ONBOARD R/V ATLANTIS – An international team of scientists from eight institutions has arrived over a domed and corrugated underwater mountain in the middle of the Atlantic Ocean, where they are deploying the research submarine Alvin and other high-tech tools to evaluate unusual and poorly understood geological processes underway in a zone where new ocean crust is created.
Their investigation is being chronicled in twice-weekly dispatches from onboard the research ship R/V Atlantis, for the expedition web site entitled "Descent to Mid-Atlantic Ridge: A Live Expedition" (http://earthguide.ucsd.edu/mar) hosted by the Scripps Institution of Oceanography in La Jolla, Calif. The Atlantis is operated by Woods Hole Oceanographic Institution in Massachusetts.
Duke University science writer Monte Basgall is providing the narratives and photographs for the site, covering not only the geological science behind the expedition, but the instruments the scientists are using and the adventures of undertaking a scientific exploration. The site also includes animated diagrams illustrating scientific concepts.
New entries will be posted on Tuesdays and Thursdays, except on Thanksgiving Day, through Thursday, Dec. 14. Entries have already been posted for Nov. 14 and 16, covering plate tectonics and instruments and vehicles.
The targeted underwater mountain towers more than 12,000 feet above the deep ocean floor. That height reaches about 5,000 feet more than is usual along the Mid-Atlantic Ridge, one of the elevated volcanically-active zones below which the huge crustal plates that underlie Earth's surface are thought to be spreading apart.
As those plates diverge, molten magma upwells to the ocean floor surface as lava, which repaves the ocean floor as it cools. According to the theory of plate tectonics, the plates have both oceanic and continental parts, so the Atlantic floor's spreading motion means North America is moving slowly away from Europe and South America from Africa. The crust-producing ocean ridges stitch the entire globe like a baseball's seams.
However, there is evidence that a different-than-usual process occurred to form this mountain, which rises just northwest of the intersection of the Mid-Atlantic Ridge and the "Atlantis Transform" at the latitude of northern Florida.
Such transforms are earthquake-prone faults that connect individual segments of the ocean ridges, which are actually offset from each other rather than running straight along one unbroken line. According to theories of plate tectonics, these offsets are relics of the jagged way that plates broke many millions of years ago when they first began to spread apart. Like grooves on a sliding door, transforms are then necessary to accommodate the lateral slip of the spreading that followed.
Donna Blackman, a Scripps geophysicist who is chief scientist on this National Science Foundation-funded expedition, believes the forces that develop at the "inside corners" of ridge-transform intersections, such as this one, may promote the raising of such high dome-shaped underwater peaks, known to geologists as massifs.
"This cruise is really to try and understand how this type of mountain feature under the sea forms," Blackman said. "There appear to be time periods when the plate spreading system works differently. The interplay between faulting and magmatic activity changes. We're really starting to get into the guts of how the Earth works."
According to Blackman, about a dozen similar massifs have been identified along ocean ridges that are slow-spreading, such as the Atlantic Ocean's. The average annual spreading rate in the expedition's target area is about 24 millimeters, or 0.8 inches. "But the detailed motion gets more complicated at ridge-transform intersections," she said.
A key feature of these massifs, in addition to their unusual height, is less than normal volcanic activity. One theory is that a magma shortage prevents the cracks that form during plate separation from being "healed" with upwelling magma as they would normally. In the crust the result would be "taffy pull tectonics," said Jeffrey Karson, a Duke University structural geologist who is a co-principal investigator for the study. "As it pulls apart horizontally, it gets thinner vertically." Under those circumstances, blocks of underlying crust may be upended and lifted to form such tall domes, in the process pulling up and exposing crust from Earth's middle-region, the mantle.
Another process may also be at work, noted Deborah Kelley a geochemist from the University of Washington in Seattle's School of Oceanography who is another expedition co-principal investigator. If the "taffy pull" dredges up hot mantle rock, principally made of the mineral olivine, overlying seawater may percolate through the faults and fractures to chemically change the unstable olivine into serpentine minerals. "When serpentine minerals form, they swell and increase the volume of the rock significantly," said Kelley. "We think some of this expansion may aid in the formation of these domes."
In order to understand the formation of such massifs, their features will be compared to continental "basin and range" mountain-forming processes of such high and dry regions as the southwestern United States.
Also participating in the expedition are researchers from the University of Wyoming, University of Leeds in England, University of Houston, University of Illinois at Champagne-Urbana and ETH in Zurich, Switzerland.
Expedition scientists will probe the mountain with sophisticated underwater devices operated and maintained by a group of Woods Hole Oceanographic Institution engineers and technicians on the R/V Atlantis under the direction of expedition leader Patrick Hickey. First deployed is the DSL-120, a towed side-scan sonar that uses sound wave reflections and absorptions to create electronic and paper maps of the geology it passes over.
That will soon be followed by Argo II, a camera-bearing towed instrument package that Atlantis can maneuver into surprisingly tight spaces to provide digital and photographic images of the seafloor. The most spectacular tool is Alvin, a titanium-hulled submarine that can carry two researchers and a pilot at a time to depths as much as 14,764 feet for as long as eight hours.
Wedged in Alvin's cramped cockpit, the scientists will peer through the submarine's small 3 ½-inch thick viewports and use an array of video and still cameras to make invaluable personal evaluations of normally inaccessible terrain. The submarine also has two external robot arms to grasp and break off rock samples for later chemical and physical studies.
At the expedition's peak, the decks of R/V Atlantis will become a separate study in group dynamics as scientists, engineers, technicians and the ship's crew work in shifts around the clock to launch, operate and retrieve the various submersible vehicles, sort and study recovered specimens, monitor instruments and video screens, log data, transcribe their taped dictations during Alvin dives and somehow manage to find time to eat and sleep.
The above story is based on materials provided by Duke University. Note: Materials may be edited for content and length.
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