Tuesday, February 26, 2013 ... Deutsch/Español/Related posts from blogosphere

Mauritia: microcontinents must have been around for eons

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.

Add to del.icio.us Digg this Add to reddit

snail feedback (9) :

reader Petr C. said...

Lubos, do you really 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." ? With the moon being much closer than it is today wouldn't the tidal forces alone make higher and steeper mountains harder to exist?

reader Luboš Motl said...

Dear Petře, maybe I am doing a mistake but my answer is that it wouldn't. But it's because I don't quite understand your reasoning.

The Moon-controlled tides are another process - but I think it's largely independent from the earthquakes. Moreover, I think that its impact has the same sign. Tides also cause some erosion that tends to make the Earth rounder, I would guess.

A perfectly round Earth has the minimum gravitational potential energy among similar arrangements of the mass and this is where we're converging.

At the beginning, I find it conceivable if not likely that there existed 20-km and maybe even much higher mountains for millions or hundreds of millions of years. The two excessive and sharp ones would be removed or collapsed quickly. More modest ones could exist for a much longer time.

Tides could have been stronger as well if the Moon was closer but I don't understand why it would imply that taller mountains were impossible. Quite on the contrary, I am imagining that everything - tides as well as mountains - were taller than today, things were just less uniform. I don't see a contradiction.

The average height of top 20 mountains probably decays roughly as some exponential, exp(-Ct), with the time from the birth of the Earth. If there's some physical impossibility in these claims, could you please be more explicit about it? Thanks.

reader Chad said...

As you state, rocks are generically heavier than water, but there is a lot of water, and there are a lot of rocks under water. And, on a related issue (I think) do we need to consider glacial periods with their differing amounts of landmass/ice cover. During glacial periods, a lot of water is advected poleward lowering the levels of the oceans by 100m and depositing more mass at the poles. Dunno.

If memory serves me, the Pangaea concept was formulated from evidence gathered from the similarity of a large quantity of species on the various continents and islands. But that does not disprove your thought about landmasses on opposite sides of the earth, as it could just be that life originated on that particular landmass named Pangaea.

reader Rami Niemi said...

Should the geophysicists agree the mechanism driving the plates one could say if the forces where stronger earlier, and mountains higher. However the convection of mantle must have been stronger in history because the energy comes from fission in deep. Cant say much about subduction zone sinking in history.

reader Bernhard Jordan said...

australia is not a continent , it cant be . In the same continent is located papua new guinea.

Greenland is not an island to Greenland is in the north America continent

reader Bernhard Jordan said...

if the continents are composed by floaty rocks , are the ocean flour composed by heavier ones ? If so wouldn't that be an indication of heavier and valuable elements on the ocen rocks ?

reader Erik Bloem said...

To study the oldest times one also must look at the oldest shields that contain the oldest rocks on earth, called preCambrian or Archaean. Those rocks can be found on most continents, like in Scandinavia, large parts of Canada and Western Australia for example. The rocks comprise mostly of intrusive granites and some 'greenstone belts', the latter being volcanic rocks. Little sediments are found. This indicates that the earth at that time was very active with volcanism, granites being igneous rocks from rest-lavas rising through the crust. The large amounts of granits indicate that the shields are relatively light-weight compared to oceanplate rocks, mostly comprised of heavy basalt (extrusive) volcanic rocks.

That's why I think that during the early age of the earth (Archaean) there wasn't really a division yet in continents and real oceans, but rather there were 'proto-continents' of the granite-greenstone belts described above. The greenstone belts (volcanic rocks) were most abundant, see them as proto-ocean floors, they are in fact of the same density and origin (volcanic). The continents were created mostly where the granitic rocks where more abundant, or simply where the granitic rocks were rising through the greenstone belts, simply because the granites are lighter. These areas were then little by little rising (lighter), so they are in fact 'floating'. The continents grew slowly by more granitic uprising through the crust and little by little by more sediments, that came down from the first proto-continents. Since water is going to the lowest places, the continents became finally above water. I guess that during the early times the earth was completely covered with water, but not as deep as today. The ocean floors got (and get) destroyed all the time in subduction zones, so we do not know anything about the earliest proto-oceans, but they were probably just from 'greenstone belt' origin.

Therefore, I guess that in the early times the problem of two landmasses probably did not exist (it was just one big greenstone belt covered by water where granites were intruding) and it is also difficult to say when exactly we can speak about real continents as we know them today. I guess the proterozoic or late-archaean (1.8 Billion years ago) is where they first appeared.

Nonetheless, an interesting article and the continental drift from the proterozoic until today indeed took place moving continents around the globe and creating several supercontinents and breaking them up again.

reader Erik Bloem said...

Not necessarily so. Basalts are relatively poor in silica, granites (continents) relatively rich. Basalts are richer in Magnesium and Calcium, but those elements are not so valuable. Nonetheless, there are geologists now exploring the ocean floors for valuable elements. Nautilus Minerals in Australia was one of the first starting around 25 years ago.

reader Erik Bloem said...

You are correct to state that Papua New Guinea belongs to the same 'Australian' continent. But we still, for sake of simplicity, call Australia (with PNG) a continent.

(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){ (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o), m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m) })(window,document,'script','//www.google-analytics.com/analytics.js','ga'); ga('create', 'UA-1828728-1', 'auto'); ga('send', 'pageview');