BharatNotes Why this chapter matters for UPSC: This is Chapter 4 of Fundamentals of Physical Geography in the current rationalised NCERT (Reprint 2026-27). It traces how ideas about the positions of continents and oceans developed: Wegener's continental drift (1912), the post-war ocean-floor discoveries, Hess's sea-floor spreading (1961) and plate tectonics (1967). It ends with the movement of the Indian plate. The Mains bank has four direct questions on it (continental drift 2013, fold mountains 2014, mantle plumes 2018, the Circum-Pacific zone 2020), and Prelims 2025 asked for the evidence of continental drift in NCERT's own words.
Contemporary hook: The Himalaya continues to rise by more than 1 cm a year because the Indian and Eurasian plates are still converging (USGS, This Dynamic Earth). The same plate boundary, where the India plate subducts beneath the Burma micro-plate, produced the M 9.1 Sumatra–Andaman earthquake and Indian Ocean tsunami of 26 December 2004 (USGS).
🧠 First Principles — Read This First
The positions of land and sea on a world map are not permanent. They have changed through geological time and will keep changing. This chapter explains how geographers came to know this and what makes the land move.
Start with two layers. The lithosphere is the rigid outer shell: the crust plus the top part of the mantle. Under it lies the asthenosphere, hot and soft enough to flow very slowly. The lithosphere is broken into large slabs called plates, and the plates slide over the asthenosphere at a few centimetres a year. That is roughly the speed at which fingernails grow.
The idea arrived in two steps. In 1912 Alfred Wegener argued that all the continents were once joined in a single landmass, Pangaea, which later broke apart. He supported this with matching coastlines, rocks and fossils on opposite sides of the Atlantic. But he could not name a force strong enough to move continents, and most scholars rejected his explanation. After the Second World War, mapping of the ocean floor showed that new crust is created at mid-ocean ridges and destroyed at deep trenches. This is sea-floor spreading. It led in 1967 to plate tectonics: what moves is not a continent but the whole plate that carries it.
Plate edges are where plates pull apart, collide or slide past each other. That is why earthquakes, volcanoes, fold mountains and trenches are concentrated in long, narrow belts instead of being scattered.
UPSC uses this chapter to test the evidence for drift, the types of plate boundary and the landforms each produces, and India's tectonic setting, which explains the Himalaya, the Deccan Traps and the country's earthquake risk.
PART 1 — Quick Reference
Table 1: Timeline of ideas (NCERT)
| Year | Person(s) | Contribution |
|---|---|---|
| 1596 | Abraham Ortelius (Dutch map maker) | First proposed that the Americas, Europe and Africa were once joined |
| — | Antonio Pellegrini | Drew a map showing the three continents together |
| 1912 | Alfred Wegener (German meteorologist) | Comprehensive "continental drift theory" |
| 1930s | Arthur Holmes | Convection currents in the mantle, driven by heat from radioactive elements |
| Post-1945 | Ocean-floor mapping expeditions | Mid-oceanic ridges, trenches, young ocean-floor rocks |
| 1961 | Harry Hess | Sea-floor spreading |
| 1964 | Bullard | Computer best-fit of the Atlantic margins, matched at the 1,000-fathom line |
| 1967 | McKenzie and Parker; Morgan (independently) | Plate tectonics |
Source: NCERT FoPG (rationalised), Chapter 4, pp. 27–34.
Table 2: Continental drift theory (Wegener, 1912)
| Aspect | Details |
|---|---|
| Supercontinent | Pangaea ("all earth") |
| Surrounding mega-ocean | Panthalassa ("all water") |
| Break-up | Began about 200 million years ago (NCERT) |
| First split | Laurasia (northern) and Gondwanaland (southern), which later broke into today's continents |
| Forces proposed | Pole-fleeing force (related to the Earth's rotation and the equatorial bulge) and tidal force (attraction of the Moon and the Sun) |
| Verdict | Most scholars considered these forces totally inadequate |
Table 3: Evidence for continental drift (NCERT)
| Evidence | What NCERT says |
|---|---|
| Jig-saw fit | Facing shorelines of Africa and South America match; Bullard's 1964 computer fit, made at the 1,000-fathom line rather than the shoreline, was nearly perfect |
| Rocks of same age across the oceans | Belts of 2,000-million-year-old rocks on the Brazil coast match those of western Africa; the earliest marine deposits on both coasts are Jurassic, so the ocean did not exist before then |
| Tillite | Glacial sedimentary rock. India's Gondwana sediments have counterparts in six Southern Hemisphere landmasses (Africa, Falkland Islands, Madagascar, Antarctica, Australia among them), with thick tillite at the base, showing extensive and prolonged glaciation |
| Placer deposits | Ghana's coast has rich gold placers but no source rock; the gold-bearing veins are in Brazil, so the gold came from the Brazil plateau when the two continents lay side by side |
| Distribution of fossils | Lemurs in India, Madagascar and Africa led some to propose a land bridge, "Lemuria". Mesosaurus, a small reptile of shallow brackish water, is found only in the Southern Cape province of South Africa and the Iraver formations of Brazil, now 4,800 km apart |
Source: NCERT FoPG (rationalised), Chapter 4, pp. 27–28.
