Plate margins, earthquakes and volcanoes, their impacts and how risk is managed.
Notes and quizzes readyCore topic
Named examples on this page are revision choices. Use your school’s examples if they differ.
Revise the key ideas
Plates and hazard distribution
Earth has a solid inner core, a liquid outer core, a mantle and a thin crust. The rigid lithosphere includes the crust and the uppermost mantle and is broken into tectonic plates.
Plates move slowly over the mantle beneath them. Slab pull at subduction zones and ridge push help drive movement; mantle circulation is part of this system. The mantle is mainly solid rock that can flow very slowly, not a global liquid magma layer.
Oceanic crust is generally thinner and denser than continental crust. These differences help explain why oceanic plates can sink beneath other plates at destructive margins.
Most earthquakes and volcanoes occur in belts along plate margins, including around the Pacific Ocean. Earthquakes also occur away from margins, and some volcanoes form above hotspots such as Hawaii.
An earthquake happens when stress builds up and rocks suddenly slip along a fault. The released energy travels as seismic waves. The focus is the point inside Earth where rupture starts; the epicentre is directly above it at the surface.
Constructive, destructive and conservative margins
At a constructive margin, plates move apart. Mantle rock rises and partially melts as pressure falls. Magma rises into the gap and cools to form new crust, producing volcanoes and usually shallow earthquakes.Plates move apart. Arrows show divergent plate movement and upward magma movement; not to scale.
The Mid-Atlantic Ridge is a constructive margin. Iceland lies on the ridge between the North American and Eurasian plates and also has a hotspot beneath it.
At a destructive margin involving oceanic crust, the denser oceanic plate sinks into the mantle in a process called subduction. Friction and sudden slip cause earthquakes, and an ocean trench forms near the margin.
Water released from the descending plate helps the mantle above it melt. Magma may rise to form volcanoes. Thick, gas-rich magma can produce explosive eruptions; subduction is not simply a plate melting through friction.Oceanic plate subduction. The denser oceanic plate sinks beneath the overriding plate. Arrows show movement; not to scale.
The Nazca Plate subducts beneath the South American Plate along western South America. This creates earthquakes and the volcanic Andes.
When two continental plates collide, neither sinks easily. Compression folds and thickens the crust into mountains, as in the Himalayas. Collision causes earthquakes but usually no chain of volcanoes.
At a conservative margin, plates slide past each other. Friction can lock them until stress produces sudden slip and an earthquake. Crust is neither created nor destroyed, and this movement does not normally generate volcanoes.Sliding plate boundary. Opposite arrows show relative movement along the fault. This movement causes earthquakes, not a volcanic chain.
The San Andreas Fault in California is a conservative boundary between the Pacific and North American plates.
Effects and responses
Primary effects happen directly because of the event: shaking can collapse buildings, break roads and injure people. Lava, ash and hot flows can damage settlements during an eruption.
Secondary effects follow from primary effects: damaged pipes can interrupt drinking water, fires can start from broken gas lines, and blocked roads can delay aid. Earthquakes may trigger landslides or tsunamis under suitable conditions.
Immediate responses include rescue, emergency medical care, temporary shelters, food and clean water. Longer-term responses include repairing infrastructure, rebuilding homes and improving preparedness.
To compare places of contrasting wealth, consider building quality, access to emergency care, transport and money for recovery. Also consider depth, magnitude, population exposure and time of day; wealth alone does not determine the outcome.
Living with tectonic hazards
People stay near volcanoes for fertile soils, geothermal energy, tourism and mineral resources. Jobs, family ties, confidence in protection and the cost of moving also explain why people live in earthquake zones.
Monitoring volcanoes includes measuring small earthquakes, ground swelling, gas emissions and temperature changes. These may indicate rising magma, but forecasts still have uncertainty.
Scientists can estimate earthquake probabilities over periods of time, but cannot reliably predict an earthquake's exact place, time and magnitude. Monitoring and prediction are different from guaranteeing a warning.
Protection includes earthquake-resistant buildings with reinforced structures, flexible connections or base isolation. Buildings must be designed and maintained properly; no structure is completely earthquake-proof.
Planning includes hazard maps, evacuation routes, drills and emergency supplies. Keeping important facilities away from the most hazardous areas can reduce losses.
Tsunami warning systems use earthquake data and sea-level instruments to warn coasts. People close to the source may need to evacuate immediately after strong shaking because official warnings can arrive too late.
Contrasting examples: Chile 2010 and Nepal 2015
Chile's Maule earthquake struck on 27 February 2010 with magnitude 8.8. It occurred where the Nazca Plate subducts beneath the South American Plate. Chile had substantially more resources for protection and recovery than Nepal.
Shaking damaged buildings, roads and services in Chile. A tsunami then flooded coastal settlements. Around 500 people died in the earthquake and tsunami; strong building standards reduced collapse but did not prevent every loss.
Chile's immediate responses included search and rescue, emergency shelter, medical care and restoring essential services. Military support helped with logistics and security; communication and tsunami-warning failures showed weaknesses in preparedness.
Longer-term work in Chile included rebuilding housing and infrastructure and reviewing warning and emergency arrangements. Existing institutions, engineering expertise and financial resources supported recovery.
Nepal's Gorkha earthquake struck on 25 April 2015 with magnitude 7.8. Movement on the collision zone between the Indian and Eurasian plates caused the event; a major magnitude 7.3 aftershock followed on 12 May.
Buildings collapsed in Kathmandu and rural settlements, and historic sites were damaged. The earthquake sequence killed nearly 9,000 people and left many households without safe homes.
Landslides and avalanches were secondary effects in Nepal. They damaged routes and isolated mountain communities, making rescue and delivery of supplies harder.
Immediate help came from local people, Nepal's emergency services and international teams. Rescue, temporary shelter, medical support, food and water were priorities, with difficult access slowing work in remote areas.
Nepal's longer-term housing reconstruction used grants, engineering support and training for safer construction. International financing helped households rebuild, but recovery required years of work.
Chile's quake had a larger magnitude, yet Nepal suffered far more deaths. Building vulnerability, resources and access help explain this contrast; differences in physical conditions and population exposure also matter.
Test yourself
50 questions · Random sets of 10. These quick checks support revision; practise longer explanations and justified judgements too.
Mr Kidman's Classroom