Distribution of Oceans and Continents, Class 11 Geography

What Distribution of Oceans and Continents is about

Weightage: This chapter is part of Unit II, The Earth, of Fundamentals of Physical Geography, which carries 9 of the 70 theory marks in the school annual exam together with The Origin and Evolution of the Earth and Interior of the Earth. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Continental drift theory

NCERT section Continental Drift

Background: the symmetry of the Atlantic coastlines suggested that the two Americas, Europe and Africa were once joined.

• Abraham Ortelius (Dutch map maker) first proposed this in 1596.
• Antonio Pellegrini drew a map with the three continents together.
• Alfred Wegener, a German meteorologist, put forward the comprehensive continental drift theory in 1912.

Wegener's idea:

• All continents formed one mass, Pangaea ('all earth'), surrounded by a mega-ocean, Panthalassa ('all water').
• About 200 million years ago Pangaea began to split, first into Laurasia (northern) and Gondwanaland (southern), which later broke into today's continents.

Force for drifting (Wegener):

• Pole-fleeing force: related to the earth's rotation; the earth bulges at the equator because it rotates.
• Tidal force: due to the attraction of the moon and sun that creates ocean tides.
• Wegener thought these would work over many millions of years, but most scholars found them totally inadequate: the weak link of his theory.

Worked example

Why was Wegener's theory not accepted in his time, even though his evidence was strong? (2 marks)

Try it first, then show the solutionSolution

1. His evidence (fit of continents, fossils, tillite, rocks) showed that drift had happened.
2. But his proposed forces, pole-fleeing and tidal, were considered totally inadequate to move continents, so the mechanism was rejected.

Answer: Because the forces he proposed were too weak to explain the movement.

Evidence in support of continental drift

NCERT section Evidence in Support of the Continental Drift

1. Jig-saw fit: the facing shorelines of Africa and South America match remarkably. Bullard (1964) used a computer programme to find the best fit of the Atlantic margins, matched at the 1,000-fathom line (not the present shoreline); it was nearly perfect.
2. Rocks of the same age across oceans: radiometric dating shows a belt of ancient rocks (2,000 million years) on the Brazil coast matching rocks of western Africa. The earliest marine deposits on both coasts are Jurassic, so the ocean did not exist before then.
3. Tillite: sedimentary rock formed from glacial deposits. India's Gondwana system of sediments has counterparts in six southern landmasses (Africa, Falkland Island, Madagascar, Antarctica, Australia); thick tillite at its base shows extensive, prolonged glaciation. Similar histories prove these lands were together, and tillite is clear evidence of palaeoclimates and drift.
4. Placer deposits: rich placer gold on the Ghana coast but no source rock there; the gold-bearing veins are in Brazil, so the gold came from the Brazil plateau when the two continents lay side by side.
5. Distribution of fossils: identical land or freshwater species on either side of seas. Lemurs in India, Madagascar and Africa led some to imagine a linking landmass, 'Lemuria'. Mesosaurus, a small reptile of shallow brackish water, is found only in the Southern Cape province of South Africa and the Iraver formations of Brazil, now 4,800 km apart across an ocean.

Worked example

How do the gold deposits of Ghana support continental drift? (2 marks)

Try it first, then show the solutionSolution

1. Ghana's coast has rich placer gold, but no gold-bearing source rock exists in that region.
2. The gold-bearing veins are in Brazil; the gold must have come from the Brazil plateau when Africa and South America were joined.

Answer: The gold's source rock is on another continent, so the two must once have been side by side.

Post-drift studies: convection currents and ocean floor mapping

NCERT section Force for Drifting; Mapping of the Ocean Floor

Most drift evidence came from continents (flora, fauna, tillite). After World War II, new discoveries, especially from the ocean floor, added new dimensions.

Convection current theory (Arthur Holmes, 1930s):

• Radioactive elements cause thermal differences in the mantle, generating convection currents.
• Holmes argued a system of such currents exists in the entire mantle.
• It was an attempt to answer the 'force' problem that had made scientists reject drift.

