Earth Scientists Discover How Continent Broke

Photograph: Michael Steckler.

Earth scientists have uncovered the origins of the world's youngest nascent ocean, the Red Sea, forging a new 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 Columbia's Lamont-Doherty Earth Observatory and the University of Pennsylvania reported in the Nov. 24 issue of Science. The finding contradicts 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 a 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 Columbia and Penn scientists believe both mechanisms were at work when the African-Arabian continent divided. They are Michael Steckler, senior research scientist at Lamont-Doherty, Columbia's earth science research institute in Palisades, N.Y., and Gomaa Omar, lecturer in geology, at the University of Pennsylvania.

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 continents into separate land masses with new oceans 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 mid-ocean ridge has formed. Molten material rises there to create new seafloor that spreads outward and further separates Africa and Arabia, currently 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 research by Steckler and Omar confirms for the first time when the first large-scale rift 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 25 million and 21 million years ago.

Scientists had known about a large-scale separation that occurred 21-25 million years ago because during that time seawater gradually spilled into the nascent Red Sea ocean basin, bringing in marine life whose fossils could help date nearby rocks. But the initial burst of rifting at 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 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.

Steckler and Omar 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. However, heat can repair the damage to the crystal, so that the track marks become shorter and can disappear over time. That occurs in deeply buried rocks, which are hotter than those near the surface.

As a rift flank erodes, however, long-buried rock becomes exposed, cools down and starts to accumulate new fission tracks. By analyzing the number and length of newly accumulated fission tracks in rocks on the northwest and southeast sides of the Red Sea, Steckler and Omar pinpointed when rapid erosion began, and hence when rifting thrust up the cliffs.

The research was supported by the National Science Foundation.

Columbia University Record -- December 1, 1995 -- Vol. 21, No. 11