Distribution of Oceans and ContinentsClass 11 Fundamentals of Physical Geography NCERT Solutions
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Q11. 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
Solution
(c) Abraham Ortelius
Explanation: According to the source text, "From the known records of the history of science, it was Abraham Ortelius, a Dutch map maker, who first proposed such a possibility as early as 1596."
Q21. Multiple choice questions.
(ii) Polar fleeing force relates to:
(a)
Revolution of the Earth
(b)
Gravitation
(c)
Rotation of the earth
(d)
Tides
Solution
(c) Rotation of the earth
Explanation: The text states, "The polar-fleeing force relates to the rotation of the earth. You are aware of the fact that the earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth."
Q31. Multiple choice questions.
(iii) Which one of the following is not a minor plate?
(a)
Nazca
(b)
Arabia
(c)
Philippines
(d)
Antarctica
Solution
(d) Antarctica
Explanation: The chapter lists Antarctica as one of the seven major plates: "Antarctica and the surrounding oceanic plate". Nazca, Arabia, and Philippines are listed under the important minor plates.
Q41. Multiple choice questions.
(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.
Solution
(c) Distribution of fossils in different continents.
Explanation: The distribution of fossils was a key piece of evidence used by Alfred Wegener to support his theory of Continental Drift. The concept of sea floor spreading was developed later based on evidence from ocean floor mapping, such as volcanic activity at ridges, magnetic striping, and the age of oceanic rocks.
Q51. Multiple choice questions.
(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
Solution
(d) Continent-continent convergence
Explanation: The text explicitly states, "The subduction zone along the Himalayas forms the northern plate boundary in the form of continent-continent convergence."
Q12. Answer the following questions in about 30 words.
(i) What were the forces suggested by Wegener for the movement of the continents?
Solution
Alfred Wegener suggested two forces responsible for continental drift. The first was the pole-fleeing force, which he related to the rotation of the Earth causing a bulge at the equator. The second was the tidal force, caused by the gravitational attraction of the sun and the moon.
Q22. Answer the following questions in about 30 words.
(ii) How are the convectional currents in the mantle initiated and maintained?
Solution
Convectional currents in the mantle are initiated by thermal differences caused by radioactive elements within the Earth's interior. Hot, less dense material from the mantle rises, spreads, cools, and then sinks back down. This continuous circular flow, or convection cell, is maintained by the constant heat from radioactive decay and residual heat.
Q32. Answer the following questions in about 30 words.
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Solution
The major difference is that at transform boundaries, the Earth's crust is neither created nor destroyed; the plates simply slide horizontally past each other. In contrast, at divergent boundaries new crust is generated, and at convergent boundaries, the crust is destroyed as one plate subducts under another.
Q42. Answer the following questions in about 30 words.
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
Solution
During the formation of the Deccan Traps, which began around 60 million years ago, the Indian subcontinent was located close to the equator. This was during its long northward journey after breaking away from Gondwanaland and before its collision with the Eurasian plate.
Q13. Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?
Solution
Alfred Wegener provided several pieces of evidence to support his theory of continental drift. These are:
- The Matching of Continents (Jig-Saw-Fit): The coastlines of Africa and South America show a remarkable and unmistakable match, suggesting they were once joined. This fit is even more perfect when matched at the continental shelf margin.
- Rocks of Same Age Across the Oceans: Radiometric dating has shown that ancient rock formations in Brazil match perfectly with those in Western Africa, indicating they were once part of a continuous belt.
- Tillite: Glacial sedimentary deposits, known as tillite, from the Gondwana system in India have counterparts in Africa, South America, Madagascar, Antarctica, and Australia. This suggests these landmasses experienced the same prolonged glaciation when joined together.
- Placer Deposits: The presence of rich gold deposits on the Ghana coast, despite the absence of source rock, is explained by the fact that the gold-bearing veins are in Brazil. These deposits would have formed when the two continents were adjacent.
- Distribution of Fossils: The fossils of certain land-based or freshwater species, like Mesosaurus and Lemurs, are found in continents now separated by vast oceans. This distribution is best explained by the existence of a contiguous landmass in the past.
Q23. Answer the following questions in about 150 words.
(ii) Bring about the basic difference between the drift theory and Plate tectonics.
Solution
The basic differences between Alfred Wegener's Continental Drift Theory and the modern theory of Plate Tectonics are fundamental in terms of both mechanism and scope.
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What Moves: Wegener's theory proposed that the continents (sial) moved or plowed through the oceanic crust (sima). In contrast, the theory of Plate Tectonics states that it is not just the continents that move, but the entire lithospheric plates. These massive plates, which can be composed of both continental and oceanic crust, move as rigid units over the semi-molten asthenosphere.
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Driving Force: Wegener suggested that the forces driving the drift were the 'pole-fleeing force' (due to Earth's rotation) and 'tidal force' (from the sun and moon). These forces were widely rejected by the scientific community as being far too weak. Plate Tectonics, on the other hand, is driven by powerful convection currents within the Earth's mantle. Heat from the core causes mantle material to rise, spread, and sink, creating a mechanism powerful enough to move the massive lithospheric plates.
In essence, Continental Drift was a pioneering idea that identified that continents move, while Plate Tectonics is a more comprehensive and scientifically supported theory that explains how and why they move.
Q33. Answer the following questions in about 150 words.
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Solution
After Wegener's theory was initially dismissed, several major discoveries, particularly in the post-World War II era, revived scientific interest in the mobility of continents and oceans. These discoveries provided a new evidence base that led to the development of Plate Tectonics.
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Mapping of the Ocean Floor: Detailed expeditions revealed that the ocean floor was not a flat, featureless plain. Scientists discovered vast underwater mountain ranges called mid-oceanic ridges, deep trenches near continental margins, and a rugged topography. This indicated a geologically active ocean floor.
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Convectional Current Theory: In the 1930s, Arthur Holmes proposed that convection currents in the mantle could be the driving force for continental movement. This provided a plausible mechanism that Wegener's theory had lacked.
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Evidence from Mid-Oceanic Ridges: It was found that mid-oceanic ridges were sites of intense volcanic activity and shallow earthquakes. Furthermore, analysis of oceanic crust revealed that rocks were youngest at the crest of the ridges and progressively older further away. Rocks equidistant on either side of a ridge showed remarkable similarities in age, chemical composition, and magnetic properties (palaeomagnetism), which suggested new crust was being formed at the ridges and pushing older crust aside.
These findings led to the concept of Sea Floor Spreading, proposed by Harry Hess in 1961, which became the cornerstone of the unifying theory of Plate Tectonics.