GeographyISC Class 11 · Plate Tectonics and Endogenous Processes

Continental Drift, Sea Floor Spreading and Plate Boundaries (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Wegener (1912) proposed that the continents formed one supercontinent, Pangaea, which split and drifted; Hess (1961) explained how, through sea floor spreading at mid-ocean ridges. Plate tectonics combines both: rigid plates move over the asthenosphere at divergent, convergent and transform boundaries, driven by mantle convection currents suggested by Arthur Holmes.

Understand Continental Drift, Sea Floor Spreading and Plate Boundaries

Continental drift (Alfred Wegener, 1912). All continents were once joined as Pangaea, surrounded by the ocean Panthalassa. About 200 million years ago Pangaea split into Laurasia in the north and Gondwanaland in the south.

Evidence: the fit of South America and Africa; matching rocks on both sides of the Atlantic; glacial deposits (tillite) of the same age in India, Africa, South America, Antarctica and Australia; fossils of the freshwater reptile Mesosaurus and the plant Glossopteris on far-apart continents; placer gold in Ghana whose source lies in Brazil.
Weakness: the forces he proposed were far too weak, so the theory was rejected for decades.

Sea floor spreading (Harry Hess, 1961). Magma rises at mid-ocean ridges, forms new crust and pushes older crust away; old crust sinks at trenches.

Evidence: ocean crust is youngest at ridges and older away from them; no ocean floor is older than about 200 million years while some continental rocks exceed 3,000 million years; magnetic stripes lie symmetrically on both sides of ridges; sediments thicken away from ridges.

Plate tectonics (from 1967). The lithosphere is broken into rigid plates over the asthenosphere. Major plates: Antarctic, North American, South American, Pacific, Indo-Australian, African and Eurasian; minor ones include Nazca, Cocos, Arabian, Philippine and Caroline.

Divergent: plates move apart, new crust forms (Mid-Atlantic Ridge, East African rift).
Convergent: plates collide and one subducts: ocean-continent (Andes), ocean-ocean (Japan island arcs), continent-continent (Himalayas).
Transform: plates slide past each other (San Andreas Fault).
Rates: from less than 2.5 cm a year on the Arctic Ridge to more than 15 cm a year on the East Pacific Rise.
Driving force: mantle convection currents (Holmes, 1930s).

Indian plate. It broke from Gondwanaland and moved north; about 140 million years ago it lay near 50 degrees S; the Deccan Traps erupted on the way; it collided with Asia 40 to 50 million years ago, closing the Tethys Sea and raising the Himalayas, which are still rising.

Continental Drift, Sea Floor Spreading and Plate Boundaries: key facts

Spreading rate: $R a t e (c m / y r) = d i s t an ce (k m) x 100, 000/ t im e (y r) ∣1 k m = 100, 000 c m$
Age of crust: $A g e = d i s t an ce / r a t e ∣ C o n v er t k m t oc m f i r s t$
Ocean widening: $W i d e nin g = 2 x ha l f - r a t e x t im e ∣ B o t h s i d es m o v e$
Boundaries: Divergent ridges
Convergent trenches, fold mountains
Transform San Andreas

Divergent	ridges
Convergent	trenches, fold mountains
Transform	San Andreas

How to answer Continental Drift, Sea Floor Spreading and Plate Boundaries questions

Expect evidence questions (4 marks), boundary comparisons and spreading-rate sums (3 to 4 marks).

1Name the scientist and year, then the central idea.
2List evidence in numbered points.
3For calculations, convert km to cm (x 100,000) and write the formula.
4For boundaries, give movement, process, landform and example.

Continental Drift, Sea Floor Spreading and Plate Boundaries in real life

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

Seafloor conveyor

Suppose a conveyor belt feeds fresh goods at its middle and carries them towards both ends at a steady speed.

Which part of the ocean does the middle represent, and what would the age pattern be?

Try it first, then show the solutionSolution

The middle is the mid-ocean ridge where new crust forms.
Crust gets older with distance from the ridge on both sides.

Answer: The ridge; ages increase symmetrically away from it.

Memory hook: New at the middle, old at the edges.

Rate of drift

Suppose two islands on opposite sides of a ridge are 120 km further apart after 4 million years.

Find the total opening rate and each side's half-rate.

Try it first, then show the solutionSolution

120 km = 12,000,000 cm; 12,000,000 / 4,000,000 = 3 cm/yr total.
Each side moves 1.5 cm/yr.

Answer: 3 cm/yr total; 1.5 cm/yr each side.

Memory hook: Total rate is twice the half-rate.

Tricks to remember Continental Drift, Sea Floor Spreading and Plate Boundaries

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

Wegener then Hess
Wegener said "they moved"; Hess said "the floor spreads"; plates said "everything moves together".
Use it when: Sequence.
DCT
Divergent apart, Convergent collide, Transform slide.
Use it when: Boundary types.

Continental Drift, Sea Floor Spreading and Plate Boundaries: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Supercontinent halves · 1 mark · MCQBoard level

The northern landmass formed when Pangaea split was called:
(a) Gondwanaland
(b) Laurasia
(c) Panthalassa
(d) Tethys

Try it first, then show the solutionSolution

Northern part.

Answer: (b) Laurasia.

Example 2: Boundary types · 3 marks · Short answerBoard level

Name the three types of plate boundary and give one example landform or place for each.

Try it first, then show the solutionSolution

Divergent: Mid-Atlantic Ridge.
Convergent: Himalayas or Andes.
Transform: San Andreas Fault.

