Class 11 Geography Notes Chapter 4 (Distribution of oceans and continents) – Fundamental of Physical Geography Book
Alright class, let's delve into Chapter 4: Distribution of Oceans and Continents. This is a fundamental chapter that explains how the large-scale features of our planet's surface came to be arranged as they are today. Understanding this is crucial not just for geography but also for grasping concepts in geology and disaster management, making it important for your competitive exams.
Chapter 4: Distribution of Oceans and Continents - Detailed Notes
1. Introduction: The Puzzle of Continents
- Early mapmakers and scientists noticed the apparent 'fit' of continents, particularly South America and Africa.
- This observation sparked curiosity about whether the continents were once joined together.
2. Continental Drift Theory (Alfred Wegener, 1912)
- Proponent: Alfred Wegener, a German meteorologist.
- Core Idea: Proposed that all continents were once part of a single supercontinent called Pangea (meaning 'all earth'). This supercontinent was surrounded by a mega-ocean called Panthalassa (meaning 'all water').
- Breakup: Wegener suggested that around 200 million years ago (Mesozoic Era), Pangea began to break apart.
- It first split into two large landmasses:
- Laurasia: Northern part (North America, Greenland, Eurasia).
- Gondwanaland: Southern part (South America, Africa, Madagascar, India, Antarctica, Australia).
- These landmasses further fragmented and drifted to their present positions.
- It first split into two large landmasses:
- Evidence Presented by Wegener:
- (a) The Apparent Fit of Continents (Jigsaw Fit): The coastlines of South America and Africa facing each other have a remarkable match. Bullard later produced a better fit using the continental shelf edge (at 1000-fathom line).
- (b) Rocks of Same Age Across the Oceans: Radiometric dating of rock formations across different continents shows matches. Example: Ancient rocks belts in Brazil match those in Western Africa. The early marine deposits along the coastlines of South America and Africa belong to the Jurassic age, suggesting the ocean did not exist prior to that time.
- (c) Tillite Deposits: Tillite is sedimentary rock formed from glacial deposits. Gondwana system sediments found in India, Africa, Falkland Islands, Madagascar, Antarctica, and Australia have similarities, indicating past connections and shared glaciation.
- (d) Placer Deposits: Rich placer deposits of gold are found on the Ghana coast (Africa), but the source rocks are in Brazil (South America), suggesting they were joined.
- (e) Distribution of Fossils: Identical species of plants and animals adapted to living on land or in fresh water are found on widely separated continents.
- Mesosaurus: Small reptile fossils found only in Southern Africa and Brazil.
- Glossopteris: Fossil flora found across India, Australia, South Africa, South America, Antarctica, and Madagascar.
- Lystrosaurus: Land reptile fossils found in India, Africa, and Antarctica.
- Forces Proposed by Wegener (Later Proven Inadequate):
- Pole-fleeing Force: Related to the Earth's rotation, causing continents to move towards the equator.
- Tidal Force: Attraction of the sun and moon, causing westward drift.
- Criticism: Scientists largely rejected Wegener's theory initially because the proposed forces were considered far too weak to move entire continents.
3. Post-Drift Studies: New Evidence Emerges
- Interest in continental drift revived in the mid-20th century due to new findings.
- Convection Current Theory (Arthur Holmes, 1930s):
- Holmes suggested the possibility of convection currents operating in the Earth's mantle.
- These currents are generated by radioactive decay causing heat differences within the mantle.
- He argued these currents could provide the immense force needed to move continents.
- Mapping of the Ocean Floor (Post World War II):
- Extensive surveys revealed the ocean floor is not just a flat plain.
- Key features discovered:
- Mid-Oceanic Ridges (MORs): Underwater mountain ranges, often with central rift valleys. Characterized by volcanic activity and earthquakes. (e.g., Mid-Atlantic Ridge).
- Deep Sea Trenches: Extremely deep, narrow depressions, usually found near continental margins. Associated with strong earthquakes and volcanic activity. (e.g., Mariana Trench).
