NCERT Solutions - Distribution of Oceans and Continents

Q1: 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) Edmond Hess
Ans: (c) 
Explanation: Abraham Ortelius in the 16th century noted the complementary coastlines of Africa and South America and suggested they were once joined, predating Wegener's formal theory.

(ii) Polar fleeing force relates to:
(a) Revolution of the Earth
(b) Gravitation
(c) Rotation of the earth
(d) Tides
Ans: (c) 
Explanation: The polar-fleeing force is essentially a centrifugal effect arising from Earth's rotation, which Wegener suggested could help move continental masses very slowly over time.

(iii) Which one of the following is not a minor plate?
(a) Nazca 
(b) Arabia
(c) Philippines
(d) Antarctica
Ans: (d) 
Explanation: The Antarctic plate is a major plate; Nazca, Arabia and the Philippine are classified as smaller (minor) plates.

(iv) Which one of the following facts was not considered by those whilediscussing 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.
Ans: (c) 
Explanation: Sea-floor spreading evidence is based on oceanic features (volcanism, magnetic striping and rock ages); distribution of fossils across continents relates more directly to continental drift than to sea-floor spreading.

(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
Ans: (d) 
Explanation: The Himalayas were formed by collision between the Indian continental plate and the Eurasian continental plate, a continent-continent convergence.

Q2: Answer the following questions in about 30 words.

(i) What were the forces suggested by Wegener for the movement of the continents?
Ans: Wegener suggested two main forces: the pole-fleeing (centrifugal) force due to Earth's rotation and tidal forces from the Moon and Sun. He believed these small but persistent forces acting over millions of years could cause continental drift.

(ii) How are the convectional currents in the mantle initiated and maintained?
Ans: Heat from Earth's interior (radioactive decay and residual heat) warms mantle material. Hot, less-dense material rises, cools near the lithosphere and then sinks, creating continuous convection cells that help drive plate motion.

(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Ans: The difference:

• Along divergent or convergent boundaries, crust is either produced (at ridges) or destroyed (at trenches).
• Along transform boundaries, plates slide past each other horizontally, so crust is neither created nor destroyed, though seismic activity is common.

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

Ans: The Deccan Traps erupted about 60 million years ago when the Indian landmass lay well south of the equator, moving northwards toward the Asiatic plate during that period.
Q3: Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?
Ans: The evidences in support of the continental drift theory are:

• The Matching of Continents (Jig-Saw-Fit): The coastlines of Africa and South America fit together closely, suggesting these landmasses were once joined.
• Rocks of Same Age Across the Oceans: Similar aged rock belts (for example, 2,000 million-year old rocks) and comparable marine deposits on facing coasts indicate prior contiguity of those landmasses.
• Tillite: Widespread glacial deposits (tillite) of the Gondwana system occur across southern continents (India, Africa, South America, Antarctica, Australia), implying these were once united near polar regions.
• Placer Deposits: The presence of rich placer gold on the Ghana coast, without local source rocks, aligns with matching gold-bearing sources in Brazil when the continents were adjacent.
• Distribution of Fossils: Identical terrestrial or freshwater fossils (for example of certain plants or animals) found on now distant continents indicate former land connections that allowed their spread.

(ii) Bring about the basic difference between the drift theory and Plate tectonics.
Ans:

• Drift theory: Proposed by Alfred Wegener, it emphasises that continents were once joined (Pangaea) and later drifted apart. Wegener assembled observational evidence (fit of continents, fossils, rocks) but lacked a convincing mechanism for how continents moved.
• Plate Tectonics: Developed later, this theory describes the lithosphere as broken into plates (major and minor) that move over the asthenosphere. It incorporates sea-floor spreading, subduction and mantle convection as mechanisms, thus providing the physical processes that Wegener's hypothesis lacked.

(iii) 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 during the post-war period added new information to geological literature. Particularly, detailed mapping of the ocean floor produced evidence that helped explain how continents and oceans evolve and move.

• Volcanic eruptions are common along mid-oceanic ridges, bringing large volumes of lava to the seafloor.
• Rocks equidistant on either side of ridge crests show symmetric patterns in age, chemistry and magnetic polarity; those nearest the ridge are youngest and often show normal polarity.
• Ocean-crust rocks are much younger than continental rocks; seafloor is generally no older than about 200 million years.
• Sediment cover on the ocean floor is thinner than expected. Deep trenches are associated with deep-focus earthquakes, while mid-ocean ridges have shallow earthquake foci.

These observations-particularly magnetic striping, systematic rock ages across ridges, and patterns of earthquakes-supported the ideas of sea-floor spreading and ultimately the theory of plate tectonics.