Class 11 Geography Notes Chapter 14 (Movements of ocean water) – Fundamental of Physical Geography Book

Detailed Notes with MCQs of Chapter 14: Movements of Ocean Water from our Physical Geography textbook. This is a crucial chapter, not just for understanding our planet's systems, but also frequently tested in various government exams. Pay close attention.

Ocean water is never static. Its movements are dynamic and occur at different scales, driven by various forces. We broadly categorize these movements into three types: Waves, Tides, and Ocean Currents.

1. Waves

• Definition: Waves are essentially the oscillatory movement of surface water, characterized by a rise and fall. In a wave, it's the energy that moves across the ocean surface, not the water itself (water particles move largely in small circles).
• Formation: Primarily generated by the friction of wind blowing over the water surface. The stronger the wind and the longer the duration and fetch (distance over which wind blows), the larger the waves. Gravitational pull (tides) and underwater disturbances like earthquakes or volcanic eruptions (tsunamis) can also generate waves.
• Characteristics of Waves:
  • Crest: The highest point of a wave.
  • Trough: The lowest point of a wave between two crests.
  • Wave Height: The vertical distance from the trough to the crest.
  • Wave Amplitude: Half of the wave height.
  • Wavelength: The horizontal distance between two successive crests or troughs.
  • Wave Period: The time interval between two successive crests or troughs passing a fixed point.
  • Wave Speed: Rate at which the wave moves through the water (related to wavelength and period).
  • Wave Frequency: The number of waves passing a given point per unit of time.
• Wave Breaking: As waves approach the coast, the depth of the water decreases. The friction with the seabed slows down the base of the wave, while the crest continues moving faster. This causes the wave height to increase and the wavelength to decrease until the wave becomes unstable and collapses or 'breaks', creating surf.
• Tsunamis: These are not wind-generated waves. They are extremely long-wavelength waves caused by sudden displacement of large volumes of water, typically due to underwater earthquakes, volcanic eruptions, or landslides. In deep oceans, they have low height but travel at very high speeds. As they reach shallow coastal waters, their speed decreases, but their height increases dramatically, causing immense destruction.

2. Tides

• Definition: Tides are the periodic and predictable rise and fall of sea level, typically occurring twice a day (though variations exist). They are essentially very long-period waves.
• Causes:
  • Gravitational Pull: Primarily the Moon's gravity, and to a lesser extent, the Sun's gravity, pulls the water on Earth towards them, creating a bulge.
  • Centrifugal Force: As the Earth rotates, a centrifugal force acts outwards, opposite to the gravitational pull of the Moon/Sun. This creates a second bulge on the side of the Earth facing away from the Moon/Sun.
  • The interaction of these forces results in two high tides and two low tides approximately every 24 hours and 50 minutes (because the Moon also orbits the Earth).
• Types of Tides (Based on Frequency):
  • Semi-diurnal: Two high tides and two low tides each day, of approximately equal height (e.g., Atlantic coast of North America).
  • Diurnal: Only one high tide and one low tide each day (e.g., Gulf of Mexico).
  • Mixed: Tides vary greatly in height; often two high and two low tides per day but with significant differences in height (e.g., Pacific coast of North America).
• Types of Tides (Based on Sun, Moon, Earth Positions):
  • Spring Tides: Occur when the Sun, Moon, and Earth are aligned in a straight line (syzygy), either during the new moon (conjunction) or full moon (opposition). The gravitational forces of the Sun and Moon combine, resulting in unusually high high tides and unusually low low tides (large tidal range).
  • Neap Tides: Occur when the Sun and Moon are at right angles to each other relative to the Earth (quadrature), during the first and third quarter moon phases. The gravitational forces partially cancel each other out, resulting in lower high tides and higher low tides (small tidal range).
• Importance of Tides:
  • Navigation: High tides allow larger ships to enter shallower ports and harbours. Tidal currents affect navigation routes.
  • Fishing: Tidal flows concentrate nutrients and fish in certain areas.
  • Coastal Ecology: Tides influence the zonation of life in intertidal zones.
  • Tidal Energy: The predictable rise and fall of water can be harnessed to generate electricity.
• Tidal Bore: When a tide enters a narrow, shallow estuary or river, the leading edge of the incoming tide forms a wave (or waves) that travels up the river against the current.

