CONFIGURATION OF THE OCEAN FLOOR

DISTRIBUTION OF WATER AND LAND

A casual look at the terrestrial globe shows that the area of oceans is far greater than
that of land. According to the earlier estimates of Wegner, water covers 71.7% per cent
of the globe while land occupies only 28.3 per cent of the earth’s surface. Krummel
gave the figures as, water 70.8% and land 29.2% and these figures still hold good.
Minor differences between these two estimates was caused by our limited knowledge of
the polar regions. But the differences are too small considering the dimensions of the
earth and 71 per cent and 29 per cent are usually taken as round figures for water and
land respectively. According to most accurate measurements of water and land, water
area of the earth is 361,059,000 sq km i.e. 71 per cent and land area is 148,892, 000 sq
km i.e. 29 per cent. Thus ratios of extent of land and water is about 1: 2.43.

TABLE 29.1 Areas of Oceans and Seas

Oceans Area (Square kilometers)

Pacific Ocean 155,557,000
Atlantic Ocean 76,762,000
Indian Ocean 68,556,000
Southern Ocean 20,327,000
Arctic Ocean 14,056,000
Seas
Pacific Ocean
South China Sea 2,974,600
Bering Sea 2,268,000
Sea of Okhotsk 1,528,000
East China and Yellow Sea 1,249,000
Sea of Japan 1,008,000

Geography by Rushikesh Dudhat

Gulf of California 1,62,000
Bass Strait 75,000
Atlantic Ocean
Caribbean Sea 2,766,000
Mediterranean Sea 2,516,000
Gulf of Mexico 1,543,000
Hudson Bay 1,232,000
North Sea 575,000
Black Sea 462,000
Baltic Sea 422,170
Gulf of St. Lawrence 2,38,000
Indian Ocean
Red Sea 438,000
Persian Gulf 2,39,000
Distribution of land and water is very uneven over the surface of the earth. Most of the
land area is concentrated in the northern hemisphere whereas bulk of water is found in
the southern hemisphere. In the northern hemisphere water covers only 60.7%,
whereas in the southern hemisphere it is 80/9% of the total area. It follows that out of
the total water, 43% is in the northern hemisphere and 57% is in the southern
hemisphere.

The most remarkable feature of world’s ocean basins that strikes the mind when we
look at the globe is that they are interconnected and thus, are parts of one World
Ocean.

The Hypsometric Curve and Hypsometric Graph

Also known as hypsographic curve, hypsometric curve is the representation of the

respective elevations of land and depths of sea with reference to sea level. Hypsometric
graph is a plot of the percentage of elevation and depth distribution of continents and
oceans. The primary object of hypsometric curve is to represent the topography of relief
in relative values with reference to sea level. The magnitude of elevations or

Geography by Rushikesh Dudhat

depressions represented by the hypsometric curve brings out clearly relative importance
of features found on the land surface and those found at the sea floor. It also shows the
areas occupied by different depths and heights on the surface of the earth.

In early 20th century, attempts were made to determine the average height of land and
average depth of ocean and also the proportion of land at various altitudes and the
proportion of ocean floor at various depths. Sir John Murray gave the following figures
regarding the extent and proportion of land and sea floor area at various altitudes and
depths.

The following four important general features of the ocean floor:

1. Continental shelf lying next to the land and sloping gently from the shore.
2. Continental slope immediately outside the continent shelf, and sloping more
steeply.
3. Deep-sea plain, a broad nearly level area forming the greater part of the ocean
floor.
4. Deeps are the deepest parts of the ocean, forming depressions in the floor,
relatively small in area, and with comparatively steep sides.

