Notes Class 8 Chapter 4 Geography Maharashtra Board

Notes For All Chapters – Geography Class 8

Structure of Ocean Floor

Introduction

• Continents and oceans are part of the lithosphere and hydrosphere, respectively.
• Oceans cover about 71% of the Earth’s surface.
• Just like the surface of the land, the ocean floor is also uneven and contains various landforms.
• Ocean floor landforms can be classified based on depth and shape.

Relief of Ocean Floor

The land submerged under oceanic water is called the ocean floor.

• The average depth of the oceans = 3700 metres
• The structure of the ocean floor is not uniform and varies from one ocean to another.
• As we move away from the coast, the structure of the ocean floor changes.

Major Landforms of the Ocean Floor

1. Continental Shelf

• Shallow part of the ocean floor near the coast.
• Slope = gentle.
• Depth = up to 200 metres below sea level.
• Extent: Varies – narrow along some coasts and extends hundreds of kilometers along others.
• Importance:
  • Rich in marine life due to sunlight penetration.
  • Algae and plankton grow, supporting fish populations → Good fishing grounds.
  • Contains natural resources like mineral oil and natural gas.
  • Example: Mumbai High (source of mineral oil and natural gas).

2. Continental Slope

• Begins where the continental shelf ends.
• Steeper slope compared to the continental shelf.
• Depth = 200 m to 3600 m (sometimes more).
• Importance: Boundary between continents and ocean floor.

3. Abyssal Plains

• Beyond the continental slope.
• Flat and smooth part of the ocean floor.
• Consists of various submerged landforms like hills and plateaus.

4. Submerged Hills and Mountains

• Hills and mountains on the ocean floor.
• Width = hundreds of kilometers
• Length = thousands of kilometers
• Peaks of some hills emerge above sea level and form marine islands.
  • Example:
    • Iceland (Atlantic Ocean)
    • Andaman and Nicobar Islands (Bay of Bengal)
• Some marine islands have flat, extensive summits called submarine plateaus.
  • Example: Chagos Plateau (Indian Ocean).

5. Marine Deeps and Trenches

• Deep, narrow, and steep landforms on the ocean bed.
• Shallow formations = Marine Deeps
• Deep formations = Trenches
• Depth = Thousands of meters below sea level.
  • Example:
    • Mariana Trench (Pacific Ocean) = 11,034 metres deep (deepest in the world).
• Geologically Active Zones:
  • Earthquakes and volcanic eruptions common.
  • Formation of tsunamis due to underwater earthquakes.

6. Marine Deposits

• Various materials deposited on the ocean floor.
• Sources of Deposits:
  • Pebbles, clay, soil, etc. from rivers and glaciers.
  • Volcanic ash and lava.
  • Remains of marine plants and animals → Forms marine oozes (30% of deposits).
• Sedimentary Rocks:
  • Formed by the pressure of sea water over deposited layers.
• Pollution:
  • Human waste (sewage, chemicals, plastics, radioactive material)
  • Harmful to marine life and environment.

Mean Sea Level

• Used as a reference for measuring elevation and depth.
• Average of the highest high tides and the lowest low tides = zero level
• Example:
• Mount Everest = +8848 metres
• Mariana Trench = -11034 metres
• In India, the reference level is based on the sea level at Chennai.

Age of the Ocean Floor

• Ocean floor deposits = Not older than 200 million years
• Rocks on continents = Up to 3200 million years old
• Conclusion:
• Ocean floor is younger than the Earth’s surface.
• Supports the theory of plate tectonics.

Importance of Ocean Floor Study

• Helps in understanding marine life.
• Helps in locating natural resources (oil, gas, minerals).
• Provides insights into geological processes (earthquakes, volcanoes, tsunamis).
• Supports fishing industry.
• Contributes to naval and defense strategies.

Environmental Hazards

• Natural Hazards:
• • Earthquakes
  • Volcanic eruptions
  • Tsunamis
• Human-Induced Hazards:
  • Pollution from plastic, chemicals, radioactive waste
  • Destruction of marine ecosystems
  • Overfishing and loss of biodiversity

Key Takeaways

• The ocean floor has a diverse and complex structure.
• Continental shelf is economically valuable due to fishing and mineral resources.
• Deep-sea trenches and marine deeps are geologically active and prone to tsunamis.
• Marine deposits provide valuable information about marine life and resources.
• Human-induced pollution is a threat to marine biodiversity.