4. Distribution of oceans and continents

 NCERT Questions:

1. Multiple choice questions.

(i)  Who among 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) Edmund Hess

Ans. c) Abraham Ortelius

 (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

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

a) Nazca

b) Arabia

c) Philippines

d) Antarctica

Ans.d) Antarctica

 (iv) Which one of the following facts was not considered by those while discussing the concept of seafloor spreading?

a) Volcanic activity along the mid oceanic ridges

b) Strips 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.b) Strips of normal and reverse magnetic field observed in rocks of ocean floor

 (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

2. Answer the following questions in about 30 words:

(i) What were the forces suggested by Wegener for the movement of the continents?

Ans. The forces suggested by Wegener for the movement of the continents were polar fleeing force and 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.

b) The tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegner believed that these forces would become effective when applied for many million years.

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

Ans.

a) The convection currents are generated due to the radioactive elements causing thermal differences in the mental portion.

b) It is the motion of molten rock in the mantle as radioactive decay heats up magma, causing it to rise and driving the global scale flow of magma.

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

Ans.

a) The sites where the plates move away from each other are called spreading sites or divergent boundaries.

b) These are also named as constructive plate boundaries, example Mid Atlantic Ridge.

c) Where the crust is destroyed as one plate dived under another (subduction zone) are convergent plate boundaries.

d) There are three ways in which convergence can occur. These are:

i) Between an oceanic and continental plate

ii) Between two oceanic plates

iii) Between two continental plates

e) Where the crust is neither produced nor destroyed as the plates slide horizontally passed each other are transform faults. These are generally perpendicular to the mid oceanic ridges.

f) As the eruptions do not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth.

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

Ans.

a) During the movement of the Indian plate towards the Eurasian plate, a major event that occurred was the outpouring of lava and formation of the Deccan traps.

b) This started somewhere around 60 million years ago and continued for a long period of time.

c) At that time, the subcontinent was still close to the equator.

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 evidences were offered in support of the continental drift. Some of these are:

a) The shorelines jig-saw fit of Africa and South America facing each other has a remarkable and unmistakable match.

b) The radiometric dating methods (rock of same age across the oceans) developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean.

c) The earliest marine deposits along the coastline of South America and Africa are of the Jurassic Age.

d) At the base, the system has thick tillite indicating extensive and prolonged glaciation.

e) Counterparts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia.

f) The Gondwana system of sediments from India is known to have its counterparts in six different land masses of the southern hemisphere in Africa, Falkland Islands, Madagascar, Antarctica and Australia.

g) The belt of ancient rock of 2000 million years from Brazil coast matches with those from western Africa.

h) The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from Brazil Plateau when the two continents lay side by side.

i) The observation that lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmark ‘Lemuria’ linking these three landmasses.

j) Mesosaurus was a small reptile adapted to shallow brackish water.

k) 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 are presently 4800 kilometre apart with an ocean in between them.

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

Ans.

(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) In the continental drift theory, most of the evidences were collected from the continental areas in the form of distribution of flora and fauna or deposits like tillite.

b) Post World War II added new information to geological literature; particularly the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.

c) Arthur Holmes (1990 to 1965) in 1930s discussed the possibility of convection currents operating in the mantle portion due to radioactive elements causing thermal differences in the mental portion.

d) Expeditions to map the ocean floor in the post-World War II provided a detailed picture of the ocean floor relief and indicated the existence of submerged mountain ranges as well as deep trenches mostly located closer to the continent margins.

e) The mid-oceanic ridges were found to be most active in terms of volcanic eruptions.

f) The dating of the rocks from the oceanic crust revealed the fact that they are much younger than the continental areas.

g) The rocks on either side of the crest of oceanic ridges and having equidistant locations from the crest were found to have remarkable similarities, both in terms of their constituents and their age.

Question Bank:

1.The earliest marine deposits along the coastline of the South America and Africa are of:

a) Triassic age

b) Permian age

c) Eocene age

d) Jurassic age

Ans. d) Jurassic age

Before the Jurassic age, the ocean did not exist along the South America and Africa.

