NCERT Textbook Chapter 4 - Distribution of Oceans and Continents, Class 11, Geography UPSC Notes | EduRev

Geography (Prelims) by Valor Academy

UPSC : NCERT Textbook Chapter 4 - Distribution of Oceans and Continents, Class 11, Geography UPSC Notes | EduRev

 Page 1


In the previous chapter, you have studied the
interior of the earth. You are already familiar
with the world map. You know that continents
cover 29 per cent of the surface of the earth
and the remainder is under oceanic waters.
The positions of the continents and the ocean
bodies, as we see them in the map, have not
been the same in the past. Moreover, it is now
a well-accepted fact that oceans and
continents will not continue to enjoy their
present positions in times to come.  If this is
so, the question arises what were their positions
in the past? Why and how do they change their
positions? Even if it is true that the continents
and oceans have changed and are changing
their positions, you may wonder as to how
scientists know this. How have they determined
their earlier positions? You will find the answers
to some of these and related questions in this
chapter.
CONTINENTAL DRIFT
Observe the shape of the coastline of the Atlantic
Ocean. You will be surprised by the symmetry
of the coastlines on either side of the ocean. No
wonder, many scientists thought of this
similarity and considered the possibility of the
two Americas, Europe and Africa, to be once
joined together. From the known records of the
history of science, it was Abraham Ortelius, a
Dutch map maker, who first proposed such a
possibility as early as 1596. Antonio Pellegrini
drew a map showing the three continents together.
However, it was Alfred Wegener—a German
meteorologist who put forth a comprehensive
argument in the form of  “the continental drift
DISTRIBUTION OF OCEANS
AND CONTINENTS
CHAPTER
theory” in 1912. This was regarding the
distribution of the oceans and the continents.
According to Wegener, all the continents
formed a single continental mass and mega
ocean surrounded the same.  The super
continent was named PANGAEA, which meant
all earth. The mega-ocean was called
PANTHALASSA, meaning all water. He argued
that, around 200 million years ago, the super
continent, Pangaea, began to split. Pangaea first
broke into two large continental masses as
Laurasia and Gondwanaland forming the
northern and southern components
respectively. Subse-quently, Laurasia and
Gondwanaland continued to break into various
smaller continents that exist today. A variety of
evidence was offered in support of the
continental drift. Some of these are given below.
Evidence in Support of the Continental Drift
The Matching of Continents (Jig-Saw-Fit)
The shorelines of Africa and South America
facing each other have a remarkable and
unmistakable match. It may be noted that a
map produced using a computer programme
to find the best fit of the Atlantic margin was
presented by Bullard in 1964. It proved to be
quite perfect. The match was tried at 1,000-
fathom line instead of the present shoreline.
Rocks of Same Age Across the Oceans
The radiometric dating methods developed in
the recent period have facilitated correlating the
rock formation from different continents across
© NCERT
not to be republished
Page 2


In the previous chapter, you have studied the
interior of the earth. You are already familiar
with the world map. You know that continents
cover 29 per cent of the surface of the earth
and the remainder is under oceanic waters.
The positions of the continents and the ocean
bodies, as we see them in the map, have not
been the same in the past. Moreover, it is now
a well-accepted fact that oceans and
continents will not continue to enjoy their
present positions in times to come.  If this is
so, the question arises what were their positions
in the past? Why and how do they change their
positions? Even if it is true that the continents
and oceans have changed and are changing
their positions, you may wonder as to how
scientists know this. How have they determined
their earlier positions? You will find the answers
to some of these and related questions in this
chapter.
CONTINENTAL DRIFT
Observe the shape of the coastline of the Atlantic
Ocean. You will be surprised by the symmetry
of the coastlines on either side of the ocean. No
wonder, many scientists thought of this
similarity and considered the possibility of the
two Americas, Europe and Africa, to be once
joined together. From the known records of the
history of science, it was Abraham Ortelius, a
Dutch map maker, who first proposed such a
possibility as early as 1596. Antonio Pellegrini
drew a map showing the three continents together.
However, it was Alfred Wegener—a German
meteorologist who put forth a comprehensive
argument in the form of  “the continental drift
DISTRIBUTION OF OCEANS
AND CONTINENTS
CHAPTER
theory” in 1912. This was regarding the
distribution of the oceans and the continents.
According to Wegener, all the continents
formed a single continental mass and mega
ocean surrounded the same.  The super
continent was named PANGAEA, which meant
all earth. The mega-ocean was called
PANTHALASSA, meaning all water. He argued
that, around 200 million years ago, the super
continent, Pangaea, began to split. Pangaea first
broke into two large continental masses as
Laurasia and Gondwanaland forming the
northern and southern components
respectively. Subse-quently, Laurasia and
Gondwanaland continued to break into various
smaller continents that exist today. A variety of
evidence was offered in support of the
continental drift. Some of these are given below.
