Africa's Big Break

The continent may be splitting apart, and so is consensus among geologists.

By Shannon Hall|Tuesday, May 08, 2018
RELATED TAGS: EARTH SCIENCE
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Geologists thought they knew how Africa’s rifts, such as Ethiopia’s Da’Ure vent, would shape the continent. But new data suggest they don’t have the full picture.
Lorraine Field

In the Afar Depression — a vast desert in northeast Ethiopia — pastoral tribes survive amid an alien landscape of steaming vents, boiling geysers and even a lake of lava. But on a September day in 2005, the Afar herders witnessed a scene unlike anything they had ever seen.

The ground beneath their feet shuddered violently with the first of hundreds of earthquakes that would sweep through the region over the course of two weeks. A huge crack — nearly 40 miles long and up to 25 feet wide — opened in Earth’s crust. And a volcanic eruption from a second crack launched clouds of ash that dimmed the sky for days. Terrified, the nomads believed Allah was punishing them for not strictly adhering to religious rules.

The truth was no less dramatic. For millions of years, a bubble of hot, possibly molten rock has been slowly swelling up beneath the surface, heating Earth’s crust and causing it to stretch and crack.

Geologists know this story well. More than 200 million years ago, when dinosaurs roamed the planet, Earth’s continents were joined together in a single enormous landmass called Pangaea. Over time, deep rifts opened within that mighty supercontinent, causing it to fracture and ultimately spawn seven new continents — a number that may be growing.

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Lava bubbles out of Erta Ale volcano, site of the world’s oldest continuously active lava lake. The area may one day give way entirely, allowing a new ocean to flow within Africa — though geologists aren’t as sure of this as they once were.
Joel Santos/Barcroft Media via Getty Images

The signs of unrest hint that the Afar might be the site of the first continental breakup since modern humans evolved. The data from that day in 2005 suggest that within 10 million years, one of the hottest and driest deserts on Earth will give birth to a small new ocean as Africa splits apart.

But conventional ideas about how such a dramatic breakup might progress could be wrong. Recent results have spilled unexpected discoveries onto the pages of prominent journals, implying that the rift has diverged from expectations. It’s a suggestion that might ultimately help scientists understand why some rifts succeed in cleaving continents into pieces while others fail, allowing continents to recover from their wounds. The whole of Africa — not to mention the textbook definition of continental breakup — is at stake.

Slip-Sliding Continents

With Africa’s future partly on her shoulders, Ellen Knappe, a graduate student in geosciences at the University of Montana, drops a 40-pound duffel bag into thick yellow grass and tall green weeds. Hours ago, that ground was covered in dew — the only mark that it’s winter at the equator. It’s January, and we’re just outside the small town of Debark in northwest Ethiopia, gathering data from one of nine instruments Knappe has set up across the country. She hunches, deep in concentration, by a large white box hidden behind a solar panel, unaware that an armed Ethiopian guard is curiously eyeing her from a distance, or that young schoolchildren have sprinted to a nearby stick-lined fence to watch with broad smiles.

Instead, her mind is on what’s in that large white box. During the previous year, a GPS antenna roughly 10 feet away from Knappe pinged satellites every 30 seconds to determine its own precise point on Earth’s surface. Any movement of that point will help geologists pin down exactly how the crust is stretching in East Africa. The antenna sends that information to a receiver — the instrument inside the box — that stores the data. If everything is working, a tiny green light will be on, and Knappe is anxious to see it.

She pulls the door open and lets out a sigh of relief before giving me a thumbs-up. Knappe plops down on the dusty ground, pulls her long, chestnut brown hair into a loose ponytail and starts transferring the stored data to her laptop. She’s eager to see how the rift’s story might evolve with one more year of recent data to add to the eons of confusion.

