(a) Discuss in detail the notion of 'continental drift' and the theories of plate tectonics as they relate to palaeogeography…

The three topics link structural geology and remote sensing: drift and plate tectonics explain large-scale palaeogeography, aerial photography maps its surface expressions, and linear structures record deformation.

Continental drift and plate tectonics. Wegener’s continental drift proposed that continents once formed Pangaea and have drifted apart. Evidence included the jigsaw fit of South America and Africa, fossil correlations such as Glossopteris and Mesosaurus across Gondwana, and palaeoclimatic indicators like glacial striations and coal deposits in now tropical regions. The idea was initially rejected because no mechanism was accepted. Plate tectonics supplied mechanisms: mantle convection, slab pull, ridge push, and transform motion. Seafloor spreading, supported by young oceanic crust and symmetrical magnetic stripes, linked drift to plate tectonics. Divergent boundaries create oceanic crust at mid-ocean ridges; convergent boundaries consume it at trenches, producing subduction, collision and orogeny; transform boundaries accommodate lateral slip. This makes plate tectonics a dynamic framework: it explains not only present boundaries but also past reconstructions, rates of drift, and the timing of mountain building. Indian examples include the Himalayan collision zone where the Indian plate has converged with Eurasia, the Andaman-Nicobar subduction zone, and the diffuse Indo-Australian plate boundary, a broad zone of deformation rather than a simple transform. Palaeomagnetic data, including Apparent Polar Wander Paths from Deccan Trap basalts, support India’s northward movement and Gondwana reconstruction. The Wilson cycle and supercontinent cycles—Rodinia, Gondwana, Pangea—show repeated assembly and breakup. Palaeogeography thus uses fit, fossils, climate, magnetics and plate kinematics to restore past positions, ocean basins and collision timing.

Aerial photography. Aerial photography records the Earth’s surface from aircraft; related remote-sensing images use other platforms. Its principles include stereoscopic vision, which gives three-dimensional perception from overlapping images; parallax, the apparent displacement used to estimate heights and relief; scale, the ratio of image distance to ground distance; resolution, the ability to distinguish adjacent features; and overlap, usually 60 per cent longitudinal and 20 to 30 per cent lateral, needed for stereoscopy and coverage. Classification is by platform: terrestrial, aerial and space; by tilt: vertical and oblique; by film or sensor: panchromatic, infrared and false-colour composite; and by scale: large, medium and small. These classifications help select the right image for structural mapping, lineament detection and thematic interpretation. In India, GSI aerial surveys have been used for geological mapping and mineral targeting, while Landsat and IRS satellite imagery have helped map lineaments in the Dharwar craton. Bhuvan integrates such datasets for geological and remote-sensing analysis.

Linear structures. Linear structures are elongated features in rocks. Primary lineations include bedding and flow cleavage, formed during deposition or magmatic flow; they may be inherited and later deformed. Secondary lineations are deformational: fold axes, intersection lineations, mineral lineations, mullions and boudin axes. They are important strain markers and kinematic indicators: their orientation, plunge and associated asymmetry help determine the direction of shortening, extension and transport. A simple sketch would show a folded bed with a fold axis, a planar foliation cut by a mineral lineation, and boudinaged layers with long axes parallel to extension. In maps, lineaments may be mapped from aerial or satellite images, then verified in the field. In India, E-W trending lineaments in the Singhbhum shear zone mark the shear-zone trend, and associated E-W stretching lineations would indicate E-W transport; N-S trending lineations in the Aravalli fold belt reflect fold axes and shortening directions; and mineral lineations in the Kerala khondalite belt indicate transport direction during ductile shear. Together, these structures help reconstruct palaeostresses and plate-boundary evolution.

Thus, integrating palaeogeographic reconstructions, remote-sensing classification and structural lineations provides a way forward for mapping crustal evolution and guiding mineral exploration.