Theme 2: The Natural Environment — Plate Tectonics

What you'll learn

This revision guide covers the tectonic processes section of Theme 2 in CIE IGCSE Geography. You'll understand the structure of the Earth, how tectonic plates move and interact, and the hazards that result from these movements. This topic regularly appears in Paper 1 and Paper 2, with questions worth 4-7 marks requiring both knowledge recall and application to case studies.

Key terms and definitions

Plate tectonics — the theory that Earth's outer shell is divided into several rigid plates that move relative to one another over the semi-molten mantle beneath

Convection currents — circular movements of heated material in the mantle caused by temperature differences, which drive plate movement

Subduction — the process where a denser oceanic plate is forced beneath a less dense continental plate at a destructive plate boundary

Epicentre — the point on the Earth's surface directly above the focus (origin point) of an earthquake

Magma — molten rock beneath the Earth's surface; becomes lava when it reaches the surface

Fault line — a fracture or zone of fractures in the Earth's crust along which movement has occurred

Richter scale — a logarithmic scale (0-10) measuring the magnitude of an earthquake based on seismic wave amplitude

Fold mountains — large mountain ranges formed by the compression and upward folding of rock layers at collision zones

Core concepts

Structure of the Earth

The Earth consists of four distinct layers, each with different compositions and physical properties:

Crust

• Thinnest outer layer (5-70 km thick)
• Oceanic crust: 5-10 km thick, dense basaltic rock
• Continental crust: 30-70 km thick, less dense granitic rock
• Cool and brittle, broken into tectonic plates

Mantle

• Extends to approximately 2,900 km depth
• Semi-molten rock (magma) in the upper mantle
• Temperature: 1,000-3,700°C
• Contains convection currents that drive plate movement

Outer core

• Liquid iron and nickel
• 2,900-5,100 km depth
• Temperature: approximately 4,500°C

Inner core

• Solid iron and nickel despite extreme heat
• Immense pressure prevents melting
• Temperature: up to 5,500°C
• Radius of approximately 1,200 km

Plate movement and boundaries

Tectonic plates move at rates of 2-10 cm per year, driven primarily by convection currents in the mantle. Heat from the Earth's core causes magma to rise, spread horizontally beneath the crust, cool, and sink again in a continuous cycle.

Constructive (divergent) plate boundaries

Plates move apart from each other, creating new crust:

• Magma rises through the gap, cools and solidifies
• Forms new oceanic crust and underwater mountain ranges (mid-ocean ridges)
• Volcanic activity is frequent but relatively gentle
• Shallow-focus earthquakes occur as plates crack and separate
• Example: Mid-Atlantic Ridge (separating Eurasian and North American plates)

Landforms created:

• Mid-ocean ridges
• Rift valleys (where continents split, e.g., East African Rift Valley)
• Submarine volcanoes, occasionally forming islands (e.g., Iceland, Surtsey)

Destructive (convergent) plate boundaries

Plates move toward each other, with denser oceanic plate subducting beneath continental plate:

• Oceanic plate forced down into mantle at subduction zone
• Friction causes powerful earthquakes at various depths
• Subducted plate melts, magma rises through cracks
• Explosive volcanic eruptions occur
• Examples: Nazca Plate subducting beneath South American Plate (Andes); Pacific Plate beneath Eurasian Plate (Japan)

Landforms created:

• Deep ocean trenches (e.g., Peru-Chile Trench, 8,000 m deep)
• Fold mountains (e.g., Andes, 6,000+ m high)
• Volcanic mountain chains
• Island arcs when oceanic plate subducts beneath another oceanic plate (e.g., Aleutian Islands)

Collision plate boundaries

Two continental plates of similar density collide:

• Neither plate subducts due to equal density
• Sedimentary rocks compressed and forced upward
• Forms fold mountains with sedimentary rock layers
• Earthquakes occur but no volcanic activity (no subduction, no melting)
• Example: Indo-Australian Plate colliding with Eurasian Plate (Himalayas)

Landforms created:

• Fold mountains (e.g., Himalayas, Alps, Rockies)
• Thrust faults and complex folded rock structures

Conservative (transform) plate boundaries

Plates slide past each other horizontally:

• No crust created or destroyed
• Plates move at different rates or directions
• Friction builds until plates suddenly jerk past each other
• Causes powerful, shallow earthquakes
• No volcanic activity
• Example: San Andreas Fault, California (Pacific and North American plates)

Earthquake characteristics and impacts

Causes and measurement

Earthquakes result from sudden release of built-up stress along fault lines. The focus is the point of rupture underground; the epicentre is directly above on the surface. Seismic waves radiate outward, causing ground shaking.

