Page 212 - NS-2 Textbook

OCEANOGRAPHY 207

continent shore line average ocean depth: 200 fJ average ocean depth: 2% mi

r--c'ont.inental shelf (average breadth: 42 miles)--+--+j

The continental shelf topography as it generally appears off the continental shores. There is some variation in width and smoothness through-
out the world. Submarine canyons often cut through the shelf.

Biologically, the continental shelves are sunlit areas Some submarine canyons are much larger than the
that support most of the sea vegetation and saltwater Grand Canyon of Arizona. The Hudson Canyon in the
fishes and animals on Earth. Even today, om knowledge western North Atlantic, for example, extends from wa-
about the ocean is mostly limited to the continental shelf ters with a depth of 300 feet at the canyon head, 90 nllies
regions. It is here that most fishing is done. Exploration southeast of New York HarbOl, to a depth of 7,000 feet
for, and production of, oil and other minerals is done al- some 150 miles offshore. The 50-mile-long canyon is
most entirely on the continental shelves. It is here that 4,000 feet deep in places and has a number of big tribu-
nations are most liable to confront each other as their taries entering it. It cuts through the continental slope
growing populations increase their demands for fuels, and joins a low spot in the continental shelf that marks
minerals .. and food. the entrance of the Hudson River channel off New York
Beyond the continental shelf, no matter how far from Harbor. The Hudson Canyon is continuously scoured by
the land, the bottom drops off suddenly. This is where currents containing large amounts of silt coming out of
the continental crust of granitic rocks ends and the bot- the Hudson River. The silt is eventually deposited on an
tom drops off to the sediments on the ocean floor, which enormous plain of mud called a submarine fan. Similar
has a base of basaltic rock. The sharp descent is called the fans extend hundreds of miles out to sea from the
cOlltillental slope. Here is where the deep sea truly begins. mouths of other great rivers of the world, notably the
Oceanographers and geologists have found that the con- Mississippi, Indus, and Ganges.
tinental slopes generally drop from 100 to 500 feet per The ocean floor lies at the foot of the continental
mile, but with increasing depth they tend to flatten out slope and is the true bottom of the ocean. T1,e deep ocean
and merge into the deep ocean floor. floor extends seaward from the continental slope and
Humans find this area a bleak and uncornlortable takes up one-third of the Atlantic and Indian Oceans and
world. There is no light and no plant life. The pressure, three-quarters of the Pacific Ocean. They are the last
cold, and silence increase as one descends. The bottom large areas to be explored, truly the "last frontier" on
sediments are mainly mud and clay; with small amolmts Earth.
of sand and gravel. There may be rocks in areas with ac- Oceanographers have determined that most of the
tive volcanoes. In some areas the steepness of the slope is Pacific deep ocean basin consists of hills forming a rough
dramatic} as along the ·western coast of South America, topography, while plains are widespread in the Atlantic.
where there is an 8-mile descent from the top of the All these plains are connected by canyons or other chan-
Andes Mountains to the bottom of the Peru-Chile Trench nels to sources of sediments on land. These sediments are
in a horizontal distance of less than 100 miles. transported by turbidity currents down the slope to be
The continental slopes have some of the most rugged deposited on the plains.
features on Earth. 111ey are scarred with spectacular fea- Oceall Ridges. Every deep ocean floor has impressive
hues like sublnarine canyons, steep cliffs! and winding mow1tain ranges called ridges. The great Mid-Atlantic
valleys. Some places have terraces and plateaus, while Ridge soars more than 6,000 feet above the nearby sea
others have sheer drop-offs of several thousand feet. floor in some places, and rises above the surface to form
Submarille callyons in the continental slope are similar islands such as the Azores and Iceland. It extends from
to canyons fOlmd in the southwestern Unlted States. north of Iceland to below the tip of South Africa. It con-
111ey are often carved out of the shelf and slope by past tinues around Africa and joins the Mid-Indian Ocean
glaciation, tidal currents, other lmderwater currents, and Ridge coming down from the Arabian Penlnsula. The
landslides. Rapidly moving underwater currents carry- Mid-Indian Ridge continues eastward south of Australia
ing debris and sediments are called turbidity currents. and New Zealand, joining the East Pacific Rise.
They scour the canyon \vaBs much like river or wind ero- The East Pacific Rise is the main underwater feature
sion does on continental surfaces. in the southern and southeastern Pacific Ocean. Located