Death of a Continent, Birth of an Ocean

Africa’s Afar region gives glimpses of geology in action

To those who live there, east Africa’s Afar region is “the place the devil plows.” One of the hottest and lowest areas on Earth, it is a landscape of baking desert and barren lava flows. To scientists, though, Afar means something more promising: geology in the raw.

As Afar is pulled and jostled from below, fissures form across the landscape. Some areas are already below sea level. Lorraine Field/Univ. of Bristol

TENSION ON LAND Afar (shaded) sits where the East African Rift and two ocean-splitting centers meet, meaning the region is being pulled in many directions. Intense geologic activity results: In 2005, researchers detected a flurry of earthquakes and upwelling magma along the Dabbahu rift segment. Lava flows were also observed at the Erta Ale volcano (shown above) in 2010. Image: Lorraine Field/Univ. of Bristol; map: Graphi-Ogre, adapted by E. Feliciano

HOW TO MAKE AN OCEAN | Over time, the rift valley at Afar will become the world’s newest ocean. Ocean formation can take millions of years, but ongoing geologic activity offers researchers clues to the process. E. Feliciano

ON THE MOVE In 2005, the Envisat satellite captured how tectonic plates pulling apart in the Afar region caused the greatest ground deformation ever seen from space. At left, the rainbow pattern reveals which parts of the ground surrounding the Dabbahu rift segment moved between May and October of that year — due almost entirely to a September rifting event. Other analyses of ground motion (middle and right) reveal that movement was concentrated along a thin band in the rift zone. T.J. Wright et al/Nature 2006

There, on the edge of Africa, the continent is splitting apart. Pulled inexorably by the grind of tectonic plates, Afar is ripping asunder like a gateway to hell. Molten rock wells up from below, pouring onto the sweltering surface.

Yet for all the fire and brimstone, Afar is on its way to a watery end. A million years or so from now, the geological processes that rip the continent will give birth to a new seafloor. And Afar will lie at the bottom of Earth’s freshest ocean.

Until then, researchers have a front seat to an unparalleled physical spectacle. “It’s a really unique opportunity to understand how continents break apart,” says Tim Wright, a remote-sensing expert at the University of Leeds in England. Wright leads a large international consortium that began studying the region in 2005, when the splitting picked up pace.

Afar’s geological violence comes in many forms. Magma welling up from the depths sometimes erupts through existing volcanoes. Other times it pools underground, cooling to form giant vertical sheets called dikes. As it rises, the moving magma causes the ground to tremble in an earthquake drumroll. For the past couple of years scientists have listened to the landscape’s clamor, trying to discover what Afar has to say about the death of continents.

New findings reveal that the dikes stack up against each other — fresh ones pushing their way into places where the rock is least stressed, crackling with seismicity as the magma arrives. Other discoveries include the first-ever glimpse at how magma flows from storage reservoirs into such dikes along an intricate system of volcanic plumbing. For the first time, researchers have seen some of the planet’s most common geologic activity transpire nearly in real time, and on land where they can watch.

To its list of superlatives — hottest, lowest, least hospitable — Afar can now add the title of best-studied birth of an ocean.

A violent story

A single earthquake, of magnitude 4.5, first alerted scientists to the tale unfolding in Afar. At Addis Ababa University in Ethiopia, in September 2005, seismologist Atalay Ayele saw the sign of an Afar quake pop up on his monitors. Then more quakes appeared, bigger ones, and then yet more. Something unusual, he realized, was going on.

“It was a surprise,” Ayele says. “We didn’t know at the beginning how big it was going to be. We just recorded all the quakes as we normally do.”

Ayele called Cynthia Ebinger, a geophysicist then at Royal Holloway College at the University of London, who arrived within days with extra seismometers to monitor the quaking ground. Ebinger in turn asked Wright to check for satellite imagery that, by bouncing radar waves off the ground and measuring their return, can reveal how much the ground is shifting, and where. If a big eruption were going on just under the surface, she reasoned, the satellites should have captured it.

“I kept bugging him and bugging him,” remembers Ebinger, now at the University of Rochester in New York. And then one day Wright called with striking news.

Because the Afar quakes hadn’t gotten much bigger than magnitude 5.5, Wright says, “we didn’t expect to see very much. But when we downloaded the data what we saw was astounding — the biggest signal we’d ever seen in terms of ground deformation. At that point it was immediately clear that something really unusual had happened.”

