Hydrosphere
ICSE Class 9 · Geography · Chapter 11
What this chapter covers
- Meaning of hydrosphere
- Tides: meaning and formation
- Pattern of tides
- Ocean currents: meaning and causes
- Circulation pattern of ocean currents
- Effects of ocean currents
- Currents of the North Atlantic Ocean
- Currents of the North Pacific Ocean
Key points
- Hydrosphere is all the water on, below and above the earth's surface; oceans hold about 97 per cent of it and cover about 71 per cent of the earth's surface.
- Less than 3 per cent of the earth's water is fresh, and most of that is frozen in ice caps and glaciers.
- Tides are the periodic rise and fall of sea level caused mainly by the moon's pull and partly by the sun's.
- Two high tides form at once, one facing the moon (moon's pull) and one on the opposite side (centrifugal force); low tides lie between them.
- Successive high tides are about 12 hours 25 minutes apart, and the tides come about 50 minutes later each day.
- Spring tides (greatest range) occur at new and full moon, when the sun, moon and earth are in a straight line.
- Neap tides (least range) occur at the first and last quarters, when the sun and moon are at right angles at the earth.
- Tides help navigation, keep harbours clean, aid fishing and salt making, and can generate electricity.
- Ocean currents are caused by planetary winds, differences in temperature and salinity, the rotation of the earth and the shape of coastlines.
- Currents form gyres that turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere; the North Indian Ocean currents reverse with the monsoon.
- Warm currents raise coastal temperatures and bring rain; cold currents lower temperatures and are linked with dry coasts and deserts.
- Meeting zones of warm and cold currents produce fog and rich fishing grounds.
- The warm Gulf Stream and its continuation, the North Atlantic Drift, give north-western Europe mild winters and ice-free ports.
- The cold Labrador Current chills Labrador and Newfoundland, freezes their ports and carries icebergs; it meets the Gulf Stream at the Grand Banks.
- The warm Kuro Shio warms southern Japan; the cold Oya Shio chills northern Japan; they meet off north-eastern Japan, creating fog and a great fishery.
Questions answered
Define hydrosphere. Name the three forms in which water exists in it.
Hydrosphere is the water realm of the earth, made up of all the water found on, below and above the earth's surface. Water exists in it as a liquid (oceans, rivers, groundwater), a solid (ice caps and glaciers) and a gas (water vapour).
About what share of the earth's water is fresh water, and where is most of it stored?
Less than 3 per cent of the earth's water is fresh water, and most of it is stored as ice in ice caps and glaciers.
What are tides? State the main cause of tides.
Tides are the periodic rise and fall of the level of the sea. They are caused mainly by the gravitational pull of the moon, helped or weakened by the pull of the sun.
Give a reason why the moon has a greater effect on tides than the sun, although the sun is far more massive.
The moon is much nearer to the earth than the sun, and the tide-raising force falls off very rapidly with distance, so the moon's tide-raising force is more than twice that of the sun.
Explain why a high tide occurs on the side of the earth facing away from the moon.
On the far side the moon's pull is weakest, so the centrifugal force produced by the revolution of the earth and the moon around their common centre of gravity is stronger, and the water bulges outwards away from the moon.
High tide occurs at a port at 9:40 p.m. At about what time will the next high tide occur?
Successive high tides are about 12 hours 25 minutes apart, so the next high tide will occur at about 10:05 a.m. the next day.
Why do high tides occur about 50 minutes later each day?
While the earth rotates once in 24 hours, the moon moves forward in its orbit, so the earth must turn for about 50 minutes more to bring a place directly under the moon again.
Distinguish between spring tides and neap tides.
Spring tides occur at new moon and full moon, when the sun, the moon and the earth are in a straight line; the tidal range is greatest. Neap tides occur at the first and last quarters, when the sun and the moon are at right angles at the earth; the tidal range is least.
Past board exam questions
ICSE 2019 sample paper · 2 marks
How does the rotation of the earth influence the direction of the ocean currents?
Show answer
Because of the rotation of the Earth (Coriolis force), ocean currents are deflected to the right (clockwise) in the Northern hemisphere and to the left (anti-clockwise) in the Southern hemisphere. Rotation from west to east also makes water pile up along the eastern coasts of continents, so currents flow westward near the equator and form circular gyres.
ICSE 2019 sample paper · 2 marks
State four factors that affect the movement of ocean currents.
Show answer
Any four: 1. Planetary (prevailing) winds. 2. Rotation of the Earth (Coriolis force). 3. Differences in temperature of ocean water. 4. Differences in salinity (and density) of ocean water. 5. Shape and position of the coastline / landmasses. 6. Seasons.
ICSE 2019 sample paper · 2 marks
Draw a neat and well labelled diagram showing the two types of tides.
Show answer
Diagram 1 - Spring tide: Sun, Moon and Earth in a straight line (at new moon or full moon); the pull of sun and moon combine, giving very high high-tides and very low low-tides. Labels: Sun, Moon (new moon/full moon position), Earth, high tide bulges, low tide.
Diagram 2 - Neap tide: Sun and Moon at right angles to each other with respect to the Earth (first and last quarter); their pulls oppose each other, giving lower high-tides and higher low-tides. Labels: Sun, Moon (quarter), Earth, lunar high tide, solar tide, low tide.
ICSE 2019 sample paper · 3 marks
Give a reason for each of the following: (i) Newfoundland has rich fishing grounds. (ii) The eastern coast of Japan is warm even in January. (iii) The tidal range differs from one water body to the other.
Show answer
(i) Off Newfoundland the warm Gulf Stream meets the cold Labrador Current; the mixing of warm and cold water favours the growth of plankton, the food of fish, and the shallow Grand Banks provide breeding ground, so fish are abundant.
(ii) The warm Kuroshio (Japan) Current flows along the eastern coast of Japan and raises the temperature of the coastal areas, keeping them warm even in winter (January).
(iii) The tidal range depends on the shape and depth of the water body, the shape of the coastline and the width of its opening to the ocean; e.g. funnel-shaped bays and shallow seas have high ranges (Bay of Fundy) while enclosed seas such as the Mediterranean have small ranges.
ICSE 2019 sample paper · 3 marks
State three ways in which oceans are important to us.
Show answer
Any three: 1. They moderate the climate of coastal areas and are the main source of moisture for rainfall. 2. They provide food (fish and other sea food) and minerals, salt and petroleum. 3. They provide cheap water routes for international trade and transport. 4. Tides and waves can be used to generate electricity. 5. They support tourism and recreation.