And pairs of opposing large continents may have been more typical than a single unified one
Tons of articles including one at NPR have been written about a new finding.
Lava sands with zircon xenocrysts found on the beaches of Mauritius (island which is East from Madagascar) support the idea of a microcontinent dubbed Mauritia that existed between the continents (...) of Madagascar and India for tens of millions of years sometime 70 million years ago. I don't want to be excessively accurate because I don't think that their reconstructed layout may be trusted this accurately.
Mauritia is supposed to be just a tiny sliver inserted between India and Madagascar in the left upper corner. At that time, the liberals in San Francisco belonged to the African union with Congo and were Hispanics. Baltic states haven't been occupied by Stalin yet and they had the Amazon forest in their yard. ;-)
The papers by Torsvik et al. and Niocaill et al. have appeared in Nature Geoscience; they're linked to at this Nature review. Although the precise arguments leading to the dating and other claims aren't comprehensible to me, it still seems like too much hype given the importance of the finding.
When we first hear about the continental drift, we are usually told about the supercontinent named Pangaea that was around between 300 million and 200 million BC. That universal continent was surrounded by Panthalassa, the universal global sea, the story says.
But was it this simple?
The first thing to notice is that 200-300 million years is just 4-6 percent of the age of the Earth. So the era of Pangaea is a relatively recent phenomenon. Why would all the continents be unified into one this recently even though they're split again? The key part of the answer is that Pangaea wasn't the first supercontinent; on the contrary, it was the most recent one (and the first one to be historically debated). The plates have been rearranging and splitting and merging for a long time and as much as billions of years ago, many other supercontinents existed.
They perhaps included Vaalbara, Ur, Kenorland, Rodinia (Russian for Fatherland, roughly speaking). These supercontinents incorporated most of the landmass when they were around. But there were also supercontinents that contained about 1/2 of the landmass when they were around – these were the Cold War periods of the supercontinent cycle. ;-) Gondwana and Laurasia – 510-180 million years BC or so – are the most famous example. Gondwana was composed of components including South America and Africa that fit together so neatly. Some other hypothetical continents in the past have been submerged. And there could have been ordinary-size continents whose names coincide with countries and other regions – Kazakhstania, North China, Siberia, India, and others.
Many of the details are unknown and many of the known details may be wrong. That's equally true about the future projections of supercontinents. Will there be Amasia (China will merge with the U.S., not only when it comes to their currency union haha), Novopangaea, or Pangaea Ultima? These are highly inequivalent scenarios how the contemporary continents may merge in the future (hundreds of millions of years in the future).
The idea of supercontinents that covered "almost everything" is very tempting. In some sense, it is as almost as tempting as the unification of forces (and other concepts) in physics. But when we return to relatively recent eras, it may be misleading and I would guess that it probably is. There's really no reason why 95% of the landmass should be connected at any moment less than 500 million years ago – when the Earth was already 90 percent of the actual current age old.
These days, we have continents such as Australia and large islands such as the Greenland. And the rest is divided to several mostly disconnected parts, too. I guess that at most moments in the past, the decomposition was comparable so there always existed separate continents and icelands that occupied a similar percentage of the global landmass.
Let me mention that the difference between continents and islands goes beyond the continents' being larger. There should also be an intrinsic geological difference and indeed, there are at least two major ones. First, continents should be made of low-density rocks so that they "float" while islands are just extensions of ocean floor that happen to reach above the sea level in some cases and should be composed of heavier rocks. Needless to say, I think it's preposterous to imagine that there is always a clear separation between the two concepts. Second, landmasses that sit at their own tectonic plate (e.g. Australia) should better be called continents.
So I find it "more likely than not" that even during the most unified moments of Rodinia or Pangaea, there used to be disconnected continents that were larger than the Greenland, for example, and perhaps many smaller continents existed at various moments, too.
The very first years of the Earth could paint a different story, however. Yes, I do think that the mountains used to be taller and steeper; the Earth is getting rounder after every earthquake as some potential energy of the rocks is converted to heat. And yes, I think that there was a chance that the continents and islands were less fragmented than they are today. However, for some reason, I find it a bit more likely that there were two major landmasses – nearly opposing each other – when the Earth was very, very young.
What's the reason behind this idiosyncratic claim of mine? When we approximate the early solid Earth by a random perturbed ellipsoid, its longest semiaxis is likely to show the positions of the two major supercontinents – on the opposite sides of the globe. Because the rocks are generically heavier than water, I find the idea of water on one side and rocks on the other side to be highly imbalanced. Much like tides, it seems more sensible to assume that the rocks get out of the water level – an equipotential surface – at two opposing places at the same moment.
To a large extent, I would make the same guess about most "highly unified" moments in the geological history, both in the past and in the future. It really does seem to me that the people who try to reconstruct the details of the continental drift don't fully incorporate the change of the equipotential surfaces – gravity – caused by the accumulation of the landmasses. If they did so, they would arguably realize that the sea level near excessively large continents inevitably goes up (so the mountains on too large continents get lowered relatively to the surrounding sea level) while the sea level goes down on the opposite side of the globe which makes it more likely that some continent or island will emerge on the opposite side of the globe.
Do you agree with that? The main loophole I may imagine is that the original solid Earth was highly non-uniform so the actual center of mass could have been shifted away from the "apparent geometric center" by a significant distance, making it likely that the continent would only appear on one side. I haven't tried to quantify how strong this effect could have been, relatively speaking.
These days, the Earth is kind of balanced. The Eastern Hemisphere contains the bulk of Eurasia and Africa but the Western Hemisphere boasts Americas. The Northern Hemisphere contains significantly more landmass than the Southern one but even this difference isn't "overwhelming". Moreover, the average thickness of ice in the Antarctica is 2 km which helps to add at least some balance.