Tuesday, September 30, 2008

Birth of an Ocean: The Evolution of Ethiopia's Afar Depression

Ghostly salt deposits near Afdera volcano testify to ancient inundations in Ethiopia's Afar region. In the past 200,000 years the Red Sea flooded Afar's lowlands at least three times; the salt stayed behind as the seawater evaporated. One day the ersatz seascape will likely become the real thing.

  • Africa is splitting apart at the seams—literally. From the southern tip of the Red Sea southward through Eritrea, Ethiopia, Kenya, Tanzania and Mozambique, the continent is coming un­­stitched along a zone called the East African Rift.
  • Like a shirtsleeve tearing under a bulging bicep, the earth’s crust rips apart as molten rock from deep down pushes up on the solid surface and stretches it thin—sometimes to its breaking point. Each new slit widens as lava fills the gap from below.
  • This spectacular geologic unraveling, already under way for millions of years, will be complete when saltwater from the Red Sea floods the massive gash. Ten million years from now the entire rift may be submerged.

In northeastern Ethiopia one of the earth’s driest deserts is making way for a new ocean. This region of the African continent, known to geologists as the Afar Depression, is pulling apart in two directions—a process that is gradually thinning the earth’s rocky outer skin. The continental crust under Afar is a mere 20 kilometers from top to bottom, less than half its original thickness, and parts of the area are over 100 meters below sea level. Low hills to the east are all that stops the Red Sea from encroaching.

Such proximity to the planet’s scorching interior has transformed the region into a dynamic landscape of earthquakes, volcanoes and hydrothermal fields—making Afar a veritable paradise for people, like me, eager to understand those processes. Yet few outsiders, scientists included, have ever set foot in Afar. Daytime temperatures soar to 48 degrees Celsius (118 degrees Fahrenheit) in the summer, and no rain falls for much of the year. But I knew I faced more than treacherous geology and climate. Nasty geopolitical struggles—namely, war between Ethiopia and neighboring Eritrea—combine with those natural hardships to make Afar utterly inhospitable.

Geologists predict another million years of the land stretching and sinking, combined with a massive deluge from the Red Sea, could put Afar at the bottom of a new ocean. For now, this incip­ient seabed is a desolate landscape where lava stifles vegetation, hellish heat makes acid boil, devilish formations emit toxic fumes, and the salty legacy of ancient Red Sea floods provides nomadic tribes of Afar with a precious export.

The highest point in sunken Afar is Erta Ale, or “smoking mountain” in the language of the local people. Erta Ale is the northernmost volcano in a long chain that follows the so-called East African Rift.

This rift is the not yet submerged equivalent of mid-ocean ridges—chains of under­­sea volcanoes that produce new seafloor. Indeed, Erta Ale spews the same kind of basaltic lava that erupts at mid-ocean ridges; past expulsions have covered the surrounding plain with so much fresh basalt that vegetation struggles to take hold (1).

Atop Erta Ale is one of the earth’s few quasi-permanent lava lakes. The flux of heat from the earth’s interior is rarely sufficient to keep rock molten under the cooling effect of the atmosphere. Even on Erta Ale the heat sometimes slackens enough so that portions of the lake surface “freeze” into a black crust (2) . Typically, though, blocks of basalt float like icebergs on the fiery liquid rock, which reaches 1,200 degrees C (2,190 degrees F) (3). Most of the Afar people do not approach the volcano, because it is thought to harbor evil spirits. Seeing an Afar warrior on the volcano’s summit is unusual; this man, Ibrahim, was my guide (4). Lava emerging from cracks in the lake is particularly spectacular at night (5), when the sight evokes the phantoms of local lore.

One hundred kilometers north of Erta Ale, near the Eritrea border, is the Dallol crater. There molten magma simmering below the surface fuels a vast plumbing network of superheated water. The result is a 1.6-kilometer-wide field of hydrothermal vents, geysers and hot springs (6) that call to mind the similar but more accessible environment in Yellowstone National Park in the western U.S. The mineral sulfur produces the lemon-yellow color in this earthly palette (7); blended with the signature red of oxidized iron, the sulfur stains turn orange (8). Only a few steps away from this vivid scene are drab, desiccated reminders of a hot spring’s ephemeral nature (9). When an earthquake or other natural process clogs a vent’s buried conduits, its minerals can lose their florid flush within a year.

The surreal landscape of the Dallol crater results as rain­water percolates deep underground, heats up as it contacts hot magma and rises to the surface through thick layers of salt, dissolving the salt as it travels. Recrystallization of the salt at ground level can sculpt massive structures (10) or formations as delicate as an eggshell (11) . But the beauty of the sculptures can be deceiving: toxic vapors emanating from these so-called aeration mouths are yet another contributorto Afar’s devilish reputation—and often require visitors to wear gas masks. More than once a surge of the ominous gas forced me to stop shooting photographs and don my mask for safety.

Near reddish pools of bubbling-hot, iron-rich water (12), the strong odor of hydrocarbon is a telltale sign of danger. Animals sometimes stop for a drink—not realizing it will be their last. I saw several ill-fated birds swirling in the scalding pools. But I was comforted by the irony that one organism’s poison is another’s elixir. The same emanations that can kill birds, insects and mammals also nourish complex communities of microbes, which thrive in many of Dallol’s acidic waters. Not surprisingly, these terrestrial hot-springs communities bear striking similarities to their counterparts along submerged mid-ocean ridges.

The salt sculptures on the opposite page and others that decorate Afar serve as a reminder that the birth of an ocean is not a singular event but rather an ongoing saga. During the 30 million years this region has been stretching thin, global sea level has fluctuated, at times filling Afar with seawater. Most recently, about 80,000 years ago, the waters of the Red Sea rose high enough to breech the low hills east of Afar, carving deep canyons (13) as they flooded the lowlands. When sea level dropped and Afar was once again cut off from the sea, the floodwaters evaporated. Wind and water sculpted the salty traces of these past inundations over the ensuing millennia, sometimes carving bizarre formations called salt mushrooms (14). In other areas, alternating layers of salt and reddish marine sediment are visible in eroded canyon walls (15).

Salty traces of past deluges give the modern people of Afar a modest means to benefit from their baked and barren homeland. These nomadic herders collect the salt by hand, wielding wooden stakes and hatchets to break the thick layers into manageable blocks (16). The closest places to sell or exchange the salt are located in the Ethiopian highlands to the west—about a six days’ walk for the camel caravans used to transport this unlikely export (17).

Most years the greatest concern for the Afar people is finding adequate water. But the rains were unusually heavy in late 2006, and many of the salt fields remained flooded throughout my visit in January 2007. This unusual environmental circumstance afforded one of the most lasting impressions of my visit to Afar: as the camel caravans waded through the floodwaters, they appeared from a distance as a surreal montage of the present and future of this ocean floor in the making (18).

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