From: emmanuel (emmanuel@bayou.com)
Date: Mon Sep 29 2008 - 18:11:55 EDT
http://www.sciam.com/article.cfm?id=birth-of-an-ocean
Scientific American Magazine - September 29, 2008
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Birth of an Ocean: The Evolution of Ethiopia's Afar Depression
Key Concepts
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.
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Formation of an ocean is a rare event, one few scientists have ever
witnessed. Yet this geophysical nativity is unfolding today in one of
the hottest and most inhospitable corners of the globe. Visit the site
in safety through this extraordinary photographic essay
By Eitan Haddok
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.
Click here to view this photo essay as a slide show
RISING ABOVE
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).
LAKE OF LAVA
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.
HELLISH HEAT
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.
LETHAL FUMES
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.
POISON OR ELIXIR?
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.
FATEFUL FLOODS
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).
SALT OF THE EARTH
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).
MIRAGE OR HALLUCINATION?
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).
Note: This article was originally printed with the title, "Birth of an
Ocean".
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