Ridge Push and Gravitational Head

Thanks Percy. Since this is a provisional thread I gather I should just edit the first post?The slab-pull view was justified as being called the primary mechanism from a fairly impressive couple of graphs of plate velocity vs length of the ridge and length of the subduction.There was little correlation between plate velocity and the length of MORs but there was a readily apparent positive correlation between the length of subduction zones and the velocity of the plate. So slab pull looks like the dominant force.Maybe slab pull is more important, but it's not the whole story. My structural text (Structural Geology, by Davis and Reynolds) says that based on data on the current stress fields of world (from Zoback, 1992), ridge push is the primary compressive force on midplate North America today. And this force is apparently driven by topography through a process called "gravitational head"--the force of gravity on MORs and also high mountain regions like the Himalayas, the Andes, and even the Rockies does not outpace the bouyancy force--which is somehow increased by plate-driving forces?--in the mantle and this causes extension. Evidence was cited that ridge-push is real (shallow earthquakes in the crust just off the ridges that represent compression) and a value of about 20-30 MPa of stress was ascribed to the process.Writing this summary makes me feel like I understand it better than the last I time I read it through, so I think I'll ask a more general question: is this most of the story, or are there other components that should be included on ridge-push? Does this extension at the MOR in any way pump additional magma in?{Note from Adminnemooseus - MOR = mid ocean ridge}This message has been edited by gene90, 09-14-2005 05:35 PMThis message has been edited by Adminnemooseus, 09-16-2005 01:08 AM

Welcome back!The push vs. pull issue came up a in a thread a few months ago, but it didn't receive enough discussion. Slab-pull as the primary mechanism is, of course, consistent with Baumgardener's proposal of runaway subduction. Ridge-push has a more traditional feel to it, but when push came to shove (so to speak) during the discussion, we couldn't find any conclusive research either way. The evolutionists concluded that neither ridge-push nor slab-pull could be the primary factors, since each should be revealed in either compressive or stretching stress and strain in the region between ridge and subduction zone, and there was nothing we could find that indicated any evidence of this. The Creationists concluded that slab-pull was the primary mechanism.Anyway, if you can elaborate on your question a bit more to describe why your profs bought into slab-pull, what you thought was lacking in the textbooks, and why ridge push interests you as a possibility, then I'll release this. Nothing long, a couple paragraphs should be sufficient to introduce the topic.

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--	Percy
	EvC Forum Director

Bump for Percy; I don't think he realizes the OP was edited.This information's out of my league, so I feel uncomfortable promoting it on my own.

I'm not sure what the relevance to the C/E debate is.It seems to be a potentially interesting, albeit highly technical discussion of sea floor spreading mechanisms, of limited appeal to most of the forums membership.That said, I'll give it a promotion.Adminnemooseus

Thread copied to the Ridge Push and Gravitational Head thread in the Geology and the Great Flood forum, this copy of the thread has been closed.