Download Deep Ocean Basins

Basins The Oceans and Their Basins
The oceans cover 71 per cent of the surface of the world, and have a volume of 1,370
million cubic kilometers. The average depth is 3,795 meters, four and one-half times the
average height of dry land above sea level; 83 per cent of the sea bottom lies between
3,000 and 6,000 meters in depth. Only 1.3 per cent is deeper than 6,000 meters, and such
areas are called deeps. Deeps usually occur fairly close to continental masses, rarely in
the middle of oceanic basins. The greatest depths known occur off Japan and off the
Philippines where depths between 10,000 and 11,000 meters have been recorded.
Slopes
At the edge of most landmasses the sea floor slopes out very gently to depths of 110 to
146 meters. This gentle slope is the continental shelf. At the edge of the continental shelf
a much steeper slope, the continental slope, falls away rapidly to the great depths of the
ocean.
Ocean floor
Owing to the great difficulties of making soundings in really deep water, very few such
soundings existed until recently. On the basis of these few soundings it was thought that
the deep ocean floor was a monotonous plain. Depth can now be determined in seconds
instead of hours by automatically timing the passage of sound echoes from the ocean
bottom to a ship; and we now know that the deep ocean floor is not unlike the land we
see. Ocean basins are divided by mountain-like ridges into a great number of practically
separate basins.
Trenches
An oceanic trench is a long, narrow depression of the seabed with relatively steep sides
(see ocean and sea). Trenches are generally deeper than 6,000 m (19,685 ft) and are the
most abyssal, or deepest, regions on the Earth (see hadal zone). The deepest oceanic
trench, the Mariana Trench in the Western Pacific Ocean, has a depth of about 11,034
m (36,201 ft) in the Challenger Deep subsector, the site of the bathyscaphe Trieste's
abyssal dive in 1960.
Trenches, in a geophysical sense, are explained in terms of plate tectonics.
They form the surface expression of subduction, the process in which an
oceanic plate descends beneath a continental plate or a second oceanic plate
as a result of seafloor spreading. A subduction zone is a region of earthquake
activity, island arc formation, and volcanism (see volcano). When the plate
convergence involves a continental plate, with its relatively more buoyant
and thicker crust, the oceanic trench typically forms within about 100 km
(62 mi) of the continental coastline. This is observed, for example, along the
west coasts of Central and South America (the Middle America Trench and
the Peru-Chile Trench).
Where two oceanic plates converge with each other, a more complex
situation exists because the densities and thicknesses of the two plates are
similar. Typically, one plate will subduct beneath the second, and in the
process an island arc will develop on the upper plate within 100 km of the
trench. This island arc is a line of volcanoes created by the melting of the
subducting plate. The Aleutian Islands of Alaska are an example of a welldeveloped island arc. A zone of seafloor spreading often develops behind an
island arc, in a system called a back-arc basin. Trenches with welldeveloped back-arc basins include the Mariana Trench and the Tonga
Trench in the Western Pacific Ocean.
Deeps and Troughs
These terms may be used to describe any local depression in the ocean
floor without implying how it formed. Some deeps and troughs are parts of
oceanic trenches (such as Challenger Deep), whereas others (such as NovaCanton Trough in the western equatorial Pacific Ocean) are deep sections of
oceanic fracture zones. The term trough is also occasionally used to describe
larger ocean-floor depressions. Mariana Trough, for example, is the name of
the back-arc basin from west of the Mariana Trench and is similar in size to
California.
Deep ocean basin
The deep ocean basin, which is about 2.5 to 3.5 miles deep, covers 30 % of Earth's
surface and has features, such as abyssal plains, deep-sea trenches and seamounts.
The Abyssal Plain
The abyssal plain is the flat, deep ocean floor. It is almost featureless because a thick
layer of sediment covers the hills and valleys of the ocean floor below it.
