Unlock a deeper understanding of Earth’s evolving surface with our expertly crafted resource featuring Class 11 Geography Chapter-4: Distribution of Oceans and Continents Solutions. Designed to help you navigate complex geological inquiries with ease, this guide provides structured, precise answers to all textbook exercises. Whether you are aiming to ace your school board assessments or strengthen your conceptual foundation for competitive exams, these solutions ensure complete clarity on critical topics like Wegener’s Continental Drift, sea floor mapping, and the mechanics of tectonic plate movements.
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
Answer: (c) Abraham Ortelius
- Reason: Abraham Ortelius, a Dutch mapmaker, first proposed this physical possibility in 1596.
(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
- Reason: Earth’s rotation creates a polar-fleeing force that causes an equatorial bulge.
(iii) Which one of the following is not a minor plate?
- (a) Nazca
- (b) Arabia
- (c) Philippines
- (d) Antarctica
Answer: (d) Antarctica
- Reason: Antarctica (along with its surrounding oceanic plate) is classified as one of the seven major tectonic plates.
(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.
- Reason: Fossil distribution supported Wegener’s earlier Continental Drift Theory. Harry Hess based Sea Floor Spreading on ocean floor mapping, rock ages, ridge volcanism, and magnetic reversal patterns.
(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: (d) Continent-continent convergence
- Reason: The northern boundary of the Indian plate involves a subduction and collision zone between the Indian and Eurasian continental landmasses.
Answer the following questions in about 30 words.
(i) What were the forces suggested by Wegener for the movement of the continents?
Wegener proposed two primary forces for continental drift:
- Polar-fleeing force: Relates to Earth’s rotation and its equatorial bulge.
- Tidal force: Driven by the gravitational attraction of the Sun and Moon.
(ii) How are the convectional currents in the mantle initiated and maintained?
Convection currents are initiated by thermal differences within the mantle. They are continuously maintained by heat originating from two sources:
- Ongoing radioactive decay of elements.
- Primal residual heat from Earth’s core.
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
At transform boundaries, lithospheric crust is neither created nor destroyed as plates slide horizontally past each other. Conversely, convergent boundaries destroy crust through subduction, and divergent boundaries construct new oceanic crust.
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
During the Deccan Traps volcanic outpourings (~60 million years ago), the Indian landmass was located close to the equator as it drifted northward toward the Eurasian plate.
Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?
Alfred Wegener offered five major categories of evidence to support his 1912 Continental Drift Theory:
- Jig-Saw-Fit of Coastlines: The opposing coastlines of South America and Africa show a striking physical match. In 1964, Bullard confirmed this fit using a computer program along the 1,000-fathom depth line.
- Ancient Rock Belts: Radiometric dating correlates 2,000-million-year-old rock formations from coastal Brazil directly with western Africa. Additionally, marine deposits along both coastlines date only to the Jurassic age.
- Glacial Tillite: Basal glacial tillite deposits in India’s Gondwana system match identical formations in Africa, Madagascar, Antarctica, Australia, and the Falkland Islands, proving shared palaeoclimatic histories.
- Placer Deposits: Ghana’s coast features rich gold placer deposits despite completely lacking local gold-bearing source rocks. These deposits derived directly from Brazil’s gold veins when both continents were united.
- Fossil Distribution: Identical land species exist across deep ocean barriers. Lemur fossils link India, Madagascar, and Africa (Lemuria), while the freshwater reptile Mesosaurus is found exclusively in South Africa and Brazil.
(ii) Bring about the basic difference between the drift theory and Plate tectonics.
Continental Drift Theory and Plate Tectonics differ fundamentally in their proposed mechanisms, moving units, and crustal dynamics:
| Parameter | Continental Drift Theory | Plate Tectonics Theory |
| Proposer & Timeline | Alfred Wegener (1912) | McKenzie, Parker, and Morgan (1967) |
| Basic Premise | Continents cut through static ocean basins independently. | Rigid lithospheric plates carrying continents and oceans move over the asthenosphere. |
| Driving Force | Polar-fleeing force (rotation) and tidal force (Moon/Sun attraction). [Rejected] | Thermal convection currents in the mantle driven by radioactive decay and residual heat. |
| Ocean Floor Role | Passive background; assumed to be fixed and permanent. | Dynamic boundary zones where crust is continuously generated (ridges) and consumed (trenches). |
| Supercontinent Concept | Pangaea was the original, static starting landmass of Earth. | Pangaea was a temporary convergent assembly of dynamic moving plates. |
While Wegener focused purely on drifting continental blocks, Plate Tectonics provides a unified global model explaining ocean creation, destruction, and mountain building.
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Post-World War II oceanographic explorations and geological studies uncovered critical evidence that transformed theories on global land distribution:
- Convectional Current Theory: In the 1930s, Arthur Holmes proposed thermal convection cells in the mantle driven by radioactive heat, offering the missing physical force for crustal movement.
- Ocean Floor Mapping: Detailed relief surveys revealed complex submerged topography, including continuous mid-oceanic ridges and deep oceanic trenches near continental margins.
- Submarine Volcanism: Mid-oceanic ridges were identified as highly active volcanic zones continuously extruding fresh basaltic lava.
- Young Age of Oceanic Crust: Radiometric dating proved ocean floor rocks are nowhere older than 200 million years, whereas continental rocks date up to 3,200 million years.
- Symmetrical Magnetic Stripes: Rocks equidistant on either side of ridge crests match perfectly in age, composition, and magnetic polarity reversals.
- Thin Oceanic Sediments: Ocean sediment columns were unexpectedly thin, proving ocean basins are geologically young and constantly renewing.