Mapping of the ocean floor:

• The floor is not a vast plain but full of relief: submerged mountain ranges and deep trenches, mostly near continental margins.
• Mid-oceanic ridges are the most volcanically active.
• Oceanic crust rocks are much younger than continental ones.
• Rocks equidistant from a ridge crest on either side are remarkably similar in constituents and age.

Ocean floor divisions (by depth and relief):

• Continental margins: continental shelf, slope, rise and deep-oceanic trenches (trenches matter most for this chapter).
• Deep-sea basins with abyssal plains between margins and ridges, where continental sediments carried beyond the margins are deposited.
• Mid-oceanic ridges: an interconnected chain, the longest mountain chain on earth though submerged; a central rift at the crest (intense volcanic activity), a fractionated plateau and flank zone.

Worked example

What problem in Wegener's theory did Arthur Holmes try to solve, and how? (2 marks)

Try it first, then show the solutionSolution

1. Wegener's theory lacked an adequate force to move continents.
2. Holmes suggested convection currents in the whole mantle, generated by thermal differences due to radioactive elements.

Answer: The force problem; he proposed radioactivity-driven convection currents in the mantle.

Distribution of earthquakes and volcanoes

NCERT section Mapping of the Ocean Floor

On the world map of seismic activity and volcanoes (Figure 4.2):

• A line of dots runs through the central Atlantic, almost parallel to the coasts, and extends into the Indian Ocean. A little south of the Indian subcontinent it bifurcates: one branch goes into East Africa, the other meets a similar line from Myanmar to New Guinea. This line coincides with the mid-oceanic ridges.
• A second, shaded belt of concentration follows the Alpine-Himalayan system and the edge (rim) of the Pacific.

Depth of foci:

• Mid-oceanic ridges: shallow earthquake foci.
• Alpine-Himalayan belt and Pacific rim: deep-seated earthquakes.

Volcanoes show a similar pattern. The Pacific rim is called the 'rim of fire' (Ring of Fire) because of its active volcanoes.

Worked example

Earthquakes at mid-ocean ridges are shallow, while those around the Pacific rim are deep. Suggest a reason using sea floor spreading. (2 marks)

Try it first, then show the solutionSolution

1. At ridges, plates pull apart and new crust forms near the surface, so energy is released at shallow depth.
2. At the Pacific rim, ocean floor sinks into trenches and is consumed; the sinking slab carries earthquake foci deep down.

Answer: Ridges: shallow crust formation; Pacific rim: deep subduction at trenches.

Sea floor spreading

NCERT section Mapping of the Ocean Floor

Facts revealed by ocean floor mapping and palaeomagnetic studies:
1. Volcanic eruptions are common all along mid-oceanic ridges, bringing huge amounts of lava to the surface.
2. Rocks equidistant on either side of the crest are similar in period of formation, chemical composition and magnetic properties. Rocks closest to the ridge have normal polarity and are the youngest; age increases away from the crest.
3. Oceanic crust is much younger than continental rock: nowhere older than 200 million years, whereas some continental rocks are 3,200 million years old.
4. Ocean floor sediments are unexpectedly thin; no sediment column older than 200 million years.
5. Deep trenches have deep-seated earthquakes; ridges have shallow foci.

Hess's hypothesis (1961), sea floor spreading:

• Constant eruptions at the ridge crest rupture the oceanic crust; new lava wedges in and pushes the crust on either side apart: the ocean floor spreads.
• Since oceanic crust is young and the spreading of one ocean does not shrink another, Hess reasoned that oceanic crust must be consumed.
• The pushed ocean floor sinks at oceanic trenches and is consumed (Figure 4.3).

Worked example

Why did Hess conclude that the ocean floor must be consumed somewhere? (2 marks)

Try it first, then show the solutionSolution

1. New crust is constantly added at ridges, yet oceanic crust is nowhere older than 200 million years, and one ocean spreading does not make another shrink.
2. So old ocean floor must be destroyed; Hess proposed it sinks and is consumed at oceanic trenches.

Answer: Because crust is constantly created but stays young and the earth does not grow; it must be consumed at trenches.

Plate tectonics: plates and boundaries

NCERT section Plate Tectonics

Origin: in 1967, McKenzie and Parker, and independently Morgan, combined available ideas into plate tectonics.

