Clearer view under crust
WOODS HOLE, MASS. — As he spreads a pair of colorful sea-floor maps across a table, Henry Dick pauses and confides that ever since graduate school, he's wanted to make some small contribution to understanding how Earth renews its crust and sculpts continents.
Now it appears Dr. Dick and his colleagues at the Woods Hole Oceanographic Institution may have contributed more than he bargained for. Drawing on results from undersea mapping and rock-dredging at opposite ends of the globe, they appear to have discovered a new class of ocean ridge, giving researchers new insights into key regions between Earth's ever-shifting tectonic plates.
The work also may answer longstanding puzzles surrounding the mechanisms that build those plates and split continents. In the long run, the results could point to potentially valuable deep-sea mineral deposits.
This represents the "biggest discovery" about the workings of ocean ridges in 20 years, says Jason Phipps Morgan, who heads the department of marine geodynamics at the GEOMAR Research Center for Marine Geosciences in Kiel, Germany.
These ridges, which crease the sea floor in oceans worldwide, are sites where material from deep within the Earth wells up to form new crust. They are said to host 90 percent of all Earth's volcanic activity. There, in the ridges and rift valleys that form plate boundaries, magma oozes from undersea volcanoes and fissures, spreading at rates that typically range from 2 to 18 centimeters (0.78 to 7 inches) every year.
At least two sections of prominent ridges beneath the Arctic and Southwestern Indian oceans were known to be among the slowest of the tectonic slowpokes. But little else was known about them, says Dick. Theories held that these ridge segments held little in the way of volcanic activity. This relegated them to the ranks of the boring. They were also hard to get to. One lies under an ocean sheathed in ice all year, while the other lies beneath seas where relentless 30-foot swells would be considered good conditions.
For Dick and his colleagues, however, the ultraslow ridges held a particular attraction. Theories held that the crust at these locations would be mantle rock, not the basalts formed through volcanic activity. Thus, compared with more-active ridges, where the crust averages 6.5 kilometers thick, the regions along the ultra-slow spreaders might actually have no crust at all. Instead, mantle rock would be rising directly into the sea with little or no volcanic processing.
To sample this relatively pristine mantle, the southwestern Indian Ocean looked like the best bet.
In December 2000, Dick and his team dredged rock samples and developed a highly detailed maps of a little-studied section of the Southwest Indian Ridge. The structure of faults, valleys, and other features looked different than patterns on other, more closely studied ridges.
Then, in the summer of 2001 he turned his attention to the Arctic Ocean and its Gakkel Ridge. The ridge had been partially mapped during 1999 by a team of oceanographers aboard a US Navy nuclear submarine. The sub's team discovered that the Gakkel hosted volcanoes, but lacked fracture features often found along mid-ocean ridges. Something wasn't fitting the standard picture.
Using US and German icebreakers, Dick's surface expedition developed a more detailed map of the ridge and included the segment the submarine didn't get.
"It was a heroic effort," he says of the trip and his colleagues' work.
Some doubted that sonar would work, because any sound wave bouncing off the sea floor would be swamped by the sounds of crunching ice. But additional work allowed researchers to filter out the crunch, leaving the sea floor data clean. The US icebreaker Healy plowed through the ice, followed by the German Polarstern with the sonar.
The ridge's patterns of faults, rift valleys, and volcanoes and the rock samples dredged up bore a stunning resemblance to the Indian Ocean ridge segments Dick had mapped six months earlier. And while the new crust - in this case blocks of mantle - was being introduced through nonvolcanic processes, portions of the ridge also displayed an unexpectedly high level of volcanic and hydrothermal activity.
This feature, particularly in the Arctic, has excited marine biologists. They reason that since the Arctic Ocean sits in an enclosed basin, these undersea hot spots could well host life forms seen nowhere else on the planet.
For Dick, the alternating volcanic- nonvolcanic pattern along the Gakkel and Southwest Indian ridges represents a new class of ridge. And he sees their nonvolcanic stretches as a new form of plate-boundary structure. "We're staring right down into the mantle," Dr. Morgan says. The work is detailed in Thursday's issue of the journal Nature.
Morgan also suggests that these "amagmatic" ridge segments represent a possible key to a longstanding conundrum.
Many people hold that when a continent splits, the tectonic divorce is accompanied by extensive volcanic activity. Yet over the past 20 years, researchers have been finding more undersea areas where solidified molten rock isn't evident.
At the ridge sites Dick has identified, Morgan says, "we're seeing in real time on the mid-ocean ridge places where there's no real melting when there's a rift."