Download Title: Physiography of the Ocean Basins

Title: Physiography of the Ocean Basins
Origin of the Ocean Basins
The ocean basins are the result of tectonic forces and processes. All of the ocean
basins were formed from volcanic rock that was released from fissures located at the midoceanic ridges. The oldest rocks found in these basins are approximately 200 million years
old. This is a lot younger than the oldest continental rocks which have ages greater than 4
billion years. The reason for this discrepancy is simple. Tectonic processes destroy old
oceanic rocks! Oceanic rock is returned to the Earth's mantle when oceanic crust is
subducted. Many of these subduction zones occur at the continental margins where oceanic
crust meets continental crust. Subduction also creates the ocean's deep trenches.
Topography of the Ocean Basins
The ocean basins are not featureless Earth surfaces. Much of our knowledge about the
topographic features that exist here are derived from the following technologies: seismic
surveying; echo sounder; side-scan sonar; and the measurement of the height of sea surfaces
by satellites. Most of the general information concerning the depth of the ocean basins were
made after World War I when the echo sounder was developed for military purposes. This
instrument accurately determines the time between the emission of a strong acoustic pulse
and the detection of its echo. Using this principle scientists can determine the distance from
the sounder to the ocean bottom.
The image displays the topography of the Earth's terrestrial land surface and ocean
basins. Data for the image comes from satellite and ship depth soundings, and U.S.
Geological Survey digital elevation maps (DEM) of the Earth's land surface. In the ocean
basin, the gradation from red to yellow to green to blue indicates increasing depth. A number
of topographic features associated with the ocean basin can be seen in this image. The red
area that borders the various landmasses is the continental shelf. This feature is structurally
part of the continental landmasses despite the fact that it is under water. The yellow to green
zone around the continental shelf is the continental slope and continental rise. The blue
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins
region in the various ocean basins constitutes the ocean floor. In the center of ocean basins,
the mid-oceanic ridges can be seen with a colour ranging from green to yellow to orange.
Some of the dominant topographic features associated with the ocean basins include:
a. Continental shelf
Continental shelf is a shallow (average depth 130 meters) gently sloping part of the
continental crust that borders the continents. The extent of this feature varies from tens of
meters to a maximum width of about 1300 kilometers. The continental shelf is the extended
perimeter of each continent and associated coastal plain. The shelf surrounding an island is
known as an insular shelf (interested only in your own country or group and not willing to accept
different or foreign ideas). Much of the shelf was exposed during glacial periods, but it is now
submerged under relatively shallow seas (known as shelf seas) and gulfs and was similarly
submerged during other interglacial periods.
The shelf usually ends at a point of increasing slope (called the shelf break) (Fig. 4.6.).
The sea floor below the break is the continental slope. Below the slope is the continental rise,
which finally merges into the deep ocean floor, the abyssal plain. The continental shelf and
the slope are part of the continental margin. The shelf area is commonly subdivided into the
inner continental shelf, mid continental shelf, and outer continental shelf, each with their
specific geomorphology and marine biology.
The character of the shelf changes dramatically at the shelf break, where the continental slope
begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of
roughly 140 m (460 ft); this is likely a hallmark of past ice ages, when sea level was lower
than it is now. The continental slope is much steeper than the shelf; the average angle is 3°,
but it can be as low as 1° or as high as 10°. The slope is often cut with submarine canyons.
The physical mechanisms involved in forming these canyons were not well understood until
the 1960s.[
Economic significance
The relatively accessible continental shelf is the best understood part of the ocean
floor. Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, and
hydrocarbon extraction, takes place on the continental shelf. Sovereign rights over their
continental shelves up to a depth of 200 metres or to a distance where the depth of waters
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins
admitted of resource exploitation were claimed by the marine nations that signed the
Convention on the Continental Shelf drawn up by the UN's International Law Commission in
1958. This was partly superseded by the 1982 United Nations Convention on the Law of the
Sea. which created the 200 nautical mile exclusive economic zone and extended continental
shelf rights for states with physical continental shelves that extend beyond that distance.
Fig. 4.6. Marginal features found at the interface of the continents and the ocean basins.
b. Continental slope:
The continental slope extends from the continental shelf at an average depth of about
135 meters. The base of this steeply sloping (from 1 to 25°, average about 4°) topographic
feature occurs at a depth of approximately 2000 meters, marking the edge of the continents.
The width of the slope varies from 20 to 100 kilometers. Both the continental shelf and slope
are considered structurally part of the continents, even though they are below the sea surface.
The boundary between the continental slope and shelf is called the continental shelf break.