2. The mid oceanic ridges were found to be most active in terms of:

a) Volcanic eruption

b) Earthquake

c) Tides

d) Tsunami

Ans. a) Volcanic eruption

At the mid oceanic ridges along the divergent plate boundaries, where new ocean floor is created as the Earth’s tectonic plates diverge. As the plates diverge, molten rock rises to the seafloor producing enormous volcanic eruptions of basalt.

3. The longest mountain chain, on the earth crust, submerged under ocean water is:

a) Continental margin

b) Mid – Oceanic ridges

c) Continental shelf

d) Abyssal plain

Ans. b) Mid Oceanic ridge

Mid oceanic ridges is the submarine range, shaped by plate tectonics. It occurs when convectional currents rise in the mantle below the oceanic crust and create magma where two tectonic plates move at divergent boundary.

4. The area where the continental sediments that move beyond the margins get deposited is called:

a) Continental margin

b) Mid- oceanic ridges

c) Continental shelf

d) Abyssal plain

Ans. d) Abyssal plain

An abyssal plain consists of bed of volcanic rocks under the sediments, up to thousands of feet thick. The sediments wash off the continents and are carried to the depths by dense currents. With time, the sediments spread out to provide a smooth level surface.

5. The tectonic plate which lies between the Asiatic plate and Pacific plate is:

a) Cocos plate

b) Nazca plate

c) Arabian plate

d) Philippine plate

Ans. d) Philippine plate

The Philippine plate is an oceanic plate between the Pacific plate to the east and Asiatic plate to the west.

6. The ‘Rim of Pacific’ is also called the ring of fire due to the existence of:

a) Active volcanoes

b) Warm ocean currents

c) Transform boundary

d) Coastal areas

Ans. a) Active volcanoes

The ‘Rim of Pacific’ is positioned at the boundaries of the Pacific plate and other minor tectonic plates. The Pacific plate is colliding with the sliding underneath the other plates around it. This causes the release of the tremendous amount of energy, which can easily melt a rock into magma. It erupts on the surface as lava and forms volcanoes.

7. The slowest rate of plate movement is of:

a) East Pacific

b) Arctic Ridge

c) South Pacific Region

d) Mid Oceanic Ridge

Ans. b) Arctic Ridge

The Arctic ridge has the slowest rate (less than 2.5 cm/yr.) and the East Pacific Ridge near Eastern Island, in South Pacific about 3,400 Km west of Chile, has the fastest rate (more than 15 cm/ yr).

8. Which one is the major plate among the following?

a) Nazca plate

b) Arabian plate

c) Philippine plate

d) Antarctica plate

Ans. d) Antarctica plate

The theory of plate tectonic proposes that the earth’s lithosphere is divided into seven major and some minor plates. Young fold Mountain ridges, trenches or faults surround these major plates. Antarctica plate is a major plate whereas the rest of them are important minor plates.

9. Along the convergent margin the crust is:

a) Added

b) Destroyed

c) Unchanged

d) Slipped

Ans. b) Destroyed

In convergent boundaries the crust is destroyed as one plate dives under another. The location where the sinking of a plate occurs is called subduction zone.

10. Alfred Wegener gave the theory of continental drift in the year:

a) 1920

b) 1919

c) 1912

d) 1925

Ans. c) 1912

He was a German meteorologist. He proposed continental drift theory in 1912.

11. About 140 million years before the present Indian Sub-continent was located at:

a) 60˚ South latitude

b) 30˚ South latitude

c) 50˚ South latitude

d) 40˚ South latitude

Ans. 50˚South latitude

India was a large island situated at 50˚ south latitude, off the Australian coast.

12. The ‘Rim of Pacific’ is also called:

a) Ring of fire

b) Ring of waves

c) Ring of ridges

d) Ring of slopes

Ans. a) Ring of Fire

The ‘ring of Pacific’ is also called ‘Ring of Fire’.

13. Tillite is formed due to:

a) Deposits on volcanoes

b) Deposits on glaciers

c) Deposits on gold

d) Deposits on mountains

Ans. b) Deposits of glaciers

It is a sedimentary rock formed because of the glaciers deposits.

