How Continents Do the Splits

African study reveals how land gives way to ocean crust.

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By Alexandra Witze, Science News

Breaking up is easy to do, if you’re East Africa. Researchers have discovered new details of how the Earth’s continental crust is tearing apart in Ethiopia, which will one day give birth to a new ocean.

In the final throes of breakup, it turns out, the crust thins dramatically, allowing a flood of magma to rise from deep in the Earth and erupt onto the surface, the scientists report online March 13 in Nature Geoscience. The work reveals how an old continent gives way to a young ocean.

“It shows that we’re starting to understand some of the processes of how we go from continental to oceanic crust,” says James Hammond, a seismologist at the University of Bristol in England who was not involved in the research.

Earth’s surface is broken into more than a dozen large tectonic plates, which drift around, collide and create geological phenomena such as earthquakes and volcanoes. The plates are driven by magma that wells up from within the planet, then cools and solidifies to form fresh oceanic crust. Older, lighter crust makes up the continents.

In East Africa, continental crust is losing its battle for existence. Tugged by tectonic forces from either side, the crust here is destined to rip apart and create a new ocean, like the neighboring Red Sea and Gulf of Aden.

To geologists, eastern Africa shows plate tectonics in the raw. “Ethiopia is an ideal natural laboratory because we can see all these things happening,” says Ian Bastow of the University of Bristol, a coauthor of the new paper.

Bastow and Derek Keir, a geologist at the University of Southampton, reanalyzed data from the 1970s, collected when other scientists set off explosive charges and watched how the resulting vibrations traveled through the ground.  How fast those waves travel through the crust allows researchers to determine how thick the crust is.

Of particular interest, says Bastow, is a region in northernmost Ethiopia where the crust is so thin that much of the ground has dropped below sea level. Here earthquakes shake the ground constantly, and volcanoes pour out lava across the hot, flat landscape.

The new analysis shows that this lava erupts because of the recent stretching of the plate. “This very abrupt thinning is what is causing the melting,” Bastow says. “If that thinning had occurred slowly, you couldn’t explain this large magma pulse and the voluminous young lava flows at the surface.”

Bastow says the work explains not only why northern Ethiopia looks the way it does, but also what might have happened at other places where continents tore apart. The Atlantic Ocean, for instance, started to form about 200 million years ago when Europe and North America pulled away from each other.

Scientists might now be able to use what they’ve learned about Ethiopia’s split to better understand what happened as Europe and North America ripped asunder.

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