The theory that states “pieces of the Earth’s crust are in constant, slow motion driven by movement in the mantle” is called

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2023 Q24 — Geography PYQ with Answer

State PCS 2023 Q24 — Geography PYQ with Answer