14. The radiometric dating method refers to:

a) Analysing the rock formation

b) Correlating the rock formation of different continents

c) Correlating the different continents of the world

d) Correlating different oceans of the world

Ans. b) Correlating the rock formation of different continents

The radiometric dating methods developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean.

15. Jigsaw fit means:

a) Continents and oceans

b) Only continents

c) Matching of continents

d) Matching of oceans

Ans. c) Matching of continents

Jig-saw fit means matching of continents.

16. The Pangaea was surrounded by an ocean called:

a) Tethys

b) Panther

c) Panthalassa

d) Pacific

Ans. c) Panthalassa

Panthalassa means all water. Pangaea was surrounded by Panthalassa.

17. The Pangaea was split into:

a) Gondwana land

b) Laurasia

c) Gondwana and Laurasia

d) Panthalassa

Ans. c) Gondwana and Laurasia

The Pangaea was split into Gondwana and Laurasia. It further broke into various smaller continents that exist today.

18. The part of south India, one compact landmass known as Gondwana land belongs to:

a) Triassic period of the Mesozoic Era

b) Mesozoic or tertiary era

c) Great ice age

d) Carboniferous age

Ans. a) Triassic period of the Mesozoic Era

The part of south India, one compact landmass known as Gondwana land belongs to Triassic period of the Mesozoic Era

19. Pangaea means:

a) All Earth

b) Old Earth

c) New Earth

d) All Water

Ans. a) All Earth

According to Wegener, all the continents formed a single continental mass, a mega ocean surrounded by the same. The super continent was named Pangaea, meaning all earth. The Mega Ocean was called Panthalasa, meaning all water.

20. The hypothesis of seafloor spreading was proposed by:

a) Hess

b) Hadley

c) Hewlett

d) Wegener

Ans. a) Hess

A detailed study of magnetic properties of the rocks on either side of the mid oceanic ridge led Harry Hess (1961) to propose his hypothesis, known as ‘Sea Floor Spreading’. Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. Thus, the ocean floor spreads.

21. The boundary between the Indian plate and the Antarctic plate is a:

a) Convergent boundary

b) Divergent boundary

c) Transform fault boundary

d) Transform boundary

Ans. b) Divergent boundary

The boundary between the Indian plate and the Antarctic plate is a divergent boundary. These plates move away from each other and are also called spreading sites.

22. Convection currents are generated because of the:

a) Radioactive elements in the mantle

b) Nickel and iron in the core

c) Aluminium and silica in the crust

d) Silica and magnesium in mantle

Ans. a) Radioactive elements in the mantle

Convection currents are generated because of the radioactive elements in the mantle.

23. The continental drift theory was propounded by:

a) Alfred Wegener

b) Ortelius

c) Hall and Danna

d) Abraham Ortelius

Ans. a) Alfred Wegener

Alfred Wegener a German Meteorologist put forth a comprehensive argument in the form of ‘Continental Drift Theory’.

24. One of the submarine features is the:

a) Continental crust

b) Continental slope

c) Alluvial fans

d) Aeolian plains

Ans. b) Continental slopes

Continental crust is found over the continents. Alluvial fans are made by rivers. An Aeolian plain is made by the winds in the arid areas of the world. Hence, continental slopes are submarine features.

25. Fuji Plate lies to the:

a) North west of Australia

b) South west of Australia

c) North east of Australia

d) South east of Australia

Ans. c) North east of Australia

Fuji Plate is one of the minor plates. It lies to the north east of Australia.

26. _________ is the minor plate

a) North American Plate

b) Pacific Plate

c) African Plate

d) Arabian Plate

Ans. d) Arabian plate

Cocos, Nazca, Philippine, Arabian plate are some of the important minor plates.

27. The earth’s lithosphere is divided into a number of major and minor tectonic plates. How many major tectonic plates are there?

a) 8

b) 7

c) 6

d) 5

Ans. b) 7

On earth there are 7 major and many minor plates. Where plates meet their relative motion determines the type of boundary; convergent, divergent or transform.