Evidence in Support of the Continental Drift
The Matching of Continents (Jig-Saw-Fit)
The shorelines of Africa and South America
facing each other have a remarkable and
unmistakable match. It may be noted that a
map produced using a computer programme
to find the best fit of the Atlantic margin was
presented by Bullard in 1964. It proved to be
quite perfect. The match was tried at 1,000-
fathom line instead of the present shoreline.
Rocks of Same Age Across the Oceans
The radiometric dating methods developed in
the recent period have facilitated correlating the
rock formation from different continents across
© NCERT
not to be republished
DISTRIBUTION OF OCEANS AND CONTINENTS 31
the vast ocean. The belt of ancient rocks of
2,000 million years from Brazil coast matches
with those from western Africa.  The earliest
marine deposits along the coastline of South
America and Africa are of the Jurassic age.
This suggests that the ocean did not exist prior
to that time.
Tillite
It is the sedimentary rock formed out of
deposits of glaciers. The Gondawana system
of sediments from India is known to have its
counter parts in six different landmasses of the
Southern Hemisphere. At the base the system
has thick tillite indicating extensive and
prolonged glaciation. Counter parts of this
succession are found in Africa, Falkland Island,
Madagascar, Antarctica and Australia besides
India.  Overall resemblance of the Gondawana
type sediments clearly demonstrates that these
landmasses had remarkably similar histories.
The glacial tillite provides unambiguous
evidence of palaeoclimates and also of drifting
of continents.
Placer Deposits
The occurrence of rich placer deposits of gold
in the Ghana coast and the absolute absence
of source rock in the region is an amazing fact.
The gold bearing veins are in Brazil and it is
obvious that the gold deposits of the Ghana
are derived from the Brazil plateau when the
two continents lay side by side.
Distribution of Fossils
When identical species of plants and animals
adapted to living on land or in fresh water are
found on either side of the marine barriers, a
problem arises regarding accounting for such
distribution.  The observations that Lemurs
occur in India, Madagascar and Africa led some
to consider a contiguous landmass “Lemuria”
linking these three landmasses.  Mesosaurus
was a small reptile adapted to shallow brackish
water.  The skeletons of these are found only
in two localities : the Southern Cape province
of South Africa and Iraver formations of Brazil.
The two localities presently are 4,800 km apart
with an ocean in between them.
Force for Drifting
Wegener suggested that the movement
responsible for the drifting of the continents
was caused by pole-fleeing force and tidal force.
The polar-fleeing force relates to the rotation
of the earth. You are aware of the fact that the
earth is not a perfect sphere; it has a bulge at
the equator. This bulge is due to the rotation
of the earth. The second force that was
suggested by Wegener—the tidal force—is due
to the attraction of the moon and the sun that
develops tides in oceanic waters. Wegener
believed that these forces would become
effective when applied over many million years.
However, most of scholars considered these
forces to be totally inadequate.
Post-Drift Studies
It is interesting to note that for continental drift,
most of the evidence was collected from the
continental areas in the form of distribution of
flora and fauna or deposits like tillite. A number
of discoveries during the post-war period
added new information to geological literature.
Particularly, the information collected from the
ocean floor mapping provided new dimensions
for the study of distribution of oceans and
continents.
Convectional Current Theory
Arthur Holmes in 1930s discussed the
possibility of convection currents operating in
the mantle portion. These currents are
generated due to radioactive elements causing
thermal differences in the mantle portion.
Holmes argued that there exists a system of
such currents in the entire mantle portion. This
was an attempt to provide an explanation to
the issue of force, on the basis of which
contemporary scientists discarded the
continental drift theory.
Mapping of the Ocean Floor
Detailed research of the ocean configuration
revealed that the ocean floor is not just a vast
plain but it is full of relief. Expeditions to map
the oceanic floor in the post-war period
provided a detailed picture of the ocean relief
and indicated the existence of submerged
© NCERT
not to be republished
Page 3


In the previous chapter, you have studied the
interior of the earth. You are already familiar
with the world map. You know that continents
cover 29 per cent of the surface of the earth
and the remainder is under oceanic waters.
The positions of the continents and the ocean
bodies, as we see them in the map, have not
been the same in the past. Moreover, it is now
a well-accepted fact that oceans and
continents will not continue to enjoy their
present positions in times to come.  If this is
so, the question arises what were their positions
in the past? Why and how do they change their
positions? Even if it is true that the continents
and oceans have changed and are changing
their positions, you may wonder as to how
scientists know this. How have they determined
their earlier positions? You will find the answers
to some of these and related questions in this
chapter.
CONTINENTAL DRIFT
Observe the shape of the coastline of the Atlantic
Ocean. You will be surprised by the symmetry
of the coastlines on either side of the ocean. No
wonder, many scientists thought of this
similarity and considered the possibility of the
two Americas, Europe and Africa, to be once
joined together. From the known records of the
history of science, it was Abraham Ortelius, a
Dutch map maker, who first proposed such a
possibility as early as 1596. Antonio Pellegrini
drew a map showing the three continents together.
