This MCQ module is based on: Plate Tectonics, Boundaries & Indian Plate Journey + Exercises
TOPIC 8 OF 29
Plate Tectonics, Boundaries & Indian Plate Journey + Exercises
🎓 Class 11
Social Science
CBSE
Theory
Ch 4 — Distribution of Oceans and Continents
⏱ ~28 min
🌐 Language: [gtranslate]
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4.9 The Sea Floor Speaks — A New Kind of Evidence
Part 1 ended with the post-war revival of interest in continental drift. The crucial new evidence came from the ocean — the part of the Earth that Wegener could not study because no one had mapped it in any detail. Two new lines of investigation transformed the science: the geophysical mapping of the ocean floor (already discussed) and a brand-new technique known as palaeomagnetism? — the study of the magnetic record locked in oceanic rocks.
📍 What This Lesson Covers
Five facts that puzzled post-war geologists → Hess's Sea Floor Spreading hypothesis (1961) → the Plate Tectonics Theory (1967) by McKenzie, Parker and Morgan → seven major plates and six minor plates → rates of plate movement → driving force (mantle convection — Holmes, Hess) → three types of plate boundaries (divergent?, convergent?, transform?) → movement of the Indian plate → ALL NCERT exercises with answers.
4.9.1 Five New Facts About the Ocean Floor
By the early 1960s, the post-drift surveys and palaeomagnetic studies had built up five striking facts:
- Volcanic eruptions are extremely common all along the mid-oceanic ridges, bringing huge amounts of fresh lava to the surface.
- Rocks at equal distances on either side of the crest of a ridge show the same period of formation, the same chemistry, and the same magnetic properties. Rocks closest to the ridge crest are the youngest and have normal magnetic polarity; ages increase as you move away from the ridge.
- The oceanic crust as a whole is much younger than the continental crust. No oceanic rock is older than 200 million years, while some continental rocks are as old as 3,200 million years.
- The sediment column on the ocean floor is unexpectedly thin. Had the oceans been as old as the continents, geologists would have expected hundreds of millions of years' worth of mud. Instead, the entire sediment layer represents at most about 200 million years.
- Earthquakes in the deep trenches are deep-seated; quakes near the mid-oceanic ridges have shallow foci.
Together these five facts pointed to one inescapable conclusion: the ocean floor is actively being created at the ridges and actively being destroyed at the trenches.
4.10 Sea Floor Spreading — Hess (1961)
The American geologist Harry Hess, in 1961, put the five facts together into a hypothesis he called sea floor spreading?. Hess argued that the constant volcanic eruptions at the crest of a mid-oceanic ridge rupture the oceanic crust. New lava wedges into the gap and pushes the older crust outwards on either side. The ocean floor therefore spreads away from the ridge.
The fact that the oceanic crust is uniformly young, and that one ocean's spreading does not cause another ocean to shrink, forced Hess to add a second idea — the crust must also be consumed somewhere. He proposed that the floor pushed outwards by ridge volcanism eventually sinks back into the mantle at the deep oceanic trenches. Production at the ridge balances destruction at the trench, and the total surface area of the Earth stays the same.
Sea Floor Spreading — Cross-Section
Bloom: L3 ApplyFigure 4.4 (after NCERT 4.3): new ocean floor is created at the mid-oceanic ridge and consumed at the deep trenches — Hess's 1961 hypothesis.
📜 Source — Vine and Matthews (1963)
Two years after Hess proposed his hypothesis, the British geologists Fred Vine and Drummond Matthews discovered that the rocks of the ocean floor preserve parallel stripes of normal and reversed magnetic polarity — symmetrical mirror-images on either side of every ridge. These palaeomagnetic stripes were the smoking-gun proof of sea floor spreading.
