NCERT Class 11 Geography Chapter 4 Distribution of Oceans and Continents

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NCERT Class 11 Geography Chapter 4 Distribution of Oceans and Continents

Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. CBSE Class 11 Geography Solutions are part of All Subject Solutions. Here we have given NCERT Class 11 Geography Part I: Fundamentals of Physical Geography, Part II: Indian: Physical Environment, Part III: Practical Work in Geography. NCERT Class 11 Geography Chapter 4 Distribution of Oceans and Continents Notes, NCERT Class 11 Geography Textbook Solutions for All Chapters, You can practice these here.

Distribution of Oceans and Continents

Chapter: 4

GEOGRAPY [ PART – I ]

SHORT ANSWER TYPE QUESTIONS

Q.1. What was Pangaea?

Ans. In the beginning, there was a single continental landmass on the earth. This supercontinent was known as Pangaea. 

Q.2. Who first propounded the theory of continental drift? 

Ans. Alfred Wegener in 1912 first of all propounded the theory of continental drift.

Q.3. Name the major plates of the earth.

Ans. The seven major plates of the earth are:

(i) Antarctica and the surrounding oceanic plate.

(i) North American Plate.

(iii) South American Plate.

(iv) Pacific Plate.

(v) India-Australia-New Zealand Plate.

(vi) Africa with the eastern Atlantic floor plate.

Q.4. Which plates is composed of mainly oceanic crust?

Ans. Zone of divergence are boundaries along which plate is composed of mainly oceanic crust.

Q. 5. In which two components Pangaea was split?

Ans. Pangaea split in the two components-Laurasia representing the northern part and Gondwanaland representing the southern part.

Q.6. What is polar wandering?

Ans. Changes in the position of the poles due to the rotation of the earth is called polar wandering.

Q.7. How did the Himalayas rise?

Ans. The collision between Indian plate and Eurasian plate took place giving rise to the Himalayas. The suture between Indian and Eurasian plates in the Himalayan region has been along the Indus and Brahmaputra rivers.

Q.8. What is meant by sea floor spreading?

Ans. The crust spreads away from the ridge and the ocean basin widens. This phenomena is known as sea floor spreading.

Q. 9. Answer the following questions:

(i) Name the southern continent which broke from Pangaea.

Ans. Gondwanaland.

(ii) Centres of volcanic activity.

Ans. Hot Spots.

(iii) Periodic change in the position of magnetic pole.

Ans. Polar wandering.

(iv) The process in which one plate overrides the other and the overridden plate slips down into the mantle and is absorbed.

Ans. Subduction.

Q.10. What process is known as subduction?

Ans. The process of sinking of a plate below another plate along convergent boundaries is called subduction. The crust is destroyed in the process.

SHORT ANSWER TYPE QUESTION

Q.1. Describe the two main types of platea.

Ans. When a plate pushes beneath the plate, it is called converging plate. It is called a subduction zone. This is caused in three ways:

(i) Between a continental and oceanic plate.

(ii) Between two oceanic plates.

(iii) Between two continental plates.

Q.2. What was the location of the Indian landmass during the formation of the Deccan Traps?

Ans. Initially India was a large island situated off the Australian coast, in a vast ocean. The Tethys sea separated it from Asian landmass about 225 million years ago. When the Indian plate was moving towards Asian Plate, somewhere around 60 million years ago outpouring of lava occurred and Deccan Traps were formed. At that time the Indian landmass was still very close to equator.

Q.3. What were the forces suggested by Wegener for the movement of the continents?

Ans. 1. Pole fleeing force. and 

2. Tidal force.

Q.4. How are the convectional currents in the mantle initiated and maintained?

Ans. The convectional currents in the mantle are generated due to radioactive elements causing thermal differences in the mantle portion. The currents move in radial direction and develop in cell patterns.

Q.5. What do you mean by Tillite? Where are these found?

Ans. Tillite are the sedimentary rocks formed out of deposits of glaciers. The Gondwana system of sediments from India is known to have its counter parts in six different land masses of southern hemisphere such as Africa Folkland island Medagaskar Antarctic and Australia. The counter part of the succession are found in Africa Folkland island and Australia. Over all resamblance of the Gondwana type sediments clearly demonstrate that these landmark had remarkably similar histories.

