Distribution of Oceans and Continents
Class 11 | Geography | Chapter 4 | Questions and Answers | NCERT
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 (b) Antonio Pellegrini (c) Abraham Ortelius (d) Edmund Hess
(ii) Polar fleeing force relates to:
(a) Revolution of the earth (b) Gravitation (c) Rotation of the earth (d) Tides
(iii) Which one of the following is not a minor plate?
(a) Nazca (b) Arabia (c) Philippines (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 movements of the continent?
In 1912, Alfred Wegener proposed a theory about the movements of continents. He suggested two forces that are responsible for the continental drift. These are – Polar fleeing force and Tidal force. The former occurs as a result of the rotation of the earth while the latter is due to the attraction between the moon and the sun which causes tides in the oceans.
(ii) How are the convectional currents in the mantle initiated and maintained?
In 1930, Arthur Holmes stated the possibility of convection currents. According to him, these are generated in the mantle due to the radioactive elements which also increase its temperature. The convection currents in the mantle are initiated and maintained by difference of temperature within the earth. The temperature difference is due to the movement of hot magma upwards and cold lava downwards. As a result of convection currents, continents have been thought to break apart.
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Along the divergent or convergent boundaries, either the crust is produced or destroyed.
In the transform boundaries, neither of this happens as the plates gently slides horizontally past each other.
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
During the formation of the Deccan Traps, Tethys sea separated the Indian and the Eurasian landmasses and the Tibet landmass was close to the Indian. This happened almost 140 years ago and at that time, the location of the Indian landmass was towards the south of the equator.
3. Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?
Following are the evidences in support of the continental drift theory –
- The matching of continents – If the continents are combined, maximum chances are there that they will fit each other. There is a remarkable and undisputable match between the shorelines of Africa and South America facing each other.
2. Rocks of same age across the oceans – Through implementation of radiometric dating methods, it has been found that the age of rocks in both the shores of oceans is same. The age of rocks of Brazil in South America is 2000 million years. On the other hand, the age is also same is case of Western Africa.
3. Tillite – This is one kind of sedimentary rock that is formed out of deposits of glaciers. Similar kind of Indian sedimentary rocks – the Gondawana system are also found in six different landmasses of the Southern Hemisphere. The other areas where this rock exist are – Africa, Falkland Island, Madagascar, Antarctica and Australia. This proves that these areas share same geographical history.
4. Placer Deposits – The occurrence of rich placer deposits of Gold in the Ghana Coast establishes the fact that formerly Africa and South America were together. Because gold is found only in the Ghana Coast and the gold bearing veins are in Brazil.
5. Distribution of Fossils – Same kind of fossil deposits can be seen in both coasts of the oceans. The fossils of Lemur has been found in India, Africa and Madagascar. On the other hand, fossils of an organism called Mesosaurus has been found in Southern Cape Province of South America and Iraver formations Brazil. But now, the distance of both the regions is 4800 km with the Atlantic Ocean in between them.
(ii) Bring about the basic difference between the drift theory and Plate tectonics.
The basic difference between the drift theory and Plate tectonics are given in the table below-
| Drift theory | Plate tectonics |
| 1. It assumes all the present continents to have arisen from the breaking and then drifting of the components. | 1. It assumes the earth’s lithosphere to be formed of major and minor plates which are forever moving. |
| 2. Considers only horizontal movement. | 2. Considers both sideways and downward movement. |
| 3. It relies on the evidences like Jig-Saw-Fit, fossils, placer deposits etc. | 3. It is based on the scientific analysis of processes inside the earth’s surface. |
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Scientist collected information from the ocean floor mapping which provided a ray towards more study of the distribution of oceans and continents.
- The volcanic eruptions are scattered all along the mid-oceanic ridges which bring huge amounts of lava to the surface in this area.
2. 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.
3. The rocks present in the ocean crust are much younger than the continental rocks. The former are almost 200 million years old while the continental rock formations are as old as 3200 million years.
4. The sediments on the ocean floor are unexpectedly very thin. This confirms that ocean crust rocks are relatively younger than the continental crust rocks.
5. The deep trenches have deep-seated earthquake occurrences while in the mid-oceanic ridge areas, the quake foci have shallow depths.
Additional Questions:
When was continental drift theory proposed?
In 1912, German Scientist Alfred Wegener proposed the continental drift theory.
2. What is Pangaea and Panthalassa?
According to Wegener, the present day continents existed as one massive supercontinent in the beginning which is called Pangaea. On the other hand, this super continent was surrounded by a huge or super ocean which is called as Panthalassa.
3. What is Laurasia and Gondwana land?
The Pangaea broke into two halves – Northern and Southern. The Northern part was known is Laurasia land while the Southern as Gondwana land.
4. When was plate tectonics theory suggested?
In 1969, McKenzie, Parker, Morgan and a number of other scientists suggested the plate tectonics theory.
5. What is the fastest plate?
The Pacific Plate is the fastest. It’s speed is more than 15 cm per year.
6. Who proposed the theory of sea floor spreading?
Hess, in 1961 gave the theory of sea floor spreading.