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Jay Smith

The first geologic activity in the area dates back 30 million years, when the Arabian Peninsula was fastened to the African continent. Then, the region began to surge upward as a mantle plume — a vast lump of heated rock that arises within the Earth’s innards like globs in a lava lamp — pushed toward the surface. The ground strained as it ballooned, and several fissures developed, a little like those atop a freshly baked cake.

Molten rock rose up to the surface in these newly formed gashes, creating thin sheets of cooled magma, or basalt — a rock so dense that the crust slumped lower and lower until this new valley broadened in width and depth, ultimately sinking below sea level. Then, water from the Indian Ocean flooded in, and the region gave birth to the Gulf of Aden and the Red Sea, separating the Arabian and Nubian tectonic plates. But the drama had only just begun.

Roughly 10 million to 20 million years later, the same mantle plume gradually created a second valley nearly perpendicular to the previous one. Called the East African Rift System, the depression began in Ethiopia and slowly spread nearly 4,000 miles south toward Mozambique, once again breaking the African plate in two main pieces: the Nubian plate, which straddles the equator in most of Africa and the Atlantic Ocean; and the Somalian plate, which straddles the equator in eastern Ethiopia, Somalia, eastern Kenya, eastern Tanzania and the Indian Ocean.

At the heart of it all is the Afar, which sits at the junction of three tectonic plates, each of which is slowly slipping away from the others at different rates. If you hold the Nubian plate still, the Arabian plate appears to speed northeast at 0.8 inch a year — about the rate your toenails grow. That’s causing the Afar to spread toward the northeast, but the desert is also moving toward the southeast as the Somalian plate similarly appears to creep toward Australia at a rate of 0.2 inch a year.

To complicate matters, the Somalian plate isn’t separating from the Nubian plate all at once, with the same rate across the rift. Instead, the East African Rift System looks like a zipper that’s slowly opening. Roughly speaking, it’s oldest and widest in the north, and younger and narrower the farther you travel south, creating a tornado-shaped valley easily visible in any satellite image of East Africa. As such, studying the older, deeper portions of the rift, called the Main Ethiopian Rift — a spot where the splitting up is most complete — should help geologists understand the final stages of continental breakup, as well as the hazards associated with it. So, for the past three years, Knappe has spent her winters in Ethiopia, adding data to that record. “This is really the only natural laboratory where we can look at this,” she says. “There’s no other place on Earth where you can watch a continent get subdivided into two.”

Recipe for a Rift

The splitting of the continents, however, might not proceed as geologists expect. Scientists had always thought that stretching a continental plate would be similar to slowly pulling a cold block of cheese apart, says MIT geophysicist Michael Floyd. “You can feel it elastically stretch in your hands, but after a while, there will be a little chink in the armor — a little crack that forms,” he says. And at that point, the stress you’re creating is no longer spread out across the whole block of cheese, but becomes concentrated at that weakness — the crack — allowing it to expand and form a deep gash.

That’s what happened in the Afar in 2005. A spot of heated crust gave way, causing the rift to abruptly split open by as much as 26 feet — something that Tim Wright, a geophysicist at the University of Leeds, calls “Hollywood-style geology.” That crack, equivalent to 400 years of normal plate movement, ripped apart in a matter of days, resulting in the destruction that scared the nomads. The crack also filled with about 660 billion gallons of molten rock — about a ton of rock for every human on the planet — and it wasn’t the only one. Over the next five years, molten rock shot into 13 nearby fractures, just the latest cycle of scars in a valley that has been straining to stretch for 30 million years.

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Geoscientist Ellen Knappe pulls data from a receiver in Lalibela, Ethiopia. These gather data from GPS antennas that Knappe’s team has placed around the country, telling researchers how the crust has moved.
Shannon Hall

Now, thanks to events like the one in 2005, the Afar is chock-full of vertical sheets of basalt, whose density has plunged the crust more than 300 feet below sea level. This thins and further weakens the crust, allowing the mantle plume to continue attacking it. Scientists assumed that, in a self-reinforcing cycle, any further extension should take place where the first weaknesses began: inside the rift.

