From: Biniam Tekle (biniamt@dehai.org)
Date: Mon Sep 29 2008 - 14:26:18 EDT
September 29, 2008
Birth of an Ocean: How It Works Africa is splitting apart at the seam,
making way for a new ocean.
Scientific American Magazine - September 29, 2008
Birth of an Ocean: The Evolution of Ethiopia's Afar Depression
http://www.sciam.com/article.cfm?id=birth-of-an-ocean
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
http://www.sciam.com/slideshow.cfm?id=birth-of-an-ocean
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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