Chapter 4 – Distribution of Oceans and Continents | CBSE Notes

GEOGRAPHY | CLASS XI | NCERT

Book: Fundamentals of Physical Geography | Chapter 4

Distribution of Oceans and Continents

⭐ Topper Level 💬 Easy Language 📌 Point-Wise 🌍 Mind Maps Included

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1. Learning Objectives

After reading these notes, you will be able to:

Understand Wegener’s Continental Drift Theory — what it says, its evidences, and its limitations.

Know the concept of Sea Floor Spreading proposed by Hess in 1961 and the facts supporting it.

Understand Plate Tectonics — major and minor plates, and types of plate boundaries.

Know the Ocean Floor Configuration — Continental Margins, Abyssal Plains, Mid-Oceanic Ridges.

Trace the movement of the Indian Plate and understand how the Himalayas were formed.

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2. Continental Drift Theory

Continents cover 29% of Earth’s surface; the rest is under oceanic waters. The positions of continents and oceans have not always been the same and will continue to change in the future. Alfred Wegener, a German meteorologist, put forth the comprehensive Continental Drift Theory in 1912.

Abraham Ortelius

Dutch map maker who first proposed the possibility of continents being joined — in 1596

Alfred Wegener

German meteorologist who gave the complete Continental Drift Theory in 1912

PANGAEA

Super-continent meaning “All Earth”. All continents were once joined as one single mass.

PANTHALASSA

The mega-ocean surrounding Pangaea meaning “All Water”

🌍 Breakup of Pangaea — Timeline

About 200 million years ago, Pangaea began to split → first broke into Laurasia (northern) and Gondwanaland (southern) → these further broke into the various continents that exist today.

📋 Evidences in Support of Continental Drift

🧩 Jig-Saw-Fit (Matching of Continents)

The shorelines of Africa and South America facing each other have a remarkable match. Bullard (1964) produced a computer map showing a near-perfect fit at the 1,000-fathom line (not present shoreline).

🪨 Rocks of Same Age Across Oceans

Belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa. Earliest marine deposits of South America and Africa are of the Jurassic age — suggesting ocean did not exist before that.

🧊 Tillite

Sedimentary rock formed from glacial deposits. The Gondwana system from India has counterparts in Africa, Falkland Island, Madagascar, Antarctica and Australia. Thick tillite at the base indicates extensive glaciation — proves palaeoclimates and continental drift.

🥇 Placer Deposits

Rich gold placer deposits on Ghana coast but no source rock there. The gold-bearing veins are in Brazil — proving Ghana’s gold came from Brazil when the two continents lay side by side.

🦎 Distribution of Fossils

Lemurs found in India, Madagascar and Africa → suggested a landmass ‘Lemuria’ linking them. Mesosaurus (small reptile) skeletons found only in Southern Cape of South Africa and Iraver formations of Brazil — now 4,800 km apart with an ocean between.

⚡ Force for Drifting

Wegener suggested two forces: (1) Pole-fleeing force — due to Earth’s rotation causing equatorial bulge; (2) Tidal force — due to attraction of Moon and Sun. Most scholars considered these forces totally inadequate.

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3. Concept of Sea Floor Spreading

Post-World War II ocean floor mapping and palaeomagnetic studies provided new information. Arthur Holmes (1930s) first proposed Convectional Current Theory — currents generated in mantle due to radioactive elements causing thermal differences. Harry Hess (1961) then proposed the Sea Floor Spreading hypothesis.

🔬 Key Facts Revealed by Ocean Floor Mapping

All along the mid-oceanic ridges, volcanic eruptions are common — bringing huge amounts of lava to the surface.
Rocks equidistant on either side of the crest of mid-oceanic ridges show remarkable similarities in period of formation, chemical composition and magnetic properties.
Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest. Age increases as one moves away from the crest.
Ocean crust rocks are much younger than continental rocks. Age of oceanic crust = nowhere more than 200 million years. Some continental rocks are as old as 3,200 million years.
Sediments on ocean floor are unexpectedly very thin. Nowhere was the sediment column found older than 200 million years.
Deep trenches have deep-seated earthquake occurrences; in mid-oceanic ridge areas, earthquake foci have shallow depths.

