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It would seem, on the face of it, improbable that the continental land-masses would accumulate into a single composite, yet it has happened numerous times, and is expected to again in the future.

There must likely then be some aspect of plate tectonics which favors these arrangements.

Can anyone provide an explanation?

EDIT: This is not, as I see it, a duplicate of the 'What are the causes of the supercontinent cycle?' question. This question goes to what process drives the formation of any & all supercontinent formations, which I assert should be improbable, made more improbable by their recurrence, not so much the cycle itself. The other question did not address this more fundamental aspect, or in any case receive a pertinent account of its resolution. If anyone wants to engage on this, or doesn't see the distinction, please do so in the comments or a chat.

theRiley
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    Why is this improbable? – Gimelist Nov 24 '18 at 10:35
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    well, if you imagine a small number of pieces randomly placed upon a sphere, what is the likelihood they would all end up clumped together? that's my thought process, anyway. even assuming some cohesive force -holding- them together, they still have to be combined in the first place. – theRiley Nov 24 '18 at 10:38
  • i'm surprised you aren't surprised, or hadn't been at some point, to be honest. do you have an answer? :) – theRiley Nov 24 '18 at 10:40
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    I have, by the way, at many points looked for an answer to this question elsewhere, without success. That is why I am grateful for this site on stack exchange, where truly knowledgeable people are gathered with the requisite interest/expertise to grapple with such a question. – theRiley Nov 24 '18 at 11:01
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    What are the causes of the supercontinent cycle? – Keith McClary Nov 24 '18 at 23:04
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    @Keith McClary - thanks Keith. I don't think that goes to my question, though, which is - what causes the supercontinents, any of them, to form in the first place. the break-up is of no particular relevance, and seems to be straight-forward - plate movement under the cratons. this is not so difficult to envision. – theRiley Nov 24 '18 at 23:12
  • @arkaia - see my comment to keith, and the edit in my question. i disagree that it is a duplicate. if you think it is, please elaborate. – theRiley Nov 26 '18 at 00:09
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    The key problem as I see it, is that you have assumed that continental crust is randomly distributed upon the surface of the Earth. It isn't. The positions of the continents are a result of the forces acting on them that are produced by the movement of Oceanic crust, which is being formed at mid ocean ridges, and recycled into the mantle at subduction zones. – Arkenstein XII Nov 26 '18 at 00:53

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Just to reiterate some of the points previously said here:

  1. Continental crust fragments that collided are sticky. This results in mountain belts that essentially "glue" the two crusts together.

  2. Subduction of a plate that contains both oceanic and continental crusts underneath continental crust will inevitably lead the the complete loss of the oceanic crust, and the gluing of the continental crust to the other one. India and the Himalayas are an example of something like this with all oceanic crust lost to the mantle, and the Mediterranean is an example of an almost completely subducted oceanic crust (aka the Tethys), with the formation of a mountain belt (Alps) and soon have two continental crusts glued together (Europe and Africa).

it could also be subducting AWAY from an adjacent plate also containing continental crust

"Away" on a sphere means "towards" in the other direction. It's all relative here.

or at least lower the probability of all continental crust being annealed at one time

They don't have to be annealed at one time. They just have to be annealed for long enough, without rifting, so all end up in one place.

is how all these pieces land together again & again so improbably frequently

Improbably frequently? The last one existed about 300 million years ago, and the next one will form only in about 200 millions (more or less, depending who you're asking). In geological history, there are only two confirmed supercontinents, Pangea and Rodinia. Anything earlier than that is pure speculation (although educated speculation). I wouldn't call "twice" in 4.5 billion years "frequent".

I can also demonstrate by example. Today, there is only one rift that is properly breaking up a continent. This is the East African rift, and the Red Sea is a nascent ocean that is spreading now. But, there are more than one collision zones. The Australian plate is moving northwards, and soon will stick to SE Asia. India is already stuck there. Asian plate is stuck to European plate (Urals in Russia are the suture zone). Arabian plate and African plate are moving northwards as well (Alps, Zagros, all those mountain belts are the suture zones). The Pacific ocean used to be a whole lot bigger (Panthalassa Ocean). One oceanic plate is already underneath North America. The subduction rate of the Pacific Plate is the fastest of all subduction zones, and in not too long ago America (particularly North) will stick to Asia. The Atlantic Ocean has no subduction zones, so it will grow to become the new Panthalassa Ocean, with most other continents forming one single supercontinent.

the break-up is of no particular relevance, and seems to be straight-forward - plate movement under the cratons

Actually no. The "cratons" are already a single plate, glued together from several other plates. The break-up usually occurs because of

build-up of mantle heat under the insulating continental lid, will ultimately cause rifts to begin and start to form young oceanic crust (Arkenstein XII from the comments).

