Oceans and continents of the world- their distribution and positions

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This booklet provides information about oceans and continents of the world. It will tell about the distribution of oceans and continents of the world.

1. DISTRIBUTION OF OCEANS
AND CONTINENTS
In the previous chapter, you have studied the theory” in 1912. This was regarding the
interior of the earth. You are already familiar distribution of the oceans and the continents.
with the world map. You know that continents According to Wegener, all the continents
cover 29 per cent of the surface of the earth formed a single continental mass and mega
and the remainder is under oceanic waters. ocean surrounded the same. The super
The positions of the continents and the ocean continent was named PANGAEA, which meant
bodies, as we see them in the map, have not all earth. The mega-ocean was called
been the same in the past. Moreover, it is now PANTHALASSA, meaning all water. He argued
a well-accepted fact that oceans and that, around 200 million years ago, the super
continents will not continue to enjoy their continent, Pangaea, began to split. Pangaea first
present positions in times to come. If this is broke into two large continental masses as
so, the question arises what were their positions Laurasia and Gondwanaland forming the
in the past? Why and how do they change their northern and southern components
positions? Even if it is true that the continents respectively. Subsequently, Laurasia and
and oceans have changed and are changing Gondwanaland continued to break into various
their positions, you may wonder as to how smaller continents that exist today. A variety of
scientists know this. How have they determined evidence was offered in support of the
their earlier positions? You will find the answers continental drift. Some of these are given below.
to some of these and related questions in this
chapter. Evidence in Support of the Continental Drift
CONTINENTAL DRIFT The Matching of Continents (Jig-Saw-Fit)
Observe the shape of the coastline of the Atlantic The shorelines of Africa and South America
Ocean. You will be surprised by the symmetry facing each other have a remarkable and
of the coastlines on either side of the ocean. No
unmistakable match. It may be noted that a
wonder, many scientists thought of this
map produced using a computer programme
similarity and considered the possibility of the
to find the best fit of the Atlantic margin was
two Americas, Europe and Africa, to be once
presented by Bullard in 1964. It proved to be
joined together. From the known records of the
quite perfect. The match was tried at 1,000-
history of science, it was Abraham Ortelius, a
fathom line instead of the present shoreline.
Dutch map maker, who first proposed such a
possibility as early as 1596. Antonio Pellegrini
Rocks of Same Age Across the Oceans
drew a map showing the three continents together.
However, it was Alfred Wegener—a German The radiometric dating methods developed in
meteorologist who put forth a comprehensive the recent period have facilitated correlating the
argument in the form of “the continental drift rock formation from different continents across
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2. DISTRIBUTION OF OCEANS AND CONTINENTS 31
the vast ocean. The belt of ancient rocks of Force for Drifting
2,000 million years from Brazil coast matches
Wegener suggested that the movement
with those from western Africa. The earliest
responsible for the drifting of the continents
marine deposits along the coastline of South
was caused by pole-fleeing force and tidal force.
America and Africa are of the Jurassic age. The polar-fleeing force relates to the rotation
This suggests that the ocean did not exist prior of the earth. You are aware of the fact that the
to that time. earth is not a perfect sphere; it has a bulge at
the equator. This bulge is due to the rotation
Tillite of the earth. The second force that was
It is the sedimentary rock formed out of suggested by Wegener — the tidal force — is
deposits of glaciers. The Gondawana system due to the attraction of the moon and the sun
of sediments from India is known to have its that develops tides in oceanic waters. Wegener
counterparts in six different landmasses of the believed that these forces would become
Southern Hemisphere. At the base, the system effective when applied over many million years.
has thick tillite indicating extensive and However, most of scholars considered these
prolonged glaciation. Counterparts of this forces to be totally inadequate.
succession are found in Africa, Falkland Island,
Post-drift Studies
Madagascar, Antarctica and Australia. Overall
resemblance of the Gondawana-type sediments It is interesting to note that for continental drift,
clearly demonstrates that these landmasses most of the evidence was collected from the
had remarkably similar histories. The glacial continental areas in the form of distribution of
tillite provides unambiguous evidence of flora and fauna or deposits, like tillite. A
palaeoclimates and also of drifting of continents. number of discoveries during the post–World
War II period added new information to
Placer Deposits geological literature. Particularly, the
information collected from the ocean floor
The occurrence of rich placer deposits of gold mapping provided new dimensions for the
in the Ghana coast and the absolute absence study of distribution of oceans and continents.
of source rock in the region is an amazing fact.
The gold bearing veins are in Brazil and it is Convectional Current Theory
obvious that the gold deposits of the Ghana
Arthur Holmes in 1930s discussed the
are derived from the Brazil plateau when the
possibility of convection currents operating in
two continents lay side by side.
the mantle portion. These currents are
generated due to radioactive elements causing
Distribution of Fossils
thermal differences in the mantle portion.
