Quick Revision Notes

Class 11 Geography: Distribution of Oceans and Continents

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### 1. Introduction

• Continents and Oceans: The Earth's surface is unevenly divided into continents (landmasses, ~29%) and oceans (water bodies, ~71%).
• Significance: Understanding their distribution helps explain global climate, plate tectonics, seismic activity, and the distribution of flora and fauna.

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### 2. Continental Drift Theory (महाद्वीपीय विस्थापन सिद्धांत)

Proposed by Alfred Wegener in 1912, this theory attempts to explain the present-day distribution of continents and oceans.

• Initial State (Carboniferous Period):
  • Pangaea (पेंजिया): A single giant supercontinent meaning "All Earth".
  • Panthalassa (पैंथलासा): A vast mega-ocean surrounding Pangaea meaning "All Water".
• Break-up of Pangaea (around Mesozoic Era):
  • Laurasia (लॉरेशिया / Angaraland): The northern landmass (North America, Europe, Asia).
  • Gondwanaland (गोंडवानालैंड): The southern landmass (South America, Africa, India, Australia, Antarctica).
  • Tethys Sea (टेथिस सागर): The shallow sea that separated Laurasia and Gondwanaland.

Evidence in Support of Continental Drift:

1. Matching of Continents (Jigsaw Fit / जिग-सॉ फिट): The shorelines of Africa and South America (especially the bulge of Brazil fitting into the Gulf of Guinea) match remarkably well.
2. Rocks of Same Age across Oceans: Radiometric dating shows matching rock belts along the coasts of South America and Africa.
3. Tillite (टिलाइट): Sedimentary rock formed out of glacial deposits. India's Gondwana system sediments have exact counterparts in six different landmasses of the Southern Hemisphere.
4. Placer Deposits (प्लॉसर निक्षेप): Rich deposits of gold occur in the Ghana coast (Africa), but the source rocks are found in Brazil (South America).
5. Distribution of Fossils: Identical fossils of freshwater reptiles like Mesosaurus are found exclusively in South Africa and South America.

Force for Drift (Wegener’s Proposal):

• Pole-fleeing force (ध्रुव-प्रसारेण बल): Caused by the rotation of the Earth (bulge at the equator).
• Tidal force (जوارनद भाटा बल): Caused by the gravitational pull of the Moon and the Sun. (Note: Modern science proved these forces are inadequate to move continents, leading to the rejection of Wegener's mechanism, though his drift concept was correct.)

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### 3. Convectional Current Theory (संवहन धारा सिद्धांत)

• Proposed by: Arthur Holmes in 1930.
• Concept: Radioactive elements inside the mantle generate immense thermal heat. This creates convection currents (संवहन धाराएँ) in the mantle, which act as the driving force behind the movement of crustal plates.

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### 4. Mapping of the Ocean Floor (महासागरीय तल का मानचित्रण)

• Advances in sonar (sound navigation and ranging) technology during and after World War II revealed that the ocean floor is not a flat plain.
• Key Relief Features:
  • Continental Shelf (महाद्वीपीय मग्नतट): Shallow, gently sloping extension of continents.
  • Continental Slope (महाद्वीपीय ढाल): Steep slope connecting shelf to deep ocean basin.
  • Deep Sea Plains / Abyssal Plains (गिरे-समुद्री मैदान): Vast, flat plains covering the deepest parts.
  • Mid-Oceanic Ridges (मध्य-महासागरीय कटक): Continuous chain of mountain ranges on the ocean floor formed by volcanic activity.
  • Oceanic Trenches (महासागरीय खाइयाँ): Deepest parts of the oceans (e.g., Mariana Trench).

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### 5. Concept of Sea Floor Spreading (सागर नितल विस्तार)

• Proposed by: Harry Hess (1961).
• Key Principles:
  1. Volcanic eruptions along the Mid-Oceanic Ridges constantly push up fresh magma, forming new oceanic crust.
  2. Older rocks are pushed away from the ridge symmetrically on both sides.
  3. Oceanic crust is eventually consumed (subducted) into deep ocean trenches and recycled into the mantle.
  4. Evidence: Paleomagnetism (normal and reverse magnetic striped patterns parallel to the ridges) proves the crust is expanding.

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### 6. Plate Tectonics Theory (प्लेट विवर्तनिकी सिद्धांत)

• Formulated by: McKenzie, Parker, and Morgan (1967).
• Lithospheric Plates (स्थलमंडल प्लेटें): The Earth’s crust is broken into several rigid slabs called tectonic plates (major and minor plates).

Types of Plate Boundaries (प्लेट सीमाएँ):

1. Divergent Boundaries (अपसारी सीमाएँ):
   • Plates move away from each other.
   • New crust is created (e.g., Mid-Atlantic Ridge).
   • Also known as constructive boundaries.
2. Convergent Boundaries (अभिसारी सीमाएँ):
   • Plates collide or move towards each other.
   • Crust is destroyed/subducted (e.g., Himalayas formed by Indian and Eurasian plate collision).
   • Also known as destructive boundaries.
3. Transform Boundaries (रूपांतरित सीमाएँ):
   • Plates slide horizontally past each other along a fault line.
   • No crust is created or destroyed, but major earthquakes occur (e.g., San Andreas Fault).

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### 7. Movement of the Indian Plate (भारतीय प्लेट का संचलन)

• India was originally part of Gondwanaland.
• Around 140 million years ago, it began moving northward.
• About 40-50 million years ago, it collided with the Eurasian Plate, resulting in the massive tectonic uplift of the Tethys Sea sediments to form the Himalayas.
• Volcanic activity during this northward journey over the Réunion hotspot created the Deccan Traps (basaltic lava flows).