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If you had stood where the Himalaya now rises, around a hundred million years ago, you would have been looking at a sea. Fossils of marine creatures are found high in the mountains today. How an ocean became the world's highest mountain range is the story of plate tectonics — one of the most important ideas in physical geography, and one that UPSC tests in both Prelims and Mains.
From continental drift to plate tectonics
The theory developed in stages.
Continental drift (Alfred Wegener, 1912). Wegener proposed that the continents were once joined in a supercontinent, Pangaea, surrounded by an ocean, Panthalassa, and had since drifted apart. His evidence included:
- the jigsaw fit of the coastlines of South America and Africa;
- matching rock formations and fossils (such as Glossopteris and Mesosaurus) on continents now far apart;
- evidence of past glaciation in places that are now tropical.
His theory was not widely accepted, largely because he could not explain what force moved the continents.
Sea-floor spreading (1960s). Mapping of the ocean floor revealed mid-ocean ridges, where new crust forms from rising magma and spreads outwards. Magnetic striping on either side of the ridges, and the fact that ocean-floor rocks are much younger than continental rocks, supported the idea. Harry Hess is closely associated with the theory.
Plate tectonics. The two ideas combined into the modern theory: the Earth's rigid outer layer, the lithosphere, is broken into plates that move over the softer asthenosphere below, driven by forces connected with mantle convection, ridge push and slab pull.
Types of plate boundaries
| Boundary | Plates move | Features | Examples |
|---|---|---|---|
| Divergent | Apart | Mid-ocean ridges, rift valleys, new crust | Mid-Atlantic Ridge; East African Rift |
| Convergent: ocean–ocean | Towards each other | Deep trenches, volcanic island arcs | Mariana Trench; Japan |
| Convergent: ocean–continent | Towards each other | Oceanic plate subducts; fold mountains with volcanoes | Andes |
| Convergent: continent–continent | Towards each other | Collision; high fold mountains, little volcanism | Himalaya |
| Transform | Slide past each other | Earthquakes; no creation or destruction of crust | San Andreas Fault |
The journey of the Indian plate
- India was part of the southern supercontinent Gondwana, along with Africa, South America, Antarctica and Australia.
- As Gondwana broke up, the Indian plate separated and moved rapidly northward across the Tethys Sea, the ocean that lay between Gondwana and Eurasia.
- As it moved, the oceanic crust in front of it subducted beneath Eurasia.
- Around 50 million years ago, the Indian continental plate began to collide with the Eurasian plate. Continental crust is too buoyant to subduct easily, so the crust crumpled, thickened and rose.
- The sediments of the Tethys were folded and thrust upward — which is why marine fossils are found in the Himalaya.
The Indian plate is still moving north-eastward, at a rate of a few centimetres a year, and the Himalaya continues to rise and to be eroded.
The divisions of the Himalaya
From north to south:
| Range | Also called | Features |
|---|---|---|
| Greater Himalaya | Himadri | Highest; perennially snow-covered; peaks such as Everest, Kanchenjunga, Nanda Devi; granite core |
| Lesser Himalaya | Himachal | Middle ranges such as the Pir Panjal and Dhauladhar; hill stations; valleys such as Kashmir and Kullu |
| Outer Himalaya | Shiwaliks | Youngest and lowest; made of unconsolidated sediments; flat valleys called duns (e.g. Dehradun) |
North of the Greater Himalaya lies the Trans-Himalaya (Karakoram, Ladakh, Zaskar ranges).
These ranges are separated by major thrust faults — including the Main Central Thrust and the Main Boundary Thrust — along which rocks have been pushed southward over younger rocks. These fault zones are sources of earthquakes.
What the collision created beyond the mountains
- The Indo-Gangetic plain — the weight of the rising Himalaya caused the crust to its south to sag, forming a deep depression (a foredeep) that rivers filled with sediment, creating one of the world's most fertile plains.
- Earthquakes — the collision makes the Himalaya and north-east India among the most seismically active regions in the world. Most of this belt falls in India's highest seismic zones.
- The Deccan Traps — the vast basalt flows of the Deccan, formed around 66 million years ago as the Indian plate passed over a volcanic hotspot, are part of the same northward journey.
- Rivers older than the mountains — rivers such as the Indus, Sutlej and Brahmaputra kept cutting down as the land rose, forming deep gorges. They are called antecedent rivers.
Why this matters for Mains
Plate tectonics links to several GS topics:
- Disaster management — earthquake risk in the Himalaya and the north-east, building codes, and the vulnerability of Himalayan towns.
- Development in the Himalaya — the fragility of young mountains to roads, tunnels and hydropower projects, a recurring theme after landslides and land subsidence events.
- Resources — minerals and energy associated with tectonic history.
Practice questions
- Prelims-type: Which type of plate boundary produced the Himalaya? What are duns?
- Mains-type (150 words): Explain how the theory of plate tectonics accounts for the formation of the Himalaya.
- Mains-type (250 words): "The youth of the Himalaya is both its grandeur and its vulnerability." Discuss with reference to development and disasters.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Union Public Service Commission website .
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