Duke Expedition Will Visit Dark Underwater Chasm To Study How Ocean Crust Forms

March 09, 1999

DURHAM, N.C. -- Scientists and students from Duke University will sail Friday toward an area in the Pacific Ocean off Ecuador above a dark, Grand Canyon-sized chasm to learn more about how new oceanic crust forms along Earth's 37,000-mile mid-ocean ridge network, which belts the planet like the seams of a baseball. Led by Jeffrey Karson, professor and chairman of the Division of Earth and Ocean Sciences at Duke's Nicholas School of the Environment, the expedition will travel southwest from Manzanillo, Mexico aboard the Woods Hole Oceanographic Institution research vessel RV Atlantis to visit a place where the edge of ocean bottom ridge has been cross cut by a plunging rift known as Hess Deep.

The upper rim of Hess Deep is located a mile underwater, and the rift plunges as deep as 9,000 more feet at its lowest spot. It is formed by the separation of two huge crustal plates -- the Cocos and the Nazca -- that make up part of the ocean floor just west of Central and South America.

The researchers will spend 24 days floating over the Hess Deep study site just north of the equator -- from March 16 to April 8. They will carry out their studies aboard a 23-foot titanium- hulled submarine, Alvin, capable of carrying two researchers at a time to depths as deep as 14,764 feet and remaining submerged for as long as 10 hours. The scientists also will use a side-scan sonar system towed behind the ship, and a camera-carrying towed sled that can dive to depths of 18,000 feet.

Except when they are diving in the Alvin, students and scientists will live and work on the RV Atlantis, a 274-foot oceanographic research vessel built in 1997. Completely air-conditioned, it features sleeping and working space for 24 researchers, including 3,710 square feet of laboratory space. There is also a library/lounge, a washer and dryer (especially important for students cleaning rock samples), and telephone, e-mail and fax links with the outside world. Hess Deep, named for Harry Hess, the father of the hypothesis that new crust gets created through seafloor spreading along mid-ocean ridges, is especially valuable to science because it slices through a slowly-moving treadmill of recently created crust produced at one of the planet's fastest-spreading mid-ocean ridges, the East Pacific Rise. Associate Professor Emily Klein, a co-principal investigator on the mission, said it is a unique phenomenon. The rift exposes spots where lava upwelled only in the last million years -- a mere heartbeat on the geological time scale -- and was then pushed away from its point of origin.

What scientists who visit the site will see is frozen cross-sectional records of past eruption events, displayed one after another on the walls of Hess Deep like cutout paper dolls. These complex structures, which demand the utmost in geological knowledge and observational ability to interpret, are normally buried underneath the seafloor.

Klein said Hess Deep provides a crucial window into one of Earth's important geological processes: how it makes new skin. "It's like taking a knife to a layer cake," she said. "It allows you to look at the base of that layer cake, which is unique."

According to Hess's hypothesis, molten magma made of basaltic materials constantly upwells along the crests of these mid-ocean ridges. As that outpouring reaches the surface as lava, it cools and then gets pushed away from the ridge flanks and spreads out as new oceanic crust.

At the same time, older crust located at the far edges of oceanic plates gets remelted and recycled after being forced underneath continental masses. That's now happening where the Cocos plate dives underneath Central America and the Nazca under South America.

While a variety of evidence supports the general seafloor-spreading hypothesis, a host of details are far from resolved, and that is the reason for geological expeditions like Hess Deep. "The main objective for us is to document what the internal structure of the uppermost oceanic crust looks like," said Karson, who will serve as chief scientist. "We know that it's covered with this frosting of basalt, but what's underneath that?"

Karson wonders, for example, how episodic the eruptions of lavas are. Do active phases get followed by quiet periods during which the underlying magma chamber deflates like a spent balloon? And he wonders exactly where all that newly created magma goes after it is released from the crests of the modest-sized mid-ocean ridges and then begins to cool.

"We don't build up a giant mountain of material," he said. "We don't have a big hole to fill, either. The obvious answer is we have to keep dropping the bottom out underneath, and keep filling it in.

"But how do you get that material out of the way? Does it subside along faults or fractures at the edges of dikes? Does it collapse in a chaotic way like a pile of gravel, or does it flow out of the way in a more uniform fashion?

"The details of that internal structure are essential for us if we want to understand what is going on immediately underneath that ridge axis when the oceanic crust is created."

Dikes, among the geological structures that the Hess Deep researchers will be examining, are the now-solidified pipes through which molten minerals from deep within the Earth traveled to the surface in the past.

To attempt to answer questions like these, Duke's expedition will concentrate only on the upper 6,000 feet of the north wall of Hess Deep, high enough to expose interesting rock strata that would degenerate into rubble fields further down the slope.

The first task will be two days of explorations with the side scan sonar system, a 10.8-foot vehicle that is towed to the side of the ship's wake so it can bounce unimpeded 120 kilohertz sound waves off the rocks below. With suitable electronic processing, the returning sonar beams will paint shadowy, black and white mosaic portraits of the underlying geology, based on the degree to which particular kinds of rocks absorb or reflect sound.

Next will come "something that has never been done before in this sort of place," Karson said. The Duke researchers will spend four days, and perhaps some additional nights, documenting everything interesting with an electronic camera carried on the 15-foot diving sled. Equipped with an array of bright lights, the sled called Argo 11 will allow the scientists to make a "continuous mosaic" of digital images on a scale of hundreds of yards across, which will be invaluable to geological research, Karson said.

That will be followed by 16 days of dives to Hess Deep aboard Alvin, which is equipped with video cameras and a camera, as well as with mechanical arms that can break off geological samples to return to the surface for geological analysis. Even more important, its viewing ports will give scientists the opportunity to personally view the details of the rock face.

"The most important thing scientists can do is describe what they see out the window," Karson said.

Each Alvin dive will include at least one senior scientist, a flexibility that will also allow students on the voyage to make forays to the bottom and record their impressions. And after those students resurface, "they will be absolutely buried in trying to write up the results of that dive," Karson said. "We require that everybody sit down and produce a complete written transcript of the voice log they record." They will also have to integrate the taped comments with the photographic and videotaped records so that "we really have a detailed report on each dive before those people dive again."

Others from Duke on the expedition include doctoral degree students Daniel Curewitz and Michael Stewart; masters degree student Carrie Lee, undergraduate senior Aisha Renee Morris, staff assistant Peter Rivizzigno, and senior science writer Monte Basgall.

Duke University

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