However, it was Alfred Wegener—a German
meteorologist who put forth a comprehensive
argument in the form of  “the continental drift
DISTRIBUTION OF OCEANS
AND CONTINENTS
CHAPTER
theory” in 1912. This was regarding the
distribution of the oceans and the continents.
According to Wegener, all the continents
formed a single continental mass and mega
ocean surrounded the same.  The super
continent was named PANGAEA, which meant
all earth. The mega-ocean was called
PANTHALASSA, meaning all water. He argued
that, around 200 million years ago, the super
continent, Pangaea, began to split. Pangaea first
broke into two large continental masses as
Laurasia and Gondwanaland forming the
northern and southern components
respectively. Subse-quently, Laurasia and
Gondwanaland continued to break into various
smaller continents that exist today. A variety of
evidence was offered in support of the
continental drift. Some of these are given below.
Evidence in Support of the Continental Drift
The Matching of Continents (Jig-Saw-Fit)
The shorelines of Africa and South America
facing each other have a remarkable and
unmistakable match. It may be noted that a
map produced using a computer programme
to find the best fit of the Atlantic margin was
presented by Bullard in 1964. It proved to be
quite perfect. The match was tried at 1,000-
fathom line instead of the present shoreline.
Rocks of Same Age Across the Oceans
The radiometric dating methods developed in
the recent period have facilitated correlating the
rock formation from different continents across
© NCERT
not to be republished
DISTRIBUTION OF OCEANS AND CONTINENTS 31
the vast ocean. The belt of ancient rocks of
2,000 million years from Brazil coast matches
with those from western Africa.  The earliest
marine deposits along the coastline of South
America and Africa are of the Jurassic age.
This suggests that the ocean did not exist prior
to that time.
Tillite
It is the sedimentary rock formed out of
deposits of glaciers. The Gondawana system
of sediments from India is known to have its
counter parts in six different landmasses of the
Southern Hemisphere. At the base the system
has thick tillite indicating extensive and
prolonged glaciation. Counter parts of this
succession are found in Africa, Falkland Island,
Madagascar, Antarctica and Australia besides
India.  Overall resemblance of the Gondawana
type sediments clearly demonstrates that these
landmasses had remarkably similar histories.
The glacial tillite provides unambiguous
evidence of palaeoclimates and also of drifting
of continents.
Placer Deposits
The occurrence of rich placer deposits of gold
in the Ghana coast and the absolute absence
of source rock in the region is an amazing fact.
The gold bearing veins are in Brazil and it is
obvious that the gold deposits of the Ghana
are derived from the Brazil plateau when the
two continents lay side by side.
Distribution of Fossils
When identical species of plants and animals
adapted to living on land or in fresh water are
found on either side of the marine barriers, a
problem arises regarding accounting for such
distribution.  The observations that Lemurs
occur in India, Madagascar and Africa led some
to consider a contiguous landmass “Lemuria”
linking these three landmasses.  Mesosaurus
was a small reptile adapted to shallow brackish
water.  The skeletons of these are found only
in two localities : the Southern Cape province
of South Africa and Iraver formations of Brazil.
The two localities presently are 4,800 km apart
with an ocean in between them.
Force for Drifting
Wegener suggested that the movement
responsible for the drifting of the continents
was caused by pole-fleeing force and tidal force.
The polar-fleeing force relates to the rotation
of the earth. You are aware of the fact that the
earth is not a perfect sphere; it has a bulge at
the equator. This bulge is due to the rotation
of the earth. The second force that was
suggested by Wegener—the tidal force—is due
to the attraction of the moon and the sun that
develops tides in oceanic waters. Wegener
believed that these forces would become
effective when applied over many million years.
However, most of scholars considered these
forces to be totally inadequate.
Post-Drift Studies
It is interesting to note that for continental drift,
most of the evidence was collected from the
continental areas in the form of distribution of
flora and fauna or deposits like tillite. A number
of discoveries during the post-war period
added new information to geological literature.
Particularly, the information collected from the
ocean floor mapping provided new dimensions
for the study of distribution of oceans and
continents.
Convectional Current Theory
Arthur Holmes in 1930s discussed the
possibility of convection currents operating in
the mantle portion. These currents are
generated due to radioactive elements causing
thermal differences in the mantle portion.
Holmes argued that there exists a system of
such currents in the entire mantle portion. This
was an attempt to provide an explanation to
the issue of force, on the basis of which
contemporary scientists discarded the
continental drift theory.
Mapping of the Ocean Floor
Detailed research of the ocean configuration
revealed that the ocean floor is not just a vast
plain but it is full of relief. Expeditions to map
the oceanic floor in the post-war period
provided a detailed picture of the ocean relief
and indicated the existence of submerged
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 32
mountain ranges as well as deep trenches,
mostly located closer to the continent margins.