Table 4: The plates (NCERT list)
| Major plates (7) | Minor plates named by NCERT |
|---|---|
| Antarctica and the surrounding oceanic plate | Cocos — between Central America and the Pacific plate |
| North American (with the western Atlantic floor, separated from the South American plate along the Caribbean islands) | Nazca — between South America and the Pacific plate |
| South American (with the western Atlantic floor) | Arabian — mostly the Saudi Arabian landmass |
| Pacific | Philippine — between the Asiatic and Pacific plates |
| India-Australia-New Zealand | Caroline — between the Philippine and Indian plates, north of New Guinea |
| Africa with the eastern Atlantic floor | |
| Eurasia and the adjacent oceanic plate |
Source: NCERT FoPG (rationalised), Chapter 4, pp. 32–33. Other texts also name the Caribbean and Juan de Fuca plates; they are not in NCERT's list.
Table 5: Plate boundaries
| Boundary | NCERT definition | Landforms | Examples |
|---|---|---|---|
| Divergent (constructive) | New crust is generated as plates pull apart; these are "spreading sites" | Mid-ocean ridges; rift valleys on land | Mid-Atlantic Ridge (American plates separate from the Eurasian and African plates); Iceland; East African Rift |
| Convergent: ocean–continent | Crust is destroyed as one plate dives under another at a subduction zone | Trench, volcanic mountain chain on the continent | Nazca under South American plate: Peru–Chile Trench and Andes |
| Convergent: ocean–ocean | Same | Trench and volcanic island arc | Pacific plate against Philippine plate: Mariana Trench and Mariana Islands |
| Convergent: continent–continent | Same, but neither continental plate sinks | Fold mountains; no volcanic arc | India–Eurasia: Himalaya and Tibetan Plateau |
| Transform (conservative) | Crust neither produced nor destroyed; plates slide horizontally past each other | Transform faults, generally perpendicular to mid-oceanic ridges | San Andreas fault (on land) |
Sources: NCERT FoPG Chapter 4, p. 33; examples from USGS, This Dynamic Earth, "Understanding plate motions".
Table 6: Measured figures beyond NCERT
| Figure | Value | Source |
|---|---|---|
| Spreading on the Mid-Atlantic Ridge | About 2.5 cm/yr on average | USGS, This Dynamic Earth |
| San Andreas fault | Pacific plate moving past the North American plate at about 5 cm/yr on average, for about 10 million years; fault zone about 1,300 km long | USGS, This Dynamic Earth |
| Uplift of the Himalaya | More than 1 cm a year | USGS, This Dynamic Earth, "The Himalayas" |
| Challenger Deep, Mariana Trench | About 10,935 m. Older books give about 11,034 m; USGS says "nearly 11,000 m" | NOAA National Ocean Service, "How deep is the ocean?" (accessed 1 Oct 2026) |
| Ring of Fire | More than 450 volcanoes along nearly 40,250 km, horseshoe-shaped | NOAA Ocean Exploration, "What is the Ring of Fire?" (last updated 17 Dec 2020) |
PART 2 — Concepts & Narrative
Continental drift and why it was rejected
Wegener's theory explained a pattern that map readers had noticed for centuries: the coastlines on either side of the Atlantic look as if they once fitted together. Table 2 sets out his main claims.
His evidence was strong (Table 3). His mechanism was not. He proposed a pole-fleeing force, linked to the bulge the Earth's rotation creates at the equator, and a tidal force from the Moon and Sun. He thought these would work if applied over many millions of years. Most scholars judged them totally inadequate, so the theory was set aside.
