VIDYA VIHAR CLASS 8 TERM 2 Geography
CONTRIBUTOR :-
CHAPTER 4:- Structure of Ocean Floor
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Q 1. Choose the correct option:
(a) Like there are landforms on land, ocean floor also has submerged landforms because ……
(i) There is land under water
(ii) There are volcanoes under water
(iii) Land is continuous and there is water in deeper parts.
(iv) Though land is continuous, its level is not the same everywhere like that of water
Answer:- (iv) Though land is continuous, its level is not the same everywhere like that of water
(b) Which part of the ocean floor is most useful to the man ?
(i) Continental shelf
(ii) Continental slope
(iii) Abyssal plains
(iv) Marine deeps
Answer:- (i) Continental shelf
(c) Which one of the following option is related to marine deposits?
(i) Rivers, glaciers, remains of plants and animals
(ii) Volcanic ash, continental shelf, remains of plants and animals
(iii) Volcanic ash, lava, fine particles of soil
(iv) Volcanic ash, remains of plants and animals, abyssal plains
Answer:- (iii) Volcanic ash, lava, fine particles of soil
Q 2. (a) Name the landforms shown in the figure.
Answer:-
1. Submerged hills and mountains
2. Mid-Oceanic Ridge
3. Abyssal plain
4. Marine Trench
5. Continental Slope
6. Continental shelf
(b) Which of these landforms is useful for deep sea research?
Answer:- Th emarine deep and marine trenches are the most useful landforms for deep sea research as they are very deep from the sea level.
(c) Which of these are appropriate to be used for the protection of marine borders and naval-base building?
Answer:- The continental shelf and the continental slope are appropriate to be used for the protection of marine boarders and naval-base building. Submarines can be placed here and the marine borders can be protected.
Q 3. Give geographical reasons:
(a) The study of ocean floor is useful to man
Answer:- i. Land submerged below the oceanic water is called the ocean floor. The ocean floor relief consists of all submerged landforms.
ii. The continental shelf is very important from the point of view of humans. It has extensive fishing grounds, and abundant resources like natural gas, mineral oil and various minerals can be obtained by mining the continental shelf.
iii. The mid-oceanic submerged hills and marine trenches are geologically the most active areas of the ocean bed in the world.
iv. There are many active volcanoes here. These areas are also earthquake prone areas. Earthquakes and volcanic eruptions occurring in the ocean bed give rise to tsunamis in nearby coastal areas.
v. Moreover, the study of ocean floor is useful to man because it helps one to understand ocean pollution, climate changes, untapped resources like mineral resources, natural gas, etc. changes in currents, behaviour of marine organisms, etc.
(b) The continental shelf is a paradise for fishing activity.
Answer:- i. The land near the coast, submerged under the sea is called continental shelf. This is the shallowest part of the ocean bed.
ii. As this part is shallow, sunlight reaches its bed.
iii. Algae, plankton, etc. which grow here are fish food. Fishes come to eat it.
Therefore, the continental shelf is a paradise for fishing activity.
(c) Some marine islands are actually the peaks of sea mountains
Answer:-
i. The hills and mountains found on the ocean bed are called submerged hills and mountains.
ii. These hills are hundreds of kilometres wide and thousands of kilometres long.
iii. Peaks of few submerged hills reach above the sea level and are visible as marine islands.
iv. For e.g. Iceland in Atlantic Ocean, Andaman and Nicobar Islands in the Bay of Bengal, etc.
Thus, some marine islands are actually the peaks of Sea Mountains.
(d) The continental slope is considered to be the boundary of continents.
Answer:- i. Once the extent of continental shelf ends, the slope of the sea bed becomes steeper which is called as the continental
slope.
ii. The depth of the slope is from 200 m to 3600 m. In some places, it is more.
iii. The continental slope is narrow and its lower boundary is considered to be the boundary of continents.
(e) The disposal of waste materials in the oceans by man is harmful to the environment.
Answer:-
i. Deposition of many materials coming from land keeps occurring in the seas and oceans. This is in the form of natural deposits and sediments.
ii. But, man disposes waste materials in the sea, like sewage, solid waste, radioactive material, waste chemicals, plastics, etc. which pose a danger to the ocean bed and sea water (hydrosphere).
iii. Moreover, they are extremely harmful to the marine life and environment.
Therefore, the disposal of waste materials in the oceans by man is harmful to the environment.
Q 4. Observe the map on Pg 27 in ‘Give it a try’ and answer the following questions:
(a) Madagascar and Sri Lanka are related to which landform of the ocean floor?
Answer:- Both Madagascar and Sri Lanka are Marine Islands.
(b) Near which continent are these land forms located?
Answer:- Sri Lanka is located near the continent of Asia whereas Madagascar is located near in the continent of Africa.
