It might seem that the world's landmasses are fixed, but as Richard Fisher discovers, there are major changes coming.Nearly 500 years ago, the Flemish cartographer Geradus Mercator produced one of the world's most important maps.
It certainly wasn't the first attempt at a world atlas, and it was not particularly accurate either: Australia is absent, and the Americas are only roughly drawn. Since then, cartographers have produced ever-more precise versions of this continental arrangement, correcting for Mercator's errors, as well the biases between hemispheres and latitudes created by his projection.
But Mercator's map, along with others produced by his 16th-Century contemporaries, revealed a truly global picture of Earth's landmasses – a perspective that has persisted in people's minds ever since.
What Mercator didn't know is that the continents have not always been arranged this way. He lived around 400 years before the theory of plate tectonics was confirmed.
When looking at the positions of the seven continents on a map, it's easy to assume that they are fixed. For centuries, human beings have fought wars and made peace over their share of these territories, on the assumption that their land – and that of their neighbours – has always been there, and always will be.
From the Earth's perspective, however, the continents are leaves drifting across a pond. And human concerns are a raindrop on the leaf's surface. The seven continents were once assembled in a single mass, a supercontinent called Pangaea. And before that, there's evidence for others stretching back over three billion years: Pannotia, Rodinia, Columbia/Nuna, Kenorland and Ur.
Geologists know that supercontinents disperse and assemble in cycles: we're halfway through on now. So, what kind of supercontinent might lie in Earth's future? How will the landmasses as we know them rearrange over the very long-term? It turns out that there are at least four different trajectories that could lie ahead. And they show that Earth's living beings will one day reside on a very different planet, which looks more like an alien world.
For geologist Joao Duarte at the University of Lisbon, the path to exploring Earth's future supercontinents began with an unusual event in the past: an earthquake that struck Portugal one Saturday morning in November 1755. It was among the most powerful quakes of the past 250 years, killing 60,000 people and sending a tsunami across the Atlantic Ocean. What made it particularly odd was its location. "You should not have big earthquakes in the Atlantic," says Duarte. "It was strange."
Earthquakes of this scale usually happen on or near major subduction zones, where oceanic plates plunge beneath the continents and are melted and consumed in the hot mantle. They involve collision and destruction. The 1755 quake, however, happened along a "passive" boundary, where the ocean plate underlying the Atlantic transitions smoothly into the continents of Europe and Africa.
In 2016, Duarte and colleagues proposed a theory for what might be going on: the stitches between these plates could be unravelling, and a major rupture may be looming. "It could be a kind of infectious mechanism," he explains. Or like the glass splintering between two small holes in a car windscreen. If so, a subduction zone could be poised to spread out from the Mediterranean along western Africa and perhaps all the way up past Ireland and the UK, bringing volcanoes, mountain-building and earthquakes to these regions.
Duarte realised that, if this happens, it could lead to the Atlantic eventually closing. And if the Pacific continued to close too – which is already occurring along the subducting "Ring of Fire" circling it – a new supercontinent would eventually form. He called it Aurica, named because the former landmasses of Australia and the Americas would sit at its centre. After Duarte published his proposal for Aurica, he wondered about other future scenarios. After all, his was not the only supercontinent trajectory that geologists had proposed.
So, he began chatting with oceanographer Matthias Green at Bangor University in Wales. The pair realised they needed someone with the computational chops to create digital models. "That person had to be someone a little bit special, who didn't mind studying something that will never happen in human timescales," he explains. That turned out to be his colleague Hannah Davies, another geologist at Lisbon University. "My job was to turn drawings and illustrations from past geologists into something that is quantitative, geo-referenced and in a digitised format," explains Davies. The idea was to create models that other scientists could build on and refine.
But it wasn't straightforward. "What we were nervous about is it's an incredibly blue-sky topic. It's not in the same kind of vein as a regular scientific paper," says Davies. "We wanted to say, 'Okay, we understand this much about plate tectonics after 40 years or 50 years. And we understand this much about mantle dynamics, and all of the other components of the system. How far can we take that knowledge into the future?'"
