Today we have discussed about the
different theories proving the origin of continents and basins
If you will look at the syllabus you will
find that in GS – 1, there is section namely important geographical
phenomenon.
So the upcoming modules will be titled as
important geographical phenomenon, where we would cover every aspects of world
geography namely physical, social and economic.
We would cover all conceptual aspects
under this module,
Important geographical phenomenon
1. In
order to understand important geographical phenomenon of geography, as per the
syllabus of GS it is necessary to understand the origin of continents and ocean
basins.
2. Continent
and ocean basins are fundamental relief feature of the globe.
3. They
are considered as relief features of the first order, and different theories
regarding the origin of the continents and oceans are being put forth by the
scientists.
4. About
70.8 percent of the total surface area of the globe is represented by the
oceans and 29.2 are represented by the continents.
Distributional pattern of the continents and ocean basins
1. There
is overwhelming dominance of land areas in the northern hemisphere; more than
75% of total globe is situated to the north of the equator.
2. Continents
are arranged in roughly triangular shape; most have their bases (of triangle)
in the north while their apices are pointed towards south.
3. The
oceans are also triangular in shape, contrary to the continents the base of
oceans is in the south while their apices are in the north.
4. The
North Pole is surrounded by oceanic water, while South Pole is surrounded by
land area (of the Antarctic Ocean).
5. There
is antipodal situation of continents and oceans, only 44.6 percent oceans are
situated opposite to oceans.
6. 1.4
percent of the total land area of the globe is opposite to land area; more than
95% of the total land area is situated opposite to water bodies.
7. The
great Pacific Ocean basin occupies almost one – third of the entire surface
area of the globe.
The above
mentioned characteristic would help us to validate the authenticity of any
hypothesis or theory dealing with the origin and evolution of the continents
and ocean basins.
There are
various hypotheses regarding the origin of the earth, but we would discuss the
following one, they are considered to be much more scientific.
1. Tetrahedral
hypothesis postulated by Lowthian Green
2. Continental
drift theory of Taylor
3. Plate
tectonic theory
4. Sea
floor spreading
Tetrahedral hypothesis
1. It
is based on the fundamental principles of geometry
2. Elie
de Beaumont is considered to be first the attempted in this field
3. But
the theory of Lowthain (in 1875) is most significant of all hypotheses based on
geometric principles.
4. He
based his hypothesis on the two basic principle of geometry
(i) A sphere is
that which contains the largest volume with respect to its surface area
(ii) A tetrahedron
is that body which contains the largest volume with respect to its surface area.
Basic assumption
1. It
is based on the characteristics of a tetrahedron which is a solid body having
four equal plane surfaces.
2. Each
of which is an equilateral triangle. He postulated it after considering the
characteristics of the distributional pattern of the land and water over the
globe.
3. There
is dominance of land area in the northern hemisphere and water areas in the
southern hemisphere
4. Triangular
shape of continent and oceans,
5. Situation
of continuous ring of land around north polar sea and location of south pole in
land area (Antarctica) surrounded by water from all sides
6. Antipodal
arrangement of continent and oceans
7. Largest
extent of Pacific Ocean covering one third area of the globe
8. Location
of chain folded mountains around Pacific Ocean.
9. According
to him when earth was originated it was in sphere form
10. In
the beginning earth was very hot but it gradually began to cool down due to
loss of heat.
11. First
the quarter part of the earth cooled down and thus was formed the crust but the
inner part was continued to cool down
12. Consequently
the inner part of the earth was subjected to much more contraction, thus there
was marked reduction in the inner part of the earth.
13. Upper
part of the earth (crust) was already cooled down, and could not be subjected
to further contraction.
14. This
resulted into possible gap between the upper and inner parts of the earth.
15. Consequently
the upper part collapsed on the inner part and ultimately the earth began to
assume the shape of a tetrahedron.
16. He
also clarified that due to structural variations; there may be some deviations
from a true tetrahedron.
17. Four
oceans (the Pacific Ocean, the Atlantic Ocean, the Indian Ocean and the Arctic
Ocean) were created on the four plane faces of the terrestrial tetrahedron.
18. These
plane faces retain water because of the fact that these were lower than the
level of the apices or coigns of the terrestrial tetrahedron.
Fig-
Distribution of land and water on a tetrahedron
Criticism
1. To
major extent it successfully explains the characteristics feature of the
distributional pattern of the present day continents and ocean basin.
2. But
because of certain basic defects and errors the theory is not acceptable to the
modern scientific community.
3. It
is argued that balance of the earth in the form of a tetrahedron while rotating
on an apex cannot be maintained.
4. The
earth rotating so rapidly on its axis that the spherical earth cannot be
converted into a tetrahedron while contracting on cooling.
5. This
hypothesis believes more or less in the permanency of continents and ocean
basins, while plate tectonic theory has validated the concept of continental
drift.
Continental drift theory of Taylor
1. F.B
.Taylor postulated his concept of horizontal displacement of the continents in
1908.
