Longman Panorma Geography Solutions for Class 7 Social science Chapter 8 - Ocean Waters And Their Circulation

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The three types of ocean water movements are as follows:

1. Tides
2. Waves
3. Currents

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Waves are generally caused by the action of winds on the surface of ocean water. Winds cause the particles of the surface water of the ocean to be pushed down. The water particles again rise above the surface. This continuous up and down motion is passed on to other surface particles and is known as wave.

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The Asian Tsunami was caused by a massive underwater earthquake of magnitude 9.0 on the Richter scale, which is known by the names of Indian Ocean earthquake or Sumatra–Andaman earthquake. The epicentre of this devastating earthquake was off the west coast of Sumatra islands in Indonesia.

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The warning signs of a tsunami are as follows:

1. Sudden receding of water on the shoreline, thereby exposing the submerged areas
2. Unusual behaviour or fleeing of animals to higher grounds

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The periodic rise and fall of water level in large water bodies like seas and oceans is termed as tide. Tides are caused due to the gravitational pull of the Moon and the Sun.

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The horizontal movements of the cohesive streams of ocean water in a definite direction are known as ocean currents.

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Following are the four important factors that give rise to ocean currents or modify their course:

1. Rotation of the Earth.
2. The shapes of the coastlines.
3. Effects of planetary winds on the ocean surface.
4. Difference in the density of ocean water due to variations in temperature and salinity.

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The two examples showing the influence of ocean currents on the climate of coastal regions are as follows:

1. The western coast of Europe and eastern coast of the USA receive significant amount of rainfall due to the effect of warm currents of North Atlantic Drift and Gulf Stream, respectively.
2. The coast of Norway is free from ice even in winters due to the effect of warm North Atlantic Drift.

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Tides are very helpful in trade activities. Their importance can be understood with the help of the following points:

• Some ports are dependent on tides for trade activities like Kandla Port of Gujarat and Diamond Harbour of West Bengal. The water level increases during high tides; this allows ships to have a safe entry and exit from the ports.
• In the river ports of London, New York, Hamburg, etc., the water level increases during high tides; this facilitates the safe movement of ships.
• Tides wash away the silt deposited by the rivers at the mouth, thus keeping the river mouth open for vessels to ply easily.
• Tides are useful for fishing, as fish are found in large numbers during high tides. This, in turn, enables fishermen to earn their livelihood.

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The Asian tsunami of 2004 is counted among one of the most serious natural disasters so far. It was responsible for the death of thousands of people and unprecedented loss of property and resources. The worst impact was in the parts of south and southeast Asian countries. Tsunami wrecked havoc on the coastal areas of Indonesia, Thailand, Sri Lanka and India. It also affected the countries along the Indian Ocean like Malaysia, Myanmar, Bangladesh and Maldives. The impact was not limited to Asian countries; some African countries like Tanzania, Madagascar, Kenya and Somalia were in the line of impact of tsunami. In India, the coastal areas of Tamil Nadu, Andhra Pradesh, Kerala, Puducherry and the Andaman and Nicobar Islands suffered the wrath of tsunami. The Andaman and Nicobar Islands were worst affected by this crisis, as the southernmost point of the islands, also known as Indira Point, was completely submerged in the ocean. The Asian tsunami claimed the lives of more than 1.8 lakh people, according to an estimation. Nearly 42,000 people were never found. Houses were completely flattened, trees and poles were uprooted, fields were submerged in water and the means of transport were diminished to the core. Many villages got wiped off. Such were the horrifying scene and devastating extent of the tsunami of 2004.

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The importance of tides can be explained as follows:

1. High tides at coasts are of great importance, as they allow big ships to enter or leave harbours safely. Diamond Harbour of Kolkata and Kandla Port of Gujarat are dependent on tidal conditions for the safe passage of vessels.
2. In case of river ports, tides allow giant ships to navigate safely. Various ports around the world take advantage of the tides for their transportation.
3. Tides also support fishing. High tides increase the volume of water. As a result, more fish are obtained and coastal fishermen get benefitted.
4. Tides also help to remove the silt deposited by the rivers at their mouths, thereby maintaining a free flow all around the year.
5. Tidal energy can be harnessed during high tides, as seawater rushes into the coastal areas. Tidal energy can be converted into electrical energy as done in countries like Japan, Canada and Italy.

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Ocean currents influence the climate and economic activities of coastal regions in the following ways:

1. Warm currents raise the temperature along the coast, while cold currents drop the temperature along the coast. The ports of Norway remain ice-free even in winters because of the effect of warm currents, while the coast of Newfoundland remains frozen for almost nine months owing to the cold currents.
2. Winds blowing over warm currents result in a good amount of rainfall, as they get moisture-laden. On the other hand, winds blowing over cold currents are dry; therefore, rainfall is scanty. The west coast of Europe receives a significant amount of rainfall because of the warm North Atlantic Drift, while the Atacama Desert of South America is the result of the influence of the cold Peru Current.
3. Ocean currents also help in navigation and transport. A lot of fuel and time are saved if ships move along the direction of currents.
4. The mixing of warm and cold currents causes dense fog and reduced visibility. For example, Newfoundland, where the warm Gulf Stream and the cold Labrador Current meet, is densely foggy, posing a great risk of accidents.
5. The places where cold and warm currents meet are the biggest fishing grounds in the world. Small organisms known as phytoplanktons thrive in these places; they also make an excellent food for the fish.

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The correct answer is option (b).

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The correct answer is option (a).

Explanation: The word 'tsunami' is made up of two Japanese words: 'tsu,' which means a harbour, and 'nami,' which means a wave.

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The correct answer is option (d).

Explanation: The Earth, the Moon and the Sun are in a straight line during full moon and new moon days. The combined gravitational pull of the Sun and the Moon occurs on the surface of ocean water, which causes high tides called spring tides.

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The correct answer is option (c).

Explanation: The Peru Current and the West Wind Drift are the cold currents of the Pacific Ocean. The Labrador Current is the cold current of the Atlantic Ocean. The Gulf Stream is the only warm current in the given options; it is a part of currents in the Atlantic Ocean.

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