Why the Nile River hasn’t changed course in 30 million years

Scientists have finally solved the geologic mystery of the Nile River’s unchanging path. And they made another discovery: the river is about 30 million years old—six times as old as previously believed.

Ancient Egyptians considered the Nile River to be the source of all life. The steady northward path of the river has nourished the fertile valleys of northeast Africa for millions of years and in doing so, shaped the course of human civilization.

Long-lived rivers usually move over time. Researchers have cracked the case of why the Nile is different by linking the river’s flow to the movement of rock in the Earth’s deep mantle.

They discovered that if not for the mantle movement keeping the river on course, the Nile would have turned west long ago, probably changing the course of history along with it.

The findings appear in Nature Geosciences.

“One of the big questions about the Nile is when it originated and why it has persisted for so long,” says lead author Claudio Faccenna, a professor at the Jackson School of Geosciences at the University of Texas at Austin. “Our solution is actually quite exciting.”

Nile River’s ‘conveyor belt’ topography

The results should settle a long-running debate about the age of the river and provide evidence that the slow movement of the deep mantle is one of the key forces shaping our Earth’s landscape and geological processes.

The Earth’s mantle is composed of solid rock that flows like a fluid over long periods. Like currents in an ocean, different areas of the mantle have different circulation patterns.

The researchers connected the tilted nature of the Nile’s topography to a conveyor belt of mantle rock pushing up against the Ethiopian Highlands in the south and pulling the surface down in the north. From beginning to end, the gentle gradient keeps the Nile on a consistent northward course.

The research involved tracing the geological history of the Nile by studying ancient volcanic rock in the Ethiopian Highlands and correlating it with enormous deposits of river sediment buried under the Nile Delta. This told the researchers that after rising dramatically, the Ethiopian Highlands have remained at a similar height for millions of years, which the researchers attribute to the support of the mantle rock from below.

“We know that the high topography of the Ethiopian plateau was formed about 30 million years ago,” says research scientist Thorsten Becker, a professor at the Jackson School.

Until now, however, it was unclear what has maintained the topography for so long.

River simulations

The team used computer simulations that re-created 40 million years of Earth’s plate tectonic activity to verify their findings.

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 a conveyor belt in the mantle that persists to this day.

The simulation reproduced changes in the landscape almost exactly as the scientists had expected—including small details in the landscape such as whitewater rapids found along the length of the Nile.

The ability of the model to refine such small details was a big surprise and a significant research finding, says Petar Glisovic, a research collaborator at the University of Quebec. “I think this technique gives us something we didn’t have in the past,” he says.

Combining diverse geological data with state-of-the-art, geophysical modeling was key for the research, says Eric Kirby, a professor at Oregon State University. “Without either piece, you wouldn’t get such a compelling result,” says Kirby, who was not involved in the study.

The team now hopes to apply the technique to other rivers such as the Yangtze and Congo.

Additional researchers from the University of Florida, the University of Milano-Bicocca, the Geological Survey of Israel, and the Hebrew University of Jerusalem contributed to the work, which received support from NASA and the Italian government’s Ministry of Education, University and Research.

Source: University of Texas at Austin

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