Tectonic (lithospheric) plate: a massive, irregular slab of solid rock, generally of both continental and oceanic lithosphere, moving horizontally over the asthenosphere as a rigid unit. Lithosphere thickness: 5-100 km in oceanic parts, about 200 km in continental areas. A plate is called oceanic or continental by which part is larger: the Pacific plate is largely oceanic; the Eurasian plate continental.

Seven major plates (bounded by young fold mountain ridges, trenches and/or faults):
1. Antarctica and surrounding oceanic plate
2. North American plate (with western Atlantic floor; separated from South America along the Caribbean islands)
3. South American plate (with western Atlantic floor)
4. Pacific plate
5. India-Australia-New Zealand plate
6. Africa with the eastern Atlantic floor
7. Eurasia and adjacent oceanic plate

Important minor plates: Cocos (Central America-Pacific), Nazca (South America-Pacific), Arabian (mostly Saudi Arabian landmass), Philippine (Asiatic-Pacific), Caroline (Philippine-Indian, north of New Guinea).

Plates, not continents, move: continents are parts of plates; all plates have moved and will keep moving. Pangaea itself was the result of converging continental masses. Palaeomagnetic data fix past positions; India's is traced from rocks of the Nagpur area.

Three boundaries:

Boundary	What happens	Example/feature
Divergent	Plates pull apart; new crust generated; spreading sites	Mid-Atlantic Ridge (American plates separate from Eurasian and African)
Convergent	One plate dives under another; crust destroyed; subduction zone	Ocean-continent, ocean-ocean, continent-continent
Transform	Plates slide past horizontally; crust neither produced nor destroyed	Transform faults, roughly perpendicular to ridges, because eruptions do not occur along the whole crest at once and the earth's rotation affects separated blocks

Worked example

The San Andreas Fault in California is a transform boundary. If two towns lie on opposite sides of it, what will happen to the distance between them over time, and will any new crust form between them? (2 marks)

Try it first, then show the solutionSolution

1. At a transform boundary the plates slide horizontally past each other, so the towns shift sideways relative to each other along the fault.
2. Crust is neither produced (as at divergent boundaries) nor destroyed (as at convergent boundaries), so no new crust forms between them.

Answer: They slide past each other sideways; no crust is created or destroyed.

Rates and force of plate movement

NCERT section Rates of Plate Movement; Force for the Plate Movement

Rates: measured from the strips of normal and reverse magnetic field parallel to the ridges.

• Slowest: Arctic Ridge, less than 2.5 cm/yr.
• Fastest: East Pacific Rise near Easter Island (South Pacific, about 3,400 km west of Chile), more than 15 cm/yr.

Force: in Wegener's time the earth was thought solid and motionless; sea floor spreading and plate tectonics showed surface and interior are dynamic.

• Mobile rock under the rigid plates moves in a circular manner: heated material rises, spreads, cools and sinks back: a convection cell (convective flow).
• Earth's internal heat comes from radioactive decay and residual heat.
• Holmes first considered the idea (1930s); it influenced Harry Hess on sea floor spreading.
• The slow movement of hot, softened mantle below the rigid plates is the driving force.

Worked example

How far would the sea floor at the Arctic Ridge (2.5 cm/yr) and at the East Pacific Rise (15 cm/yr) move in one million years? (2 marks)

Try it first, then show the solutionSolution

1. Arctic Ridge: 2.5 cm × 1,000,000 = 2,500,000 cm = 25 km.
2. East Pacific Rise: 15 cm × 1,000,000 = 15,000,000 cm = 150 km.

Answer: About 25 km and 150 km (using the boundary values of the stated rates).

Movement of the Indian plate

NCERT section Movement of the Indian Plate

Extent: Peninsular India and the Australian continental portion.

Boundaries:

• North: subduction zone along the Himalayas: continent-continent convergence.
• East: through the Rakinyoma (Arakan Yoma) Mountains of Myanmar towards the island arc along the Java Trench; further east a spreading site (oceanic ridge) east of Australia in the SW Pacific.
• West: Kirthar Mountains of Pakistan, along the Makran coast, joining the spreading site from the Red Sea rift south-eastward along the Chagos Archipelago.
• South: boundary with the Antarctic plate is an oceanic ridge (divergent) running roughly west-east, merging with the spreading site south of New Zealand.

