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The concept of a supercontinent is irresistible: what happens when the world's drifting continents clump together in one big lump, surrounded by a single world ocean?
Alfred Wegener, starting in 1912, was the first scientist to discuss supercontinents seriously, as part of his theory of continental motion. He combined a body of new and old evidence to show that the Earth's continents had once been united in a single body, back in late Paleozoic time. At first, he simply called it "Urkontinent" but soon gave it the name Pangaea ("all Earth").
Wegener's theory was the basis of today's plate tectonics. Once we had a grasp of how continents had moved in the past, scientists were quick to look for earlier Pangaeas. These were spotted as possibilities as early as 1962, and today we have settled on four. And we already have a name for the next supercontinent!
What Supercontinents Are
The idea of a supercontinent is that most of the world's continents are pushed together. The thing to realize is that today's continents are patchworks of pieces of older continents. These pieces are called cratons ("cray-tonns"), and specialists are as familiar with them as diplomats are with today's nations. The block of ancient continental crust under much of the Mojave Desert, for instance, is known as Mojavia. Before it became part of North America, it had its own separate history. The crust beneath much of Scandinavia is known as Baltica; the Precambrian core of Brazil is Amazonia, and so on. Africa contains the cratons Kaapvaal, Kalahari, Sahara, Hoggar, Congo, West Africa and more, all of which have wandered about during the last two or three billion years.
Supercontinents, like ordinary continents, are temporary in the eyes of geologists. The common working definition of a supercontinent is that it involved about 75 percent of the existing continental crust. It may be that one part of the supercontinent was breaking up while another part was still forming. It may be that the supercontinent included long-lived fissures and gaps-we simply can't tell with the information available, and may never be able to tell. But naming a supercontinent, whatever it really was, means that specialists believe there's something to discuss. There is no widely accepted map for any of these supercontinents, except for the latest one, Pangaea.
Here are the four most widely recognized supercontinents, plus the supercontinent of the future.
The evidence is sketchy, but several different researchers have proposed a version of a supercontinent that combined the craton complexes Vaalbara, Superia and Sclavia. Various dates are given for it, so it's best to say that it existed around 2500 million years ago (2500 Ma), in the late Archean and early Proterozoic eons. The name comes from the Kenoran orogeny, or mountain-building event, recorded in Canada and the United States (where it's called the Algoman orogeny). Another name proposed for this supercontinent is Paleopangaea.
Columbia is the name, proposed in 2002 by John Rogers and M. Santosh, for an aggregation of cratons that finished coming together about 2100 Ma and finished breaking up around 1400 Ma. Its time of "maximum packing" was around 1600 Ma. Other names for it, or its larger pieces, have included Hudson or Hudsonia, Nena, Nuna, and Protopangaea. The core of Columbia is still intact as the Canadian Shield or Laurentia, which today is the world's largest craton. (Paul Hoffman, who coined the name Nuna, memorably called Laurentia "the United Plates of America.")
Columbia was named for the Columbia region of North America (the Pacific Northwest, or northwestern Laurentia), which was supposedly connected to eastern India at the time of the supercontinent. There are as many different configurations of Columbia as there are researchers.
Rodinia came together around 1100 Ma and reached its maximum packing around 1000 Ma, combining most of the world's cratons. It was named in 1990 by Mark and Diana McMenamin, who used a Russian word signifying "to beget" to suggest that all of today's continents are derived from it and that the first complex animals evolved in the coastal seas around it. They were led to the idea of Rodinia by evolutionary evidence, but the dirty work of putting the pieces together was done by specialists in paleomagnetism, igneous petrology, detailed field mapping, and zircon provenance.
Rodinia appears to have lasted about 400 million years before fragmenting for good, between 800 and 600 Ma. The corresponding giant world ocean that lay around it is named Mirovia, from the Russian word for "global."
Unlike the previous supercontinents, Rodinia is well established among the community of specialists. Yet most of the details about it-its history and configuration-are strongly debated.
Pangaea came together about 300 Ma, in late Carboniferous time. Because it was the latest supercontinent, the evidence of its existence has not been obscured by a lot of later plate collisions and mountain-building. It appears to have been a complete supercontinent, encompassing up to 90 percent of all continental crust. The corresponding sea, Panthalassa, must have been a mighty thing, and between the great continent and the great ocean, it is easy to envision some dramatic and interesting climatic contrasts. The southern end of Pangaea covered the South Pole and was heavily glaciated at times.
Starting about 200 Ma, during the Triassic time, Pangaea broke apart into two very large continents, Laurasia in the north and Gondwana (or Gondwanaland) in the south, separated by the Tethys Sea. These, in turn, separated into the continents we have today.
The way things are going today, the North American continent is heading toward Asia, and if nothing changes dramatically the two continents will fuse into a fifth supercontinent. Africa is already on its way to Europe, closing the last remnant of the Tethys that we know as the Mediterranean Sea. Australia is currently moving northward toward Asia. Antarctica would follow, and the Atlantic Ocean would expand into a new Panthalassa. This future supercontinent, popularly called Amasia, should take shape starting in about 50 to 200 million years (that is, -50 to -200 Ma).
What Supercontinents (Might) Mean
Would a supercontinent make Earth lopsided? In Wegener's original theory, Pangaea did something like that. He thought that the supercontinent split apart because of the centrifugal force of the Earth's rotation, with the pieces we know today as Africa, Australia, India, and South America splitting off and going separate ways. But theorists soon showed that this wouldn't happen.
Today we explain continental motions by the mechanisms of plate tectonics. Movements of the plates are interactions between the cold surface and the hot interior of the planet. Continental rocks are enriched in the heat-making radioactive elements uranium, thorium and potassium. If one continent covers one large patch of the Earth's surface (about 35 percent of it) in a big warm blanket, that suggests that the mantle underneath would slow down its activity while under the surrounding oceanic crust the mantle would liven up, the way a boiling pot on the stove quickens when you blow on it. Is such a scenario unstable? It must be, because every supercontinent so far has broken up rather than hanging together.
Theorists are working on the ways this dynamic would play out, then testing their ideas against the geologic evidence. Nothing yet is settled fact.