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[GavJ]

Wondering if anybody out there has some answers about confusing tectonic plate stuff (for a world generator I'm making, see signature).

I'm wondering what happens when a LARGE tectonic plate gets surrounded on all sides by divergent rifts? All other situations seem like they have straightforward consequences, but this one confuses me. Such as the African and Antarctic plates on Earth today. The African one especially, since it's also making yet another divergent rift in the middle (presumably due to a new convection plume):



Where does all that rock/pressure go? I have several possible thoughts, but no idea what might be correct:
1) It eventually cracks somewhere and starts subducting under itself (i.e. new plates).
2) It just buckles up in the middle and either forms an interior moutnain range (at a hypothetical weak spot) or just very gradual thickening everywhere (if all ~equally strong)
3) One of the rifts overpowers another one. For instance, maybe the Atlantic rift is stronger and closes up the Indian ocean one and Africa slides east. This seems unlikely, since it is currently forming new rifts which appears to be the opposite of closing them up.
4) All the convection cells get tumbled up and the whole system shifts into a new stable state with all kinds of stuff different?

or something else?

[10ebbor10]

Something else, IIRC.

The African plate in this example stays exactly where it is, while the other plates move away from it. A divergent zone just means that plates move away from each other, after all.

[Eric Blank]

That is correct. The plate in question does nothing unusual. It would drift to one side if volcanic activity is stronger on one than the other, I guess. Remember that as surface area is added to one part of the globe an equal amount will have to be lost (planet radius/density/composition does not change, conservation of mass blahblah) somewhere else. Oceanic crust is subducted, normally, and continental crust riding on it ends up more or less on the other plate. Or squashed into other bits of continental crust on other plates, making mountains.

[GavJ]

But
A) That implies they drew the map incorrectly, because it has big prominent arrows pointing inward into the African plate, whereas if the ocean ridges were only expanding away from it (the "not much is happening to Africa" hypothesis would necessitate this), it should only point out, yes?
B) Across scores of websites and about a dozen books on the topic I've looked into, I've never seen a single diagram or description of a rift that only expands to one side. Some rifts being stronger than others, absolutely, but not unidirectional pillow lava. How would that even work?

Note that I don't disagree that the plate would stay still, potentially. But it would have to stay still AND get squished or deform or crack, or something else has to give, because it has almost diametrically opposite expansion into it from two sides.

By analogy, when you put something in a table vice and start screwing, yeah sure its center of gravity stays still. But staying still is not the same thing as "nothing happening"

A divergent zone just means that plates move away from each other, after all.

Just in case what I'm pointing out is unclear, look at the SE corner of Africa, and then look at the NW corner. How does it move apart from other plates in two nearly opposite directions?
The antarctic plate is even more extreme, with three arrows pretty much all pointing at each other (easier to see on less terrible map projections)

[i2amroy]

I'd like to point out that both the African and Antarctic plates do have some boundaries that aren't divergent rifts. Africa's is where it meets the Arabian plate, and the Antarctic plate's is where it meets South America.

[GavJ]

Right, but all of Africa getting squeezed underneath Turkey is even more extreme deformation than any of my original suggestions by a lot. Plus that's probably going to stop as soon as the eastern Mediterranean closes up anyway, because continents don't tend to subduct under each other much.  MAYBE the lowland Egypt area would, but then stop when it gets to the tibesti ranges or thereabouts. More likely it would just make a small mountain in egypt then stop going that way.

[Jelle]

1) It eventually cracks somewhere and starts subducting under itself (i.e. new plates).

I thought that this was the case, what with the eastern part of the african plate splitting off and pushing eastwards, but then as I recall there's no subduction involved since the two plates are pulling away from one another. There's also the high concentration of mountains and volcanic activity in that area. I don't know nearly enough about tectonic plates to make sense of all that myself.

[Orange Wizard]

I can't help but notice that New Zealand remains an island in that prediction.

[Orange Wizard]

Stop ruining my attempts at being an elitist.

[GavJ]

The future maps helped a bit, but they didn't show plates. I just found this though

https://www.youtube.com/watch?v=swRFnPq-lJo

(3:00 is the relevant animation).
It seems to suggest a combination of solutions wherein
1) Africa gets squished and deformed a bit
2) The Atlantic rift gets overpowered eventually and returns to subduction
3) It looks as if a new subduction forms at west African coast?

So like, everything suggested happens? I don't know, the last one sounds fishy. I might just stick to the first two for my model. I also came up with a plan of how to use veroni cells as convection cells for hotspots under large plates to help determine hen they split, which should work pretty well.