NCERT's later correction matters for answers. Wegener thought the continents moved through the ocean floor. Plate tectonics says the plates move, carrying both continents and ocean floor. NCERT also notes that Pangaea was not the original state: continents have been wandering through geological time, and Pangaea itself was formed when separate continental masses converged.
Pangaea, Panthalassa, Laurasia, Gondwanaland. Pangaea ("all earth") is Wegener's single supercontinent. Panthalassa ("all water") is the mega-ocean around it. When Pangaea split, the northern part became Laurasia and the southern part Gondwanaland. India's Gondwana rock system, with its basal tillite and its coal, takes its name from this southern landmass.
Post-drift studies
Most of Wegener's evidence came from the continents: fossils, plants and deposits like tillite. Two lines of work after the Second World War shifted attention to the oceans.
Convection currents (Holmes, 1930s). Arthur Holmes suggested that heat from radioactive elements creates temperature differences in the mantle and sets up convection currents across the whole mantle. This was an attempt to supply the force that Wegener lacked.
Mapping the ocean floor. Expeditions showed that the ocean floor is not a flat plain. It has submerged mountain ranges and deep trenches, the trenches mostly close to continental margins. What these surveys found about the ridges is listed under sea-floor spreading below.
NCERT divides the ocean floor into three parts:
- Continental margins: the transition from the shore to the deep-sea basin. They include the continental shelf, slope, rise and the deep-oceanic trenches. The trenches matter most for this chapter.
- Abyssal plains: extensive plains between the continental margins and the mid-oceanic ridges, where sediments carried beyond the margins settle.
- Mid-oceanic ridges: an interconnected chain of mountains, the longest mountain chain on the Earth's surface though under water. A central rift at the crest is a zone of intense volcanic activity.
Distribution of earthquakes and volcanoes. On NCERT's map (Figure 4.2) a line of earthquake dots runs down the central Atlantic, nearly parallel to the coasts, and continues into the Indian Ocean. South of the Indian subcontinent it splits: one branch goes into East Africa and the other joins a line from Myanmar to New Guinea. This line coincides with the mid-oceanic ridges, where earthquake foci are shallow. A second belt follows the Alpine-Himalayan system and the rim of the Pacific, where earthquakes are deep-seated. Volcanoes follow a similar pattern, and the Pacific rim is called the "rim of fire" (popularly the Ring of Fire) because of its many active volcanoes.
Sea-floor spreading (Hess, 1961)
NCERT lists five facts from ocean-floor mapping and palaeomagnetic study that Wegener did not have:
- Volcanic eruptions are common all along the mid-oceanic ridges and bring huge amounts of lava to the surface.
- Rocks equidistant on either side of a ridge crest match in age, chemical composition and magnetic properties. Rocks next to the ridge have normal polarity and are the youngest; age increases away from the crest.
- Ocean-crust rocks are nowhere older than 200 million years. Some continental rocks are 3,200 million years old.
- Ocean-floor sediments are unexpectedly thin, and no sediment column is older than 200 million years.
- Deep trenches have deep-seated earthquakes; mid-oceanic ridges have shallow ones (the pattern on the map above).
From these facts Hess proposed in 1961 that constant eruptions at the ridge crest split the oceanic crust and new lava wedges into it, pushing the older crust to either side. The ocean floor spreads. Because the Earth is not getting bigger, and the spreading of one ocean does not shrink another, Hess reasoned that crust must also be consumed. He placed this at the oceanic trenches, where the pushed-away floor sinks and is destroyed.
How the magnetic stripes date the ocean floor. Lava erupting at a ridge contains magnetic minerals. As it cools, those minerals record the direction of the Earth's magnetic field at that moment. The field reverses from time to time, so successive strips of new crust record "normal" and "reverse" polarity. Spreading carries each strip away from the ridge, producing matching bands on both sides. Because the dates of reversals are known, the width of each band gives the speed of spreading. NCERT notes this is how plate-movement rates are measured, and asks in the exercises which fact was not used for sea-floor spreading: the distribution of fossils on different continents. Fossils were drift evidence, not spreading evidence.
Plate tectonics (1967)
In 1967 McKenzie and Parker, and separately Morgan, brought these ideas together as plate tectonics. NCERT defines a tectonic (lithospheric) plate as a massive, irregularly shaped slab of solid rock, generally made of both continental and oceanic lithosphere, that moves horizontally over the asthenosphere as a rigid unit. The lithosphere is 5–100 km thick under the oceans and about 200 km under the continents. A plate is called continental or oceanic depending on which part is larger: the Pacific plate is largely oceanic, the Eurasian plate largely continental. Young fold mountains, ridges, trenches and faults surround the major plates.