(c) Which islands in our country are examples of peaks of submerged mountains?
Answer:- The Andaman and Nicobar Islands are examples of peaks of submerged mountains.
Some Additional Questions from workbook
Q1 . Choose the correct answer and complete the sentences.
- Like there are landforms on land, ocean floor also has submerged landforms because ………….
(i) there is land under water
(ii) there are volcanoes under water
(iii) land is continuous and there is water in deeper parts
(iv) though land is continuous, its level is not the same everywhere like that of water
Answer:- (iv) though land is continuous, its level is not the same everywhere like that of water
2.) Which part of the ocean floor is most useful to the man?
(i) Continental shelf
(ii) Continental slope
(iii) Abyssal plains
(iv) Marine deeps
Answer:- (i) Continental shelf
3) Which one of the following option is related to marine deposits?
(i) Rivers, glaciers, continental shelf, remains of plants and animals
(ii) Volcanic ash, continental shelf, remains of plants and animals
(iii) Volcanic ash, lava, fine particles of soil
(iv) Volcanic ash, remains of plants and animals, abyssal plants
Answer:- (iii) Volcanic ash, lava, fine particles of soil
4. Continental slope has slope.
(a) flatter
(b) broader
(c) steeper
(d) lower
Answer:
(c) steeper
5. The average depth of the oceans is around metres.
(a) 7300
(b) 3700
(c) 3900
(d) 3300
Answer:
(b) 3700
Que 2:- Examine the following statements and correct the incorrect ones:
Question 1
Abyssal plain lies beyond the continental shelf.
Answer:
Incorrect.
Correct statement: Abyssal plain lies beyond the continental slope
Question 2.
Around 51 per cent of the surface of the earth is covered by water.
Answer:
Incorrect.
Correct statement: Around 71 per cent of the surface of the earth is covered by water.
Question 3.
Natural gas and mineral oil can be obtained from the continental shelf.
Answer:
Correct.
Question 4.
For survey purpose in India, the height of sea level at Kanyakumari is considered to be zero.
Answer:
Incorrect.
Correct statement: For survey purpose in India, the height of sea level at Chennai is considered to be zero.
Question 5.
Active volcanoes are mostly seen in marine trenches.
Answer:
Correct.
Que3 :- Answer the following questions in one sentence each:
Question 1.
What is called continental shelf?
Answer:
The land near the coast and submerged under the sea is called continental shelf.
Question 2.
What is called continental slope?
Answer:
The steeper slope of the sea bed beyond the extent of continental shelf is called continental slope.
Question 3.
What is called abyssal plain?
Answer:
The flat part of the sea bed that lies beyond the continental slope is called abyssal plain.
Question 4.
What is called submerged hills?
Answer:
The hundreds of kilometres wide and thousands of kilometres long hills found on the ocean bed are called submerged hills.
Question 5.
What is called marine islands?
Answer:
Peaks of some of the submerged hills that come above the sea level are called marine islands.
Question 6.
What is called submarine plateau?
Answer:
The flat and extensive summit of marine island is called submarine plateau.
Match the columns and complete the chain:
‘A’ Column | ‘B’ Column | ‘C’ Column |
1. Andaman – Nicobar | (i) Submarine plateau | (a) Pacific Ocean |
2. Chagos | (ii) Located on continental shelf | (b) Bay of Bengal |
3. Mariana | (iii) Islands | (c) Arabian Sea |
4. Mumbai High | (iv) Marine deep | (d) Indian Ocean |
Answer:-
‘A’ Column | ‘B’ Column | ‘C’ Column |
1. Andaman – Nicobar | (iii) Islands | (b) Bay of Bengal |
2. Chagos | (i) Submarine plateau | (d) Indian Ocean |
3. Mariana | (iv) Marine deep | (a) Pacific Ocean |
4. Mumbai High | (ii) Located on continental shelf | (c) Arabian Sea |
Que 5:- Answer the following questions in brief:
Que 1 :- What is marine deposits?
Answer:
- Marine deposit is a type of marine deposition.
- The rivers and glaciers flowing from the continent bring pebbles, clay, soil, etc. with them when they meet sea or ocean.
- These materials get deposited on the continental shelf. The deposits are known as marine deposits.
- Due to pressure of sea water and deposition of layers over layers of sediments, sedimentary rocks are formed.
- Marine life and the availability of minerals in the sea bed can be studied with the help of marine deposits.
Que 2 :- What is marine oozes?
Answer:
- Marine oozes is a type of marine deposit.
- Lava and ash erupting out of volcano, fine soil particles, remains of plants and animals, etc. get deposited on marine abyssal plains. Deposits of these materials on the abyssal plains are known as marine oozes.
