Why do tectonic plates move? It is not a conveyor belt beneath your feet.
In short: For decades, scientists imagined tectonic plates riding atop mantle convection cells like luggage on a conveyor belt. Modern research reveals a more powerful engine: the plates themselves. As they cool and densify, they sink into the Earth, pulling the rest of the plate along with them in a process called slab pull.
The concept of slab pull fundamentally shifted our understanding of plate tectonics. In 1975, researchers Forsyth and Uyeda utilized inverse theory to demonstrate that the weight of cold, dense oceanic plates sinking into the mantle at trenches acts as the primary driver of plate motion. While early models relied on the friction of mantle convection to push plates, contemporary consensus suggests the asthenosphere does not directly drive motion in this way. Instead, slab pull and slab suction account for nearly all the force behind tectonic movement.
The physics of this mechanism is defined by the density contrast between the subducting slab and the surrounding asthenosphere. The force is calculated based on the slab's length above the 670-kilometer boundary and its mean density difference, which is estimated at 80 kilograms per cubic meter. This downward pull manifests in diverse geological outcomes, ranging from the aseismic back-arc extension seen in the Izu–Bonin–Mariana Arc to the intense seismic activity and back-arc thrusting characteristic of the Aleutian and Chile regions.
The evolution of the Farallon Plate offers a vivid case study of these forces in action. Over time, the plate underwent significant changes, including the Nevada, Sevier, and Laramide orogenies, followed by the Mid-Tertiary ignimbrite flare-up. Eventually, the slab broke off, leading to the formation of the Juan de Fuca and Cocos plates and the extension of the Basin and Range Province. While slab pull dominates in subduction zones, other forces like ridge push—which contributes only 5 to 10 percent of the total force—remain responsible for the movement of plates that are not currently being subducted, such as the North American and African plates.
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- Summary by Intellect · source: Slab pull
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