Mainstream plate tectonics model is nowhere near quantitatively correct

JM, I hope you can clarify me a couple of questions.
I understand that, with time, as ocean plates become older they get thicker and denser by underplating. So eventually the plate does not float any longer and begin subducting into the mantle. With this, the movement of the plates reverse, until all plates rejoin in one big supercontinent and the cycle restarts once again.
But then, the real engine of the plates movement would be, the pull of the subducting plate, and not the convection currents.?

Percipient, thank you for the answer. What you say it's what I thought until I read the book "Plate Tectonics" by Jon Erickson.
If I understand correctly what he says, the movement westward of the Atlantic plate pushed by the mid-ocean ridge, will not bring together East-Asia with West-America: Before that happens, the plate will be so dense and massive that it will sink by gravity due to its own weight, and will force the reversal of the movement, so America will rejoin Europe again in the future. ??
I think in favor of this is the fact that the points of fracture of the previous supercontinents seem to be the same once and again. Do they not?

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[This message has been edited by Andor, 06-03-2002]

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Originally posted by Joe Meert:
However, subduction will occur when the plate becomes negatively buoyant as well. How long this takes depends on the viscosity/density contrast between the oceanic plate and the asthenosphere. Thus subduction could, at some point in the future, begin to occur beneath eastern North America even though the oceanic and continental portions of the NAP are now moving the same direction. This may be precipitated by a change in plate motion elsewhere.

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Thanks JM.
So, the subduction of the plate could change the direction of the convection currents under the plate?

(Percy, I think is also evident that I'm not a geologist; I did only a basic introductory course of Geology, but I like it, and I'm very interested).
From what Joe and Egde say, it seems to me that with so many factors at play, we shouldn't look for exact patterns in the movement of plates, or in the formation and breaking of the supercontinents. So the similarity in the formation and rupture between Rodinia and Pangea should be considered only a coincidence?

minnemooseus, I had already bookmarked that place, but thank you any way.
JM, I have found a couple of references about "neopangea" or "pangea ultima", and this alternative map of the plates position 250 Ma in the future: http://www.scotese.com/future2.htmForgive me if I'm always trying to find patterns. Probably what we have more often in nature are systems in the edge of chaos.