Class | Subject | Book | Chapter | Session | Questions covered |
|---|---|---|---|---|---|
XI | Geography | Fundamentals of Physical Geography (NCERT) | 13 – Movements of Ocean Water | 2026-27 | 11 of 11 |
Chapter at a glance
Topic | Key points |
|---|---|
Waves | Horizontal movement of energy, not water; generated mainly by wind; crest, trough, wave height, length, period |
Tides | Periodic rise and fall of sea level once or twice a day, caused by gravitational pull of the moon and sun and centrifugal force |
Types of tides | Semi-diurnal, diurnal, mixed; spring tides (sun, moon, earth in line) and neap tides (at right angles) |
Perigee and apogee | Moon closest (perigee) → unusually high tides; earth closest to sun (perihelion) about 3 January |
Currents | Steady flow of water in a definite direction; caused by insolation, winds, gravity, Coriolis force, density differences |
Effects | Warm currents raise coastal temperatures (Gulf Stream–North Atlantic Drift warms NW Europe); cold currents cause deserts and fog |
Exercise Questions
Question 1 (i)
Upward and downward movement of ocean water is known as the:
(a) tide (b) wave (c) current (d) none of the above
Answer
The periodic rise and fall (vertical movement) of sea level is the tide.
- tide
Question 1 (ii)
Spring tides are caused:
(a) As result of the moon and the sun pulling the earth gravitationally in the same direction. (b) As result of the moon and the sun pulling the earth gravitationally in the opposite direction. (c) Indention in the coast line. (d) None of the above.
Answer
Spring tides occur at new moon and full moon, when the sun, moon and earth are in a straight line and their pulls combine.
- As result of the moon and the sun pulling the earth gravitationally in the same direction.
Question 1 (iii)
The distance between the earth and the moon is minimum when the moon is in:
(a) Aphelion (b) Perigee (c) Perihelion (d) Apogee
Answer
- Perigee
Question 1 (iv)
The earth reaches its perihelion in:
(a) October (b) September (c) July (d) January
Answer
The earth is closest to the sun around 3 January.
- January
Question 2 (i)
What are waves?
Answer
Waves are the oscillatory movements of the ocean surface in which energy travels across the water, but the water particles mainly move in small circles and do not travel forward. They are generated mostly by wind, and break when they reach shallow water near the shore.
Up-and-down oscillations of the ocean surface that transmit energy (mostly from wind) while water particles move in circles.
Question 2 (ii)
Where do waves in the ocean get their energy from?
Answer
Waves get most of their energy from the wind: friction between wind and the water surface transfers energy to the water. Size depends on wind speed, duration and fetch. (Large waves can also arise from earthquakes and volcanic eruptions — tsunamis.)
Mainly from the wind blowing over the water surface (and, for tsunamis, from undersea earthquakes).
Question 2 (iii)
What are tides?
Answer
Tides are the periodical rise and fall of sea level, once or twice a day, caused mainly by the gravitational attraction of the moon and, to a lesser extent, the sun. The high water is called high tide and the low water low tide.
The periodic rise and fall of sea level, once or twice a day, caused mainly by the moon’s and sun’s attraction.
Question 2 (iv)
How are tides caused?
Answer
Tides are caused by the gravitational pull of the moon and the sun together with the centrifugal force of the earth–moon system. On the side of the earth facing the moon, the moon’s pull is greater, causing a tidal bulge. On the opposite side, the centrifugal force exceeds the pull, causing a second bulge. As the earth rotates, each place passes through these bulges, giving two high tides and two low tides a day.
By the gravitational pull of the moon (and sun) plus centrifugal force, creating two tidal bulges through which places pass as the earth rotates.
Question 2 (v)
How are tides related to navigation?
Answer
Tides help navigation by raising the water level near the coast and in harbours, allowing large ships to enter shallow ports, estuaries and river mouths during high tide (e.g. Kolkata port on the Hooghly). Tidal currents also help ships move in and out of ports. Knowledge of tidal times is essential for safe navigation.
High tides raise water levels so large ships can enter shallow harbours and river mouths; tide timings are needed for safe navigation.
Question 3 (i)
How do currents affect the temperature? How does it affect the temperature of coastal areas in the N. W. Europe?
Answer
Effect of currents on temperature
- Warm currents flow from low to high latitudes and raise the temperature of coastal areas, keeping ports ice-free and causing rain.
- Cold currents flow from high to low latitudes and lower the temperature of coastal areas; they cool the air, causing fog and making coasts dry (e.g. deserts along west coasts in tropical latitudes — Atacama by the Peru current, Namib by the Benguela current).
- Where warm and cold currents meet, fog forms and plankton thrive, creating rich fishing grounds (e.g. Grand Banks of Newfoundland).
North-west Europe: the warm Gulf Stream and its extension, the North Atlantic Drift, flow north-eastward across the Atlantic to the coasts of north-west Europe (Britain, Norway). Along with the westerly winds that carry the warmth inland, they give NW Europe a mild, moist climate with warmer winters than other places at the same latitude. Ports such as those of Norway remain ice-free in winter.
Warm currents raise and cold currents lower coastal temperatures; the warm Gulf Stream–North Atlantic Drift, with westerlies, keeps NW Europe mild in winter and its ports ice-free.
Question 3 (ii)
What are the causes of currents?
Answer
Primary forces
- Heating by solar energy: water near the equator expands, so the sea level there is slightly higher (about 8 cm), creating a slight gradient and water flows down the slope.
- Wind: winds blowing over the ocean surface push water forward; friction makes water move in the wind’s direction (e.g. trade winds drive equatorial currents).
- Gravity: tends to pull water down to pile up, creating gradients.
- Coriolis force: deflects currents to the right in the northern hemisphere and to the left in the southern, forming large circular gyres.
Secondary factors
- Differences in water density caused by temperature and salinity: dense (cold, salty) water sinks and less dense water rises, producing vertical and deep-water circulation.
- Shape of coastlines and ocean floor, which deflect and divide currents.
Solar heating, winds, gravity and the Coriolis force are the primary causes; density differences (temperature and salinity) and coastline shape act as secondary factors.
Topic checklist
- Waves: characteristics and causes
- Tides: causes, types, spring and neap tides, importance
- Ocean currents: causes, types, major currents
- Effects of ocean currents