Distribution of Oceans and Continents

Why this chapter matters for UPSC: This is Chapter 4 of Fundamentals of Physical Geography in the current rationalised NCERT (Reprint 2026-27). It traces how ideas about the positions of continents and oceans developed: Wegener's continental drift (1912), the post-war ocean-floor discoveries, Hess's sea-floor spreading (1961) and plate tectonics (1967). It ends with the movement of the Indian plate. Prelims asks for NCERT's evidence of drift and the plate names; Mains GS1 asks for the evidence, for the link between plate margins and the belts of fold mountains, earthquakes and volcanoes, and for mantle plumes.

Contemporary hook: The Himalaya continues to rise by more than 1 cm a year because the Indian and Eurasian plates are still converging (USGS, This Dynamic Earth). The same plate boundary, where the India plate subducts beneath the Burma micro-plate, produced the M 9.1 Sumatra–Andaman earthquake and Indian Ocean tsunami of 26 December 2004 (USGS).

🧠 First Principles — Read This First

The positions of land and sea on a world map are not permanent. They have changed through geological time and will keep changing. This chapter explains how geographers came to know this and what makes the land move.

Start with two layers. The lithosphere is the rigid outer shell: the crust plus the top part of the mantle. Under it lies the asthenosphere, hot and soft enough to flow very slowly. The lithosphere is broken into large slabs called plates, and the plates slide over the asthenosphere at a few centimetres a year. That is roughly the speed at which fingernails grow.

The idea arrived in two steps. In 1912 Alfred Wegener argued that all the continents were once joined in a single landmass, Pangaea, which later broke apart. He supported this with matching coastlines, rocks and fossils on opposite sides of the Atlantic. But he could not name a force strong enough to move continents, and most scholars rejected his explanation. After the Second World War, mapping of the ocean floor showed that new crust is created at mid-ocean ridges and destroyed at deep trenches. This is sea-floor spreading. It led in 1967 to plate tectonics: what moves is not a continent but the whole plate that carries it.

Plate edges are where plates pull apart, collide or slide past each other. That is why earthquakes, volcanoes, fold mountains and trenches are concentrated in long, narrow belts instead of being scattered.

NCERT's starting point is simple: continents cover 29 per cent of the earth's surface and the rest is under oceanic waters, and neither has always been where it is now.

PART 1 — Quick Reference

Table 1: Timeline of ideas (NCERT)

Source: NCERT FoPG (rationalised), Chapter 4, pp. 27–34.

Table 2: Continental drift theory (Wegener, 1912)

Table 3: Evidence for continental drift (NCERT)

Source: NCERT FoPG (rationalised), Chapter 4, pp. 27–28.

Table 4: The plates (NCERT list)

Source: NCERT FoPG (rationalised), Chapter 4, pp. 32–33. The pre-2023 edition also listed a sixth minor plate, the "Fuji plate" (north-east of Australia); the rationalised text dropped it.

Table 5: Plate boundaries

Sources: NCERT FoPG Chapter 4, p. 33; examples from USGS, This Dynamic Earth, "Understanding plate motions".

Table 6: Measured figures beyond NCERT

PART 2 — Concepts & Narrative

Continental drift and why it was rejected

Wegener's theory explained a pattern that map readers had noticed for centuries: the coastlines on either side of the Atlantic look as if they once fitted together. Table 2 sets out his main claims.

His evidence was strong (Table 3). His mechanism was not. He proposed a pole-fleeing force, linked to the bulge the Earth's rotation creates at the equator, and a tidal force from the Moon and Sun. He thought these would work if applied over many millions of years. Most scholars judged them totally inadequate, so the theory was set aside.

NCERT's later correction matters for answers. Wegener thought the continents moved through the ocean floor. Plate tectonics says the plates move, carrying both continents and ocean floor. NCERT also notes that Pangaea was not the original state: continents have been wandering through geological time, and Pangaea itself was formed when separate continental masses converged.

Post-drift studies

Most of Wegener's evidence came from the continents: fossils, plants and deposits like tillite. Two lines of work after the Second World War shifted attention to the oceans.

Convection currents (Holmes, 1930s). Arthur Holmes suggested that heat from radioactive elements creates temperature differences in the mantle and sets up convection currents across the whole mantle. This was an attempt to supply the force that Wegener lacked.

