Water (oceans)

Hydration is the process of adding water to a salt, to help regain its colour. The water cycle does not have any such steps.

A is incorrect. The water cycle is initiated by the evaporation of water from the water bodies.

C is incorrect. Precipitation is the mechanism by which the water returns from the atmosphere to the earth.

D is incorrect. Condensation is the process by which clouds are formed.

The picture shows the relative extent of continental slope, when compared to other geographical features in the sea.

it does not constitute a minor relief feature in the ocean. It refers to trenches, which deep sea depressions with narrow basins, which are associated with strong earthquake and volcanic activity.

A is incorrect. Sea-mount is a minor relief feature. It refers to a mountain with a summit, which does not rise all the way to the surface of the ocean.

B is incorrect. Atolls are tropical islands formed by coral reefs.

D is incorrect. Guyot is a seamount with a flat top.

Salinity is used to measure the amount of salt present in water, and is expressed as the weight of salt, in grams, dissolved in every 1000 grams of water.

Other options are factually incorrect.

(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.

(i) The various parts of the hydrological cycle are inter-related by the exchange of water. Water is a resource which cycles between atmosphere, land and water bodies. This cyclical nature is prompted by evaporation and condensation, followed by precipitation.

Water from the oceans enters the atmosphere through evaporation, and causes precipitation over land. This recharges our rivers, ground water systems and glaciers. The glaciers, in turn melt, and supply water to the perennial rivers.

(ii) The temperature distribution of oceans is influenced by many factors:

• Latitude: As we move away from the equator, the temperature of oceans decreases. This is because the direct rays of the sun do not reach those areas.

• Presence of Land: The oceans receive greater heat in the northern hemisphere, due to their extensive contact with the landmass

• Wind systems: Two competing wind systems exist. The winds that blow from the land drive the warm water away from the coast, and bring up cold water from below. But the onshore winds drive the warm water towards the coastal areas.

• Ocean Currents: Warm currents like Gulf Stream raise the temperature of the ocean water in colder areas, whereas cold currents like the Labrador Current reduce the temperature of water in warmer, equatorial areas.

The physical map above shows the geography of the oceanic relief of the oceans of the world.

The Mid-Indian Ridge is a continuance of the mid-Atlantic Ridge. It separates the African and Indo-Australian Plate. The Owen Fracture Zone marks the northern end of the CIR, separating Arabian and Indo-Australian Plates.

It divides into 2 branches near the Rodrigues Island, with one branch moving towards the Gulf of Aden, and the other moving towards Macquarie Islands. The ridge shown in the photo is the former, called Carlsberg Ridge.