Determining depth and nature of the ocean floor

• Echo Sounding. Echo sounding is done with the help of a transmitter mounted
on a vassel. An automatic transmitter fitted on a ship continues to record the
depth of the sea floor on a graph. Sound waves are produced by the instrument
and these waves are reflected by the sea floor and are recorded by a listening
device called hydrophone. By calculating the total time and dividing it by two, we
get the time required by the sound waves to reach sea floor from the ship. If the
speed of sound waves in the water is known, depth of sea floor can easily be
calculated. Usually, sound waves in oceans travel at the speed of 1,460 metres
per second. Now if the time taken by sound waves in going from the ship to the
sea floor and coming back from the sea floor to the ship in 8 nseconds, then the
depth of the sea floor can be calculated by following formula:

Geography by Rushikesh Dudhat

 d=½XtXv
where, d = depth of sea floor
t = time taken by the sound waves in going to and coming back from sea
floor
v = speed of the sound waves in sea water

by putting the values in above formula, we get

d = ½ x 8 x 1460 = 5840 meters

We can know about the topography of the sea floor by calculationg the depth of the sea
floor and ascertaining the position of the ship.

• Side Scan Sonar. Based on the principle of echo-sounding, side scan sonar
method is used to survey the ocean floor which lies to either side of an
observation ship. The graphic display of the incoming signals gives a rough three
dimensional map of the portion of the ocean bottom traversed by the side
scanning transmitting equipment.
• Continuous Seismic Reflection. In addition to plotting the configuration of the
ocean floor, the continuous seismic reflection gives details of attitudes (positions)
and thickness of sediments and rocks beneath the ocean floor. This method uses
a stronger energy source and lower sound frequencies than echo sounding.
These sound waves are able to penetrate the ocean floor and are reflected by
the rock layers. The reflected waves comprise the returning signals which are
detected by a hydrophone. The reflected waves comprise the returning signals
which are detected graphically to create a seismic reflection profile which
represents the sub bottom geological structure along the course of the ship. This
technique is helpful in detecting even those rock layers which are several
kilometers deep below the ocean floor.
• Seismic Refraction. Wave pulses suffer not only reflection but also refraction
when they strike rocks of different densities. Densities, depths, thickness. Etc, of
rock layers lying on the ocean floor can be determined by recording the time
taken by the refracted waves for travelling from sound source to sound receiver.

Geography by Rushikesh Dudhat

 The high energy and low frequencies of sound pulse help in seismic refraction to
depict the rock structure deep into the earth’s mantle.
• In addition to the above mentioned four methods two more methods namely
Mechanical Photography and Diving operations are also used to study the
ocean floor.

Ocean Bottom Relief. In the beginning, much information about the ocean bottom was
not available and the ocean bottom was considered as monotonous and featureless
surface which aroused little interest among scholars of different parts of the world.
However, with the advancement of science and technology, exploration of the ocean
floor was undertaken rather vigorously with the help of more sophisticated instruments,
and the secrets of topographic variability of the ocean floor were unraveled. The
science of bathymetry, concerned with the measurement of depth of different parts of
the ocean basins, made a significant contribution in understanding the relief of the
ocean floor. The term bathymetry consists of two words bathos meaning depth and
metry meaning measurement. Depending upon various studies made by a number of
oceanographers, following five marine provinces may be recognised:

1. Continental Shelf
2. Continental Slope
3. Deep Sea Plain
4. Mid Ocean Ridge
5. Ocean Trench

To these may be added some other features such as abyssal hills and plains,
submarine canyons, island arcs and ocean banks etc.

Continental Shelf

Continental shelf is a shallow water area which surrounds the main

continental land mass. Geologically, it is a part of the continent which is
submerged under the oceanic waters. It is a shallow area near the coast
which is usually 100 fathoms deep. The co ntinental shelf slopes gently
towards the ocean at an average rate of 20 meters per kilometer with angle
of slope varying from 1º to 3º. Usually the isobaths (line joining places of

Geography by Rushikesh Dudhat

equal depth below sea level) of 100 fathoms demarcates the boundary of the
continental self. However, in certain areas, the depth of the continental shelf
may be as little as 65 fathoms or as great as 300 fathoms.
Continental shelf- A few important definitions
1. “The continental shelf is the zone around a continent, extending from the line
of permanent immersion to the depths at which there is a marked increase of
slope to grater depths.” Philip Lake (1952)

2. “Around the coast between low tide level and about the 100 fathom (180 m)
mark is a platform known as continental shelf, structurally part of the continent
itself.” -F.J. Monkhions (1971)

