Geomorphology: plate tectonics, landforms & rock cycle
Plate tectonics, the rock cycle and major landforms for UPSC Geography: theory, mechanisms, Indian instances and exam-tuned high-yield facts.
From Continental Drift to Plate Tectonics
Alfred Wegener, in Die Entstehung der Kontinente und Ozeane (1912, expanded 1915), proposed continental drift: a single supercontinent, Pangaea, surrounded by the ocean Panthalassa, fragmented from the Mesozoic onward. He marshalled the jigsaw fit of the Atlantic coasts, matching Mesosaurus and Glossopteris fossils across South America, Africa, India, Antarctica and Australia, and identical Permo-Carboniferous glacial striations on these now-tropical landmasses. Wegener's fatal weakness was mechanism: his proposed driving forces (Polflucht and tidal forces) were dynamically inadequate, and the theory was rejected by most geophysicists until the 1950s.
The mechanism arrived through the sea-floor spreading hypothesis of Harry Hess (1962, 'History of Ocean Basins') and Robert Dietz. Hess argued that mantle convection upwells along mid-oceanic ridges, creating new oceanic crust that spreads laterally and is destroyed at trenches. The decisive proof was the Vine–Matthews–Morley hypothesis (1963): symmetrical magnetic-stripe anomalies on either side of the ridge record reversals of Earth's magnetic field, confirming the conveyor-belt of crust.
The Theory of Plate Tectonics
Synthesised c. 1967–68 (McKenzie, Parker, Morgan), plate tectonics holds that the lithosphere is divided into rigid plates—seven major (Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic) and several minor (Arabian, Nazca, Cocos, Caribbean, Philippine, Scotia, Juan de Fuca)—gliding over the plastic asthenosphere. Plate motion is driven chiefly by slab pull and ridge push, with mantle convection (Holmes, 1928–29) and plume activity as deeper engines.
Three boundary types define the tectonic regime:
- Divergent (constructive): plates separate; new crust forms. Examples: the Mid-Atlantic Ridge, the East African Rift, Iceland.
- Convergent (destructive): plates collide. Ocean–continent convergence yields subduction, volcanic arcs and trenches (Andes, Cascades); ocean–ocean yields island arcs (Japan, Aleutians); continent–continent yields fold mountains—the Himalaya, raised by the Indian plate colliding with the Eurasian plate from c. 50 million years ago, after the breakup of Gondwana drove India northward across the Tethys Sea.
- Transform (conservative): plates slide past each other; crust is neither created nor destroyed. The San Andreas Fault is the classic case.
The Pacific 'Ring of Fire', where the Pacific, Nazca, Cocos and Philippine plates subduct, concentrates ~75% of the world's active volcanoes and ~90% of earthquakes. India's own Deccan Traps—flood basalts erupted c. 66 million years ago over the Réunion hotspot—illustrate intra-plate plume volcanism, a favourite UPSC link to the Cretaceous–Palaeogene extinction debate.
