Plate Tectonics: Continental Drift, Sea-Floor Spreading, Plate Boundaries, Driving Forces and the Indian Plate (UPSC Notes + PYQs)

Continental drift: Wegener’s idea

In 1912 German meteorologist Alfred Wegener argued that all continents were once joined in a supercontinent, Pangaea, surrounded by one ocean, Panthalassa. Pangaea split into Laurasia (north) and Gondwanaland (south), which broke into today’s continents. His evidence: the jigsaw fit of South America and Africa; matching rocks and mountain belts across the Atlantic; fossils such as Mesosaurus (a freshwater reptile found only in Brazil and southern Africa) and the plant Glossopteris across the southern continents and India; tillite (glacial deposits) in India, Africa and Australia; and placer gold on the Ghana coast without a local source rock. Wegener could not explain what moved the continents (he suggested polar-fleeing force and tidal force, both too weak), so most geologists rejected the idea.

Sea-floor spreading

Post-war ocean mapping found long mid-ocean ridges and deep trenches. In the early 1960s Harry Hess proposed that magma rises at ridges, forms new crust and pushes the sea floor apart; old crust sinks back at trenches. Proof came from palaeomagnetism: rocks on either side of ridges show matching stripes of normal and reversed magnetism (Vine and Matthews, 1963). Ocean crust is youngest at ridges and nowhere older than about 200 million years, while some continental rocks are about 4 billion years old. Sediment thickness also increases away from ridges.

Plate tectonics: the synthesis

By 1967-68, McKenzie, Parker and Morgan combined these ideas: the lithosphere (crust plus upper mantle, about 100 km thick) is divided into plates. Major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian (often split into Indian and Australian) and Antarctic. Minor plates include Nazca, Cocos, Philippine, Arabian, Caribbean, Juan de Fuca and Scotia.

Types of plate boundaries

What drives the plates

Arthur Holmes suggested in the 1930s that heat from radioactive decay sets up convection currents in the mantle. Today slab pull (the weight of a cold, sinking plate at a subduction zone) is thought to be the strongest force, with ridge push (gravity sliding plates off the raised ridge) adding to it. Plates with long subducting edges, like the Pacific, move fastest.

The Indian plate

India broke from Gondwana roughly 140-120 million years ago, separated from Madagascar around 90 million years ago and raced north at about 15-20 cm a year, among the fastest known plate speeds. It passed over the Réunion hotspot, which erupted the Deccan Traps about 66 million years ago. Collision with Eurasia began around 50 million years ago, closing the Tethys Sea and raising the Himalaya and the Tibetan Plateau. The plate still moves north-north-east at about 4-5 cm a year, which is why the Himalaya keeps rising and the region has frequent earthquakes. On its eastern edge, subduction under the Burma plate created the Andaman arc and the 2004 Sumatra-Andaman earthquake (magnitude 9.1) and tsunami. The Peninsular block is old and stable, though not quake-free (Latur 1993, Koyna 1967).

Significance

• Explains the global pattern of earthquakes and volcanoes, such as the Pacific Ring of Fire.
• Helps locate minerals and hydrocarbons, which collect along old plate margins and basins.
• Guides seismic zoning in India (zones II to V; Zone V covers the North-East, J&K, Himachal, Uttarakhand, Rann of Kutch and Andaman-Nicobar).
• Shapes long-term climate through mountains and ocean gateways.

Hotspots and island chains

Not all volcanoes sit at plate boundaries. A hotspot is a plume of hot mantle rising under a plate. As the plate moves over it, a chain of volcanoes forms, oldest farthest from the plume. The Hawaiian-Emperor chain is the classic example; a bend in it records a change in the Pacific plate’s direction about 47 million years ago. Iceland lies where a hotspot meets the Mid-Atlantic Ridge. In India, the Deccan Traps, vast lava flows erupted about 66 million years ago, are linked to the Réunion hotspot, which the Indian plate passed over as it moved north; the Lakshadweep-Chagos ridge marks its track.

Earthquakes and volcanoes along the boundaries

About 80% of the world’s earthquakes and most active volcanoes lie on the Pacific Ring of Fire, where ocean plates sink under others at subduction zones. The 2004 Sumatra-Andaman earthquake, of magnitude 9.1, happened where the Indian plate dives beneath the Burma microplate, and its tsunami killed over two lakh people across the Indian Ocean. India’s only active volcano, Barren Island in the Andamans, sits on the same subduction arc. The Himalaya, where the Indian plate pushes into Eurasia, is why most of north India lies in seismic zones IV and V, and why the region faces “great” earthquakes such as Bihar-Nepal (1934) and Nepal (2015).

PYQ trend

Prelims asks the forces behind surface changes (2013), the Peninsular block (2026) and parallel ranges (2011). Mains GS-I 2025 asked how tectonic movements change the shape of continents and ocean basins. Draw a simple boundary diagram in Mains answers.

Plate Tectonics: UPSC angle

Plate Tectonics UPSC PYQs (Prelims)

Questions UPSC has actually asked, from the official papers. Pick an option to check yourself.

Mains PYQs on this topic

Questions from the official UPSC Mains papers (2023-2026 in the Battle IAS Mains bank), newest first. Each links to a model answer.

1. UPSC Mains 2025 · GS Paper I · Q16 · 15 marks

   Discuss how the changes in shape and sizes of continents and ocean basins of the planet take place due to tectonic movements of the crustal masses.

   Model answer and approach

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