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Ocean Bumps and Dips
| Article
# : |
20443 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1992 |
1,572 Words |
| Author
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David T. Sandwell
David T. Sandwell is associate professor of geophysics at
Scripps Institution of Oceanography in La Jolla, California.
He studies geological structures of deep-ocean basins using
satellites and ships and has published over 40 papers on these
topics. His current research is focused on understanding
poorly charted areas of the South Pacific. |
The surface of our planet consists of about 30 percent land and 70 percent ocean. Many of us could sketch the outlines of the continents and perhaps label the major mountain ranges and river basins. Yet most people are unfamiliar with the large topographic features beneath the deep oceans. For example, the Pacific-Antarctic Rise, which has an area about equal to South America, is a broad rise of the ocean floor caused by seafloor spreading between two major tectonic plates. To the west of the rise lies the Louisville Ridge, which is a chain of large undersea volcanoes having a length equal to the distance between New York and Los Angeles. The Louisville Ridge was first detected in 1972 and charted in greater detail in 1986, although many of the 4,000-meter-tall seamounts along this ridge remains uncharted. In an age when we are mapping the surfaces of Venus and Mars in great detail, it is difficult to believe that so little is known about our own planet.
The basic problem is that the seafloor is masked by 3,000-5,000 meters of salty water. Electromagnetic waves cannot penetrate to this depth, so a satellite carrying radar or laser equipment cannot be used to directly measure the seafloor topography. Prior to World War II, ocean depths were measured using a lead weight attached to a very long wire. Clearly, this is an inefficient and inaccurate method of mapping large areas. It takes hours to deploy and retrieve the wire, and during the time the ship can drift so that the wire is no longer vertical. Using this tedious method, explorers and scientists were able to establish the broad-scale variations in ocean depth. Most details were completely unknown, however, and most maps portrayed the ocean floor as a featureless plain.
While the ocean is impenetrable to electromagnetic waves, acoustic weaves propagate quite efficiently in seawater. In the late 1940s echo sounders were developed to rapidly measure ocean depth. The apparatus consists of a transponder (underwater speaker) and a hydrophone installed on the hull of a ship. An acoustic pulse, emitted by the transponder, travels through the ocean and reflects from the closes rock surface on the ocean floor. The two-way travel time (about 3-6 seconds) provides a measure of the ocean depth. With such rapid measurements, scientists could collect continuous profiles while steaming at full speed.
The installation of echo sounders on several research vessels dramatically increased the number of depth soundings and proved that the ocean floor was not featureless. Indeed, compilation of these soundings into topographic charts revealed the highly structured nature
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