Deep-Sea Trenches
Deep-sea trenches are the deepest parts of the ocean. The deepest one, the Marianas
Trench in the South Pacific Ocean, is more than 35,000 feet (10,668 meters), or almost
6.6 miles (10.6 kilometers) deep. A Navy-owned submarine, the Trieste, still holds the
record for diving to the bottom of the deepest part of the Marianas Trench, the Challenger
Deep, on January 23,1960.
Continental Shelf
Submerged part of the continent
Gently sloping (less than one tenth of a degree)
Up to 1500 km wide; averages 80 km wide
Water depth at seaward edge averages 130 m (about 400 feet)
Locally cut by canyons (eroded by rivers during the Ice Age low sea level stand)
Continental Slope
Boundary between continental and oceanic crust
Steeply sloping compared to shelf (averages about 5 degree slope, up to 25 degrees)
May be about 20 km wide
Continental Rise
At base of continental slope; slope angle decreases
May be hundreds of km wide
Thick accumulation of sediment transported downslope from continental shelf
At mouths of submarine canyons, deep-sea fans are present
May be carved by turbidity currents - bottom-currents carrying suspended
sediment downslope
As the currents slow, the suspended sediments begin to settle out; larger and
heavier grains settle first.
Graded beds called turbidities are deposited by turbidity currents.
Abyssal Plain Part of the deep ocean basin (see below)
Deep Ocean Basins
Contain abyssal plains, deep sea trenches, and seamounts
Cover about 30% of Earth's surface
Abyssal Plain
Flat, deep ocean floor.
Depth may be 2 - 3 miles or more
Thick accumulations of sediment bury topography of oceanic crust
Types of sea floor sediments:
Terrigenous sediment
Mineral grains from weathered continental rocks
Fine-grained sediment (clay, mud)
Accumulates slowly (5000 to 50,000 years to deposit 1 cm)
Color may be black, red or brown
Biogenous sediment
Biological origin - primarily shells and skeletons of microscopic plankton



Calcareous oozes
Remains of foraminifera and coccolithophores
May form chalk
Siliceous oozes
Remains of radiolarians and diatoms
May form diatomite or chert
Phosphatic material
From bones, teeth and scales of fish
Hydrogenous sediment
(Authigenic or diagenetic minerals)
Minerals that precipitate from sea water by chemical reactions.
Example: manganese nodules
Deep Sea Trenches
The deepest part of the oceans
May exceed 10,000 m deep (30,000 feet, or nearly 5 miles)
Deepest is Mariana trench in Pacific Ocean (more than 11,000 m or 33,000 ft)
Occur at subduction zones where oceanic crust is forced downward into the mantle
Associated with earthquakes (Benioff Zones) and volcanoes
Seamounts
Undersea volcanic peaks which formed along mid-ocean ridges or over hot spots
May be eroded flat on top and called guyots
Subsidence occurs after volcanic activity ceases; crust moves away from ridge or off hot
spot.
May be ringed by coral reefs called atolls (circular reef surounding lagoon over nowsubmerged volcanic peak)
Bibliography:
Charnock, Henry, and Edmond, J. M., eds., The Deep Sea Bed (1991)
Heezen, Bruce C., The Ocean Floor (1982).
Pickering, Kevin T., et al., Deep Marine Environments (1989).
Seibold, Eugen, and Berger, Wolfgang H., The Sea Floor, 3d ed. (1996).
Talwani, Manik, and Pitman, Walter C., III, eds., Island Arcs, Deep-Sea Trenches
and Back-Arc Basins (1977).
FEATURES OF THE OCEAN FLOOR AND THE PROCESSES THAT
HAVE SHAPED IT
Once scientists were able to map the ocean floor, they discovered something surprising.
The bottom of the ocean was not a flat, sandy plain stretching between the continents, as
many people once thought. In fact, the ocean floor was rocky and dramatically uneven.
Many previously unknown features of the seafloor were discovered.