Scientists have used palaeomagnetic data to fix where each landmass lay in past geological periods (NCERT Figure 4.4 shows 540 million years of motion). The position of Peninsular India has been traced from rocks of the Nagpur area.
The three boundary types: what Table 5 does not show
Table 5 gives the definitions and standard examples. Four points need more explanation.
Iceland and the rifts. Iceland straddles the Mid-Atlantic Ridge and is splitting along the spreading centre between the North American and Eurasian plates. On land, USGS describes the Red Sea as formed by spreading that tore Arabia away from Africa, and the East African Rift as a possible new spreading centre (USGS, "Understanding plate motions").
Which plate sinks. At an ocean–continent boundary the denser oceanic plate always goes down. At an ocean–ocean boundary one oceanic plate sinks under the other; at the Marianas it is the fast-moving Pacific plate going under the slower Philippine plate (USGS). At a continent–continent boundary neither sinks: the crust buckles and is pushed up and sideways.
Where subduction magma comes from. USGS states that the source is debated: partial melting of the descending slab, of the overlying plate, or both. Write "partial melting at the subduction zone", not "the oceanic plate melts".
Why transform faults cut across ridges. NCERT explains that eruptions do not happen along the whole ridge crest at once, so different parts of a plate move away from the ridge axis by different amounts, and the Earth's rotation also affects the separated blocks. Most transform faults are on the ocean floor; the San Andreas fault is one of the few on land (USGS).
Why the Andes have volcanoes and the Himalaya does not. Both ranges were built by convergence. In the Andes, an oceanic plate sinks into the mantle, and partial melting at depth generates magma that rises to form volcanoes. In the Himalaya, two continental plates meet. Continental crust is too buoyant to sink deep enough to melt, so it folds, faults and thickens instead. The result is the world's highest mountains with strong earthquakes but no volcanic arc. The type of crust on each side of a convergent boundary therefore predicts whether a mountain belt will be volcanic.
Earthquakes at plate boundaries
All three boundary types produce earthquakes. NCERT's contrast between shallow foci at ridges and deep-seated foci at trenches is given above; two further points:
- At transform faults earthquakes are generally shallow (USGS); the San Andreas fault is the land example.
- At subduction zones earthquakes occur from shallow to great depth, and the largest events happen here. The M 9.1 Sumatra–Andaman earthquake of 26 December 2004 was a megathrust earthquake on the interface where the India plate subducts beneath the Burma micro-plate (USGS). The M 8.3 earthquake of 9 June 1994 under Bolivia, 636 km deep in the Nazca–South America subduction zone, was one of the deepest large earthquakes recorded in South America (USGS).
Hotspots: volcanoes away from plate edges
Some volcanoes sit in the middle of plates. The Hawaiian Islands lie more than 3,200 km from the nearest plate boundary. J. Tuzo Wilson's hotspot theory (1963) explains them as the product of a long-lasting hot region beneath the plate. As the Pacific plate moves over it, a chain of volcanoes forms that gets older away from the hotspot: Kauai's oldest rocks are about 5.5 million years old, while the Big Island's are under 0.7 million years. USGS notes more than a hundred hotspots active in the past 10 million years, most under plate interiors and some near ridges, such as under Iceland. It also notes an ongoing scientific debate about whether hotspots are deep and fixed. A mantle plume is the rising column of hot mantle thought to feed a hotspot; Mains GS1 2018 asked for its role in plate tectonics.
So Mauna Loa and Kilauea in Hawaii are hotspot (intraplate) volcanoes, not divergent-boundary volcanoes. Iceland is the classic case of volcanism on a divergent boundary.
Rates and forces of plate movement
The magnetic stripes parallel to mid-oceanic ridges give the rates. NCERT gives the range: the Arctic Ridge is slowest (less than 2.5 cm/yr) and the East Pacific Rise near Easter Island, about 3,400 km west of Chile, is fastest (more than 15 cm/yr). Today GPS and other space-geodesy methods measure plate motion directly, and their results agree with the long-term averages from the stripes (USGS).