— after Vine & Matthews, "Magnetic Anomalies over Oceanic Ridges", Nature, 1963
4.11 Plate Tectonics — The Unifying Theory (1967)
Sea floor spreading explained how the ocean was being made and consumed, but the larger question remained — what was the geometry of the moving pieces? In 1967, two teams of geophysicists working independently arrived at the same answer. Dan McKenzie and Robert Parker on one side, and Jason Morgan on the other, gathered all the available evidence and unveiled a new framework called the theory of Plate Tectonics?.
According to plate tectonics, the rigid outer shell of the Earth — the lithosphere — is broken into a number of large slabs called tectonic or lithospheric plates. Each plate is irregular in shape and made of solid rock. Plates may include both continental and oceanic lithosphere. They move horizontally over the soft, plastic asthenosphere below as rigid units. The lithosphere itself is between 5 and 100 km thick in oceanic regions, and about 200 km thick in continental regions.
A plate is called continental or oceanic depending on which type of lithosphere covers most of its area. The Pacific plate is largely oceanic; the Eurasian plate is largely continental. Plates are surrounded by young fold-mountain ridges, deep trenches and/or great fault lines.
📖 Definition — Tectonic Plate
A tectonic plate (also called a lithospheric plate) is a massive, irregularly-shaped slab of solid rock made up of both continental and oceanic lithosphere. Plates move horizontally as rigid units over the asthenosphere. The earth's lithosphere is divided into seven major plates and several minor plates.
4.11.1 The Seven Major Plates
| Plate | Brief description |
|---|---|
| I. Antarctic plate | Antarctica and the surrounding oceanic floor |
| II. North American plate | North America with the western Atlantic floor (separated from the South American plate along the Caribbean islands) |
| III. South American plate | South America with the western Atlantic floor (separated from the North American plate along the Caribbean islands) |
| IV. Pacific plate | Largely oceanic — the largest and most active of all the plates |
| V. India–Australia–New Zealand plate | Includes Peninsular India, Australia, New Zealand and the surrounding sea floor |
| VI. African plate | Africa with the eastern Atlantic floor |
| VII. Eurasian plate | Europe, most of Asia and the adjacent oceanic floor |
4.11.2 Important Minor Plates
(i) Cocos plate
Lies between Central America and the Pacific plate.
(ii) Nazca plate
Lies between South America and the Pacific plate.
(iii) Arabian plate
Mostly the Saudi Arabian landmass.
(iv) Philippine plate
Lies between the Asiatic landmass and the Pacific plate.
(v) Caroline plate
Lies between the Philippine and Indian plates, north of New Guinea.
(vi) Fuji plate
Small plate in the south-western Pacific Ocean, near Fiji.
⚠ Important — Plate vs Continent
Wegener thought the continents moved through the ocean floor. Plate tectonics corrects this: it is the plate that moves, carrying both its continental and oceanic crust together. Pangaea was therefore not the original starting point but a temporary configuration created when several wandering plates happened to converge. All plates, without exception, have moved in the past and continue to move today.
4.12 Rates of Plate Movement
How fast do plates move? The strips of normal and reversed magnetic field in the rocks beside the mid-oceanic ridges (palaeomagnetic stripes) provide a natural calendar. By measuring the width of each stripe and matching it to the known time of magnetic reversal, geologists can calculate the speed of spreading.
The speeds vary dramatically. The Arctic Ridge is the slowest, spreading at less than 2.5 cm per year. The East Pacific Rise near Easter Island — about 3,400 km west of Chile in the South Pacific — is the fastest, spreading at more than 15 cm per year.
Comparative Rates of Plate Spreading (cm / year)
Indicative rates from the chapter and standard references: from the slow Arctic Ridge to the fast-spreading East Pacific Rise. The Mid-Atlantic Ridge sits between the two extremes.
4.13 Force for Plate Movement — Convection at Last
When Wegener proposed his theory in 1912, most scientists believed the Earth was a solid, motionless body. The triumph of plate tectonics changed this view fundamentally. Both the surface and the deep interior are now known to be dynamic — never still.
