Hydrosphere and distribution of water
Rain, river, well, lake, sea and ice are all forms of water. All this water on the Earth together is called the hydrosphere.
The hydrosphere includes all forms of water on the Earth: oceans, rivers, lakes, reservoirs, groundwater and atmospheric vapour. As most of the Earth is covered by water it is called the ‘Blue Planet’ or ‘Water Planet’.
Key idea 1 : Saline and fresh water — About 97 per cent of the total water on the Earth is in the oceans and it is saline. Only 3 per cent is fresh water. This shows how little drinkable water there is.
Key idea 2 : Share of fresh water — Of fresh water about 70 per cent is in the form of icebergs and glaciers, 29 per cent is groundwater and only 1 per cent is in other forms (rivers, lakes, vapour). So usable water is very limited.
Key idea 3 : Saving water — As usable water is less it is necessary to store rain water, stop leakages, increase groundwater and use water carefully.
- Easy : How much water is in oceans? — About 97 %.
- Medium : Fresh water is how much of total water? — About 3 %.
- Exam-type : Biggest store of fresh water? — Icebergs and glaciers (70 %).
In the figure: Distribution of water.
97 saline, 3 fresh; 70-29-1.
It is not that drinking water is plenty because there is lots of water; only 3 per cent is fresh and most of it is ice.
Memory clue: 97 % saline, 3 % fresh. Memory clue: "97 salty, 3 sweet".
Spot the difference
| Type | Share | Place |
|---|---|---|
| Saline water | 97 % | Oceans |
| Fresh water | 3 % | Ice, groundwater, rivers |
| Ice in fresh water | 70 % | Icebergs, glaciers |
Practice questions
- 1What is the hydrosphere?
- 2Why is the Earth called the ‘Blue Planet’?
- 3State the proportion of saline and fresh water.
- 4How is fresh water distributed?
- 5Why is usable water less?
- 6State three measures to increase usable water.
- 7What are the sources of groundwater?
- 8Write the uses of ocean water.
Show answers (try them yourself first)
- 1All forms of water on the Earth.
- 2Most of it is covered by water.
- 3Saline 97 %, fresh 3 %.
- 470 % ice, 29 % groundwater, 1 % others.
- 5Fresh water is 3 % and most of it is ice.
- 6Store rain, stop leakage, economy.
- 7Water seeping into the ground from rain, rivers, lakes.
- 8Salt, fish, transport, climate control.
Oceans and ocean floor
As land has hills, plains and valleys, there are similar ups and downs under the sea water too; we just cannot see them.
A vast expanse of saline water lying between two or more continents is called an ocean. The structure of the ocean floor is as varied as landforms on the continents; the average depth of oceans is about 3,730 m (as per the textbook).
Key idea 1 : Five oceans — There are five oceans: Pacific, Atlantic, Indian, Southern (Antarctic) and Arctic. The Pacific is the largest and the Arctic the smallest. Areas as per the textbook: Pacific about 166 million, Atlantic 87 million, Indian 73 million, Southern 20 million and Arctic 13 million sq. km.
Key idea 2 : Continental shelf and slope — The submerged part of the continent near the coast with gentle slope and shallow water is the continental shelf. Large fishing grounds, crude oil and natural gas are found here; e.g. Mumbai High in the Arabian Sea. After it comes the steep continental slope (as per the textbook depth 200 to 3000 m).
Key idea 3 : Deep ocean floor — The gently sloping bottom after the slope is the abyssal plain; submerged highlands, mountains and plateaus are found there. If mountain ranges of the sea come out of the water they form oceanic islands (e.g. Andaman and Nicobar); the Chagos plateau is in the Indian Ocean. A narrow and very deep part is called an oceanic trench; the Mariana trench in the Pacific is the deepest.
- Easy : The largest ocean? — Pacific.
- Medium : Mumbai High is in which part? — Continental shelf (Arabian Sea).
- Exam-type : The deepest trench? — Mariana.
In the figure: Ocean floor from coast to deep sea.
The textbook gives the depth of the Mariana trench as 11,034 m; as per a newer precise survey the depth of Challenger Deep is taken as about 10,935 m (±6 m), and figures of other expeditions differ by some metres (as per several sources; check with the book).
Shelf → slope → plain → trench.
The sea bottom is not flat; it has mountains, plateaus, plains and trenches.
Memory clue: Shelf (shallow) → slope → abyssal plain → trench (deep). Memory clue: "Shelf shallow, trench deep".
Spot the difference
| Part | Feature | Importance |
|---|---|---|
| Continental shelf | Shallow, gentle slope | Fishing, oil, gas |
| Continental slope | Steep slope | Depth 200–3000 m |
| Oceanic trench | Narrow, very deep | Mariana trench |
Practice questions
- 1What is an ocean?
- 2Name the five oceans.
- 3Write the importance of the continental shelf.
- 4State the difference between continental slope and abyssal plain.
- 5How are oceanic islands formed? Give an example.
- 6What is an oceanic trench? Which is the deepest?
- 7What is the difference in area of Pacific and Arctic (in crore sq. km)?
- 8Show the five oceans on a map in different colours.
Show answers (try them yourself first)
- 1A vast expanse of saline water between continents.
- 2Pacific, Atlantic, Indian, Southern, Arctic.
- 3Fishing, oil, gas and minerals.
- 4Slope is steep; plain is gently sloping at the deep base.
- 5When sea mountains emerge from water; Andaman and Nicobar.
- 6A narrow very deep part; Mariana.
