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Plate Tectonics: Influence of Gravity on Plate Movement

AS, A-Level
AQA, Edexcel, OCR, Eduqas, WJEC

Last updated 22 Mar 2021

More recent theories of plate movement consider the effect of gravitational forces acting within the crust that also contribute to plate movement, although the impact is thought to be weaker than the effect of convectional movement in the mantle.

However, there is debate about the relative influence of the various forces and they may account for different plate motions in contrasting parts of the earth’s surface.

Gravitational sliding away from a spreading ocean ridge takes place with plate movement driven because of the higher elevation of plates at ocean ridges.

As fresh magma wells up at mid-ocean ridges to form new young, oceanic lithosphere, a higher elevation is formed at spreading ridges. The new oceanic crust gradually cools and thickens with age and is pushed ‘downhill’ as new magma emerges from the active zone of divergence behind it (and thus adds distance from the ridge). This force is regarded as a secondary force and is referred to by some as ‘ridge push’.

Slab pull is thought to be a more significant gravitational force acting on plates. In the current understanding of plate motion the movement is driven by the weight of cold, older, dense plate material sinking into the mantle at deep ocean trenches and pulling the rest of the plate slab with them as gravity causes them to slide downwards.

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