Pangaea, the comeback

It's the year 250,000,000 and Earth is alive and well. Humans have long since perished, but the planet is still home to a bewildering array of life forms. Yet apart from a few mysterious fossils there is no trace that we ever existed.

If we could visit this future Earth we would barely recognise it. The continents have crashed together to form a single gigantic supercontinent, surrounded by a global ocean. Much of the land is inhospitable desert, while the coast is battered by ferocious storms. The oceans are turbulent on the surface, stagnant at depth and starved of oxygen and nutrients. Disease, war, or asteroid collisions have pushed humans and many of the species we know today to extinction and competition has seen off all but the hardiest of the rest.

This supercontinent isn't the first on Earth, and it won't be the last. Geologists now suspect that the movements of the Earth's continents are cyclical, and that every 500 to 700 million years they clump together. Unfolding over a period three times as long as it takes our solar system to orbit the centre of the galaxy, this is one of nature's grandest patterns. So what drives this cycle, and what will life be like next time the continents meet?

The continents move because of circulation in the Earth's mantle beneath the seven major tectonic plates. Where the plates meet, one is forced below the other in a process called subduction. This pulls apart the crust at the other side of the plate, allowing new molten rock to well up to the surface to fill the gap. This process means that oceanic crust is constantly being created and destroyed, but because the continents are made from less dense rock than the heavier and thinner oceanic crust that forms the ocean floor, they ride higher in the mantle and escape subduction.

As a result, the continents hold their shape for hundreds of millions of years as they glide slowly around the planet. Inevitably, though, continents collide, and sometimes clump together to form a supercontinent.

The most recent, Pangaea, formed 300 million years ago and was already breaking up 100 million years later as the dinosaurs evolved. Some 1.1 billion years ago, another supercontinent, called Rodinia, formed, breaking up 250 million years later. Before that, another, and there were almost certainly many more still earlier, but since the formation of one supercontinent tends to destroy evidence of its predecessor, no one can be certain about exactly how many there have been. What is generally agreed is that there have been two true supercontinents containing all or nearly all the land on Earth - Pangaea and Rodinia - and there may have been many more true or partial supercontinents, including Pannotia, Columbia, Kenorland and Ur (see Diagram).

Right now, we are halfway through a cycle. The Pacific is gradually closing, as oceanic crust sinks into subduction zones in the north Pacific, while the Mid-Atlantic Ridge is feeding out new ocean floor as the Americas move apart from Europe and Africa. Africa is moving northward, heading for the southern coast of Europe, while Australia is also on its way north towards south-east Asia. The continents are moving at about 15 millimetres per year - similar to the speed your fingernails grow.

Roll the clock forward 50 to 100 million years and it's easy to get a rough idea where things are going. But seeing further into the Earth's future takes more than just projection of the continents' current movements. Christopher Scotese of the University of Texas, Arlington, likens the problem to predicting your drive along a highway. "You can make a guess at where you're going to be in 5 or 10 minutes, but there are always accidents, people change lanes, or the road may diverge and you have to make a choice."

There are two main ways today's continents could fit together. If the Atlantic continues to widen, the Americas will eventually crash into Asia. Alternatively, a subduction zone might somehow open up in the Atlantic and reel the sea floor back in, forcing Europe and America back together. This would essentially re-create Pangaea.

In 1992 geologist Chris Hartnady, from the University of Cape Town in South Africa took up the challenge of "pre-constructing" the next supercontinent. As the Atlantic continues to widen, "the Americas, swinging clockwise about a pivot in north-eastern Siberia, seem destined to fuse with the eastern margin of the future supercontinent", which Harvard University geologist Paul Hoffman called "Amasia". In this vision of the future, Australia will continue northward while Africa stays more or less in its present position. Antarctica won't join the supercontinent, remaining at the South Pole. "It's not attached to any subduction zone so there is no reason for it to move," Hoffman says.

Roy Livermore, now at the University of Cambridge, came to a similar conclusion. In the late 1990s he created his own version of Amasia - a future supercontinent he called Novopangaea. "I have taken the liberty of opening up a new rift between the Indian Ocean and the North Atlantic," he says. "We know the East African Rift is active, so we project that into the future by opening a small ocean. East Africa and Madagascar move across the Indian Ocean to collide with Asia; Australia has already collided with south-east Asia." South of what is now India, a mountain chain has risen from the sea along a new subduction zone. Just south of it is Antarctica.

In Livermore's future, all the present continents take part. "I don't believe Antarctica is going to stay at the pole," he says. "I want it to come north." For this to happen, he postulates a new subduction zone will open up to drag it that way. "The beauty of all this is that no one will ever be able to prove me wrong," he says.

That may be true, but other researchers disagree on how the future planet will look. Scotese has spent much of his career reconstructing where today's continents used to lie, and now applies this knowledge to project the continents into the future. He sees the planet's distant future very differently to Hoffman and Livermore.

