Mapping the Margins

It’s a violent world at the edges of our continental shelves, which could serve as a geology textbook

Plate tectonics, dissolving rock, salt gushers and plain old erosion give widely different forms to the slopes where our continent meets the ocean floor. The images on these pages provide our clearest pictures yet of what is happening where. Features as small as an acre can be seen, a resolution 5,000 times that of similar images produced from satellite data.

more terrain maps below

Sediments have come down the Mississippi River and covered layers of salt at the bottom of the Gulf of Mexico. We are offshore, south of Baton Rouge, looking northeast toward Pensacola, Florida. Craters form when salt resists compression, mushrooms upward and collapses. Salt also makes the sediment edges bulge.

When the United States in 1983 extended its territorial waters out to 200 miles, surveys were ordered. Ships using multibeam echo depth sounders steamed up, down and across the continental shelves, amassing billions of data points, which were archived at the National Geophysical Data Center (NGDC) in Boulder, Colorado.

Lincoln Pratson and William Haxby, both marine geophysicists at the Lamont-Doherty Earth Observatory (part of Columbia University), then turned the NGDC information into these extraordinary underwater views. The images have been distorted by making the vertical dimension four times as high as it should be to show geological features more clearly. Black areas are those for which no data are available.

By John P. Wiley jr.

Off Florida's Gulf Coast, the shelf ends in mile-high cliffs. Freshwater seeping underneath dissolves rock at the base; the rest then collapses.

Looking southward offshore from Oregon, oceanward is right, landward is left. The Juan de Fuca Plate, at right, covered by a smooth layer of sediment, collides with and is pushed under the North American Plate. As this happens, sediment is scraped off and piled up in ridges on the North American Plate.  

Looking north from Point Sur to Humboldt Bay, we see a series of submarine canyons. San Francisco is in the black area near the top. Notice how, in some places, the continental slope appears to bulge out at its base. This occurs as the Pacific Plate, which underlies the area, slides past the North American Plate.

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