A huge rift that appeared last year along a fault in the Afar desert in Ethiopia, where the African and Arabian tectonic plates meet, has provided the strongest indication yet of how the plates are separating to create a new sea.

Geologists believe that they are witnessing a tectonic process similar to the one that formed the Atlantic Ocean, as adjacent plates push apart over millions of years to alter the shape of the continents.

While the precise course of this continental drift is difficult to predict, the movement of the fault promises eventually to widen the Red Sea between Africa and the Arabian peninsula and extend it southwards, cutting a marine inlet inland.

Eritrea and northeast Ethiopia, which are joined to Africa but lie on the Arabian Plate, would become marooned and would form a new island.

The world’s continents are not static, as was once thought, but instead rest on tectonic plates that shift around with the movements of molten rock, or magma, beneath the Earth’s crust, joining at unstable fault lines.

In some faults, the plates rub against each other in opposite directions, like gears, and in others a plate disappears beneath another in what is known as a subduction zone. The Afar fault is a third variety, a spreading fault, in which magma bubbles up through the surface and pushes two plates apart.

The most celebrated of these spreading faults is the MidAtlantic Ridge, which pushes the North and South American plates away from the Eurasian and African plates. It widens the Atlantic by a centimetre or two each year, similar to the rate at which fingernails grow.

For several million years a similar process has been slowly separating the African and Arabian plates, forming the Red Sea in the north and the Great Rift Valley in East Africa.

The new research on the Afar fault, which is published today in the journal Nature, offers fresh insights into the process that could eventually form a new sea or ocean.

A team of geologists from Royal Holloway, University of London, the University of Oxford and Addis Ababa University in Ethiopia monitored a heavy period of activity in the fault during September last year, which included a volcanic eruption.

The scientists used data from the Envisat satellite to show that an 8-metre rift developed along a 37-mile (60km) stretch of the fault in just three weeks.

It is the first time that such a developing rift has been monitored by satellite in such detail, and shows how the spreading process does not take place smoothly, but in a series of sudden, large, rupture events.

The scientists, led by Cindy Ebinger, of Royal Holloway, and Tim Wright, of Oxford, established that as the rift was torn it was filled by magma bubbling up from chambers lying underneath two volcanoes at its northern end. This magma will eventually harden and, when submerged, will form a new ocean floor.

“It’s amazing,” Dr Ebinger said. “It is the first large event we have seen like this in a rift zone since the advent of some of the space-based techniques we are now using. These techniques give us a resolution and a detail to see what’s really going on and how the Earth’s processes work. The Afar region provides a unique study area for continental break-up and formation of new ocean basins.”

Derek Keir, of Royal Holloway, who also contributed to the research, said: “Every rift system has to evolve in stages, and the rift system in the Afar desert is proceeding as if it were a mid-ocean rift. We think it has the potential eventually to form an ocean.”

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