Longman Panorma Geography Solutions for Class 7 Social science Chapter 7 - Distribution Of Major Water Bodies

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Earth is known as watery planet because of following two reasons:
  1. Almost 71% of the Earth's entire surface is covered with water.
  2. Earth is the only known planet in our solar system that has vast amount of water, enabling it to support life.

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Fresh water is available in various forms on the Earth, which can be broadly classified into two groups:

  1. Inaccessible to human beings: In the form of glaciers and ice-caps at polar regions and mountain peaks
  2. Available to human beings: In the form of ground water, as water vapour in the air and as surface water in rivers and lakes

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The amount of salt dissolved in per unit volume of ocean water is known as the salinity of ocean water. The average salinity of ocean water is 35 per thousand, which means 35 grams of dissolved salt in per 1,000 grams of ocean water.

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Salinity of ocean water depends on a number of factors:
  1. Rate of evaporation
  2. Amount of precipitation
  3. Amount of fresh water that is discharged into ocean water

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The Indian Ocean is located to the south of Asia and directly below India. To its west lies the African continent, while Australia lies to its east. In the south, it merges with the Atlantic Ocean and the Pacific Ocean, forming another ocean, that is, the Southern Ocean.

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Oceans modify the climate of coastal regions in the following two ways:

1. Since oceans are great sites of evaporation, they are responsible for humidity in atmosphere and precipitation in coastal regions.
2. Oceans also have a moderating effect on the climate of coastal regions, reducing the annual and diurnal range of temperatures.

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The salinity of oceans in the equatorial region is not high because

1. there is heavy rainfall throughout the year in the equatorial region
2. in this region, large rivers like Zaire and Amazon discharge a large quantity of fresh water into the ocean

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The Atlantic Ocean is the busiest ocean in terms of passengers and cargo traffic because of following reasons:

1. The coastline along the Atlantic Ocean is very uneven and irregular, thereby giving rise to lots of modern day ports and harbours.
2. It provides passage for navigation among the continents of North America. South America, Europe and Africa.

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Oceans are great sites of evaporation, which, in turn, leads to condensation. This repeated cycle of evaporation and condensation increases the amount of water vapour in the air and thus acts as the main source of precipitation in the atmosphere. That is why oceans are regarded as the prime source of humidity in the atmosphere.

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Water cycle: Water is an essential component for living beings. It is found in various forms on the Earth. It moves in a cyclic manner from the surface to the atmosphere and from the atmosphere back to the surface. 
Evaporation takes place when the water in oceans or other water bodies gets heated up due to sunlight. The resultant water vapour rises in the atmosphere and starts cooling down again with a decrease in temperature in upper layers of air. This process is termed as condensation. Heavy droplets of water combine together to form clouds. These clouds further result into precipitation and bring back the water on the surface in the form of rain, hail or snow. Some of the water seeps down the layers of earth, some is absorbed by plants and the rest falls into the rivers, which again drain into the ocean. This continuous cycle of evaporation, condensation and precipitation of water is known as water cycle or hydrological cycle.

The following diagram gives the detail of the process.

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Pacific Ocean

Location and extent: Pacific Ocean lies to the west of North and South American continent and to the east of Asia and Australia. In the north, it connects to Arctic ocean via Bering Strait and in the south, it forms the Southern Ocean along with the Atlantic Ocean and the Indian Ocean.
Special features: Pacific ocean is circular in its form. It is the largest ocean in the world, covering about one-third of the Earth's total surface. Its area is more than the cumulative area of all continents. It is not only the largest but also the deepest ocean in the world. ​Mariana Trench, the deepest known place on the Earth,  belongs to the Pacific Ocean. It has a large number of islands, mostly volcanic in nature.

Atlantic Ocean

Location and extent: The Atlantic Ocean extends from Europe and Africa on the east to North America and South America in the west. In the north, it stretches up to the Arctic circle and in south, it merges with the Pacific Ocean and the Indian Ocean.
 
Special Features: It is the second largest ocean in the world. The coastline of the Atlantic Ocean is very indented, making it suitable for the growth of various ports and harbours. Therefore, it is the busiest ocean in terms of trade, commerce and transport. Some of the most famous cities like London, Liverpool, New York and Philadelphia are also located on the coast of the Atlantic Ocean.
 

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Oceans are important due to the following reasons:

1. They are the prime source of precipitation and humidity in the atmosphere as maximum evaporation takes place on their surface itself.
2. They are home to a wide range of aquatic and marine life, which is the source of food and livelihood for many people.
3. They have a moderating effect on the climate of coastal areas making it very pleasant.
4. They have huge deposits of petroleum and minerals, which are very essential for the growth of modern industry.
5. They are a renewable source of power generation. Tidal and wave energy is being converted into electricity in many countries these days.
6. The saline water of oceans is also converted into portable water by desalination process, which can be used by humans.
7. Oceans have served as a great source of transport since long time. In the modern world, ocean transport plays an important role in international trade and commerce.

Answer:

The correct answer is (a).

Explanation: About 97% of total water on the Earth has a large quantity of dissolved salts in it, thereby making it unsuitable for human consumption. Thus, the amount of fresh water is on the Earth is only 3%.

Answer:

The correct answer is option (c).

Explanation: Pacific Ocean is the largest ocean in the world. It covers almost one-third of the Earth's entire surface. Its area is bigger than the combined area of all continents.

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The correct answer is option (b).

Explanation: The salinity of most of the oceans is mainly because of the presence of sodium chloride, magnesium chloride and sulphates of magnesium or calcium. Ferrous oxide is not among the salts or minerals found in oceans.

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The correct answer is option (d).

Explanation: The southern extension of three major oceans, namely, the Pacific Ocean, the Atlantic Ocean and the Indian Ocean, combine together and form a new ocean around Antarctica, which is known as the Southern Ocean.

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