That had been the thinking since the birth of plate tectonics in the 1960s. But with the first natural laboratory splitting open in East Africa in 2005, Rebecca Bendick, Knappe’s Ph.D. adviser at the University of Montana, decided to double-check the theory. In 2011, she set her eyes (and her instruments) on a region no one had considered before: outside the rift.

That year, Bendick and her colleagues installed 11 GPS instruments that ran from Sudan to Somalia. Should she find that other areas in Ethiopia are stretching, it would overturn a generation of theoretical models and uproot geology’s understanding of how rifts ultimately broke Pangaea (and previous supercontinents) apart. Already, the first few years of data have shown the extension might not be limited to just the rift — and scientists are hungry for more data.

So Knappe has spent her winters investigating the rift from more than 60 miles away, in the neighboring areas of Africa. When I accompanied her in January 2017, we spent four days crisscrossing the Ethiopian highlands, a land that transitioned from a patchwork of cultivated fields to grassy moorlands scarred with deep basaltic canyons. It was a world constantly in motion. Men harvested teff (a grain unique to Ethiopia and Eritrea) by tossing it in the air, women walked to and from town carrying baskets of goods on their heads, and boys herded flocks of animals down the road. Most of the time, we drove from one instrument to the next, and peering out the window was a visual treat.

“Some places just steal your heart,” said Knappe as we sat in the dirt-speckled Land Cruiser, watching the countryside and people roll past. “It’s like living in color for the first time — that’s the only way I can describe it to people.”

Splits in the Science

Every instrument on Knappe’s route was perfectly intact, save one. All together, they’ll add another year to Bendick’s growing research showing the rift is not proceeding according to plan. Any textbook would suggest that a mature rift, like the segment in Ethiopia, would be the site of 100 percent of the region’s extension. But Bendick’s latest paper, published in Geophysical Research Letters in 2016, shows something different. As much as 20 percent of the extension takes place outside the rift, deep in the Ethiopian highlands. The cheesy crust of East Africa appears to be stretching in places it shouldn’t. The question is why.

Virginia Tech geologist D. Sarah Stamps thinks gravity is partially to blame. The Ethiopian highlands — sometimes called the Roof of Africa — can tower nearly 15,000 feet above sea level. But they can’t escape gravity’s pull. “If you have a spherical ball of putty and you set it on a flat surface, over time it will flatten,” Stamps says. This flattening can be seen in mountain chains around the world, from the Andes to the Tibetan Plateau, and could explain why there’s some extension beyond the rift.

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Jay Smith

This theory is made especially likely given a second surprising finding. A year or so after Bendick first deployed her GPS instruments, Katie Keranen, a geophysicist at Cornell University, set up 35 seismometers at 40 rotating sites across Ethiopia. The instruments record slight ground motions, allowing scientists to detect local and even distant earthquakes in order to map hot, less dense spots of partially molten rock beneath Earth’s crust.

Keranen hadn’t expected to see much heat beyond the rift — like Bendick, she was just double-checking established theory. Previous research had already pinpointed a mantle plume beneath the Afar depression, and scientists mostly anticipated that the land away from the rift would be cold. But Keranen found just the opposite: multiple tiny hot spots she suspects are fingers off the rift’s main plume below the Ethiopian highlands. Not only is Africa’s cheesy crust stretching in places it shouldn’t, but it’s also sizzling where it shouldn’t.

That added heat might come to gravity’s aid. Think about that spherical ball of putty: If you warm it up, it will droop and flatten more easily. Still, scientists can’t yet say whether the Ethiopian highlands are stretching mostly due to gravity or heat. Nor can they say if this added sizzling and stretching is actually typical for a rift’s final breakup. “Maybe this is the standard model, and we’ve gotten the standard model wrong for the past 50 or so years,” Bendick says. Regardless, it’s clear that the rift has diverged from the recipe revealed in any modern textbook.

