(i) Earth is called a Blue Planet because over 78% of Earth’s surface is covered by water bodies. Hence, when viewed from space, it reflects the universe and appears blue. No other planet, with the exception of Earth, has water on its surface. This is due to the optimal distance of earth from the sun, which allows water to exist in liquid form on our planet.

(ii)

The continental margin is found on the edge of the continent, and is comprised of three features: continental rise, continental slope and continental shelf.

It covers over 28% of the total oceanic area in the world.

(iii)

(iv) The boundary region between the upper areas of the ocean, where the temperature decreases smoothly, and the lower areas, where the temperature fall is rapid, is called a THERMOCLINE.

It is located at a depth of 100-400 metres. Over 90% of total water lies beneath the thermocline. The temperature in this region approaches the freezing point.

(v) The various layers encountered are:

• The first layer, with the highest temperatures

• The thermocline layer, with rapidly decreasing temperatures

• The third layer, where water temperature approaches 0 degrees and there is mild variation in temperature

The temperature of water changes with depth because the surface layer receives direct heat from the sun. Hence, it is the warmest. This heat moves convectively to the lower layers of the water body. The heat convection is the primary reason for differential temperatures.

(vi) Salinity measures the amount of salt dissolved in water. It is calculated as the amount of salt, in grams, present for every 1000 grams of sea water. Salinity is influenced by wind, temperature, processes of evaporation and precipitation and ocean currents. An upper limit of 18.6 ppt is recognised for demarcation of brackish waters.