- Abyssal Plains: Vast, relatively flat areas between MORs and continental margins.
- Continental Margins: Transition zones between continental crust and oceanic crust (including shelf, slope, rise).
- Age of Ocean Floor Rocks: Surprisingly, rocks from the ocean floor were found to be much younger (nowhere older than 200 million years) than continental rocks (some over 3 billion years old).
4. Sea Floor Spreading Theory (Harry Hess, early 1960s)
- Based on ocean floor mapping and Holmes' convection currents.
- Core Idea: Constant volcanic eruptions occur at the crests of MORs, causing new oceanic crust to form. This new crust pushes the older crust sideways, causing the seafloor to "spread." The older crust eventually descends into the mantle at deep-sea trenches (subduction).
- Evidence Supporting Sea Floor Spreading:
- (a) Age of Rocks: Rocks are youngest at the MOR crests and progressively older further away on either side.
- (b) Magnetic Anomalies (Paleomagnetism):
- Ocean floor rocks (basalts) contain magnetic minerals that align with the Earth's magnetic field when they cool and solidify.
- Earth's magnetic field has reversed polarity multiple times throughout history.
- Studies revealed alternating bands of normal and reversed polarity mirrored symmetrically on both sides of the MORs. This pattern strongly supports the idea of new crust forming at the ridge and spreading outwards.
- (c) Distribution of Earthquakes and Volcanoes: Deep earthquakes are concentrated in trench areas (subduction zones), while shallow earthquakes and volcanic eruptions are common along MORs.
- (d) Sediment Thickness: Ocean floor sediments are thinnest near the MORs and thickest further away, consistent with the age pattern of the crust beneath.
5. Plate Tectonics Theory (Developed in the late 1960s)
- This theory combines Continental Drift and Sea Floor Spreading into a comprehensive model.
- Core Idea: The Earth's outer rigid layer, the lithosphere (crust + upper solid mantle), is broken into several large and small segments called tectonic plates or lithospheric plates.
- These plates float on the semi-molten asthenosphere (upper mantle) and are constantly moving relative to each other, driven by mantle convection currents.
- Types of Plates:
- Major Plates:
- Antarctica Plate
- North American Plate
- South American Plate
- Pacific Plate (largest, mostly oceanic)
- India-Australia-New Zealand Plate
- Africa Plate
- Eurasian Plate
- Minor Plates: Cocos, Nazca, Arabian, Philippine, Caroline, Fuji, Scotia, Juan de Fuca, etc.
- Major Plates:
- Plate Boundaries: Most geological activity (earthquakes, volcanoes, mountain building) occurs along plate boundaries. There are three main types:
- (a) Divergent Boundaries (Constructive Margins):
- Plates move apart.
- Magma rises from the mantle to create new crust.
- Features: Mid-Oceanic Ridges (e.g., Mid-Atlantic Ridge), Rift Valleys (e.g., East African Rift).
- Associated with shallow earthquakes and volcanic activity.
- (b) Convergent Boundaries (Destructive Margins):
- Plates move towards each other.
- Crust is destroyed as one plate subducts (sinks) beneath the other.
- Three types of convergence:
- Ocean-Continent: Denser oceanic plate subducts under the continental plate. Features: Deep sea trenches, volcanic mountain ranges on the continent (e.g., Andes), strong earthquakes.
- Ocean-Ocean: One oceanic plate subducts under another. Features: Deep sea trenches, volcanic island arcs (e.g., Mariana Islands, Japan), strong earthquakes.
- Continent-Continent: Neither plate subducts easily due to low density. Plates collide, buckle, and fold. Features: Major fold mountain ranges (e.g., Himalayas, Alps), intense earthquakes, minimal volcanism.
- (c) Transform Boundaries (Conservative Margins):
- Plates slide horizontally past each other.
- Crust is neither created nor destroyed.
- Features: Transform faults (e.g., San Andreas Fault).
- Associated with significant earthquake activity.
- (a) Divergent Boundaries (Constructive Margins):
- Driving Force: Mantle convection currents are the widely accepted primary driving force. Hot, less dense material rises, cools, and sinks, creating circular motion that drags the overlying plates. Ridge push (gravitational sliding from elevated ridges) and slab pull (pull exerted by sinking cold, dense slabs at subduction zones) are also considered contributing factors.
- Rates of Plate Movement: Vary considerably, from less than 2.5 cm/year (Arctic Ridge) to over 15 cm/year (East Pacific Rise).
6. Movement of the Indian Plate
- India was part of the Gondwanaland supercontinent.
- Around 200 million years ago, it began drifting northward after breaking away from the larger landmass.
- Travelled a long distance, crossing the equator.
- About 40-50 million years ago, it collided with the Eurasian Plate.
- This collision led to the buckling and folding of the crust, resulting in the formation of the Himalayas. The process is still ongoing, causing the Himalayas to rise and resulting in significant seismic activity in the region.
Conclusion:
The theory of Plate Tectonics provides a unified explanation for the distribution of continents and oceans, the formation of major landforms like mountains, ridges, and trenches, and the occurrence patterns of earthquakes and volcanoes. It revolutionized our understanding of the Earth's dynamic nature.
Multiple Choice Questions (MCQs)
-
Who proposed the Continental Drift Theory in 1912?
(a) Arthur Holmes
(b) Harry Hess
(c) Alfred Wegener
(d) Tuzo Wilson -
According to Wegener, the single supercontinent that existed about 200 million years ago was called:
(a) Laurasia
(b) Gondwanaland
(c) Panthalassa
(d) Pangea -
Which of the following was NOT used by Wegener as evidence for Continental Drift?
(a) Jigsaw fit of continents
(b) Paleomagnetic data from the ocean floor
(c) Fossil distribution (e.g., Glossopteris)
(d) Tillite deposits in different continents -
The theory that proposed convection currents in the mantle as the driving force for continental movement was put forward by:
(a) Alfred Wegener
(b) Arthur Holmes
(c) Harry Hess
(d) McKenzie and Parker -
Mid-Oceanic Ridges are associated with which type of plate boundary?
(a) Convergent Boundary
(b) Transform Boundary
(c) Divergent Boundary
(d) Subduction Zone -
The process where one tectonic plate sinks beneath another at a convergent boundary is called:
(a) Rifting
(b) Spreading
(c) Subduction
(d) Transformation -
Where is the youngest oceanic crust typically found?
(a) Along continental shelves
(b) Near deep-sea trenches
(c) At the crests of Mid-Oceanic Ridges
(d) On abyssal plains far from ridges -
The symmetrical pattern of magnetic anomalies (stripes) on either side of Mid-Oceanic Ridges is key evidence for:
(a) Continental Drift only
(b) Sea Floor Spreading
(c) The age of continental rocks
(d) The formation of abyssal plains -
The formation of the Himalayan Mountains is a result of which type of plate collision?
(a) Ocean-Continent Convergence
(b) Ocean-Ocean Convergence
(c) Continent-Continent Convergence
(d) Divergent Boundary activity -
Which of the following is considered a major tectonic plate?
(a) Cocos Plate
(b) Nazca Plate
(c) Arabian Plate
(d) Pacific Plate
Answer Key for MCQs:
- (c) Alfred Wegener
- (d) Pangea
- (b) Paleomagnetic data from the ocean floor (This evidence came later with Sea Floor Spreading)
- (b) Arthur Holmes
- (c) Divergent Boundary
- (c) Subduction
- (c) At the crests of Mid-Oceanic Ridges
- (b) Sea Floor Spreading
- (c) Continent-Continent Convergence
- (d) Pacific Plate
Make sure you revise these concepts thoroughly. Understanding the evidence for each theory and the characteristics of different plate boundaries is key for your exams. Good luck with your preparation!