When identical species of plants and animals Holmes argued that there exists a system of
adapted to living on land or in fresh water are such currents in the entire mantle portion. This
found on either side of the marine barriers, a was an attempt to provide an explanation to
problem arises regarding accounting for such the issue of force, on the basis of which
distribution. The observations that Lemurs contemporary scientists discarded the
occur in India, Madagascar and Africa led some continental drift theory.
to consider a contiguous landmass ‘Lemuria’
linking these three landmasses. Mesosaurus Mapping of the Ocean Floor
was a small reptile adapted to shallow brackish Detailed research of the ocean configuration
water. The skeletons of these are found only revealed that the ocean floor is not just a vast
in two localities: the Southern Cape province plain but it is full of relief. Expeditions to map
of South Africa and Iraver formations of Brazil. the oceanic floor in the post–World War II period
The two localities are presently 4,800 km apart provided a detailed picture of the ocean relief
with an ocean in between them. and indicated the existence of submerged
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3. 32 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
mountain ranges as well as deep trenches, Abyssal Plains
mostly located closer to the continent margins.
The mid-oceanic ridges were found to be most These are extensive plains that lie between the
active in terms of volcanic eruptions. The dating continental margins and mid-oceanic ridges.
of the rocks from the oceanic crust revealed The abyssal plains are the areas where the
the fact that they are much younger than the continental sediments that move beyond the
continental areas. Rocks on either side of the margins get deposited.
crest of oceanic ridges and having equi-distant
locations from the crest were found to have Mid-Oceanic Ridges
remarkable similarities both in terms of their This forms an interconnected chain of
constituents and their age. mountain system within the ocean. It is the
longest mountain-chain on the surface of the
Ocean Floor Configuration earth though submerged under the oceanic
In this section we shall note a few things related waters. It is characterised by a central rift
to the ocean floor configuration that help us in system at the crest, a fractionated plateau and
the understanding of the distribution of flank zone all along its length. The rift system
continents and oceans. You will be studying at the crest is the zone of intense volcanic
the details of ocean floor relief in Chapter activity. In the previous chapter, you have been
13. The ocean floor may be segmented into introduced to this type of volcanoes as mid-
three major divisions based on the depth oceanic volcanoes.
as well as the forms of relief. These divisions
are continental margins, deep-sea basins and Distribution of Earthquakes and Volcanoes
mid-ocean ridges. 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
Figure 4.1 : Ocean Floor
are at shallow depths whereas along the
Alpine-Himalayan belt as well as the rim of the
Continental Margins Pacific, the earthquakes are deep-seated ones.
The map of volcanoes also shows a similar
These form the transition between continental
pattern. The rim of the Pacific is also called rim
shores and deep-sea basins. They include
of fire due to the existence of active volcanoes in
continental shelf, continental slope, continental
this area.
rise and deep-oceanic trenches. Of these, the
deep-oceanic trenches are the areas which are
CONCEPT OF SEA FLOOR SPREADING
of considerable interest in so far as the
distribution of oceans and continents is As mentioned above, the post-drift studies
concerned. provided considerable information that was not
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4. DISTRIBUTION OF OCEANS AND CONTINENTS 33
Figure 4. 2 : Distribution of earthquakes and volcanoes
available at the time Wegener put forth his (iv) The sediments on the ocean floor are
concept of continental drift. Particularly, the unexpectedly very thin. Scientists were
mapping of the ocean floor and palaeomagnetic expecting, if the ocean floors were as old
studies of rocks from oceanic regions revealed as the continent, to have a complete
the following facts : sequence of sediments for a period of much
(i) It was realised that all along the mid- longer duration. However, nowhere was the
oceanic ridges, volcanic eruptions are sediment column found to be older than
common and they bring huge amounts of 200 million years.
lava to the surface in this area. (v) The deep trenches have deep-seated
(ii) The rocks equidistant on either sides of the earthquake occurrences while in the mid-
crest of mid-oceanic ridges show oceanic ridge areas, the quake foci have
remarkable similarities in terms of period shallow depths.
of formation, chemical compositions and These facts and a detailed analysis of magnetic
magnetic properties. Rocks closer to the properties of the rocks on either sides of the
mid-oceanic ridges have normal polarity mid-oceanic ridge led Hess (1961) to propose
and are the youngest. The age of the rocks his hypothesis, known as the “sea floor
increases as one moves away from the spreading”. Hess argued that constant
crest. eruptions at the crest of oceanic ridges cause
(iii) The ocean crust rocks are much younger the rupture of the oceanic crust and the new
than the continental rocks. The age of rocks lava wedges into it, pushing the oceanic crust
in the oceanic crust is nowhere more than on either side. The ocean floor, thus spreads.