Answer: Divergent (Mid-Atlantic Ridge), convergent (Himalayas) and transform (San Andreas Fault).

Example 3: Rate and age · 4 marks · Long answerHarder board level

A section of ocean floor 150 km from a ridge formed 6 million years ago. (a) Find the spreading rate in cm/yr. (b) Find the age of crust 360 km away if spreading is 4.5 cm/yr.

Try it first, then show the solutionSolution

Rate = 150 km = 15,000,000 cm; 15,000,000 / 6,000,000 = 2.5 cm/yr.
360 km = 36,000,000 cm.
Age = 36,000,000 / 4.5 = 8,000,000 years.
Answer: 2.5 cm/yr and 8 million years.

Answer: 2.5 cm/yr; 8 million years.

Example 4: Compare boundaries · 4 marks · Long answerHarder board level

Compare divergent and convergent boundaries on plate movement, crust process, landforms and an example.

Try it first, then show the solutionSolution

Divergent: plates move apart; convergent: plates move together.
Divergent forms new crust; convergent destroys it by subduction or builds mountains.
Landforms: ridges and rift valleys versus trenches, island arcs and fold mountains.
Examples: Mid-Atlantic Ridge versus the Himalayas or Andes.

Answer: Divergent: apart, new crust, ridges; convergent: together, subduction or folding, trenches and mountains.

Example 5: Ocean-floor case · 5 marks · Case-basedHOTS / competency

Case (invented): Three drill cores 0, 200 and 400 km east of a ridge give ages of 0, 4 and 8 million years. (a) Find the spreading rate. (b) Predict the age at 1,000 km. (c) State two other evidences that agree with this pattern.

Try it first, then show the solutionSolution

(a) 200 km = 20,000,000 cm in 4,000,000 yr = 5 cm/yr (check 400 km: 40,000,000 / 8,000,000 = 5).
(b) 1,000 km = 100,000,000 cm; 100,000,000 / 5 = 20,000,000 yr = 20 million years.
(c) Symmetrical magnetic stripes and thicker sediments away from the ridge.

Answer: (a) 5 cm/yr (b) 20 million years (c) magnetic stripes and thicker sediments.

Where students lose marks in Continental Drift, Sea Floor Spreading and Plate Boundaries

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

Mistake: Saying continents, not plates, move on their own
Plate tectonics is oversimplified.
Do this: Plates move; continents are carried as parts of plates.
Usually costs: 1 mark.
Mistake: Forgetting km to cm
Rate sums give wrong powers of ten.
Wrong working
150 km / 6 million yr = 0.000025 cm/yr
Right working
15,000,000 cm / 6,000,000 yr = 2.5 cm/yr
Do this: Multiply km by 100,000 before dividing.
Usually costs: 2 marks.
Mistake: Mixing Wegener and Hess
Years swapped.
Do this: Wegener 1912; Hess 1961.
Usually costs: 1 mark.

Exam tips

Give name, year and idea for every theory.
Use 100,000 cm per km in rate sums.
Name an example for each boundary type.
Avoid saying ocean crust is older than 200 million years.

Practice questions on Continental Drift, Sea Floor Spreading and Plate Boundaries

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

Q1 · 1 mark · MCQBoard level
Which fossil plant supports continental drift?
(a) Glossopteris
(b) Mesosaurus
(c) Tillite
(d) Placer
Hint
Plant, not reptile.
Answer
(a) Glossopteris.
Q2 · 3 marks · Short answerHarder board level
A plate boundary moves apart at a half-rate of 2 cm/yr for 5 million years. Find the total widening.
Hint
2 x half-rate x time.
Answer
2 x 2 cm/yr x 5,000,000 yr = 20,000,000 cm = 200 km.
Q3 · 4 marks · Long answerHarder board level
Explain why Wegener's theory was rejected and how sea floor spreading supplied what it lacked.
Hint
Forces too weak.
Answer
The forces Wegener proposed were far too weak to move continents. Hess showed that new crust forms at ridges and moves away, giving the mechanism for movement.
Q4 · 4 marks · Long answerHarder board level
Describe three events in the movement of the Indian plate.
Hint
Gondwanaland, Deccan, collision.
Answer
It broke from Gondwanaland and moved north; the Deccan Traps erupted during the journey; about 40-50 million years ago it collided with Asia, closing the Tethys Sea and raising the Himalayas.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A hypothetical plate moves 4,500 km north in 45 million years. (a) Find its average speed in cm/yr. (b) Compare it with the East Pacific Rise rate of more than 15 cm/yr. (c) Explain the landform when it meets a continental plate.
Hint
4,500 km = 450,000,000 cm.
Answer
(a) 450,000,000 / 45,000,000 = 10 cm/yr. (b) Slower than the East Pacific Rise. (c) Continent-continent convergence builds fold mountains such as the Himalayas.

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.Name of the supercontinent?Show answer
Pangaea.
2.Who proposed sea floor spreading?Show answer
Harry Hess, 1961.
3.Example of a transform boundary?Show answer
San Andreas Fault.
4.Who suggested convection currents?Show answer
Arthur Holmes.
5.Year of Wegener's theory?Show answer
1912.

Continental Drift, Sea Floor Spreading and Plate Boundaries: frequently asked questions

Why are ridge rocks youngest?

New magma forms the crust there.

Why are the Himalayas still rising?

The Indian plate is still pushing into Asia.

Is the transform boundary destructive?

No, crust is neither created nor destroyed.