Mapping the ocean floor. Expeditions showed that the ocean floor is not a flat plain. It has submerged mountain ranges and deep trenches, the trenches mostly close to continental margins. What these surveys found about the ridges is listed under sea-floor spreading below.

NCERT divides the ocean floor into three parts:

• Continental margins: the transition from the shore to the deep-sea basin. They include the continental shelf, slope, rise and the deep-oceanic trenches. The trenches matter most for this chapter.
• Abyssal plains: extensive plains between the continental margins and the mid-oceanic ridges, where sediments carried beyond the margins settle.
• Mid-oceanic ridges: an interconnected chain of mountains, the longest mountain chain on the Earth's surface though under water. A central rift at the crest is a zone of intense volcanic activity.

Distribution of earthquakes and volcanoes. On NCERT's map (Figure 4.2) a line of earthquake dots runs down the central Atlantic, nearly parallel to the coasts, and continues into the Indian Ocean. South of the Indian subcontinent it splits: one branch goes into East Africa and the other joins a line from Myanmar to New Guinea. This line coincides with the mid-oceanic ridges, where earthquake foci are shallow. A second belt follows the Alpine-Himalayan system and the rim of the Pacific, where earthquakes are deep-seated. Volcanoes follow a similar pattern, and the Pacific rim is called the "rim of fire" (popularly the Ring of Fire) because of its many active volcanoes.

Sea-floor spreading (Hess, 1961)

NCERT lists five facts from ocean-floor mapping and palaeomagnetic study that Wegener did not have:

1. Volcanic eruptions are common all along the mid-oceanic ridges and bring huge amounts of lava to the surface.
2. Rocks equidistant on either side of a ridge crest match in age, chemical composition and magnetic properties. Rocks next to the ridge have normal polarity and are the youngest; age increases away from the crest.
3. Ocean-crust rocks are nowhere older than 200 million years. Some continental rocks are 3,200 million years old. (NCERT's rule describes the open oceans; a rare older survivor is the crust beneath the eastern Mediterranean, dated from its magnetic anomalies to about 340 million years ago: Granot, Nature Geoscience, 2016.)
4. Ocean-floor sediments are unexpectedly thin, and no sediment column is older than 200 million years.
5. Deep trenches have deep-seated earthquakes; mid-oceanic ridges have shallow ones (the pattern on the map above).

From these facts Hess proposed in 1961 that constant eruptions at the ridge crest split the oceanic crust and new lava wedges into it, pushing the older crust to either side. The ocean floor spreads. The younger age of the oceanic crust, and the fact that the spreading of one ocean does not cause another to shrink, made Hess think of the consumption of oceanic crust. He placed this at the oceanic trenches, where the pushed-away floor sinks and is destroyed.

Plate tectonics (1967)

In 1967 McKenzie and Parker, and separately Morgan, brought these ideas together as plate tectonics. NCERT defines a tectonic (lithospheric) plate as a massive, irregularly shaped slab of solid rock, generally made of both continental and oceanic lithosphere, that moves horizontally over the asthenosphere as a rigid unit. The lithosphere is 5–100 km thick under the oceans and about 200 km under the continents. A plate is called continental or oceanic depending on which part is larger: the Pacific plate is largely oceanic, the Eurasian plate largely continental. Young fold mountains, ridges, trenches and faults surround the major plates.

Scientists have used palaeomagnetic data to fix where each landmass lay in past geological periods (NCERT Figure 4.4 shows 540 million years of motion). The position of Peninsular India has been traced from rocks of the Nagpur area.

The three boundary types: what Table 5 does not show

Table 5 gives the definitions and standard examples. Four points need more explanation.

Iceland and the rifts. Iceland straddles the Mid-Atlantic Ridge and is splitting along the spreading centre between the North American and Eurasian plates. On land, USGS describes the Red Sea as formed by spreading that tore Arabia away from Africa, and the East African Rift as a possible new spreading centre (USGS, "Understanding plate motions").

Which plate sinks. At an ocean–continent boundary the denser oceanic plate always goes down. At an ocean–ocean boundary one oceanic plate sinks under the other; at the Marianas it is the fast-moving Pacific plate going under the slower Philippine plate (USGS). At a continent–continent boundary neither sinks: the crust buckles and is pushed up and sideways.

Where subduction magma comes from. USGS states that the source is debated: partial melting of the descending slab, of the overlying plate, or both. Write "partial melting at the subduction zone", not "the oceanic plate melts".