⭐ Hess’s Sea Floor Spreading — Key Idea

Constant eruptions at the crest of oceanic ridges cause rupture of the oceanic crust. New lava wedges in, pushing the oceanic crust on either side — so the ocean floor spreads. The pushed ocean floor then sinks down at the oceanic trenches and gets consumed. This explains why ocean floors are young but oceans don’t keep growing indefinitely.

🌋 Ocean Floor Configuration

🏝️ Continental Margins

Form the transition between continental shores and deep-sea basins. Include: continental shelf, continental slope, continental rise and deep-oceanic trenches. Deep-oceanic trenches are of great importance for studying distribution of oceans and continents.

🏜️ Abyssal Plains

Extensive plains lying between continental margins and mid-oceanic ridges. Areas where continental sediments that move beyond the margins get deposited.

⛰️ Mid-Oceanic Ridges

Interconnected chain of mountain systems within the ocean. The longest mountain-chain on Earth’s surface (though submerged). Characterised by a central rift system at the crest — zone of intense volcanic activity.

📌 Ring of Fire

The rim of the Pacific Ocean is called the “Ring of Fire” due to the existence of active volcanoes. The Alpine-Himalayan belt and the rim of the Pacific have deep-seated earthquakes. Mid-oceanic ridge areas have shallow earthquake foci.

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4. Plate Tectonics

In 1967, McKenzie, Parker and Morgan independently came up with the concept of Plate Tectonics. A tectonic plate (also called lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units.

7 Major Plates

Earth’s lithosphere is divided into 7 major and some minor plates

5–100 km

Thickness of lithosphere in oceanic parts

~200 km

Thickness of lithosphere in continental areas

1967

Year when Plate Tectonics concept was proposed by McKenzie, Parker & Morgan

🌍 7 Major Plates

No.	Major Plate	Type
I	Antarctica and surrounding oceanic plate	Mixed
II	North American plate (with western Atlantic floor)	Continental
III	South American plate (with western Atlantic floor)	Continental
IV	Pacific plate	Oceanic
V	India-Australia-New Zealand plate	Mixed
VI	Africa with eastern Atlantic floor plate	Continental
VII	Eurasia and adjacent oceanic plate	Continental

📌 Important Minor Plates

Cocos Plate

Between Central America and Pacific plate

Nazca Plate

Between South America and Pacific plate

Arabian Plate

Mostly the Saudi Arabian landmass

Philippine Plate

Between Asiatic and Pacific plate

Caroline Plate

Between Philippine and Indian plate (North of New Guinea)

🔀 Three Types of Plate Boundaries

🔀 Mind Map — Types of Plate Boundaries

Plate Boundaries (3 Types)

↔️ Divergent
Plates move apart
New crust created
Example: Mid-Atlantic Ridge

↕️ Convergent
Plates collide
Crust destroyed
Subduction zone formed

↕️ Transform
Plates slide past
No crust created
or destroyed

Boundary Type	What Happens	Crust	Example
Divergent	Plates pull away from each other (spreading sites)	New crust created	Mid-Atlantic Ridge — American plate separates from Eurasian & African plates
Convergent	One plate dives under another (subduction zone). 3 ways: ocean-continent, ocean-ocean, continent-continent	Crust destroyed	Himalayas — continent-continent convergence
Transform	Plates slide horizontally past each other. Transform faults perpendicular to mid-oceanic ridges	Neither created nor destroyed	San Andreas Fault

⚡ Rates of Plate Movement

Determined by studying strips of normal and reverse magnetic fields that parallel the mid-oceanic ridges.
Slowest: Arctic Ridge — less than 2.5 cm/yr.
Fastest: East Pacific Rise (near Easter Island, ~3,400 km west of Chile) — more than 15 cm/yr.