The gradient of the slope is lowest off stable coasts without major rivers and highest off
coasts with young mountain ranges and narrow continental shelves. Most Pacific slopes are
steeper than Atlantic slopes. Gradients are flattest in the Indian Ocean. About one-half of all
continental slopes descend into deep-sea trenches or shallower depressions, and most of the
remainder terminate in fans of marine sediment or in continental rises. The transition from
continental crust to oceanic crust usually occurs below the continental slope.
About 8.5 percent of the ocean floor is covered by the continental slope-rise system.
This system is an expression of the edge of the continental crustal block. Beyond the shelfslope break, the continental crust thins quickly, and the rise lies partly on the continental crust
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins
and partly on the oceanic crust of the deep sea. Although the continental slope averages about
4°, it can approach vertical on carbonate margins, on faulted margins, or on leading-edge,
tectonically active margins. Steep slopes usually have either a very poorly developed
continental rise or none at all and are called escarpments.
Canyons are V-shaped canyons cut into the continental slope to a depth of up to 1200
meters. The submarine canyons are cut perpendicular to the running direction of the
continental slope. Many canyons are associated with major rivers such as the Congo, Hudson,
and others.
c. Continental rise:
The continental rise is found at the base of the continental slope and The depth of the
rise ranges from 2000 to 5000 meters deep. Its breadth is up 300 kilometers wide. This
feature was created by the merging of accumulated deposits at the mouths of the many
submarine canyons. Each canyon's thick fan-shaped sedimentary deposit is called an abyssal
fans. At the bottom of the continental slope, one will find the continental rise, an underwater
hill composed of tons of accumulated sediments. The general slope of the continental rise is
between 0.5 degrees and 1.0 degrees. Beyond the continental rise stretches the abyssal plain,
an extremely flat area of the sea floor which is also incredibly deep. The abyssal plain hosts
many unique life forms which are uniquely adapted to survival in its cold, high pressure, and
dark conditions. The flatness of the abyssal plain is interrupted by massive underwater
mountain chains near the tectonic boundaries of the Earth's plates.
d. Abyssal plain:
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths
between 3000 and 6000 m. Lying generally between the foot of a continental rise and a midocean ridge, abyssal plains cover more than 50% of the Earth’s surface. They are among the
flattest, smoothest and least explored regions on Earth. Abyssal plains are key geologic
elements of oceanic basins (the other elements being an elevated mid-ocean ridge and
flanking abyssal hills). In addition to these elements, active oceanic basins (those that are
associated with a moving plate tectonic boundary) also typically include an oceanic trench
and a subduction zone.
Abyssal plains were not recognized as distinct physiographic features of the sea floor until
the late 1940s and, until very recently, none had been studied on a systematic basis. They are
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins
poorly preserved in the sedimentary record, because they tend to be consumed by the
subduction process. The creation of the abyssal plain is the end result of spreading of the
seafloor (plate tectonics) and melting of the lower oceanic crust. Magma rises from above the
asthenosphere (a layer of the upper mantle) and as this basaltic material reaches the surface at
mid-ocean ridges it forms new oceanic crust. This is constantly pulled sideways by spreading
of the seafloor. Abyssal plains result from the blanketing of an originally uneven surface of
oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is
deposited by turbidity currents that have been channelled from the continental margins along
submarine canyons down into deeper water. The remainder of the sediment is composed
chiefly of pelagic sediments. Metallic nodules are common in some areas of the plains, with
varying concentrations of metals, including manganese, iron, nickel, cobalt, and copper.
These nodules may provide a significant resource for future mining ventures.
Owing in part to their vast size, abyssal plains are currently believed to be a major reservoir
of biodiversity. The abyss (a very deep hole which seems to have no bottom) also exerts
significant influence upon ocean carbon cycling, dissolution of calcium carbonate, and
atmospheric CO2 concentrations over timescales of 100–1000 years. The structure and
function of abyssal ecosystems are strongly influenced by the rate of flux of food to the
seafloor and the composition of the material that settles. Factors such as climate change,
fishing practices, and ocean fertilization are expected to have a substantial effect on patterns
of primary production in the euphotic zone. This will undoubtedly impact the flux of organic
material to the abyss in a similar manner and thus have a profound effect on the structure,
function and diversity of abyssal ecosystems.
The ocean floor is found at the base of the continental rise in water 4000 to 6000 meters deep.
The ocean floor accounts for nearly 30% of the Earth's surface. The composition of the ocean
floor consists of a relatively thin layer (on average 5 kilometers thick) of basaltic rock with an
average density of 3.0 grams per cubic centimeter (continents - granite rocks - density 2.7
grams per cubic centimeter).
Numerous volcanoes populate the floor of the ocean basins. Scientists estimate that there are
approximately 10,000 volcanoes on the ocean floor.
e. Mid-oceanic ridge:
