BPSC 69th CCE Prelims General Studies · Question 24 of 144

Correct — C, the theory of plate tectonics. Read the stem as a definition and it contains two clauses, each of which points only at plate tectonics. First, 'pieces of the Earth's crust' — plural, rigid fragments, not continents. Plate tectonics holds that the outer shell of the Earth is broken into a small number of rigid slabs; what actually moves is the lithosphere, meaning the crust plus the cool, rigid uppermost mantle welded to it, about 100 km thick, sliding over the hotter and more ductile asthenosphere below. Seven major plates carry the whole surface: the Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic. Second, 'driven by movement in the mantle' — this is the mechanism clause, and it is precisely what plate tectonics supplied and continental drift never had. Slow convection in the mantle, reinforced by ridge push at the spreading centres and slab pull at the subduction zones, keeps the plates in motion at speeds of a few centimetres a year, comparable to the growth of a fingernail. The theory was assembled quickly between 1965 and 1967 and was first called 'New Global Tectonics'. J. Tuzo Wilson added the transform fault in 1965 and completed the set of boundary types; at the 1967 meeting of the American Geophysical Union, W. Jason Morgan proposed that the surface consists of twelve rigid plates moving relative to each other, and Xavier Le Pichon published a complete quantitative model two months later. India is the theory's most spectacular working example: the Indian Plate broke away from Gondwana about 100 million years ago, drove north at up to 20 cm a year, and has been colliding with Eurasia since roughly 55 million years ago — the collision that raised the Himalaya and that is still going on today at about 5 cm a year.

• (a)the theory of continental drift — Alfred Wegener's 1912 hypothesis, and the closest wrong answer, because it also says the surface moves. It fails on both clauses of the stem. Wegener moved CONTINENTS, imagining lighter granitic landmasses ploughing through the solid basaltic ocean floor, whereas plate tectonics moves whole plates that carry ocean floor and continent together. More decisively, Wegener had no mantle mechanism: he invoked a pole-fleeing force and tidal forces that physicists showed were orders of magnitude too weak, and that missing driver is exactly why the geological establishment rejected him for half a century.
• (b)the theory of Pangaea — A category error rather than a wrong theory. Pangaea is not a theory at all — it is the name Wegener gave to the single supercontinent that he argued existed about 250 million years ago, surrounded by the world ocean Panthalassa, and which later split into Laurasia and Gondwana. It is a reconstructed object, a piece of evidence within continental drift, not an explanation of anything. The option is there to catch a candidate who recognises the word from the same chapter and stops reading.
• (d)the theory of plate boundaries — There is no theory of this name. Plate boundaries are a classification inside plate tectonics, not a rival theory: they come in three kinds — divergent, where new crust is made at mid-ocean ridges; convergent, where a plate is subducted into a trench; and transform, where two plates slide past each other, as along the San Andreas Fault. Naming a component of a theory as though it were the theory is a standard distractor device.

Plate tectonics is the unifying theory of the Earth sciences, and it was built by fusing two earlier ideas. Alfred Wegener's continental drift (1912) supplied the observation that the continents had once been joined: the jigsaw fit of South America and Africa, matching Precambrian rock belts across the Atlantic, identical Gondwana glacial tillites and coal sequences on six southern landmasses, and shared fossils such as Glossopteris, Mesosaurus and Lystrosaurus on continents now separated by oceans. Harry Hess and Robert Dietz's seafloor spreading (1961–62) supplied the missing mechanism by showing that the ocean floor itself is being manufactured at ridges and consumed at trenches, so continents need not plough through anything. Plate tectonics puts the two together: the rigid lithosphere is divided into plates that move over the ductile asthenosphere, driven by mantle convection with ridge push and slab pull, and essentially all of the world's earthquakes, volcanoes, young fold mountains and deep trenches are concentrated along their edges. Almost every major landform question in a state or UPSC paper — the Himalaya, the Andaman volcanoes, the Pacific Ring of Fire, the East African Rift — is ultimately an application of this one theory.