Measurement scales:

• Richter scale: measures magnitude (energy released), 0-10 logarithmic scale
• Mercalli scale: measures intensity (observed effects), I-XII based on damage

Primary effects (immediate impacts):

• Ground shaking damages/collapses buildings and infrastructure
• Ground rupture along fault lines
• Deaths and injuries from collapsing structures
• Damage to roads, bridges, railways, water pipes

Secondary effects (follow-on impacts):

• Fires from ruptured gas pipes and electrical lines
• Tsunamis if epicentre is underwater (displacement of ocean floor)
• Landslides on unstable slopes
• Disease spread due to contaminated water
• Economic disruption and unemployment
• Homelessness and refugee movements

Factors affecting impact severity:

• Population density in affected area
• Building quality and earthquake resistance
• Depth of focus (shallow earthquakes cause more surface damage)
• Distance from epicentre
• Level of economic development and preparedness
• Time of day (more deaths if earthquake strikes when people indoors/asleep)

Volcanic activity and landforms

Volcanic eruptions at different boundaries

Destructive boundaries:

• Explosive eruptions due to viscous, gas-rich magma
• Composite (stratovolcano) cones with steep sides
• Pyroclastic flows, ash clouds, lava bombs
• Example: Mount Pinatubo (Philippines, 1991)

Constructive boundaries:

• Gentle eruptions with runny basaltic lava
• Shield volcanoes with gently sloping sides
• Lava flows dominate
• Example: Eyjafjallajökull (Iceland, 2010)

Primary effects of eruptions:

• Lava flows destroy buildings, farmland, forests
• Pyroclastic flows (superheated gas and rock) kill living things
• Ash fall buries settlements, collapses roofs
• Volcanic bombs (large rock fragments) cause damage
• Poisonous gases suffocate people and animals

Secondary effects:

• Lahars (mudflows from melted ice/heavy rain mixing with ash)
• Climate effects (ash in atmosphere blocks sunlight, cooling temperatures)
• Acid rain from sulphur dioxide
• Long-term agricultural disruption
• Tourism decline or increase
• Economic costs of rebuilding

Benefits of volcanic activity:

• Fertile soils from weathered volcanic ash (supports intensive agriculture)
• Geothermal energy production
• Tourism revenue
• Mining of volcanic minerals and precious stones
• New land creation

Responses to tectonic hazards

Prediction and monitoring

Earthquakes:

• Seismographs detect ground movements
• Satellite monitoring of ground deformation
• Difficult to predict timing accurately
• Focus on long-term probability and preparedness

Volcanoes:

• More predictable than earthquakes
• Gas emissions monitoring (sulphur dioxide increase)
• Ground deformation (bulging) measurement
• Increased seismic activity detection
• Temperature changes in crater lakes
• Allows evacuation warnings

Protection measures

Earthquake-resistant building design:

• Reinforced steel frames that flex
• Deep foundations anchored in bedrock
• Cross-bracing and shear walls
• Counterweights and dampers
• Automatic shutters on windows
• Examples: Tokyo skyscrapers, San Francisco buildings

Planning and preparation:

• Earthquake drills in schools and workplaces
• Emergency supply kits (water, food, first aid)
• Exclusion zones around active volcanoes
• Evacuation route planning
• Public education campaigns
• Building codes and land-use zoning

Why people continue living in hazard zones:

• Fertile volcanic soils for agriculture
• Established communities and family ties
• Economic opportunities (tourism, geothermal energy)
• Lack of financial resources to relocate
• Perception that major event won't occur in their lifetime
• Adaptation through improved building standards

Worked examples

Example 1: Describe the characteristics of a destructive plate boundary. [4 marks]

Model answer: At a destructive plate boundary, oceanic and continental plates move towards each other [1]. The denser oceanic plate is subducted beneath the less dense continental plate [1]. As the oceanic plate descends, friction generates earthquakes at varying depths [1]. The subducted plate melts in the mantle, producing magma that rises to form explosive volcanoes [1]. Features include ocean trenches, fold mountains and volcanic mountain chains [1 for any feature].

Examiner guidance: This question requires accurate description using specific terminology. Award yourself one mark per valid point. Notice the use of precise terms (subducted, oceanic/continental, friction, magma).

Example 2: Explain why earthquakes at destructive plate boundaries can be more destructive than those at constructive boundaries. [6 marks]

Model answer: Earthquakes at destructive boundaries often occur in more densely populated areas [1], such as Japan and the west coast of South America, whereas constructive boundaries are mostly underwater [1]. This means more people and buildings are at risk from the ground shaking [1].

Destructive boundary earthquakes can have deeper focus points due to the subduction of the oceanic plate [1], but also produce powerful shallow earthquakes from friction along the subduction zone [1], releasing more energy than the shallow earthquakes at constructive boundaries [1].

