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 To one who can't swim, the "deep sea"
may be just a little above ankle level in the ocean. So what do scientists mean when they refer to the "deep sea"?
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. It is the submerged portion
of the continents. The shelf has features similar to those we see on land, including hills, ridges, and canyons.
The size of the shelf varies. It may be virtually non-existent in some areas; elsewhere it may extend from shore
for several hundred miles. The shelf's average distance is about 64 kilometers (40 mi).
It is beyond the continental shelf that the "deep sea" begins. The shelf ends at a depth of about 200 meters
(660 ft), giving way to the steeper continental slope, which descends about 3,700 meters (12,000 ft) to
the deep ocean basin.
Here, the ocean floor deepens sharply and its features again resemble those on land, only on a much larger scale,
with great plains and mountains.
In fact, the Earth's longest mountain range lies under the sea. Over 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. It is one of the most
geologically active areas on Earth. It is where new seafloor is being born, giving rise to hydrothermal vents and
volcanoes.
Volcanic arcs and oceanic trenches partly encircling the Pacific Basin form the so-called Ring of Fire,
a zone of frequent earthquakes and volcanic eruptions. The trenches are shown in blue-green. The volcanic island
arcs, although not labelled, are parallel to, and always landward of, the trenches. For example, the island arc
associated with the Aleutian Trench is represented by the long chain of volcanoes that make up the Aleutian Islands.
Map and info. courtesy of U.S. Geological Survey.
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