Braided river systems like those found in the Arctic once covered the whole Earth (Image: B&C Alexanda/ArcticPhoto)
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From its source high in the Peruvian Andes to its mouth on the coast of Brazil 6400 kilometres away, the Amazon river flows across almost the entire breadth of a continent. The mountain range it springs from is even longer, stretching 7000 kilometres from the tropical jungles of Venezuela in the north to the icy wilderness of Tierra del Fuego in the far south.
Impressive on today’s Earth. But quite piddling by the standards of the deep past.
The first stop for your time machine is way back, a billion years ago, when Earth’s landmasses were fused in a supercontinent called Rodinia. Peer out of the window and you’ll see an unfamiliar world. All life then was unicellular and entirely marine, so Rodinia’s vast expanse is completely barren. But what it lacks in biological richness it makes up for in vast river systems and mountain ranges.
As the continental building blocks that made Rodinia crashed together about 1.2 billion years ago, large portions of crust were lifted up, much as the ongoing collision between the Indian and Eurasian plates is generating the Himalayas now.
It is tempting to think that such great forces would push up absolutely enormous mountains. Although we have no way of knowing how high they were, it is unlikely that anything much loftier than the Himalayas has ever existed on Earth, says geologist David Rowley of the University of Chicago. Weathering erodes away mountains as they form, and gravity dictates how much load Earth’s crust can bear without buckling. Account for that, and Mount Everest is about as high as a mountain can be.
But by another measure, Rodinia’s mountains are mind-blowing. Imagine taking the Andes, Rockies, Himalayas, Alps, Atlas and Urals and stringing them together end-to-end and you’re getting close to the length of Rodinia’s principal mountain chain.
“The range stretched across the entire supercontinent, maybe 15 to 20,000 kilometres,” says Robert Rainbird of the Geological Survey of Canada in Ottawa. Its eroded remains can still be found across North America and Europe, including parts of the Appalachians and the Highlands of Scotland.
And just as mountain ranges like the Andes and Himalayas give rise to great rivers today, so too did those of Rodinia – but with a big difference. “There was no vegetation to constrain the rivers so they would have just flowed unconstrained across the barren landscape,” says Rainbird.
Similar river systems, characteristically braided into many smaller channels, exist in the vegetation-free high Arctic today, but on Rodinia they were vastly bigger. Rainbird and his colleagues have found sediments from the Rodinian mountains 3000 kilometres away on the other side of North America, as well as in India, Antarctica, Scandinavia and Siberia, indicating river systems that spread across the whole supercontinent. “They would have been very dominant features, far longer and wider than the Amazon,” says Rainbird. The rivers would also have fed huge inland seas far bigger than anything we have on Earth now, he says ().
Rodinia began to break apart about 750 million years ago, splitting its vast mountain range into pieces. By the time the landmasses reassembled into the next supercontinent, Pangaea, around 300 million years ago, the land was covered in vegetation. So while Pangaea might also have been home to huge mountain ranges, the great rivers of Rodinia are possibly unique in Earth’s history.
If monster rivers and massive mountain ranges are not your thing, another feature of Rodinia might make it worth a visit: shorter days. The moon will look larger than you’ve ever seen it, because it was closer to Earth back then. That made the planet rotate faster, like a spinning ice skater with arms and legs tucked in. Sedimentary rocks which contain a record of the height of the tides around 900 million years ago indicate a Rodinian day lasted somewhere between 19 and 21 hours.
Read more: “A time traveller’s guide to Earth”
This article appeared in print under the headline “River wide, mountain high”
