GS Paper 1 15 marks · 250w 14 min Medium
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.
Subtopic: Geography · plate tectonics and ocean basins
How to structure your answer
Introduction (plates and driving forces) → Divergence and seafloor spreading enlarge basins → Convergence: subduction shrinks basins; arcs and collision reshape continents → Terrane accretion and transform shear → Wilson cycle and supercontinent cycle → Conclusion
Written within the word limit
279 words · target 250 words · 14 min
The lithosphere is broken into rigid plates riding on the asthenosphere; their relative motions — driven by mantle convection, slab pull and ridge push — continuously reshape continents and ocean basins.
Processes That Reshape Basins and Continents
- Divergence: continental rifting thins and splits landmasses — the East African Rift is a continent beginning to break, the Red Sea a young ocean, and the Atlantic, opening along the Mid-Atlantic Ridge, widens a few centimetres yearly, enlarging its basin.
- Seafloor spreading: new oceanic crust forms at mid-ocean ridges and moves symmetrically outward — recorded in mirror-image magnetic stripes — so basins grow wherever spreading outpaces destruction.
- Convergence: at subduction zones old ocean floor is consumed in trenches, which is why the Pacific is shrinking even as the Atlantic grows; volcanic arcs (Andes, Japan) weld new material onto continental edges, altering their shape.
- Continental collision: when an intervening ocean closes, continents suture — the Tethys closed as India met Eurasia, raising the Himalaya and enlarging Asia; ancient sutures such as the Urals record older closures.
- Terrane accretion and transform shear: microcontinents and island arcs plaster onto margins (western North America), while transform faults like the San Andreas slice and offset coastlines.
The Long Cycle
- The Wilson cycle — rifting, ocean opening, subduction, closure, collision — recycles ocean basins over 200–500 million-year rhythms; no ocean floor is older than about 200 million years, while continental cores are billions of years old.
- Supercontinents assemble and disperse (Rodinia, Pangaea); Pangaea's breakup over the last 200 million years produced today's configuration, and geologists project a future supercontinent.
Continents and ocean basins are thus temporary arrangements on a slow planetary conveyor: basins are born at ridges, die in trenches, and continents are the accumulating, re-sculpted residue of the tectonic cycle.
What an examiner expects to see
- Driving forces: mantle convection with slab pull (dominant) and ridge push moving lithospheric plates.
- Divergent margins create and widen basins — the East African Rift → Red Sea → Atlantic sequence shows an ocean's life stages.
- Symmetric magnetic striping and sediment ages about ridges prove seafloor spreading (Hess; Vine–Matthews).
- Subduction consumes ocean floor: the Pacific shrinks, deep trenches form, and volcanic arcs accrete to continental margins.
- Collision closes oceans and enlarges continents: Tethys closure raised the Himalaya; the Urals mark an older suture.
- Wilson cycle recycles basins — oceanic crust is nowhere older than about 200 million years, unlike Archean continental shields.
- Supercontinent cycle: Pangaea's breakup since about 200 million years ago produced the present continent–ocean layout.
Concrete cases, schemes and judgments
- Red Sea — a nascent ocean opening between the Arabian and African plates
- Himalaya — product of Tethys closure after the India–Eurasia collision (~50 million years ago)
- Mariana Trench — subduction destroying old Pacific floor
- Vine–Matthews magnetic anomalies across the Mid-Atlantic Ridge
- San Andreas transform fault offsetting the Californian margin
Terminology to weave into the answer
plate tectonicsseafloor spreadingsubductionWilson cycleterrane accretionsupercontinent cycle