Journey:

• India was a large island off the Australian coast; the Tethys Sea separated it from Asia until about 225 million years ago.
• It began moving north about 200 million years ago, when Pangaea broke.
• About 140 million years ago it lay as far south as 50°S.
• About 60 million years ago lava outpouring formed the Deccan Traps, continuing for a long time, while India was still close to the equator.
• India collided with Asia 40-50 million years ago, causing rapid uplift; from 40 million years ago the Himalayas formed, and they are still rising.

Worked example

What type of plate boundary lies along the Himalayas, and what is its result? (2 marks)

Try it first, then show the solutionSolution

1. India and Eurasia, both continental, meet along the Himalayas: continent-continent convergence (a subduction zone).
2. The collision 40-50 million years ago caused rapid uplift of the Himalayas, which continue to rise.

Answer: Continent-continent convergence; it raised the Himalayas.

Distribution of Oceans and Continents in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Continental Drift Theory: Wegener's Pangaea and the Evidence
• Sea Floor Spreading, Plate Tectonics and the Movement of the Indian Plate

Distribution of Oceans and Continents: key facts

Drift timeline

1. 1.Ortelius 1596
2. 2.Wegener 1912
3. 3.Holmes 1930s
4. 4.Hess 1961
5. 5.Bullard 1964
6. 6.plate tectonics 1967

Plate tectonics: McKenzie and Parker, and Morgan, independently.

Pangaea split

1. 1.Pangaea (all earth) + Panthalassa (all water)
2. 2.about 200 million years ago
3. 3.Laurasia (N) + Gondwanaland (S)

Then smaller continents.

Evidence (5)

Jig-saw fit, same-age rocks, tillite, placer gold, fossils

Bullard fit at the 1,000-fathom line; Mesosaurus localities 4,800 km apart.

Ages

Oceanic crust ≤ 200 million years; some continental rock 3,200 million years

Ocean sediments also never older than 200 million years.

Rates

Arctic Ridge < 2.5 cm/yr; East Pacific Rise > 15 cm/yr

Read from normal and reverse magnetic strips.

Distance moved

$Distance(km)=rate(cm/yr)\times years÷100,000$

1 km = 100,000 cm.

Boundaries

$Divergent=crustmade;convergent=crustdestroyed(subduction);transform=neither$

Convergence: ocean-continent, ocean-ocean, continent-continent.

Lithosphere thickness

Oceanic 5-100 km; continental about 200 km

Plates move over the asthenosphere.

India's journey

1. 1.Tethys until 225 Ma
2. 2.moves north 200 Ma
3. 3.50°S at 140 Ma
4. 4.Deccan Traps from 60 Ma
5. 5.collision 40-50 Ma

Ma = million years ago; Himalayas still rising.

Distribution of Oceans and Continents: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Distribution of Oceans and Continents: map work

The places on the official CBSE map list for this chapter. Find each one on an outline map of India and practise marking it.

How the school exam asks Distribution of Oceans and Continents

The question patterns to expect, and the method that works for each.

Tricks to remember Distribution of Oceans and Continents

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Who and when

  'Old Wise Hosts Help Build Plates': Ortelius 1596, Wegener 1912, Holmes 1930s, Hess 1961, Bullard 1964, Plate tectonics 1967.

  Use it when: Timeline MCQs.

• Drift evidence

  'Just Rocks Till Gold Fossils': Jig-saw, Rocks of same age, Tillite, Gold placers, Fossils.

  Use it when: 5-mark evidence answers.

• Minor plates

  'CoNA PhiCa': Cocos, Nazca, Arabian, Philippine, Caroline.

  Use it when: Minor plate MCQs and map work.

• Boundaries

  'Divergent Delivers, Convergent Consumes, Transform Touches': crust made, destroyed, neither.

  Use it when: Boundary questions.

• India's dates

  '225-200-140-60-45': Tethys separation, journey starts, 50°S, Deccan Traps, collision (40-50).

  Use it when: Indian plate answers.

