Topography of ocean floors

Seas and Oceans are blue beauties of the planet earth.
Oceans are vast body of saline water occupying the great depressions on the earth. The surface beneath the oceanic waters is characterized by a lot of relief features.
The structure, configuration and relief features of the oceans also vary from each other.On the basis of Bathymetry and other studies, the morphology of Ocean basins contains a lot of relief features. This module highlights many of those features.

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TOPOGRAPHY OF OCEAN FLOORS

by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth
Science
University of Mysore, India

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Introduction:

Seas and Oceans are blue beauties of the planet
earth.
Oceans are vast body of saline water
occupying the great depressions on the earth.
They belong to the hydrosphere and cover
almost 97% of it.
Oceans cover 361 million Sq.km of the earth’s
surface.

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The volume of water is 1.37 billion cubic km.
They are extensive, exhaustive and exploitable
marine ecosystems.
Very huge masses of water with varied
dimension exist as oceans.
On-shore and off-shore marine ecosystems are
characterized by very distinct features with
unique fauna and flora.
They hold many marine habitat and contain a
lot of natural resources.

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When we move towards the seashore, we may
see the beaches with splashing tides and
dancing waves.
There is a bluish water body seen as a straight
flat plane denoting the sea-level.
Waves coming towards the land break while
nearing towards the coast and subsides while
touching the beach.
The coastline is the boundary between land
and ocean.

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The surface beneath the oceanic waters is
characterized by a lot of relief features.
The structure, configuration and relief features
of the oceans also vary from each other.
All major oceans and seas vary in their sizes,
shape, depth, areal extension and in all other
aspects like temperature, density, salinity and
chemical composition.
The marine geological, chemical and biological
information and data are very vast and useful
for studying and analyzing the oceans.

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On the basis of Bathymetry and other studies,
the morphology of Ocean basins include:
1) Continental margins encompassing
Continental shelf & Continental slope,
2) Deep ocean basins encompassing the
Abyssal plains ,
abyssal hills, ocean trenches,
Mid-oceanic ridges and deep ocean floor.

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In this episode, the following aspects are
going to be seen:
 Historical ocean explorations
 Continental Shelf
 Continental Slope & Submarine canyons
 Features of Deep Ocean basins
 Mid Oceanic Ridges

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1. Historical ocean explorations

Ever since people started sailing the oceans,
attempts have been made to map them.
Ptolemy’s oldest map is an example.
Modern knowledge of the ocean began with
voyages of discovery of Christopher
Columbus (1492-1494), Vasco da Gama
(1497-1499), Ferdinand Magellan (1519-
1522), and many others.

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They laid the foundation for global trade routes
stretching from Spain to the Philippines in the
early 16th century.
The routes were based on a good working
knowledge of trade-winds, the westerlies, and
western boundary currents in the Atlantic and
Pacific.
The early European explorers were soon
followed by scientific voyages of discovery led
by (among many others),James Cook (1728-
1779), Charles Darwin (1809-1882).

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Sir James Clark Ross and Sir John Ross. All of
them surveyed the Arctic and Antarctic
regions from the Victory, the Isabella, and
the Erebus.
Edward Forbes (1815-1854) studied the
vertical distribution of life in the oceans.

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Others collected oceanic observations and
produced useful charts, including Edmond
Halley who charted the trade-winds and
monsoons and Benjamin Franklin who charted
the Gulf Stream.

Slow ships of the 19th and 20th centuries gave
way to satellites, drifters, and autonomous
instruments toward the end of the 20th century.

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Topography refers to the elevation of a land
with reference to the sea level.
Bathymetry refers to the depth of water column
below the sea level.

Bathymetry refers to the depth zones of Ocean
Basins depicted as charts and maps prepared
using depth sounding techniques.

The words Bathos means depth, Metry means
Measurement.

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The measurement of ocean depths in the
Mediterranean sea was started as back as 85
B.C.
In 1872, the HMS Challenger expedition was
the first to use fully the methods of
Bathymetry.

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The Challenger was the first vessel used to
systematically record information about all the
oceans except the Arctic, including their
depths, circulations, temperatures, and organic
life.
In 1925, the Meteor, one of the oldest South
Atlantic Ocean expedition – Used
echosounder, for depth measurements using
sound waves.

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In 1950s, the sophisticated Precision Depth
Recorders were invented and used.

Ocean Bathymetry was used as the basis to
conduct all expeditions and experiments in
different depth zones of oceans.

The precision depth Recorders of 1950s and
multi beam echo sounders help to efficiently
and accurately map the ocean relief features.

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A Hypsographic curve is a graphical
representation of the area of earth’s surface
above any given elevation (or) depth. This
may be above the sea level or below the sea
level.
This diagram shows the hypsographic curve of
the earth’s surface, showing how the surface
area of the globe is distributed.

