Mainstream plate tectonics model is nowhere near quantitatively correct

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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?

Joe, I'm probably asking you to repeat something, but I'd like to know more about subduction that doesn't result from plate collisions. One specific question, is there a contemporary example of this somewhere in the world?Andor, follow Joe on this. He knows much more than me on this topic.--Percy

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Originally posted by Percipient:
Joe, I'm probably asking you to repeat something, but I'd like to know more about subduction that doesn't result from plate collisions. One specific question, is there a contemporary example of this somewhere in the world?

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Andor, follow Joe on this. He knows much more than me on this topic.

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--Percy

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Actually, most oceanic plates are trying to sink. That's why the cooler, older parts of it are usually deeper. In a way, subduction begins at the divergent boundary. I think it is hard to reverse direction on a plate because of this. Think of it as the topography of the lithosphere/asthenosphere contact. Which way is the gradient?However, the plarity of a subduction zone can reverse and we see this several times/places in the geological record.

What does it mean for the polarity of a subduction zone to reverse, and what is the geological evidence for such events. Thanks!--Percy

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Originally posted by Percipient:
What does it mean for the polarity of a subduction zone to reverse, and what is the geological evidence for such events. Thanks!

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--Percy

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Oh, yeah. I forgot to mention that polarity reversals usually can only happen in ocean to ocean collisions such as in the Marianas arc. It means that the overiding plate can switch to the subducting plate. The evidence for this is in the fabric and distribution of rock types. This is probably just trivia as far as the evolution connection is concerned, but it shows the complexity of these systems.

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Originally posted by Andor:
Thanks JM.
So, the subduction of the plate could change the direction of the convection currents under the plate?

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JM: Indeed. Think of it as sticking a cold finger down into the mantle. Mantle circulation will change as subduction proceeds.CheersJoe Meert

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Originally posted by Percipient:
Joe, I'm probably asking you to repeat something, but I'd like to know more about subduction that doesn't result from plate collisions. One specific question, is there a contemporary example of this somewhere in the world?

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JM: That's a good question. As far as I know, the Loiusade Plateau (solomon islands) is one place where subduction seems incipient. As edge points out subduction polarity reversal is common in the South Pacific.CheersJoe Meert

Joe, Edge,Thanks! Had never heard of this before.About underplating, in general is this a significant contributor to plate thickness? Does deplating (you can tell from the terminology that I'm a professional geologist ) also occur? In fact, does it sometimes occur that the lithosphere wears very thin or wears away completely?Underplating isn't in the site's geology glossary. Can someone provide a laymen's level definition that I could add? And for it's opposite if there is such a term? Thanks!--Percy

Hi Joe,Took a closer look at your picture. Really neat! I didn't realize Australia and Antactica are moving north that rapidly. Also neat is that you have eastern Africa rifting off into the Indian Ocean to become the Somali Plate. It must be moving much slower than Australia. What's the little yellow piece between the African fragment and Antarctica?--Percy

Nice pic Joe - I'd often wondered what the extrapolation would be.

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Originally posted by Percipient:
About underplating, in general is this a significant contributor to plate thickness?

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It is significant, though I do not remember any numbers. In my own mind, part of the significance is that it slowly increases the gradient of the lithospher/asthenosphere contact. To me, pushing and pulling don't work and this is the only way to get significant plate velocities. Body centered forces, like gravity. Think of it as standing on a treadmill that isn't moving and slowly increasing the gradient. What happens?

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Does deplating (you can tell from the terminology that I'm a professional geologist ) also occur? In fact, does it sometimes occur that the lithosphere wears very thin or wears away completely?

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The only place I can see this is at a hot spot where temperatures effectively thin the lithosphere. Unless you are talking about scraping off the supracrustal sediments and volcanics at the subduction zone.

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Underplating isn't in the site's geology glossary. Can someone provide a laymen's level definition that I could add? And for it's opposite if there is such a term? Thanks!

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Joe probably can better define it, but I would call it the thickening of the lithosphere due to cooling of the crust and mantle as the slab moves away from the ridge where it formed. In a way, the addition of sediments also adds to the overal thickness of the lithosphere.

[QUOTE]Originally posted by Percipient:
[B]Hi Joe,Took a closer look at your picture. Really neat! I didn't realize Australia and Antactica are moving north that rapidly. Also neat is that you have eastern Africa rifting off into the Indian Ocean to become the Somali Plate. It must be moving much slower than Australia. What's the little yellow piece between the African fragment and Antarctica? [/QUOTE]JM: I always tell my students that I will pay them each a million dollars if I am wrong . The little yellow piece is Madagascar. As for 'deplating', in some sense it happens everytime continents break apart. The Somali plate shown in the Amasia cartoon is thinned at this point (forming the east African rift). The alkaline lakes that make up the EAR will eventually link up to form a shallow ocean and spreading center (at least that's how I see things happening). What's intriguing is the the EAR is following (more or less) the suture of Gondwana assembly as it cuts through the Mozambique Belt. This is particularly striking when the EAR hits northern Kenya and skirts around the old Archean cratonic nucleus (The Tanzanian craton). The rift form to both the east and west of the craton and then they merge again at the southern boundary of the Tanzanian craton in Malawi. One could almost map the craton by the rift lake boundaries. This shows it although it is not the most precise map. By the way a better name for 'deplating' would be lithospheric thinning.CheersJoe Meert

(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?

Just an information source link (discovered while looking for something else):
http://pubs.usgs.gov/publications/text/dynamic.htmlThis is a United States Geological Survey (USGS) online publication, covering plate tectonics.The full title is This Dynamic Earth: The Story Of Plate Tectonics.Please note the text info, towards the bottom of page. Book is available in a number of forms, including .pdf file.Moose

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.