2. The
main purpose of his hypothesis was to explain the problems of the origin of the
folded mountains of tertiary period.
3. The
concept of Taylor is considered to be the first attempt in the field of
continental drift, though Antonio Snider presented his views about drift in the
year 1858 in France.
Basic Assumption
1. According
to him there were two land masses during Cretaceous period.
2. Lauratia
and Gondwanaland were located near the north and south pole respectively.
3. The
continent were made of sial which was practically absent in the oceanic crust.
4. Continent
moved towards the equator, the main driving force of the continental drift was
tidal force.
5. Continents
were displaced in two ways:
(i)
Equator wards movement
(ii)
Westward movement
Hypothesis
1. Lauratia
started moving away from the North Pole because of enormous tidal force of the
moon towards the equator in a radial manner.
2. This
movement of landmass resulted into tensional force near the north pole which
caused stretching, splitting and rupture in landmass.
3. The
displacement of the Gondwana land from the South Pole towards the equator
caused splitting and disruption.
4. Arctic
sea was formed between Greenland and Serbia due to earthward movement of
Lauratia.
5. Atlantic
and Indian Ocean were supposed to have been formed because of filling of gaps
between the drifting continents with water.
6. Taylor
assumed that the landmasses began to move in the lobe form while drifting through
the zones of lesser resistance.
7. Thus
mountain and island arcs were formed in the frontal part of the moving lobes.
8. The
Himalayas. Caucasus and Alps are considered to have been formed during equator
ward movement of the Lauratia and Gondwanaland from the north and south poles respectively
9. The
Rockies and Andes formed due to westward movement of the landmasses.
Criticisms
1. Taylor’s
main aim was to explain the origin of the tertiary folded mountains and hence
he made the continents to move at a very large scale.
2. He
described displacement of the landmasses for thousands of kilometres, but
displacement of landmasses up to 32-64 Km would have been sufficient enough for
the purpose.
3. Tidal
force was used as mode of displacement , if it was so enormous during
cretaceous period that it could displace the landmasses for thousands of
kilometres
4. Apart
then it might have also put a break on the rotatory motion of the earth and
thus rotation of the earth might have been stopped within a year.
5. Though
the concept of F.B. Taylor is not acceptable but his hypothesis is considered
to be significant on the ground that
6. He
raised his voice very forcefully through deductive postulation against the
prevalent concept of the permanency of the continents and ocean basins.
1. It
was a great scientific achievement of the decade of 1960s; it is based on the
two major scientific concepts
(i) The
concept of continental drift
(ii) The
concept of sea – floor spreading
2. Lithosphere
is internally made of rigid plates (first used by Canadian geophysicist J.T.
Wilson in 1965), six major plates and 20 minor plates have been identified so
far.
3. Six
major plates are Eurasian plate, Indian – Australian plate, American plate,
Pacific plate, African plate and Antarctic plate.
4. W.J.Morgan
and Le Pichon elaborated the various aspects of plate tectonics in 1968
5. Now
the continental drift and the displacement are considered a reality on the
basis of plate tectonics.
6. Tectonic
plate boundaries are most important because all tectonic activities occur along
the plate margins.
Fig-
Main aspects of plate tectonics
Plate margins are generally divided into three groups
Constructive plate margins
1. These
are also called divergent plate margins or accreting plate margins.
2. It
represents zone of divergence where there is continuous upwelling of molten
material (lava) and thus new oceanic crust is continuously formed.
Destructive plate margins
1. These
are also called as consuming plate margins or convergent plate margins.
2. Two
plates move towards each other or two plates converge along a line and leading
edge of one plate overrides the other plate.
3. The
overridden plate is sub ducted or thrust into the mantle and thus part of the
crust (plate) is lost in the mantle.
Conservative plate margins
1. These
are also called as shear plate margins
2. Two
plates pass or slide past one another along a transform faults and thus crust
is neither created nor destroyed.
Fig-
Different types of plate margins
Conclusion
1. H.
Hess postulated the concept of plate tectonics in 1960, in support of
continental drift.
2. The
continents and ocean move with the movement of these plates.
3. The
present shape and arrangement of continent and ocean basins could be attained
because of continuous movement of different plates.
4. Plate
tectonic theory is based on the evidences of sea floor spreading and
palaeomagnetism .
Sea – floor spreading
1. Propounded
by the Harry Hess of the Princeton University in the year 1960.
2. He
propounded that mid – oceanic ridges were situated on the rising thermal convection
currents, coming up from the mantle.
3. The
oceanic crust move in opposite directions from mid – oceanic ridges.
4. This
molten lava cool down and solidify to form new crust along the trailing ends of
divergent plates (oceanic crust).
5. Thus
there is continuous creation of new crust along the mid – oceanic ridges and
the expanding crusts (plates) are destroyed along the oceanic trenches.
6. These
facts prove that continents and ocean basins are in constant motion.
Fig – pattern of thermal
connective currents and plate movements
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