3. “The continental shelf is defined as a shelflike zone extending from the shore
beneath the ocean surface to a point at which a marked increase in slope angle
occurs. This point is referred to as the Shelf break and the steeper portion
beyond the shelf break is known as the continental slope.” -Thurman and
Trujillo (1999)

4. “The nearly flat plains, or terraces, at the top of the sedimentary wedge
beneath the drowned edges of the continents are continental shelves.”
P.R. Pinet (2000)

Physiographically the continental shelf is flatter than the flattest surface exposed on
land because of very gentle gradient. Although the shelf is usually considered as
smooth and flat, yet on close examination it reveals some relief. About 60% of the
shelf’s surface is covered by low hillocks with a relief of 18 m or more, 35%is
characterized by shallow basins or valleys. Recent marine beds cover about 35% of the
present shelf area.

Continental shelves are influenced to a great extent by any change in the sea
level because they possess very gentle slope. Sea floor spreading which took place
about 120 million years ago was associated with expansion of the mid ocean ridges. As
the mid ocean ridges expanded, they displaced sea water and caused rise in the sea
level. It is estimated that at that time, sea level was about 300 meters higher than the
present sea level. This led to flooding of about 35 per cent of the continents resulting in

Geography by Rushikesh Dudhat

larger spread of the continental shelves. During the last glacial epoch of the Pleistocene
about 2.5 million years ago, water derived from the sea was locked in ice sheets and
glaciers and the sea level fell nearly 100metres below the present level. The continental
shelves were almost completely exposed and the surface area of the continents was
about 18% greater than the present area. Waves eroded previously submerged land
and rivers brought sediments far out on the shelf. As the ice sheets and glaciers melted,
there was rise in the sea level. Coastal areas were flooded and sediments were
deposited. Today about 70% of the continental shelves are covered with thick deposits
of silt, sand, mud and sediment derived from the land.

Spatial variations in the width of the continental shelf are the result of the
following factors:

• There is inverse relation between the slope of the continental shelf and its width,
i.e., continental shelf with gentle slope is wider and that with steep slope is
narrow.
• Continental shelves are narrow where high mountains are very close and parallel
to the coast. For example, off the west coast of South America where the Andes
mountains are very close and parallel to the coast, the shelf is only a few
hundred metres wide. In contrast, the eastern coast of South America has very
wide shelves which measure over hundreds of kilometers. The maximum width of
560 km is off the coast of Rio de La Plata. Similarly, there is narrow shelf along
the western coast of North America where the Rockies approach the coast. On
the other shelf along the eastern and northern coasts of North America.
• Generally the continental shelves are wider in front of river mouths. Broad
shelves of north Siberia, Yellow sea and Gulf of Thailand are examples of such
shelves. But all river months of the world do not have broad shelves. For
example, continental shelf off the Mississippi river mouth is exceptionally narrow.
• The coasts which have undergone glaciations have shelves of greater depth but
such shelves are full irregularities.
• Folded and faulted coasts have narrow shelf as compared to the flat areas.

Geography by Rushikesh Dudhat

Continental shelf covers about 7.6% of the total sea area including the adjacent seas,
whereas in individual oceans it covers 13.3% in the Atlantic Ocean, 5.7% in the Pacific
Ocean and 4.2%in the Indian Ocean. In India, continental shelves occur both along the
eastern and the western coast where they are 50 km and 150 km wide respectively. The
shelves are narrow, varying from 30 to 35 km off the mouths of the Ganga, the
Mahanadi and Godavari, the Krishna and the Couvery rivers whereas they are wider off
estuaries of the Narmada, the Tapi and the Mahi rivers.

Importance of the Continental Shelves

Continental shelves comprise the shallowest part of oceans, usually less than
200 m deep where sun’s rays can easily penetrate. The sediment brought down by the
rivers is also deposited here. These natural conditions give ample opportunities to sea
animals and plants to grow and flourish properly. Almost all the food articles are
obtained form the continental shelf areas. All the major fishing grounds of the world are
found on the continental shelves. The shelves also comprises a huge store house for a
large number of minerals. Recent explorations have given evidence that continental
shelves can provide about half of the world’s oil resources. Already about 20% of the
world’s oil and natural gas is obtained from the continental shelves. These areas are
expected to play an increasing role in future as land resources are rapidly exhausted
due to increasing demand. Sand and pebbles brought on the shore by sea waves from
the continental shelves are used as building materials.

