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 tec-tonophysicist 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
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.
Rifting begins when hot currents carry magma up from the
mantle, causing the crust above
to bulge.
Continental crust
Magma
As currents spread, they pull the
crust, which splits to form a rift
valley. Pieces of crust break off
and sink, volcanoes form and
earthquakes rumble.
Magma continues to well up and
volcanic rock is deposited in the
rift valley. The valley drops below
sea level and an ocean forms.
Upwelling magma spills
out into undersea mountains and volcanoes, and
the rift becomes a true
ocean-spreading center.
Ocean
Ocean crust
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.
E. FELICIANO
www.sciencenews.org
