Earth scientists have uncovered the origins of the world's youngest nascent ocean, the Red Sea, and advanced an understanding of how continents break apart.
The Red Sea opened abruptly along its full length 34 million years ago when Arabia and Africa tore apart, researchers at Penn and Columbia University's Lamont-Doherty Earth Observatory reported in the Nov. 24 issue of Science. Their findings contradict a widely held theory that a small tear in the African continental crust gradually spread northward, slowly "unzipping" Arabia from Africa.
The new research sheds light not only on how Africa split apart, but on what caused it. The scientists found that the great rift thrust up steep cliffs on either side of a fissure that became the Red Sea, and at the same time, set off a massive upwelling of molten rock from the earth's interior near the southern end of the sea. Scientists had no previous evidence that rifting, uplift and volcanism all started simultaneously when Africa and Arabia broke apart.
According to one theory, movements of the earth's rigid crustal plates stretch and thin a continent until it separates. Hot magma from the mantle wells up and fills the void--but only after a long geological time, not immediately. According to another theory, localized plumes of magma, called hotspots, rise from earth's mantle to melt and weaken plates, acting as a hot wedge that causes the initial break in the continental crust. The Penn and Columbia scientists believe both mechanisms were at work when the African-Arabian continent divided. The scientists are Gomaa Omar, lecturer of geology at Penn, and Michael Steckler, senior research scientist at Lamont-Doherty, Columbia's earth science research institute in Palisades, N.Y.
Their finding that the Red Sea rifted along its entire length all at once backs the theory that plate movements exert the forces that break plates. The splitting of a continent into separate land masses with bodies of water between them is a fundamental but still poorly understood phenomenon that has shaped the earth. The same process created the Atlantic Ocean starting 200 million years ago as the supercontinent Pangaea rifted into what became Africa and North and South America.
The theory that a small tear propagated through the African continent and gradually unzipped Arabia had been bolstered by the present-day behavior of the Red Sea floor. In the fissure that has become the Red Sea, a midocean ridge has formed. Molten material has risen there to create new sea floor that has spread outward, further separating Africa and Arabia at a rate of up to almost an inch per year. The ridge first formed near the southern end of the Red Sea and has propagated northward and southward.
The Penn-Columbia research, supported by the National Science Foundation, confirms for the first time the timing of the first large-scale rift that appeared in the African continent--about 34 million years ago. The scientists also found that a second major pulse of rifting and uplift followed sometime between 21 million and 25 million years ago.
Scientists had known about a large-scale separation that occurred then because seawater gradually spilled into the nascent Red Sea ocean basin during that time, bringing in marine life whose fossils could help date nearby rocks. But the initial burst of rifting 34 million years ago had never been confirmed because, without Red Sea marine life, rocks could not be easily dated.
Although scientists knew by dating volcanic rocks that volcanic activity increased in the region during those same two time periods, they could never before ascertain whether the volcanism occurred before, during or after rifting.
The scientists' finding that the Red Sea rifting began at the same time as volcanism and massive magma upwelling at the sea's southern end indicates that a hotspot played a strong role, as well.
Drs. Omar and Steckler suggest that a hotspot known to exist at the Red Sea's southern end, called the Afar hotspot, could not alone cause the Africa plate to rift. But once the continental rift began, it easily connected with the hotspot. That opened the hotspot's floodgate, sending forth massive amounts of molten rock that further weakened the crust and intensified rifting.
To make their discoveries, the scientists analyzed rocks from Egypt and Saudi Arabia. Using a technique called fission-track analysis, they determined the precise times when the cliffs on either side and along the entire length of the Red Sea were uplifted.
The newly created steep cliffs set the stage for rapid erosion, exposing previously buried rocks that contain a mineral called apatite. Apatite has small amounts of naturally occurring uranium, which spontaneously fissions into two particles with lighter nuclei that repel each other. As the particles streak through the rock crystals, they leave trails of damage that, under microscopes, look almost like skid marks. Heat, however, can repair the damage to the crystal so that the track marks become shorter and can even disappear. That occurs in deeply buried rocks, which are hotter than those near the surface.
As a rift flank erodes, though, long-buried rock becomes exposed, cools down and starts to accumulate new fission tracks. By dating precisely when new fission tracks started to accumulate in rocks on the northwest and southeast sides of the Red Sea, Drs. Omar and Steckler pinpointed when rapid erosion began, and hence when rifting thrust up the cliffs.