Hotbed of Volcanic Activity Found Beneath Arctic Ocean
for National Geographic News
|June 25, 2003|
Findings reported from the first ever detailed exploration of the Gakkel Ridgethe northernmost segment of the worldwide mid-ocean ridge system that snakes for 1,100 miles (1,800 kilometers) beneath the Arctic Oceanunderscore the waiting discoveries on the frontiers of Earth science.
For decades scientists longingly eyed the Gakkel Ridge. But since it lies beneath a cover of sea ice, access to it has been limited. Apart from a single submarine study, much of what was known about the undersea region's geology was extrapolated from studies of other, more accessible, ocean ridges.
Then in August 2001 an international team of scientists embarked on a rare exploratory mission aboard two icebreakers specially-equipped with scientific instruments to map the seafloor, collect samples, and analyze what really lies three miles (five kilometers) beneath the ice-covered seas.
The team returned nine weeks later with findings that are changing preconceived notions of how the ocean floor forms, said Emily Klein, a geologist at Duke University in Durham, North Carolina.
Researchers discovered that, contrary to expectations, the exceedingly slowly spreading ridge is rife with hydrothermal vents and a hotbed of irregular volcanic activity.
Klein noted that extraordinary technical achievements were needed in order to make the expedition. "People truly thought it could not be done," said Klein, author of a perspective on the Gakkel Ridge expedition and its findings that is published in the current issue of the science journal Nature.
Charles Langmuir, an Earth scientist at Harvard University in Cambridge, Massachusetts, and co-leader of the expedition said, "One thing that brought the three chief investigators into harmony on this expedition was how wrong we all were about what we would find. But nothing could be more exciting than to be so wrong. Discovery means to learn something not known before." Langmuir co-authored a related expedition research paper also published in Nature.
Volcanoes and Vents
Mid-ocean ridges form where two tectonic plates pull apart and hot magma from deep within the Earth oozes up to fill the gap. The ridges circle the globe and the magma that solidifies between the plates forms the ocean floor.
The Gakkel Ridge was known to be the slowest spreading ridge in the world, averaging just 0.4 inches (1 centimeter) per year. By contrast, other volcanically active mid-ocean ridges elsewhere in the world spread at rates as fast as 7.5 inches (19 centimeters) per year.
But based on rock samples dredged from the ocean floor and detailed sonar mapping, researchers were surprised to discover that the Gakkel Ridge is actually a hotbed of volcanic activity and hydrothermal vents.
"We expected very few fresh volcanic lavas. Yet the first maps and samples revealed a highly active volcanic province," said Langmuir, noting that the abundant and recent volcanic activity was both surprising and remarkable.
Researchers discovered that volcanic activity along the ridge did not correlate directly to its rate of spread.
"Instead of finding a linear relationship between the spreading rate and amount of volcanic activity, we found that the intermediate part of the ridge was really the least active," said Peter Michael, a geoscientist at the University of Tulsa in Oklahoma and an expedition co-leader. "It tells us that there are other factors in the mantle that are controlling the amount of volcanic activity we see," said Michael, co-author of a related expedition research paper also published in Nature.
The expedition team found much more hydrothermal venting activity than expected. To get hydrothermal activity, water must percolate down through cracks in the ocean floor to where it interacts with a heat source such as magma. The heated water then reverses direction and, as it rises, leaches elements from the surrounding rock. When the element-rich hot water reaches the ocean floor, it spouts out.
"The hydrothermal signal was so omnipresent that we thought there must be something wrong with the instrument," said Langmuir. "In fact, when we sent a message from the ship to the specialist who built the instrument on shore about the data we were getting. He said there must be something wrong because the results were not reasonable."
One explanation for the unexpected hydrothermal venting is the fact that because the ridge is spreading but magma is only rising at a few focused areas, many cracks in the ocean floor are not being plastered over with fresh magma. This in turn gives seawater an enhanced pathway to penetrate the crust and results in more venting, according to Klein.
Wilfried Jokat, a marine scientist at the Alfred Wegner Institute for Polar Research in Bremerhaven, Germany, and colleagues used an array of seismic sensors to map the thickness of the ridge crust. Models had predicted that at slower spreading rates there should be less magma and thus a thinner oceanic crust.
As predicted, the researchers discovered that oceanic crust along the Gakkel Ridge is exceptionally thin. But they were surprised to find that the model did not hold as the ridge trends to the east.
The discovery of such a thin crust in conjunction with abundant volcanic activity and hydrothermal vents is unusual, added Jokat. He said the data suggests that crustal thickness is not only a function of spreading rates but also depends on the three-dimensional character of ridge magmatism.
"This expedition affirms that new observations in new places with new tools allows the Earth to lead us to discovery beyond the limitations of our theories and imagination," said Langmuir.
|© 1996-2008 National Geographic Society. All rights reserved.|