Volume 7, Number 1, 1997

On Research

Mapping 20,000 feet beneath the sea

Deep below the turbulent sea, between the tip of Africa and the Antarctic Circle, lies the Southwest Indian Ridge, created by the slowest moving set of continental plates on Earth. Here, Africa and the Antarctica move away from each other at a rate of three-fifths of an inch a year, slowly tearing at the planet's crust and allowing molten lava to seep out of the ensuing fissures and cracks.

John Madsen, associate professor of geology, is part of a marine geophysical investigating team using remote sensing devices to map this least-studied area of the Mid-Ocean Ridge System. Scientists from major European countries, Japan, South Africa and Korea are participating in the project.

In the spring of 1996, Madsen and a team of scientists spent 48 days aboard the R/V Knorr, operated by the Woods Hole Oceanographic Institute, mapping an area "the size of New England." The ship covered 10,833 nautical miles in 40 degree Fahrenheit weather, all but three of those days in seas of 10 to 20 feet. It was the longest cruise the Knorr had ever taken.

The topographical maps the trip has produced will help scientists understand the physical characteristics of the sea and ocean floor. Madsen says their work will be useful to the entire oceanographic community, but especially to those studying the biophysical, seismological and geophysical aspects of the area.

"Scientists studying microbacteria believe life may have originated at hydrothermal vents beginning at 10,000 feet below the surface," he says. That far down, the water is close to freezing, and when the 650 degree Fahrenheit magma flowing through the vents hits the 37-degree water, sulfide deposits are formed.

Aboard the Knorr, Madsen and his colleagues tested a new remote sensing device called the Sea Beam 2112, a multibeam sonar system that details not only depth but also the nature of the ocean floor.

"The multibeam sound signal travels in a variety of directions. For each ping sent to the ocean floor, 128 signals are received back on the ship," he says. These signals are recorded by computers and viewed on monitors. The ship moves ahead 10 knots and the Sea Beam pings again. All of this information ultimately becomes color-coded topographical maps of the evolving Southwest Indian Ridge.

During the voyage, the team made a surprising discovery. While charting an area 1,000 miles south of Africa's Cape of Good Hope and above the Antarctic where glaciers begin to thicken into land mass, they discovered a dramatic drop in the ridge, descending from 500 to 20,000 feet in only 10 miles. "It's the steepest change in elevation along the ocean floor that's been mapped any place on the globe," Madsen says.

He says the early results of the research suggest that unusual geologic activity and volcanic processes are occurring along the Southwest Ridge, and this information needs to be incorporated into existing knowledge about the global ridge system.

-Barbara Garrison