A Undersea View of Our Earth's Geography
The ocean bottom is divided into three major areas: the continental shelf, the continental slope, and the deep ocean basin. The continental shelf extends underwater from each of the major land masses and can best be characterized as the submerged portion of the continents. The shelf has features similar to those we see on land, including hills, ridges, and canyons. The shelf varies in size--it may be virtually non-existent in some areas, while elsewhere it may extend out several hundred miles from the shore. The shelf's average distance is about 64 kilometers (40 miles). It is beyond the continental shelf that the 'deep sea' begins.
The shelf ends at a depth of about 200 meters (660 feet), giving way to the steeper continental slope, which descends about 3,700 meters (12,000 feet) to the deep ocean basin. Here, the ocean floor deepens sharply and its features again resemble those on land (i.e., great plains and mountains) only on a much larger scale. In fact, the Earth's longest mountain range lies under the sea. More than 56,000 kilometers (35,000 mi) long, this mountain range, called the Mid-Ocean Ridge system, snakes its way around the globe. The Mid-Ocean Ridge marks the areas where the Earth's crustal plates are moving apart and is one of the most geologically active areas on Earth. It is here that new sea floor is created, giving rise to hydrothermal vents and volcanoes.
The deepest known point on Earth is at the bottom of the Mariana Trench, a depression in the floor of the western Pacific Ocean, just east of the Mariana Islands. This trench is 1,554 miles long and 44 miles wide. Near its southwestern extremity (210 miles southwest of Guam) lies the deepest point on Earth. This point--known as the Challenger Deep--is where the ocean bottom lies at a depth of nearly 7 miles (variously reported to be at least 36,198 to 38,518 feet deep). In 1960, the Trieste, a manned submersible owned by the U.S. Navy, descended to the bottom of the Mariana Trench. The depth reported is still only an estimate, based on a mathematical conversion from measured pressure (more than 8 tons per square inch), interpreted in terms of the integral of water density from the surface to the bottom.
NOAA National Oceanic and Atmospheric Administration