RBSE Solutions for Class 11 Geography Chapter 4 Distribution of Oceans and Continents

Rajasthan Board RBSE Solutions for Class 11 Geography Chapter 4 Distribution of Oceans and Continents Textbook Exercise Questions and Answers.

RBSE Class 11 Geography Solutions Chapter 4 Distribution of Oceans and Continents

RBSE Class 11 Geography Distribution of Oceans and Continents Textbook Questions and Answers 

Multiple Choice Questions

1. Choose the right answer from the four alternatives given :
(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
(b) Antonio Pellegrini
(c) Abraham Ortelius
(d) Edmond Hess.
Answer:
(c) Abraham Ortelius.

(ii) Polar fleeing force relates to :
(a) Revolution of the earth
(b) Gravitation
(c) Rotation of the earth
(d) Tides.
Answer:
(c) Rotation of the earth.

(iii) Which one of the following is not a minor plate?
(a) Nazca 
(b) Arabia
(c) Philippines 
(d) Antarctica.
Answer: 
(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.
Answer: 
(c) Distribution of fossils in different continents.

(v) Which one of the following is the type © of plate boundary of the Indian plate '. along the Himalayan mountains?
(a) Ocean-continent covergence
(b) Divergent boundary
(c) Transform boundary
(d) Continent-continent convergence.
Answer: 
(d) Continent-continent convergence. 

Short Answer Type Questions

2. Answer the following questions in about 30 words :
(i) What were the forces suggested by Wegner-/ for the movement of continents?
Answer: 
Force for Drifting: Wegner suggested that the movement-drifting of continents was caused by 

1. pole fleeing force and 
2. tidal force. Wegner believed that these forces would become effective when applied over many million years. However, most of the scholars considered these forces to be totally inadequate.

(ii) How the convectional currents in the mantle are intitated and maintained?
Answer: 
Arthur Holmes in 1950s discussed the possibility of convection currents operating in the mantle portion. These are the currents generated due to radioactive elements causing thermal differences in the mantle portion. The currents move in radial direction and develop in cell patterns. Holmes suggested a system of such convection currents in mantle. It explains the force of drift on the basis of which continental theory wras discarded.

(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Answer: 
The three types of the plate boundaries are :
(a) Divergent boundary. Where the plates pull away from each other.
(b) Convergent boundary. When the edge of one plate over rides the other.
(c) Transform boundary. When the plates slide horizontally part each other.

(iv) What was the location of Indian landmass during the formation of Deccan trap?
Answer: 
Above 60 million years ago, Deccan trap was formed. At that time, the subcontinent of Indian landmass was located at South as 50°S latitude. At that time, the Indian plate moved towards Asiatic plate.

Essay Type Questions

3. Answer the following questions in about 150 words :
(i) What are the evidences in support of
Continental Drift Theory?
Or
Examine critically Wegner’s Continental V Drift Theory.
Answer: 
Continental Drift Theory. Continental drift means the horizontal movement of the continents on a vast scale. The idea of continental drift was first advocated by Alfred Wegner, in 1912. Wegner was primarily a meteorologist. His studies related to the problem of major variation of climate in the past. The equatorial, tropical and arctic climates were found in latitudes far removed from lands where these climates are found today.

Wegner explained this fact by suggesting that continents drifted apart from each other in the past. The present distribution of continents and oceans is the result of movement of lands. Outlines of theory, (t) Wegner supposed that continents were made of Sial Layers. Before the Silurian Period (280 million years ago), several land masses joined together to form a large continent or a super continent called Pangea. It was surrounded by an ocean named Panthalassa. The continent had a wide belt of sea in the middle part known as Tethy’s sea.

(ii) The northern land mass consisting of North America, Europe and Asia was known as Laurasia, while the southern land mass consisting of Antarctica, Australia, India, Africa and South America was known as Gondwana Land.

(iii) The Pangea broke into pieces during the Late Palaeozoic era (200 to 500 million years ago). The breaking up and drifting apart of these pieces were due to the differential gravitational force and Tidal force affecting the earth. These forces moved land mass in two directions-one towards the equator and the other westward.

(iv) The differential gravitational force caused the Gondwana Land (African Block) and Laurasia to drift towards the equator during the Mesozoic era (70 to 200 million years ago). The tidal forces drifted the America westward in the Tertiary Period (2 to 70 million years ago). The drift created the Atlantic Ocean. By compression of these drifts, the mountain chains of Himalayas, Rocky and Andes were created.

(v) Later on, during different periods, the different continents drifted apart in different directions. Australia and Antarctica were separated from Gondwana Land. Indian part of Gondwana Land moved norhtwards. It created Indian Ocean.

(vi) Labrador, Newfoundland and Greenland were separated from Europe during Quarternery Period (about 2 million years ago). West Indies drifted apart from Americas. Madagascar drifted away from Africa. Evidence in support of the theory. Wegner collected evidences from different branches of science in support of his theory. The theory was subjected to criticism and was not accepted in its early period. But the central idea of drift has been accepted. The following evidences support the theory :

1. Jig-saw-fit. Jig-saw-fit of the coast lines of South America and Africa suggest the similarities of opposed coast lines of Atlantic Ocean. The matching of shoreline of continents proved is be quite

perfect. A map produced using a computer programme to find the best fit of Atlantic margin was presented by Edward Bullard in 1964. The match was tried at the 1000 metre (500 fathom) submarine depth contour, which gave the present shore line.
 
2. Geological Structure. Similarity of geological structure of Africa, Deccan Plateau of India, Brazil and Australia suggest that these were once a part of Gondwana Land. The ancient mountain belt 470 million years old were created as a belt. The ancient mountain belt is 70 million years old were created as a belt.

3. Fold lines. The similarity of fold lines of two coasts of Atlantic Ocean.

4. Fauna and flora. Similarity of fossil faunas and floras in different parts of Gondwana Land.
Some fossils tell us that the continents were once joined. For example, fossil of the plant Glossoptesis and the animals Mesosurus and Lystrosaurus are all found on all continents of Gondwanaland. Climatic conditions suggest that various type of climatic zones are widely separated from each other by large oceans.

5. The recent theory of plate tectonics suggests that the continents are moving as plates on a semi-liquid surface.

(ii) Bring about the basic difference between the drift theory and plate tectonics.
Answer:
According to Wegner’s drift theory, the super continent Pangea was made up of many continents. These continents changed their position due to drift. But according to plate tectonic theory, solid continents rest on plates. Pangea was made up of many plates. These plates are moving continually.

(iii) What were the major post drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Answer:
Many post drift discoveries from the ocean floor mapping provided new dimension for the study of distribution of oceans and continents.
(a) The convection currents in mantle.
(b) Ocean floor mapping
(c) The mid oceanic ridges
(d) Dating of rocks
(e) Plate tectnoic theory.