3. Ocean Currents

• Definition: Ocean currents are the continuous, directed movement of large masses of seawater, flowing like rivers within the oceans. They circulate water globally.
• Forces Influencing Currents:
  • Primary Forces (Initiate Movement):
    • Solar Heating: Causes water to expand. Water near the equator is warmer and slightly higher than in mid-latitudes, creating a gentle slope.
    • Wind: Drags the surface water through friction, setting it in motion.
    • Gravity: Tends to pull the water down the slope created by heating differences or wind piling up water.
    • Coriolis Force: Due to Earth's rotation, deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes large-scale circular patterns called Gyres.
  • Secondary Forces (Modify Flow):
    • Differences in Water Density: Colder, saltier water is denser and sinks, while warmer, less salty water is less dense and rises. This drives deep ocean circulation (thermohaline circulation).
    • Configuration of Coastlines: Landmasses obstruct and divert current flow.
• Types of Ocean Currents:
  • Based on Depth:
    • Surface Currents (approx. 10%): Driven mainly by wind, constitute the upper 400m of ocean water. Form large gyres.
    • Deep Water Currents (approx. 90%): Driven by density differences (temperature and salinity), known as Thermohaline Circulation or the 'Global Conveyor Belt'. This is a slow but massive circulation connecting all ocean basins.
  • Based on Temperature:
    • Warm Currents: Flow from lower latitudes (equator) towards higher latitudes (poles), bringing warmer water to colder regions. Generally flow along the east coasts of continents in lower/middle latitudes. (e.g., Gulf Stream, Kuroshio).
    • Cold Currents: Flow from higher latitudes towards lower latitudes, bringing colder water to warmer regions. Generally flow along the west coasts of continents in lower/middle latitudes. (e.g., Canary Current, California Current, Peru Current).
• Major Ocean Currents (Know the key ones and their locations/type):
  • Atlantic: North Equatorial (W), South Equatorial (W), Equatorial Counter Current, Gulf Stream (W), North Atlantic Drift (W), Canary (C), Labrador (C), Benguela (C), Brazil (W), Falkland (C).
  • Pacific: North Equatorial (W), South Equatorial (W), Equatorial Counter Current, Kuroshio (W), North Pacific Drift (W), Oyashio (C), California (C), Peru/Humboldt (C), East Australian (W).
  • Indian: North Equatorial (reverses seasonally with Monsoons - SW Monsoon Drift (W), NE Monsoon Drift (W)), South Equatorial (W), Mozambique (W), Agulhas (W), West Australian (C).
• Effects of Ocean Currents:
  • Climate Moderation: Warm currents raise temperatures of adjacent coasts (e.g., North Atlantic Drift keeps Western Europe milder). Cold currents lower temperatures and can increase aridity (e.g., Peru Current and Atacama Desert).
  • Rainfall: Winds blowing over warm currents pick up moisture and bring rain to coastal areas. Winds blowing over cold currents are dry.
  • Fog Formation: Mixing of warm and cold currents creates dense fog (e.g., Newfoundland - Gulf Stream meets Labrador Current).
  • Fishing Grounds: Areas where warm and cold currents mix are rich in plankton, supporting major fisheries (e.g., Japan, Newfoundland).
  • Navigation: Ships can save fuel and time by travelling with currents. Icebergs carried by cold currents pose hazards.
  • Desert Formation: Cold currents off the west coasts in tropical/subtropical latitudes contribute to the formation of coastal deserts (Atacama, Namib, Baja California).

This covers the core concepts of ocean water movements. Remember the driving forces and the distinct characteristics and effects of waves, tides, and currents. Now, let's test your understanding with some MCQs.

Multiple Choice Questions (MCQs):

1. The primary force responsible for generating most surface ocean waves is:
   a) Gravitational pull of the Moon
   b) Wind friction
   c) Underwater earthquakes
   d) Differences in water density

2. Spring tides occur when:
   a) The Sun and Moon are at right angles relative to the Earth.
   b) The Earth is closest to the Sun.
   c) The Sun, Moon, and Earth are aligned in a straight line.
   d) Only during the summer solstice.

3. The deflection of ocean currents to the right in the Northern Hemisphere is caused by:
   a) Gravity
   b) Wind patterns
   c) Temperature differences
   d) Coriolis force

4. Which type of tide is characterized by having two high tides and two low tides of significantly different heights each day?
   a) Diurnal tide
   b) Semi-diurnal tide
   c) Mixed tide
   d) Neap tide

5. The Gulf Stream in the Atlantic Ocean is an example of a:
   a) Cold current flowing towards the equator
   b) Warm current flowing towards the poles
   c) Deep water thermohaline current
   d) Seasonal monsoon drift

6. Which of the following is NOT a direct characteristic used to describe a wave?
   a) Wavelength
   b) Wave height
   c) Water salinity
   d) Wave period

7. Thermohaline circulation (deep ocean currents) is primarily driven by differences in:
   a) Wind speed and direction
   b) Air pressure
   c) Water density (temperature and salinity)
   d) Coastal configuration

8. The meeting of the warm Gulf Stream and the cold Labrador Current near Newfoundland results in:
   a) Formation of rich fishing grounds and dense fog
   b) Creation of tropical cyclones
   c) Unusually high spring tides
   d) A strong diurnal tidal pattern

9. The Peru Current (Humboldt Current) along the west coast of South America is a _______ current and contributes to the _______ conditions of the Atacama Desert.
   a) Warm / Wet
   b) Warm / Arid
   c) Cold / Wet
   d) Cold / Arid

10. A Tsunami is a type of wave caused by:
    a) Strong monsoon winds
    b) The alignment of Sun, Moon, and Earth
    c) Displacement of large volumes of water due to underwater disturbances
    d) The mixing of warm and cold ocean currents

Answer Key:

1. b
2. c
3. d
4. c
5. b
6. c
7. c
8. a
9. d
10. c

Study these notes thoroughly. Understanding the dynamics of ocean water is fundamental to physical geography. Let me know if any part needs further clarification.