The mid-oceanic ridges were found to be most
active in terms of volcanic eruptions. The dating
of the rocks from the oceanic crust revealed
the fact that they are much younger than the
continental areas. Rocks on either side of the
crest of oceanic ridges and having equi-distant
locations from the crest were found to have
remarkable similarities both in terms of their
constituents and their age.
Ocean Floor Configuration
In this section we shall note a few things related
to the ocean floor configuration that help us in
the understanding of the distribution of
continents and oceans. You will be studying
the details of ocean floor relief in Chapter
13. The ocean floor may be segmented into
three major divisions based on the depth
as well as the forms of relief. These divisions
are continental margins, deep-sea basins and
mid-ocean ridges.
Abyssal Plains
These are extensive plains that lie between the
continental margins and mid-oceanic ridges.
The abyssal plains are the areas where the
continental sediments that move beyond the
margins get deposited.
Mid-Oceanic Ridges
This forms an interconnected chain of
mountain system within the ocean. It is the
longest mountain-chain on the surface of the
earth though submerged under the oceanic
waters. It is characterised by a central rift
system at the crest, a fractionated plateau and
flank zone all along its length. The rift system
at the crest is the zone of intense volcanic
activity. In the previous chapter, you have been
introduced to this type of volcanoes as mid-
oceanic volcanoes.
Distribution of Earthquakes and Volcanoes
Study the maps showing the distribution of
seismic activity and volcanoes given in Figure
4.2. You will notice a line of dots in the central
parts of the Atlantic Ocean almost parallel to
the coastlines. It further extends into the Indian
Ocean. It bifurcates a little south of the Indian
subcontinent with one branch moving into
East Africa and the other meeting a similar line
from Myanmar to New Guiana. You will notice
that this line of dots coincides with the mid-
oceanic ridges. The shaded belt showing
another area of concentration coincides with
the Alpine-Himalayan system and the rim of
the Pacific Ocean. In general, the foci of the
earthquake in the areas of mid-oceanic ridges
are at shallow depths whereas along the
Alpine-Himalayan belt as well as the rim of the
Pacific, the earthquakes are deep-seated ones.
The map of volcanoes also shows a similar
pattern. The rim of the Pacific is also called rim
of fire due to the existence of active volcanoes in
this area.
CONCEPT OF SEA FLOOR SPREADING
As mentioned above, the post-drift studies
provided considerable information that was not
Figure 4.1 : Ocean Floor
Continental Margins
These form the transition between continental
shores and deep-sea basins. They include
continental shelf, continental slope, continental
rise and deep-oceanic trenches.  Of these, the
deep-oceanic trenches are the areas which are
of considerable interest in so far as the
distribution of oceans and continents is
concerned.
© NCERT
not to be republished
Page 4


In the previous chapter, you have studied the
interior of the earth. You are already familiar
with the world map. You know that continents
cover 29 per cent of the surface of the earth
and the remainder is under oceanic waters.
The positions of the continents and the ocean
bodies, as we see them in the map, have not
been the same in the past. Moreover, it is now
a well-accepted fact that oceans and
continents will not continue to enjoy their
present positions in times to come.  If this is
so, the question arises what were their positions
in the past? Why and how do they change their
positions? Even if it is true that the continents
and oceans have changed and are changing
their positions, you may wonder as to how
scientists know this. How have they determined
their earlier positions? You will find the answers
to some of these and related questions in this
chapter.
CONTINENTAL DRIFT
Observe the shape of the coastline of the Atlantic
Ocean. You will be surprised by the symmetry
of the coastlines on either side of the ocean. No
wonder, many scientists thought of this
similarity and considered the possibility of the
two Americas, Europe and Africa, to be once
joined together. From the known records of the
history of science, it was Abraham Ortelius, a
Dutch map maker, who first proposed such a
possibility as early as 1596. Antonio Pellegrini
drew a map showing the three continents together.
However, it was Alfred Wegener—a German
meteorologist who put forth a comprehensive
argument in the form of  “the continental drift
DISTRIBUTION OF OCEANS
AND CONTINENTS
CHAPTER
theory” in 1912. This was regarding the
distribution of the oceans and the continents.
According to Wegener, all the continents
formed a single continental mass and mega
ocean surrounded the same.  The super
continent was named PANGAEA, which meant
all earth. The mega-ocean was called
PANTHALASSA, meaning all water. He argued
that, around 200 million years ago, the super
continent, Pangaea, began to split. Pangaea first
broke into two large continental masses as
Laurasia and Gondwanaland forming the
northern and southern components
respectively. Subse-quently, Laurasia and
Gondwanaland continued to break into various
smaller continents that exist today. A variety of
evidence was offered in support of the
continental drift. Some of these are given below.
Evidence in Support of the Continental Drift
The Matching of Continents (Jig-Saw-Fit)
The shorelines of Africa and South America
facing each other have a remarkable and
unmistakable match. It may be noted that a
map produced using a computer programme
to find the best fit of the Atlantic margin was
presented by Bullard in 1964. It proved to be
quite perfect. The match was tried at 1,000-
fathom line instead of the present shoreline.