On the driving force, NCERT says that the mobile rock beneath the plates moves in a circular manner: heated material rises, spreads, cools and sinks, forming convection cells. The heat comes from radioactive decay and residual heat. Holmes first considered this idea in the 1930s, and it influenced Hess. NCERT concludes that "the slow movement of hot, softened mantle that lies below the rigid plates is the driving force behind the plate movement".
Current view of the driving force. USGS (This Dynamic Earth, "Some unanswered questions") records a change in emphasis. Until the 1990s, mantle convection and ridge push were seen as the main drivers. Most scientists now give more weight to forces linked to subduction: the gravity-driven sinking of a cold, dense oceanic slab into the mantle, called slab pull, which drags the rest of the plate. USGS adds that no proposed mechanism explains every aspect of plate motion and none can be tested directly. For an NCERT-based answer, write convection first, then add that slab pull is now considered dominant, with ridge push and mantle flow also contributing.
Movement of the Indian plate
NCERT describes the Indian plate's boundaries as follows:
| Margin | Nature |
|---|---|
| North | Subduction zone along the Himalaya, in the form of continent–continent convergence |
| East | Through the Rakinyoma mountains of Myanmar towards the island arc along the Java Trench |
| Far east | A spreading site east of Australia, an oceanic ridge in the south-west Pacific |
| West | Along the Kirthar mountains of Pakistan, then the Makran coast, joining the spreading site from the Red Sea rift south-eastward along the Chagos Archipelago |
| South | An oceanic ridge (divergent) between the Indian and Antarctic plates, running roughly west–east and merging into the spreading site south of New Zealand |
Source: NCERT FoPG (rationalised), Chapter 4, p. 34.
India's journey, in NCERT's dates:
- India was a large island off the Australian coast. The Tethys Sea separated it from Asia until about 225 million years ago.
- India began moving north about 200 million years ago, when Pangaea broke up.
- About 140 million years ago the subcontinent lay as far south as 50°S.
- The outpouring of lava that formed the Deccan Traps started around 60 million years ago and continued for a long time. The subcontinent was then still close to the equator.
- India collided with Asia about 40–50 million years ago, causing rapid uplift. Himalayan formation took place from 40 million years ago onward, and the process continues.
Two modern figures differ from NCERT's rounding. Uranium-lead dating of the Deccan lavas places their main eruptive pulses around the Cretaceous–Paleogene boundary, about 66 million years ago, with one pulse starting tens of thousands of years before the mass extinction (Schoene et al., Science, 2019). The eruptions are widely linked to the head of the mantle plume whose tail now feeds Réunion Island, though some geologists dispute the plume model. USGS gives about 50 million years for the start of the collision on one page and 40–50 million years on another; NCERT's 40–50 Ma covers both. In the exam, use NCERT's figures and add the modern ones only as a note.
The collision also explains marine fossils in the Himalaya. Sediments laid down in the Tethys Sea were folded and lifted as India pushed into Asia; USGS describes fossil-bearing Tethys sandstones north of Lhasa that show Tibet was closer to the equator about 105 million years ago.
PART 3 — UPSC Integration
PYQ pattern (from the bank)
| Paper, year | Bank ID | Question (summary) | Chapter section |
|---|---|---|---|
| Mains GS1 2013 (5M) | gs1-pyq-2013-19a | Theory of continental drift and its evidences | Continental drift; Table 3 |
| Mains GS1 2014 (10M) | gs1-pyq-2014-09 | Why fold mountains lie along continental margins; their association with earthquakes and volcanoes | Convergent boundaries |
| Mains GS1 2018 (10M) | gs1-pyq-2018-06 | Mantle plume and its role in plate tectonics | Hotspots |
| Mains GS1 2020 (10M) | gs1-pyq-2020-05 | Geophysical characteristics of the Circum-Pacific zone | Earthquakes and volcanoes; subduction |
| Prelims 2025 | geo-068 (geography bank) | Which statements are evidence of continental drift (Brazil–West Africa rocks, Ghana gold, Gondwana sediments) | Table 3 |
The Prelims 2025 question tested three of NCERT's evidences (rock belts, placer gold, Gondwana sediments). The Mains questions ask for the same content in explanatory form.
Mains frameworks
- Evidence, then mechanism, then modern theory. For drift questions: Wegener's evidence (jig-saw fit, rocks, tillite, placers, fossils), the weakness (inadequate forces), then the post-drift discoveries that revived the idea and the move from drifting continents to moving plates.
