Distribution of Oceans - Solutions

 CBSE Class 11 Geography

NCERT Solutions
Chapter 4
Distribution of Oceans and Continents

1. 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) Abraham Ortelius ( He was a Dutch map maker,the possibility was considered  in 1596) 

(ii) Polar fleeing force relates to:

(a) Revolution of the Earth

(b) Gravitation

(c) Rotation of the earth

(d) Tides.

Ans. (c) Rotation of the earth.( Polar fleeing force is a geophysical concept invoked in 1915 by Alfred Werner to explain his idea of continental drift.) 

(iii) Which one of the following is not a minor plate?

(a) Nazca

(b) Arabia

(c) Philippines

(d) Antarctica.

Ans. (d) Antarctica.( It's a tectonic plate containing the continent Antarctica and extending outward under the surrounding oceans) 

(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.

Ans. (c) Distribution of fossils in different continents. (Seafloor spreading is a process that occursyat mid-ocean ridges, where new oceanic crust is formed through volcanic activity.) 

(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) Continent-continent convergence.( During collisions between two continental plates, large mountain ranges such as Himalayas are formed.)  

2. 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 that the movement responsible for the drifting of the continents was caused by two forces. A) Pole-fleeing force and B) Tidal force. (A)  The polar-fleeing force relates to the rotation of the earth. The earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth. Wegener believed that Pangaea originated near the South Pole and that the centrifugal force of the planet caused the protocontinent to break apart and the resultant continents to drift towards the Equator. He called this the Pole-fleeing force. 
(B) The Tidal force is a force that is the secondary effect of the force of gravity; it is responsible for the phenomenon of tides. It is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed that these forces would become effective when applied over many million years.

(ii) How are the convectional currents in the mantle initiated and maintained?

Ans. Convection currents in the Earth's mantle are caused by the rise of hot material rising towards the crust, becoming cooler and sinking back down. Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion. These currents are generated due to radioactive elements causing thermal differences in the mantle portion. According to Holmes, there exists a system of such currents in the entire mantle portion. This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory. Convection currents are established within the Earth, which bring warm material to the surface and  send cooler material to the interior.

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

Ans. The major difference between the transform boundary and the convergent or divergent boundaries of plates are as follows:
(a) Transform Boundaries: Where the crust is neither produced nor destroyed as the plates slide horizontally past each other. In this type of boundary, the lithosphere is neither created nor destroyed. Example of this type of boundary is California's San Andreas Fault. 
(b) Convergent Boundaries:  Where the crust the is destroyed as one plate dived under another, it is called convergent boundaries. It is also known as destructive plate boundary. The Washington-Oregon coastline of the United States is an example of this type of convergent plate boundary. 
(c) Divergent Boundaries:  Where new crust is generated as the plates pull away from each other, these are called divergent boundaries. It is also known as a constructive boundary of an extensional boundary. Examples of this are Mid-Atlantic Ridge and the Great Rift Valley.  

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

Ans. About 140 million years before the present, the subcontinent was located as south as 50°8' latitude. During the movement of the Indian plate towards the Asiatic plate, a major event that occurred was the outpouring of lava and formation of the Deccan Traps. This started somewhere around 60 million years ago and continued for a long period of time. Note that the subcontinent was still close to the equator. From 40 million years and thereafter, the event of formation of the Himalayas took place. Scientists believe that the process is still continuing and the height of the Himalayas is rising even to this date.Hence, the Indian landmass was located South of the Equator during the formation of the Deccan Trap. 

3. Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?

Ans. A variety of evidence was offered in support of the continental drift. Some of these are given below:
(a) The Matching of Continents (Jig-saw-Fit): The shorelines of Africa and South America facing each other have a remarkable and unmistakable match. The East Coast of South America and the west coast of Africa look as though they would fit together like the pieces of a jigsaw puzzle.  With a bit of rearranging most of the continents can be put together too. 
(b) Rocks of Same Age Across the Oceans: Wegener discovered that identical rocks could be found on both sides of the Atlantic Ocean. These rocks were the same type and same age. He understood that the rocks had formed side-by-side and that the land has since moved apart. The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa. The earliest marine deposits along the coastline of South America and Africa are to the Jurassic age. This suggests that the ocean did not exist prior to that time.
(c) Tillite: It is the sedimentary rock formed out of deposits of glaciers. The Gondawana system of sediments from India is known to have its counter parts in six different landmass of  the Southern Hemisphere. The best system has thick tillite indicating extensive and  prolonged glaciation. Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India. Overall resemblance of Gondawana type sediments clearly demonstrates that these landmass had remarkably similar histories. The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.
(d) Placer Deposits: A placer deposit or placer is the accumulation of valuable minerals formed by gravity separation during sedimentary processes. The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact. The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.
(e) Distribution of Fossils: Fossils are the preserved traces of animals or plants found inside rocks. They are useful in dating geological material, because they indicate which species were alive at the time the rock was formed.  The observations that occur in India, Madagascar and Africa led some to consider a contiguous landmass "Lemuria" linking these three landmasses. Mesosaurus was a small reptile adapted to shallow brackish water. The skeletons of these are found only in two localities : the Southern Cape province of South Africa and Iraver formations of Brazil. The two localities presently are 4,800 km apart with an ocean in between them.

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

Ans. Drift theory: According to the theory of continental drift, the world was made up of a single continent through most of the geologic time.  Alfred Wegener a German meteorologist put forth "the continental drift theory". According to him, all continents formed a single continental mass called PANGAEA. All oceans formed a single universal ocean called PANTHALASSA. He argued that, around 200 million years ago, the super continent, Pangaea, began to split. Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively. Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.
Plate Tectonics: According to the generally accepted plate tectonics theory, scientists believe that the Earth's surface is broken into a number of shifting slabs of plates, which average about 50 miles in thickness. This theory emerged in 1967, by McKenzie and Parker and also Morgan. A tectonic plate is also called lithospheric plate. It is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units. The lithosphere includes the crust and top mantle with its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas. Pacific plate is largely an oceanic plate whereas the Eurasian plate may be called a continental plate. The theory of plate tectonics proposes that the earth's lithosphere is divided into seven major and some minor plates.

(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 discoveries during the post-drift period added new information to geological literature. These informations were not available at the time Wegener put forth his concept of continental drift. These discoveries led to the rejuvenated interest in the study of distributions of ocean and continents. Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.
(a) These currents are generated due to radioactive elements causing thermal differences in the mantle portion. Holmes argued that there exists a system of such currents in the entire mantle portion. This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.
(b) Detailed research of the ocean configuration revealed that the ocean floor is not just a vast plain but it is full of relief.
(c) Expeditions to map the oceanic floor in the post-war period provided a detailed picture of the ocean relief and indicated the existence of submerged mountain ranges as well as deep trenches, mostly located closer to the continent margins.
(d) The mid-oceanic ridges were found to be most active in terms of volcanic eruptions. The dating of the rocks from the oceanic crust revealed the fact that they are much younger than the continental areas.
(e) Rocks on either side of the crest of oceanic ridges and having equi-distant locations from the crest were found to have remarkable similarities both in terms of their constituents and their age.