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The vertical scale refers to the height (or) depth
in metre and the horizontal scale refers to the
percentage of total are equated to 5.2 million
sq.km.
The Morphology of Ocean Basin means the
configuration of the ocean basins in terms of
relief features.
This could be analysed by using the
bathymetric records.

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Great ocean bathymetric mapping, marine
explorations and expeditions have provided us
enormous data and information about the
world’s oceans.

The world’s ocean floor as mapped by Bruce
Heezen and Marie Tharp is presented here.
This map shows the spreading zones,
subduction zones with their trenches.

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Oceanic dimensions range from around 1500
km for the minimum width of the Atlantic to
more than 13,000 km for the north-south extent
of the Atlantic and the width of the Pacific.

Typical depths are only 3-4 km. So horizontal
dimensions of ocean basins are 1,000 times
greater than the vertical dimension.
The amount of land above sea level is very
much less than the volume of the sea.

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It is seen from the graph that the average
height of land is 840 m and the average depth
is about 3795m.

This diagram gives us an idea about the general
shape of the oceans.
Only a small part goes deeper than 6000 m and
the deepest troughs are beyond 10 km.

Satellite oceanography is a milestone in ocean
analysis.

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The launching in 1978 of Seasat, the first
oceanographic satellite, revolutionized
measurements of physical properties of the
ocean.
Within a few years, the sea-surface
temperature, wave height, variations in sea
surface contours, ice cover, chlorophyll
content, and other parameters were measured
and reported almost instantly from satellites.

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Satellites now observe the oceans, air, and
land.
Thousands of drifters observe the upper two
kilometers of the ocean.
Data from these systems, when fed into
numerical models allows the study of Earth as
a system.
2. Continental shelf
A schematic profile of the continental margin
to the deep-sea shows the following zones:

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1. The coastal plain is the first part of the
water's edge.
It includes the beaches, mainly covered by
unconsolidated sediments.
These are subjected to wave energy that
moves these sediments by converging on
headlands.

2. The inner and outer continental shelf. It is
about 70 km wide, bordering the continent –
topography.

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It generally resembles that of the coastal plain.
It has a gentle slope of about 0.1 degree
(1.9m/km).

Sediments are transported across this shelf
region by the currents.

3. The continental slope is rather steep slope
with 4 degrees (76m/km), following the
continental shelf break.

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Some slopes are cut by the submarine canyons.
These canyons are spaced closer together as
the slope increases.
Deep-sea fans are also formed where these
canyons empty out their sediments onto the
deep-sea floor.
4. The upper and lower continental rise comes
next. These are usually smooth due to sediment
cover. The slope may be of 0.5 degrees
(9.5m/km).

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The final part is the Deep-ocean Basin.

The underwater zone bordering the continents
is known as the continental shelf.
It begins from the shoreline and gently slopes
down to a depth of about 130 metres.

The Continental shelves vary in their width
from almost zero up to the 1,500-km-wide.

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The maximum is seen at the Siberian shelf in
the Arctic Ocean.

The average width of the continental shelf goes
to about 75 kilometres.

The edge of the shelf, called as the shelf-break,
occurs at a depth of 20m to 550 m , averaging
to 130 m. This break is marked by an abrupt
increase in the gradient or slope to an average
of about 4°.

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In places like the Arctic region, the
continental shelf extends upto a distance of
1,600 kilometres.
In some other areas, particularly those
bordering much of the Pacific, it measures only
1.6 kilometres or less.

The extensions of major rivers and their
valleys of varying depths cut through the
continental shelf region.

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The continental shelf region collects much of
the sediments (deposits of sand and mud) that
are carried by the rivers and deposited into the
ocean.

The continental shelves are the most exploited
oceanic regions commercially.
It is here, all the petroleum, sand and gravel
deposits, and fishery resources are found in
enormous amounts.

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It is also the location where most of the wastes
are getting dumped.

The shelves consist of vast deposits of sands,
muds, and gravels, overlying crystalline rocks
and appreciable thicknesses of consolidated
sedimentary rocks.

Deep submarine canyons are sometimes found
cutting across the continental shelf and slope
regions.

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They are often found to be extending from the
mouths of major rivers of continents.
The notable rivers having such submarine
canyons are the Congo, Amazon, Ganges, and
Hudson rivers.

3. Continental slope & submarine canyons
Continental slope marks the boundary of the
continental granitic mass and defines the
boundary between continental crust and
oceanic crust.

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Continental slopes are the longest and highest
slopes on the earth.

The slope may be straight or curved.
The width of continental shelf zone may go
upto 150 km inside the sea.
The water depth may range from 200m to
2000m.
This is a zone of steep slope.
The slope may range from 5°to more than 60°.

