NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 4: Distribution of Oceans and Continents

Chapter at a glance

Exercise Questions

Question 1 (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

The Dutch map-maker Abraham Ortelius first proposed this possibility in 1596.

3. Abraham Ortelius

Question 1 (ii)

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

Answer

3. Rotation of the earth

Question 1 (iii)

Which one of the following is not a minor plate?
(a) Nazca (b) Arabia (c) Philippines (d) Antarctica

Answer

The Antarctic plate is one of the seven major plates.

4. Antarctica

Question 1 (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

Distribution of fossils was evidence for continental drift, not for sea-floor spreading.

3. Distribution of fossils in different continents.

Question 1 (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

Answer

4. Continent-continent convergence

Question 2 (i)

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

Answer

Wegener suggested two forces:

1. Pole-fleeing force — related to the earth’s rotation; because the earth bulges at the equator, it pushes continents towards the equator.
2. Tidal force — due to the attraction of the Moon and the Sun, which develops tides in oceanic waters.

Most scholars considered these forces too weak to move continents.

The pole-fleeing force (from earth’s rotation) and the tidal force (from the attraction of the Moon and Sun).

Question 2 (ii)

How are the convectional currents in the mantle initiated and maintained?

Answer

Arthur Holmes (1930s) suggested that radioactive elements in the mantle produce heat. Because of differences in temperature, hot material rises and cooler material sinks, setting up convection currents in the mantle. The continuous supply of heat from radioactive decay maintains these currents, which form a system of convection cells that can move the plates.

Heat from radioactive decay causes temperature differences in the mantle; hot material rises and cooler sinks, and continuous radioactive heating maintains the currents.

Question 2 (iii)

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

Answer

• At divergent boundaries, plates move apart and new crust is created (e.g. Mid-Atlantic Ridge).
• At convergent boundaries, plates move towards each other and crust is destroyed as one plate sinks under another (subduction).
• At transform boundaries, plates slide past each other horizontally; crust is neither created nor destroyed (e.g. San Andreas fault).

At transform boundaries plates slide past each other and crust is neither created nor destroyed, unlike divergent (crust created) and convergent (crust destroyed) boundaries.

Question 2 (iv)

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

Answer

During the formation of the Deccan Traps (about 60 million years ago), the Indian landmass was located near the equator, moving north towards Asia. The lava eruptions continued for a long period as the plate drifted over a hot spot (Réunion).

Near the equator, about 60 million years ago, while the Indian plate was drifting north towards Asia.

Question 3 (i)

What are the evidences in support of the continental drift theory?

Answer

1. Matching of continents (jig-saw fit): the shorelines of Africa and South America facing each other fit remarkably well, especially at the 1,000-fathom line (Bullard, 1964).
2. Rocks of the same age across the oceans: radiometric dating shows that 2,000-million-year-old rock belts of Brazil match those of western Africa; early marine deposits on both coasts are of the Jurassic age.
3. Tillite: glacial deposits (Dwyka tillite) in India, Africa, Falkland Islands, Madagascar, Antarctica and Australia show that these lands had a common glacial history — the Gondwana system.
4. Placer deposits: rich placer gold deposits on the Ghana coast without a source rock nearby; the gold-bearing veins are in Brazil, showing the two were once joined.
5. Distribution of fossils: identical species found on widely separated continents — e.g. Lemurs in India, Madagascar and Africa (leading to the idea of “Lemuria”), and Mesosaurus (a small freshwater reptile) in South Africa and Brazil.

Jig-saw fit of coastlines, rocks of the same age on both sides of the Atlantic, tillite deposits across Gondwana lands, placer gold in Ghana sourced from Brazil, and identical fossils (Lemurs, Mesosaurus) on separated continents.

Question 3 (ii)

Bring about the basic difference between the drift theory and Plate tectonics.

Answer

Drift theory said only continents moved through the ocean floor by weak tidal and pole-fleeing forces; plate tectonics says whole lithospheric plates (continents and ocean floor) move, driven by mantle convection, with plate boundaries explaining earthquakes and mountains.

Question 3 (iii)

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

Answer

1. Convection current theory (Arthur Holmes, 1930s): radioactive heat drives currents in the mantle, providing a force for movement.
2. Mapping of the ocean floor (post-World War II): revealed mid-oceanic ridges, abyssal plains, trenches and seamounts.
3. Palaeomagnetism: rocks on either side of mid-oceanic ridges show alternating stripes of normal and reversed magnetism, symmetrical about the ridge.
4. Age of ocean floor rocks: rocks near the ridges are youngest, becoming older away from them; ocean crust is much younger than continental crust (not older than 200 million years).
5. Distribution of earthquakes and volcanoes along ridges and trenches.
6. Sea-floor spreading (Harry Hess, 1961): new crust forms at ridges and is destroyed at trenches.

These led to the plate tectonics theory.

Holmes’s convection currents, ocean-floor mapping (ridges and trenches), palaeomagnetic stripes, young and symmetrical ocean-floor rock ages, earthquake/volcano belts, and Hess’s sea-floor spreading — leading to plate tectonics.

Topic checklist

• Continental drift theory and its evidence
• Forces for drift; convection currents
• Ocean floor mapping; sea-floor spreading
• Plate tectonics: major and minor plates, plate boundaries, rates of movement
• Movement of the Indian plate

Total questions answered: 12 of 12