Rocks of Same Age Across the Oceans
The radiometric dating methods developed in
the recent period have facilitated correlating the
rock formation from different continents across
© NCERT
not to be republished
DISTRIBUTION OF OCEANS AND CONTINENTS 31
the vast ocean. The belt of ancient rocks of
2,000 million years from Brazil coast matches
with those from western Africa.  The earliest
marine deposits along the coastline of South
America and Africa are of the Jurassic age.
This suggests that the ocean did not exist prior
to that time.
Tillite
It is the sedimentary rock formed out of
deposits of glaciers. The Gondawana system
of sediments from India is known to have its
counter parts in six different landmasses of the
Southern Hemisphere. At the base the system
has thick tillite indicating extensive and
prolonged glaciation. Counter parts of this
succession are found in Africa, Falkland Island,
Madagascar, Antarctica and Australia besides
India.  Overall resemblance of the Gondawana
type sediments clearly demonstrates that these
landmasses had remarkably similar histories.
The glacial tillite provides unambiguous
evidence of palaeoclimates and also of drifting
of continents.
Placer Deposits
The occurrence of rich placer deposits of gold
in the Ghana coast and the absolute absence
of source rock in the region is an amazing fact.
The gold bearing veins are in Brazil and it is
obvious that the gold deposits of the Ghana
are derived from the Brazil plateau when the
two continents lay side by side.
Distribution of Fossils
When identical species of plants and animals
adapted to living on land or in fresh water are
found on either side of the marine barriers, a
problem arises regarding accounting for such
distribution.  The observations that Lemurs
occur in India, Madagascar and Africa led some
to consider a contiguous landmass “Lemuria”
linking these three landmasses.  Mesosaurus
was a small reptile adapted to shallow brackish
water.  The skeletons of these are found only
in two localities : the Southern Cape province
of South Africa and Iraver formations of Brazil.
The two localities presently are 4,800 km apart
with an ocean in between them.
Force for Drifting
Wegener suggested that the movement
responsible for the drifting of the continents
was caused by pole-fleeing force and tidal force.
The polar-fleeing force relates to the rotation
of the earth. You are aware of the fact that the
earth is not a perfect sphere; it has a bulge at
the equator. This bulge is due to the rotation
of the earth. The second force that was
suggested by Wegener—the tidal force—is due
to the attraction of the moon and the sun that
develops tides in oceanic waters. Wegener
believed that these forces would become
effective when applied over many million years.
However, most of scholars considered these
forces to be totally inadequate.
Post-Drift Studies
It is interesting to note that for continental drift,
most of the evidence was collected from the
continental areas in the form of distribution of
flora and fauna or deposits like tillite. A number
of discoveries during the post-war period
added new information to geological literature.
Particularly, the information collected from the
ocean floor mapping provided new dimensions
for the study of distribution of oceans and
continents.
Convectional Current Theory
Arthur Holmes in 1930s discussed the
possibility of convection currents operating in
the mantle portion. These currents are
generated due to radioactive elements causing
thermal differences in the mantle portion.
Holmes argued that there exists a system of
such currents in the entire mantle portion. This
was an attempt to provide an explanation to
the issue of force, on the basis of which
contemporary scientists discarded the
continental drift theory.
Mapping of the Ocean Floor
Detailed research of the ocean configuration
revealed that the ocean floor is not just a vast
plain but it is full of relief. Expeditions to map
the oceanic floor in the post-war period
provided a detailed picture of the ocean relief
and indicated the existence of submerged
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 32
mountain ranges as well as deep trenches,
mostly located closer to the continent margins.
The mid-oceanic ridges were found to be most
active in terms of volcanic eruptions. The dating
of the rocks from the oceanic crust revealed
the fact that they are much younger than the
continental areas. Rocks on either side of the
crest of oceanic ridges and having equi-distant
locations from the crest were found to have
remarkable similarities both in terms of their
constituents and their age.
Ocean Floor Configuration
In this section we shall note a few things related
to the ocean floor configuration that help us in
the understanding of the distribution of
continents and oceans. You will be studying
the details of ocean floor relief in Chapter
13. The ocean floor may be segmented into
three major divisions based on the depth
as well as the forms of relief. These divisions
are continental margins, deep-sea basins and
mid-ocean ridges.
Abyssal Plains
These are extensive plains that lie between the
continental margins and mid-oceanic ridges.
The abyssal plains are the areas where the
continental sediments that move beyond the
margins get deposited.
Mid-Oceanic Ridges
This forms an interconnected chain of
mountain system within the ocean. It is the
longest mountain-chain on the surface of the
earth though submerged under the oceanic
waters. It is characterised by a central rift
system at the crest, a fractionated plateau and
flank zone all along its length. The rift system
at the crest is the zone of intense volcanic
activity. In the previous chapter, you have been
introduced to this type of volcanoes as mid-
oceanic volcanoes.
