Ridge Push and Gravitational Head

Hello gene90,According to Turcotte and Schubert [1] the following forces are driving plate tectonics

• Ridge push
• Trench pull

Trench pull is the body force on the descending lithosphere which is the combination of two forces:
A gravitational body force “due to the temperature deficit relative to the adjacent mantle“ which increases the density and therefore the relative weight of the slab and the force due to the elevation of the olivine-spinel phase change . That means because of the lower temperature of the descending lithosphere the phase change occurs at shallower depth than in the surrounding mantle, the phase change in turn causes a higher density of the slab and therefore increases its weight .
Ridge push is a consequence of the elevation of the ridges. This creates a pressure head, which “drives the flow horizontally away from the center of the ascending plume”. That means basically that ridge push is an example of gravitational sliding, caused by the elevation difference between ridge and deep ocean basin. When we compare ridge push and trench pull - the calculations can be found on page 283 of [1] - we find that ridge push is by an order of magnitude smaller than trench pull ( ridge push for 100 million year old lithosphere is calculated with 3.41*10¹² N m^-1, whereas trench pull amounts to 4.9*10¹³ N m^-1). Turcotte and Schubert note that this difference probably mostly is set off by resistive forces encountered when the slab descends.-BerndP.S.
By the way, the calculation of trench pull does not consider frictional heating.

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References[1]Turcotte, Schubert (2002)
Geodynamics
Cambridge University Press

Hello Edge,Up to now I was under the impression that ridge push and trench (or slab) pull are the general accepted terms to denote the forces which drive plate tectonic. And while I can see why you judge ridge push as a misnomer - describing gravitational sliding as push may indeed lead to wrong conclusions - I’m not convinced that your critic of the term trench pull is valid. You wrote: I think that the following paragraph of Turcotte and Schubert (p. 9) describes such a condition: (An example of the elastic behavior of the lithosphere is the bending of elastic lithosphere at an ocean trench, described on page 127 of [1])Concerning whether there is a difference between ridge push and trench pull, it’s probably instructive to compare how they are calculated:Slab pull: with ρ₀ - density of slab, g - acceleration of gravity, α_v - coefficient of thermal expansion, b - length of descending slab, T_c-T₀) - temperature difference between slab and mantle, κ - thermal diffusivity, λ - overall length of slabThe slab is modeled analog to a descending plume of a two dimensional thermal convection cell in a fluid layer heated from below. The relevant term here is which is the downward buoyancy force per unit volume on an element of the plume (see [1], page 274-276)Ridge push: With g - acceleration of gravity, ρ_m - density of mantle, ρ_w - density of water, *α_v - coefficient of thermal expansion, (T₁-T₀) - temperature difference between mantle and seafloor, κ - thermal diffusivity, t - age of seafloorRidge push in contrast to slab pull increases proportional with the age of the sea floor. It is modeled based on thermal, isostatic compensation ([1], p. 221 and p. 283) As far I can see - and I have to admit that's not very far, for I'm not a trained geologist - there are relevant differences between these two forces.-Bernd

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References[1] Turcotte, Schubert(2002)
Geodynamics
Cambridge University PressThis message has been edited by bernd, 18-Sep-2005 11:25 PM