Why transform faults cut across ridges. NCERT explains that eruptions do not happen along the whole ridge crest at once, so different parts of a plate move away from the ridge axis by different amounts, and the Earth's rotation also affects the separated blocks. Most transform faults are on the ocean floor; the San Andreas fault is one of the few on land (USGS).

Earthquakes at plate boundaries

All three boundary types produce earthquakes. NCERT's contrast between shallow foci at ridges and deep-seated foci at trenches is given above; two further points:

• At transform faults earthquakes are generally shallow (USGS); the San Andreas fault is the land example.
• At subduction zones earthquakes occur from shallow to great depth, and the largest events happen here. The M 9.1 Sumatra–Andaman earthquake of 26 December 2004 was a megathrust earthquake on the interface where the India plate subducts beneath the Burma micro-plate (USGS). The magnitude 8.2 earthquake of 9 June 1994 beneath Bolivia, at a depth of about 631 km (USGS catalogue), shows how deep subduction-zone earthquakes reach.

Hotspots: volcanoes away from plate edges

Some volcanoes sit in the middle of plates. The Hawaiian Islands lie more than 3,200 km from the nearest plate boundary. J. Tuzo Wilson's hotspot theory (1963) explains them as the product of a long-lasting hot region beneath the plate. As the Pacific plate moves over it, a chain of volcanoes forms that gets older away from the hotspot: Kauai's oldest rocks are about 5.5 million years old, while the Big Island's are under 0.7 million years. USGS notes more than a hundred hotspots active in the past 10 million years, most under plate interiors and some near ridges, such as under Iceland. It also notes an ongoing scientific debate about whether hotspots are deep and fixed. A mantle plume is the rising column of hot mantle thought to feed a hotspot; NCERT does not use the term.

So Mauna Loa and Kilauea in Hawaii are hotspot (intraplate) volcanoes, not divergent-boundary volcanoes. Iceland is the classic case of volcanism on a divergent boundary.

Rates and forces of plate movement

The magnetic stripes parallel to mid-oceanic ridges give the rates. NCERT gives the range: the Arctic Ridge is slowest (less than 2.5 cm/yr) and the East Pacific Rise near Easter Island, about 3,400 km west of Chile, is fastest (more than 15 cm/yr). Today GPS and other space-geodesy methods measure plate motion directly, and their results agree with the long-term averages from the stripes (USGS).

On the driving force, NCERT says that the mobile rock beneath the plates moves in a circular manner: heated material rises, spreads, cools and sinks, forming convection cells. The heat comes from radioactive decay and residual heat. Holmes first considered this idea in the 1930s, and it influenced Hess. NCERT concludes that "the slow movement of hot, softened mantle that lies below the rigid plates is the driving force behind the plate movement".

Movement of the Indian plate

NCERT describes the Indian plate's boundaries as follows:

Source: NCERT FoPG (rationalised), Chapter 4, p. 34.

India's journey, in NCERT's dates:

• India was a large island off the Australian coast. The Tethys Sea separated it from Asia until about 225 million years ago.
• India began moving north about 200 million years ago, when Pangaea broke up.
• About 140 million years ago the subcontinent lay as far south as 50°S.
• The outpouring of lava that formed the Deccan Traps started around 60 million years ago and continued for a long time. The subcontinent was then still close to the equator.
• India collided with Asia about 40–50 million years ago, causing rapid uplift. Himalayan formation took place from 40 million years ago onward, and the process continues.

Two modern figures differ from NCERT's rounding. Uranium-lead dating of the Deccan lavas places their main eruptive pulses around the Cretaceous–Paleogene boundary, about 66 million years ago, with one pulse starting tens of thousands of years before the mass extinction (Schoene et al., Science, 2019). The eruptions are widely linked to the head of the mantle plume whose tail now feeds Réunion Island. Earlier geochemical links between the two were ambiguous; a 2017 isotope study (Peters and Day) found the same Sr-Nd-Os signature in the Réunion and Deccan primary magmas. Some geologists still dispute the plume model. USGS gives about 50 million years for the start of the collision on one page and 40–50 million years on another; NCERT's 40–50 Ma covers both. In the exam, use NCERT's figures and add the modern ones only as a note.

