Why tectonic plates move

The forces on the table

For most of the twentieth century, explanations emphasised mantle convection and seafloor spreading. Some geologists also argued that magma intruding at spreading ridges supplies an additional propelling force — ridge push.^[1]

Since the 1990s the weight of opinion has moved to slab pull: the gravity-controlled sinking of cold, dense oceanic lithosphere into a subduction zone, dragging the rest of the plate behind it. Subduction, on this view, matters more than spreading.^[1]

Lithoen records no single sufficient mechanism. Convective flow, ridge push and slab pull have each been argued to contribute, and their relative importance has shifted with the evidence rather than being settled by it.^[1]

What the plates are

The moving objects are lithospheric plates — crust plus rigid uppermost mantle — riding over a weaker, slowly flowing asthenosphere. They are not continents, and what they move over is not liquid.^[1,2]

What limits the answer

Convection cannot occur without a heat source, and Earth has two: radiogenic decay and residual heat from formation. That much is clear. The geometry of the resulting circulation is not.^[1,2]

How many convection cells exist, where they originate and what structure they take remain open questions, and how heat concentrates into cells at all is unresolved. A confident account of what drives the plates would have to answer these, and none does yet.^[1,2]

The constraint that holds regardless

Whatever the force balance, the bookkeeping is fixed: Earth’s size has not changed significantly in the last 600 million years, so crust must be destroyed at about the rate it is created. Creation happens at spreading centres, destruction at subduction zones.^[1,2]

Commonly misunderstood