🔥 Force for Plate Movement

💡 Convection Cells

Hot mantle rock below rigid plates moves in a circular manner. Heated material rises → spreads → cools → sinks back to deeper depths. This cycle repeats to generate convection cells / convective flow. Heat comes from two sources: radioactive decay and residual heat. The slow movement of hot softened mantle below rigid plates is the driving force behind plate movement.

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5. Movement of the Indian Plate

The Indian plate includes Peninsular India and the Australian continental portions. The northern boundary is the subduction zone along the Himalayas (continent–continent convergence). Position of Indian subcontinent traced through rocks analysed from the Nagpur area.

🗺️ Boundaries of the Indian Plate

North: Subduction zone along the Himalayas → continent–continent convergence.
East: Extends through Rakinyoma Mountains of Myanmar → island arc along the Java Trench.
East (far): Spreading site — east of Australia — oceanic ridge in SW Pacific.
West: Follows Kirthar Mountain of Pakistan → extends along Makrana coast → joins spreading site from Red Sea rift along Chagos Archipelago.
South: Boundary with Antarctic plate marked by oceanic ridge (divergent boundary) running W-E direction.

📅 Timeline of India’s Journey

Time (Million Years Ago)	Event
225 million years ago	India was a large island near Australian coast. Tethys Sea separated it from Asian continent.
200 million years ago	India started her northward journey when Pangaea broke.
140 million years ago	Indian subcontinent was located as far south as 50°S latitude.
~60 million years ago	Deccan Traps formed — outpouring of lava. Subcontinent was still close to the equator.
40–50 million years ago	India collided with Asia → rapid uplift of the Himalayas.
Present	Himalayas are still rising. Process of plate convergence is continuing.

📌 Key Point about India’s Movement

It is not the continent that moves (as Wegener believed) — it is the plate that moves and the continent is just a part of that plate. India’s position in different geological periods was determined by scientists using palaeomagnetic data.

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Summary — Quick Revision

Abraham Ortelius (1596) first proposed continents were joined. Alfred Wegener (1912) gave the complete Continental Drift Theory with evidences.

Pangaea (all earth) = super-continent; Panthalassa (all water) = mega-ocean. Split ~200 million years ago into Laurasia (north) and Gondwanaland (south).

Evidences for drift: Jig-saw-fit, same-age rocks, Tillite, Placer deposits (Ghana-Brazil gold), Fossil distribution (Mesosaurus, Lemurs).

Wegener’s forces — pole-fleeing force (Earth’s rotation) and tidal force (Moon & Sun attraction) — were considered totally inadequate by scholars.

Arthur Holmes (1930s): Convectional Current Theory in mantle. Harry Hess (1961): Sea Floor Spreading — new crust at ridges, old crust consumed at trenches.

Ocean crust rocks = max 200 million years old. Continental rocks = up to 3,200 million years old. Ocean sediments are unexpectedly thin.

Plate Tectonics (1967) — McKenzie, Parker & Morgan. Earth’s lithosphere divided into 7 major and some minor plates moving over the asthenosphere.

Plate boundaries: Divergent (crust created; Mid-Atlantic Ridge), Convergent (crust destroyed; subduction zone), Transform (no creation/destruction; San Andreas Fault).

Plate movement rates: Slowest = Arctic Ridge (<2.5 cm/yr); Fastest = East Pacific Rise (>15 cm/yr). Driving force = convection currents in mantle.

India started northward journey ~200 million years ago. Collided with Asia ~40–50 million years ago → Himalayas formed. Tethys Sea once separated India from Asia.

