The Turkana Trench in East Africa is considered a cradle of humanity. More than 1,200 fossils of pre- and early humans from the last four million years were found there. A study now shows that the earth’s crust in this region is becoming increasingly thinner – a precursor to the splitting up of the continent. The East African Rift Valley system thus provides important insights into fundamental processes of plate tectonics and continental drift. The new findings also provide an explanation for why so many fossils from our evolutionary history have been preserved in the Turkana Trench of all places.
Stretching across Kenya and Ethiopia is the Turkana Rift, a 500-kilometer-wide, low-lying region that is part of the East African Rift Valley system. Here the African and Somali tectonic plates are drifting apart at a rate of about 4.7 millimeters per year. Due to plate shifting, the region is a hotspot of volcanic activity. It is also one of the richest treasure troves for pre- and early human fossils. For example, the “Turkana Boy,” a juvenile Homo erectus, was discovered here, as well as more than 1,200 other pieces of evidence of human evolution over the last four million years.
Thin earth crust
A team led by Christian Rowan from Columbia University in New York has now used seismic measurements to shed more light on the tectonic processes at the Rift Valley. The researchers discovered that the earth’s crust where the plates drift apart is already thinner than previously assumed. Using a combination of different depth imaging techniques, they came to the conclusion that the earth’s crust there is only about 13 kilometers thick. Further away from the center of the Rift Valley, however, it measures 35 kilometers.
This result suggests that a process known in geology as “necking” is already underway in the Turkana Trench. The earth’s crust is stretched as tectonic plates drift apart. Rowan compares this to pulling apart a chewy candy: the center of the candy becomes thinner and more elongated, while the ends remain the same. Something similar is apparently currently happening in East Africa. “The thinner the crust becomes, the weaker it becomes, which promotes progressive fission,” says Rowan. Ultimately, the crust breaks apart, magma flows through the cracks and forms the basis for a newly formed sea.
Insights into trench formation
But even if the splitting of the continent is imminent on geological timescales, according to Rowan and his colleagues, it will still take millions of years before it actually happens. After all, the drifting apart of the once united tectonic plates began around 45 million years ago and, according to researchers’ estimates, necking has been taking place for around four million years.
The Turkana Trench is the first identified continental trench in the world that is currently in a necking phase. It therefore plays a special role in the research of fundamental tectonic processes. “We now have a front-row seat to a crucial phase of rift formation that has fundamentally shaped all rift edges worldwide,” says Rowan’s colleague Folarin Kolawole.
Human evolution preserved
The geological findings also shed new light on the region’s fossil wealth. Until now, scientists have assumed that the Turkana Trench and its surroundings played a unique role in the evolution and diversification of our ancestors. But perhaps there were no more hominids living there than anywhere else in Africa. However, the conditions for preserving their remains were unique. Because of the necking that began around four million years ago, the Turkana Trench sank, causing fine-grained sediments to quickly accumulate. “These conditions were ideal for preserving a complete fossil record,” says Rowan.
Source: Christian Rowan (Columbia University, New York, USA) et al., Nature Communications, doi: 10.1038/s41467-026-71663-x