Geologists Discover Clockwork Motion By Ocean Floor Microplates
Durham NC (SPX) Feb 24, 2005
A team of geologists from Duke University and the Woods Hole
Oceanographic Institution has discovered a grinding, coordinated
ballet of crustal "microplates" unfolding below the equatorial east
Pacific Ocean within a construction zone for new seafloor.
The scientists deduced that relatively small sections of the ocean
floor there, and perhaps in other similar places, may be slowly
rotating like imperfectly meshing cogs in a machine.
The unexpected findings provide new insights into the way several
ocean ridge segments that border the microplates evolved into their
current positions to form part of what is known as a "triple
junction," according to the researchers. And these results may be
applicable to systems elsewhere, they added.
"As often happens in science, what you think you're going to learn
doesn't always end up being the exciting thing that you learn," said
Emily Klein, the Lee Hill Snowdon Professor at Duke's Nicholas School
of the Environment and Earth Sciences, who is the lead author of a
report on the findings published in the Thursday, February 24, 2005
issue of the journal Nature.
Other authors include Deborah Smith, Clare Williams and Hans Schouten
of the Woods Hole Oceanographic Institution in Massachusetts. The
group's study, begun aboard the San Diego-based research ship R/V
Melville, was supported by the National Science Foundation.
Klein, whose specialty is geochemistry, said the scientists' original
focus was the chemistry and structure of the Incipient Rift, the
smallest and newest of four ocean ridge segments in a region of the
ocean floor northwest of the Galapagos Islands.
Ocean ridges are linear features on the ocean floor where molten magma
originating in the earth's mantle rises and solidifies to form new
The Incipient Rift and other ridge segments in the area intersect with
the East Pacific Rise, part of a globe-circling mid-ocean ridge system
and the region's largest ocean crust producer.
All these intersecting ridge segments also form parts of boundaries
separating what the study revealed to be subsections of the Galapagos
Microplate, which wedges between three other larger plates in the
region's complex ocean floor topography.
"The exciting story is about the tectonics and the kinematics of the
whole Galapagos microplate, which before our cruise was little
understood," Klein said in an interview. Tectonics are the crustal
deformation of plates; kinematics describe their motion over the mantle.
"The Galapagos microplate shares a complex plate boundary
configuration with the surrounding Cocos, Nazca and Pacific plates. We
learned a lot on this cruise and have many new questions to pursue,"
Smith said of the study.
At the outset, the scientists grew puzzled when they began analyzing
data from sensitive sonar beams they were using to map the extremely
jumbled terrain of previously uncharted geological features along the
A previous study by other researchers led them to expect that rift
would grow consistently wider, in the manner of a ship's wake, as they
mapped increasingly eastward from the East Pacific Rise.
Instead, their sonar imaging showed the rift becoming narrower and
deeper as their distance from the East Pacific Rise grew larger.
Narrowing at both ends and widest in the middle, the trough thus
assumed the overall shape of an elongated diamond - which they termed
a "lozenge." That finding implied that more complex dynamics are at
work there, said Klein.
Meanwhile, underwater photography and rock magnetic measurements by
the group suggested that molten lava was periodically erupting within
the area where the Incipient Rift re-narrows. Such eruptions would
provide further evidence for "active rifting and continuing
reorganization of the microplate's boundary," Klein said.
Using those collective observations, Smith, Schouten and Williams of
Woods Hole applied their own expertise in modeling to deduce the
likely present, past and future motion of the Incipient Rift and the
Galapagos microplate it borders.
In the process, the scientists found that "what was previously
considered one coherent microplate must, in fact, form two separate
microplates," according to the Nature report. They also deduced that
those separate Galapagos microplates should be "rotating," and turning
"in opposite directions."
Duke's Klein and the Woods Hole modelers then went on to infer the
kinematics of these contiguous microplates.
Both microplates appeared to be turning - the northern one
counter-clockwise and the southern one clockwise - in a coordinated
way, reported the Nature report's authors, who located likely rotating
points on each of the three ocean ridge segments.
Eventually, further rifting may force the Incipient Rift to cut
further from the East Pacific Rise in a direction that pierces the
walls of an adjoining rift. If that happens, the "driving torque" will
cease, "and the microplates will stop rotating," the Nature report
"It's like ball bearings moving past each other," Klein said. "This
finding has huge implications for how complex plate boundaries
interact and evolve and change their orientations and kinematics
The key is "edge driven" action caused by the microplate rotations,
according to the paper. But what drives the rotations themselves
remains "truly an unanswered question," Klein acknowledged.
Map of relationship of Insipient Rift to Hess Deep and Easter Pacific
Rise. "The exciting story is about the tectonics and the kinematics of
the whole Galapagos microplate, which before our cruise was little
understood". See larger image.