The driving force of plate motion is the slow circulation of hot, soft mantle rock beneath the rigid plates. Heated material rises towards the surface, spreads sideways, cools, and sinks back into the depths. This continuous loop is called a convection cell. The heat that powers it comes from two sources: radioactive decay of elements like uranium and thorium inside the Earth, and residual heat left over from the planet's formation. Arthur Holmes first proposed this idea in the 1930s; his work later inspired Harry Hess's 1961 hypothesis of sea floor spreading. Today the slow movement of the soft, hot asthenosphere beneath the rigid plates is accepted as the engine of plate motion.
4.14 Three Types of Plate Boundaries
Plates meet each other along three kinds of boundaries.
4.14.1 Divergent Boundaries
At a divergent boundary, two plates pull away from each other, and new crust is generated by the rise of fresh lava in the gap between them. Such sites are called spreading sites. The best-known example is the Mid-Atlantic Ridge, where the American plate(s) is/are moving away from the Eurasian and African plates.
4.14.2 Convergent Boundaries
At a convergent boundary, two plates collide and one of them is destroyed by being driven down beneath the other into the mantle. The site of sinking is called a subduction zone?. Three types of convergence are recognised:
- (i) between an oceanic plate and a continental plate (e.g., Nazca plate plunging under South America to raise the Andes);
- (ii) between two oceanic plates (forming island arcs);
- (iii) between two continental plates (the Indian plate against the Eurasian plate, raising the Himalayas).
4.14.3 Transform Boundaries
At a transform boundary, neither new crust is created nor old crust destroyed. Two plates simply slide horizontally past each other. The most famous example is the San Andreas Fault in California, where the Pacific plate slides north-westwards past the North American plate. Transform faults are usually perpendicular to the mid-oceanic ridges. They form because volcanic eruptions do not occur evenly along the entire crest of a ridge at the same time — different stretches of the ridge move at slightly different speeds, and the rotation of the Earth causes the separated blocks to slide past each other.
Three Types of Plate Boundaries
Bloom: L3 ApplyFigure 4.5 (after NCERT 4.5): three boundary types — at the first new crust forms, at the second old crust is consumed, at the third crust simply slides.
LET'S EXPLORE — How is the rate of plate movement determined?
Bloom: L3 Apply
The chapter asks: How do you think the rate of plate movement is determined? Use the keywords "palaeomagnetic stripes", "magnetic reversal" and "ridge crest" to write a 3-step explanation of how a geologist measures the speed at which the East Pacific Rise spreads.
✅ Answer
Step 1: A magnetometer towed across an ocean ridge records strips of normal and reversed magnetic polarity in the rocks of the ocean floor. These palaeomagnetic stripes are mirror-symmetrical on either side of the ridge crest. Step 2: Each magnetic reversal of the Earth has been independently dated. So the geologist can label every stripe with a known age. Step 3: Dividing the distance from the ridge crest to a particular stripe by the age of that stripe gives the average rate of spreading. For the East Pacific Rise this works out to more than 15 cm/year — the fastest in the world.
4.15 Movement of the Indian Plate
The Indian plate is one of the most travelled plates in the world. Today it includes Peninsular India and the Australian continental portion — a single plate carrying two distant landmasses. Its boundaries are of every kind:
Northern boundary — Himalayas
The subduction zone along the Himalayas marks the northern edge of the plate. This is a continent–continent convergence with the Eurasian plate.
Eastern boundary — Java Trench
Extends through the Rakinyoma Mountains of Myanmar towards the island arc along the Java Trench.
East of Australia — spreading site
The eastern margin of the Australian portion is a spreading site in the form of an oceanic ridge in the south-west Pacific.
Western boundary — Kirthar & Makrana
Follows the Kirthar Mountain of Pakistan and the Makrana coast, then joins the spreading site from the Red Sea rift south-eastwards along the Chagos Archipelago.
The boundary between the Indian and Antarctic plates is also marked by an oceanic ridge (a divergent boundary) running roughly W–E and merging into the spreading site a little south of New Zealand.
