<p>I think the mechanisms that you're looking for are subduction, paired with the "stickiness" of continental crust. </p>

<p>The subduction of oceanic crust under continental crust inevitably creates a net movement of crustal material <em>toward</em> 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. </p>

<p>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.</p>

<p>In every case, subduction inevitably pulls continents together.</p>

<p>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.</p>

CC BY-SA 4.0
2018-11-26T00:04:43.117Z
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
CC BY-SA 4.0
2018-11-26T00:21:08.253Z
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
CC BY-SA 4.0
2018-11-26T00:58:41.160Z
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
CC BY-SA 4.0
2018-11-26T01:26:20.053Z
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
CC BY-SA 4.0
2018-11-26T01:27:55.083Z
thanks Arkenstein - you've given me some things to think about.
theRiley
CC BY-SA 4.0
2018-11-26T01:30:58.173Z
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
CC BY-SA 4.0
2018-11-26T01:36:15.043Z
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
CC BY-SA 4.0
2018-11-27T04:47:34.970Z
@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
CC BY-SA 4.0
2018-11-27T05:07:09.750Z
@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
CC BY-SA 4.0
2018-11-27T13:46:11.080Z
@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
CC BY-SA 4.0
2018-11-25T23:09:42.917Z
2018-11-25T23:09:42.917Z
Arkenstein XII
<p>Just to reiterate some of the points previously said here:</p>

<ol>
<li><p>Continental crust fragments that collided are sticky. This results in mountain belts that essentially "glue" the two crusts together.</p></li>
<li><p>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).</p></li>
</ol>

<blockquote>
  <p>it could also be subducting AWAY from an adjacent plate also containing continental crust</p>
</blockquote>

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

<blockquote>
  <p>or at least lower the probability of all continental crust being
  annealed at one time</p>
</blockquote>

<p>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.</p>

<blockquote>
  <p>is how all these pieces land together again &amp; again so improbably
  frequently</p>
</blockquote>

<p>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".</p>

<hr>

<p>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.</p>

<hr>

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

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

<blockquote>
  <p>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).</p>
</blockquote>

CC BY-SA 4.0
2018-11-26T12:18:03.203Z
2018-11-26T12:18:03.203Z
Gimelist
<p>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.  </p>

<p>There must likely then be some aspect of plate tectonics which favors these arrangements.</p>

<p>Can anyone provide an explanation?</p>

<p>EDIT:  This is <em>not</em>, 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 <strong>formation</strong> of any &amp; 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.</p>

CC BY-SA 4.0
2018-11-24T10:35:09.923Z
Why is this improbable?
Gimelist
CC BY-SA 4.0
2018-11-24T10:38:42.630Z
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
CC BY-SA 4.0
2018-11-24T10:40:40.993Z
i'm surprised you aren't surprised, or hadn't been at some point, to be honest.  do you have an answer?  :)
theRiley
CC BY-SA 4.0
2018-11-24T11:01:21.553Z
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
CC BY-SA 4.0
2018-11-24T23:04:15.650Z
[What are the causes of the supercontinent cycle?](https://earthscience.stackexchange.com/questions/2421/what-are-the-causes-of-the-supercontinent-cycle)
Keith McClary
CC BY-SA 4.0
2018-11-24T23:12:08.817Z
@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
CC BY-SA 4.0
2018-11-26T00:09:25.747Z
@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
CC BY-SA 4.0
2018-11-26T00:53:58.600Z
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
<geophysics><plate-tectonics><earth-history><continent>
Why Do Supercontinents Form?
theRiley
# Why Do Supercontinents Form?

(Asked by: username_0 on 2018-11-23)

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.

**Tags:** geophysics, plate-tectonics, earth-history, continent

> **Comments:**
> **username_1**: Why is this improbable?  
> **username_0**: 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.  
> **username_0**: i'm surprised you aren't surprised, or hadn't been at some point, to be honest.  do you have an answer?  :)  
> **username_0**: 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.  
> **username_2**: [What are the causes of the supercontinent cycle?](https://earthscience.stackexchange.com/questions/2421/what-are-the-causes-of-the-supercontinent-cycle)  
> **username_0**: @username_2 - thanks username_2.  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.  
> **username_0**: @arkaia - see my comment to username_2, and the edit in my question.  i disagree that it is a duplicate.  if you think it is, please elaborate.  
> **username_3**: 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.  

---

## 2 answers

### Answer by username_3 on 2018-11-25. Score: 1 | Accepted answer

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.

> **Comments:**
> **username_0**: 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?  
> **username_3**: 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.  
> **username_0**: 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?  
> **username_3**: 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.  
> **username_0**: thanks username_3 - you've given me some things to think about.  
> **username_3**: You're quite welcome. I'd strongly suggest looking into the initiation of subduction. That might be a helpful area to become familiar with. :)  
> **username_0**: 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.  
> **username_4**: @username_0 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.  
> **username_0**: @username_4 - 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.  
> **username_4**: @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.  

---

### Answer by username_1 on 2018-11-26. Score: 4

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 (username_3 XII from the comments).


---