Whereas the ground might move a few centimeters during most volcanic eruptions or earthquakes, places in Afar had moved eight meters in just 10 days, a world record. By the time Ayele and colleagues arrived in the region to check what had happened, fresh fissures and steep cliff faces yawned across the landscape, created by the massively shifting ground. Brand-new lava glistened in the desert sun.

All this geological action traces to the fact that Afar sits at the intersection of three segments of Earth’s crust that are pulling apart, or rifting.

Like pieces in a moving jigsaw puzzle, the planet’s tectonic plates constantly elbow against one another, carrying continents great distances and allowing new oceans to be born and die. In large part, this plate jostling is driven by fresh magma that wells up from seams that run along the centers of oceans, like the underwater mountain chain that splits the Atlantic Ocean in two. Molten rock erupts onto the seafloor there, then cools and rifts away from the ridge on either side in a process known as seafloor spreading. Geologists can take a peek at this in Iceland, where the Atlantic’s mid-ocean ridge surfaces above the waves.

Plate tectonics can also pull continents apart. Instead of magma cleanly forming fresh ocean crust, continental rifts often have a wide, messy zone where parallel valleys form, accompanied by spasms of eruptions and earthquakes. Such is the case with the Great Rift Valley that runs down eastern Africa on dry land.

In Afar, that continental rift meets two ocean rifts, one bisecting the Red Sea and the other the Gulf of Aden. This tectonic “triple junction,” pulling at Afar from all directions, is the geologic equivalent of being drawn and quartered.

In eastern Africa, the current transition between continental and oceanic rift lies somewhere between Afar and the southern end of the Red Sea. As eastern Africa keeps stretching, though, its continental crust gets thinner and thinner. “Once there’s no continental plate left, once it’s completely thinned and gone, then that’s the end of continental breakup,” says Derek Keir, a former student of Ebinger now at the National Oceanography Centre in Southampton, England.

At that point, Afar will become a true seafloor spreading ridge. Magma welling up from below will be richer in heavy elements like iron, so that the newborn crust will be denser and sink lower in elevation compared with the rest of the African continent. Waters from the Red Sea will rush in, forming a new ocean.

Geologists have known the end is coming for Afar, and they got a preview beginning in 1978, in a part of the rift zone located in Djibouti. There, a small earthquake swarm popped up as magma intruded underground to form a dike. French scientists, who had seismometers and other ground-measuring instruments installed across Djibouti, watched the whole thing happen. “It really helped us understand the processes involved with rifting,” says Cécile Doubre, a tectonophysicist at the Institut de Physique du Globe in Strasbourg.

The main Afar show, though, began in 2005. Geologists knew something was coming, and to some extent they knew pretty much what to expect. The next time magma welled up into the rift zone, Keir had predicted in his doctoral thesis, it would appear below a particular 60-kilometer-long segment of the rift zone with a volcano called Dabbahu at its northern end. But the sheer size of the September 2005 eruption astonished researchers. Over just a couple of days, some 2.5 cubic kilometers of molten rock squirted toward the surface.

Much of this magma had been lurking beneath the region for some time, Wright explains. The magma begins some 20 kilometers down, in the quasi-molten region known as the Earth’s mantle. From there, the magma can make its way toward the surface into shallow chambers, like temporary storage reservoirs, where it sits for some time. Eventually, pressure in these reservoirs gets too great, and the magma forces its way up again — either to erupt out as lava on the surface, or cool and solidify just underground.

In 2005, most of the magma cooled as a 70-kilometer-long dike without making it onto the surface. Since then, 13 other dikes have appeared beneath Afar, most of them much smaller, about 10 kilo­meters long. Scientists can track where the dikes appear and how big they are by monitoring patterns of earthquakes as well as by mapping changes in the ground’s electrical conductivity.

The dikes form one next to another, like a row of marching toy soldiers. In a paper published last year in Nature Geoscience, Wright, his student Ian Hamling and colleagues described how each new dike changes stress fields within the ground. Because magma likes to squirt into regions of lower stress, scientists could predict where the next dike in the sequence would appear.

Earthquakes rippling out from the central rift show how molten rock moves along natural underground pipes. A new study of five of the 14 dikes found that seismicity migrated away from the rift center for about 10 to 15 kilometers, just as it does at ocean spreading centers. Doubre and colleagues, led by Raphaël Grandin of the École Normale Supérieure in Paris, reported the finding in April in Geochemistry, Geophysics, Geosystems.

