Distribution of Oceans and Continents
NCERT Solutions • Class 11 Geography • Chapter 41. 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?
(c) Abraham Ortelius
Reason: While Alfred Wegener formulated the Continental Drift Theory in 1912, Abraham Ortelius, a Dutch map maker, was the first to propose this possibility as early as 1596 based on the symmetry of coastlines.
Reason: While Alfred Wegener formulated the Continental Drift Theory in 1912, Abraham Ortelius, a Dutch map maker, was the first to propose this possibility as early as 1596 based on the symmetry of coastlines.
(ii) Polar fleeing force relates to:
(c) Rotation of the earth
Reason: The “Polar Fleeing Force” (Polflucht) suggested by Wegener is caused by the centrifugal force generated by the rotation of the earth, which bulges the earth at the equator.
Reason: The “Polar Fleeing Force” (Polflucht) suggested by Wegener is caused by the centrifugal force generated by the rotation of the earth, which bulges the earth at the equator.
(iii) Which one of the following is not a minor plate?
(d) Antarctica
Reason: The Antarctic plate is one of the seven major tectonic plates. Nazca, Philippines, and Arabia are minor plates.
Reason: The Antarctic plate is one of the seven major tectonic plates. Nazca, Philippines, and Arabia are minor plates.
(iv) Which one of the following facts was not considered by those while discussing the concept of sea floor spreading?
(c) Distribution of fossils in different continents
Reason: Fossil distribution was a key evidence for the older Continental Drift Theory. Sea Floor Spreading relied on newer data like volcanic activity at ridges, magnetic field stripes, and the age of ocean rocks.
Reason: Fossil distribution was a key evidence for the older Continental Drift Theory. Sea Floor Spreading relied on newer data like volcanic activity at ridges, magnetic field stripes, and the age of ocean rocks.
(v) Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains?
(d) Continent-continent convergence
Reason: The Himalayas formed due to the collision between the Indian Plate (Continental) and the Eurasian Plate (Continental).
Reason: The Himalayas formed due to the collision between the Indian Plate (Continental) and the Eurasian Plate (Continental).
2. Short Answer Questions
(i) What were the forces suggested by Wegener for the movement of the continents?
Wegener suggested two forces for Continental Drift:
- Pole-fleeing Force: Caused by the rotation of the earth (centrifugal force).
- Tidal Force: Caused by the gravitational attraction of the sun and the moon, which generates tides in oceanic waters.
(ii) How are the convectional currents in the mantle initiated and maintained?
Convection currents are initiated by the difference in temperature within the mantle. These thermal differences arise due to radioactive decay of elements and residual heat. The heated material rises to the surface, spreads, cools, and sinks back, creating a continuous cycle of currents (Arthur Holmes, 1930).
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
[Image of plate boundaries types convergent divergent transform]
The major difference is in the creation or destruction of crust:
The major difference is in the creation or destruction of crust:
- Transform Boundary: Plates slide horizontally past each other. Crust is neither created nor destroyed.
- Convergent/Divergent: In Convergent, crust is destroyed (subduction). In Divergent, new crust is created (spreading).
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
During the formation of the Deccan Traps (approx. 60 million years ago), the Indian landmass was located in the southern hemisphere, moving northward. It was passing over a stationary hot spot near the present-day Reunion Island in the Indian Ocean, causing massive volcanic eruptions.
During the formation of the Deccan Traps (approx. 60 million years ago), the Indian landmass was located in the southern hemisphere, moving northward. It was passing over a stationary hot spot near the present-day Reunion Island in the Indian Ocean, causing massive volcanic eruptions.
3. Long Answer Questions
(i) What are the evidences in support of the continental drift theory?
Alfred Wegener provided varied evidence to support that continents were once joined in a supercontinent (Pangaea):
- The Matching of Continents (Jigsaw Fit): The shorelines of Africa and South America fit together perfectly.
- Rocks of Same Age across Oceans: An ancient belt of rocks (2,000 million years old) in Brazil matches perfectly with those in Western Africa.
- Tillite: Glacial deposits found in India, Africa, Falkland Island, Madagascar, Antarctica, and Australia indicate these lands were once covered by the same ice sheet near the South Pole (Gondwanaland).
- Placer Deposits: Gold deposits are found in Ghana, but the source rocks (gold veins) are in Brazil, suggesting they were once adjacent.
- Distribution of Fossils: Fossils of Mesosaurus (a reptile) and Glossopteris (a fern) are found across continents separated by oceans today, implying land bridges existed.
(ii) Bring about the basic difference between the drift theory and Plate tectonics.
| Continental Drift Theory (Wegener) | Plate Tectonics Theory (McKenzie, Parker, Morgan) |
|---|---|
| Believed that only Continents move. | Believes that Lithospheric Plates move (which include both continent and ocean floor). |
| Proposed that continents plough through the ocean floor like a ship. | Proposed that continents are embedded in the plates and move with the ocean floor. |
| Mechanism: Relied on weak forces like Tidal and Polar-fleeing force. | Mechanism: Relies on strong Convection Currents in the mantle (Sea Floor Spreading). |
| Could not explain why the movement occurred effectively. | Scientifically explains the movement, earthquakes, and volcanism along plate margins. |
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
After World War II, ocean mapping revealed facts that Wegener’s theory couldn’t explain, leading to the Sea Floor Spreading theory:
- Mapping of Ocean Floor: Sonar mapping revealed that the ocean floor is not a flat plain but full of mountain ranges (Mid-Oceanic Ridges) and deep trenches.
- Volcanic Activity: Active volcanoes were found along the mid-oceanic ridges, indicating magma upwelling.
- Age of Rocks: Ocean crust rocks were found to be much younger (max 200 million years) than continental rocks (over 3,200 million years), suggesting the ocean floor is constantly being created.
- Paleomagnetism: Studies showed magnetic stripes on the ocean floor parallel to ridges, with alternating polarity. This proved that new floor is created at ridges and spreads outwards.
Project Work
Topic: Collage on Earthquake Damages
(Guidance for Student Activity)
To prepare a collage:
To prepare a collage:
- Collect newspaper clippings or images from the internet of recent major earthquakes (e.g., Turkey-Syria 2023, Nepal 2015, Japan 2011).
- Categorize images:
– Structural Damage: Collapsed buildings, bridges, roads.
– Human Impact: Rescue operations, displacement camps.
– Secondary Disasters: Tsunamis, landslides, fires caused by the quake. - Arrange them artistically on a chart paper with captions indicating the location and magnitude of the quake.