Gimelist
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The simple answer is that when continental plates collide, they tend to stick together, until eventually all 7 are stuck together at once. Currently Eurasia, Africa, and India have already recombined. Australia will recombine relatively soon. The Americas and Antarctica still have a long way to go.

The reason why continents stick together has to do with the positive feedback mechanism of "seafloor spreading" and "mantle convection". Once the continents begin to break up, the process of seafloor spreading ensures that the initially small oceans will continue to spread until there is once again a single large ocean. The driver of seafloor spreading is mantle convection, which in turn is powered by the weight of the cool, dense, subducting slabs that descend to the planet core. When the continents begin to break apart, subducting only occurs in the existing large ocean (think Mariana Trench). Once started, this subducting is difficult to stop, and it acts as a vacuum, sucking the continents toward it. Since the earth is a sphere, what goes around comes around. This supercontinent cycle takes about 300-500ma.

https://en.wikipedia.org/wiki/Seafloor_spreading has a map of the current Age of the Oceanic Lithosphere that shows how seafloor spreading only occurs in the middle of oceans.

https://en.wikipedia.org/wiki/Mantle_convection includes a picture illustrating mantle convection.

theRiley
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  • i don't see how plate subduction accounts for all the clatons colliding with one another, let alone repeatedly. something is still missing, I believe. perhaps deformations in the plates caused by the weight of the cratons creates a gradient in the direction of plate flow - that is just a wild guess, however - i'm looking for some agent of directionality which makes such collisions so (otherwise improbably) frequent. you have basically summarized the mechanics of plate tectonics, it seems. – theRiley Nov 25 '18 at 04:03
  • I dont think, just to try to be clear, even random motion would cause the inevitable collisions resulting in supercontinents, Matt. that is an assertion - I don't see a mechanism which accounts for the observations. – theRiley Nov 25 '18 at 04:23
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    I placed 7 ice cubes in a water filled bowl with enough surface area so the ice cubes are about 40% of surface. Mixing them with random motions there are generally enough collisions so there are only 1-3 groups of cubes when they settle. But that isn't the crux of the argument, the positive feedback is. Any system where there is positive feedback will tend to oscillate to extremes. One of the extremes in this case all the continents stuck together. Watching an animation of Pangaea breaking up, you can already see that Africa, India and Eurasia have recombined, the Americas are on their way. –  Nov 25 '18 at 04:49
  • what is the positive feedback? – theRiley Nov 25 '18 at 05:11
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    With the ice cubes it would be surface tension. With plate tectonics it's the difference between rising and falling density of mantle convection due to surface cooling. I think your idea with the weight of the cratons causing a gradient is valid, but more likely as a cause for breakup. I did calculations showing that oceanic areas are actually more dense than continental areas (see my related recent question regarding Pangaea). Tomorrow I will try to simplify my answer, and maybe take it from another viewpoint easier to understand. Simplest answer is that when continents collide, they stick. –  Nov 25 '18 at 09:55
  • Matt, let me say I appreciate your participation. The idea of cohesion makes sense. What I have a problem understanding, and this may not even be settled science you understand, is how all these pieces land together again & again so improbably frequently, given the many other potential non craton-colliding plate interactions. there needs to be a gradient guiding this process of some sort, I'm thinking. I'm not a geoscientist, so these are just speculations of a computer scientist who knows the basics, and finds this phenomenon not readily explained in popular literature, at least. – theRiley Nov 25 '18 at 10:12
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    Yeah, I'm not a geoscientist either, but good at physics, engineering and programming. I too see a lack of a simple clear answer. See my rewritten answer. I think it is much more simple and clear. You are right about this not being settled science. If you read the wikipedia entry for plate tectonics there are sections "Driving forces related to mantle dynamics" and "Driving forces related to gravity". Gravity (with gradients like you proposed) is generally considered secondary and mantle dynamics primary. But the relative importance of the factors is not completely proven and accepted. –  Nov 25 '18 at 16:46
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    @theRiley The movement of the plates is not at all random, since it is driven by an organised system of mantle convection that has a strong dependence on the history of the system. So it really isn't that improbable that all the continents will end up colliding. Plus they do tend to stick together once they have. – bon Nov 25 '18 at 18:01
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    If one considers the Bering land bridge, it would seem that Eurasia and the Americas are already a supercontinent. Not our fault that it's too warm to have a proper amount of water locked up in glaciers :-) – jamesqf Nov 25 '18 at 18:29
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I think the mechanisms that you're looking for are subduction, paired with the "stickiness" of continental crust.

