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Author | Topic: Ridge Push and Gravitational Head | |||||||||||||||||||||||
gene90 Member (Idle past 3851 days) Posts: 1610 Joined: |
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 PM This message has been edited by Adminnemooseus, 09-16-2005 01:08 AM
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Admin Director Posts: 13040 From: EvC Forum Joined: Member Rating: 2.2 |
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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AdminBen Inactive Member |
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.
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Adminnemooseus Administrator Posts: 3976 Joined: |
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
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Adminnemooseus Administrator Posts: 3976 Joined: |
Thread moved here from the Proposed New Topics forum.
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gene90 Member (Idle past 3851 days) Posts: 1610 Joined: |
Thanks to the mods. I suppose if it doesn't get any bites you can retire it. I have noticed that sea-floor spreading still seems to be current topic in some threads.
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Percy Member Posts: 22502 From: New Hampshire Joined: Member Rating: 4.9 |
I'm still very interested in this topic, but my available discussion time is limited at present.
--Percy
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bernd Member (Idle past 4009 days) Posts: 95 From: Munich,Germany Joined: |
Hello gene90,
According to Turcotte and Schubert [1] the following forces are driving plate tectonics
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. -Bernd P.S.By the way, the calculation of trench pull does not consider frictional heating. References [1]Turcotte, Schubert (2002)Geodynamics Cambridge University Press
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edge Member (Idle past 1734 days) Posts: 4696 From: Colorado, USA Joined: |
With all respect to Turcotte and Schbert, IMHO, 'push' and 'pull' are misnomers. There is no true 'push' from the ridges as most people think of a push. I know of no compressive type deformation at MORs that should develop from a true 'push'. Such a force that would move entire lithospheric plates, or even a portion of them should have some manifestation in the geologic structure or earthquake solutions.
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 agree with T&S's description of the dynamic scenarios, but the terminology seems a bit misleading. And it is true that gravitational head from an elevated ridge seems small compared to the huge mass of the plate, but one must remember that we are not talking about actual elevation of the seafloor, but elevation of the gliding surface, which one would expect to be just slightly steeper because of the cooling effect. In a way, from their own description, it becomes hard to tell the 'push' from the 'pull'...
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bernd Member (Idle past 4009 days) Posts: 95 From: Munich,Germany Joined: |
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 ρ0 - density of slab, g - acceleration of gravity, αv - coefficient of thermal expansion, b - length of descending slab, Tc-T0) - 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, ρm - density of mantle, ρw - density of water, *αv - coefficient of thermal expansion, (T1-T0) - 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
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edge Member (Idle past 1734 days) Posts: 4696 From: Colorado, USA Joined: |
Slab pull: Fb1=2*0*g*v*b*(Tc-T0)(*/2**u0)1/2 with 0 - density of slab, g - acceleration of gravity, v - coefficient of thermal expansion, b - length of descending slab, Tc-T0) - temperature difference between slab and mantle, - thermal diffusivity, - overall length of slab This is all very interesting, but my problem with it is twofold. One is that there is no factor for the strength and continuity of the plate. In other words, when a slab finally begins to accelerate into the asthenosphere, it tends to break away and drop like a falling icicle. THat is, there is no more 'pull'. Second, if slab pull, as most people think of it, occurs, why is there often a fore-trench bulge indicating a decelerating rather than accelerating slab at the trench. Furthermore, what does this all say about the variable slope of Benioff Zones and what does it predict for steep zones versus shallow zones? I haven't thought about this yet. In fact, I haven't thought about plate tectonics very much in decades, so my information is dated and my understanding quite rusty. Mathematical modeling is nice, but we need to look at the rocks, too.
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gene90 Member (Idle past 3851 days) Posts: 1610 Joined: |
quote: I am having a conceptual problem here. How would the 'pull' not have to be propagated across the plate?
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edge Member (Idle past 1734 days) Posts: 4696 From: Colorado, USA Joined: |
That is the way I think of it. I do not follow T&S well enough (I just don't have time) to know if that is their concept.
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Minnemooseus Member Posts: 3945 From: Duluth, Minnesota, U.S. (West end of Lake Superior) Joined: Member Rating: 10.0 |
January 2006 issue, p 60. They don't seem to have anything from this issue online yet (Discover.com)
Quick summary: Brown University graduate student Catherine Rychert, based on high-frequency seismic wave studies, has determined that the lithosphere/asthenosphere boundry is a sharper zone of about 7 miles, as opposed to the previously thought 25 miles. The study suggests that rather than the previous long-held assumption that asthenospheric convection moves the tectonic plates around, the driving force is gravitational pulling at the subduction zones. Disclaimer - The above is my quick summary of the article - I wasn't directly quoting and thus I might have somewhat inaccurately presented the information presented. Moose
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edge Member (Idle past 1734 days) Posts: 4696 From: Colorado, USA Joined: |
quote: I've been waiting for something like this for a long time. I haven't read the article, but my guess is that deformation results in a preferred orientation of crystals, which, if they are seismically anisotropic, should give an idea of the orientation of crystals. THis would reflect the state of strain in the asthenosphere. The only problem is that, in this scenario, slab-pull is indistinguishable from a ridge-push. What it does tell us is that a convection current model may be on the way out. If you get a link, let us know, thanks.
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