(a) What do you understand by continental drift ? Discuss various geological evidences in favour of continental drift. (20…

(a) Continental Drift and Geological Evidences

Alfred Wegener (1912) postulated the Continental Drift hypothesis, proposing that Earth's landmasses were once assembled into a single supercontinent, Pangea, surrounded by the super-ocean Panthalassa, before rifting and drifting apart during the Mesozoic. Multiple lines of empirical evidence corroborate this reconstruction:

Morphological Jigsaw-Fit: The complementary geometric match between continental margins across the Atlantic, most notably the Atlantic coastlines of South America and Africa, verified by computer fitting at about the 1,000-metre (500-fathom) isobath by Edward Bullard and colleagues (1965).

Stratigraphic and Structural Continuity: Pre-drift orogenic belts and cratonic boundaries terminate abruptly at continental edges and resume seamlessly across oceans. The Paleozoic Appalachian belt of North America aligns directly with the Caledonian fold belt of the British Isles and Scandinavia. Similarly, Archean cratons and mobile belts match precisely between Brazil and West Africa.

Palaeontological Correlations: Terrestrial and freshwater fossil taxa with restricted dispersal capabilities occur across widely separated Gondwanan continents. The tongue-fern Glossopteris flora, the early aquatic reptile Mesosaurus (found only in South Africa and eastern South America), and terrestrial reptiles Lystrosaurus and Cynognathus provide compelling proof of contiguous landmasses; Alexander du Toit (1937) assembled this Gondwana evidence, including India's Gondwana Supergroup.

Palaeoclimatic Markers: Permo-Carboniferous glacial deposits, such as the Talchir tillites of India, Dwyka tillites of South Africa, and contemporaneous diamictites in South America, Australia, and Antarctica, show radiating glacial striations from a common polar centre, impossible under current latitudinal configurations.

Palaeomagnetism: Apparent Polar Wander (APW) paths from different cratons diverge historically but converge into a single path when continents are restored to Wegenerian fits.

While Wegener's mechanism—invoking tidal friction and differential gravitational forces—proved physically inadequate, the hypothesis was later vindicated by seafloor spreading (Hess, 1962) and symmetric magnetic stripes (Vine–Matthews, 1963), and integrated into modern Plate Tectonics driven mainly by slab pull and ridge push with mantle convection.

(b) Geomorphic Diversity: Endogenic and Exogenic Controls

Landscape diversity is governed by the coupled interaction between endogenic processes, which generate crustal relief and structural templates, and exogenic processes, which denude and sculpt these structures under varying climatic regimes.

Endogenic controls involve volcanism, faulting, folding, and isostasy. In peninsular India, Cretaceous flood basalt volcanism formed the distinct step-like trappean topography of the Deccan Traps. In northeast India, intraplate compressive stresses drive the pop-up tectonic uplift of the Meghalaya Plateau along the Dauki fault to the south and the Oldham (Brahmaputra) fault to the north.

Exogenic controls—weathering, mass wasting, fluvial, aeolian, and glacial erosion—work to modify these endogenic templates. Climate dictates the dominant agent: in hyper-arid western Rajasthan, wind action sculpts the barchans and seif dunes of the Thar desert upon an otherwise stable craton. Conversely, along the passive rifted margin of the Western Ghats, heavy monsoonal precipitation drives intense chemical weathering, scarp retreat, and deep fluvial incising. Geomorphic diversity is thus the transient expression of tectonic uplift counterbalanced by climatically modulated surface denudation.

(c) Morphotectonic Features of Rejuvenation

Rejuvenation occurs when a geomorphic cycle is interrupted by a drop in base level, caused either by epeirogenic/tectonic uplift or glacio-eustasy, which restores potential energy to fluvial systems. In mountainous terrains, characteristic morphotectonic signatures include:

Knickpoints and Waterfalls: Abrupt gradient changes along river longitudinal profiles, indicating the upstream migration of an incision wave (e.g., escarpment waterfalls such as Jog Falls on the Sharavati along the Western Ghats scarp, and knickpoints where Himalayan rivers cross active thrusts).

Incised Meanders: Entrenched and ingrown meanders produced when low-gradient, mature meandering streams are subjected to rapid vertical incision without altering their planform geometry, as seen in entrenched meanders of Lesser Himalayan rivers such as the Kosi in Kumaon; the valley-in-valley (V-in-V) cross-profiles of Himalayan rivers record the same pulse of incision.

Fluvial Terraces: Step-like, paired and unpaired abandoned floodplains flanking valleys, documenting pulsed tectonic uplift in the Siwalik foothill belt and the intermontane Dun valleys.

Antecedent Drainage: Rivers that maintain their pre-existing courses by incising at a rate equal to or exceeding tectonic uplift, carving profound transverse gorges (e.g., Indus, Sutlej, and Brahmaputra across the rising Himalayan ranges).

Elevated Planation Surfaces and Fault-generated Forms: Relict peneplains uplifted to high elevations and undergoing fresh dissection, together with fresh fault scarps, triangular facets, wind gaps and deflected or beheaded streams along active faults such as the Himalayan Frontal Thrust.

Modern geomorphology recognises that these rejuvenation features represent a complex interplay between active tectonic orogeny and continuous isostatic rebound triggered by erosional unloading. Morphometric indices (stream-length gradient index, steepness index, hypsometric integral) from high-resolution DEMs, combined with cosmogenic nuclide and OSL dating, quantify of whether individual landforms stem primarily from deep-seated geodynamic drift, localized tectonic rejuvenation, or climatic oscillations.