The process where the Earth’s rigid outer layer, the lithosphere, is dragged downward into the mantle by the force of gravity is called Slab Pull. This movement is one of the primary mechanisms driving plate tectonics, the theory that describes the large-scale motion of plates that make up our planet’s outer shell. The lithosphere is composed of the crust and the uppermost part of the mantle, acting as a cool, stiff layer that floats atop the hotter, more ductile asthenosphere.
The Mechanism Called Slab Pull
Slab Pull is a force generated by the negative buoyancy of the oceanic lithosphere as it ages and cools. As a section of oceanic plate moves away from a mid-ocean ridge, it loses heat, causing it to contract and increase in density. This colder, denser material eventually becomes heavier than the surrounding, hotter mantle material it rests upon. Gravity then acts upon this dense, over-weighted section of plate, causing it to sink deeper into the Earth’s interior. The force of this sinking segment, often referred to as the “slab,” pulls the entire attached tectonic plate behind it.
Subduction Zones: Where the Pull Begins
Slab Pull occurs at a subduction zone, which is the geological setting marking a convergent plate boundary. Subduction is the process where one tectonic plate, typically the denser oceanic lithosphere, descends beneath a less dense plate. This downward movement forms a deep oceanic trench at the surface. As the leading edge of the plate bends and plunges, it creates the “slab,” which is the physical entity that experiences the gravitational pull.
Slabs can descend at angles typically ranging from 25 to 75 degrees, with steeper angles often associated with faster plate movement. The subducting slab can penetrate hundreds of kilometers into the mantle, sometimes reaching the boundary between the upper and lower mantle at about 670 kilometers depth. The deeper the slab can sink while maintaining its structural integrity, the greater the downward pull exerted on the rest of the plate.
Slab Pull Versus Other Tectonic Forces
Slab Pull is considered the dominant force contributing to plate motion, alongside Ridge Push and Mantle Drag. Ridge Push is a gravitational sliding force that originates at mid-ocean ridges, where new oceanic lithosphere is created. As hot, buoyant mantle material rises and creates a topographic high, gravity causes the newly formed plate to slide away from the elevated ridge crest. This force acts as a gentle push from behind, helping to move the plate laterally.
Mantle Drag, also known as viscous drag, refers to the forces exerted by the convective currents in the asthenosphere. Mantle flow can both drag plates along or resist their motion, depending on the local flow direction relative to the plate. Scientific consensus holds that Slab Pull is the most powerful force, often accounting for the majority of the total energy that drives plate motion. Estimates suggest that Slab Pull can provide upwards of 90% of the driving force for plates that contain large subduction zones.