The collision also explains marine fossils in the Himalaya. Sediments laid down in the Tethys Sea were folded and lifted as India pushed into Asia; USGS describes fossil-bearing Tethys sandstones north of Lhasa that show Tibet was closer to the equator about 105 million years ago.

PART 3 — UPSC Integration

Frames that score.

1. Evidence, then mechanism, then modern theory. For drift questions: Wegener's evidence (jig-saw fit, rocks, tillite, placers, fossils), the weakness (inadequate forces), then the post-drift discoveries that revived the idea and the move from drifting continents to moving plates.
2. Boundary type predicts landform. Divergent gives ridges and rifts; ocean–continent gives a trench and a volcanic range; ocean–ocean gives a trench and an island arc; continent–continent gives fold mountains without volcanoes; transform gives shallow-focus earthquakes. This one table answers questions on fold mountains at continental margins and on the Circum-Pacific belt.
3. India as a case study. The Indian plate's boundaries and journey connect the chapter to the Himalaya, the Deccan Traps and the subduction zone along the Andaman–Sumatra arc.

Exam Strategy

Prelims fact-traps (from NCERT's text and exercises):

• The first person to suggest the continents were once joined was Abraham Ortelius (1596), not Wegener. Wegener (1912) made it a full theory.
• Pole-fleeing force relates to the rotation of the Earth, not its revolution.
• Antarctica is a major plate, not a minor one. NCERT's minor plates are Cocos, Nazca, Arabian, Philippine and Caroline.
• The fact not used for sea-floor spreading is the distribution of fossils; that was drift evidence.
• The Indian plate's boundary along the Himalaya is continent–continent convergence.
• Mesosaurus lived in shallow brackish water (NCERT), not the open sea; its two localities are 4,800 km apart.
• Hess proposed sea-floor spreading in 1961; plate tectonics dates from 1967 (McKenzie and Parker; Morgan).
• Ocean crust is nowhere older than 200 million years; some continental rocks are 3,200 million years old.
• Earthquakes under mid-oceanic ridges are shallow; under trenches they are deep-seated.
• Hawaii is a hotspot, in the middle of the Pacific plate, not on a divergent boundary.

Mains question patterns:

• "Discuss the evidences" questions want NCERT's five evidence types with the specific example for each (2,000-million-year rock belts, Ghana gold without source rock, Gondwana tillite in six landmasses, Mesosaurus).
• "Geophysical characteristics" of a zone want boundary type, earthquake depth pattern, volcano type, trench and island arc, with named examples.
• Mantle-plume questions want hotspot volcanism within plates (Hawaii), plume links to flood basalts (Deccan–Réunion, with the caveat that it is debated) and the plume's place among plate-driving forces.

Practice Questions

Practice (UPSC-pattern, not past papers). Questions 1 to 5 are NCERT's own exercise MCQs.

Prelims:

1. Who amongst the following was the first to consider the possibility of Europe, Africa and America having been located side by side?
   (a) Alfred Wegener
   (b) Antonio Pellegrini
   (c) Abraham Ortelius
   (d) Edmond Hess

2. Polar fleeing force relates to:
   (a) Revolution of the earth
   (b) Gravitation
   (c) Rotation of the earth
   (d) Tides

3. Which one of the following is not a minor plate?
   (a) Nazca
   (b) Arabia
   (c) Philippines
   (d) Antarctica

4. Which one of the following facts was not considered by those discussing the concept of sea floor spreading?
   (a) Volcanic activity along the mid-oceanic ridges
   (b) Stripes of normal and reverse magnetic field observed in rocks of the ocean floor
   (c) Distribution of fossils in different continents
   (d) Age of rocks from the ocean floor

5. Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains?
   (a) Ocean-continent convergence
   (b) Divergent boundary
   (c) Transform boundary
   (d) Continent-continent convergence

6. Consider the following statements, which NCERT offers as evidence of continental drift: 1. Belts of ancient rocks on the Brazil coast match those of western Africa. 2. The gold of Ghana's coast came from veins in the Brazil plateau. 3. The magnetic stripes on either side of the mid-oceanic ridges match. Which of these are evidence for drift (rather than for sea-floor spreading)?
   (a) 1 and 2 only
   (b) 2 and 3 only
   (c) 1 and 3 only
   (d) 1, 2 and 3
   The magnetic stripes are sea-floor spreading evidence, found after Wegener.

