It’s a bit of a stretch, but the analogy works. Sometimes Earth’s tectonic plates pull apart from one another like taffy.
In eastern Africa, that taffy is already thin and weak, and the tugging forces are concentrated in certain spots. The Turkana Rift Zone is undergoing “necking,” a critical transition toward continental breakup that has never been observed before, researchers reported April 23 in Nature Communications. The region may be closer to splitting, and doing so faster, than scientists once thought, the data suggest.
The Turkana Rift Zone straddles Kenya and Ethiopia, and it is best known for its wealth of fossils of our human ancestors. “Scientists were initially drawn there for its world-famous hominin fossil record,” says Christian Rowan, a geoscientist at Columbia University. But the rocks beneath the Turkana Rift Zone hold their own secrets.
Rowan and his colleagues used a suite of archival data, including acoustic-based measurements originally collected to explore oil and gas resources, to take a look beneath the surface. By sending acoustic waves into the planet and measuring how they’re reflected, it’s possible to create images of what’s underground, Rowan says. “It’s almost like an ultrasound of the upper crust.”
The team focused on a layer of metamorphic rocks beneath the planet’s surface. Those old, hard rocks form the Earth’s continental crust, with the stuff we think of as the ground merely piled on top. Rowan and his collaborators traced those crustal rocks downward toward Earth’s mantle and found one region where they were just under 13 kilometers thick. That was a big surprise, Rowan says. “Typical crustal thicknesses are about 30 kilometers.”
It was clear that Earth’s crust had been stretched substantially and wasn’t uniform—some spots have stretched more than the rest. The data match the necking stage of continental breakup in computer models of rifting. “It’s kind of the point of no return,” says Sascha Brune, a geodynamicist at the GFZ Helmholtz Centre for Geosciences in Germany, not involved in the research.
A handful of divergent plate boundaries elsewhere on Earth have already completed necking, but this process has never been observed in action. That’s what makes the Turkana Rift Zone so special scientifically, Brune says. “This is the place to go.”
To estimate how long the Turkana Rift Zone has been in this breakup stage, Rowan and his collaborators traced how a layer of volcanic rock, once at Earth’s surface, has been pulled downward over time as the crust stretched and sank. “The center of the rift has completely dropped out,” Rowan says. The rift has been necking for roughly 4 million years, the data suggest.
The Turkana Rift Zone’s treasure trove of fossils may in fact be due to that subsidence, the team suspects. That’s because sediments, some of which contain fossils, naturally accumulate in low-lying regions.
If the Turkana Rift Zone continues necking, it could enter the final phase of continental rifting: oceanization. At that point, Earth’s crust tears and the mantle underneath punches through, allowing magma to ooze across Earth’s surface. That magma eventually cools and forms new oceanic crust. Because oceanic crust is denser than continental crust, it tends to sink and collect water. Over millions of years, an ocean could develop that would separate parts of eastern Africa into a distinct landmass, Rowan says. “Eventually, eastern Africa will break apart.”
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