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In this steep topographic gradient, there may be
no marine deposits.
Many of the sediments may directly move
down into the deep ocean basins.
The continental slope zone occupy 8.5 % of the
total are of ocean basins.
Within Atlantic ocean it is about 12.4% .
Within the Pacific ocean it is about 7.0% . In
the Indian Ocean, it is about 6.5%.

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Continental slope is geologically not belonging
to the oceanic crust, but a part of the
continental margin covered by oceans.

The most outstanding features of the
continental slopes are the presence of features
like submarine canyons.
Sub marine canyons
Submarine canyons are steep sided and V-
shaped valleys with tributaries, similar to those
of river cut canyons seen on land.

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These are formed due to continental rifting.
The distributary channels are called as levees.

Submarine canyons are located in transverse
direction to continental shelves.
The fan-shaped deposits of these canyons
which are dumped at the bottom are known as
deep-sea fans.
These are distinct underwater alluvial fans.
They are transported and deposited by turbidity
currents.

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Submarine canyons resemble river canyons on
land, usually having steep, rocky walls.
They are found along most continental slopes.
Those of the Grand Bahama Canyon, which are
thought to be the deepest, cut nearly 3 mi (5
km) deep into the continental slope.
Most submarine canyons extend only about 30
mi (50 km) or less, but a few are more than 200
mi (300 km) long

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Modern submarine canyons vary considerably
in their dimensions.
The average lengths of canyons has been
estimated to be about 34 mi (55 km); although
the Bering Canyon is more than 680 mi (1100
km) long and is the world's longest submarine
canyon.

The shortest canyons are those of the Hawaiian
Islands, and average about 6 mi (10 km) in
length.

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Submarine canyons are characterized by
relatively steep gradients.
The average slope of canyon floors is 309 ft/mi
(58 m/km).
In general, shorter canyons tend to have higher
gradients.
For example, shorter canyons of the Hawaiian
group have an average gradient of 766 ft/mi
(144 m/km), whereas the Bering Canyon has a
slope of only 42 ft/mi (7.9 m/km).

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Continental Rise:
At the base of continental slope, the
topographic gradient decreases to 1° (or) less
and the ocean enters into the abyssal plains or
hills.
This portion of the sea floor is known as
continental rise.
This is a product of deposition by turbidity
current, underwater landslides and many other
processes.

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Features of deep ocean basins

The schematic profile of an ocean basin shows
the following ten features:
1. Submarine volcanoes.
2. Seamounts are isolated submarine hills,
steep sides, volcano-shaped - more than 1 km
in relief.
3. Mid-ocean rises and ridges.
4. Abyssal plains.

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5. Knolls - or abyssal hills (mostly in Pacific)
30-1,000m in relief, several nm wide, origin
unknown, very common less than 1000m
elevation, not completely smooth.
6. Trenches - mostly in the Pacific - mark the
transition between the continents and the ocean
basins.
These are steep sided - as deep as 11000m.
Some are "higher" than the Mt. Everest from
the base.

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7. Island arcs - on landward side of trenches -
areas of active mountain building - often with
high mountains on the adjacent continent.
8. Fracture zones in the oceans are generally
east-west oriented.
They extend several 1000km across the major
Oceans.
9. Guyots or flat topped seamounts.
These gets eroded seamounts during exposure
at the surface. They are more than 1 km in
relief.

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10. Atolls.

The abyssal plains lying at depths of 5000-
6000m are nearly flat.
This is mainly due to sedimentation.
They are found mostly in the Atlantic and are
absent in the eastern Indian and most of the
Pacific ocean floors.
If there are no plains, then there may be
oceanic trenches.

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Ocean trenches:

Trenches are long narrow depressions in the
ocean floor.
They reach a depth of about 7000m to 11000m
below sea level.
They occupy 1% of the surface of the earth.

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The following are the morphological features
of the deep ocean-basin:
a) the width of the Deep Sea Trenches varies
from 30km-100 km, the Relief of features
go upto > 2 km. Water depth may range from
5000-12000 m.
b) The width of Abyssal Hills may range from
100m to 100,000 m. The Relief may range
from 1m – 1000 m.

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Most of the Trenches lie parallel to mountain
ranges.

There are 26 trenches in the world ocean:
 3 in the Atlantic Ocean
 1 in the Indian Ocean
 22 in the Pacific Ocean.

There are numerous Ocean Trenches beyond
continental rise.

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The greatest depths of oceans can be seen only
in the form of ocean trenches.
These are steep sided, long narrow and hollow
portions.
They may be 300-500 km in length and 30-100
km wide with a depth of 6 km.
Initially, their sides slope at 4 to 8° and then
goes upto 16° to depths more than 10 km.
For those parts of a depression which exceed
6000 m in depth, the term Ocean deep is used.