4.15.1 The Long Journey of India
India has travelled the longest distance of any major continent. About 225 million years ago, India was a large island sitting off the Australian coast in the wide ocean. The Tethys Sea separated it from the Asian continent. Then, when Pangaea began to split about 200 million years ago, India started moving northward. Roughly 140 million years ago, the Indian sub-continent lay as far south as 50° S latitude; the two great plates were separated by the Tethys Sea, with the Tibetan block lying closer to the Asian landmass.
During this northward journey, around 60 million years ago, an enormous outpouring of lava in central India built up the Deccan Traps. This volcanism continued for a long time. At that point the sub-continent was still close to the equator. From 40 million years ago, India crashed into the Asian landmass. The collision raised the Himalayas — and the process is still going on. The Himalayas are rising even today, by a few millimetres every year.
Northward Journey of the Indian Plate
Bloom: L4 AnalyseFigure 4.6 (after NCERT 4.6): from 50° S latitude to its present position — including the Deccan Traps eruption (~60 mya) and the start of the Himalayan orogeny (~40 mya).
THINK ABOUT IT — Why does the Himalaya keep growing?
Bloom: L4 Analyse
The Himalayan range is rising even today, even though India and Asia have been "in contact" for around 40 million years. Why has the collision not stopped, and what would it take to make the Himalayas finally stop growing?
✅ Reasoning
The convection currents in the mantle that pushed India north are still active. The Indian plate continues to push against the Eurasian plate at a few centimetres a year. Because both plates are continental and equally light, neither can subduct cleanly — they instead crumple and pile up, raising the Himalayas higher. The mountain range will only stop growing when (a) the convection cells driving the Indian plate change direction or shut down, or (b) the leading edge of the Indian plate finally breaks off and sinks into the mantle, removing the push.
📝 Competency-Based Questions — Spreading, Plates & Boundaries
A team of geophysicists is mapping the Mid-Atlantic Ridge with a research ship. They tow a magnetometer across the ridge and find perfectly symmetrical stripes of normal and reversed polarity on either side. They also notice that the rocks on both sides at equal distance from the crest have the same age. Meanwhile, on the other side of the world, a seismograph network detects deep-focus earthquakes along the Java Trench.
Q1. The mirror-symmetrical magnetic stripes either side of a mid-oceanic ridge are best explained by:
L1 RememberAnswer: (b) — As fresh lava cools at the ridge crest it locks in the prevailing direction of the Earth's magnetic field. The crust is then pushed outwards on both sides, producing a symmetrical "barcode" record of every magnetic reversal — a direct proof of sea floor spreading (Hess 1961; Vine and Matthews 1963).
Q2. The deep-focus earthquakes along the Java Trench are most likely caused by which type of plate boundary? Justify briefly.
L3 ApplyModel Answer: The Java Trench is a convergent boundary where the Indian plate dives beneath the Sunda (Eurasian) plate in a subduction zone. As the descending slab plunges hundreds of kilometres into the mantle, it shears against the surrounding rock and produces deep-focus earthquakes. Mid-oceanic ridges, by contrast, host only shallow-focus quakes.
Q3. Compare and contrast the Mid-Atlantic Ridge and the San Andreas Fault as plate boundaries.
L4 AnalyseModel Answer: The Mid-Atlantic Ridge is a divergent boundary: the American plate(s) are moving away from the Eurasian and African plates, and new crust is generated at the ridge crest. The San Andreas Fault is a transform boundary: the Pacific plate slides horizontally past the North American plate, and crust is neither created nor destroyed. Both are seismically active, but the Mid-Atlantic Ridge produces shallow ridge-type quakes and basaltic volcanism, whereas the San Andreas produces strike-slip earthquakes only.
HOT Q. If the convection currents in the mantle were to suddenly stop, predict TWO long-term consequences for plate tectonics on Earth and explain each.