Continental shelves are important from the ecological point of view also because
they provide ideal habitats for marine life (both plants and animals). They are the only
areas in the world where coral reefs can thrive well. Coral reefs are capable of
absorbing most of the disruptive forces of storms, tidal surges and killing tsunamis.
Thus, they weaken such forces, act as frontline natural buffers and save the
inhabitants of coastal areas from the onslaught of such natural hazards and disasters. It
is worth mentioning that rich coral reefs on the continental shelves of Maldives
absorbed much energy of tsunami that took place as December 26,2004 due to high
intensity earthquake (measuring 8.9 on the Richter Scale) having its epicenter off the

Geography by Rushikesh Dudhat

 coast of Sumatra (Indonesia) and saved large number of human lives as well damage to
property. Shallow continental shelves near the coast are ideal places for the growth of
rich mangrove forests which provide natural habitats for marine animals as well as
land animals. Bengal tigers in Sundarban of West Bengal provide an ideal example.
Mangroves of Orissa and Tamil Nadu acted as protective walls against the onslaught of
tsunami’s that struck the coasts of India on 26 December, 2004. It is a sad commentary
that these mangroves are destroyed indiscriminately by greed of man.

Origin of the Continental Shelves

Continental shelves are so complex in their nature, composition, extension, depth

etc. that no single mechanism or process is able to explain their origin. Consequently
different scholars have come up with different views in this regard. Some of the views
are explained as under:

(i) Change in Sea Level. Many scholars feel that continental shelf is a continuation of
the surface of land beneath the level of the sea and there is no change of angle
where water begins. Thus several writers, especially in America, hold the view that
the continents really end at the outer margin of the shelf. At some former time the sea
reached only to the top of the continental slope and due to some season the sea had
overflowed the edges of the continents. This view is supported by the fact that many
river valleys are continued beneath the sea, across the continental shelf, and open
on the continental slope.
According to this view the continental shelves were formed either by rise in the sea
level or fall in the level of land. Change in the sea level at such a large scale can be
brought only by tectonic forces. Since the shelf is almost continuous around the
shores of the Atlanitic Ocean, the movement must have affected half the globe, and it
must have been remarkabley uniform in amount. Unless there has been an increase
in the quantity of water in the ocean it is difficult to understand how a change of level
so widespread and regular could be brought about. Some scholars have expressed
the view that vast areas of the earth were covered by ice in the Pleistocene Ice Age
about 2.5 million years ago. At the end of the ice age, the ice melted and water thus

Geography by Rushikesh Dudhat

 generated entered the sea and raised its level. But it is difficult to explain that the
amount of water resulting from the melting of ice could be sufficient to raise the sea
level by 100 fathoms.

(ii) Erosion. Some scholars have tried to explain the origin of continental shelves on the
basis of erosion. According to this stream of thought, if the relative level of land and
sea remains unchanged, the waves and currents of the sea will gradually wear away
the edge of the land, cutting a notch in the original profile. It will reach from the high
water mark to the depth where waves and currents stop erosional work. The breadth
of the platform depends upon several factors such as resistance of the waves and
currents and the length of time during which the level of land and sea remains
unchanged.
Shelves formed by the erosional work of sea are found round Iceland and Faroe
Islands. On the coast of Norway, a similar wave cut platform has been lifted above
the sea. It may be mentioned that shelves formed by erosion alone are difficult to find
and if at all formed, they are nothing more than a fringe. The erosional effect of
waves decreases rapidly downwards and is hardly felt at a depth of 30 metres. Even
the currents have little or no erosional effect at depths greater than 100 fathoms.
Moreover, with the increase in the width of the platform, the force of waves
decreases towards the shore and their ability to erode the cliff also decreases.
In the cold regions, erosional work by glaciers has played a significant role in shelf
formation. Features formed by the erosional work of glaciers are often found on such
continental shelves. Sometimes, moraine edges are formed which are eroded by
waves and currents. According to F.P. Shepard, “the most clear cut history of shelf
development is found around the glaciated areas. Glaciers have deepened the
continental shelves in these areas, Shelves are broadened also by the erosional
work of glaciers.

