GEOGRAPHY CLASS 11 – CHAPTER 4

DISTRIBUTION OF OCEANS AND CONTINENTS

1. INTRODUCTION

• The earth’s surface is made up of continents and oceans arranged in a distinct pattern.
• This pattern did not exist since the formation of the Earth.
• The present distribution is a result of long geological processes including continental drift, seafloor spreading, and plate tectonics.
• The chapter explains how continents and oceans have moved, the forces responsible, and the scientific evidence supporting these theories.

2. DISTRIBUTION OF OCEANS AND CONTINENTS

2.1 Present Arrangement of Continents

• The Earth has seven major continents: Asia, Africa, North America, South America, Antarctica, Europe, and Australia.
• Continents are not evenly distributed:
  • The Northern Hemisphere contains major landmasses (Eurasia, North America).
  • The Southern Hemisphere has extensive oceans (Pacific, Indian, Southern Ocean).
• Continents differ in shape and size:
  • Asia: largest and most populated.
  • Australia: smallest continent.
• They are separated by large water bodies and plate boundaries.

2.2 Distribution of Oceans

• Earth has five major oceans: Pacific, Atlantic, Indian, Southern, and Arctic.
• Oceans occupy about 71% of Earth’s surface.
• Pacific Ocean: largest and deepest; known for the Ring of Fire (volcanic activity).
• Atlantic Ocean: S-shaped; separates North & South America from Europe & Africa.
• Indian Ocean: triangular ocean located mainly in the Southern Hemisphere.
• Arctic Ocean: smallest and shallowest; surrounded by continents.
• Southern Ocean: encircles Antarctica.

3. EVIDENCE IN SUPPORT OF CONTINENTAL DRIFT

3.1 Continental Drift Theory (Alfred Wegener, 1912)

• Proposed that continents were once joined as a single landmass called Pangaea.
• Pangaea broke into two parts:
  • Laurasia (north)
  • Gondwana (south)
• Later split into present-day continents.
• Drift occurred over millions of years.

3.2 Evidence Categories

Wegener supported his theory using geological, biological, climatic, and geographical evidence.

A. Geological Evidence

• Matching coastlines:
  • The coasts of South America and Africa fit like a jigsaw puzzle.
• Rock formations and mountain chains match:
  • Appalachians in North America align with mountains in Scotland and Norway.
• Similar geological structures found across different continents.

B. Biological Evidence

• Fossils of identical species found on continents now separated by oceans:
  • Mesosaurus fossils found in South America and Africa.
  • Glossopteris (fern) fossils found in India, Australia, Antarctica, and South America.
• Indicates that these landmasses were once connected.

C. Climatic Evidence

• Past climates do not match present locations:
  • Coal deposits found in cold regions like Antarctica indicate earlier warm climate.
  • Glacier deposits in India, Africa, and Australia show they were closer to South Pole earlier.

D. Geographical Evidence

• Continents appear to fit together as part of one supercontinent.
• Similar sedimentary sequences across continents show common origin.

3.3 Limitations of Continental Drift

• Wegener could not explain:
  • The forces responsible for drifting.
  • The mechanism of continental movement.
• This limited acceptance of the theory until later discoveries.

4. FORCES FOR DRIFTING

Wegener proposed two forces which were later found insufficient.

4.1 Pole-Fleeing Force

• Based on Earth’s rotation.
• Centrifugal force pushes continents away from the poles.
• Too weak to move continents.

4.2 Tidal Force

• Gravitational pull of the Sun and Moon.
• Thought to move continents westward.
• Also too weak and geologically unrealistic.

Modern Understanding

• These forces cannot explain large-scale continental movement.
• Later discoveries in the mid-20th century led to acceptance of mantle convection, seafloor spreading, and plate tectonics as correct mechanisms.

5. POST-DRIFT STUDIES

After Wegener, several studies provided new data.

5.1 Mapping of Ocean Floors (1920s–1950s)

• Sonar technology (echo sounding) revealed:
  • Mid-oceanic ridges
  • Deep sea trenches
  • Abyssal plains
  • Seamounts and guyots

5.2 Heat Flow Studies

• High heat flow recorded along mid-ocean ridges.
• Low heat flow recorded along trenches.
• Supported the idea of mantle rising and sinking.

5.3 Paleomagnetism

• Rocks preserve magnetic signatures.
• Revealed polar wandering paths and magnetic stripes on the ocean floor.
• Showed continents have moved.

6. OCEAN FLOOR CONFIGURATION

Ocean floor is not flat; it has distinct structures.

