Considered the source of life by the ancient Egyptians, the majestic Nile — Africa’s longest river — follows a seemingly improbable northward course from Ethiopia.

Looking at a map, one might wonder why the river didn’t flow east into the nearby  Red Sea — while geologists long believed it once drained west, to the Atlantic.

The discovery of ancient sediments under the Nile Delta that came from the Blue Nile’s source in Ethiopia shows it has instead been forging north for 30 million years.

This makes the revered river around six times older than previously thought. 

Geologists found that a ‘conveyor belt’ of rising magma under the Earth’s crust began pushing up the Ethiopian Highlands 30 million years ago.

At the same time, the crust under Egypt was sinking down — forming a gentle gradient that guided the Nile along its northward path.

Had this not formed, Africa’s topography would have drained the Nile westward to the Atlantic, dramatically altering the course of civilisation in the region.  

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Looking at a map, one might wonder why the river didn't flow east into the nearby Red Sea — while geologists long believed it once drained west, to the Atlantic. The discovery of ancient sediments under the Nile Delta that came from the Blue Nile's source in Ethiopia shows it has instead been forging north for 30 million years

Looking at a map, one might wonder why the river didn’t flow east into the nearby Red Sea — while geologists long believed it once drained west, to the Atlantic. The discovery of ancient sediments under the Nile Delta that came from the Blue Nile’s source in Ethiopia shows it has instead been forging north for 30 million years

Long-lived rivers usually move over time — making the Nile’s stability over time something of a mystery.

‘One of the big questions about the Nile is when it originated and why it has persisted for so long,’ said geologist Claudio Faccenna of Italy’s Roma Tre University.

‘Our solution is actually quite exciting,’ he added.

To solve the puzzle of the Nile’s unchangeability, Professor Faccenna and colleagues set out to trace the river’s history.

To do this, they analysed ancient, volcano-derived rocks in the Ethiopian highlands, matching these to the deposits of sediments carried down the river and ultimately buried under the Nile Delta.

The researchers determined that after rising rapidly,  the Ethiopian Highlands have remained a relatively similar height for millions of years, supported by rock in the Earth’s mantle, beneath.

‘We know that the high topography of the Ethiopian plateau was formed about 30 million years ago,’ said paper author and geophysicist Thorsten Becker of the University of Texas. 

According to the researchers, however, it had not previously been clear how this topography was maintained over such a length of time. 

From analyses of rocks along the river and simulations, experts determined that the river's northward flow has been maintained by movements in the Earth mantle beneath. Pictured, the Nile as it appears upstream, in Uganda

From analyses of rocks along the river and simulations, experts determined that the river’s northward flow has been maintained by movements in the Earth mantle beneath. Pictured, the Nile as it appears upstream, in Uganda

To verify their findings, the team turned to computer simulations, which modelled around 40 million years of the Earth’s plate tectonic activity.

The team found that the arrival of a so-called mantle plume — an upwelling of hotter rock from deep within the Earth — likely provided the lava which generated the Ethiopian Highlands.

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This plume would also have forged a sort-of conveyor belt of magma in the mantle that persists, supporting the highlands, up to the present day. 

This mantle activity — lifting up the south and pulling the north down at the same time — would have kept the Nile on a gentle gradient that maintained its consistent, northward-wending course. 

The simulation was also able to reproduce changes in the African landscape almost exactly as the team expected — even down to such tiny aspects as the whitewater rapids that can be found along the length of the Nile.

‘I think this technique gives us something we didn’t have in the past,’ commented paper author and geophysicist Petar Glisovic — now of the University of Quebec — noting that the model’s ability to refine such small details came as a surprise.

The researchers analysed ancient, volcano-derived rocks in the Ethiopian highlands, matching these to the deposits of sediments carried down the river and ultimately buried under the Nile Delta. Pictured, Professor Faccenna studies rocks along the Nile

 The researchers analysed ancient, volcano-derived rocks in the Ethiopian highlands, matching these to the deposits of sediments carried down the river and ultimately buried under the Nile Delta. Pictured, Professor Faccenna studies rocks along the Nile

Eric Kirby — a geologist from the Oregon State University who was not involved in the present study — said that integrating diverse sources of geological data with state-of-the-art models is key to research like this. 

‘Without either piece, you wouldn’t get such a compelling result,’ he added.

With their initial study complete, the researchers are now hoping to apply their technique to other giant rivers like the Congo and the Yangtze.

The full findings of the study were published in the journal Nature Geoscience

'One of the big questions about the Nile is when it originated and why it has persisted for so long,' said geologist Claudio Faccenna of Italy's Roma Tre University. Pictured, a village on the banks of the Nile in 1891 — a recent memory for the long-lived river

‘One of the big questions about the Nile is when it originated and why it has persisted for so long,’ said geologist Claudio Faccenna of Italy’s Roma Tre University. Pictured, a village on the banks of the Nile in 1891 — a recent memory for the long-lived river

WHAT ARE TECTONIC PLATES?

Tectonic plates are composed of Earth’s crust and the uppermost portion of the mantle. 

Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.

The Earth has fifteen tectonic plates (pictured) that together have molded the shape of the landscape we see around us today

The Earth has fifteen tectonic plates (pictured) that together have molded the shape of the landscape we see around us today

Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other. 

Earthquakes rarely occur in the middle of plates, but they can happen when ancient faults or rifts far below the surface reactivate. 

These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.

 



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