200 million years old. Some of the continental The younger age of the oceanic crust as well
rock formations are as old as 3,200 million as the fact that the spreading of one ocean does
years. not cause the shrinking of the other, made Hess
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5. 34 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Figure 4. 3 : Sea floor spreading
think about the consumption of the oceanic PLATE TECTONICS
crust. He further maintained that the ocean Since the advent of the concept of sea floor
floor that gets pushed due to volcanic spreading, the interest in the problem of
eruptions at the crest, sinks down at the distribution of oceans and continents was
oceanic trenches and gets consumed. revived. It was in 1967, McKenzie and Parker
The basic concept of sea floor spreading has and also Morgan, independently collected the
been depicted in Figure 4.3. available ideas and came out with another
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)
Figure 4.4 : Position of continents through geological past
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6. DISTRIBUTION OF OCEANS AND CONTINENTS 35
concept termed Plate Tectonics. A tectonic I Antarctica and the surrounding
plate (also called lithospheric plate) is a oceanic plate
massive, irregularly-shaped slab of solid rock, II North American (with western Atlantic
generally composed of both continental and floor separated from the South American
oceanic lithosphere. Plates move horizontally plate along the Caribbean islands) plate
over the asthenosphere as rigid units. The III South American (with western Atlantic
lithosphere includes the crust and top mantle floor separated from the North American
with its thickness range varying between plate along the Caribbean islands) plate
5 and100 km in oceanic parts and about 200 IV Pacific plate
km in the continental areas. A plate may be V India-Australia-New Zealand plate
referred to as the continental plate or oceanic VI Africa with the eastern Atlantic floor plate
plate depending on which of the two occupy a VII Eurasia and the adjacent oceanic plate.
larger portion of the plate. Pacific plate is Some important minor plates are listed below:
largely an oceanic plate whereas the Eurasian (i) Cocos plate : Between Central America
plate may be called a continental plate. The and Pacific plate
theory of plate tectonics proposes that the (ii) Nazca plate : Between South America
earth’s lithosphere is divided into seven major and Pacific plate
and some minor plates. Young Fold Mountain (iii) Arabian plate : Mostly the Saudi Arabian
ridges, trenches, and/or faults surround these landmass
major plates (Figure 4.5). The major plates (iv) Philippine plate : Between the Asiatic and
are as follows : Pacific plate
Figure 4.5 : Major and minor plates of the world
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7. 36 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
(v) Caroline plate : Between the Philippine Transform Boundaries
and Indian plate (North of New Guinea)
Where the crust is neither produced nor
(vi) Fuji plate : North-east of Australia.
destroyed as the plates slide horizontally past
These plates have been constantly moving
each other. Transform faults are the planes of
over the globe throughout the history of the
separation generally perpendicular to the mid-
earth. It is not the continent that moves as
oceanic ridges. As the eruptions do not take
believed by Wegener. Continents are part of a
all along the entire crest at the same time, there
plate and what moves is the plate. Moreover, it
is a differential movement of a portion of the
may be noted that all the plates, without
plate away from the axis of the earth. Also, the
exception, have moved in the geological past,
rotation of the earth has its effect on the
and shall continue to move in the future as well.
separated blocks of the plate portions.
Wegener had thought of all the continents to
have initially existed as a super continent in How do you think the rate of plate
the form of Pangaea. However, later discoveries movement is determined?
reveal that the continental masses, resting on
the plates, have been wandering all through the
Rates of Plate Movement
geological period, and Pangaea was a result of
converging of different continental masses that The strips of normal and reverse magnetic field
were parts of one or the other plates. Scientists that parallel the mid-oceanic ridges help
using the palaeomagnetic data have determined scientists determine the rates of plate
the positions held by each of the present movement. These rates vary considerably. The
continental landmass in different geological Arctic Ridge has the slowest rate (less than 2.5
periods (Fig 4.4). Position of the Indian sub- cm/yr), and the East Pacific Rise near Easter
continent (mostly Peninsular India) is traced Island, in the South Pacific about 3,400 km
with the help of the rocks analysed from the west of Chile, has the fastest rate (more than
Nagpur area. 15 cm/yr).
