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:
As to 'pull', I know of no conditions under which a lateral, tensional force can exerted on the oceanic lithosphere and be transmitted throughout the length of the plate from trench to ridge. This is what the lay person would think of as 'pull', as in pulling on a chain. As near as I can tell, that is not what T&S describe, however.
I think that the following paragraph of Turcotte and Schubert (p. 9) describes such a condition:
The negative buoyancy of the dense rocks of the descending lithosphere results in a downward body force. Because the lithosphere behaves elastically, it can transmit stresses and act as stress guide. The body force acting on the descending plate is transmitted to the surface plate, which is pulled towards the ocean trench. This is one of the important forces driving plate tectonics and continental drift. It is known as slab pull.
(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:
Fb1=2*ρ0*g*αv*b*(Tc-T0)(κ*λ/2*π*u0)1/2
with ρ
0 - density of slab, g - acceleration of gravity, α
v - coefficient of thermal expansion, b - length of descending slab, T
c-T
0) - temperature difference between slab and mantle, κ - thermal diffusivity, λ - overall length of slab
The 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
ρ0*g*αv*b*(Tc-T0)
which is the downward buoyancy force per unit volume on an element of the plume (see [1], page 274-276)
Ridge push:
FR=g*ρm*αv*(T1-T0)[1+2/π*(ρm*αv*(T1-T0))/( ρm-ρv)]*κ*t
With g - acceleration of gravity, ρ
m - density of mantle, ρ
w - density of water, *α
v - coefficient of thermal expansion, (T
1-T
0) - temperature difference between mantle and seafloor, κ - thermal diffusivity, t - age of seafloor
Ridge 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
References
[1] Turcotte, Schubert(2002)
Geodynamics
Cambridge University Press
This message has been edited by bernd, 18-Sep-2005 11:25 PM