Making mountains

Like them, he predicts that over the next 50 million years Africa will continue north, closing the Mediterranean and driving up a Himalayan-scale mountain range in southern Europe. Australia will rotate and collide with Borneo and south China. But 200 million years later, everything will change, he says. Subduction starts up on the west side of the Atlantic. The widening stops and the Atlantic begins to shrink, bringing most of the world's land masses back together as North America comes crashing into the merged Euro-African continent. Scotese originally called the resulting supercontinent Pangaea Ultima, but has recently renamed it Pangaea Proxima, meaning the next Pangaea. "The name Ultima bothered me because it implies that it's the last supercontinent," Scotese says. "This process will continue for another couple of billion years."

He says a new Atlantic subduction zone could start if a small existing subduction zone, such as part of the Puerto Rico trench in the Caribbean, spread up and down the American coast as a result of changing stresses on the planet. Under the right circumstances, he says, the crust could start to tear along this line, signalling the beginning of the end for the Mid-Atlantic Ridge. Today it lies halfway between Europe and the Americas, but "if we were to start subduction in either the western Atlantic or the eastern Atlantic, then the ridge would be forced to move toward the subduction zone", he says. "Eventually it would be subducted and we'd have an ocean with a subduction zone but no ridge. That means we close the ocean, and we close it pretty fast."

For now there is nothing to show whose model is right, but what everyone agrees on is that life on the next supercontinent - however it forms - will be tough. "Supercontinents create extremes," says Paul Valdes, a climatologist at the University of Bristol, UK. We can tell what Pangaea's climate was like from geological evidence: the positions of climate-sensitive deposits such as coal, which originates in warm, wet conditions, for example, or the mineral deposits called evaporites that form when lake sediments dry out in a hot climate. This evidence can then be used to build computer models to forecast what the climate might be like in the future. The models that result suggest that supercontinents are prone to violently changing seasons.

"In Pangaea, tropical latitudes could be quite hot, up to perhaps 44 °C. Mid-latitudes had very hot summers with very cold winters when it could get down to -20 or -30 °C with very heavy snowfall," Valdes says. "In summer it would all melt, producing major flooding." Despite this, vast areas of the interior would have been dry, because rain clouds would not have been able to penetrate far inland. In such extreme climates, only a small proportion of the land could support life. On Pangaea, Valdes says, the best real estate was probably in a narrow zone just outside the tropics on the north coast of the Tethys Sea.

The vastness of the supercontinent's land mass will also provoke extreme weather. "Monsoons form because of temperature differences between the land and ocean. If you have a huge land mass, it warms up a lot and stimulates a mega-monsoon," Valdes says.

The next supercontinent's weather could be even worse. If the supercontinent happens to form at the end of an active volcanic phase, leaving behind an atmosphere rich in carbon dioxide and a warmer planet, warm surface waters could drive extreme hurricanes or "hypercanes". These huge weather systems, thousands of kilometres across and some 50 per cent stronger than today's strongest hurricanes, would batter the landscape with wind speeds of more than 400 kilometres per hour.

Life will also be difficult in the oceans. The global conveyor system of currents that keeps today's oceans oxygenated and stocked with essential nutrients depends on the size and shape of the ocean basins, and therefore the positions of the continents. Move the continents and these conveyors could cease to exist. As a result, below a few hundred metres the waters will become stratified and anoxic, and little will be able to survive.

The reef-fringed coasts close to the equator will be full of life, but even here life won't be easy. As the continents crowd together, there will be a vast reduction in the area of shallow seas that will probably lead to a mass extinction as species from all over the world are thrown together and forced to compete. Something similar will happen on land. The formation of Pangaea has been implicated in the greatest species loss of all time, the Permian mass extinction, due in part to the huge reduction in available habitats.

Life has a knack of making the best of new situations, however. As Pangaea formed and the southern icecaps melted 290 million years ago, there emerged perhaps the Earth's eeriest ever ecosystem. Dense forests of now-extinct Glossopteris trees stood up to 25 metres tall on the southern coast of the Tethys Sea and stretched inland to within 20 degrees of the South Pole.

Despite having only a summer of feeble light to sustain them, they were able to survive months of unremitting winter darkness. Trees close to the coast were lashed by mega-monsoon winds and rains roaring in from the Tethys, with thick cloud obscuring the already weak sunshine. As winter approached, Glossopteris's tongue-like leaves would fall to the oxygen-starved peat before six months of total darkness. Not surprisingly, analysis of fossilised growth rings shows that Glossopteris grew frenetically when it could.

Whatever life has to cope with on the next supercontinent, humans won't be around to see it. The next supercontinent is no more than a glint in the planet's eye, but already it has valuable lessons to teach us: clever we may be, but the Earth marches on, with or without us.