Additionally, destructive boundaries often have secondary hazards such as tsunamis if the earthquake epicentre is offshore [1], and landslides in mountainous areas [1], which increase the destructive impact beyond the initial earthquake [1].

Examiner guidance: For 6-mark questions, develop your points with explanation (not just description). Link cause and effect clearly. This answer provides developed points worth 2 marks each.

Example 3: Using a named example, assess the responses to a tectonic hazard. [7 marks]

Model answer: The 2011 Tōhoku earthquake (magnitude 9.0) in Japan demonstrated both effective and limited responses to tectonic hazards [1].

Japan's prediction systems provided early warning seconds before the earthquake struck [1], allowing automatic shutdown of bullet trains and factory machinery, preventing additional casualties [1]. Buildings constructed to strict earthquake-resistant codes largely survived the shaking [1], with most damage caused by the subsequent tsunami rather than ground motion [1].

However, the prediction systems could not prevent the tsunami that reached heights of 40 metres in some areas [1]. Seawalls proved inadequate against such extreme waves [1]. The Fukushima nuclear power plant suffered catastrophic failure despite safety measures [1], suggesting that even highly developed nations cannot fully protect against the most extreme tectonic events [1].

The rapid emergency response, including military deployment and international aid coordination [1], helped reduce further casualties, while regular earthquake drills meant the population knew evacuation procedures [1]. Overall, preparation reduced earthquake damage but could not prevent all impacts of this exceptional event [1].

Examiner guidance: 'Assess' requires evaluation and judgement. Use a specific, named example with facts and figures. Balance positive and negative points, then reach a conclusion. Top-level answers (6-7 marks) show sophisticated understanding of both successes and limitations.

Common mistakes and how to avoid them

• Confusing plate boundary types: Learn the direction of movement for each boundary. Constructive = plates move apart; destructive = plates move together (oceanic under continental); collision = two continental plates together; conservative = plates slide past. Draw simple diagrams to memorize.

• Mixing up focus and epicentre: The focus is the point underground where the earthquake originates; the epicentre is the point on the surface directly above. Examiners regularly test this distinction.

• Describing effects without explaining causes: Don't just state "buildings collapse" — explain why (ground shaking exceeds structural tolerance, poor building standards, shallow focus increases surface wave intensity). Always link effect to cause.

• Forgetting to name examples: When questions ask for "a named example," you must provide specific place names, dates, and figures. Generic answers like "a volcano in a developing country" score zero marks.

• Listing primary and secondary effects without distinguishing them: Primary effects are immediate and direct (ground shaking, lava flow); secondary effects are subsequent consequences (fires, tsunamis, disease). Keep them separate in your answers.

• Ignoring command words: "Describe" means what/where (characteristics); "Explain" means why/how (reasons and processes); "Assess" means weigh up strengths/weaknesses and reach a judgement. Match your answer style to the command word.

Exam technique for "Theme 2: The Natural Environment — Plate Tectonics"

• Command word recognition: "Describe" questions (typically 3-4 marks) require characteristics and features only — no explanation needed. "Explain" questions (typically 4-6 marks) require reasons and processes using connectives like "because," "this causes," "as a result." "Assess" or "evaluate" questions (6-7 marks) need balanced arguments and a concluding judgement.

• Use of case studies: When a question states "using a named example," you must provide specific details (location, date, magnitude/VEI, impacts with figures, responses with examples). Generic answers receive minimal credit. Prepare 2-3 detailed case studies: one earthquake in a developed country, one in a developing country, and one volcanic eruption.

• Diagram annotation: For 4-mark "draw and label" questions on plate boundaries, draw a clear cross-section showing both plates, direction of movement (arrows), key features (trench, volcano, etc.), and labels for crust, mantle, subduction zone, and resulting landforms. Neatness and accuracy matter.

• Mark allocation awareness: Allocate approximately one developed point per mark available. For 6-mark questions, write three developed points (each worth 2 marks) or six simple points. Check the mark scheme guidance in brackets — if it says [4], don't write eight points; focus on quality over quantity.

Quick revision summary

Plate tectonics explains how Earth's rigid outer plates move over the semi-molten mantle driven by convection currents. At constructive boundaries, plates diverge creating new crust and gentle volcanic activity. At destructive boundaries, oceanic plates subduct beneath continental plates causing explosive volcanoes and powerful earthquakes. Collision boundaries form fold mountains without volcanic activity. Conservative boundaries produce earthquakes as plates slide past each other. Tectonic hazards vary in impact based on population density, economic development, and preparedness. Responses include monitoring, prediction, earthquake-resistant design, and evacuation planning.