This led to four scenarios. As well as modelling a more detailed picture of Aurica, they explored three other possibilities, each projecting ahead roughly 200-250 million years from now.
The first was what could happen if the status quo continues: the Atlantic stays open and the Pacific closes. In this scenario, the supercontinent that forms will be called Novopangaea. "It is the most simple, and most plausible based on what we understand right now," says Davies.
However, there could also be geological events in the future that lead to different arrangements.
One example is a process called "orthoversion" where the Arctic Ocean closes and the Atlantic and Pacific remain open. This changes the dominant orientations of tectonic spreading, and the continents drift northward, all arranging around the North Pole, except Antarctica.
In this scenario, a supercontinent called Amasia forms: Finally, it's also possible that the seafloor spreading in the Atlantic could slow down. In the middle of the ocean, there's a giant ridge bisecting two plates, running through Iceland all the way down to the Southern Ocean. Here, new lithosphere is forming, feeding out like a conveyor belt. If this spreading slowed or stopped, and if a new subducting plate boundary formed alongside the east coast of the Americas, you'd get a supercontinent called Pangaea Ultima, which looks like an enormous atoll: These four digital models now mean that geologists have a base to test other theories. For example, the scenarios could help scientists to understand the effects of different supercontinental arrangement on the tides, as well as the climate of the deep future – what would the weather be like on a world with a massive ocean and giant landmass?
To model the climate of a supercontinent, "you cannot use the IPCC
models, full stop, because they are not designed to do that", says Duarte. "You cannot change the variables that you need to change."The models of Earth's future supercontinents can also serve as a proxy for understanding the climate of exoplanets. "The future Earth is completely alien," says Davies. "If you were in orbit above Aurica, or Novopangaea, you probably wouldn't recognise it as Earth, but another planet that had similar colours." This insight led the trio to collaborate with Michael Way, a physicist at the Nasa Goddard Institute for Space Studies. He and his colleagues seek to study climates on alien worlds by modelling the variations of our own over deep time. "We only have so many examples of what a temperate climate can look like. Well, we have one example to be honest: Earth, but we have Earth through time," says Way. "We have the past scenarios, but by moving to the future and using these wonderful tectonic models for the future, it gives us another ensemble to add to our collection."
You need such models because it can be difficult to know what to look for when analysing potentially habitable exoplanets from afar. Ideally you want to know if a planet has a supercontinent cycle, because the presence of life and active plate tectonics may well be entwined. The continental arrangement could also affect the likelihood of liquid water. Through telescopes, you can't see the continents, and the atmospheric composition can only be inferred. So, models of climate variations could reveal some indirect signature that astronomers could detect. Way's modelling of the supercontinent climates – which took months using a supercomputer – revealed some striking variations between the four scenarios. Amasia, for example, would lead to a much chillier planet than the rest. With land concentrated around the North Pole and the oceans less likely to carry warm currents to cooler latitudes, ice sheets would build up. Aurica, by contrast, would be balmier, with a dry core but coasts akin to Brazil's today, with more liquid water.
All this is helpful to know, because if an Earth-like exoplanet has plate tectonics, we won't know which stage of the supercontinent cycle it is currently in, and therefore we will need to know what to look out for to infer its habitability. We shouldn’t assume that the landmasses will be dispersed, mid-cycle, like our own.
As for our own planet's future, Davies acknowledges that the four supercontinent scenarios they have modelled are speculative, and there may be unanticipated geological surprises that change the outcome. "If I had a Tardis to go and see, I wouldn't be surprised if, in 250 million years, the supercontinent didn't look anything like any of these scenarios. There are so many factors involved," she says.
However, what can be said for certain is that the landmasses we take for granted will one day rearrange into an entirely new configuration. Countries once isolated from one another will be close neighbours. And if Earth still hosts intelligent beings, they will be able to travel between the ancient ruins of New York, Beijing, Sydney and London without ever seeing an ocean.
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