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Mysteries of the Nile: how to age an ancient river

Mysteries of the Nile: how to age an ancient river Nina R.
26 Nov
2019
The Nile is home to mysteries both ancient and modern – these scientists may have got to the bottom of two long-debated questions

The great river which sustained the Ancient Egyptians, producing a fertile plain on which to cultivate crops and build an extraordinary civilisation, was in some ways a mystery to them – or so the story goes. Dubbed the river Ar (or Aur), meaning ‘black’ due to the colour of the sediment left after the annual flood, what we now call the river Nile was considered unusual for the way it flowed from south to north, in contrast to other rivers in the region.

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This directional mystery, at least, has been solved (simply put, the river flows this way because the land tilts downhill, from the high Ethiopian plateau to the Nile basin and finally out into the Mediterranean), but other conundrums still remain. In particular, two questions are now high on the agenda: how old is the river Nile, and why has it never changed path?

Previous research in this area has suggested that the Nile has been following its current path for around six million years, but a more recent study has placed doubt on that theory. Led by Claudio Faccenna, a professor at the UT Jackson School of Geosciences, the collaborative study provides evidence that the Nile has been carving its way across East Africa for 30 million years. The longevity of this route is unusual as most long-lived rivers change course over time (though the researchers state that the actions keeping the Nile so steady may prove relevant in other regions too).

figure 1 copyMap showing how sediment is transported from the Nile’s source high in the Ethiopian Highlands to the Nile Delta in the Mediterranean Sea. By studying the layers of sediment carried by the river over millions of years, the researchers were able to recreate the story of uplift in the region (Image: Claudio Faccenna et al.)

According to the scientists behind this study, it all comes down to processes deep beneath the Earth’s surface. Utilising on-the-ground geologic research and computer models, the team was able to demonstrate that movement of solid rock in the Earth’s mantle, at a depth of 200 to 500km, has over the years sustained the path of the river by controlling the tilted nature of the Nile's topography. They liken the movement of the mantle to a ‘conveyor belt’ of rock pushing up against the Ethiopian Highlands in the south and pulling the surface down in the north.

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‘The mantle is circulating and convecting, because you have dissipating heat,’ explains Faccenna. ‘The hot part is going up and the cold is going down. We launched the idea that the convection in the mantle forms like a conveyor belt, popping up in Ethiopia and coming down in the Mediterranean.’ It’s this gentle gradient keeps the Nile on the consistent northward course that so confused the ancient peoples who lived on its banks.

The team verified its findings using computer simulations which recreated 40 million years of the Earth’s plate tectonic activity. The model showed the arrival of a hot mantle plume that probably led to the outpouring of lava that formed the Ethiopian Highlands while activating the conveyor belt in the mantle that persists to this day. The team was particularly pleased to find that this simulation reproduced changes in the landscape almost exactly as it had expected – including small details such as whitewater rapids found along the length of the Nile.

Faccenna in fieldClaudio Faccenna (pictured) and the research team brought together a huge catalogue of geological evidence to unravel the evolution of the Ethiopian plateau. Their finding, that the tilted landscape had persisted for 30 million years, means that the eternal river is around six times older than previously thought (Image: Claudio Faccenna)

It was a combination of this computer-based research and work done at the river itself that allowed the scientists to put an age to the Nile. During the course of the research, some scientists studied the ancient volcanic rock in the Ethiopian Highlands, while others examined river sediment buried under the Nile Delta, thousands of kilometres away. ‘At the Nile Delta they found that the very old deposits started depositing around 30 million years ago, and they found traces of the Ethiopian material down there. That made a connection directly between the two,’ says Faccenna. This indicated to the researchers that after rising dramatically, the Ethiopian Highlands have remained at a similar height for millions of years. From there, the river has maintained its ever-steady path to the sea.

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