An earthquake low on the Richter scale has shaken geologists. Contrary to what that scientific community widely thought, the Central Afar region in Ethiopia will probably never become a deep ocean. This is the conclusion reached by two geologists from the University of Fribourg working with an international team. Published in the journal Geology, their research affords a better understanding of the formation of oceanic plateaus.

Anneleen Foubert and Valentin Rime of the Department of Geosciences at the University of Fribourg know Afar like the back of their hand, having been all over this arid zone in Ethiopia for years. Known especially for its fossils – paleontology’s Lucy hails from this part of the world – Afar also represents a veritable paradise for geologists, where they can observe fundamental geological processes in the open air, as it were. Afar is indeed considered the archetype of continental drift, the separating of tectonic plates. “It’s the very spot where the African and Arabian plates are breaking apart. This rift is very visible when you look at a map of the Red Sea and the Gulf of Arden,” Rime points out. Until now, scientists had thought that this rifting had been causing a very slow and steady thinning of the Earth’s crust there, like pie dough that is being stretched. It was even generally admitted that this process represented the initial phase in the formation of an ocean. New data on the structure and origins of this rift, however, have shaken up the theory. “We realized that the Earth’s crust was much thicker there than under the rest of the Red Sea and the Gulf of Aden, and was still far from being thin enough to form an ocean,” Rime explained. “We found that intriguing and sought an alternative hypothesis.” The two Unifr geologists decided to turn their attention to oceanic plateaus although at first glance it had nothing to do with their area of research.

Crucial zones for the evolution of our planet
Oceanic plateaus have only been studied since the 1970s. They are vast regions that cover nearly 3% of the Earth’s surface. Comprising a thick crust, these plateaus lie at far shallower depths than the surrounding seabed. “We can see it very clearly with Iceland, which sits atop one of the best known oceanic plateaus and forms an island, whereas the rest of the Atlantic is several kilometers deep.” Rime pointed out. The oceanic plateaus play a fundamental role in sea currents and the dispersal of species. “Iceland, for example, formed a land bridge between Europe and America until about 40 million years ago. That’s what allowed several species to cross from one continent to the other while the Atlantic was already open,” Anneleen Foubert, professor at the Department of Geosciences, explained. For all these reasons, the Unifr researchers found it pertinent to compare Central Afar with the Iceland region, a zone that also has the advantage of having been closely studied.

To substantiate their hypothesis, Foubert and Rime had help from their colleagues at the universities of Florence (IT), Southampton (UK), and Derby (UK), and Wayne State University (USA). As Foubert put it, “Just like in Afar, the North Atlantic’s Iceland Plateau is a region whose origin is connected with the rifting of tectonic plates. However, we know that a hotspot, which is a massive mantle plume, that is enormous amounts of matter rising from the Earth’s mantle, greatly thickened the crust there, which exceeds 30 km in some areas.”

No new ocean
Like the region around Iceland and other oceanic plateaus, the formation of Central Afar was also molded by massive additions of magma that took place around 30 million years ago. “Here, too, the Earth’s crust is more that 25 km thick. Early and sustained input of enormous amounts of magma there hindered the thinning of the Earth’s crust despite its being stretched. This makes ocean formation improbable,” Rime concluded. Central Afar would represent then the early stage in the evolution of an oceanic plateau, like the region centered on Iceland. “And that difference probably had an effect on the human species since our ancestors most likely used this almost completely emerged region to migrate out of Africa during the Quaternary period,” Foubert added.

Published in the journal Geology, the researchers’ study also serves as a model that helps us better understand the origin of oceanic plateaus. Professor Foubert, who teaches in the university’s Department of Geosciences, had a final word about the broader importance of their work, “This discovery provides decisive information on the geological processes involved in the creation of these vast structures under the sea.”

Central Afar: An analogue for oceanic plateau development, in: Geology. 2024 DOI: https://doi.org/10.1130/G52330.1