And that could have implications for why some rifts fail while others do not — an open question in geology. Roughly 1.1 billion years ago, a similar rift ran more than 1,000 miles from Lake Superior to Kansas, threatening to cleave North America down the middle. Then, for reasons unknown, the rift failed, leaving North America intact. “If the sequence of steps is way more complicated than the simple model would suggest, then there are lots of opportunities for rifts to fail,” Bendick says. “The more steps you have in a process, the more places it can go wrong.”

Or perhaps the added heat is a rift’s ultimate downfall. Keranen would not be surprised if the pockets of heat below the Ethiopian highlands make it harder for the rift to ultimately succeed. “The fact that it has all of these multiple fingers that are all hot and likely weak could mean that when you pull on it from side to side, you spread that strain out, and it’s less likely to really break,” she says. Think back to that block of cheese. If the strain were not contained within a crack, but across far more of the block, the cheese would stretch for much longer before it snaps — if it ever did. This could point toward another way in which rifts fail.

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Looking out from the northern Ethiopian highlands, often called the Roof of Africa, geology’s influence is clear. With enough data, perhaps one day geoscientists will be able to predict how the continent’s landscape will change in the future.
Shannon Hall

Keranen, however, argues that scientists can’t yet foresee East Africa’s future. “Because we are looking at such a short snapshot of time, it’s hard to know what will happen in Ethiopia,” she says. Some geophysicists, like MIT’s Floyd, are certain the end point will remain the same as scientists initially predicted, however much the process diverges from normal. “There’s absolutely no doubt that we see a great amount of extension on the rift itself and it will continue that way,” he says. “It will be the case that yes, the Red Sea starts to inundate farther south into the Afar triangle and eventually into the Main Ethiopian Rift — not for a few million years yet, but that almost certainly will happen.”

Floyd’s point underscores how much the northern portion of the rift has already transformed. The Afar is dotted with fissures jam-packed with cooled molten rock and volcanoes that have been pouring magma onto the surrounding crust for tens of millions of years. So much activity means the crust is almost entirely basalt — a characteristic of oceanic crust and not continental crust, Bendick says. “All that’s missing is just to have water come pouring in,” she says. “It’s pretty much an ocean already.”

Earth’s Ups and Downs

As Knappe and I were on the last leg of our journey, we stood at the edge of a cliff in the Ethiopian highlands, gazing at a landscape that seemed to transform in front of our very eyes. Not only was the sun sinking lower in the sky, casting shadows that stretched across the wrinkled topography, but the terrain itself was split. Before us were the fertile farmlands of the highlands, but farther on, the world sunk into the rift.

We stood on a 75-mile-thick-plate, and yet we could see toward the Afar Depression, where a plate less than 20 miles thick separated the alien surface from the mantle. For the first time, I felt like I truly was on the Roof of Africa.

It’s easy to see why this work has stolen Knappe’s heart. Her research in East Africa plays a pivotal role in understanding where hazards might lie — and her latest research shows that they aren’t confined to the Afar. The stretching and sizzling of the Ethiopian highlands means they, too, could be at higher risk of geologic dangers like the earthquakes, volcanic eruptions and geothermal eruptions that have devastated the Afar.

Despite the hazards, scientists of various disciplines have flocked to the rift over the years, braving the physical and political dangers. (During our time there, Ethiopia was in a state of emergency after hundreds of people had been killed in government protests.) The opportunity to see a continent rip in two was well worth the risk. Geology does not usually happen in real time. More often than not, geologists dig through layers of sediment, excited by events that happened thousands of years in the past. The discipline tends to dwarf human lifetimes.

The rift is an exception. “It’s not like you’re looking at something stagnant now,” Knappe says. “It’s still really active.” The Earth beneath our feet has been changing since long before the age of the dinosaurs, and it’s not done yet.

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