Grade 7 Water Classnotes PDF - Tides, Tsunamis, Currents

INDIAN SST – GRADE 7 WATER CLASSNOTES Match the content of column A with that of column B: (i) Caspian Sea (a) Streams of water moving in a definite path. (ii) Tide (b) St...

INDIAN SST – GRADE 7 WATER CLASSNOTES Match the content of column A with that of column B: (i) Caspian Sea (a) Streams of water moving in a definite path. (ii) Tide (b) Strong seismic waves. (iii) Tsunami (c) Largest lake. (iv) Ocean currents (d) Periodic rise and fall of water. Answers: (i) Caspian Sea (c) Largest lake. (ii) Tide (d) Periodic rise and fall of water. (iii) Tsunami (b) Strong seismic waves. (iv) Ocean currents (a) Streams of water moving in a definite path. True or False: 1. Low tides are useful in navigation. False 2. The Indira point got submerged in the Tsunami during 2004. True 3. The gravitation pull of the moon and the sun is the major cause of the tide. True 4. The largest tsunami ever measured was 700 m high. False 5. Low tides help in navigation. False 6. Californian current flow to the east of North America. True 7. Labrador current is an example of cold current. True Questions: 1. Where can we find the best fishing grounds of the world? Give examples. The areas where the warm and cold currents meet provide the best fishing grounds of the world. Seas around Japan and the eastern coast of North America are such examples. 2. Which factors affect the movement of waves? The factors which effect the movement of waves are: (i) Temperature. (ii) Winds. (iii) Gravitational pull of the sun. (iv) The earth. (v) The moon. (vi) The warm and cold currents. 3. What are the factors affecting the height of the waves? The factors affecting the height of the waves are: The shape and size of the wave depends on the speed of the wind. On calm days the waves are small but on windy days they grow larger. During storms, the winds blowing at very high-speed form huge waves. It may rise to great heights and even become destructive. Earthquakes and storms cause huge waves called tidal waves or tsunamis. 4. Explain ‘spring tides’ and ‘neap tides with the help of a diagram. During the full moon and new moon days, the sun, the moon and the earth are in the same line. These tides are called spring tides. The combined gravitational force of the sun and the moon is very strong and so tides are higher than the usual. When the moon is in its first and last quarter, the ocean waters get drawn in diagonally opposite directions by the gravitational pull of sun and moon resulting in low tides. The sun and moon are positioned so that their gravitational pull acts on the earth at a right angle to each other, so the tides have lesser heights. They are called neap tides.