Rewriting the Story of Earth's Formation... Slowly

Henry Dick dredges up a new seafloor ridge

By Shelley Dawicki

Source: Woods Hole Currents

When Henry Dick started college in 1965, plate tectonics-the theory that Earth's surface is made up of great crustal plates in constant motion-was viewed as a wild hypothesis. By the time he was a young marine geologist in the early 1970s, it had become widely accepted theory. Dick and other young marine geologists thought all the major advances in their field had been made.

Happily, he was wrong.

Now a senior scientist in the WHOI Department of Geology and Geophysics (G&G), Dick and G&G colleagues Jian Lin and Hans Schouten recently identified a new type of ridge on the ocean floor. The "ultraslow spreading ridge," as the team called it in a November 2003 article in the journal Nature, could fundamentally change some aspects of plate tectonic theory.

"Marine geology textbooks will be rewritten," said David Epp, director of the marine geology and geophysics program at the National Science Foundation. As much as one-third of the seafloor may form differently than geologists had previously thought.

Crust in slow motion
In plate tectonic theory, Earth's surface is made up of about a dozen large sections of crust, or plates, all in constant motion. The boundaries form the mid-ocean ridge system, a 30,000-mile chain of underwater mountains and valleys that circle the globe like seams on a baseball.

Along sections of the ridge where plates are moving apart, new crust is formed. Scientists have believed since the 1970s that these ridges are either slow-spreading-the Mid-Atlantic Ridge creeps along at one to two inches a year-or fast-spreading-the East Pacific Rise races at five to seven inches per year.

By contrast, the Southwest Indian Ridge (SWIR) in the Indian Ocean and the Gakkel Ridge below the Arctic Ocean spread no more than fractions of an inch per year. Scientists have long said these regions are extreme examples of slow spreading ridges. That explanation did not satisfy Henry Dick.

"These ridges were rarely studied until the early 1990s," Dick said. "These places are hard to reach, we didn't have some of the necessary tools, and the intellectual leap to a different type of ridge was just too big, so funding was tough to secure."

Lacking support for a full expedition to study the geology of SWIR, he cobbled together a few days here and there on other scientists' cruises. Between 1976 and 2003, Dick made fourteen visits to the ridge to collect evidence.

He made a breakthrough in 1987. After years of dredging for rock samples and mapping the seafloor in increasing detail, he found that the crust along the Southwest Indian Ridge was much thinner than at ridges found elsewhere in the ocean. "My eyes grew to the size of dinner plates when I looked at the first high-resolution maps," Dick recalled.

Dick and colleagues finally had convincing evidence that SWIR was in a class by itself. But they needed more evidence to make the case for a new type of ridge.

More evidence under ice
Dick believed a similar process was at work in the Gakkel Ridge, but studying the seafloor three miles beneath the ice cap of the Arctic Ocean was a challenge. The arrival of a new Coast Guard icebreaker, Healy, brought Dick and colleagues Peter Michael (University of Tulsa) and Charles Langmuir (Harvard University), a fresh opportunity in 2001. But no one expected them to accomplish much.

During a nine-week summer cruise the research team exceeded all expectations. They proved that Gakkel Ridge is volcanically active, but also found that the ridge spreads so slowly that large chunks of Earth's mantle, instead of volcanic magma, are deposited directly onto the seafloor.

In fast-spreading and even slow-spreading ridges, magma rises to the seafloor through submarine volcanoes and fractures in the crust, filling in the gaps as the plates move apart. The process creates layers of volcanic rock up to six miles thick.

But at ultraslow ridges, the Earth cracks apart so slowly that magma from the mantle cools before it reaches the seafloor. There is no layer of volcanic rock. Instead, great slabs from Earth's interior are slowly pushed up directly onto the seafloor, providing geologists with their first direct look into the Earth's mantle.

They might provide more than that. Hot springs are far more abundant at these ridges than anyone had suspected. The ultraslow springs are likely the longest lived on the seafloor and may produce the largest potential ore deposits for nickel, zinc, and copper.

"All my career I have wanted to make a small contribution to the field of plate tectonics, but I didn't expect this," Dick said. "I can't wait to go back. We've truly just started to explore."