Researchers have found that the dikes all appear to feed off a main chamber in the center of the Dabbahu rift segment,  Keir says; there isn’t an elongated chamber underlying the entire segment. Similar volcanic plumbing has been observed beneath mid-ocean ridges.

Looking at lava

While the dikes usually don’t make it to the surface, at other places in Afar magma does break through in bona fide volcanic eruptions. Some 110 kilometers north of Dabbahu, for instance, sits the well-known volcano Erta Ale. Most of the time, this volcano doesn’t spit out lava dramatically. Instead, a lava lake constantly burbles around within its crater, like a heated pot of water that never quite boils over.

But during a field trip to Afar in November 2010, Keir and Lorraine Field, a volcanology student at the University of Bristol in England, decided to check out Erta Ale. They climbed up its side, looked down, and realized that the lava lake was higher than scientists had seen it in years — overflowing the side in places. “We had 48 hours of lava heaven,” says Field. “I watched my rocks being born.”

Erta Ale’s lava is thinner and less sticky than that at Dabbahu, suggesting that it is erupting directly from the mantle rather than sitting in reservoir chambers for a while, says Field. (Magma undergoes chemical changes when it sits in a reservoir, such as by melting the surrounding rocks and incorporating their minerals.) Erta Ale is also much closer to the Red Sea’s spreading center, so the ground there may more closely resemble ocean crust than the Dabbahu rift zone does at the moment. How the two areas of volcanic activity are related — and how they fit into the bigger tectonic triple junction picture — remains to be explored.

In some ways, Afar’s chronic volcanism has become an everyday part of life in the region. The Afar people have adapted to gather water for themselves and their goats from natural fumaroles, or steam vents. First, says Field, the locals hold a piece of obsidian glass up to a vent; if it turns cloudy, that signifies too many poisons are in the steam. But if the obsidian stays clear, the people lay reeds down into the vent, then use a can to collect the water that condenses on and drips off the reeds.

In other ways, modern life has not adapted so well to local geology. The newly built regional capital of Afar, Semera, and a nearby dam lie atop the many fault lines that crisscross the region. Both were planned long before the September 2005 eruption and are unlikely to be decommissioned in a place where starvation and disease are more pressing concerns than geologic hazards.

Still, Ayele says he and his colleagues spend a lot of time working to educate the local government and people about the earthquake risk.

In the long term, Afar may need to brace for volcanoes and earthquakes for quite a while. Scientists aren’t sure exactly how long Afar will remain highly active, but they do have one point of comparison: the Krafla eruptions in northern Iceland, which took place over nearly a decade in the 1970s and 1980s. Eruptions at Krafla poured out lava for several years, then quieted down, then burst out again with a lot of magma right at the end.

At Afar, “things have been suspiciously quiet since May 2010,” Wright says, with no dikes or eruptions along the Dabbahu segment. But if Krafla offers a comparison, Afar might yet expect a lot of magma to pour out in another couple of years. Afar also has more magma underlying it to start with than Krafla did.

“We’re not done yet,” says Ebinger.

Already, researchers have recorded a flurry of earthquakes to the east, about 100 kilometers offshore in the Gulf of Aden. That activity, in December 2010, could mean a dike was injected there below the seafloor, which may be related to the activity at Dabbahu. “It’s likely when stress is relieved at one point it can trigger another point that is critically close to failure,” Ayele says. But scientists can’t take a ship to study the region, because of the threat of Somali pirates.

For now, scientists must content themselves with the wealth of data they have gathered from a five-year push in Afar. The data have yielded surprise discoveries, like details on an eruption north of Erta Ale that happened in November 2008. There, for the first time, scientists found and watched magma flow from a very shallow chamber stretched out along the rift axis.

Such “axial magma chambers” are common on the seafloor but almost never studied, says Wright, because they have been so inaccessible — until now. “There’s a huge amount of ocean floor formed through magma in these chambers,” he says, “and now we can actually see how they behave.”

Other remaining questions include where exactly the magma resides lower down in the mantle, from which it pipes to feed storage reservoirs closer to the surface. By piecing together details of the underground plumbing, Ebinger says, scientists can better understand some of the most everyday volcanic processes on Earth.

All from a little bit of rumbling in a remote corner of Africa. “It was just such an exciting event,” says James Hammond, a consortium seismologist at the University of Bristol. “It was a once-in-a-lifetime scientific opportunity.”

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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