The subduction of oceanic crust under continental crust inevitably creates a net movement of crustal material toward a continental plate. Any oceanic plate that is carrying continental material will therefore always drag that continent toward the continental plate that it is subducting underneath, always resulting in eventual collision.

If an oceanic plate has subduction occurring on both sides, the ocean will inevitably narrow until it closes, thereby causing the continental plates on either side to collide.

In every case, subduction inevitably pulls continents together.

Furthermore, once continental plates collide, they have a tendency to stick together for long periods of time, increasing the likelihood that all continental material will eventually accumulate there.

Arkenstein XII
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  • the rest of your answer is clear, but this i don't see: "The subduction of oceanic crust under continental crust INEVITABLY creates a net movement of crustal material TOWARD a continental plate." why? if the edge of a continental crust-containing plate is oceanic, it could also be subducting AWAY from an adjacent plate also containing continental crust. do you really demonstrate to your own satisfaction the inevitability of this in every case? – theRiley Nov 26 '18 at 00:04
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    Oceanic crust moves away from a mid-ocean ridge, not away from a continental plate. The difference in density between mature oceanic crust and continental crust is what causes subduction, and it always results in oceanic crust moving toward the subduction zone. – Arkenstein XII Nov 26 '18 at 00:21
  • and yet, annealed continental plates give rise to rifting - which means there are forces working to combine continental plates and divide them, which might be expected to be working at counter-purposes all along. doesn't rifting contradict the inevitability spoken of earleir, or at least lower the probability of all continental crust being annealed at one time? – theRiley Nov 26 '18 at 00:58
  • The process that breaks up supercontinents is actually a result of that continent reaching a kind of "critical mass". The supercontinent ends up surrounded on all sides by subduction zones, which deliver volatiles under the continental plate. This increases magmatism in the heart of the supercontinent, and combined with a build-up of mantle heat under the insulating continental lid, will ultimately cause rifts to begin and start to form young oceanic crust. Be aware of the distinction between mature and young oceanic crust, where maturity is required for subduction to begin. – Arkenstein XII Nov 26 '18 at 01:26
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    thanks Arkenstein - you've given me some things to think about. – theRiley Nov 26 '18 at 01:27
  • You're quite welcome. I'd strongly suggest looking into the initiation of subduction. That might be a helpful area to become familiar with. :) – Arkenstein XII Nov 26 '18 at 01:30
  • i believe i understand subduction fairly well. it isn't that complicated, in principle. it is the net effects of irregular plates all rifting AWAY from the mid-ocean ridge system that i think is the missing conceptual link. this is not a concept, in my reviews, that is often or well explained. – theRiley Nov 26 '18 at 01:36
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    @theRiley it helps if you consider the "life cycle of a subduction zone, subduction zones keeps running until continental crust meets continental crust, or a continent overruns it. this is why continental crust as a net effect are moving together. As soon as a supercontinent breaks up all the pieces are on a collison course again on the other side of the planet. – John Nov 27 '18 at 04:47
  • @John - very succinctly stated.- i'm assuming at that point in the cycle, the mid-ocean ridge is no longer present, but rather setting up anew amid the supercontinental rifting zone. – theRiley Nov 27 '18 at 05:07
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    @Riley, it may still be there but oceanic crust has a built in limit on size the sink as they age and they are powered by convention cells which can only get so big but the size of convention cells also means supercontinents are inherently unstable as well, they are too big for a single convention cell so once they form they have a limited lifespan. It also helps if you think of spreading centers less as lines and more like single points that connect if they are close enough. – John Nov 27 '18 at 13:46