Tectonic hazards · GCSE Geography

Plate tectonics

The Earth's crust is split into tectonic plates that move slowly, driven by convection currents in the mantle and slab pull.
Most earthquakes and volcanoes happen along plate margins.

Destructive: an oceanic plate sinks (subducts) under a continental plate, causing explosive volcanoes and strong earthquakes.
Constructive: plates move apart and magma rises to make new crust, with gentler volcanoes (e.g. Iceland).
Conservative: plates slide past each other, causing earthquakes but no volcanoes (e.g. the San Andreas Fault).

Primary effects happen straight away: buildings collapse, roads crack, people are injured or killed.
Secondary effects happen later: fires, tsunamis, landslides, disease, loss of jobs and income.

Immediate responses: search and rescue, medical care, emergency shelter, food and water.
Long-term responses: rebuilding homes and infrastructure, stricter building codes, restoring the economy.

Chile (a higher-income country), 27 February 2010: a magnitude 8.8 earthquake off the coast, near Concepción. About 500 people died, and a tsunami hit coastal towns. The damage cost about US$30 billion. Strict building codes and quick emergency responses kept the death toll low, and the country recovered quickly.
Nepal (a low-income country), 25 April 2015: a magnitude 7.8 earthquake in the Gorkha district, about 80 km north-west of Kathmandu. Nearly 9,000 people died, many old buildings collapsed, and landslides and avalanches followed, including on Mount Everest. Nepal needed international aid, and rebuilding took many years.

Monitoring: seismometers, tiltmeters and gas measurements can warn that a volcano may erupt.
Protection: earthquake-resistant buildings with deep foundations, cross-bracing and shock absorbers.
Planning: hazard maps, evacuation routes and drills.
People live near hazards because of fertile soil, geothermal energy, tourism, jobs and family ties.

Tectonic plate: A large slab of the Earth's crust that moves slowly.
Convection current: Circulation of heat in the mantle that helps move the plates.
Destructive margin: Where an oceanic plate subducts under a continental plate.
Constructive margin: Where two plates move apart and new crust forms.
Conservative margin: Where two plates slide past each other.
Subduction: One plate sinking beneath another into the mantle.
Epicentre: The point on the surface directly above where an earthquake starts.
Primary effect: An effect caused directly by the hazard, straight away.
Secondary effect: An effect that happens later as a result of the primary effects.