Submarine Topography
The shape of the ocean floor is known to scientists as submarine topography. The floor contains mountains, plains, canyons, plateaus, basins, and other topographic features that are found on land. Usually, the ocean bottom is divided into three major zones: the continental margin, the ocean-basin floor, and the mid-ocean ridge.
Immediately offshore from the continents and some large islands is the continental shelf. It is a slightly inclined platform under shallow seas and is composed primarily of sediments. The shelf, which varies greatly in width, terminates on its seaward side with a sharp decline, called the continental slope. At the slope's base lies the continental rise, an area where the decline is much less pronounced. In some areas, such as the west coast of Central and South America, there is no continental rise; instead, there is a trench.
Other topographic features are also found along the continental margin. There are submarine canyons, deep V-shaped depressions found mainly in the continental slope, and marginal plateaus, steplike formations at the edge of the continental shelf. Also present, mainly on the shelf, are barely submerged features, such as banks, shoals, and reefs.
Beyond the continental margin, as far as the mid-ocean ridge, is the ocean-basin floor. It lies generally at great depths and is marked by various topographic features. Ridges, resembling low mountain ranges, and rises, elongated portions of the ocean floor with smooth, gentle slopes, are widespread. Abyssal plains are large, almost perfectly flat areas, usually near a continental rise. Adjoining many of the plains are expanses of abyssal hills, submerged volcanic peaks that rise less than 3,000 feet (914 m) above the ocean floor. Seamounts are submerged volcanic peaks that rise more than 3,000 feet above the ocean floor. Similar peaks with flat tops are called guyots, or tablemounts. Volcanic islands are peaks that rise above the surface of the ocean.
Beneath the sea, in some places about midway between the continents, is a vast mountain chain that is almost continuous around the world. It is some 40,000 miles (64,000 km) long, up to 2,500 miles (4,000 km) wide, and generally 6,000 to 12,000 feet (1,829 to 3,658 m) high. In a few places, such as the Azores, volcanic peaks along the ridge protrude above the surface as islands. Numerous fracture zones cut across the ridge, creating unusually irregular topography. One deep fracture, called the rift valley or mid-ocean rift, runs virtually the entire length of the ridge along its crest. According to the theory of plate tectonics, the rift forms the boundary between sections of the earth's crust that are moving apart. New sea-floor material is gradually formed along the rift as the plates diverge.
The ocean contains thousands of islands. Continental islands, such as the British Isles, Vancouver Island, and Sri Lanka, once were joined by land to nearby continents. Oceanic islands, or islands that rise from the ocean-basin floor rather than from the continental margins, are usually of volcanic origin. Mauna Loa, an active volcano in Hawaii, is taller than Mount Everest if its height from submerged base to peak is considered. It rises 13,680 feet (4,170 m) above sea level, and its undersea height is about 18,000 feet (5,486 m).
The ocean bottom is covered with sediments. Sediments are derived from four kinds of materials: rock, decomposed sea creatures, minerals in sea-water, and debris from meteors. In some places the sediment layer is 6,000 feet (1,800 m) thick. Sediments composed mainly of the remains of microscopic sea organisms are called oozes.
By studying marine sediments, oceanographers can obtain information on the history of the earth, including the nature of its climate millions of years ago. Sediment samples are obtained with coring devices, which are driven into the ocean bottom by long drills suspended from surface ships. Under-sea robots with mechanical arms that can scoop up sediment samples have also been developed.
The measuring of ocean depth is known as sounding. By taking a number of soundings over a given region of ocean, oceanographers can identify ocean-bottom features. An early method of sounding consisted of tossing overboard a line with a weight on its end and noting the length of line that was payed out before bottom was reached.
Today, sounding is usually done with an instrument called an echo sounder, a form of sonar. The echo sounder contains a transmitter that sends sound waves to the ocean bottom and a receiver that intercepts the sound waves reflected from the bottom. The depth is equal to half the distance sound travels in the time interval between the sending and receiving of the sound waves. The receiver is usually connected to a continuous recorder, so that, as the ship proceeds, an uninterrupted record of depth is traced on a paper chart.
A laser sounder is similar to an echo sounder, but it uses light waves instead of sound waves. It transmits laser beam pulses toward the ocean bottom and detects light that is reflected from the bottom.
Photographs of the ocean bottom can be taken with cameras that are installed in submersible craft or suspended by cable from surface vessels. Waterproof television cameras can also be lowered into the ocean by cable, and the closed-circuit television pictures viewed on a screen aboard ship.