Q.6. Where the Placer deposits are found?

Ans. Placer deposits: The occurence of rich placer deposits of gold in Ghana coast and 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 Ghana are derived from the Brazil Plateau when the two continents by side by side.

Q.7. What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?

Ans. The three types of plates boundaries are:

(i) Divergent boundary: Where the plates pull away from each other.

(ii) Convergent boundary: When the edge of one plate over rides the other.

(iii) Transform boundary: When the plates slide horizontally part each other.

Q.8. What do you understand by Mid-oceanic Ridges?

Ans. Mid-oceanic ridges form 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 characterized 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.

Q.9. What do you mean by Plate Tectonics?

Ans. Plate Tectonic: A plate is a portion of the earth’s lithosphere which moves horizontally over the Asthenosphere as an intrinsically rigid unit. The term tectonics means study of structural feature of the crust and their origin. The lithosphere Asthonosphere layering the earth used in plate tectonics is different from the traditional and familiar crustmantle core layering. The farmer is based on rigidity, the latter on seismicity and the nature of constituents. The lithosphere includes the crust and top month with its thickness range varying between 5-100 km in ocean and about 200 km in the continents.

Q.10. Describe the some minor plates.

Ans. Some important minor plates are as under:

(i) Cacoas Plate: Between Central America and Pacific plate.

(ii) Nazca Plate: Between South America and Pacific plates.

(iii) Arabian plate: Mostly the Soudi Arabian landmasses.

(iv) Philipine plate: Between Asiatic and Pacific plate.

(v) Cardine plate: Between Philipine and Indian plate (North of New Guina).

(vi) Fuji plate: North-east of Australia.

Q.11. What was the condition of India when Pangaea broke? Before that what was the status of India?.

Ans. 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 million years ago causing rapid uplift of the Himalayas.

Q.12. How was Himalaya formed?

Ans. India started her northward journey about 200 million years ago at the time when Pangaea broke. India collided with Asia about 40-50 million years ago causing rapid uplift of the Himalayas.

Q.13. How was Deccan Traps formed?

Ans. During the movement of the Indian plate towards the Asiatic plate, a major event that occurred was the outpouring of lava. It led to formation of the Deccan Traps. This started somewhere around 60 million years ago and continued for a long period of time.

LONG ANSWER TYPE QUESTIONS

Q.1. Explain the Plate Tectonic Theory.

Ans. A tectonic plate is a massive, irregularly-shaped slab of solid rock, generally composed of both oceanic and continental lithosphere. Post-drift, with the advent of the concept of sea-floor spreading, in 1967, Mckenzie and Parker and also Morgan, independently analysed the available ideas and came out with the concept of Plate Tectonics. The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates. These plates have been constantly moving over the globe throughout the history of the earth. As a result, the continental masses, resting on the plates have been wandering all through the geologic periods. The mobile rock beneath the rigid plates is believed to be moving in a circular manner. The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths. This cycle is repeated over and over again generating convection cell or convective flow. The two main sources of heat within the earth comes from: 

(i) radioactive decay. and

(ii) residual heat.

This slow movement of the softened hot mantle drives the movement of plates. There are three types of plate boundaries.

(a) Divergent Boundaries: Here the plates move away from each other and new crust is generated.

(b) Convergenat Boundaries: Here the crust is destroyed as one plate dives under the other.

(c) Transform Boundaries: Here the crust is neither produced nor destroyed as one plate slides horizontally past the another.

Q.2. What are the evidences in support of the continental drift theory?

Ans. Evidences in support of continental drift theory:

1. The matching of continents: The shorelines of Africa and South America facing each other have a remarkable and unmistakable match.

2. Rocks of same age across the oceans: The belt of ancient rocks of 2000 millions years ages from Brazil coast matches with those from western Africa.

3. Tillite: These are the sedimentary rocks formed out of deposits of glaciers. The Gondwana system of sediments from India is known to have its counterparts in six different landmasses Southern Hemisphere. The glacial tillites provide unambiguous evidences of palaeoclimates and also of drifting of continents.

