Chapter – 4

Distribution of Oceans and Continents

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 4 (Distribution of Oceans and Continents) in English. These notes are useful for the students who are going to appear in class 11 board exams.

Chapter 4: Distribution of Oceans and Continents

Introduction

• The Earth’s surface is composed of continents (29%) and oceans (71%).  
• The positions of the continents and oceans have changed over time and will continue to change in the future.  

Continental Drift

• Early Observations:
  • Abraham Ortelius (1596), a Dutch map maker, was the first to propose the possibility of continents being joined together.  
  • Antonio Pellegrini drew a map showing the three continents (Europe, Africa, and the Americas) together.  
• Alfred Wegener’s Continental Drift Theory (1912):
  • Wegener, a German meteorologist, proposed a comprehensive theory of continental drift.  
  • He suggested that all continents were once joined together in a supercontinent called Pangaea, surrounded by a mega-ocean called Panthalassa.  
  • Pangaea split into two large continental masses: Laurasia (north) and Gondwanaland (south).  
  • These masses continued to break apart, forming the continents we see today.  

Evidence for Continental Drift

• Matching of Continents (Jig-Saw-Fit):
  • The coastlines of Africa and South America fit together like a puzzle.  
  • Bullard’s 1964 map using a computer program to fit the continents at the 1,000-fathom line provided a near-perfect match.  
• Rocks of Same Age Across the Oceans:
  • Radiometric dating shows that rocks of the same age are found on continents now separated by oceans.  
  • Example: A 2,000-million-year-old rock belt from the Brazil coast matches with those from western Africa.  
• Tillite:
  • Tillite deposits, formed by glaciers, are found in India, Africa, Falkland Island, Madagascar, Antarctica, and Australia, suggesting these landmasses were once connected.  
• Placer Deposits:
  • Gold deposits in Ghana are believed to have originated from Brazil when the continents were joined.  
• Distribution of Fossils:
  • Identical fossil species are found on continents now separated by oceans, suggesting they were once connected.  
  • Examples: Lemurs in India, Madagascar, and Africa; Mesosaurus fossils in South Africa and Brazil.  

Forces for Drifting (According to Wegener)

• Pole-fleeing force:
  • Due to the Earth’s rotation and bulge at the equator, continents would drift away from the poles.  
• Tidal force:
  • Gravitational pull of the Moon and the Sun creates tides in oceanic waters, which could contribute to continental drift.  
• Inadequacy of Wegener’s Proposed Forces:
  • Most scientists considered these forces to be too weak to move continents.  

Post-drift Studies

• Ocean Floor Mapping:
  • Revealed that the ocean floor is not a flat plain but has features like submerged mountain ranges (mid-oceanic ridges) and deep trenches.  
  • Mid-oceanic ridges are sites of active volcanic eruptions.  
  • Rocks on either side of the mid-oceanic ridges are similar in age and composition.  
  • Ocean crust rocks are much younger than continental rocks (less than 200 million years old compared to 3,200 million years old).  
  • Sediments on the ocean floor are unexpectedly thin, suggesting a younger age.  
• Convectional Current Theory (Arthur Holmes, 1930s):
  • Proposed that convection currents in the mantle, caused by radioactive elements, could be the driving force behind continental drift.  

Ocean Floor Configuration

• Continental Margins:
  • Transition zone between continents and deep-sea basins.  
  • Includes continental shelf, continental slope, continental rise, and deep-oceanic trenches.  
• Abyssal Plains:
  • Flat plains between continental margins and mid-oceanic ridges.  
  • Where continental sediments accumulate.  
• Mid-Oceanic Ridges:
  • Underwater mountain ranges formed by volcanic activity.  
  • The longest mountain chain on Earth.  
  • Characterized by a central rift system, a fractionated plateau, and flank zones.  
  • The rift system is the zone of intense volcanic activity.  

Distribution of Earthquakes and Volcanoes

• Earthquakes and volcanoes are concentrated along:
  • Mid-oceanic ridges.  
  • The rim of the Pacific Ocean (the “Ring of Fire”).  
• Earthquakes along mid-oceanic ridges are shallow, while those along the Ring of Fire are deep-seated.  

Sea Floor Spreading (Hess, 1961)

• Key Observations:
  • Volcanic eruptions are common along mid-oceanic ridges.  
  • Rocks on either side of the mid-oceanic ridges are similar in age, composition, and magnetic properties.  
  • Rocks closer to the mid-oceanic ridges are younger and have normal magnetic polarity.  
  • Ocean crust rocks are much younger than continental rocks.  
  • Sediments on the ocean floor are thin.  
  • Deep trenches have deep-seated earthquakes, while mid-oceanic ridges have shallow earthquakes.  
• Hess’s Hypothesis:
  • Volcanic eruptions at mid-oceanic ridges cause the ocean floor to spread apart.  
  • New lava fills the gap, pushing older crust to the sides.  
  • The ocean floor eventually sinks and is consumed at deep-ocean trenches.  

Plate Tectonics

• Tectonic Plates:
  • Large, rigid slabs of rock that make up the Earth’s lithosphere.  
  • Composed of both continental and oceanic lithosphere.  
  • Plates move horizontally over the asthenosphere.  
  • Plates are classified as continental or oceanic depending on the dominant type of crust.  
• Major and Minor Plates:
  • The lithosphere is divided into seven major and several minor plates.  
  • Major plates: Antarctic, North American, South American, Pacific, Indian-Australian, African, Eurasian.  
  • Minor plates: Cocos, Nazca, Arabian, Philippine, Caroline.  
• Plate Boundaries:
  • Divergent: Plates move apart, creating new crust (e.g., Mid-Atlantic Ridge).  
  • Convergent: Plates collide, destroying crust (subduction zones).
    • Can occur between oceanic-continental, oceanic-oceanic, or continental-continental plates.  
  • Transform: Plates slide past each other horizontally.
    • Transform faults are generally perpendicular to mid-oceanic ridges. 

Rates of Plate Movement

• Determined by studying magnetic stripes on the ocean floor.  
• Rates vary from less than 2.5 cm/year (Arctic Ridge) to over 15 cm/year (East Pacific Rise).  

Force for Plate Movement

• Convection currents in the mantle, driven by radioactive decay and residual heat, are the main driving force.  

Movement of the Indian Plate

• The Indian plate includes Peninsular India and the Australian continental portions.  
• It has been moving northward for about 200 million years.  
• It collided with Asia about 40-50 million years ago, forming the Himalayas.  
• The Himalayas are still rising due to the ongoing collision.  
• The Deccan Traps were formed during this northward movement, around 60 million years ago.  

Major Points to Remember

• The Earth’s surface is dynamic and constantly changing.  
• Continents were once joined together in a supercontinent called Pangaea.  
• Plate tectonics is the theory that explains the movement of continents and the formation of various geological features.  
• There are three types of plate boundaries: divergent, convergent, and transform.  
• Convection currents in the mantle are the driving force behind plate movement.  
• The Indian plate has been moving northward for millions of years and collided with Asia to form the Himalayas.  

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