(iii) Deposition. Some of the continental shelves are formed by deposition. The
depositional work is done mainly by rivers which continues for thousands of years.

Geography by Rushikesh Dudhat

 The assortation of sediment from course to fine is found on the shelf. Material
brought by the river is deposited beneath the sea water, but this process is effective
near the coast only. Off the mouth of the Amazon, the sea is sometimes discoloured
by mud at a distance of about 500 km. But even the largest rivers must deposit most
of their sediment near the coast. Waves and currents may carry the material a little
farther but they cease to be effective at about 200 m beneath the surface.
Consequently the material derived from land accumulates as a submarine terrace
upon the margins of the continents. The edge of the terrace marks the limit beyond
which waves and currents cease to be effective and hence it represents the outer
limit of the shelf.

It is worth noting that limit of the shelf as mentioned above is not permanent. As the
terrace farther than before. Thus, the terrace grows outwards gradually by addition of
material at its edge.

Extension of river delta also leads to terrace formation. Rate extension has been
ascertained for most of the big deltas. Thick layer of sediment is found in the
Mississippi delta. Sea waves carry the delta sediment farther away and the size of the
terrace increases.

The main reason of wider continental shelves in the northern part of the Atlantic ocean
is partly due to deposition around the margins of great ice sheets. These ice sheets
covered larger parts of the northern part of Europe and North America in the
geological past.

Sea waves and currents redistribute the deposited material and change the shape of
the shelves. Sea waves cease to be effective at depths more than 180 m and are not
able to effect the continental shelf. This is the reason that most shelves of the world
are found at depths less than 180m.

It is rare that continental shelves are formed either purely by erosion or purely by
deposition. Shepard pleaded that both the processes (i.e., erosion and deposition)
are active simultaneously and shelf is the result of their combined effect.

Geography by Rushikesh Dudhat

(iv) Subsidence. Some of the continental shelves are supposed to be associated with
subsidence because a sinking bottom of the shelf provides space for abundant
deposition. Experiments conducted along the east coast of the U.S.A. prove that the
Appalachian rocks which disappear under the Mesozoic and Tertiary sediment at the fall
line, are affected by subsidence which lowers them to about 900 m below the sea level
at the mouth of Chesapeake Bay and to 3900 m near the outer edge of the continental
shelf. Somewhat similar conditions are found south of Nova Scotia and south of Grand
Banks of Newfoundland. Gulf of Mexico coast is also formed by ‘sedimentation
subsidence’ of the Mississippi river sediment.

(v) Faculting and Folding. Some of the continental shelves are formed by simple or step
faulting along the continental margins. Faculting causes subsidence of the marginal
land areas which are consequently submerged under the sea water. The submerged
areas become continental shelves which are consequently submerged under the sea
water. The submerged areas become continental shelves which are generally known as
tectonically formed continental shelves. Such types of continental shelves are
mostly found along Queensland coast in Australia. Along the coasts of Algeria and
Morocco, Bourcart observed the occurrence of folds in crests on the continental shelf.

(vi) Glacial Control and Marine Erosion. Continental shelves are formed as a result of
erosion of the continental margins whenever there is a fall in sea level either due to the
onset of ice ages or due to subsidence of the ocean floor. According to R.A. Daly, the
sea level fell by 38 fathoms during the Pleistocene period. Hence, the margins of the
continents, which were earlier submerged under the sea, were exposed to the
processes of glacial erosion and deposition. Some peculiar glacial features were formed
on the exposed continental margins. These margin’s were again submerged under sea
water when deglaciation occurred and there was rise in the sea level and continental
shelves were formed. This concept of origin of the continental shelves belongs to the
glacial control theory. Heavily glaciated submerged trough like features are found

Geography by Rushikesh Dudhat

 along the Norwegian shelf. The Gulf of Main shelf reveals the occurrence of drumlins at
180 m depth.

(vii) Tilling. In certain areas, there is tilling of land towards the sea which results in
submergence of continental margins and thus continental shelves are formed. The
existing continental shelves are extended by this process.

