There have been several super-continents in our earth’s history.
Pangea, Gondwana, Rodinia, etc have been created and destroyed due to plate tectonics.
Why is there a tendency for large scale landmass to move together? I would assume continental drift would be random, and therefore the creation of super-continents would be unusual.
Or, am I experiencing observer bias, because in between super-continents the tectonic plates just randomly did as they pleased, but we did not have snappy names for them? It seems odd to me that, for example, Gondwana effectively took up over 80% of the landmass, but now the various pieces of it are fairly randomly distributed across the globe.
A super-continent seems to be an anomally, yet we have evidence of at least 5 (and maybe as many as 10) supercontinents existing.
My WAG? It’s not that there’s a tendency for continents to form supercontinents - it’s that once continents get “stuck” together, it’s hard for them to get “unstuck”, because they’re pushing against each other. Think of supercontinents as continental traffic gridlocks.
Super-continents don’t form all that often. It’s been hundreds of millions of years since Pangea broke up, and it won’t coalesce into another one for hundreds of millions more.
Once a super-continent does break up the fragments will radiate outwards, so it will take a long time to come together again, on the other side of the world.
The movement of continents, plate tectonics, is driven by the motion in the mantle of the earth. This motion of mantle material, driven by heat and gravity, takes the form of convection cells. The continents are moved by the uppermost parts of these convection cells. If pieces of earth crust are above different convection cells, these pieces will be carried in different directions, just because their convection cells are different. The convection cells thus break up large continents to continents roughly of the size of the convection cells themselves. On the other hand, small nearby continents will mostly be carried by the same convection cell, in the same direction, until they reach the border of the cell, where the horizontal motion slows down, and small continents accumulate to form larger continents.
The consequence: the size of continents follows the size of the convection cells. Continents are large, even so large as to form supercontinents, because the convection cells are large.
It is a basic property of convection, that the convection cells are roughly as large as the convection layer is deep. Smaller convection cells will come to a halt because of friction (viscosity). Larger cells will break up because convection is always unstable at large scales. Thus, the continents are so large because the earth’s mantle is so deep.
Look at things this way. Pangea started to form a bit over 300 million years ago; the first tetrapods to make their way onto land evolved around 250 million years ago. This means that Pangea is the only supercontinent that land vertebrates ever populated.
It gets even more mind-blowing, though. The first evidence of terrestrial plants (as in full on plants, discounting algae mats) dates back to around 475 million years ago; there are trace fossils of amphibious arthropods on land from just under 500 MYA and evidence of air breathing arthropods by 450 MYA.
But the last supercontinent before Pangea was Pannotia (although its status is debated and its possible it shouldn’t really qualify). Pannotia, if it existed, broke up by around 500 MYA.
What does all of this mean? Well, it means that the timescales we are dealing with are so long that Earth may have had 10 supercontinents in its history, but if so the entire history of complex life on land so far fits into a single combine>break up cycle.
Do good mathematical models for this exist? That is to say, if we simulate an Earth-sized planet, with Earth-like mantle convection, do we see landmasses forming and breaking apart at roughly the frequency that they did on Earth?
Note that this is not necessarily the same thing as saying we can accurately reconstruct what happened on Earth.
Because ISTM entirely plausible that our models don’t yet account for how often the landmasses clumped (indeed, this is my recollection, but my recollection might be out-of-date).
My own WAG is that perhaps it’s similar to the phenomenon that somewhere on Earth there must always be turbulent air just because there’s no way to map air currents on a sphere that doesn’t result in this. So similarly, when we map convection currents there is always some area that is attractive from all directions. I dunno, something like that.
Sure, but that’s no guarantee of them coming together at the same time.