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 occurs when a cold, dense plate sinks at a subduction zone and pulls the rest of the plate behind it. Ridge push is gravity-driven movement away from a raised ocean ridge. These help drive plate 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 (heat from within Earth), 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 (bearings or pads that reduce the shaking passed from the ground into a building). 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: Japan 2011 and Nepal 2015
Japan's Tohoku earthquake struck offshore on 11 March 2011. USGS records magnitude 9.1; many school accounts round it to 9.0. The Pacific Plate subducts beneath the plate carrying northern Japan at the Japan Trench. Japan is a high-income country with substantial engineering and emergency-response resources.
Shaking damaged buildings and infrastructure, but the tsunami caused much of the coastal destruction. More than 15,000 people died and thousands were missing. Flooding damaged the Fukushima Daiichi nuclear power station, causing reactor meltdowns, radioactive releases and evacuation: a secondary technological disaster.
Immediate responses included earthquake and tsunami warnings, evacuation, search and rescue, emergency shelters, food, water and medical care. Japan mobilised its Self-Defence Forces and received international assistance; some tsunami defences and evacuation arrangements were overwhelmed.
Long-term responses included rebuilding homes, ports and transport, relocating some settlements to higher land and reviewing coastal defences and evacuation plans. Nuclear clean-up and displacement continued long after the emergency; wealth does not make recovery immediate or remove all risk.
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.
Compare Japan's stronger construction, warnings and financial resources with Nepal's vulnerable buildings and difficult mountain access. Japan nevertheless had a higher death toll in these events because of its much larger earthquake and devastating tsunami. Wealth affects vulnerability and response; it is not the only control on losses.
Test yourself
50 questions · Random sets of 10. These quick checks support revision; practise longer explanations and justified judgements too.
Mind map
Use the branches to recall the ideas and explain their connections. Check the revision notes for the full detail.
View G2 mind mapOpen the full-size map to zoom. Use the printable view for A4 landscape or enlarge to A3.
Mr Kidman's Classroom
