East Africa’s Turkana Rift Valley is known for both its rich record of early human fossils and intense volcanic activity caused by plate movement. Scientists now report that the region’s underground crust is much thinner than previously understood, pointing to a long-term breakup of the African continent and offering a new explanation for why so many ancient human remains were preserved there.
The survey results are nature communications.
Vast cracks formed by the movement of plates
The Turkana Rift extends for approximately 500 kilometers across Kenya and Ethiopia and forms part of the larger East African Rift system. This huge system stretches from the Afar Depression in northeastern Ethiopia to Mozambique, separating the African plate from the Arabian and Somali plates. In the Turkana region, the African and Somali plates are slowly moving apart at about 4.7 millimeters per year.
When this separation occurs, a process called rifting stretches the Earth’s crust sideways. This strain causes buckling and cracking of the surface, causing magma deep within the Earth to rise upward.
Not all rifts completely split continents apart. However, in this case, the Turkana Rift appears to be on its path.
Scientists detect unexpectedly thin crust
“We found that the cracks in this zone are more advanced than anyone realized, and the crust is thinner,” says study lead author Dr. Christian Rowan. student at Columbia University’s Lamont-Doherty Earth Observatory and part of the Columbia Climate School. “In eastern Africa, the rift process is further advanced than previously thought.”
To reach this conclusion, Rowan and colleagues analyzed rare, high-quality seismic data collected in collaboration with industry partners and the Turkana Basin Research Institute, founded by the late paleoanthropologist Richard Leakey. By studying how sound waves travel through underground layers and combining the results with other imaging techniques, the team mapped the structure of the sediment and determined the depth of the crust beneath the cracks.
The crust along the center of the rift valley is only about 13 kilometers thick. The farther it is, the more than 35km. This dramatic difference is indicative of a process known as “necking.”
‘Necking’ indicates sign of significant tectonic movement
The term describes how the crust stretches and becomes thinner in the middle, similar to the narrowed “neck” that forms when you pull apart saltwater taffy. As the earth’s crust becomes thinner, its strength also decreases, making it easier for cracks to persist.
“The thinner the Earth’s crust becomes, the weaker it becomes, which promotes continued cracking,” Rowan says. Eventually, the crust may break completely.
“We’ve reached a tipping point for crustal collapse, and we think this is why the crust becomes more likely to separate,” said Ann Bethell, a geophysicist at Lamont University and co-author of the study.
Yet these changes unfold over vast time scales. The Turkana Rift began opening about 45 million years ago, and researchers estimate necking began after widespread volcanic eruptions about 4 million years ago. It could take several million more years for the next stage, known as oceanization, to begin. At that stage, magma could rise through the cracks and form a new ocean floor, eventually allowing water to flow north from the Indian Ocean.
Evidence of previous failed lifting
The researchers also found signs of an earlier rifting episode that did not lead to a complete breakup of the continent. Instead, the Earth’s crust became thinner and weaker, ready for the current phase of activity.
“This challenges traditional thinking about how continents break up,” Rowan says.
The Turkana Rift is the first active continental rift zone currently known to be necking, providing scientists with a unique opportunity to study this important stage of tectonic deformation.
“Essentially, we now have a front row seat to a critical rupture stage that fundamentally shaped the edges of all ruptures around the world,” says Lamont and fellow co-author Folarin Kolawole. These processes are closely connected to other Earth systems and help researchers reconstruct past landscapes, vegetation, and climate patterns. “Then we can use that knowledge to understand what will happen in the future, even on shorter time scales,” Bethell says.
Rethinking the fossil record of human evolution
These discoveries also shed new light on the region’s extraordinary fossil record. The Turkana Rift has produced more than 1,200 hominin fossils over the past 4 million years, accounting for about one-third of all hominin fossils discovered in Africa. Many scientists have long considered this region to be an important center of human evolution.
Rowan and colleagues suggest another possibility.
After widespread volcanic activity about 4 million years ago, land within the rift valley subsided due to necking. This subsidence created conditions for the rapid accumulation of fine-grained sediment, ideal for preserving fossils.
“Conditions were suitable for preserving a continuing fossil record,” Rowan says.
This means that the Turkana Rift may not have been particularly important as a place where our ancestors evolved, but rather that geological conditions made it easier to record human history.
Although this idea remains hypothetical, it opens new avenues of research. “But now other researchers can use our results to explore those ideas,” Rowan says. “Furthermore, our results can be fed into climate-coupled tectonic models to really investigate how changing tectonic forces and climate have influenced our evolution.”
The research team also includes Paul Betka of Western Washington University and John Rowan of the University of Cambridge.
