Water (Oceans) Class 11 Geography Notes

This chapter explains the hydrological cycle, relief features of ocean floors, temperature distribution and salinity of ocean waters. It also discusses submarine relief features such as continental shelf, continental slope, deep sea plains and ocean trenches.

• Hydrological cycle
• Components of water cycle
• Ocean floor relief
• Continental shelf
• Continental slope
• Deep sea plains
• Ocean trenches
• Minor ocean relief features
• Ocean temperature distribution
• Thermocline
• Ocean salinity
• Halocline
• Horizontal and vertical salinity distribution

Earth as the Blue Planet

• Earth is called Blue Planet
• Large portion of Earth is covered with water
• Water is essential for life

Water as a Cyclic Resource

• Water can be reused
• Continuously circulates

Hydrological Cycle

Hydrological cycle refers to continuous circulation of water between oceans, atmosphere, land surface, underground layers and living organisms.

Forms of Water

1. Liquid form
2. Solid form
3. Gaseous form

Importance of Hydrological Cycle

• Maintains water balance
• Supports life
• Recycles water resources

Distribution of Water on Earth

• About 91% water is in oceans
• Remaining water exists in glaciers, groundwater, lakes, rivers and atmosphere

Nature of Ocean Floors

• Ocean floors are rugged
• Contain mountains, trenches and plains
• Features formed by tectonic activity, volcanism and deposition

Major Oceans

1. Pacific Ocean
2. Atlantic Ocean
3. Indian Ocean
4. Southern Ocean
5. Arctic Ocean

Major Divisions of Ocean Floors

1. Continental Shelf
2. Continental Slope
3. Deep Sea Plain
4. Oceanic Deeps or Trenches

Meaning of Continental Shelf

Continental shelf is the extended margin of continents occupied by shallow seas.

• Shallowest part of ocean
• Ends at shelf break

Characteristics

• Average width about 80 km
• Average gradient about 1°

Largest Continental Shelf

• Located in Arctic Ocean
• Called Siberian Shelf
• Width about 1,500 km

Economic Importance

• Rich in sediment deposits
• Source of fossil fuels
• Rich fishing grounds

Meaning of Continental Slope

Continental slope connects continental shelf with ocean basin.

Characteristics

• Begins at shelf break
• Gradient between 2° and 5°
• Depth ranges from 200–3,000 metres
• Contains trenches and canyons

Deep sea plains are gently sloping and among the flattest regions of the world.

• Depth between 3,000–6,000 metres
• Covered by clay and silt

Meaning of Ocean Trenches

Ocean trenches are the deepest parts of oceans.

• Narrow and steep-sided
• 3–5 km deeper than surrounding ocean floor

Importance of Trenches

• Associated with earthquakes
• Associated with volcanoes
• Related to plate movements

Distribution

• Pacific Ocean – 32 trenches
• Atlantic Ocean – 19 trenches
• Indian Ocean – 6 trenches

Mid-Oceanic Ridges

• Chains of underwater mountains
• Separated by depressions
• Peaks may rise above sea level
• Example – Mid-Atlantic Ridge

Seamounts

• Underwater volcanic mountains
• Do not reach ocean surface
• Height between 3,000–4,500 metres

Submarine Canyons

• Deep underwater valleys
• Found across shelves and slopes
• Example – Hudson Canyon

Guyots

• Flat-topped seamounts
• Formed due to subsidence

Atolls

• Coral reef islands
• Surround central lagoon
• Found in tropical oceans

Heating of Ocean Water

• Ocean water receives heat from solar radiation
• Heating and cooling slower than land

Factors Affecting Ocean Temperature

1. Latitude
2. Distribution of land and water
3. Prevailing winds
4. Ocean currents

Latitude

• Temperature decreases from equator towards poles
• Due to decrease in insolation

Prevailing Winds

• Offshore winds cause upwelling of cold water
• Onshore winds raise coastal temperature

Ocean Currents

• Warm currents increase temperature
• Cold currents lower temperature
• Examples – Gulf Stream and Labrador Current

Meaning of Thermocline

Thermocline is the zone of rapid temperature decrease with depth.

• Usually found 100–400 metres below surface

Three Thermal Layers of Ocean

First Layer

• Warm surface layer
• 500 metres thick
• Temperature between 20°–25°C

Second Layer

• Thermocline layer
• Rapid temperature decline
• Thickness 500–1,000 metres

Third Layer

• Cold deep ocean layer

Polar Oceans

• Surface water near 0°C
• Temperature change with depth is small

Meaning of Salinity

Salinity refers to total dissolved salts in seawater.

• Measured in parts per thousand (‰ or ppt)
• Represents grams of salt per 1,000 g seawater

Factors Affecting Salinity

1. Evaporation
2. Precipitation
3. River water inflow
4. Freezing and thawing
5. Wind
6. Ocean currents

Highest Salinity Water Bodies

Meaning of Halocline

Halocline is the zone of rapid salinity increase with depth.

Salinity and Density

• Higher salinity increases density
• High salinity water sinks below low salinity water

Stratification of Ocean Water

• Difference in salinity causes stratification
• Salinity generally increases with depth

Very Short Answer Questions

1. Why is Earth called Blue Planet?
2. What is thermocline?
3. What is salinity?
4. What is continental shelf?
5. What is halocline?

Short Answer Questions

1. Explain hydrological cycle.
2. Explain the continental slope.
3. What are ocean trenches?
4. Explain factors affecting ocean salinity.
5. Differentiate between thermocline and halocline.

Long Answer Questions

1. Explain the major relief features of ocean floors.
2. Discuss the factors affecting temperature distribution of oceans.
3. Explain horizontal and vertical distribution of salinity.
4. Discuss the hydrological cycle and its importance.
5. Explain the relief features of ocean floors in detail.

1. Why is Earth called the Blue Planet?

Because most of Earth’s surface is covered by water.

2. What is thermocline?

A zone where ocean temperature decreases rapidly with depth.

3. What is salinity?

Total dissolved salts in seawater.

4. Which is the deepest part of oceans?

Ocean trenches are the deepest parts of oceans.

5. What is the importance of continental shelf?

It contains rich fishing grounds and fossil fuel deposits.

• Earth:
  • Blue Planet
• Hydrological Cycle:
  • Continuous water circulation
• Major Ocean Divisions:
  • Continental shelf
  • Continental slope
  • Deep sea plain
  • Ocean trenches
• Continental Shelf:
  • Shallowest ocean part
• Deep Sea Plains:
  • Flattest ocean regions
• Ocean Trenches:
  • Deepest parts of oceans
• Minor Relief Features:
  • Ridges
  • Seamounts
  • Guyots
  • Atolls
• Factors Affecting Ocean Temperature:
  • Latitude
  • Winds
  • Currents
• Thermocline:
  • Rapid temperature decrease zone
• Average Surface Temperature:
  • 27°C
• Salinity:
  • Dissolved salt content
• Average Ocean Salinity:
  • 33‰–37‰
• Red Sea:
  • High salinity
• Bay of Bengal:
  • Lower salinity
• Arabian Sea:
  • Higher salinity
• Halocline:
  • Rapid salinity increase zone
• High Salinity Water:
  • Denser and sinks downward