Uncharted Territory

Ultraslow ridges hold new clues to crust's formation

Carolyn Gramling

At the top of the world in the late summer of 2001, the U.S. Coast Guard's icebreaker Healy carved a slow path through the ice-covered Arctic Ocean. On board, marine geologist Henry Dick sent dredge after dredge through the ice to the seafloor, searching for telltale rocks that would help shed light on how Earth's crust forms. "People said, 'You'll never get a single rock off the seafloor,'" Dick says. "They said, 'You can't dredge in the ice.'" But in fact, Dick's team collected more than 200 rocks—many of which turned out to be pieces of Earth's mantle.

Under the ice and 2 kilometers of water was a 1,800-km-long underwater mountain range known as the Gakkel Ridge. The Healy's expedition, conducted in tandem with the German icebreaker Polarstern, was the first exploration to that Arctic ridge to attempt to collect geological samples.

The surprising discovery of mantle rocks indicated that Gakkel Ridge is one of only two places known on the planet where the tectonic plates that make up Earth's hard outer crust slide apart and expose large slabs of the mantle on the seafloor. That mantle is normally buried under 6 km of crustal rock.

The other site, the 8,800-km-long Southwest Indian Ridge (SWIR), is on the far side of the world. Like the Gakkel Ridge, the SWIR is utterly remote. It's located beneath treacherous high seas.

Oceanographers are only now beginning to explore these areas in detail. They have already made surprising geological finds, including the exposed mantle. They've also uncovered evidence at both ridges of hydrothermal vents, fissures in the seafloor through which circulating, magma-heated seawater escapes. Researchers say that these two ridges may represent a new class of tectonic boundary, called an ultraslow-spreading ridge. The finding offers scientists the chance to explore new ideas about how Earth's crust forms and to study the rich ecosystems spawned by the vents.

Read more about it at Science News online.