Distribution of Oceans and Continents: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Distribution of Oceans and Continents

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying Wegener explained drift with convection currents

  Students merge the two theories.

  Wrong working

  Wegener said convection currents moved the continents.

  Right working

  Wegener proposed pole-fleeing and tidal forces; Holmes (1930s) proposed convection currents.

  ✓Do this: Wegener = rotation and tides; Holmes = heat.

  Usually costs: 1-2 marks in a 3-mark answer.

• ✗Mistake: Writing that continents move in plate tectonics

  The everyday picture of 'moving continents'.

  Wrong working

  According to plate tectonics, continents drift over the ocean floor.

  Right working

  Plates (lithosphere, continental and oceanic) move as rigid units over the asthenosphere; continents ride on them.

  ✓Do this: Plates move, continents ride.

  Usually costs: 1 mark, and the key point in a 'difference' answer.

• ✗Mistake: Listing Antarctica or Pacific as minor plates

  Confusing size of landmass with plate status.

  Wrong working

  Minor plates: Nazca, Cocos, Antarctica.

  Right working

  Minor: Cocos, Nazca, Arabian, Philippine, Caroline. Antarctica and Pacific are major.

  ✓Do this: Learn 'CoNA PhiCa' for the minor five.

  Usually costs: 1 mark per MCQ.

• ✗Mistake: Reversing rock ages across a ridge

  Students think the ridge is old because it is high.

  Wrong working

  Rocks at the ridge crest are the oldest.

  Right working

  Rocks at the crest are the youngest (normal polarity); age increases away from the crest.

  ✓Do this: New lava comes out at the crest.

  Usually costs: 1 mark in sea-floor spreading answers.

• ✗Mistake: Marking the Ring of Fire around the Atlantic

  Confusing it with the Mid-Atlantic Ridge.

  Wrong working

  Ring of Fire drawn down the Atlantic.

  Right working

  Ring of Fire around the rim of the Pacific; Mid-Atlantic Ridge down the centre of the Atlantic.

  ✓Do this: Fire rings the Pacific.

  Usually costs: 1 map mark.

NCERT exercise guide: Distribution of Oceans and Continents

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Distribution of Oceans and Continents

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Wegener's 'pole-fleeing force' arises because the earth is not a perfect sphere and has a bulge at the equator. Which motion of the earth produces this bulge?
   (a) Revolution round the sun
   (b) Rotation on its axis
   (c) The pull of the moon
   (d) Mantle convection

   Hint

   The spin, not the orbit.

   Answer

   (b) Rotation on its axis.

2. Q2 · 1 mark · MCQExam level

   A plate lies west of South America and is consumed beneath it. It is:
   (a) the Cocos plate
   (b) the Nazca plate
   (c) the Caroline plate
   (d) the Arabian plate

   Hint

   Cocos is off Central America; Caroline is north of New Guinea.

   Answer

   (b) The Nazca plate, a minor plate between South America and the Pacific plate.

3. Q3 · 1 mark · MCQHarder exam level

   Rock samples from the floor of an ocean are found to be no older than about 200 million years, while rocks on the nearby continent are as old as 3,200 million years. Which idea does this support most directly?
   (a) Pole-fleeing force
   (b) Tidal force of the sun and moon
   (c) Sea floor spreading
   (d) Jig-saw fit of coastlines

   Hint

   Where is new crust being made?

   Answer

   (c) Sea floor spreading: new oceanic crust forms at ridges, so the ocean floor is young.

4. Q4 · 2 marks · Short answerHarder exam level

   The earliest marine deposits on the coasts of South America and Africa are Jurassic. What does this tell us? (2 marks)

   Hint

   Marine deposits need a sea.

   Answer

   There were no marine deposits before the Jurassic, so the ocean between the two continents did not exist before then; they were joined and separated later.

5. Q5 · 3 marks · Short answerExam level

   A pan of thick soup heated from below shows blobs rising in the middle, spreading out and sinking at the sides. Use this to explain what starts and keeps mantle convection going and why it matters for plates. (3 marks)

   Hint

   Heat source, rising and sinking, a repeating cell.