2. Continental Slope
Continental slope is an area of steep slope which extends from the continental shelf
break to the deep sea plain. Like continental shelf, continental slope is geologically not
a part of the oceanic crust but forms an integral part of the continental margin which is
covered by oceans. The extent of the continental slope usually varies from 200 to 2,000
m depth (sometimes 3000 m) though it covers very small part of the ocean floor.
Continental slopes are by far, the longest and the highest slopes on the earth.
The exact nature of the continental slope and its characteristics have been studied by
the sonic soundings and the resulting profiles. Shepard (1841) based his observations
on 500 profiles of different parts of the ocean and concluded that the average slope is
about 4.3º although near the continental break, it ranges from 1º to 10º. There are large
variations in the angle of slope from one place to another.
Normally continental slopes do not favour thick deposits of marine materials due
to steep slope but in certain cases there is a thin veneer of deposits on these slopes.
As the material excavated by the turbidity current reaches the bottom of the
continental slope, its speed is reduced and it cannot be transported further.
Consequently the material is deposited at the base of the continental slope in the form
of a fan, more or less similar to fans produced by the rivers on the land, These are
known as deep sea fans. The base of the deep sea fan is at the base of the continental
slope and its apex points towards the mouth of the submarine canyon. These fans
extend to a distance of over 500 km and are laden by material brought by the turbidity
currents. These are graded deposits in which coarse materials are known as turbidiies
because they are deposited by turbidity currents.

Geography by Rushikesh Dudhat

 Some of the largest fans in the world are found in the Indian Ocean beyond the
deltas of the Ganga and Indus. Other large fans are Amazon and Congo fans in the
South Atlantic Ocean, Mississippi fans in the Gulf of Mexico, the Laurentian fan in the
North Atlantic and a number of smaller fans off the Pacific coast of North America.
Continental slope does not comprise major feature on the ocean floor and
occupies only 8.5 per cent of the total floor area of the ocean basins. However, it varies
widely from one ocean to the other. For example it spreads over 12.4 per cent area of
the Atlantic Ocean, 7 per cent of the Pacific Ocean and 6.5 per cent of the Indian
Ocean.

Submarine Canyons

Submarine canyons comprise the most outstand relief feature of the continental slopes.
It looks like a long steep sided V shaped valley with tributaries, similar to those of river
cut canyons on land. According to Shepard, the submarine canyons are similar to the
youthful river valleys on the land but are decidedly deeper and a few of them have
dendritic pattern of tributaries of secondary canyons. These are formed by continental
rifting because such an event leads to the formation of a rift valley similar to East Africa.
The sides of the fit valleys are affected by the erosional forces and several canyons are
cut into them. The margins of the continents are submerged as they move in opposite
directions and canyons are drowned. After this, these canyons become channels for
turbidity flows which modifies them at a later stage. Some of the canyons such as those
of the Hudson and Congo rivers are on the upper part of the continental shelf. They
were formed by the rivers during the ice age when the glaciers advanced and sea level
was lowered. At that time rivers traversed the continental shelves and cut these
canyons through the shelves. After that, the temperature increased, glaciers receded,
sea level rose and valleys were drowned. However, most canyons, particularly the
canyons with deep lower parts are formed by turbidity current. A turbidity current is a
powerful current flowing at a very high speed down the continental slope. Turbidity
currents are caused by great landslides of mud and sand which slides along the
continental slope. As these currents move forward, they act as power agents of erosion
and excavate deep submarine canyons.

Geography by Rushikesh Dudhat

 The world distribution of submarine canyons is very uneven. A look at the world
distributional pattern of submarine canyons reveals that they are more numerous along
the straight coasts than along the indented and crenulated coasts.

The Atlantic Ocean is famous for some of the big canyons of the world. The
Hudson canyon off the Atlantic coast of U.S.A. in front of the mouth of Hudson is
traceable across the shelf. This canyons is nearly 150 km long and 1129 m deep.

Off the western coast of North America, there are a few canyons near Vancouver
and Aleutian Islands. Columbia canyon penetrates about 20 km into the shelf.

