In Ethiopia's arid Afar region eruptions and earthquakes have created an open-air laboratory

The nomads were terrified. For a week the ground had shuddered violently. Cracks opened up in the soil swallowing goats and camels. Sulphur-laced smoke rose out of the dark slits. After retreating to the hills, the nomads saw chunks of obsidian rock burst through the Earth's crust "like huge black birds" and fly 30 metres into the air.A mushroom cloud of ash dimmed the sun for three days. At night the new crater breathed flashes of fire.

"They had experienced earthquakes before but never anything like this," said Atalay Ayele, a seismologist at Addis Ababa University, who interviewed the Afar tribespeople soon after the volcanic eruption 13 months ago in this remote corner of north-eastern Ethiopia. "They said that Allah must have been angry with them."

But Dr Ayele, 37, and his colleagues wanted a scientific explanation. They knew the area was geologically unstable, but the number of earthquakes - 162 measuring more than four on the Richter scale in just two weeks - made them suspect that something extraordinary had happened deep underground.

They asked a team of British-based scientists with access to satellite technology for help. When the results came back it seemed as unlikely as birds flying out of the ground. Here in the Afar desert, one of the hottest and driest places on earth, the tribe had witnessed the birth of a new ocean. Images from the European Space Agency's Envisat satellite showed that a huge rift, 37 miles long and up to eight metres (26ft) wide, had opened deep in the Earth's crust. The tear, the largest observed since the advent of satellite monitoring, was created by a violent lateral rush of molten rock, or magma, along the fault line separating the Nubian and Arabian tectonic plates.

Tim Wright, a geologist at the University of Leeds who interpreted the satellite results, was astonished by the images and what they pointed to.

"The process happening here is identical to that which created the Atlantic Ocean," said Dr Wright during a recent research expedition in Afar. "If this continues we believe parts of Eritrea, Ethiopia and Djibouti will sink low enough to allow water to flow in from the Red Sea."

The findings caused a stir in the scientific community. This year teams from the UK, France, Italy and the US mounted expeditions to Afar, a region described by the British explorer Wilfred Thesiger as a "veritable land of death".

From above you can see vast black tongues of lava lapping at the desert sands, and rust-coloured volcanos with their lids long blown off.

There are so many fissures and faults where the ground has opened and slipped that the Earth's skin looks like elephant hide.

The lunar geography reflects what lies beneath. Afar stands at the junction of three tectonic plates, which form the outer shell of the Earth and meet at unstable fault lines. The Nubian and Somali plates run along the Great Rift Valley, which spreads south from Afar. Branching out like a funnel to the north is the Arabian plate.

Tectonic plates across the globe are constantly shifting - though slowly, usually by a few centimetres a year - with the magma beneath the crust. The plates can collide, forcing the crust upwards and creating mountain ranges - as happened with the Himalayas. They can also slide past one another, as occurs along the San Andreas Fault, in California, a notorious earthquake zone.

The plates can also pull apart causing continents to break up and oceans to form. Early in this process, at the plate margins, the Earth's crust stretches and thins in the manner of toffee. Magma rises up, eventually cracking the crust and helping the plates drift apart. Between the fault lines the crust, now heavy with cooled magma, sinks to form a valley and then allows water from a nearby sea to rush in.

This is how the Atlantic was formed, separating Africa and Eurasia from the Americas. And this is what scientists believe is happening in Afar as the Arabian and Nubian plates pull apart. Parts of the region have already sunk to more than 100 metres below sea level, and only the highlands around the Danakil depression stop the Red Sea from rushing in.

Analysis of the new rift is providing an insight into the role of magma injection in cracking the Earth's crust and the pace at which continental break-up occurs. The last big "ocean spreading" occurred in Krafla, Iceland, in the mid 1970s, along the boundary of the North American and Eurasian plates that forms the Atlantic's mid-ocean ridge.

But it took nine years to achieve what has occurred in Afar in a few weeks.

"We are looking at a huge open-air laboratory here," said Gezahegn Yirgu, a geologist from Addis Ababa University, as he peered out of a military helicopter swooping low over the Afar region.

In recent months there has been more instability in Afar. After a series of earthquakes in June the rift widened by a further two metres. Hundreds of Afar nomads are still seeking refuge in a town 25 miles from the main fault zone, too afraid to go home. They may be wise; the scientists say there could be more violent earthquakes and eruptions.

The new sea is predicted to be formed within about a million years. The separation of the Nubian and Somali plates along the Great Rift Valley could take 10 times as long. But that will be even more dramatic - for then Africa will eventually lose its horn.

"Some people think that extreme natural phenomena happened only in historical times," said Cindy Ebinger, an American geologist leading the research in Afar. "But here we can see them happening right now."