The Earth’s outer shell is broken into several large, rigid tectonic plates that are constantly in motion. The Mid-Ocean Ridge (MOR) is a vast, submerged mountain chain that circles the globe. The MOR is a constructive plate boundary where hot magma rises from the mantle to create new oceanic crust. This process causes the plates on either side to move away from the ridge axis. Plate movement results from multiple powerful forces working in concert.
The Gravitational Force of Ridge Push
The Mid-Ocean Ridge is elevated because the material beneath it is hot and buoyant. As magma rises and cools at this divergent boundary, it forms new, less dense lithosphere. This new crust sits atop a broad, high-elevation bulge, sometimes rising 4,000 meters above the abyssal plain.
The force known as “ridge push” originates from the gravitational instability of this elevated structure. As the oceanic lithosphere moves away from the ridge crest, it cools, thickens, and increases in density. Gravity acts on this denser material, causing it to slide down the gentle slope. This movement exerts a force that helps push the entire tectonic plate away from the spreading center.
Ridge push is a localized force, acting primarily at the divergent boundary to initiate spreading. Models suggest this force accounts for only 5 to 10% of the total energy driving plate motion. Its importance is primarily in initiating movement for plates that are not being subducted elsewhere.
Slab Pull: The Dominant Driver of Plate Movement
While ridge push initiates movement, the most powerful and dominant mechanism driving plate motion is “slab pull.” This force acts at the opposite end of the plate boundary, in subduction zones. As the oceanic plate travels across the ocean basin, it continuously cools and thickens over millions of years, becoming progressively colder and denser.
When this old, dense oceanic lithosphere meets another plate, it sinks under its own weight back into the mantle via subduction. The descending portion, or “slab,” is significantly denser than the surrounding asthenosphere. The gravitational force acting on this sinking slab effectively pulls the rest of the plate along behind it.
The magnitude of slab pull is related to the density difference between the slab and the mantle, the length of the subducted slab, and the age of the crust. Plates attached to subducting slabs, such as the Pacific and Nazca plates, move at the fastest rates globally. This gravitational pull is the strongest force, accounting for the vast majority of the total driving force.
The Underlying Engine of Mantle Convection
The ultimate source of energy for all tectonic plate movement originates from the Earth’s internal heat, transferred through mantle convection. Heat generated by the decay of radioactive isotopes flows outward from the mantle and core. This heat transfer causes the mantle, composed of solid silicate rock, to behave like a viscous fluid over geological timescales.
Warmer, less dense material slowly rises, while cooler, denser material sinks, creating vast convection cells. This slow motion is directly responsible for the creation of new crust at the Mid-Ocean Ridge (upwelling) and the destruction of old crust at subduction zones (downwelling). Mantle convection provides the necessary conditions for the existence of both ridge push and slab pull.
The flow of the viscous mantle beneath the lithosphere also results in a force called “viscous drag” or “mantle drag.” Depending on the direction of the mantle flow, this drag can either assist or resist the motion of the overlying tectonic plate. However, convection’s primary role is to act as the underlying engine that generates the heat and movement, which manifests as the gravitational forces of ridge push and slab pull acting on the plate edges.