3. Continental Rise

Beyond the continental slope is the continental rise. It is an area of gentle slope varying
from 0.5º to 1º. Its general relief is low with increasing depth, it virtually becomes flat
and merges with the abyssal plain.

Continental rise is produced as a result of deposition by turbidity currents,

underwater landslides, and any other process that is able to carry sands, muds, silts,
etc. down the continental slope towards the deep sea plain through submarine canyons.
Continental rise represents depositional feature resulting from the coalescence of deep
sea fans, and is found to have varying morphometric characteristics. It is not a very
large feature on the oceanic floor as its relief has the amplitude of less than 40 metre
sand its width is less than 300 km. The depth of water over continental rise varies from
1.5 to 5.0 km.

4.Abyssal Plains

Where the continental rise ends, the deep sea plain known as abyssal plain or abyssal
floor begins, These are the most extensive and the flattest terrain units found on the
earth’s surface, both on the ocean floor and the continental landmasses. They are
tectonically inactive areas having a very gentle slope of less than 1: 1000 km with angle
of gradient varying from 0.5º to 1º . They are usually found at depth varying from 2000
to 6000 metres. The unique flatness of these plains is attributed to the accumulation of
fine sediment of terrigenous origin which has been transported by the submarine

Geography by Rushikesh Dudhat

turbidity currents across the submarine canyons. These deposits, about 1 km thick are
unconsolidated and layered and have buried most of the pre existing features. At a few
places there may be some volcanic deposits. Along with the terrigenous materials,
some pelagic deposits of planets, marine animals and siliceous remains may also be
found. The regional slope of these plains is from the mid oceanic ridges toward the
continental margins. The deep sea plain area comprises the most extensive feature of
the ocean floor covering about 75.9% of the total area of the ocean basins.

5. Abyssal Hills

There are thousands of small hills projecting above the abyssal plains which are
supposed to be of volcanic origin. They are known by different names depending upon
their slope and size. Some of the popular names are volcanic hills and islands, sea
mounts, sea knolls, tablemounts or guyots. Such volcanic hills are circular or elliptical,
dome shaped or elongated with extensive base. Usually they project to a height of 60-
100 m above the base and are generally 0.1 km to 100 km wide. When these hills rise
above the sea level, they appear as volcanic islands or simply islands. A brief description
of some of the abyssal hills is given as under:

Abyssal hills or seaknoll is the name given to volcanic hills of lower heights.

Seamounts are isolated steep submarine volcanic hills, usually 2-100 km wide and less
than 1,000 m high above the sea floor. The sides of the seamounts have steep slopes of
20 to 25º. Sometimes, the seamounts rise abruptly emerging above the sea level. As
such they appear as isolated islands. The Hawaii islands, the north Pacific and the
Azores in the north Atlantic are outstanding examples of such islands.

Guyots are flat topped hills, generally of the volcanic origin. Also known as tablemounts,
these small hills are formed by truncation of seamounts by wave action later or sinking
beneath the oceanic surface either by subsidence of the sea floor or by rise in sea level
due to melting of glaciers in the post glacial period. Their plateau like tops eventually
sank too deep for wave erosion to continue wearing them down. Within the tropics,
guyots are usually capped by coral reefs.

Geography by Rushikesh Dudhat

The abyssal hills may be found in groups or in isolation. When they are found in groups
they give rise to typical morphology known as abyssal hill province. They are one of the
most common features found on the ocean floor.

6. Oceanic Ridges and Rises

Oceanic ridges and rises comprise the most extensive and remarkable features of not
only the ocean basin but of the entire globe. They tectonic features are of volcanic origin
and are generally associated with sea floor spreading. .Since most of the ridges, except
those of the Pacific Ocean are located in the middle of the oceans, they are popularly
known as mid ocean ridges. The mid Atlantic Ridge and the mid Indian Ocean Ridge
are the typical examples of mid ocean ridges. In the Pacific Ocean, the ridge is not in the
middle of the ocean, rather it is located in the eastern part of the ocean.