4. Placer deposits: The occurrence of rich placer deposits of gold in Ghana coast and the complete absence of source rock in the region is an amazing fact. The gold bearing veins are in Brazil and it can be inferred that the gold deposits of Ghana were derived from the Brazil Plateau when the two continents lay side by side.

5. Distribution of fossils: The similar species of plants and animals are found in different parts of Gondwanaland.

6. Plate tectonics: The theory of plate tectonics proposes that the earth’s lithosphere is divided into plates and these plates have been moving over the globe throughout the geologic history of the earth.

Q.3. Bring about the basic differences between the drift theory and plate tectonics.

Ans.

Drift Theory	Plate Tectonics
1. Continental drift theory by Wegener assumes all the present continents to have arisen by the breaking and then drifting of the components of the Supercontinent Pangaea.	1. Plate Tectonics assumes the whole earth’s lithosphere (oceans + continents) to be divided into different ‘major’ and ‘minor’ plates which are constantly moving.
2. Continental drift theory only considers the horizontal movement.	2. Plate tectonics takes into account the subduction of plates also.
3. Continental drift theory mostly relies on circumstantial evidences of Jig-Saw-Fit, fossils, place deposits etc.	3. Plate tectonics is based on scientific analysis of the processes inside the earth’s surface.
4. Continental drift theory relies on the concept of plate tectonics to be validated.	4. Plate tectonic theory validates the drift theory and is not to be validated by the same.

Q.4. What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?

Ans. A number of post drift discoveries provided considerable information that was not available at the time when Wegener put forth his concept of continental drift. These discoveries led to a rejuvenated interest in the study of distribution of oceans and continents. Particularly the mapping of ocean floor. Mapping provided new dimensions for the study of distribution of ocean and continents. Discoveries were:

1. The convectional current in mantle.

2. Ocean floor mapping.

3. The mid-oceanic ridge.

4. Dating of rocks.

5. Plate tectonic theory.

Q.5. Briefly describe the continental drift theory.

Ans. Wegener propounded his hypothesis of continental drift in 1912. According to him all the continents were compact and one in the Palaeozoic era. This super and compact continent was called PANGAEA. It was surrounded on all sides by a huge ocean known as Panthalassa. Further down the geologic periods two split parts of Pangaea had a sea in their middle part. This was called Tethys sea by Wegener. Laurasian landmass was situated to the north of Tethys and consisted of North America, Europe and Asia. Gondwanaland was another landmass situated to the south of Tethys and comprised of Australia, Africa, South America, Antarctica and Peninsular India. According to him, the Pangaea broke up in Carboniferous period (Mesozoic Era) and began to drift. The continents had two directions of drift 

(i) Towards the equator.

(ii) Towards the west.

The movement towards the equator is considered to be caused by the relation of the forces of gravitation and buoyancy. The centre of gravity and the centre of buoyancy existed in such a position that the continents began to drift towards the equator. The movement towards west is ascribed to the tidal forces of the sun and the moon affecting the earth.

The drift in these ways caused the redistribution of continents and ocean basins resulting in the present form and shape of the earth. According to Wegener, the drift has not come to a stop but is still continuing.

Initially this theory was not acceptable to many scientists. However now it has been established by the theory of Plate tectonics that continents are moving as plates on a semi-liquid surface. This has led to the support to the theory of continental drift.

Q.6. What are the causes of plate movement?

Ans. 1. Thermal Convection: A. Holmes proposed that the sub-crustal convection currents invoke the mechanism of thermal convection. It acts as driving force for the movement of plates.

2. Movement of Currents: Hot currents rise and then cool as they reach the surface. At the same time cool currents sink down. The convectional movement moves the crustal plate.

3. Floating of Plates: The rigid plates of the lithosphere which floats on more mobile asthenosphere are in constant motion.

4. Hot Spots of Volcanic Activity: Small currents of past volcanic activity are often located far from any active plate boundary suggesting the effect of convection currents on the lithosphere. These are called the Hot Spots.

5. Volcanic Eruptions: The source of magma in the mantle remains fixed in position while the lithospheric plate above it moves steadily. In this way volcanoes are formed over a hot spot but they become extinct. These extinct volcanic form a chain that is a record of plate motion.