- 7About 16.6 − 1.3 = 15.3 crore sq. km.
- 8Student shows on the map.
Tides
On the Konkan coast the water sometimes comes near and sometimes goes far. Such periodic rise and fall of the sea water level is called tide.
The periodic rise of sea water level is high tide and its fall is low tide. In one day (24 hours) tides occur once or twice at most places.
Key idea 1 : Causes of tides — As the Moon is close to the Earth its gravity pulls sea water and raises the level. As the Sun is far its gravity has less effect. Centrifugal force created by the Earth’s rotation and gravitational force also have an effect; so high tide occurs at the same time on the side facing the Moon and the opposite side.
Key idea 2 : Why does the time change every day? — The gap between two high tides is actually 12 hours 25 minutes, because the Moon takes 50 minutes more to complete a round. So if high tide is at 6:00 am today it comes at 6:25 pm and at 6:50 am tomorrow.
Key idea 3 : Spring and neap tides — On new moon and full moon day the Sun, Moon and Earth are in a line, their forces add up and the highest high tide (spring tide) and lowest low tide occur. On the eighth day of both fortnights the Sun and Moon are at right angles, forces are divided and the difference between high and low tide is least (neap tide).
- Easy : Main cause of tide? — Moon’s gravity.
- Medium : Gap between two high tides? — 12 h 25 min.
- Exam-type : When is spring tide? — On new moon and full moon.
In the figure: Main causes of tides.
As per several sources the average gap between two consecutive high tides is about 12 hours 25 minutes and a lunar day is about 24 hours 50 minutes (as per several sources; check with the book).
12 hours 25 minutes.
High tide does not come exactly at the same time every 12 hours; it comes 25 minutes later each time.
Memory clue: Moon near = big effect; Sun far = small. Memory clue: "New-full moon = spring tide".
Spot the difference
| Point | Spring tide | Neap tide |
|---|---|---|
| When | New moon, full moon | Eighth day of both fortnights |
| Sun-Moon-Earth | In a line | At right angles |
| Effect | Highest high, lowest low | Least difference |
Practice questions
- 1What are tides?
- 2Write three causes of tides.
- 3Why does the Moon affect tides more than the Sun?
- 4If high tide is at 6 am today when will it come tomorrow?
- 5State the difference between spring and neap tides.
- 6State the advantages of tides.
- 7Why does high tide occur on the opposite side too at the same time?
- 8Note the timings of tides in the Konkan for three days.
Show answers (try them yourself first)
- 1Periodic rise and fall of sea water level.
- 2Gravity of Moon-Sun and centrifugal force of the Earth.
- 3The Moon is near the Earth.
- 4At 6:50 am tomorrow.
- 5Spring: more difference; neap: less difference.
- 6Ships enter ports, fishing, tidal energy, protecting mangroves.
- 7Centrifugal force throws water to the opposite side.
- 8Student notes own.
Biosphere
A tree grows in the ground, a bird flies in the air and a fish lives in water. All the places where living things live together make the biosphere.
The biosphere is the part of the lithosphere, hydrosphere and atmosphere in which living things live. The zone most favourable for life extends about 8 to 10 km above the ground, about 2 km in the lithosphere and about 10,000 m depth in the hydrosphere (as per the textbook).
Key idea 1 : Classification of living things — According to habitat living things are divided into three groups: terrestrial (on land, e.g. elephant), aquatic (only in water, e.g. whale, jellyfish, octopus) and amphibians. The life of amphibians has two stages: in the first larval stage they live in water and when mature on land; e.g. frog.
Key idea 2 : Plants — Plants are the most important component of the biosphere and play a key role in the food chain. Trees, climbers, grass and shrubs grow on land; algae and elodea under water; lotus and duckweed float on the water surface.
Key idea 3 : Importance of the biosphere — The biosphere provides food, water and air for life on the Earth. All living things depend on each other and on surrounding non-living elements. Plants give oxygen and control temperature; animals pollinate and spread seeds; microorganisms decompose dead remains and improve soil fertility.
- Easy : Frog belongs to which group? — Amphibian.
- Medium : Whale belongs to which group? — Aquatic.
- Exam-type : Who improves soil fertility? — Microorganisms.
In the figure: Groups of living things.
Land-water-both.
A frog does not live only in water; it lives in both water and on land, so it is amphibian.
Memory clue: Terrestrial = land; aquatic = water; amphibian = both. Memory clue: "Ubhay = both".
Spot the difference
| Group | Habitat | Example |
|---|---|---|
| Terrestrial | Land | Elephant |
| Aquatic | Only water | Whale, octopus |
| Amphibian | Water and land | Frog |
Practice questions
- 1What is the biosphere?
- 2What is the extent of the biosphere (as per book)?
- 3Write three groups of living things with an example of each.
- 4What are amphibians? Explain with the example of a frog.
- 5Name two plants growing in water.
- 6State the importance of plants in the biosphere.
- 7What is the work of microorganisms?
- 8Prepare a table of where humans, fish and frogs live.
Show answers (try them yourself first)
- 1The part of lithosphere, hydrosphere and atmosphere where living things live.
- 2About 8–10 km up, 2 km in ground, 10,000 m in water.
- 3Terrestrial: elephant; aquatic: whale; amphibian: frog.
- 4Two life stages: larva in water, adult on land.
- 5Algae, elodea, lotus.
- 6Oxygen, food, temperature control, air purification.
- 7They decompose dead remains and make soil fertile.
- 8Human: land; fish: water; frog: both.