L6 CreateHint: (1) Plate motion would gradually halt. Without the rising-and-sinking convection cells in the mantle to drag the rigid plates above, the driving force would vanish. New crust would no longer form at mid-oceanic ridges and old crust would no longer be consumed at trenches. (2) Tectonic activity would slow and the surface would erode flat. Without ridge volcanism and subduction, no new mountains, volcanoes or rifts would form. Existing mountains, including the Himalayas, would gradually wear down by erosion. The Earth would become a much quieter, smoother planet — but it would also lose much of its geothermal heat and magnetic field over geological time.
⚖️ Assertion–Reason Questions — Plate Tectonics
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): Oceanic crust is nowhere older than 200 million years, while continental rocks can be as old as 3,200 million years.
Reason (R): New oceanic crust is constantly created at mid-oceanic ridges and consumed at deep trenches through sea floor spreading and subduction.
Answer: (A) — Both true; R is the correct explanation. The oceanic crust is constantly recycled, so its maximum age is set by how long it takes a plate to travel from ridge to trench. Continental crust, being lighter, is not subducted and so survives for billions of years.
Assertion (A): The Himalayan boundary of the Indian plate is a continent–continent convergence.
Reason (R): Both the Indian plate and the Eurasian plate are predominantly continental at this margin, and neither can be cleanly subducted under the other.
Answer: (A) — Both true; R is the correct explanation. Continental crust is too light to sink into the mantle, so the two plates crumple instead, raising the Himalayas. India still pushes northward at a few cm/year, which is why the range continues to grow.
Assertion (A): Wegener's pole-fleeing force and tidal force are no longer accepted as the cause of continental drift.
Reason (R): The slow circulation of hot, soft mantle below the rigid plates — driven by radioactive decay and residual heat — is now considered the true driving force of plate movement.
Answer: (A) — Both true; R is the correct explanation. Holmes (1930s) first proposed mantle convection as the engine; the idea inspired Hess's 1961 hypothesis and is now part of the unified theory of plate tectonics.
4.16 NCERT Exercises — Chapter 4 Distribution of Oceans and Continents
1. Multiple Choice Questions
(i) Who amongst the following was the first to consider the possibility of Europe, Africa and America having been located side by side?
(a) Alfred Wegener (b) Antonio Pellegrini (c) Abraham Ortelius (d) Edmond Hess
Answer: (c) Abraham Ortelius. The Dutch map-maker Abraham Ortelius first proposed the idea in 1596, more than three centuries before Wegener's comprehensive theory of 1912.
(ii) Polar fleeing force relates to:
(a) Revolution of the Earth (b) Gravitation (c) Rotation of the earth (d) Tides
Answer: (c) Rotation of the earth. The Earth bulges at the equator because of its rotation, and this bulge generates the pole-fleeing force that pushes objects on the surface away from the poles.
(iii) Which one of the following is not a minor plate?
(a) Nazca (b) Arabia (c) Philippines (d) Antarctica
Answer: (d) Antarctica. Antarctica is one of the seven major plates of the world. Nazca, Arabia and Philippines are all minor plates.
(iv) Which one of the following facts was not considered by those while discussing the concept of sea floor spreading?
(a) Volcanic activity along the mid-oceanic ridges.
(b) Stripes of normal and reverse magnetic field observed in rocks of ocean floor.
(c) Distribution of fossils in different continents.
(d) Age of rocks from the ocean floor.
(b) Stripes of normal and reverse magnetic field observed in rocks of ocean floor.
(c) Distribution of fossils in different continents.
(d) Age of rocks from the ocean floor.
Answer: (c) Distribution of fossils in different continents. Fossil distribution was a key piece of evidence for Wegener's continental drift theory, but it played no role in Hess's 1961 hypothesis of sea floor spreading. The four facts that did matter were ridge volcanism, magnetic stripes, age of oceanic rocks and the thinness of the sediment column.
(v) Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains?