28. Along the divergent boundary the crust is:

a) Crust is consumed

b) Crust is generated

c) Fault is generated

d) Faults are consumed

Ans. b) Crust is generated

Along the divergent boundary the crust is generated. The boundary between India and Antarctic plate is also marked by oceanic ridge (divergent boundary) running in roughly west – east direction and merging into the spreading site, a little south of New Zealand.

29. McKenzie, Parker and Morgan gave the concept of:

a) Continental drift

b) Plate tectonics

c) Conventional current theory

d) Sea floor spreading

Ans. b) Plate tectonics

McKenzie, Parker and Morgan gave the concept of plate tectonics. It was given in 1967. A plate tectonic is a massive, irregular shaped slab of solid rock.

30. The concept of plate tectonics was given by McKenzie and Parker in:

a) 1965

b) 1967

c) 1969

d) 1963

Ans. b) 1967

The concept of plate tectonics was given by McKenzie and Parker in 1967.

31. Convection currents are generated because of the:
32. a) Radioactive element in the crust
33. b) Nickel and iron in the crust
34. c) Aluminium and zinc
35. d) Zinc

Ans. a) Radioactive element in the crust

Convection currents are generated because of the radioactive element in the crust.

32. The convection currents theory was propounded by:
33. a) Hall and Danna
34. b) Arthur Holmes
35. c) Hadley
36. d) Alfred Wegener

Ans. b) Arthur Holmes

Arthur Holmes in 1930 propounded the convection theory. He discussed the possibility of convection currents operating in the mantle portion.

33. The tidal force and polar fleeing forces are responsible for _______

a) The plate movement

b) The continental drift

c) Sea floor spreading

d) Movement of convection currents

Ans. b) The continental drift

Wegener suggested that two forces were responsible for continental drift. These are Polar Fleeing Force and Tidal Force. The Polar fleeing force is related to the rotation of the earth. The Tidal Force is due to the attraction of the moon and the sun which causes tides in the oceans.

34. Polar fleeing force relates to ______

a) Revolution of the earth

b) Gravitation

c) Rotation of earth

d) Tides

Ans. c) Rotation of earth

Wegener eventually proposed a mechanism for continental drift that focused on his assertion that the rotation of earth created a centrifugal force towards the equator. He believed that Pangaea originated near the South Pole and that the centrifugal force of the planet caused the proto continent to break apart and the resultant continents to drift towards the equator. He called this the Polar Fleeing Force.

35. Who amongst the following was the first to consider the possibility of Europe and North America, Africa and South America, having been located side by side?

a) Alfred Wegener

b) Antonio Snider – Pellegrini

c) Abraham Ortelius

d) Edmond Hess

Ans. c) Abraham Ortelius

Abraham Ortelius is believed to be the first person to imagine that the continents were joined together, before drifting to their present positions.

He writes that the coastlines of the continents are so similar that they seem to have been torn apart at some point of time. This is very well visible in Ortelius atlas when looking at the Latin American and African coasts.

36. Cocos plate lies between:

a) Asiatic plate

b) South America and Africa

c) Central America and Pacific plate

d) Arabian plate and Asian plate

Ans. c) Central America and Pacific plate

Cocos plate is one of the important minor plates. It lies between Central America and Pacific plate.

37. A tectonic plate is a slab of:

a) Molten rock

b) Hot rock

c) Solid rock

d) Small rock

Ans. c) Solid rock

A tectonic plate or lithospheric plate is a massive irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.

Short answer type questions:

1. Highlight the important characteristics of mid oceanic ridges.

Ans. Mid oceanic ridges are interconnected chains of mountain system within oceans; characterised by a central system at the crest, a fractionated plateau and flank zone, all along its length.

2. Define the term ‘continental margins’.

Ans. Continental margins form the transition zone between continental shores and deep sea basins. They include continental shelf, continental slope, continental rise and deep oceanic trenches.