- Boundary type predicts landform. Divergent gives ridges and rifts; ocean–continent gives trench and volcanic range; ocean–ocean gives trench and island arc; continent–continent gives fold mountains without volcanoes; transform gives shallow-focus earthquakes. This answers both the 2014 fold-mountain question and the 2020 Circum-Pacific question.
- India as a case study. The Indian plate's boundaries and journey connect the chapter to the Himalaya, the Deccan Traps and the subduction zone along the Andaman–Sumatra arc.
Cross-paper relevance
- GS1 (Geography): drift, sea-floor spreading, plate boundaries, distribution of earthquakes and volcanoes, origin of the Himalaya.
- GS3 (Disaster management): the same plate setting explains India's earthquake hazard (the Himalayan belt lies in Seismic Zones IV and V on the map in IS 1893 (Part 1):2016) and the 2004 tsunami. Bank questions gs3-pyq-2015-76, gs3-pyq-2017-38 and gs3-pyq-2021-10 build on this chapter, though they are tagged to disaster management.
Exam Strategy
Prelims fact-traps (from NCERT's text and exercises):
- The first person to suggest the continents were once joined was Abraham Ortelius (1596), not Wegener. Wegener (1912) made it a full theory.
- Pole-fleeing force relates to the rotation of the Earth, not its revolution.
- Antarctica is a major plate, not a minor one. NCERT's minor plates are Cocos, Nazca, Arabian, Philippine and Caroline.
- The fact not used for sea-floor spreading is the distribution of fossils; that was drift evidence.
- The Indian plate's boundary along the Himalaya is continent–continent convergence.
- Mesosaurus lived in shallow brackish water (NCERT), not the open sea; its two localities are 4,800 km apart.
- Hess proposed sea-floor spreading in 1961; plate tectonics dates from 1967 (McKenzie and Parker; Morgan).
- Ocean crust is nowhere older than 200 million years; some continental rocks are 3,200 million years old.
- Earthquakes under mid-oceanic ridges are shallow; under trenches they are deep-seated.
- Hawaii is a hotspot, in the middle of the Pacific plate, not on a divergent boundary.
Mains question patterns:
- "Discuss the evidences" questions want NCERT's five evidence types with the specific example for each (2,000-million-year rock belts, Ghana gold without source rock, Gondwana tillite in six landmasses, Mesosaurus).
- "Geophysical characteristics" of a zone want boundary type, earthquake depth pattern, volcano type, trench and island arc, with named examples.
- Mantle-plume questions want hotspot volcanism within plates (Hawaii), plume links to flood basalts (Deccan–Réunion, with the caveat that it is debated) and the plume's place among plate-driving forces.
Practice Questions
- Mains GS1 2013 (5 marks), bank ID gs1-pyq-2013-19a: "What do you understand by the theory of continental drift? Discuss the prominent evidences in its support."
- Mains GS1 2014 (10 marks), bank ID gs1-pyq-2014-09: "Why are the world's fold mountain systems located along the margins of continents? Bring out the association between the global distribution of fold mountains and the earthquakes and volcanoes."
- Mains GS1 2020 (10 marks), bank ID gs1-pyq-2020-05: "Discuss the geophysical characteristics of the Circum-Pacific zone."
- Prelims 2025, bank ID geo-068 (bank wording): "Which of the following are the evidence(s) of the continental drift theory? 1. Brazilian rocks match Western African rock formations. 2. Ghana's gold deposits are derived from Brazil. 3. Gondwana sediments have counterparts in the Southern Hemisphere." Options: (a) 1 and 3 only (b) 1 and 2 only (c) 1, 2 and 3 (d) 2 and 3 only. Answer: (c).
- Practice (UPSC-pattern, not a past paper): Consider the following pairs of minor plates and their locations as given by NCERT: 1. Cocos plate — between Central America and the Pacific plate; 2. Caroline plate — between the Philippine and Indian plates; 3. Nazca plate — between South America and the Pacific plate. How many of the pairs are correctly matched? Answer: all three.
📦 Revision Capsule
Hard Facts
- Ortelius (1596) first proposed the joined continents; Pellegrini drew the map; Wegener gave the continental drift theory in 1912.
- Pangaea ("all earth") surrounded by Panthalassa ("all water"); began to split about 200 million years ago into Laurasia (north) and Gondwanaland (south).
- Wegener's forces: pole-fleeing force (rotation) and tidal force; judged totally inadequate.
