What you'll learn
Tectonic hazards — earthquakes and volcanic eruptions — are caused by the movement of the plates that make up the Earth's surface. This guide covers what you need for Edexcel GCSE Geography: the structure of the Earth, the theory of plate tectonics and the forces that move plates, the types of plate boundary and the hazards found at each, hotspots, the causes and effects of earthquakes, volcanoes and tsunamis, why the impacts differ between countries at different levels of development, and how hazards are managed through prediction, protection and preparation. It uses the contrasting examples of the 2015 Nepal earthquake and the 2011 Tōhoku earthquake and tsunami in Japan.
Expect questions asking you to explain a process at a plate boundary, to compare the impacts of two hazard events, and to evaluate how people reduce tectonic risk. The highest marks go to answers that link impacts to levels of development and use specific detail from case studies.
Key terms and definitions
Crust — The thin, solid outer layer of the Earth: oceanic (dense, thin) and continental (less dense, thicker).
Mantle — The thick layer of hot, semi-molten rock beneath the crust.
Core — The Earth's centre: a liquid outer core and a solid inner core made mainly of iron and nickel.
Tectonic plate — A large section of the Earth's crust and upper mantle (lithosphere).
Convection currents — Movements in the mantle caused by heat from the core.
Slab pull — The pulling force of a dense oceanic plate sinking into the mantle at a subduction zone.
Destructive (convergent) boundary — Where plates move towards each other.
Constructive (divergent) boundary — Where plates move apart.
Conservative (transform) boundary — Where plates slide past each other.
Subduction — The sinking of a denser oceanic plate beneath another plate.
Hotspot — An area of volcanic activity away from plate boundaries, above a plume of hot mantle.
Focus — The point underground where an earthquake starts.
Epicentre — The point on the surface directly above the focus.
Magnitude — The size of an earthquake, measured on the moment magnitude scale.
Tsunami — A series of large waves caused by the sudden displacement of seawater, usually by an undersea earthquake.
Core concepts
The structure of the Earth
The Earth has four main layers: the inner core (solid, extremely hot), the outer core (liquid metal), the mantle (hot, semi-molten rock that can flow slowly) and the crust. Heat from the core and from radioactive decay drives movement in the mantle.
Why plates move
The crust and upper mantle are broken into tectonic plates that move a few centimetres a year. Forces that move them include:
- Convection currents — hot mantle rises, spreads and cools, then sinks, dragging plates with it.
- Slab pull — at subduction zones, the cold, dense edge of an oceanic plate sinks into the mantle and pulls the rest of the plate behind it; this is thought to be the most important force.
- Ridge push — at constructive boundaries, newly formed crust at the raised mid-ocean ridge slides down and away, pushing the plate.
Plate boundaries and their hazards
Destructive boundaries. Where oceanic crust meets continental crust, the denser oceanic plate is subducted. Friction causes powerful earthquakes. As the plate descends, it melts, and the magma rises to form explosive volcanoes (composite volcanoes, such as those around the Pacific "Ring of Fire"). Deep ocean trenches form where the plate sinks. Where two continental plates collide, neither subducts; the crust crumples upwards into fold mountains (such as the Himalayas) and strong earthquakes occur, but there are few volcanoes.
Constructive boundaries. Plates move apart and magma rises to fill the gap, forming new crust. This produces shield volcanoes with runny lava and gentle eruptions, and generally smaller earthquakes. Iceland lies on the Mid-Atlantic Ridge, a constructive boundary.
Conservative boundaries. Plates slide past each other in opposite directions or at different speeds. They can lock together, building pressure until it is released as a sudden earthquake. There is no volcanic activity. The San Andreas Fault in California is an example.
Hotspots. Some volcanoes occur far from plate boundaries, above an unusually hot plume in the mantle. As the plate moves over the stationary hotspot, a chain of volcanoes forms, as in Hawaii.
Earthquakes
Stress builds up where plates lock together. When it is suddenly released, energy travels outwards as seismic waves from the focus. Shaking is usually strongest near the epicentre. The effects depend on magnitude, depth of focus, distance from the epicentre, geology (soft ground can amplify shaking or liquefy), time of day, population density, building quality and the level of development.
Volcanoes
Composite volcanoes at destructive boundaries have thick, sticky (viscous) lava that traps gas, leading to explosive eruptions with ash clouds, pyroclastic flows (fast-moving clouds of hot gas and rock) and lahars (mudflows). Shield volcanoes at constructive boundaries and hotspots have runny lava and more frequent, less explosive eruptions with lava flows.
Tsunamis
When an undersea earthquake suddenly raises or lowers the seabed, it displaces a huge volume of water, sending waves outwards. In the deep ocean they are low and fast; as they reach shallow water near the coast they slow and grow taller. Often the sea first retreats, exposing the seabed, before the waves arrive. Tsunamis can travel across entire oceans.
Contrasting case studies
Nepal, 2015 (a developing country). On 25 April 2015, a magnitude 7.8 earthquake struck Nepal, where the Indian plate is colliding with the Eurasian plate. Around 9,000 people died and many thousands were injured. Hundreds of thousands of buildings were destroyed or damaged, including historic temples in Kathmandu. Avalanches were triggered on Mount Everest, and landslides blocked roads, cutting off rural villages. Many people were left homeless, and the economic losses were very large compared with the size of Nepal's economy.
