Page 211 - NS-2 Textbook

Undersea Landscapes

For many centuries people believed that the sea floor haps the top of an tmdersea mountain. Echoes that take
,vas simply a deep, slnoath basin with a bottom covered longer indicate deeper water, such as a deep mid-ocean
with oozy mud. In fact, until the twentieth cenhrry, most trench. On average, sound travels 4,800 feet per second
knowledge of the ocean floor came from the ancient in water. If an echo takes two seconds to return, then the
method of heaving a lead-weighted line overboard in sotmd has traveled two times 4,800 feet, or 9,600 feel.
shallow water and looking at the mud, weeds, and sedi- Since it is a round trip, half that distance would be the
ments that clung to the weights when retrieved. People depth of the water-in this case, 4,800 feel.
thought that this ooze covered the bottom and "swal-
Imved up" everything.-----.even sunken ships and lost civi- THE OCEAN FLOOR
lizations. It was not until echo sounders and hy-
drophones were invented by a U.S. Navy scientist to Echo sotmdings have determined that the ocean floor is
search for submarines during World War I that oceanog- divided into three distinct areas: the continental shelf; the
raphers really began to understand that the ocean bot- deep ocean basin, or abyss; and lying between them, the
tom has just as varied a geography as the land surface. continental slope.
From that time onward, an intense effort to chart the sea The contillelltal shelf borders on contioental land
floor has taken place. areas. AChtally, the margins of the continents are under
water. The sea, it can be said, spills over the brims of the
ocean basins, covering the continental shelves with rela-
RELIEF OF THE EARTH
tively shallow water. Most maritime nations of the world
The relief of the Earth refers to the different elevations have agreed that, in a legal sense, the continental shelf is
and form of its surface, called its topography. A relief a part of the land out to a depth of 200 meters (about 656
map, for instance, shows the different heights of a part feet). In that shelf area the rights of exploration and use
of the Earth's surface by use of shading, colors, or num- of resources belong to the adjacent continental nation ac-
bered cOlltollr lilies (lines along which the elevation is cording to internationalla\v.
constant). The continental shelf is a gradually sloping sea bot-
There are two Inain levels in the relief: the conti- tom surrounding all continents on Earth. The shelf
nents, or continental terraces, including theiT subluerged generally drops about 7 to 10 feet every rnile until
zones, called the colltinental shelves, and the deep ocean approaching the 75-to-l00-fathom curve (450--{i00 feet),
floor. The deep oceall floor is also called the deep sea, the and then the slope becomes very steep downward to-
deep oceall basin, or the abyss. The deep sea floor is de- ward the abyss. The average width of the contioental
scribed in terms of the individual features comprising it, shelves is about 42 miles. Off parts of North Carolina the
such as abyssal plai11s, oceanic ridges, sea floor fractures} shelf extends out to about 75 miles. In the Barents Sea
deep-sea trenches, islands, and seamolmts. It has an av- off the Arctic coast of Russia it extends 800 rniles, and off
erage depth of about 12,000 feet (about 2 to 2" miles), but the coast of California it is less than a mile in width. Off
there are regions over 7 miles deep. Though 71 percent of parts of Peru and Japan, the plunge begins almost imme-
Earth's crust is covered by vvatel~ just hvo-thirds of that diately.
is truly deep oceanic basin. The shelves are not always smooth, gradual slopes.
Echo sOllllders (sometimes called fathometers) pro- They vary from smooth plains to irregulat; rough terrain.
vide a rapid means of finding the depth of water over Many sedirnents, such as rocks, sandI mud l silt, clay, and
which a vessel is traveling. They measure the time it gravel, cover the shelves. The most common material is
takes sound pulses to travel from the vessel on the sur- coarse sand, consisting mainly of particles carried away
face to the ocean floor and return as echoes. Echoes that from the continental landmass atld deposited by rivers,
botmce back quickly indicate a shallow bottom or per- currents, ice, and ""vind during the ice age.

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