- Drift evidence: Bullard's 1964 fit at the 1,000-fathom line; 2,000-million-year rock belts of Brazil and West Africa; Gondwana tillite in six landmasses; Ghana gold from the Brazil plateau; Mesosaurus (shallow brackish water) in South Africa and Brazil, 4,800 km apart.
- Holmes: mantle convection, 1930s. Hess: sea-floor spreading, 1961. McKenzie and Parker, and Morgan: plate tectonics, 1967.
- Ocean crust nowhere older than 200 million years; some continental rocks 3,200 million years old.
- Lithosphere: 5–100 km thick under oceans, about 200 km under continents.
- 7 major plates; NCERT minor plates: Cocos, Nazca, Arabian, Philippine, Caroline.
- Spreading rates: Arctic Ridge < 2.5 cm/yr; East Pacific Rise near Easter Island > 15 cm/yr.
- India: separated from Asia by the Tethys till about 225 Ma; moved north from about 200 Ma; at 50°S about 140 Ma; Deccan Traps from about 60 Ma (NCERT; about 66 Ma by U-Pb dating); collision about 40–50 Ma.
- Challenger Deep about 10,935 m (NOAA); older figure about 11,034 m.
Core Concepts
- Plates, not continents, move; continents ride on plates.
- New crust forms at ridges and is consumed at trenches, so the Earth does not grow.
- Boundary type and crust type together predict the landform.
- Subduction produces volcanoes; continent–continent collision produces fold mountains without volcanoes.
- Hotspot volcanoes (Hawaii) form inside plates, away from boundaries.
Confused Pairs
- Continental drift (continents move; Wegener) vs plate tectonics (plates move; 1967).
- Drift evidence (fossils, tillite, placers) vs sea-floor spreading evidence (magnetic stripes, young ocean crust, thin sediments).
- Mid-oceanic ridge (shallow earthquakes, new crust) vs trench (deep-seated earthquakes, crust consumed).
- Divergent-boundary volcano (Iceland) vs hotspot volcano (Hawaii).
Data Points
- Mid-Atlantic Ridge spreading about 2.5 cm/yr; San Andreas about 5 cm/yr; Himalaya rising more than 1 cm/yr (USGS).
- Ring of Fire: more than 450 volcanoes, nearly 40,250 km (NOAA, updated 17 Dec 2020).
PYQ Pattern
- Mains GS1: drift evidence (2013), fold mountains at continental margins (2014), mantle plume (2018), Circum-Pacific zone (2020).
- Prelims 2025: NCERT's drift evidence statements (geo-068).
Sources
Sources
- NCERT, Fundamentals of Physical Geography, Class XI, Chapter 4 "Distribution of Oceans and Continents", rationalised edition, Reprint 2026-27: https://ncert.nic.in/textbook/pdf/kegy204.pdf
- USGS, This Dynamic Earth: The Story of Plate Tectonics (1996, online edition): "Understanding plate motions", https://pubs.usgs.gov/gip/dynamic/understanding.html ; "Some unanswered questions", https://pubs.usgs.gov/gip/dynamic/unanswered.html ; "Hotspots", https://pubs.usgs.gov/gip/dynamic/hotspots.html ; "The Himalayas", https://pubs.usgs.gov/gip/dynamic/himalaya.html
- NOAA National Ocean Service, "How deep is the ocean?", https://oceanservice.noaa.gov/facts/oceandepth.html (accessed 1 October 2026)
- NOAA Ocean Exploration, "What is the Ring of Fire?" (published 21 Oct 2013, last updated 17 Dec 2020), https://oceanexplorer.noaa.gov/ocean-fact/rof/
- USGS Pacific Coastal and Marine Science Center, "Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake", https://www.usgs.gov/centers/pcmsc/science/tsunami-generation-2004-m91-sumatra-andaman-earthquake
- Schoene, B. et al. (2019), "U-Pb constraints on pulsed eruption of the Deccan Traps across the end-Cretaceous mass extinction", Science 363(6429): 862–866. Author copy: https://oar.princeton.edu/bitstream/88435/pr1ms3k163/1/U-Pb_constraints_pulsed_eruption_Deccan_Traps_end-Cretaceous_mass_extinction.pdf
- "A geochemical link between plume head and tail volcanism", Geochemical Perspectives Letters, https://www.geochemicalperspectivesletters.org/article1742/ (Deccan–Réunion plume link)