6.1 Mid-Oceanic Ridges

• Underwater mountain chains.
• Example: Mid-Atlantic Ridge, East Pacific Rise.
• Zones of volcanic eruptions and creation of new crust.

6.2 Deep Sea Trenches

• Long, narrow depressions.
• Formed at subduction zones.
• Deepest: Mariana Trench.

6.3 Abyssal Plains

• Flat, deep oceanic surfaces, covered with sediments.

6.4 Seamounts and Guyots

• Seamounts: submerged volcanic mountains.
• Guyots: flat-topped seamounts.

6.5 Continental Shelf and Slope

• Shelf: shallow submerged margins of continents.
• Slope: steep boundary between continental crust and deep ocean floor.

7. DISTRIBUTION OF EARTHQUAKES AND VOLCANOES

7.1 Earthquakes

• Most earthquakes occur along:
  • Plate boundaries
  • Mid-oceanic ridges
  • Subduction zones
  • Transform faults (San Andreas Fault)

7.2 Volcanoes

• Found around boundaries of plates.
• Pacific Ring of Fire has highest concentration of volcanoes.
• Associated with:
  • Divergent boundaries – volcanic ridges
  • Convergent boundaries – volcanic arcs
  • Hotspots – Hawaii, Yellowstone

7.3 Significance

• Distribution patterns support plate tectonics.
• Seismic belts correlate with plate boundaries.

8. CONCEPT OF SEA FLOOR SPREADING

Proposed by Harry Hess (1960s).
Explains formation and movement of ocean floor.

8.1 Main Concepts

• Magma rises from the mantle at mid-ocean ridges.
• New oceanic crust is formed.
• Old crust moves away.
• Crust eventually subducts at trenches.

8.2 Evidence for Sea Floor Spreading

1. Magnetic Stripes

• Symmetrical pattern of magnetic bands on both sides of the ridge.
• Indicates repeated reversals in Earth’s magnetic field.

2. Age of Oceanic Crust

• Youngest rocks at ridges.
• Oldest near trenches.

3. Heat Flow

• High at ridges, low at trenches.

4. Sediment Thickness

• Thin near ridges.
• Thick near continental margins.

8.3 Significance

• Solved the major flaw in Wegener’s theory.
• Provided mechanism for continental drift.

9. PLATE TECTONICS

Developed in 1960s–70s integrating continental drift and seafloor spreading.

9.1 Basic Ideas

• Earth’s lithosphere is divided into seven major and several minor plates.
• Plates float on the asthenosphere.
• Plates move due to mantle convection.

9.2 Types of Plate Boundaries

A. Divergent Boundaries

• Plates move apart.
• Found at mid-ocean ridges.
• New crust created.
• Example: Mid-Atlantic Ridge.

B. Convergent Boundaries

• Plates move towards each other.
• Types:
  • Ocean–continent: Andes Mountains
  • Ocean–ocean: Japan trench
  • Continent–continent: Himalayas
• Subduction leads to trenches and volcanoes.

C. Transform Boundaries

• Plates slide past each other.
• No crust created or destroyed.
• Example: San Andreas Fault.

9.3 Driving Mechanisms of Plate Motion

• Mantle convection currents
• Slab pull
• Ridge push

9.4 Effects of Plate Tectonics

• Formation of mountains, volcanoes, earthquakes.
• Continental drift and ocean basin evolution.
• Distribution of fossil fuels and minerals.

10. MOVEMENT OF THE INDIAN PLATE

10.1 Background

• Indian Plate is composed of:
  • Peninsular India
  • Australian region
  • Adjacent oceanic crust

10.2 Northward Drift

• The Indian Plate broke from Gondwana ~140 million years ago.
• Moved northward at high speed (~15 cm/year initially).

10.3 Collision with Eurasian Plate

• Occurred ~40–50 million years ago.
• Led to formation of Himalayas.
• Movement continues today (5 cm/year).

10.4 Geological Significance

• Formation of Tibetan Plateau.
• Frequent earthquakes in Himalayan region due to ongoing convergence.
• Indian Ocean floor shows evidence of spreading activity.

11. CONCLUSION

• The distribution of continents and oceans is not static; it is the result of millions of years of geological processes.
• Wegener’s continental drift theory introduced the idea of moving continents, although lacking mechanism.
• Seafloor spreading and paleomagnetic studies provided crucial evidence for movement.
• Plate tectonics unified all earlier theories and is the current scientific explanation for Earth’s dynamic structure.
• The movement of the Indian Plate demonstrates the continuous nature of tectonic processes.
• Understanding these processes helps in predicting natural hazards and exploring natural resources.