- Marine oozes are in the form of fine clay. The percentage of remains of plants and animals in marine oozes is up to 30 per cent.
- Marine oozes are also useful to study marine life and the availability of minerals in the sea bed.
Que 3:- Write in brief about the continental shelf.
Answer:
1. The land near the seacoast and submerged under the sea is called continental shelf. The depth of continental shelf is about 200 metres below the sea level.
2. Continental shelf is found to be narrow along the coasts of some continents and broad along the coasts of some other continents.
3. Being a shallowest part of the ocean bed, sunlight reaches up to continental shelf easily. As its effect, the food for fish like algae, plankton, etc. grows on a large scale on continental shelf. Therefore, continental shelf is useful for fishing occupation.
4.Various minerals, natural gas and mineral oil, etc. can also be obtained from mining the continental shelf. For example, Mumbai High located on the continental shelf of the Arabian Sea is a source from where the mineral oil and natural gas is obtained
Que 4 :- Write in brief about the continental slope.
Answer:
- The steep slope of the sea bed beyond the extent of continental shelf is called continental slope.
- The depth of the continental slope is from 200 metres to 3600 metres below the sea level.
- The continental slope is found to be comparatively narrower.
- The lower boundary of the continental slope is considered as the boundary of I continent.
Que 5:- Write in brief about the abyssal plains.
Answer:
- The flat part of the sea bed that lies beyond the continental slope is called abyssal plains.
- Various submerged landforms like
submerged hills, submerged mountains, etc. are seen on abyssal plains. - Submarine plateaus are also found on the abyssal plains.
Que 6:- Write in brief about mean sea level.
Answer:
- The average of the highest high tide and the lowest low tide is considered to calculate the mean sea level.
- This average is considered to be zero sea level.
- The altitude of any place above the sea level is measured and expressed in positive value. (For example, Mount Everest is 8848 metres high.)
- The depth of any place below the sea level is measured and expressed in negative value. (For example, Mariana Trench is 11034 metres below sea level.)
Que 6:- Write Geographical reason.
1 .) The study of ocean floor is useful to man.
Answer:-
1. Various minerals, rocks, fine I soil particles are found on the sea floor.
2. Remains of marine plants and animals are also found on the ocean floor.
3. To study the marine life, mineral wealth as well as the process of volcanic eruption, the study of ocean floor is useful to man.
2. The continental shelf is a paradise for fishing activity.
Answer:-
1. Being a shallowest part of the ocean bed, sunlight reaches up to continental shelf easily. 2. As its effect, algae, plankton, etc. grows on a large scale on continental shelf. 3. Algae, plankton, etc. is food for fish. Therefore, fish are found on a large scale on continental shelf. Therefore, the continental shelf is paradise for fishing activity.
3. Some marine islands are actually the peaks of sea mountains.
Answer:-
1. The hundreds of kilometres wide and thousands of kilometres long hills found on the ocean bed are called submerged hills.
2. Peaks of some of the submerged hills come above the sea level.
3. These peaks are the visible land areas surrounded from all sides by water. These peaks are called islands. Thus, some marine islands are actually the peaks of sea mountains.
4. The continental slope is considered to be the boundary of continents.
Answer:-
1. Continental slope lies beyond the continental shelf.
2. The deep abyssal plain begins beyond the continental slope. Therefore, the continental slope is considered to be the boundary of continents.
5. The disposal of waste materials in the oceans by man is harmful to the environment.
Answer:-
1. The disposal of waste materials in the oceans by man increases the pollution of sea water.
2. This disposed waste materials harm the life of marine animal life and marine plant. Thus, the disposal of waste materials in the oceans by man is harmful to the environment.
Que 7 Difference between Landforms and Submerged landforms
Landforms | Submerged Landforms |
Meaning :- Natural forms of the land surface found on the surface of the earth are called landforms. | Meaning :-Natural forms of land surface found on the ocean bed are called submerged landforms. |
Parameter:- Landforms on the surface of the earth are classified on the basis of altitudes and the shape of the land there. | Parameter:-Submerged landforms are classified on the basis of depth from the sea floor and the shape of the land there. |
(2) Marine deep and Marine trench.
Ans.
| Marine deep | Marine trench |
Meaning | The landforms on the ocean bed that are comparatively less extending and shallower ones are called marine deep. | The landforms on the ocean bed that are | comparatively more extending and deeper ones are called marine trench. |
Earthquake proneness | Active volcanoes are not seen in marine | . | deep. Therefore, earthquake proneness is | less in case of marine deep. | Active volcanoes are mostly seen in marine trench. Therefore, earthquake proneness is more in case of trench. |
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Study the following map/figure/graph and answer the following questions:
Que:- Name the landforms shown in the following figure and answer the following questions: