Historical Perspectives: Early Observations
Long before modern technology, cartographers noticed the striking symmetry between the coastlines of the Atlantic Ocean.
- Abraham Ortelius: In 1596, this Dutch map maker first suggested that the Americas, Europe, and Africa were once joined and later torn apart.
- Antonio Snider-Pellegrini: In 1858, he produced a map showing these continents fitting together like a jigsaw puzzle.
The Continental Drift Theory (Alfred Wegener, 1912)
Alfred Wegener, a German meteorologist, proposed the first comprehensive theory regarding the movement of continents.
The Concept of Pangea and Panthalassa
- Pangea: Wegener argued that all continents once formed a single, massive supercontinent called Pangea (meaning “all earth”).
- Panthalassa: This supercontinent was surrounded by a mega-ocean known as Panthalassa (meaning “all water”).
- The Split: Around 200 million years ago, Pangea began to break apart into two large landmasses: Laurasia (northern part) and Gondwanaland (southern part), separated by the Tethys Sea.
Evidence Supporting Continental Drift
- The Jig-Saw Fit: The shorelines of Africa and South America facing each other have a remarkable match.
- Rocks of Same Age Across Oceans: Radiometric dating shows that the 2,000 million-year-old belt of ancient rocks from Brazil matches those in Western Africa.
- Tillite: These are sedimentary rocks formed out of deposits of glaciers. The Gondwana system of sediments from India has its counterparts in Africa, Falkland Islands, Madagascar, Antarctica, and Australia, suggesting a common glacial history.
- Placer Deposits: Rich deposits of gold in the Ghana coast (Africa) occur without a source rock, while the source rocks are found in Brazil, implying the two were once together.
- Distribution of Fossils: Identical species of plants and animals found on either side of the marine barriers. For example, Mesosaurus (a small reptile) fossils are found only in Southern Africa and Brazil.
Forces for Drifting
Wegener suggested two main forces:
- Pole-fleeing force: Related to the rotation of the earth.
- Tidal force: Due to the attraction of the moon and the sun that develops tides in oceanic waters.
- Note: Most geologists found these forces to be inadequate and rejected Wegener’s explanation of the “why” behind the drift.
Post-Drift Discoveries: Filling the Gaps
Post-World War II research provided new insights into the ocean floor, which Wegener had largely ignored.
Convectional Current Theory (Arthur Holmes, 1930s)
Holmes discussed the possibility of convection currents operating in the mantle portion. These currents are generated by radioactive elements causing thermal differences in the mantle, providing a mechanism for the movement of lithospheric plates.
Mapping the Ocean Floor
Detailed research revealed that the ocean floor is not a vast, flat plain but contains:
- Mid-Oceanic Ridges: Submerged mountain ranges where volcanic eruptions are common.
- Oceanic Trenches: Deepest parts of the ocean, often located near continental margins.
Seafloor Spreading (Harry Hess, 1961)
Hess proposed that constant eruptions at the crest of mid-oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side.
- Evidence: Rocks equidistant from the ridge crest on both sides show remarkable similarities in terms of age, magnetic properties, and chemical composition.
- Age of Rocks: The ocean floor rocks are much younger (none older than 200 million years) than the continental rocks (some as old as 3,200 million years).
Plate Tectonics: The Modern Synthesis
In 1967, Mackenzie, Parker, and Morgan independently organized these ideas into what is now called Plate Tectonics.
What is a Plate?
A tectonic plate (also called a lithospheric plate) is a massive, irregularly shaped slab of solid rock, composed of both continental and oceanic lithosphere.
- Oceanic Plate: Composed mainly of Simatic rocks (basalt) and is thinner.
- Continental Plate: Composed mainly of Sialic rocks (granite) and is thicker.
Major and Minor Plates
The Earth’s lithosphere is divided into seven major and several minor plates:
- Major Plates: Antarctica, North American, South American, Pacific, India-Australia-New Zealand, African, and Eurasian plates.
- Minor Plates: Cocos, Nazca, Arabian, Philippine, Caroline, and Fuji plates.
Types of Plate Boundaries
- Divergent Boundaries: Where new crust is generated as the plates pull away from each other (e.g., Mid-Atlantic Ridge).
- Convergent Boundaries: Where the crust is destroyed as one plate dived under another. This can happen between:
- Oceanic and continental plates.
- Two oceanic plates.
- Two continental plates (e.g., the Himalayas).
3. Transform Boundaries: Where the crust is neither produced nor destroyed as the plates slide horizontally past each other (e.g., San Andreas Fault).
Movement of the Indian Plate
The Indian plate includes Peninsular India and the Australian continental portion.
- Northward Journey: After breaking from Gondwanaland, India moved north toward the Eurasian plate.
- Deccan Traps: During its journey, the Indian plate passed over a hotspot (Reunion hotspot), leading to massive volcanic outbursts that formed the Deccan Traps.
- Himalayan Orogeny: About 40-50 million years ago, India collided with Asia, causing the uplift of the Himalayas.
UPSC Prelims: PYQs & Practice Questions
Previous Year Questions (Prelims)
Question 1 (2012)
Q: Which of the following is/are cited by scientists as evidence for the continued expansion of the universe?
1. Detection of microwaves in space.
2. Observation of redshift phenomenon in space.
3. Movement of asteroids in space.
4. Occurrence of supernova explosions in space.
Select the correct answer:
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1, 2 and 4 only
(d) 1, 2, 3 and 4
Answer: (a)
Explanation: The detection of Cosmic Microwave Background Radiation (CMBR) and the observation of redshift (light from distant galaxies shifting toward longer wavelengths) are primary evidences supporting the expansion of the universe under the Big Bang Theory. Movement of asteroids and supernova explosions are not direct evidences of universal expansion.
Question 2 (2013)
Q: Consider the following:
1. Electromagnetic radiation
2. Geothermal energy
3. Gravitational force
4. Plate movements
5. Rotation of the earth
6. Revolution of the earth
Which of the above are responsible for bringing dynamic changes on the surface of the earth?
(a) 1, 2, 3 and 4 only
(b) 2, 3, 4 and 5 only
(c) 1, 3, 5 and 6 only
(d) 1, 2, 3, 4, 5 and 6
Answer: (d)
Explanation: Dynamic changes on the Earth's surface are driven by both endogenic and exogenic forces. Geothermal energy and plate movements represent internal (endogenic) forces, while electromagnetic radiation (solar energy), gravitational force, and the Earth's rotation and revolution influence external (exogenic) processes.
Practice Questions (Prelims)
Question 1
Q: With reference to the 'Continental Drift Theory', consider the following statements:
1. Alfred Wegener suggested that the movement of continents was caused by the tidal force and pole-fleeing force.
2. The theory primarily focuses on the movement of the ocean floor rather than the continents.
Which of the statements given above is/are correct?
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
Answer: (a)
Explanation: Alfred Wegener proposed that tidal forces and pole-fleeing forces (arising due to Earth’s rotation) were responsible for continental drift. Statement 2 is incorrect because Wegener’s theory emphasized the movement of continents; the concept of ocean floor movement was later explained under the theory of Seafloor Spreading.
Question 2
Q: Which of the following evidence supports the concept of 'Pangea'?
1. The 'Jig-saw' fit of the coastlines of Africa and South America.
2. The presence of Tillite (glacial sedimentary rocks) across different continents.
3. The distribution of Mesosaurus fossils in both Brazil and South Africa.
Select the correct answer:
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (d)
Explanation: All three are classic evidences cited by Alfred Wegener to support the existence of a single supercontinent called Pangea. The jigsaw fit of coastlines, matching glacial deposits (Tillite), and identical fossil distribution across continents strongly indicate that these landmasses were once joined together.
UPSC Mains – Previous Year & Practice Questions
Previous Year Questions (Mains)
Island Arc Formation – Indonesia & Philippines (2014)
Question: Explain the formation of thousands of islands in Indonesian and Philippines archipelagos. (2014)
Focus: Discuss Convergent Plate Boundaries (Oceanic–Oceanic) and the resulting island arc formation.
Continental Drift Theory (2013)
Question: What do you understand by the theory of 'Continental Drift'? Discuss the prominent evidences in its support. (2013)
Focus: Define Wegener’s theory and detail evidences like Placer Deposits, Fossil Distribution, and Rock Age correlation.
Evolution of the Himalayas (Adapted – 2014 Theme)
Question: Explain the evolution of the Himalayas through the lens of Plate Tectonics.
Focus: Describe the collision of the Indian Plate with the Eurasian Plate and the closing of the Tethys Sea.
Mid-Oceanic Ridges & Seafloor Spreading
Question: How does the study of Mid-Oceanic Ridges help in understanding the mechanism of Seafloor Spreading?
Focus: Explain how volcanic activity at ridges creates new crust, pushing older crust away.
Distribution of Volcanoes & Earthquakes
Question: Discuss the distribution of volcanoes and earthquakes with respect to major tectonic plates.
Focus: Map the “Ring of Fire” and mid-oceanic ridges as primary zones of activity.
Mains Practice Questions
Convection Current Theory – Arthur Holmes
Question: Critically evaluate the 'Convection Current Theory' of Arthur Holmes as a precursor to modern Plate Tectonics.
Approach: Explain how Holmes' idea of mantle convection currents provided the missing "mechanism" that Wegener's Continental Drift theory lacked.
Supercontinent Cycle & Continental Movement
Question: "The current distribution of continents is merely a snapshot in a long geological history." Elaborate.
Approach: Discuss the cycle of supercontinent formation (Pangea) and breakup through the continuous movement of lithospheric plates.
Continental Crust vs Oceanic Crust
Question: Compare and contrast Continental Crust and Oceanic Crust in the context of their age, density, and composition.
Approach: Highlight that oceanic crust is younger and denser (Sima), while continental crust is older, thicker, and lighter (Sial).