3. Name the two forces responsible for drifting of continents according to Wegener.

Ans. Pole fleeing force and tidal force were the two forces responsible for drifting of the continents, according to Wegener.

4. Name seven major tectonic plates.

Ans.

1. a) Pacific plate
2. b) North American plate
3. c) South American plate
4. d) African plate
5. e) Eurasian plate
6. f) Indo-Australian plate
7. g) Antarctic plate

5. Which plate is mainly composed of oceanic crust?

Ans. The plate along the zone of divergence is mainly composed of oceanic crust.

6. Name the two continents in which Pangaea got split.

Ans. Pangaea got split up in two continents i.e., Laurasia, representing the northern part and Gondwana, representing the southern part of the earth.

7. What is Pangaea?

Ans. In the beginning there was a single continental landmass on the Earth. This super continent was known as Pangaea.

8. Who first propounded the theory of continental drift?

Ans. Sir Alfred Wegener in 1912 propounded the theory of continental drift for the first time.

9. What is Panthalassa?

Ans. Panthalassa was the vast global ancestral Ocean that surrounded the supercontinent Pangaea, during the late Palaeozoic and early Mesozoic eras. It included the Pacific Ocean to the west and north and the Tethys Ocean to the south east.

10. Name the continent which broke from Pangaea?

Ans. The continent that broke from Pangaea is called Gondwana land.

11. What is a hot spot?

Ans. Hot spots are the places within mantle where rocks melt to generate magma. Magma generated by hot spots rises through the rigid plates of the lithosphere and produces active volcanoes at the earth’s surface.

12. What is periodic change in the magnetic pole called?

Ans. The periodic change in the magnetic pole is called polar wandering.

13. What is the mechanism based on which the theory of plate tectonics develop?

Ans. The plate tectonic theory proposes that the earth’s lithosphere is divided into seven major and few minor plates, which have been constantly moving over the globe throughout the history of the earth.

14. What was the mechanism based on which the theory of plate tectonic was developed?

Ans. The plate tectonic theory proposes that the earth’s lithosphere is divided into seven major and few minor plates, which have been constantly moving over the globe throughout the history of the earth.

15. Define the term Paleomagnetism.

Ans. It is the study of magnetic properties of the earth and rocks formed during different ages which was considered a major breakthrough for the possibility of a continental mass to have been together in geologic periods.

16. How do scientists determine the past rate of plate movement?

Ans. Scientists determine the rate of plate movement by strips of normal and reverse magnetic field, which make the mid-oceanic ridges parallel.

17. How does the movement of hot currents cause plate movement?

Ans. The movements of hot currents cause plate movement as hot currents rise and then cool as they reach the surface. At the time, cool currents sink down. This convectional movement cause the movement of crustal plate.

18. What is subduction zone?

Ans. The tectonic process in which one tectonic plate is forced beneath another and sinks into the mantle as the plates converge is called subduction zone. Subduction zone exist at convergent plate boundaries where one plate of oceanic lithosphere converge with another plate.

19. What was the location of Indian landmass during the formation of Deccan Traps?

Ans. The Indian plate includes Peninsular India and Australian continental portions. About 140 million years before the present, the subcontinent was located at 50˚ S latitude. Formation of Deccan Traps started around 60 million years ago and continued for a long period of time. The subcontinent was close to the equator.

20. How are the convection currents in the mantle initiated and maintained?

Ans. The convection currents in the mantle are generated due to radioactive elements causing thermal differences in the mantle. The currents move in radial direction and develop in cell patterns.

21. What were the evidences that led to the concept of continental drift theory?

Ans. The evidences that led to the concept of continental drift were:

a) The shore lines of Africa and South America formed a perfect Jigsaw match of two continents

b) The belt of ancient rocks of 2000 million years old from Brazil coast matches with that of western Africa.

c) The discovery of rich deposits of gold in Ghana and complete absence of source rock in the region confirmed that the gold deposits of Ghana were derived from Brazil.