Extending out from a continent's edge is a gently sloping, shallow area called the
continental shelf (F). At the edge of the shelf, the ocean floor drops off in a steep
incline called the continental slope (A). The continental slope marks the true edge of the
continent, where the rock that makes up the continent stops and the rock of the ocean
floor begins. Beyond this slope is the abyssal plain (C), a smooth and nearly flat area of
the ocean floor. In some places, deep, steep-sided canyons called trenches (G) cut into
the abyssal plain. A continuous range of mountains called the mid-ocean ridge (D)
winds around Earth. There are mountains on the abyssal plain, too. Some reach above
the ocean surface to form volcanic islands (E). Others, called seamounts (B), are
completely under water.
To understand how ocean-floor features are formed, it is important to understand the
Earth's structure. The Earth consists of layers around its center, or core. The outer layer,
or crust, is thin and rocky. The thick layer between the crust and the core is the mantle.
It contains hot, liquid rock called magma. Magma flows out of the mantle and onto the
surface through cracks in the crust. Magma on the surface of the Earth is called lava,
which hardens to form new crust.
Earth's crust is broken into large plates that float on the mantle. As these plates slowly
move, they create different landforms. The mountain ranges of the mid-ocean ridge,
trenches, and underwater volcanoes are all formed by interactions of Earth's plates. At
the mid-ocean ridge, plates are diverging or moving apart. Magma squeezes up though
cracks between the plates and hardens to form new rock. Newer eruptions push the older
rock away from the ridge in a process called sea-floor spreading. Over millions of years,
sea-floor spreading created the ocean floor.
Even though new ocean floor is constantly being created at the mid-ocean ridge, Earth
always stays the same size. That is because while divergence is occurring at the midocean ridge, convergence, where two plates come together, is occurring at a different
location on the plate. When two plates converge, one plate sinks under the other plate
and old rock sinks into a trench and back into the Earth's mantle.
EXPLORING THE OCEAN
People have explored the oceans for thousands of years. Until recently, however, the
ocean floor had not been studied and much was unknown about the deep oceans. Why is
this? Studying the ocean floor is difficult; conditions are very harsh in the deep ocean.
First, it is dark since sunlight does not penetrate far below the surface. Second, the water
is cold, only a few degrees above freezing. Finally, there is tremendous pressure due to
the mass of the water pushing down from above. Because of the darkness, cold, and
extreme pressure, scientists have had to develop technology to enable them to study the
deep ocean floor. That technology includes sonar, scuba, submersibles, satellites, remote
underwater manipulators, and gravity mapping. The development of sonar was a major
break through for mapping the ocean floor. "Sonar, which stands for sound navigation
and ranging, is a system that uses sound waves to calculate the distance to an object. The
sonar equipment on a ship sends out pulses of sound that bounce off the ocean floor. The
equipment then measures how quickly the sound waves return to the ship. Sound waves
return quickly if the ocean floor is close. Sound waves take longer to return if the ocean
floor is farther away" (Simons & Wellnitz, 2000, p. 149). Using sonar, scientists were
able to develop detailed maps of the ocean floor.
Ocean Trench
Profile of an ocean floor. The ocean trench is the deepest part of the ocean and the
abyssal plains constitute most of the ocean bed.
(Image © Research Machines plc)
Submarine valley. Ocean trenches are characterized by the presence of a volcanic arc on
the concave side of the trench. Trenches are now known to be related to subduction
zones, places where a plate of oceanic lithosphere dives beneath another plate of either
oceanic or continental lithosphere. Ocean trenches are found around the edge of the
Pacific Ocean and the northeastern Indian Ocean; minor ones occur in the Caribbean and
near the Falkland Islands.
Ocean trenches represent the deepest parts of the ocean floor, the deepest being the
Mariana Trench which has a depth of 11,034 m/36,201 ft. At depths of below 6 km/3.6
mi there is no light and very high pressure; ocean trenches are inhabited by crustaceans,
coelenterates (for example, sea anemones), polychaetes (a type of worm), molluscs, and
echinoderms.
© Research Machines plc 2007. All rights reserved. Helicon Publishing is a division of
Research Machines plc.