Distribution of Earthquakes and Volcanoes
Study the maps showing the distribution of
seismic activity and volcanoes given in Figure
4.2. You will notice a line of dots in the central
parts of the Atlantic Ocean almost parallel to
the coastlines. It further extends into the Indian
Ocean. It bifurcates a little south of the Indian
subcontinent with one branch moving into
East Africa and the other meeting a similar line
from Myanmar to New Guiana. You will notice
that this line of dots coincides with the mid-
oceanic ridges. The shaded belt showing
another area of concentration coincides with
the Alpine-Himalayan system and the rim of
the Pacific Ocean. In general, the foci of the
earthquake in the areas of mid-oceanic ridges
are at shallow depths whereas along the
Alpine-Himalayan belt as well as the rim of the
Pacific, the earthquakes are deep-seated ones.
The map of volcanoes also shows a similar
pattern. The rim of the Pacific is also called rim
of fire due to the existence of active volcanoes in
this area.
CONCEPT OF SEA FLOOR SPREADING
As mentioned above, the post-drift studies
provided considerable information that was not
Figure 4.1 : Ocean Floor
Continental Margins
These form the transition between continental
shores and deep-sea basins. They include
continental shelf, continental slope, continental
rise and deep-oceanic trenches.  Of these, the
deep-oceanic trenches are the areas which are
of considerable interest in so far as the
distribution of oceans and continents is
concerned.
© NCERT
not to be republished
DISTRIBUTION OF OCEANS AND CONTINENTS 33
available at the time Wegener put forth his
concept of continental drift. Particularly, the
mapping of the ocean floor and palaeomagnetic
studies of rocks from oceanic regions revealed
the following facts :
(i) It was realised that all along the mid-
oceanic ridges, volcanic eruptions are
common and they bring huge amounts of
lava to the surface in this area.
(ii) The rocks equidistant on either sides of the
crest of mid-oceanic ridges show
remarkable similarities in terms of period
of formation, chemical compositions and
magnetic properties. Rocks closer to the
mid-oceanic ridges have normal polarity
and are the youngest. The age of the rocks
increases as one moves away from the
crest.
(iii) The ocean crust rocks are much younger
than the continental rocks. The age of rocks
in the oceanic crust is nowhere more than
200 million years old. Some of the continental
rock formations are as old as 3,200 million
years.
(iv) The sediments on the ocean floor are
unexpectedly very thin. Scientists were
expecting, if the ocean floors were as old
as the continent, to have a complete
sequence of sediments for a period of much
longer duration. However, nowhere was the
sediment column found to be older than
200 million years.
(v) The deep trenches have deep-seated
earthquake occurrences while in the mid-
oceanic ridge areas, the quake foci have
shallow depths.
These facts and a detailed analysis of magnetic
properties of the rocks on either sides of the
mid-oceanic ridge led Hess (1961) to propose
his hypothesis, known as the “sea floor
spreading”. Hess argued that constant
eruptions at the crest of oceanic ridges cause
the rupture of the oceanic crust and the new
lava wedges into it, pushing the oceanic crust
on either side. The ocean floor, thus spreads.
The younger age of the oceanic crust as well
as the fact that the spreading of one ocean does
not cause the shrinking of the other, made Hess
 Figure 4. 2 : Distribution of earthquakes and volcanoes
© NCERT
not to be republished
Page 5


In the previous chapter, you have studied the
interior of the earth. You are already familiar
with the world map. You know that continents
cover 29 per cent of the surface of the earth
and the remainder is under oceanic waters.
The positions of the continents and the ocean
bodies, as we see them in the map, have not
been the same in the past. Moreover, it is now
a well-accepted fact that oceans and
continents will not continue to enjoy their
present positions in times to come.  If this is
so, the question arises what were their positions
in the past? Why and how do they change their
positions? Even if it is true that the continents
and oceans have changed and are changing
their positions, you may wonder as to how
scientists know this. How have they determined
their earlier positions? You will find the answers
to some of these and related questions in this
chapter.
CONTINENTAL DRIFT
Observe the shape of the coastline of the Atlantic
Ocean. You will be surprised by the symmetry
of the coastlines on either side of the ocean. No
wonder, many scientists thought of this
similarity and considered the possibility of the
two Americas, Europe and Africa, to be once
joined together. From the known records of the
history of science, it was Abraham Ortelius, a
Dutch map maker, who first proposed such a
possibility as early as 1596. Antonio Pellegrini
drew a map showing the three continents together.
However, it was Alfred Wegener—a German
meteorologist who put forth a comprehensive
argument in the form of  “the continental drift
DISTRIBUTION OF OCEANS
AND CONTINENTS
CHAPTER
theory” in 1912. This was regarding the
distribution of the oceans and the continents.
