connection to mantle dynamics: the rigid-plate kinematics of motion on a sphere, the three boundary types (divergent, convergent, and transform), seafloor spreading and subduction, and the Euler-pole description of relative plate motion. It treats the forces that drive plates, especially slab pull and ridge push, the Wilson cycle of ocean opening and closing, and the link between surface plate motion and underlying mantle flow. The emphasis is on the kinematics and dynamics of plate motion.

Core questions

• How is the motion of rigid plates on a sphere described kinematically?
• What processes occur at divergent, convergent, and transform plate boundaries?
• What forces drive plate motion, and how important is slab pull versus ridge push?
• How is surface plate motion related to convection in the mantle?

Key concepts

• Lithospheric plates and the asthenosphere
• Divergent, convergent, and transform boundaries
• Seafloor spreading and subduction
• Euler poles and relative plate motion
• Slab pull, ridge push, and the Wilson cycle

Key theories

Rigid-plate kinematics on a sphere

McKenzie and Parker, and independently Morgan, showed that plate motions can be described as rigid rotations about Euler poles on a sphere, providing a precise kinematic framework that unified seafloor spreading, transform faults, and continental drift into plate tectonics.

Slab pull and plate driving forces

The dominant force on most plates is the gravitational pull of cold, dense subducting slabs, supplemented by ridge push from elevated spreading centers, integrating plate motion with the downwellings of mantle convection.

Mechanisms

New lithosphere forms and spreads at mid-ocean ridges, cooling and thickening as it ages until, dense enough, it sinks at subduction zones; the negative buoyancy of the sinking slab pulls the trailing plate while the elevated ridge pushes from behind, and these forces, coupled to viscous mantle flow, set the plate velocities, with deformation and seismicity concentrated at the boundaries between rigid plate interiors.

Clinical relevance

Plate tectonics explains the global distribution of earthquakes, volcanoes, and mountain ranges, frames long-term seismic and volcanic hazard, and accounts for the location of many mineral, hydrocarbon, and geothermal resources.

History

Building on Wegener's continental drift and Hess's seafloor spreading, the mid-1960s recognition of transform faults by Wilson and the rigid-plate formulations of McKenzie, Parker, Morgan, and Le Pichon established plate tectonics, the unifying theory of the solid Earth, within a few years.

Key figures

• Dan McKenzie
• Jason Morgan
• John Tuzo Wilson
• Xavier Le Pichon

Seminal works

• mckenzie1967
• morgan1968
• turcotte2014

Frequently asked questions

What are the three main types of plate boundary?

Divergent boundaries, where plates move apart and new crust forms, as at mid-ocean ridges; convergent boundaries, where plates collide and one may subduct beneath another; and transform boundaries, where plates slide past each other horizontally, as along the San Andreas Fault.

Why do most earthquakes happen at plate boundaries?

Plate interiors are relatively rigid and move almost as units, so the relative motion is accommodated mainly at their edges, where stresses build and release as earthquakes; this concentrates seismicity in narrow belts that trace the plate boundaries.