7. Arrange in chronological order: 1. Sea-floor spreading proposed by Hess 2. Continental drift theory of Wegener 3. Plate tectonics 4. Bullard's computer fit of the Atlantic margins
   (a) 2-1-3-4
   (b) 2-1-4-3
   (c) 1-2-4-3
   (d) 2-4-1-3
   1912, 1961, 1964, 1967.

Mains (practice, NCERT Ex. 3, 150 words each):

1. What are the evidences in support of the continental drift theory? Approach: the five evidence types of Table 3, each with NCERT's example.
2. Bring out the basic difference between the drift theory and plate tectonics. Approach: what moves (continents vs plates of lithosphere), the force (pole-fleeing and tidal vs mantle convection and slab pull), and the status of Pangaea (original state vs one stage among many).
3. What were the major post-drift discoveries that rejuvenated interest in the distribution of oceans and continents? Approach: Holmes's convection currents, ocean-floor mapping (ridges, trenches, abyssal plains), the distribution of earthquakes and volcanoes, palaeomagnetism and the young ocean floor.

Short answers (NCERT Ex. 2, about 30 words): Wegener's forces were the pole-fleeing force and the tidal force; mantle convection is started and kept going by heat from radioactive decay and residual heat; at transform boundaries crust is neither created nor destroyed, while it is created at divergent and destroyed at convergent boundaries; when the Deccan Traps formed, the subcontinent was still close to the equator.

📦 Revision Capsule

Sources

• NCERT, Fundamentals of Physical Geography, Class XI, ch 4 "Distribution of Oceans and Continents", Reprint 2026-27: kegy204.pdf; the 2019-20 edition (Fuji plate) via the Wayback copy of the book: kegy2dd.zip, 4 May 2019.
• USGS, This Dynamic Earth: The Story of Plate Tectonics (1996, online edition): "Understanding plate motions", https://pubs.usgs.gov/gip/dynamic/understanding.html ; "Some unanswered questions", https://pubs.usgs.gov/gip/dynamic/unanswered.html ; "Hotspots", https://pubs.usgs.gov/gip/dynamic/hotspots.html ; "The Himalayas", https://pubs.usgs.gov/gip/dynamic/himalaya.html
• NOAA National Ocean Service, "How deep is the ocean?", https://oceanservice.noaa.gov/facts/oceandepth.html (accessed 1 October 2026)
• NOAA Ocean Exploration, "What is the Ring of Fire?" (published 21 Oct 2013, last updated 17 Dec 2020), https://oceanexplorer.noaa.gov/ocean-fact/rof/
• USGS Pacific Coastal and Marine Science Center, "Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake", https://www.usgs.gov/centers/pcmsc/science/tsunami-generation-2004-m91-sumatra-andaman-earthquake
• USGS, earthquake catalogue record for 9 June 1994, Bolivia (M 8.2): earthquake.usgs.gov query.
• R. Granot, "Palaeozoic oceanic crust preserved beneath the eastern Mediterranean", Nature Geoscience 9 (2016): 701–705: nature.com.
• P. Bird, "An updated digital model of plate boundaries", Geochemistry, Geophysics, Geosystems 4(3) (2003): 1027: author's copy.
• "A taphonomic model for the Mesosauridae assemblage of the Irati Formation (Paraná Basin, Brazil)", Geologica Acta: redalyc.org.
• Bureau of Indian Standards, "Know your standards" record for IS 1893 (Part 1):2025, listing the standard as Withdrawn (accessed 1 Oct 2026): BIS.
• The Hindu, "Centre withdraws revised earthquake zoning after backlash from ministries, infrastructure agencies", 7 March 2026: thehindu.com.
• Schoene, B. et al. (2019), "U-Pb constraints on pulsed eruption of the Deccan Traps across the end-Cretaceous mass extinction", Science 363(6429): 862–866, doi:10.1126/science.aau2422, doi.org. Author copy: https://oar.princeton.edu/bitstream/88435/pr1ms3k163/1/U-Pb_constraints_pulsed_eruption_Deccan_Traps_end-Cretaceous_mass_extinction.pdf
• B.J. Peters and J.M.D. Day, "A geochemical link between plume head and tail volcanism", Geochemical Perspectives Letters 5 (2017), 29-34, doi:10.7185/geochemlet.1742, https://www.geochemicalperspectivesletters.org/article1742/ (Deccan–Réunion plume link)