What evidence convinced geologists that plates move?

Wegener's continental drift had strong evidence (the fit of continents, matching fossils and rocks across oceans, and palaeoclimate) but no mechanism, so it was rejected. The mechanism came from sea-floor spreading, supported by the increasing age of oceanic crust away from mid-ocean ridges and, decisively, by palaeomagnetism: basalt cooling below the Curie temperature locks in the field, and symmetrical magnetic stripes of alternating polarity about a ridge prove crust is created and carried away equally on both sides.

How do I identify a plate margin from its features in the exam?

Use the diagnostic combination. A mid-ocean ridge with shallow earthquakes and gentle basaltic volcanism is constructive. A deep ocean trench with earthquakes from shallow to about 700 km along an inclined Benioff zone and explosive andesitic volcanoes is destructive (an island arc means ocean-ocean, a continental volcanic chain means ocean-continent). Fold mountains with powerful shallow earthquakes but no volcanic arc indicate continent-continent collision. A linear transform fault with powerful shallow earthquakes and no volcanism is conservative.

What is the difference between magnitude and intensity?

Magnitude measures the energy released at the source, from the amplitude on a seismogram; it is one value per earthquake and the scales are logarithmic. The Richter scale saturates (underestimates) very large earthquakes, so moment magnitude, based on the seismic moment (fault area, slip and rigidity), is preferred for big events. Intensity measures the effects at a particular place (felt shaking and damage) on the Modified Mercalli scale, and it varies with distance, being greatest near the epicentre.

What is the single most common mistake in this module?

Reversing the anticline and syncline age rule, and confusing fault types. An anticline has the oldest rocks in the core; a syncline has the youngest, and this is how you read a fold on a map. For faults, a normal fault has the hanging wall down and records tension; a reverse or thrust fault has the hanging wall up and records compression. Getting the relative ages or the stress regime wrong loses marks on the map and cross-section questions that dominate Paper 3.

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OCR A-Level Geology Module 3 Global tectonics: plate theory, margins, earthquakes and structures

Q: What controls whether a volcano erupts gently or violently?

Silica content, viscosity and dissolved gas. Low-silica basaltic magma is runny (low viscosity), so gas escapes easily and the eruption is gentle (effusive), building broad shield volcanoes at constructive margins and hotspots. High-silica andesitic or rhyolitic magma is sticky (high viscosity), so gas is trapped, pressure builds, and the eruption is violent (explosive), producing pyroclastic flows and building steep stratovolcanoes at destructive margins.

A deep-dive OCR A-Level Geology guide to Module 3 Global tectonics. Covers the development of plate tectonics and its evidence, the three plate-margin types and their features, earthquakes and seismic waves, volcanism and eruption styles, and geological structures (folds, faults and unconformities), with the exam patterns OCR repeats.

Generated by Claude Opus 4.819 min readOCR-H414-Module-3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What Module 3 actually demands
Plate tectonics theory and evidence
Plate margins and their features
Earthquakes and seismic waves
Volcanism and eruption styles
Geological structures
How Module 3 is examined
Check your knowledge

What Module 3 actually demands

Global tectonics is the dynamic heart of OCR A-Level Geology. The module runs from the evidence that built plate tectonics, through what happens at each type of plate margin, to the earthquakes, volcanoes and structures that margins produce. The examiners test two linked skills: explaining the processes and their evidence, and reading the features (a margin, a seismogram, a fold or a fault) to deduce what is happening.

This guide walks through the five clusters of the module in a sensible build order, then sets out the exam patterns OCR repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Plate tectonics theory and evidence

The theory developed in stages. Continental drift (Wegener) had strong evidence (continental fit, matching fossils and rocks, palaeoclimate) but no mechanism, so it was rejected. Sea-floor spreading (Hess) supplied the mechanism, supported by the increasing age of oceanic crust away from ridges. The decisive evidence was palaeomagnetism: basalt cooling below the Curie temperature locks in the field, and symmetrical magnetic stripes of alternating polarity about a ridge prove crust is created and spread equally on both sides. The plates move by mantle convection, ridge push and slab pull (the strongest).

A typical quantitative task is to calculate a spreading rate from the distance and age of dated crust, using rate equals distance over time with careful unit conversion (km to cm, millions of years).

Plate margins and their features

There are three margin types. Constructive (divergent) margins pull apart, with decompression melting making basaltic magma, shallow earthquakes and gentle volcanism at mid-ocean ridges. Destructive (convergent) margins converge: the ocean-continent and ocean-ocean subtypes subduct dense oceanic crust along a Benioff zone (deep trench, shallow-to-deep earthquakes, explosive andesitic arcs), while continent-continent collision thickens the crust into fold mountains with powerful shallow earthquakes and little volcanism. Conservative (transform) margins slide past, giving powerful shallow earthquakes but no volcanism. Learn the diagnostic combination of features for each, because Paper 1 asks you to identify and explain margins.

