Editing Plate tectonics (section)

=== Driving forces related to gravity ===
Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above. In modern views, gravity is invoked as the major driving force, through slab pull along subduction zones.

Gravitational sliding away from a spreading ridge is one of the proposed driving forces: plate motion is driven by the higher elevation of plates at ocean ridges.{{sfn|Spence|1987}}{{sfn|White|McKenzie|1989}} As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increased distance from the ridge axis.

This force is regarded as a secondary force and is often referred to as "[[ridge push force|ridge push]]". This is a misnomer as there is no force "pushing" horizontally, indeed tensional features are dominant along ridges. It is more accurate to refer to this mechanism as "gravitational sliding", since the topography across the whole plate can vary considerably and spreading ridges are only the most prominent feature. Other mechanisms generating this gravitational secondary force include [[Lithospheric flexure|flexural bulging]] of the lithosphere before it dives underneath an adjacent plate, producing a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges. [[Mantle plume]]s and hot spots are also postulated to impinge on the underside of tectonic plates.

[[Slab pull]]: Scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches.{{sfn|Conrad|Lithgow-Bertelloni|2002}} Recent models indicate that [[back-arc basin|trench suction]] plays an important role as well. However, the fact that the [[North American plate]] is nowhere being subducted, although it is in motion, presents a problem. The same holds for the African, [[Eurasian plate|Eurasian]], and [[Antarctic plate|Antarctic]] plates.

Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them (see the paragraph on Mantle Mechanisms). This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of [[Reinout Willem van Bemmelen|van Bemmelen]]. This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin.{{sfn|Spence|1987}}{{sfn|White|McKenzie|1989}}{{sfn|Segev|2002}}