Nile River may be older than previously thought

Some of Earth’s largest rivers, including the Amazon and the Tigris–Euphrates system, begin in mountainous regions where tectonic plates meet. The Nile River, however, begins within the African plate, and it flows steadily from Ethiopia’s higher elevation to Egypt’s lower elevation, as shown on the map in units of meters above sea level. But the evidence found to date leaves the origin of Ethiopia’s uplift and the present-day Nile ambiguous. Now Claudio Faccenna from the University of Texas at Austin and his colleagues have found evidence that the land rose 30 million years ago when the African plate pushed closer to the Eurasian plate, not 5 million to 11 million years ago as many had thought.

Previously collected sediment cores from the Nile delta contain zircon minerals that were radiometrically dated using the uranium–lead decay chain. The analysis indicates that the oldest sediment was transported to the Nile’s drainage system about 30 million years ago. To figure out what brought the Nile to its present-day path, the researchers examined flood basalts from the Ethiopian highlands that formed around the same time. The age and elevation of the basalts indicate that the regional topography rose after mantle activity induced volcanic events and pushed the rocks hundreds of meters higher.

Faccenna and his colleagues incorporated the observations into a geophysical model that simulates the Ethiopian highlands’ rise over time as a function of mantle activity. Over long time scales, the mantle behaves as a viscous fluid and can change the surface topography by generating buoyancy in the lithosphere layer above the mantle, deforming the crust at the surface. The model reconstruction reveals that the region’s topography 40 million years ago favored a drainage pattern in which the Nile and other rivers flowed west of their current location on the way to the Mediterranean Sea.

But over the ensuing 30 million years, southern Ethiopia, northwest Sudan, and the western edge of the Red Sea basin experienced hundreds of meters of uplift while the land around the Nile delta fell. That topographical reorganization would have provided a source of sediment for the Nile and channeled it to its current route through Egypt. (C. Faccenna et al., Nat. Geosci., 2019, doi:10.1038/s41561-019-0472-x .)