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Ocean trenches are formed due to subduction
of the oceanic crust.
The faults which transect these ridges are
called as Transform faults.

Ocean trenches are found to be parallel to the
Volcanic Arcs in the oceans.
These may extend 3 to 4 Km below the level of
the surrounding ocean floor.

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The greatest known trench of the world is the
Mariana Trench which goes upto a depth of
11022 m, below sea level.

The length may go upto 2550 Km and the
width may go upto 70 Km.
The trench zones are characterized by complete
darkness and unimaginable pressures due to the
great depth.

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The pressure may go upto 16000 pounds per
square inch of the ocean floors.
This is almost inhospitable zone of deep sea for
any life to stay and survive in the oceans.

There is yet another feature within the ocean
basins at the interior zones.
They are the

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Mid oceanic ridges:

Oceanic ridges are the most remarkable
features.
These cover nearly 23% of the earth’s surface.
The mid-ocean ridges and rises are formed in
zones of tension where plates diverge.

These ridges aerially cross to more than 65000
km of the deep-ocean basins.

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These are the largest single topographic feature
in the world.
These are considered as the world’s largest
mountain ranges existing below marine waters.
The mountains of ocean ridges are not similar
to the mountains of continents.
These ridges are mountain-like structures
standing well above the deep ocean floors.
The mid oceanic ridges are, in total, 65,000 km
long and cover almost 23% of the earth’s
surface.

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They exist as chains, in the centre of the
oceanic basins.
These are rugged features.
They stand 1 to 3 km above the deep ocean
floors.
The most prominent feature of a ridge is its
steep-sided central valley called rift valley. A
rift valley may be 25 to 50 km wide and 1 to 2
km deep in its profile.

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The rift valley of a mid oceanic ridge is
bordered by rugged mountains with the tallest
peaks.
These peaks may rise upto 2 km above the sea
surface also as seen in some locations.
Upwelling of molten mantle rock has created
these rift zones.
The ocean ridges are composed entirely of
basalts and are not deformed by any folding.

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The ridges are of two types as
a) active spreading centre ridge and rise and
b) Inactive aseismic ridge.
The active spreading ridges are about 1000 km
wide and 1 to 2 km high.
There may also be ridge valleys.
The ridge valleys form due to the subsidence
and down-faulting of the central portions
where two plates are diverging.

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Oceanic ridges are traversed by series of
fractures with steep sides running
perpendicular to ridges (or) rises.

Island arcs:

Island arcs are curved chain of volcanic
islands.
They are generally convex towards the open
sea.
Due to this nature, they are called as Arcs.

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There are hundreds and thousands of extinct
volcanoes in the ocean floors.
These regions are seismically active zones.
Island arcs are topographically and structurally
continuous features.
These are formed when two oceanic plates
converge and collide.
The melting of the descending plate rises
upwards as an extended magma, to form the
islands.

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Most of these eruptions are unnoticed.
In some localities, especially in islands, one
can see the impacts as mud volcanoes and hot
springs.
Various features found in the ocean basins are
not permanent.
Some parts of the ocean may be closing due to
plate movements and some part may be
expanding.
This process of opening and closing of the
ocean basin is called as Wilson cycle.

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Seamounts and guyots
Seamounts and Guyots are some more unique
features of the ocean basins.

Seamounts are isolated steep sided submarine
volcanic mounts.
They may be 2-100 km wide and < 1 km high
above the sea floor.
Very rarely, they rise above the sea level as
seen in Hawaii islands.

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Guyots are flat-topped seamounts.
The flat-top is created due to wave action.
It is named after the Swiss scientist, A.H.
Guyot.

Seamounts are more than Guyots in the world.

Now-a-days, Digital Terrain Models of
Seamounts are generated from multibeam
sonar data.

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These methods are in a position to provide
three-dimensional pictures of the ocean floors
and morphology.

The vast ocean floor:

The ocean floors is a very vast zone.
It consist of rocks of geologically young age.
The oldest is of Jurassic in age.

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Hundreds of millions of tons of sediment are
emptied into the ocean every year by the world
streams.

Of the total volume of sediments, half of it lies
on the shelves, slopes and rises.
They are called as terrigenous sediments.
The rest are in the deep oceans.
These are clays and oozes. They are called as
pelagic sediments.

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The age of the oceans and the morphological
features of the sea floor have been extremely
important to the development of the theory of
plate tectonics, oceanic water distribution and
circulation, control of marine life and the
global climate.

The oceanic crust forms the ocean floor.
It consists of hard volcanic rocks called basalts.
The continents lie above the continental crust.

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Continental crust consists of granitic rocks.
Granites are lighter than basalts.
Due to this, the continental crust is assumed to
"float" on the mantle, above the oceanic crust.