22. What are the three major earthquake and volcano eruption zones?

Ans. The popular earthquake and volcanic zones of the world are:

a) Rim of Pacific

b) Mediterranean zone

c) Mid-Atlantic zone

23. Where are the following plates located in the world?

a) Cocos plate

b) Fuji plate

c) Caroline plate

Ans.

a) Cocos plate: Located between Central America and Pacific plate.

b) Fuji plate: Located near north east of Australia.

c) Caroline plate: Located between Philippine and Indian plate.

24. What were the major discoveries that rejuvenated interest among scientists to study the distribution of oceans and continents?

Ans. The mapping of ocean floor and Paleomagnetic studies of rocks from oceanic region provided the following facts that created interests among scientists towards the distribution of ocean and continents:

a) All along mid-oceanic ridges, volcanic eruptions are common and they bring huge amount of lava to the surface.

b) The rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities in terms of period of formation, chemical composition and magnetic properties.

c) The ocean crust rocks are much younger than the continental crust.

d) The sediments on the ocean floor are very thin.

e) The deep trenches have deep seated earthquake occurrences. However, in the mid-oceanic ridge areas, the quake foci have shallow depths.

25. How the Himalayas were formed?

Ans. The collision between Indian plate and Eurasian plate took place around 40 to 50 million years ago, which resulted in rise of the Himalayas. The suture between Indian and Eurasian plates in Himalayan region has been along the Indus and Brahmaputra rivers. It rose because of the folding processes of the Tethys Sea. 40 million years ago and thereafter, the event of the 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.

26. What were the forces suggested by Wegner for the movement of the continents?

Ans. Sir Alfred Wegener a German meteorologist suggested two forces for the movement of the continents:

a) Pole fleeing force

b) Tidal force

27. What acts as the driving force for the movement of plates?

Ans. The sub-crustal convection currents invoke the mechanism of thermal convection that acts as a driving force for the movement of the plates. Hot currents rise and then cool as they reach through the surface, at the same time, cooler currents sink down. This convectional movement moves the crustal plates. The slow movement of hot soft mantle that lies below the ridge plates is the driving force behind the movement of plates.

28. What is the major difference between the divergent and convergent boundaries of plates?

Ans. Along the divergent boundaries the new crust is generated as the plates pull away from each other. The best known example of the divergent boundaries is the Mid-Atlantic Ridge. However, along the convergent boundary, the crust is destroyed as one plate gets dived under another. The location where the sinking of a plate occurs is called a subduction zone. Eg – the Himalayas.

29. Explain the different components of oceanic floor.

Ans. The different components of oceanic floor are:

a) Continental margins: These margins form the transition between the continental shore and deep sea basins. They include continental shelf, continental slope, continental rise and deep oceanic trenches.

b) Abyssal plain: These are extensive plains that lie between the continental margins and mid-oceanic ridges. The abyssal plains are the areas where the continental sediments that move beyond the margins.

c) Mid-oceanic ridges: These ridges form an interconnected chain of a mountain system within oceans, even though the ridges are submerged under oceanic water; they are the longest mountain chain on the surface of the earth. These are characterised by a central rift system at the crest, which is a zone of intense volcanic activity, a fractionate plateau and flank zone, all along its length.

30. Differentiate continental drift theory and plate tectonic theory.

Ans.

 Long answer type questions:

1. Explain the plate tectonic theory.

Ans. A tectonic is a massive plate, irregularly shaped slab of solid rock, composed of both oceanic and continental lithosphere. In 1967 McKenzie, Parker and Morgan analysed ideas of continental drift and seafloor spreading and came out with the concept of plate tectonics.

The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and few minor plates. These plates have been constantly moving over the globe throughout the earth’s history. As a result, continental masses resting on the plates have been wandering all through the geologic periods.

Mechanism behind moving of plates:

The two sources of heat within the earth come from:

a) Radioactive decay

b) Residual heat

The movement of plates takes place along the three types of boundaries namely:

i) Divergent boundaries: here the plate moves away from each other and new crust is generated.

ii) Convergent boundaries: the crust is destroyed as one plate gets subducted under the other plate.

iii) Transform boundaries: the crust is neither produced nor destroyed, as one plate slides horizontally past the other.