(a) The concept of Plate tectonics has been derived from the isostasy and continental drift theory. Elaborate citing suitable…

(a) Plate tectonics from isostasy and continental drift

Plate tectonics combines the horizontal motion of continental drift with the vertical balance of isostasy.

Wegener (1912) argued for drift from the jigsaw fit of the Atlantic coasts, matching fossils (Glossopteris, Mesosaurus), Permo-Carboniferous glacial tillites and matching rock belts. He had no workable driving force, so the idea was rejected. Isostasy, set out by Airy (varying crustal roots) and Pratt (varying crustal density), showed that the light lithosphere floats on the denser, plastic asthenosphere. Holmes (1929) proposed mantle convection, and Hess (1962) added sea-floor spreading, backed by the magnetic striping found by Vine and Matthews. Together these produced the plate theory of McKenzie, Parker and Morgan (1967–68), in which rigid lithospheric plates move over the asthenosphere.

Drift gives the plates their horizontal motion; isostasy explains why light continental crust resists subduction while cold, dense oceanic crust sinks, so ocean basins open and close (Wilson cycle).

Examples: divergence at the Mid-Atlantic Ridge and the East African Rift; subduction at the Andes and Japan; collision of India and Eurasia raising the Himalaya, which rise isostatically; transform motion on the San Andreas Fault.

(b) Bottom topography of the Pacific Ocean

The Pacific is the largest and deepest ocean, with an average depth of about 4,280 m. Its floor is dominated by trenches and island arcs along its margins and by volcanic features in its interior.

Its continental shelves are narrow, except in marginal seas such as the Bering and East China Seas. Most of the world's deep trenches lie here: Aleutian, Kuril, Japan, Mariana, Tonga, Kermadec and Peru–Chile. The Mariana Trench holds Challenger Deep, about 10,900 m, the deepest point on Earth. Beyond them lie island arcs, the Ring of Fire.

The East Pacific Rise is a fast-spreading ridge, broad and gentle, with no deep central rift, unlike the Mid-Atlantic Ridge. Abyssal plains at 4,000–6,000 m cover most of the floor. Thousands of seamounts and guyots rise from it, and hotspot volcanism built the Hawaiian–Emperor chain. Large fracture zones (Mendocino, Clarion, Clipperton) cut the floor, and oceanic plateaus such as Ontong Java stand above it.

(c) Soil erosion and degradation as threats to food supply

Soil erosion is the removal of topsoil by water and wind. Soil degradation is the wider fall in soil quality: compaction, loss of organic matter, salinisation, acidification and contamination.

Topsoil holds most nutrients and organic matter. Its loss lowers fertility, water-holding capacity and root depth, so yields fall even when fertiliser use rises, and farmers must spend more for the same output. FAO estimates that about a third of the world's soils are degraded. India's land degradation atlas (ISRO, 2021) puts nearly 30 per cent of the country's area under degradation.

Examples in India: gully erosion in the Chambal ravines; waterlogging and salinity in canal commands of Punjab, Haryana and the Deccan black soils; leaching and acidity in lateritic soils; sheet erosion in the Shiwaliks.

Remedies include watershed development under PMKSY, contour bunding and terracing, shelterbelts and agroforestry, drainage with gypsum for saline soils, balanced fertiliser use with Soil Health Cards, and the pledge to restore 26 million ha by 2030 under UNCCD.