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins
Mid-oceanic ridge is normally found rising above the ocean floor at the center of the ocean
basins. These features are involved in the generation of new oceanic crust from volcanic
fissures produced by mantle up-welling. Some volcanic islands are part of the mid-ocean
ridge system (Iceland). The mid-oceanic ridge constitutes 23% of the Earth's surface. In the
center of the mid-oceanic ridge is a rift valley, between 30 to 50 kilometers wide, that
dissects 1000 to 3000 meters deep into the ridge system.
f. Ocean trenches:
Ocean trenches are long, narrow, steep-sided depressions found on the ocean floor that
contain the greatest depths in the ocean (11,000 meters - western Pacific). There are 26
oceanic trenches in the world: 3 in the Atlantic Ocean, 1 in the Indian Ocean, and 22 in the
Pacific Ocean. Generally, the trenches mark the transition between continents and ocean
basins, especially in the Pacific basin. Trenches are also the tectonic areas.
The oceanic trenches are hemispheric-scale long but narrow topographic depressions
of the sea floor. They are also the deepest parts of the ocean floor. Oceanic trenches are a
distinctive morphological feature of convergent plate boundaries. Along convergent plate
boundaries, plates move together at rates that vary from a few mm to over ten cm per year. A
trench marks the position at which the flexed, subducting slab begins to descend beneath
another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about
200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km (1.9 to
2.5 mi) below the level of the surrounding oceanic floor. The greatest ocean depth to be
sounded is in the Challenger Deep of the Mariana Trench, at a depth of 10,911 m (35,797 ft)
below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km2/yr.[1]
There are about 50,000 km (31,000 mi) of convergent plate margins, mostly around the
Pacific Ocean—the reason for the reference “Pacific-type” margin—but they are also in the
eastern Indian Ocean, with relatively short convergent margin segments in the Atlantic Ocean
and in the Mediterranean Sea. Trenches are sometimes buried and lack bathymetric
expression, but the fundamental structures that these represent mean that the great name
should also be applied here. This applies to Cascadia, Makran, southern Lesser Antilles, and
Calabrian trenches. Trenches along with volcanic arcs and zones of earthquakes that dip
under the volcanic arc as deeply as 700 km (430 mi) are diagnostic of convergent plate
boundaries and their deeper manifestations, subduction zones. Trenches are related to but
distinguished from continental collision zones (like that between India and Asia to form the
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins
Himalaya), where continental crust enters the subduction zone. When buoyant continental
crust enters a trench, subduction eventually stops and the convergent plate margin becomes a
collision zone. Features analogous to trenches are associated with collisions zones; these are
sediment-filled fore-deeps referred to as peripheral foreland basins, such as that which the
Ganges River and Tigris-Euphrates rivers flow along.
g. Ocean floor:
The principal feature of the bathymetry (bottom topography) is a submarine mountain range
called the Mid-Atlantic Ridge. It extends from Iceland in the north to approximately 58°
South latitude, reaching a maximum width of about 860 nautical miles (1,590 km; 990 mi). A
great rift valley also extends along the ridge over most of its length. The depth of water at the
apex of the ridge is less than 2,700 metres (1,500 fathoms; 8,900 ft) in most places, while the
bottom of the ridge is three times as deep. Several peaks rise above the water and form
islands. The South Atlantic Ocean has an additional submarine ridge, the Walvis Ridge.
The Mid-Atlantic Ridge separates the Atlantic Ocean into two large troughs with depths from
3,700–5,500 metres (2,000–3,000 fathoms; 12,100–18,000 ft). Transverse ridges running
between the continents and the Mid-Atlantic Ridge divide the ocean floor into numerous
basins. Some of the larger basins are the Blake, Guiana, North American, Cape Verde, and
Canaries basins in the North Atlantic. The largest South Atlantic basins are the Angola, Cape,
Argentina, and Brazil basins.
The deep ocean floor is thought to be fairly flat with occasional deeps, abyssal plains,
trenches, seamounts, basins, plateaus, canyons, and some guyots. Various shelves along the
margins of the continents constitute about 11% of the bottom topography with few deep
channels cut across the continental rise.
Ocean floor trenches and seamounts:

Puerto Rico Trench, in the North Atlantic, is the deepest trench at 8,605 metres (4,705
fathoms; 28,232 ft)

Laurentian Abyss is found off the eastern coast of Canada

South Sandwich Trench reaches a depth of 8,428 metres (4,608 fathoms; 27,651 ft)

Romanche Trench is located near the equator and reaches a depth of about 7,454
metres (4,076 fathoms; 24,455 ft).
Ocean sediments are composed of:
Teacher in-charge:
Irshad Ahmad Bhat
Title: Physiography of the Ocean Basins

Terrigenous deposits with land origins, consisting of sand, mud, and rock particles
formed by erosion, weathering, and volcanic activity on land washed to sea. These
materials are found mostly on the continental shelves and are thickest near large river
mouths or off desert coasts.

Pelagic deposits, which contain the remains of organisms that sink to the ocean floor,
include red clays and Globigerina and siliceous oozes. Covering most of the ocean
floor and ranging in thickness from 60–3,300 metres (33–1,800 fathoms; 200–
10,800 ft) they are thickest in the convergence belts, notably at the Hamilton Ridge
and in upwelling zones.
Teacher in-charge:
Irshad Ahmad Bhat