There are three types of plate boundaries:
Force for the Plate Movement
Divergent Boundaries
At the time that Wegener proposed his theory
Where new crust is generated as the plates pull of continental drift, most scientists believed
away from each other. The sites where the that the earth was a solid, motionless body.
plates move away from each other are called However, concepts of sea floor spreading and
spreading sites. The best-known example of the unified theory of plate tectonics have
divergent boundaries is the Mid-Atlantic Ridge. emphasised that both the surface of the earth
At this, the American Plate(s) is/are separated and the interior are not static and motionless
from the Eurasian and African Plates. but are dynamic. The fact that the plates move
is now a well-accepted fact. The mobile rock
Convergent Boundaries beneath the rigid plates is believed to be
Where the crust is destroyed as one plate dived moving in a circular manner. The heated
under another. The location where sinking of material rises to the surface, spreads and
a plate occurs is called a subduction zone. begins to cool, and then sinks back into deeper
There are three ways in which convergence can depths. This cycle is repeated over and over to
occur. These are: (i) between an oceanic and generate what scientists call a convection cell
continental plate; (ii) between two oceanic or convective flow. Heat within the earth comes
plates; and (iii) between two continental from two main sources: radioactive decay and
plates. residual heat. Arthur Holmes first considered
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8. DISTRIBUTION OF OCEANS AND CONTINENTS 37
this idea in the 1930s, which later influenced
Harry Hess’ thinking about seafloor spreading.
The slow movement of hot, softened mantle
that lies below the rigid plates is the driving
force behind the plate movement.
MOVEMENT OF THE INDIAN PLATE
The Indian plate includes Peninsular India
and the Australian continental portions. The
subduction zone along the Himalayas forms
the northern plate boundary in the form of
continent— continent convergence. In the east,
it extends through Rakinyoma Mountains of
Myanmar towards the island arc along the
Java T rench. The eastern margin is a
spreading site lying to the east of Australia in
the form of an oceanic ridge in SW Pacific. The
Western margin follows Kirthar Mountain of
Pakistan. It further extends along the Makrana
coast and joins the spreading site from the
Red Sea rift southeastward along the Chagos
Archipelago. The boundary between India
and the Antarctic plate is also marked by
oceanic ridge (divergent boundary) running
in roughly W-E direction and merging into the
spreading site, a little south of New Zealand.
India was a large island situated off the
Australian coast, in a vast ocean. The Tethys
Sea separated it from the Asian continent till
about 225 million years ago. India is supposed
to have started her northward journey about
200 million years ago at the time when Pangaea
broke. India collided with Asia about 40-50 Figure 4.6 : Movement of the Indian plate
million years ago causing rapid uplift of the plate towards the Eurasian plate, a major event
Himalayas. The positions of India since about that occurred was the outpouring of lava and
71 million years till the present are shown in formation of the Deccan Traps. This started
the Figure 4.6. It also shows the position of somewhere around 60 million years ago and
the Indian subcontinent and the Eurasian continued for a long period of time. Note that
plate. About 140 million years before the the subcontinent was still close to the equator.
present, the subcontinent was located as From 40 million years ago and thereafter, the
south as 50oS. latitude. The two major plates event of formation of the Himalayas took place.
were separated by the Tethys Sea and the Scientists believe that the process is still
Tibetan block was closer to the Asiatic continuing and the height of the Himalayas is
landmass. During the movement of the Indian rising even to this date.
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9. 38 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
EXERCISES
1. Multiple choice questions.
(i) Who amongst the following was the first to consider the possibility of
Europe, Africa and America having been located side by side.
(a) Alfred Wegener (c) Abraham Ortelius
(b) Antonio Pellegrini (d) Edmond Hess
(ii) Polar fleeing force relates to:
(a) Revolution of the Earth (c) Rotation of the earth
(b) Gravitation (d) Tides
(iii) Which one of the following is not a minor plate?
(a) Nazca (c) Philippines
(b) Arabia (d) Antarctica
(iv) Which one of the following facts was not considered by those while
discussing the concept of sea floor spreading?
(a) Volcanic activity along the mid-oceanic ridges.
(b) Stripes of normal and reverse magnetic field observed in rocks of ocean
floor.
(c) Distribution of fossils in different continents.
(d) Age of rocks from the ocean floor.
(v) Which one of the following is the type of plate boundary of the Indian plate
along the Himalayan mountains?
(a) Ocean-continent convergence
(b) Divergent boundary
(c) Transform boundary
(d) Continent-continent convergence
2. Answer the following questions in about 30 words.
(i) What were the forces suggested by Wegener for the movement of the
continents?
(ii) How are the convectional currents in the mantle initiated and maintained?
(iii) What is the major difference between the transform boundary and the
convergent or divergent boundaries of plates?
(iv) What was the location of the Indian landmass during the formation of the
Deccan Traps?
3. Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?
(ii) Bring about the basic difference between the drift theory and Plate
tectonics.
(iii) What were the major post-drift discoveries that rejuvenated the interest
of scientists in the study of distribution of oceans and continents?
Project Work
Prepare a collage related to damages caused by an earthquake.
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