Earthquakes and seismic waves

Earthquakes are generated by elastic rebound (stress builds across a fault, then it suddenly slips). They radiate P waves (fastest, through solids and liquids), S waves (slower, solids only) and surface waves (slowest, most damaging). Distinguish magnitude (energy at the source, logarithmic, moment magnitude beating Richter for large quakes) from intensity (effects at a place, Modified Mercalli, varying with distance).

The key skill is locating an epicentre: the P-S arrival gap on a travel-time graph gives the distance from a station, and triangulation from three stations pinpoints the epicentre. Practise reading the distance from a stated P-S gap and explaining why three circles are needed.

Volcanism and eruption styles

Eruption style is set by silica content, viscosity and gas. Low-silica basaltic magma is runny, gas escapes, and eruptions are gentle (effusive), building broad shield volcanoes and fissures at constructive margins and hotspots. High-silica andesitic or rhyolitic magma is viscous, traps gas, and erupts violently, producing ash and lethal pyroclastic flows and building steep stratovolcanoes and calderas at destructive margins. Viscosity and gas escape are the master controls, and the style maps directly onto plate setting.

Geological structures

Rocks deform by folding (ductile, under compression) and faulting (brittle). For folds, the age rule is decisive: anticlines have the oldest rocks in the core, synclines the youngest. For faults, the type records the stress: normal (hanging wall down, tension), reverse and thrust (hanging wall up, compression), strike-slip (shear). Joints are fractures with no displacement. Dip and strike define a tilted bed's orientation. Unconformities are gaps in the record: angular (tilted then overlain at a different angle), disconformity (parallel beds with an erosion gap) and nonconformity (sediments on eroded basement); an angular unconformity records deposition, tilting, uplift, erosion and renewed deposition.

How Module 3 is examined

A typical OCR profile for Global tectonics:

Evidence and explanation questions (Paper 1). Palaeomagnetism and spreading, why Wegener was rejected, why conservative margins have earthquakes but no volcanoes.
Calculation questions (Papers 1 and 3). Spreading rate from dated crust, and epicentre distance from a P-S gap and a travel-time graph.
Map and cross-section questions (Paper 3). Identifying folds by the age rule, classifying faults by hanging-wall movement, and reading the sequence of events at an unconformity.
Level-of-response extended answers (Papers 1 and 2). Describing a destructive margin in full, and explaining effusive versus explosive eruptions, are predictable six-mark questions.

Check your knowledge

A mix of recall and application questions covering the whole module. Attempt them under timed conditions, then check against the solutions.

Explain how symmetrical magnetic stripes provide evidence for sea-floor spreading. (4 marks)
Name the three forces that drive plate movement. (2 marks)
Describe the features (rocks, earthquakes, volcanoes) of an ocean-continent destructive margin. (4 marks)
Explain why conservative margins have earthquakes but no volcanism. (3 marks)
The P wave arrives 30 s before the S wave; a travel-time graph shows this gap corresponds to 250 km. State the distance to the epicentre and what else is needed to locate it. (3 marks)
Explain why basaltic magma erupts effusively but rhyolitic magma erupts explosively. (4 marks)
State which rocks lie in the core of a syncline, and the stress that forms folds. (2 marks)
Describe the sequence of events recorded by an angular unconformity. (4 marks)

Solutions

Q1: Basalt erupting at a ridge cools below the Curie temperature and locks in the field's polarity. The field periodically reverses, so as crust spreads outwards equally on both sides it records alternating normal and reversed periods, producing magnetic stripes symmetrical about the ridge. The symmetry proves crust is created at the ridge and carried away equally on each side.
Q2: Mantle convection, ridge push and slab pull.
Q3: Dense oceanic crust subducts beneath the continent along a Benioff zone, marked by a deep trench. Water from the slab triggers melting, generating andesitic to rhyolitic magma that forms explosive stratovolcanoes (a volcanic arc, for example the Andes). Earthquakes occur from shallow to deep along the descending slab.
Q4: Two plates slide past each other; friction locks them, stress builds, and they suddenly slip, releasing energy as shallow earthquakes. No plate is subducted or pulled apart, so there is no melting mechanism and therefore no volcanism.
Q5: A 30 s gap corresponds to 250 km, so the epicentre is 250 km from the station. This gives only a distance, so the epicentre lies on a circle of radius 250 km; two more stations are needed, and the point where the three circles intersect is the epicentre (triangulation).
Q6: Basaltic magma has low silica and low viscosity, so dissolved gas escapes easily and the eruption is gentle (lava flows). Rhyolitic magma has high silica and high viscosity, so gas is trapped and builds up under pressure until the magma fragments explosively, producing ash and pyroclastic flows.
Q7: The youngest rocks lie in the core of a syncline. Folds form under compressional stress.
Q8: The lower beds were deposited horizontally, then folded or tilted by compression, then uplifted above sea level and eroded to a flat surface (the unconformity). The land then subsided or sea level rose, and younger beds were deposited horizontally on top. The unconformity records a gap of uplift, erosion and non-deposition.