(a) Ocean–continent convergence (b) Divergent boundary (c) Transform boundary (d) Continent–continent convergence
Answer: (d) Continent–continent convergence. Both the Indian plate and the Eurasian plate are continental in nature at this boundary; their collision raises the Himalayas, which are still growing today.
2. Answer the following questions in about 30 words
(i) What were the forces suggested by Wegener for the movement of the continents?
Answer: Wegener suggested two forces. The pole-fleeing force arises from the rotation of the Earth and its equatorial bulge. The tidal force arises from the gravitational attraction of the Sun and Moon. He believed both, applied over millions of years, drifted the continents — though most scholars judged them too weak to do the job.
(ii) How are the convectional currents in the mantle initiated and maintained?
Answer: Heat from the radioactive decay of elements like uranium and thorium, together with residual heat from the Earth's formation, warms the mantle unevenly. Hot rock rises towards the surface, spreads, cools and sinks back to deeper levels. This continuous loop — a convection cell — is maintained as long as internal heat is generated.
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Answer: At transform boundaries (e.g., the San Andreas Fault) plates simply slide horizontally past each other — crust is neither created nor destroyed. At divergent boundaries new crust is generated as plates pull apart, and at convergent boundaries old crust is destroyed as one plate is subducted beneath another.
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
Answer: The Deccan Traps formed about 60 million years ago, when the Indian sub-continent was still close to the equator on its long northward journey from the Southern Hemisphere towards Asia. India had not yet collided with the Eurasian plate.
3. Answer the following questions in about 150 words
(i) What are the evidences in support of the continental drift theory?
Answer: Wegener offered six lines of evidence. (1) Matching of continents (jig-saw fit): the shorelines of South America and Africa fit together with remarkable precision; Bullard's 1964 computer programme confirmed the match at the 1,000-fathom line. (2) Rocks of same age across the oceans: a 2,000-million-year rock belt on the Brazilian coast matches one in western Africa, and the earliest South-Atlantic marine deposits on both sides are of the Jurassic age. (3) Tillite: the Gondwana sediments of India have identical counterparts (with thick basal tillite indicating prolonged glaciation) in Africa, the Falklands, Madagascar, Antarctica and Australia. (4) Placer deposits: the rich placer gold of the Ghana coast has its source rock in the Brazilian plateau across the Atlantic. (5) Distribution of fossils: Mesosaurus, Lemurs and Glossopteris are found on continents now separated by oceans (Mesosaurus skeletons sit 4,800 km apart in South Africa and Brazil). (6) Palaeoclimatic indicators: Antarctic coal beds and identical ice-age sequences across six continents prove a former union.
(ii) Bring about the basic difference between the drift theory and Plate tectonics.
Answer: The two theories agree that the surface of the Earth has changed dramatically over geological time, but differ on what moves and on the engine of that movement. Wegener's continental drift theory (1912) held that the continents themselves drift through the oceanic crust, propelled by pole-fleeing and tidal forces; the oceanic crust was treated as a passive medium. Plate tectonics (1967), framed by McKenzie, Parker and Morgan, says it is not the continent but the entire lithospheric plate — both continental and oceanic crust together — that moves as a rigid unit over the asthenosphere. The driving force is no longer pole-fleeing or tidal action but slow convection currents in the mantle, generated by radioactive heat. Plate tectonics also adds the ideas of sea floor spreading at ridges and subduction at trenches, which the drift theory could not explain. Pangaea is now seen as a temporary configuration of converging plates rather than the original arrangement.
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Answer: Three sets of post-drift discoveries revived interest in the subject. (1) Convection-current theory: in the 1930s, Arthur Holmes argued that radioactive heat generates convection currents inside the mantle, providing the engine of motion that Wegener could not. (2) Mapping of the ocean floor: post-war oceanographic expeditions revealed mid-oceanic ridges as the longest mountain chain on Earth, deep trenches near continental margins, and the surprisingly young age of all oceanic crust (no rock older than 200 million years) along with a thin sediment column. (3) Palaeomagnetic studies: rocks on either side of the ridge crests showed identical age and matching, mirror-symmetrical stripes of normal and reversed magnetic polarity. Together these discoveries led Hess (1961) to propose sea floor spreading and ultimately to the unifying theory of plate tectonics in 1967, which absorbed and transformed Wegener's original vision into modern earth science.