Responses were limited by the country's resources and geography: many villages were remote and reachable only on foot or by helicopter. International aid, search-and-rescue teams and charities provided shelter, water and medical care. Rebuilding has been slow.
Japan, 2011 (a developed country). On 11 March 2011, a magnitude 9.0 earthquake occurred off the north-east coast of Japan, where the Pacific plate subducts beneath the plate on which northern Japan sits. Japan's strict building codes meant that many buildings survived the shaking, but the earthquake generated a huge tsunami that overwhelmed sea walls and swept inland, causing most of the roughly 18,000 deaths. The tsunami damaged the Fukushima Daiichi nuclear power plant, causing a serious nuclear accident and the evacuation of surrounding areas. The economic cost was enormous in absolute terms — among the most expensive disasters ever recorded.
Japan's responses were rapid and well organised: earthquake warnings stopped high-speed trains, the military was deployed, evacuation centres opened and infrastructure was rebuilt relatively quickly.
Comparing them: Nepal had far more deaths relative to the event's magnitude, mainly because of poorly built homes, limited emergency services and remote terrain. Japan had greater economic losses because of the high value of its buildings and infrastructure, but its preparation, technology and wealth saved many lives and allowed faster recovery. The tsunami showed that even well-prepared countries can be overwhelmed by extreme events.
Why impacts differ
Countries at higher levels of development usually suffer fewer deaths because they can afford earthquake-resistant buildings, monitoring and warning systems, trained emergency services, education and drills, healthcare and insurance. Countries at lower levels of development often have poorly built housing, dense informal settlements, weaker infrastructure and fewer resources to respond and rebuild, so death tolls tend to be higher, while economic losses may be lower in absolute terms but higher as a proportion of the economy.
Managing tectonic hazards
Prediction and monitoring. Earthquakes cannot be predicted precisely, but scientists map fault lines and areas of risk. Volcanoes can often be monitored: changes in ground shape (bulging), small earthquakes, gas emissions and temperature can indicate an eruption is approaching, allowing evacuation.
Protection. Earthquake-resistant buildings use deep foundations, steel frames, cross-bracing, shock absorbers and flexible materials. Sea walls and tsunami barriers protect coasts. Land-use planning avoids building in the highest-risk areas.
Preparation. Education, earthquake drills (as practised in Japanese schools), emergency kits, warning systems such as sirens and phone alerts, and evacuation routes help people respond quickly.
Living with tectonic hazards
Millions of people live in tectonically active areas despite the risks. Reasons include fertile volcanic soils that support farming, geothermal energy (as in Iceland), tourism attracted by volcanoes and hot springs, minerals brought to the surface, and the fact that many people have family, jobs and homes there or cannot afford to move. Some people believe a major event will not happen in their lifetime, or trust in modern buildings and warning systems.
Worked examples
Example 1: Destructive boundaries
Explain why earthquakes and volcanoes occur at destructive plate boundaries. (4 marks)
Answer: The denser oceanic plate is subducted beneath the continental plate (1). Friction between the plates causes stress to build up, which is released as earthquakes (1). As the oceanic plate sinks into the mantle, it melts to form magma (1). The magma rises through cracks in the crust and erupts, forming volcanoes (1).
Example 2: Comparing impacts
Explain why the 2015 Nepal earthquake caused more deaths than might be expected for its magnitude.
Answer: Many homes in Nepal were poorly built and collapsed, and remote mountain villages were cut off by landslides, so rescue and aid were slow. Nepal had limited resources for emergency services and healthcare, so injured people were harder to treat.
Example 3: Evaluating management
Assess the importance of building design in reducing earthquake deaths.
Answer: Most earthquake deaths are caused by collapsing buildings, so earthquake-resistant design is very important; in Japan in 2011, many buildings survived very strong shaking. However, such buildings are expensive, so poorer countries may not afford them, and the 2011 tsunami showed that design alone cannot protect against all hazards. Education, warning systems and planning are also essential.
Common mistakes and how to avoid them
- Saying volcanoes occur at conservative boundaries. They do not.
- Mixing up focus and epicentre.
- Saying earthquakes can be predicted precisely. Only general risk can be assessed.
- Listing impacts without separating them. Use primary/secondary or social/economic/environmental.
- Forgetting to link impacts to development.
Exam technique for "Tectonic Hazards"
- For process questions, describe plate movement step by step and name the hazard produced.
- In case study answers, use specific details: dates, magnitudes and named places.
- When comparing countries, explain why impacts differ with reference to wealth, building quality, preparation and response.
- In evaluation questions, consider several methods and reach a judgement.
Quick revision summary
- Earth: inner core, outer core, mantle, crust.
- Plates move by convection, slab pull and ridge push.
- Destructive boundaries: subduction, earthquakes, explosive volcanoes, trenches; collision forms fold mountains.
- Constructive: new crust, shield volcanoes, smaller earthquakes. Conservative: earthquakes only. Hotspots form volcanic chains.
- Tsunamis are caused by undersea earthquakes displacing water.
- Nepal 2015 (M7.8, around 9,000 deaths) vs Japan 2011 (M9.0 and tsunami, around 18,000 deaths, Fukushima).
- Impacts depend on development; management uses prediction/monitoring, protection and preparation.