(a) Isobath - An imaginary line on a map joining the points of equal depths.
(b) Isohaline - An imaginary line on a map joining the points of equal salinity in oceans.

(i) Hypsometric curve is a graphic representation which shows the height of a certain place found on land and the height of ocean features at sea.
(ii) 'Hypso' means height in Greek.

(i) One of the most indispensable natural resources on earth is water.
(ii) The Earth is also called the Blue planet, as it holds water in abundance.
(iii) Thus stands unique among all other planets

(i) The earth consists of four spheres
(ii) They are a) Lithosphere,
b) Atmosphere,
c) Hydrosphere and
d) Biosphere

(i) The degree of concentration of salts in the sea water is called salinity.
(ii) It is usually expressed in terms of parts per thousand (ppt or ‰).
(iii) Salinity varies both horizontally and vertically.
(iv) The influence of temperature on ocean salinity depends upon the heating up of the surface water, which varies from tropics to polar regions

(i) Of all the movements of the oceans, sea waves are considered to be the strongest.
(ii) Sea waves are ripples on water caused when winds blow over the sea.
(iii) The height of these waves depends on the speed of wind, its duration and the direction from which they blow.
(iv) Sometimes waves are also caused by tremors felt on the ocean floor.
(v) Such waves are quite destructive and called Tsunami

Hydrosphere consists of water in various forms found on the earth. It is held on land as glaciers, ice caps, ground water, river, lakes and also as the water vapour in air.

(i) All of the organisms in the sea are adapted to salt water. If they were suddenly immersed in fresh water, too much water would diffuse into their bodies and they would die.
(ii) The global ecosystem would collapse, and the oceans and atmosphere would eventually revert to their composition before the evolution of photosynthesis: Almost no oxygen and lots of carbon dioxide.
(iii) The ocean currents will be completely disturbed. Hence it will trigger profound climatic changes at the global scale.

Though 71% of the Earth is covered by water, 97.5% of the Earth's water is in the oceans in the form of saltwater that cannot be used. About 2% of the water is found in the form of glaciers and ice sheets. Thus, only 0.5% of Earth's water is available for human use. It makes fresh water a critical resource.

(A) Continental Shelf
(i) A shallow and gently sloping platform extending out from the adjoining continental land mass into the sea is called Continental Shelf.
(ii) It is almost a uniform zone of sea bed with a gentle gradient.
(iii) It is significant in promoting richest fishing grounds.
(iv) Since the continental shelf in shallower, thus enables sunlight to penetrate through the water. This encourages abundant growth of grass, sea weeds and plankton to feed fishes.
(v) The continental shelves have extensive deposits of minerals and mineral fuels.
(vi) Hence, this zone becomes accessible for oil drilling and mining activities.
(B) Continental Slope
(i) A steep slope which descends from the edge of the continental shelf to the deep ocean-bed is called continental slope.
(ii) It forms a boundary between the Continental Crust and the oceanic crust.
(iii) This zone is free from deposits as they are steep.
(iv) The most important characteristic of continental slope is the presence of deep canyons and trenches.
(v) Due to the low penetration of sunlight, the slope has nearly freezing temperature.
(vi) Hence aquatic life has very slow rate of metabolism.

(i) The ocean is a continuous body of salt water that forms the major part of hydrosphere.
(ii) Geoscientists believe that the oceans were formed on Earth nearly three billion years ago.
(iii) It is difficult to believe that in the beginning there was no water on our planet.
(iv) In due course of time, when the Earth started cooling, steam escaped from the interior and entered the atmosphere to form clouds.
(v) At first, the clouds brought incessant rains.
(vi) The rain water filled the depressions for tens of thousands of years and eventually a super ocean was formed.