GEOGRAPHY - STD 8 Movements of Oceans [2016-2017] Q1. Name the two types of regular movements in the oceans. Ans. The regular movements in the ocean are of two types: 1. Horizontal Ocean currents 2. Vertical The rising of bottom water and the sinking of surface water. Q2. What causes these regular movements? Ans. They are mainly caused by the combined actions of: 1. Density particularly in vertical movements 2. Winds particularly in horizontal movements. Q3. Define tides. Ans. Tides are the alternate rise and fall of the surface ocean water twice in 24 hours. Q4. Why do tides have a rhythmic motion? Ans. Tides have a regular rhythmic motion due to the gravitational attraction exerted upon the Earth by the moon and the sun. Q5. Why do tides occur? Ans. 1) Tides are caused by the gravitational pull of the moon and to some extent by the pull of the sun which is very far from the earth. 2) The moon, though small, is very near the earth and exerts a strong gravitational pull on the earth. As the moon rises later each day by about 50 minutes than the previous day, high tide occurs later each day. 3) Naturally, the water of the earth nearest to the moon will be pulled by the moon s gravitational force. Q6. The cycle of high tide and low tide is continuous. Give reasons. OR What is the interval between tides? What is the duration of a tide? Ans. Tides strike the coast at intervals of 12 hours 26 minutes. The tide water gradually rises for around 6 hours until it reaches high tide.subsequently there is a fall in the water for about 6 hours until it reaches low tide. The cycle then begins all over again. Therefore the cycle of high tide and low tide is continuous. ( 1 )
Q7. Why does the high tide change its time everyday? Ans. High tides occur at intervals of about 12 hours 26 minutes. So if there is high tide at 7am, the next high tide will occur at 7.26pm and the subsequent will be at 7.52am and so on. This is because the moon and the earth are rotating in the same direction. Q8. Explain the term Amplitude or Range of Tides. Ans. The average difference in water level between high and low tide at any place is the amplitude (range) of the tide. The tidal range is usually 1m. to 3m. But it differs from day to day depending on the position of the sun and moon. ( 2 )
Diagram showing the positions of the Sun, the Moon and the Earth and the Tides: Q9. Distinguish between the spring tide and neap time. Spring Tide Neap tide 1. Spring Tides are high tides. 1. Neap tides are low tide. 2. When the Earth, Moon and Sun are almost in the same straight line, spring tides of the greatest range are produced lowest tides. 3. Occurs twice during the lunar month, just after the New Moon and the Full Moon. 2. The sun and the moon are at right angles to each other, they negate each other s pull and thus we have the 3. Occurs at the time of the first and the last quarters of the moon. ( 3 )
Q10. What are the effects of tides? Ans. They are as follows: 1. Tides help in the movement of debris into the sea. Thus they destroy the coastline and also help to build the creeks and inlets. 2. A strong tidal current prevents the formation of deltas but they are useful in building the lower floodplains of rivers. 3. But for tides, the famous estuaries of the rivers Thames, Atlantic Ocean currents: The North Atlantic Current: Clockwise movement 4. Emerges after passing the Caribbean Sea and Gulf of Mexico as Gulf Stream Drift. Mersey and Elbe would be mere creeks. Harbours are also kept clean by tides, which carry debris and sewage far into the sea. 5. In cold countries tides bring in salt water and by their constant motion prevent harbours from becoming icebound. 6. Low-lying areas along the sea coast which get the sea water during high tides can be used to produce salt by trapping the water. 7. The rhythm of high and low tide control the lives of fishermen who sail out and return with the tide. 8. Tidal energy can be harnessed to produce electricity.e.g. In Gujarat along the Gulf of Kutch. 9. Sea creatures like oysters, mussels and barnacles which are attached to the rocks, depend on the tide to bring them the food they require. They due to their fixed position would not get food otherwise. Q11. What is an ocean current? Ans. A regular movement of water from one part of the ocean to another is called ocean current. Q12. How are ocean currents mainly caused? Ans. The ocean currents are mainly caused by the difference in density of sea water and variation in temperature. Q13. What are the factors influencing the ocean currents? Ans. The main factors are as follows: 1. Prevailing Winds: These winds push the currents onwards. The shape and the position of landmasses, the depth of the oceans also have some influencing producing ocean currents. 