According to Wegener, all the continents
formed a single continental mass and mega
ocean surrounded the same.  The super
continent was named PANGAEA, which meant
all earth. The mega-ocean was called
PANTHALASSA, meaning all water. He argued
that, around 200 million years ago, the super
continent, Pangaea, began to split. Pangaea first
broke into two large continental masses as
Laurasia and Gondwanaland forming the
northern and southern components
respectively. Subse-quently, Laurasia and
Gondwanaland continued to break into various
smaller continents that exist today. A variety of
evidence was offered in support of the
continental drift. Some of these are given below.
Evidence in Support of the Continental Drift
The Matching of Continents (Jig-Saw-Fit)
The shorelines of Africa and South America
facing each other have a remarkable and
unmistakable match. It may be noted that a
map produced using a computer programme
to find the best fit of the Atlantic margin was
presented by Bullard in 1964. It proved to be
quite perfect. The match was tried at 1,000-
fathom line instead of the present shoreline.
Rocks of Same Age Across the Oceans
The radiometric dating methods developed in
the recent period have facilitated correlating the
rock formation from different continents across
© NCERT
not to be republished
DISTRIBUTION OF OCEANS AND CONTINENTS 31
the vast ocean. The belt of ancient rocks of
2,000 million years from Brazil coast matches
with those from western Africa.  The earliest
marine deposits along the coastline of South
America and Africa are of the Jurassic age.
This suggests that the ocean did not exist prior
to that time.
Tillite
It is the sedimentary rock formed out of
deposits of glaciers. The Gondawana system
of sediments from India is known to have its
counter parts in six different landmasses of the
Southern Hemisphere. At the base the system
has thick tillite indicating extensive and
prolonged glaciation. Counter parts of this
succession are found in Africa, Falkland Island,
Madagascar, Antarctica and Australia besides
India.  Overall resemblance of the Gondawana
type sediments clearly demonstrates that these
landmasses had remarkably similar histories.
The glacial tillite provides unambiguous
evidence of palaeoclimates and also of drifting
of continents.
Placer Deposits
The occurrence of rich placer deposits of gold
in the Ghana coast and the absolute absence
of source rock in the region is an amazing fact.
The gold bearing veins are in Brazil and it is
obvious that the gold deposits of the Ghana
are derived from the Brazil plateau when the
two continents lay side by side.
Distribution of Fossils
When identical species of plants and animals
adapted to living on land or in fresh water are
found on either side of the marine barriers, a
problem arises regarding accounting for such
distribution.  The observations that Lemurs
occur in India, Madagascar and Africa led some
to consider a contiguous landmass “Lemuria”
linking these three landmasses.  Mesosaurus
was a small reptile adapted to shallow brackish
water.  The skeletons of these are found only
in two localities : the Southern Cape province
of South Africa and Iraver formations of Brazil.
The two localities presently are 4,800 km apart
with an ocean in between them.
Force for Drifting
Wegener suggested that the movement
responsible for the drifting of the continents
was caused by pole-fleeing force and tidal force.
The polar-fleeing force relates to the rotation
of the earth. You are aware of the fact that the
earth is not a perfect sphere; it has a bulge at
the equator. This bulge is due to the rotation
of the earth. The second force that was
suggested by Wegener—the tidal force—is due
to the attraction of the moon and the sun that
develops tides in oceanic waters. Wegener
believed that these forces would become
effective when applied over many million years.
However, most of scholars considered these
forces to be totally inadequate.
Post-Drift Studies
It is interesting to note that for continental drift,
most of the evidence was collected from the
continental areas in the form of distribution of
flora and fauna or deposits like tillite. A number
of discoveries during the post-war period
added new information to geological literature.
Particularly, the information collected from the
ocean floor mapping provided new dimensions
for the study of distribution of oceans and
continents.
Convectional Current Theory
Arthur Holmes in 1930s discussed the
possibility of convection currents operating in
the mantle portion. These currents are
generated due to radioactive elements causing
thermal differences in the mantle portion.
Holmes argued that there exists a system of
such currents in the entire mantle portion. This
was an attempt to provide an explanation to
the issue of force, on the basis of which
contemporary scientists discarded the
continental drift theory.
Mapping of the Ocean Floor
Detailed research of the ocean configuration
revealed that the ocean floor is not just a vast
plain but it is full of relief. Expeditions to map
the oceanic floor in the post-war period
provided a detailed picture of the ocean relief
and indicated the existence of submerged
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 32
mountain ranges as well as deep trenches,
mostly located closer to the continent margins.
The mid-oceanic ridges were found to be most
active in terms of volcanic eruptions. The dating
of the rocks from the oceanic crust revealed
the fact that they are much younger than the
continental areas. Rocks on either side of the
crest of oceanic ridges and having equi-distant
locations from the crest were found to have
remarkable similarities both in terms of their
constituents and their age.
Ocean Floor Configuration
In this section we shall note a few things related
to the ocean floor configuration that help us in
the understanding of the distribution of
continents and oceans. You will be studying
the details of ocean floor relief in Chapter
13. The ocean floor may be segmented into
three major divisions based on the depth
as well as the forms of relief. These divisions
are continental margins, deep-sea basins and
mid-ocean ridges.