Project Work
📌 Prepare a collage related to damages caused by an earthquake.
Pointers for the Collage: Build the collage around four themes — (a) Ground effects: pictures of cracked roads, ground lurching, soil liquefaction, landslides triggered by quakes (e.g., Bhuj 2001, Sikkim 2011, Nepal 2015). (b) Built-environment damage: collapsed apartment buildings, cracked bridges, fallen flyovers, broken pipelines. (c) Secondary disasters: post-quake fires (e.g., 1906 San Francisco), tsunamis (e.g., Indian Ocean 2004, Japan 2011), dam-failure floods. (d) Human and social impact: loss of life and property, displacement of people, disruption of essential services. Add captions identifying the place, year and magnitude of each event. Finish with a short paragraph on earthquake-resilient design — flexible foundations, lightweight roofs and seismic codes — to link disaster damage with disaster mitigation. Cite news photographs and IMD/NDMA reports as your sources.
4.17 Chapter Summary
📍 Key Takeaways
- Continental Drift (Wegener, 1912): Pangaea + Panthalassa → split (~200 mya) into Laurasia + Gondwanaland → modern continents.
- Six evidences: jig-saw fit (Bullard 1964) · matching rocks (2,000 mya) · tillite · placer gold (Ghana–Brazil) · fossils (Mesosaurus, Lemur, Glossopteris) · palaeoclimate.
- Drift forces: pole-fleeing + tidal (now considered too weak).
- Post-drift studies: Holmes (1930s) — mantle convection from radioactive heat; ocean-floor mapping; palaeomagnetic stripes.
- Sea Floor Spreading (Hess, 1961): new crust at ridges, old crust consumed at trenches.
- Plate Tectonics (McKenzie, Parker, Morgan — 1967): 7 major plates + 6 minor plates moving on the asthenosphere.
- Three boundaries: divergent (Mid-Atlantic) · convergent (Andes, Himalayas) · transform (San Andreas).
- Rates: Arctic Ridge < 2.5 cm/yr · East Pacific Rise > 15 cm/yr.
- Indian plate: began moving ~200 mya · 50° S at 140 mya · Deccan Traps ~60 mya · Himalayan collision ~40 mya · still rising.
Key Terms — Quick Glossary
| Term | Meaning |
|---|---|
| Pangaea | Single super-continent ("all earth") of the past — Wegener, 1912 |
| Panthalassa | Single mega-ocean ("all water") that surrounded Pangaea |
| Laurasia | Northern half of Pangaea after the first split |
| Gondwanaland | Southern half of Pangaea after the first split |
| Tillite | Sedimentary rock formed from glacial deposits |
| Placer deposit | Concentration of dense minerals (e.g. gold) by water action |
| Sea floor spreading | Generation of new ocean crust at mid-oceanic ridges and consumption at trenches — Hess, 1961 |
| Plate tectonics | Theory (1967) that the lithosphere consists of rigid plates moving over the asthenosphere |
| Subduction zone | Convergent boundary where one plate sinks beneath another |
| Divergent boundary | Boundary where plates pull apart and new crust forms (e.g. Mid-Atlantic Ridge) |
| Convergent boundary | Boundary where plates collide and one is destroyed (e.g. Java Trench, Himalayas) |
| Transform fault | Boundary where plates slide horizontally past each other (e.g. San Andreas Fault) |
| Mid-oceanic ridge | Submarine mountain chain with central rift — longest mountain system on Earth |
| Palaeomagnetism | Study of the magnetic record locked in rocks at the time of their formation |
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