Why do “supercontinents” reassemble after breaking up?
Geologists tell that over the past billion years there have been at least three “supercontinents”, continents that consisted of nearly all the earth’s land, surrounded by a “super ocean”. Each of these supercontinents split up into the smaller pieces like we have today, and then reassemble into the next supercontinent.
I, of course, don’t question that the geologists are correct. But, construction being a lot more difficult than destruction, I find it difficult to understand how when a supercontinent breaks up into random pieces, the pieces reassemble into another supercontinent I would have thought that the poeces just slid around randomly, never reassembling.
Well, I realize they do reassemble, but I don’t understand how.
The only explanation I can think of is that the pieces of land do not move randomly;. Instead,
land not only slide around on tectonic plates, but the plates represent some sort of “tracks” that lead the broken land back to reassembly.
Is this correct? If not, why do the broken pieces reassemble into anew supercontinent?
The Earth is a (roughly) sphere. If two things start moving apart from one spot, they will run into each other at some other point on the Earth, assuming that either they are large enough (continents are quite large) or they move at roughly the same speed. It’s not too shocking, then, that landmasses that separate manage to unseparate at some later point of time over 4 billion years or so.
There are actually some different theories about this, and as I understand it there is still some debate on the subject.
One theory is that they form then drift apart and, since the Earth is a sphere and they can’t escape, they just end up running into each other somewhere else at some point. And then of course they break up again, drift apart again, and meet again, somewhere else this time.
Another theory is that as the continents drift apart, they rip open subduction zones which eventually pull them back together again. In this theory, instead of running into each other somewhere else on the sphere, they basically stretch apart, then pull back together in an endless cycle.
A third theory is kind of a combination of the two. The continents drift apart, but they become caught in subduction zones that are left over from the formation of the supercontinent, which prevents them from moving freely all the way around the Earth. In this theory, the new supercontinent forms as the various continents all get stuck on these subduction zones, forming at a roughly 90 degree shift around the Earth’s sphere from where the last supercontinent was.
If you are having trouble picturing the last one, think of it this way. North and south America, instead of continuing west and slamming into Asia, will instead get stuck sliding around the Pacific ring of fire, and will end up going north instead. Asia will also end up going north as it gets stuck around the other side of the Pacific subduction zone, and everything ends up piling up in the Arctic region (ish).
Exactly. I won’t go into the details here, but at the biggest scale, almost EVERYTHING has been basically moving northward over the past several hundred million years — the Tethys Sea closing, the remnants of Gondwana colliding with the remnants of Laurasia, the whole Pacific system slowly subducting under the Aleutians (relative to everything else); and the Americas pivoting with Eurasia around a point in Siberia, where the Chersky Mountains emerge (compression) in a line with the spreading center under the Arctic Ocean.
I’ll add that it’s helpful to keep in mind that relative plate movements occur in different ways at different scales simultaneously — much like weather systems, where clouds can be moving (say) north locally, but as part of a larger air mass that is moving east.
That’s why (for example) mid-ocean spreading centers themselves eventually subduct and disappear (or else reappear a little inside the continent) at plate boundaries. This explains a lot of the weirdness along North America’s western margin, from the Cascades to Baja California.
This is, in fact, not the case for continents. It takes more effort to rip a continent apart than it does to just move continental fragments around and accrete them - because rifting proceeds by the injection of new basaltic material (i.e. is actually a constructive process, but not mostly constructive* of continents*). By contrast, the accretion of continental fragments is mostly a side-effect of the conveyor belt of old basalt crust being dragged down because of density differences.
Doesn’t heat/insulation enter into this? I.e., once a supercontinent forms it holds in the heat in the center until a large scale hot spot forms in the middle which starts the process of breaking it up.
Oh, it’s a lot further east than the margin. West of the Rocky Mountains, if you’re interested in geology and you want to know what’s going on beneath your feet, you practically have to look it up by the county you’re in. The part I puzzle about is why sometimes when plates collide one subducts the other, and sometimes the two crumple up into mountains instead (Himalayas anyone?). I shouldn’t be disappointed, I guess. After all it’s scarcely been 100 years since Wegener came withe notion of continental drift and fifty since people stopped laughing at him.
The breakup of Pangaea didn’t start in the middle, though.
I think there’s a missaprehension that a supercontinent is all one fused continental mass. It’s not, it’s just like any continent today, made up of stable cratons and less-stable mobile belts between them.
What makes you think there wasn’t subductioninvolved in the Himalayan orogeny? Continental crust hardly ever gets subducted, anyway.
True – the effects of western-margin plate interactions extend amazingly far inland (the Laramide Orogeny being a big part of this, as you pointed out).
I was referring, though, more specifically to what happens as a mid-ocean ridge/spreading center itself gets subducted under continental crust. In other words, one SYSTEM of plates moving relative to another plate (or plate system). Example: the spreading center that USED to be in the middle of the Pacific has mostly subducted under North America, and disappeared. This may be contributing to some of the stretching in Nevada – but more directly, it has popped up again a little inland, creating the The Gulf of California (Sea of Cortez), detaching a bit of North America (Baja California) from the rest.