Abyssal Plains
These are extensive plains that lie between the
continental margins and mid-oceanic ridges.
The abyssal plains are the areas where the
continental sediments that move beyond the
margins get deposited.
Mid-Oceanic Ridges
This forms an interconnected chain of
mountain system within the ocean. It is the
longest mountain-chain on the surface of the
earth though submerged under the oceanic
waters. It is characterised by a central rift
system at the crest, a fractionated plateau and
flank zone all along its length. The rift system
at the crest is the zone of intense volcanic
activity. In the previous chapter, you have been
introduced to this type of volcanoes as mid-
oceanic volcanoes.
Distribution of Earthquakes and Volcanoes
Study the maps showing the distribution of
seismic activity and volcanoes given in Figure
4.2. You will notice a line of dots in the central
parts of the Atlantic Ocean almost parallel to
the coastlines. It further extends into the Indian
Ocean. It bifurcates a little south of the Indian
subcontinent with one branch moving into
East Africa and the other meeting a similar line
from Myanmar to New Guiana. You will notice
that this line of dots coincides with the mid-
oceanic ridges. The shaded belt showing
another area of concentration coincides with
the Alpine-Himalayan system and the rim of
the Pacific Ocean. In general, the foci of the
earthquake in the areas of mid-oceanic ridges
are at shallow depths whereas along the
Alpine-Himalayan belt as well as the rim of the
Pacific, the earthquakes are deep-seated ones.
The map of volcanoes also shows a similar
pattern. The rim of the Pacific is also called rim
of fire due to the existence of active volcanoes in
this area.
CONCEPT OF SEA FLOOR SPREADING
As mentioned above, the post-drift studies
provided considerable information that was not
Figure 4.1 : Ocean Floor
Continental Margins
These form the transition between continental
shores and deep-sea basins. They include
continental shelf, continental slope, continental
rise and deep-oceanic trenches.  Of these, the
deep-oceanic trenches are the areas which are
of considerable interest in so far as the
distribution of oceans and continents is
concerned.
© NCERT
not to be republished
DISTRIBUTION OF OCEANS AND CONTINENTS 33
available at the time Wegener put forth his
concept of continental drift. Particularly, the
mapping of the ocean floor and palaeomagnetic
studies of rocks from oceanic regions revealed
the following facts :
(i) It was realised that all along the mid-
oceanic ridges, volcanic eruptions are
common and they bring huge amounts of
lava to the surface in this area.
(ii) The rocks equidistant on either sides of the
crest of mid-oceanic ridges show
remarkable similarities in terms of period
of formation, chemical compositions and
magnetic properties. Rocks closer to the
mid-oceanic ridges have normal polarity
and are the youngest. The age of the rocks
increases as one moves away from the
crest.
(iii) The ocean crust rocks are much younger
than the continental rocks. The age of rocks
in the oceanic crust is nowhere more than
200 million years old. Some of the continental
rock formations are as old as 3,200 million
years.
(iv) The sediments on the ocean floor are
unexpectedly very thin. Scientists were
expecting, if the ocean floors were as old
as the continent, to have a complete
sequence of sediments for a period of much
longer duration. However, nowhere was the
sediment column found to be older than
200 million years.
(v) The deep trenches have deep-seated
earthquake occurrences while in the mid-
oceanic ridge areas, the quake foci have
shallow depths.
These facts and a detailed analysis of magnetic
properties of the rocks on either sides of the
mid-oceanic ridge led Hess (1961) to propose
his hypothesis, known as the “sea floor
spreading”. Hess argued that constant
eruptions at the crest of oceanic ridges cause
the rupture of the oceanic crust and the new
lava wedges into it, pushing the oceanic crust
on either side. The ocean floor, thus spreads.
The younger age of the oceanic crust as well
as the fact that the spreading of one ocean does
not cause the shrinking of the other, made Hess
 Figure 4. 2 : Distribution of earthquakes and volcanoes
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 34
think about the consumption of the oceanic
crust. He further maintained that the ocean
floor that gets pushed due to volcanic
eruptions at the crest, sinks down at the
oceanic trenches and gets consumed.
The basic concept of sea floor spreading has
been depicted in Figure 4.3.
PLATE TECTONICS
Since the advent of the concept of sea floor
spreading, the interest in the problem of
distribution of oceans and continents was
revived. It was in 1967, McKenzie and Parker
and also Morgan, independently collected the
available ideas and came out with another
 Figure 4. 3 : Sea floor spreading
Figure 4.4 : Position of continents through geological past
The motions of the continents during the past 540
million years. 1. Africa; 2. South America;
3. Antarctica; 4. Australia; 5. India; 6. China; 7. North
America; 8. Europe; 9. and 10. Siberia (Emilani, 1992)
© NCERT
not to be republished
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