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Important Terms to Remember

Continental Drift Theory: Theory proposed by Alfred Wegener (1912) stating that all continents were once joined as a single landmass (Pangaea) and have since drifted to their present positions.
Pangaea: The name given to the super-continent that existed about 200 million years ago. It means ‘All Earth’.
Panthalassa: The mega-ocean that surrounded Pangaea. It means ‘All Water’.
Laurasia: The northern component of Pangaea after its first split — included present-day North America, Europe, and Asia.
Gondwanaland: The southern component of Pangaea after its first split — included present-day South America, Africa, Antarctica, Australia, and the Indian subcontinent.
Jig-Saw-Fit: The remarkable matching of coastlines of Africa and South America facing each other — one of the key evidences of continental drift. Best fit shown by Bullard (1964) at the 1,000-fathom line.
Tillite: Sedimentary rock formed from glacial deposits. The Gondwana system tillite found in India and six other Southern Hemisphere landmasses is evidence of continental drift and past climates (palaeoclimates).
Placer Deposits: Rich mineral deposits found far from their source rock. Gold placer deposits on Ghana coast whose source is in Brazil — evidence that the two continents were once joined.
Mesosaurus: A small reptile adapted to shallow brackish water. Its fossils found only in Southern Cape of South Africa and Iraver formations of Brazil — now 4,800 km apart — proves continental drift.
Lemuria: A hypothetical landmass proposed to explain the presence of Lemurs in India, Madagascar, and Africa.
Pole-Fleeing Force: Force related to the rotation of the earth — one of the two forces Wegener suggested for continental drift. Considered inadequate by scholars.
Tidal Force: Force due to the attraction of the Moon and Sun that develops tides — the second force Wegener suggested for drift. Also considered inadequate.
Convectional Current Theory: Proposed by Arthur Holmes (1930s) — radioactive elements in the mantle cause thermal differences generating convection currents that drive plate movement.
Sea Floor Spreading: Hypothesis proposed by Harry Hess (1961) — constant eruptions at mid-oceanic ridge crests cause rupture of oceanic crust; new lava pushes the crust sideways; old crust sinks and gets consumed at oceanic trenches.
Palaeomagnetic Studies: Study of the magnetic properties of rocks from past eras. Revealed that rocks equidistant from mid-oceanic ridge crests have similar magnetic properties and age — supporting sea floor spreading.
Abyssal Plains: Extensive flat plains lying between continental margins and mid-oceanic ridges where continental sediments get deposited.
Mid-Oceanic Ridges: The longest mountain chain on Earth’s surface (though submerged). Characterised by a central rift system — zone of intense volcanic activity. New ocean floor is created here.
Continental Margins: Transition zone between continental shores and deep-sea basins. Includes continental shelf, continental slope, continental rise, and deep-oceanic trenches.
Ring of Fire: The rim of the Pacific Ocean, so called because of the large number of active volcanoes located there.
Plate Tectonics: Theory proposed in 1967 (McKenzie, Parker, Morgan) stating that Earth’s lithosphere is divided into several rigid plates that move horizontally over the asthenosphere.
Tectonic Plate / Lithospheric Plate: A massive, irregularly-shaped slab of solid rock composed of both continental and oceanic lithosphere. It moves as a rigid unit over the asthenosphere.
Divergent Boundary: Plate boundary where plates move away from each other — new crust is generated. Example: Mid-Atlantic Ridge (spreading site).
Convergent Boundary: Plate boundary where one plate dives under another — crust is destroyed. The place where a plate sinks is called the subduction zone.
Transform Boundary: Plate boundary where plates slide horizontally past each other — no crust is created or destroyed. Transform faults are perpendicular to mid-oceanic ridges.
Subduction Zone: The location where sinking of a tectonic plate occurs at a convergent boundary — the denser plate dives below the other and gets consumed.
Spreading Site: Location where two plates move away from each other at a divergent boundary — new oceanic crust is formed here.
Convection Cell / Convective Flow: The circular movement of hot mantle material — heated rock rises, spreads, cools, and sinks — generating the driving force for plate movement.
Tethys Sea: The ancient ocean that separated the Indian subcontinent from the Asian continent until about 225 million years ago. It eventually closed as India moved northward and collided with Asia.
Radiometric Dating: Method used to determine the age of rocks by measuring the decay of radioactive elements. Used to correlate rock formations from different continents across oceans.
Pacific plate: Largely an oceanic plate — one of the 7 major tectonic plates. The Eurasian plate is largely a continental plate.