Depending upon the tectonic activity, two types of ridges are generally
recognised they are: (i) active spreading centre ridge and rise and (ii) inactive
aseismic ridge. Active spreading ridges are about 1000 km wide and are elevated
1,000-2,000 m above the normal level of the sea floor. The swells with steeps slope are
called ridges and those with gentle slopes are known as rises. The important examples
of redges are the Mid Atlantic Ridge, the Carlsberg Ridge, and the Lomonosov Ridge
while the East Pacific Ridge is the example of a rise.

The origin of the oceanic ridges lies in tectonic forces as they are formed in
zones of tension resulting from the diverging lithospheric plates. As the plates move
away from each other, the pressure in the as thermosphere is lowered and there is
partial melting of peridotite. This results in the formation of basaltic magma which rises
up through the cracks and fissures. It forms a new crust when it comes up and spreads
over the old crust. The continuous accumulation of the lava results in the formation of
ridge. The general character of the oceanic ridge is a function of the rate of plate
divergence. The ridges are higher, more rugged and mountainous when the plates
diverge as a slow rate. In contrast the ridges are lower and less rugged where the
plates diverge at a fast rate. This is exemplified by the more rugged and higher Mid

Geography by Rushikesh Dudhat

Atlantic Ridge where divergence is slow and the mid East Pacific Ridge where the
divergence of plates is very fast.

Some of the oceanic ridges have longitudinal rift valleys along their crests which
are formed due to subsidence and downfaulting of the central section of the ridge
caused by divergence of two adjacent lithospheric plates. These rift valleys are 15-50
km wide and 500-1500 m deep.

Another important feature of oceanic ridges and rises is that they are traversed
by a series of fractures having steep sides and running in a direction perpendicular to
them. They are formed due to differential stretching of the lithospheric plates and are
known as transform faults.

Since the oceanic ridges are formed by the divergence of lithospheric plates,
they are characterized by numerous earthquakes have shallow foci and have their
epicenters within 70 km of the earth surface. Some of the volcanic peaks overgrow and
rise above the sea water level thus forming volcanic islands. Iceland, Azores, Ascension
and Tristan da Cunha are important examples of such islands.

Inactive aseismic ridges and rises are not associated with earthquakes and
volcanoes and are generally arranged in the form of linear chain of extinct volcanoes.
Further, they are indicators of plate movements in the geological past. Some of the
important examples of inactive seismic ridges and rises are the Galapagos Rise, Rio
Grande Rise and Walves Rise.

7. Ocean Deeps and Trenches

Ocean deeps and trenches and depressions which represent the deepest zones of the
ocean basins. They are grouped into two categories depending upon their size: (i) very
deep but less extensive depressions are called ocean deeps. Deeps are known as Tiefe
in German and Fosse in French. (ii) Long narrow linear depressions are known as
trenches. A trench is an arc shaped steep sided narrow and long depression. On an
average, it is 300-500 km long, 30-100 km wide and about 6 km deep. The sides of a
trench first slope at angles of about 4º-8º and then at 10º-16º to depths of more than

Geography by Rushikesh Dudhat

10,000 m. The walls of deeper trenches may be at angles varying from 25º to 45º. This
means that the sides of the trenches become steeper with depth, before flattening to a
floor underlain by thick sediment. Another important feature of the trenches is that its
walls on the landward side are steeper than those on the seaward side. This indicates
the direction of plate subduction. In fact, ocean trenches are formed by the subduction of
oceanic crust under oceanic or continental crust. This is the reason that most of the
ocean deeps and trenches are always found near the land areas, say coastlines, and
island areas which represent active plate margins. In such areas two plates, from
opposite directions, converge and collide, and heavier plate is subducted below the
lighter plate. This is the reason most of the ocean trenches have parallel island arcs or
young mountain belts on their seaward side. Most of the ocean trenches are found in the
eastern ands western parts of the Pacific Ocean and they are seldom found in the
middle ocean region. The trenches are curved, with concex side facing the open ocean,
because of the geometry of plate interactions on the spherical earth. Continental rises
are practically absent along those coasts which have trenches running almost parallel to
the coast because the sediment that would form the rise ends up at the bottom of the
trench.

Most of the ocean deeps lie parallel to the recently folded mountains and usually form
steep vertical descent of great magnitude from the top of the mountain to the bottom of
the deep.

Geography by Rushikesh Dudhat