Cruise to the Incipient Rift
Emily Klein 'Mows' the East Pacific to Reveal Secrets of
Magma p.4
“We’ll make a map of the whole region,”
Klein said. “It’s called “mowing the lawn.”
In essence, the whole ship became the pen for an evolving
underwater chart, with sound waves serving as ink. Each pinging
sonar swath covered about five kilometers of ocean bottom,
but it took multiple course changes, back and forth, to blanket
the whole area.
One purpose of this extensive “mowing” was to
see if the IR really stops cold 75 kilometers east of the
East Pacific Rise or instead continues at another location
on the other side. “Ridges can do those little jumps,”
Klein said.
The R/V Melville also towed a magnetometer, a device
that detects magnetic variations in ocean floor rock. When
molten lava hardens into basalt, on-board students explained,
magnetic constituents in the stone align with Earth’s
magnetic field as it existed at the time. These alignments
then begin to decay as the stone oxidizes.
Scientists can thus deduce when the rift began forming, as
well as how recently additional lava erupted. It was such
magnetic observations, said WHOI graduate student Clare Williams,
that provided the estimate that the Incipient Rift first began
cracking through the ocean floor only about 1 million years
ago—yesterday, geologically.
Ten days into the cruise, the expedition was wrapping up
its sonar and magnetic work and beginning its second stage
of operations. That’s when it first dropped the Melville’s
dragline dredge, a huge heavy jaw of iron that emptied into
a large iron net resembling chain mail.
Dragged behind a cable about 3,000 meters beneath the ship,
the dredge served as a brutish counterpoint to the wax cores
the group began dropping too. Successful wax core missions
would return with tiny bits of basaltic glass. But a good
dredge run would scoop up piles of rocks for the students
to heft from deck to laboratory for sorting and sawing into
geological samples.
Also launched was WHOI’s TowCam—nicknamed “RabbitCam”
for the slogan painted on its side. As its name implies, TowCam
was also lowered underwater by a cable and pulled behind the
ship where its digital camera made up to 1,000 pictures of
the bottom each run.
With TowCam’s 300-watt strobe light introducing wedges
of day in the deep ocean’s perpetual night, the Nicholas
School’s Web site quickly began posting crisp images
of unusual features in the unclouded water. Those included
a possible extinct hydrothermal vent (with startled crab),
and what Duke undergrad Donnelly termed a “baby giant
squid.”
Another photograph of fresh lava flowing over older rock,
coupled with the basaltic glass recovered from almost every
initial dredging run, provided Klein an early surprise. “It
is truly remarkable how magmatically active the Incipient
Rift is,” she exclaimed in an e-mail from the ship.
Also of interest from the SeaBeam sonar studies was how much
the Incipient Rift’s walls appear to have spread apart.
On their very last day, IR researchers discovered a new and
previously unknown V-shaped rift, called a “propagating
rift,” northeast of the Incipient Rift. Klein said it
was one of the exciting findings of the cruise. “This
finding may prompt us to rethink our understanding of the
whole microplate boundary system in this area.”
Klein quickly noted that final assessments and discoveries
will happen in her Duke lab, perhaps well after the cruise
is over. “The heart of this cruise is to study the geochemistry
of the lavas,” she wrote. “And we are not doing
chemical analysis at sea.”
Monte Basgall is a senior writer with Duke’s
Office of News and Communications and specializes in science
coverage.
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