The safest route here is to treat the stem as a definition to be matched clause by clause rather than as a question to be recalled. 'Pieces of the Earth's crust', plural and rigid, is the language of plates, not of continents — that alone separates (c) from (a). 'Driven by movement in the mantle' seals it, because the single reason continental drift was rejected for fifty years is that it named no adequate driver; supplying a mantle-driven mechanism is the whole difference between the 1912 hypothesis and the 1967 theory. Option (b) can be discarded on grammar as much as on geology: Pangaea is a place, not an explanation. Option (d) names a part of the theory rather than the theory, a device examiners use often. Candidates go wrong here in one of two ways — by treating continental drift and plate tectonics as synonyms, which they are not, or by assuming that because Wegener came first he must be the answer to any question about moving landmasses. The useful mental filing system is chronological: Wegener 1912 gave the evidence, Hess and Dietz around 1961–62 gave the mechanism, and Wilson, Morgan and Le Pichon between 1965 and 1967 gave the theory its final form and its name.

• Alfred Wegener (1880–1930) first proposed continental drift in a lecture at the Senckenberg Museum, Frankfurt, on 6 January 1912, and set it out fully in Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans) in 1915. His supercontinent was Pangaea, surrounded by the single ocean Panthalassa.
• Wegener's hypothesis was rejected for half a century because his proposed drivers — the pole-fleeing force (Polflucht) and tidal forces — were far too weak to move continents through solid ocean floor. Palaeomagnetic work in the 1950s and seafloor spreading in the early 1960s supplied the mechanism he lacked.
• Plate tectonics was defined in a burst of papers between 1965 and 1967, initially under the name 'New Global Tectonics'. J. Tuzo Wilson added transform faults in 1965; W. Jason Morgan proposed twelve rigid plates at the American Geophysical Union meeting in 1967; Xavier Le Pichon published a complete model two months later.
• What moves is the lithosphere — the crust plus the rigid uppermost mantle, roughly 100 km thick — riding on the ductile asthenosphere. Seven major plates cover the surface: Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic, with several minor plates including the Arabian, Philippine, Nazca, Cocos and Caribbean.
• India is the textbook case and a live one. The Indian Plate separated from Gondwana about 100 million years ago, moved north at up to 20 cm a year, and began colliding with Eurasia about 55 million years ago to raise the Himalaya. It still moves north-east at about 5 cm a year, and that continuing convergence along the Main Himalayan Thrust produced the Bihar–Nepal earthquake of 15 January 1934 — magnitude 8.0, with Munger and Muzaffarpur destroyed and between 10,700 and 12,000 dead.

• Treating continental drift and plate tectonics as the same theory — drift moves continents and offers no mechanism, plate tectonics moves lithospheric plates and names mantle convection as the driver
• Choosing Pangaea, which is a reconstructed supercontinent and not a theory at all
• Falling for an option that names a component of a theory — plate boundaries — as though it were a competing theory in its own right

BPSC gave the definition and asked for the label, with the three wrong options drawn from the same NCERT chapter — a format that rewards precise reading of the stem's two clauses over recall. UPSC has never asked the bare name; it asks around it instead, pairing theories with the scientists who proposed them (2008, where continental drift was deliberately mis-paired with Edwin Hubble), listing the geological evidences that establish drift (2025), and asking which forces keep remodelling the Earth's surface (2013).

• practice — not a real PYQ

  Who among the following first put forward the hypothesis of continental drift?

  • (a)Harry Hess
  • (b)Alfred Wegener
  • (c)J. Tuzo Wilson
  • (d)Arthur Holmes

  Answer(b) Alfred Wegener — a German meteorologist and polar researcher who proposed the idea in 1912 and published The Origin of Continents and Oceans in 1915. Hess proposed seafloor spreading, Wilson added transform faults, and Holmes suggested mantle convection as a driving force.

• practice — not a real PYQ

  Which one of the following best explains why Alfred Wegener's theory of continental drift was rejected by most geologists of his time?

  • (a)The continents were later found not to fit together at all
  • (b)He could not suggest an adequate force capable of moving the continents
  • (c)No matching fossils were ever found across the southern continents
  • (d)He denied the existence of a supercontinent in the geological past

  Answer(b) He could not suggest an adequate force capable of moving the continents — the pole-fleeing and tidal forces he proposed were shown to be far too weak. The evidence in (a), (c) and (d) was in fact in his favour; only the mechanism was missing, and seafloor spreading later supplied it.