2. Variations in Sea Water temperature: Great heat at the equator and much less at the poles set up convection currents in the oceans. The water in the equatorial region gets heated, becomes light and flows towards the poles. On the other hand water from the polar region is colder and heavier and therefore sinks, then flows towards the equator on the sea floor. 3. Rotation of the Earth and the Coriolis force: The currents are circular in shape as long as landmasses do not modify their direction. The ocean currents influenced by prevailing winds according to Ferrel s Law follow a clockwise ( to the right) direction in the northern hemisphere and anti- clockwise (to the left) in the southern hemisphere. This is known as the Coriolis Effect. 4. Salinity: Ocean currents on the surface, are generated from the areas of low salinity to the areas of high salinity. Because of the variation in the salinity of the Atlantic Ocean and the Mediterranean Sea, ocean current flows from the Atlantic Ocean to the Mediterranean Sea. ( 4 )
Q14. What are the effects of ocean currents? Ans. The effects are as follows: 1. They influence the climate by modifying the climatic conditions of the coastal regions along which they flow, e.g. the Gulf Stream Drift. 2. Winds blowing over warm currents pick moisture and bring rain while those that pass over cold currents do not bring rain but make the deserts colder and drier, e.g. Kalahari Desert. 3. Currents also influence the routes of cyclonic steams. 4. The dense fogs caused by the meeting of cold and warm currents are dangerous to ships, e.g.in 1912 the Titanic struck on an iceberg due to poor visibility of the cold Labrador Current and the warm Gulf Stream Drift. 5. Ships can sail faster if they follow the direction of the current and the opposite direction hinders speed. 6. The mixing of warm and cold currents help the formation of planktons ( fish food). THE CURRENTS At approximately 40 degrees north, the Gulf Stream flows eastwards forming the North Atlantic Current. The cool Canary Current completes the clockwise circuit in the North Atlantic Ocean by merging with the North Equatorial Current. The South Equatorial Current: Anticlockwise movement. Flows southwards as warm Brazilian Current. At about 40 degree south, it merges with cold West Wind Drift as the South Atlantic Current. Proceeds as cold Benguela Current up the west coast of Canada. Then merges with South Equatorial Current. Counter Equatorial Current: The east flowing current that exists between North and South Equatorial Currents. Q15. State the importance of the Gulf Stream. Ans. The Gulf Stream is a warm current in the North Atlantic Ocean that emerges from the Gulf of Mexico and flows along the west coast of North America. It is one of the strongest ocean currents. It is also known as the Gulf Stream Drift. Q16. State the importance of the North Atlantic Drift. Ans. At approximately 40 degree north, the Gulf Stream flows eastward forming the North Atlantic current. It reaches Europe as the North Atlantic Drift. This complete current is one of the most powerful currents carrying warm equatorial waters northwards. Q17. Mention the diversions of the North Atlantic Current. Ans. From the North Atlantic Ocean, the North Atlantic current divides and flows in 3 directions eastwards to Britain, northwards to Scandinavia and the Arctic and southwards as the cool Canary current. Q18. Give the importance of the Labrador Current. Ans. The Labrador Current is the cold current found in the North Atlantic Ocean. It drifts south eastwards between Baffin Island and West Greenland to meet the warm Gulf Stream Drift. The meeting of the warm Gulf Stream Drift and the cold Labrador Current causes fog and the ( 5 )
deposition of planktons. This give rise to famous fishing grounds called Grand Banks in Newfoundland. Q19. Why is the coast of Newfoundland found to be dangerous for navigation? Ans. Along the Newfoundland coasts, the warm Gulf Stream Drift and the cold Labrador Current meet causing fog. This fog would make it difficult for a ship to spot another ship or even a protruding coast. This makes it dangerous for navigation. Pacific Ocean Currents: Q20. Give reasons: 1. The Alaska Coast is ice free in winter. Ans. Along the Alaska Coast warm waters flow pole wards as the North Pacific Drift. These warm waters keep the Alaskan coast ice free even in winter. 2. Japan is famous for its fisheries. Ans. The meeting of the cold Oyashio Current And the warm Kuroshio Current off the coast of Japan causes fogs and the deposition of planktons giving rise to a great fishing ground in this part of the sea. Hence Japan is famous for fisheries. ( 6 )