The movement of Earth’s tectonic plates is driven primarily by two gravity-driven mechanisms: ridge push and slab pull. These forces act in concert to maintain the global system of plate tectonics, linking the creation of new crust with the destruction of old crust. Understanding these mechanisms helps explain phenomena like earthquakes and volcanoes.
The Driving Force of Ridge Push
Ridge push originates at mid-ocean ridges, which are elevated, underwater mountain ranges that form where tectonic plates diverge. As hot, buoyant magma rises from the mantle and solidifies, it creates new oceanic lithosphere at these constructive plate boundaries. The newly formed crust is significantly hotter and therefore less dense than the older crust farther away from the ridge axis.
Because of this temperature and density difference, the mid-ocean ridge stands at a higher elevation than the surrounding ocean floor. Gravity acts upon this elevated mass, causing the newer, higher crust to slide down the gentle slope away from the ridge crest. This sliding motion exerts a horizontal force, pushing the entire tectonic plate away from the ridge. Ridge push initiates the lateral movement of the plate and is a component of seafloor spreading.
The Mechanism of Slab Pull
Slab pull occurs at subduction zones, which are destructive plate boundaries where one plate is forced beneath another and sinks back into the mantle. As oceanic lithosphere moves away from the mid-ocean ridge, it cools and thickens over millions of years. This cooling causes the plate material to contract and become substantially denser than the underlying, hotter asthenosphere.
Once the oceanic plate reaches a subduction zone, its density causes it to sink under its own weight into the mantle. The descending section of the plate, known as the slab, actively pulls the rest of the plate behind it. This gravitational sinking is the slab pull force, driving the plate toward the subduction zone. The cold, dense slab creates a tensile force that propagates throughout the entire plate as it descends to depths exceeding 100 kilometers.
Comparing the Forces and Their Global Impact
These two gravity-driven forces work together but contribute differently to the overall motion of the plates. Ridge push provides the initial outward thrust that separates the plates at the constructive boundary. However, the consensus among geophysicists is that slab pull is the dominant driver of plate movement.
Plates that are attached to subducting slabs, such as the Pacific and Nazca Plates, move much faster than plates that lack a subducting edge. Slab pull is estimated to account for a majority of the total force driving plate motion. This dominance is due to the gravitational energy released by the cold, dense slab sinking deep into the Earth’s interior.
The combined action of ridge push and slab pull creates a continuous, global conveyor belt system. Ridge push contributes to the opening of ocean basins, while slab pull facilitates the closure of ocean basins and the recycling of old crust. This dynamic balance maintains the relentless movement of the Earth’s surface.