   Answer

   In the mantle the heat comes from radioactive elements and residual heat, creating thermal differences. Hotter material rises, spreads beneath the lithosphere, cools and sinks, and the loop repeats as a convection cell, as Holmes suggested in the 1930s. These cells provide the force that moves the plates.

6. Q6 · 2 marks · Short answerExam level

   Name the three types of convergence and say which one forms the Himalayas. (2 marks)

   Hint

   Combinations of oceanic and continental plates.

   Answer

   Ocean-continent, ocean-ocean, continent-continent. The Himalayas: continent-continent convergence.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a new island chain is found where two plates slide past each other with no volcanoes, and another area where a ridge produces new lava. Classify both boundaries and explain why transform faults are roughly perpendicular to ridges. (3 marks)

   Hint

   Eruptions are not simultaneous along the crest.

   Answer

   Sliding past with no new crust: transform boundary. New lava at a ridge: divergent boundary. Transform faults run roughly perpendicular to ridges because eruptions do not occur along the entire crest at the same time, so parts of a plate move away by different amounts; the earth's rotation also affects the separated blocks.

8. Q8 · 5 marks · Long answerHOTS / competency

   India lay at 50°S about 140 million years ago and is now mostly between about 8°N and 37°N. Using the chapter, describe its journey and its main geological consequences. (5 marks)

   Hint

   Tethys, Pangaea break-up, Deccan Traps, collision.

   Answer

   India was an island off Australia, separated from Asia by the Tethys Sea until about 225 million years ago. It began moving north about 200 million years ago when Pangaea broke, lay at 50°S about 140 million years ago, and was near the equator when Deccan Trap lava poured out from about 60 million years ago. It collided with Asia 40-50 million years ago (continent-continent convergence), raising the Himalayas from about 40 million years ago; they are still rising.

9. Q9 · 2 marks · Short answerHOTS / competency

   Wegener was a meteorologist. Suggest why his tillite evidence was especially strong from a climate point of view. (2 marks)

   Hint

   Palaeoclimate.

   Answer

   Tillite forms from glacial deposits, so finding it in India and other now-warm southern lands shows they once had a glacial climate together, which is best explained if they were joined and located elsewhere: evidence of palaeoclimate and drift.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What do Pangaea and Panthalassa mean?Show answer

   All earth and all water.

2. 2.Into which two masses did Pangaea first split?Show answer

   Laurasia (north) and Gondwanaland (south).

3. 3.Where are Mesosaurus fossils found?Show answer

   Southern Cape province of South Africa and Iraver formations of Brazil.

4. 4.What did Hess propose in 1961?Show answer

   Sea floor spreading.

5. 5.Which ridge has the slowest rate of plate movement?Show answer

   The Arctic Ridge, less than 2.5 cm/yr.

6. 6.What is a subduction zone?Show answer

   The location where one plate sinks under another at a convergent boundary.

7. 7.Rocks from which area trace India's past positions?Show answer

   The Nagpur area.

8. 8.When did India collide with Asia?Show answer

   About 40-50 million years ago.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Continents cover 29% of the earth's surface.
• ☐Ortelius 1596; Wegener (German meteorologist) 1912: Pangaea split about 200 million years ago.
• ☐Evidence: jig-saw fit (Bullard 1964), same-age rocks, tillite, Ghana gold from Brazil, lemurs and Mesosaurus.
• ☐Wegener's forces (pole-fleeing, tidal) were inadequate.
• ☐Holmes (1930s): radioactivity-driven convection currents.
• ☐Ocean floor: continental margins, deep-sea basins (abyssal plains), mid-ocean ridges.
• ☐Earthquakes: shallow at ridges; deep along the Alpine-Himalayan belt and Pacific rim (rim of fire).
• ☐Hess (1961): new crust at ridges, consumed at trenches; ocean crust ≤ 200 million years.
• ☐Plate tectonics (1967): 7 major and minor plates; divergent, convergent, transform boundaries.
• ☐Rates: Arctic Ridge < 2.5 cm/yr; East Pacific Rise > 15 cm/yr.
• ☐India: north from 200 million years ago; Deccan Traps from 60 million years ago; collision 40-50 million years ago; Himalayas still rising.

Distribution of Oceans and Continents: frequently asked questions