NCERT Geography Class 7 chapter 7

The watery part of the earth’s surface is the hydrosphere, it is estimated to be about 1250 million cubic km. Water from different water bodies reaches the atmosphere through evaporation. Due to loss of heat in the atmosphere, condensation occurs, in the clouds; and ultimately falls back on the ground as precipitation(rain, sleet, snow, hail). Oceans contain 97% of the total water. There are 4 main oceans- the Pacific, Atlantic, Indian, and the Arctic. The temperature of ocean water decreases vertically and increases as one moves from poles to the equator. Ocean water is saline due to the presence of a large number of dissolved salts; salinity decreases towards the equator due to heavy rainfall. The circulation in oceans takes place by three processes- Waves, Tides(spring tide & neap tide; high tide & low tide), currents(cold currents & warm currents). Tsunamis are huge oceanic waves formed due to undersea earthquakes, volcanic eruptions, or undersea landslides. A massive undersea earthquake occurred on 26 December 2004, off the Western coast of Sumatra(Indonesia). It measured 9.1 on the Richter scale. It affected the countries of India, Indonesia, Sri Lanka, Thailand, and some countries along the eastern coast of Africa. This tsunami displaced the ocean floor by about 20m. For further details on this chapter refer to my YouTube link-https://youtu.be/iacfZ2llbmg