The Ocean Floor

Like all ocean bottoms, the Atlantic's is extremely uneven, with submarine ridges, deep basins and canyons, and isolated underwater peaks, or seamounts. The Mid-Atlantic Ridge, longest of the underwater ranges, runs in an S-shaped curve for about 10,000 miles (16,000 km), approximately midway between the continents on the east and the west. The ridge lies generally one to two miles (1,600 to 3,200 m) beneath the ocean's surface.

On both sides of the Mid-Atlantic Ridge are broad, deep basins. Among those west of the ridge are the Labrador, North American, Guiana, Brazilian, and Argentine basins. Those east of the ridge include the Norwegian, West European, Canary, Cape Verde, Guinea, Angola, and Cape basins.

Between the principal basins are prominent submarine ridges, or rises. Though shorter and less prominent than the Mid-Atlantic Ridge, they nevertheless have pronounced relief. For example, the Rio Grande Rise off southern Brazil lies some 2,000 to 12,000 feet (600 to 3,600 m) beneath the water's surface and separates the Brazilian and Argentine basins, which are each about 18,000 feet (5,500 m) deep.

The Puerto Rico Trench, deepest of the Atlantic's trenches, stretches in a narrow arc north of Hispaniola and Puerto Rico. It reaches a depth of 28,232 feet (8,605 m). The South Sandwich Trench, just north of the South Sandwich Islands, has a maximum depth of 27,313 feet (8,325 m).

About three-fourths of the ocean floor is covered with deep-sea sediments. Most of these are oozes, soft sediments made up of shells and skeletons of various microorganisms. In the basins on the American side of the Mid-Atlantic Ridge are also large bottom areas of red clay.

The continental shelf, the shallow underwater extension of the continents, reaches seaward about 250 miles (400 km) southwest of England, 300 miles (480 km) southeast of Newfoundland, and 400 miles (640 km) off southern Argentina. Along Africa, however, the shelf is much narrower.

Beyond the continental shelf is the continental slope, where the ocean floor drops steeply. In some areas deep, narrow canyons cut into the continental shelf and slope. One of these is the Hudson Canyon, which leads to New York City.