Earthquakes and volcanoes

📌 Key points

1. The Earth has a solid inner core, liquid outer core, a mantle and a thin crust; the crust and upper mantle are broken into tectonic plates.
2. Oceanic crust is thinner and denser than continental crust; when they meet, the oceanic plate is subducted (sinks) beneath the continental plate.
3. At constructive (divergent) boundaries plates move apart and magma rises, forming ridges and gentle-sided volcanoes with runny basaltic lava, e.g. the Mid-Atlantic Ridge, which runs through Iceland.
4. At destructive (convergent) boundaries an oceanic plate subducts, causing violent earthquakes, explosive volcanoes and fold mountains, e.g. the Nazca plate under the South American plate (the Andes).
5. At conservative boundaries plates slide past each other; there are earthquakes but no volcanoes, e.g. the San Andreas Fault between the Pacific and North American plates.
6. Where two continental plates collide, rocks are folded upwards into fold mountains, e.g. the Himalayas.
7. An earthquake starts at the focus underground; the epicentre is the point on the surface directly above it; magnitude is measured on the Richter scale, where each whole number is 10 times greater in shaking (amplitude).
8. Volcanic hazards include lava flows, pyroclastic flows (fast, hot clouds of gas and ash), ash falls and lahars (mudflows); earthquake hazards include building collapse, fires, landslides, liquefaction and tsunamis.
9. People live near volcanoes for fertile soils, geothermal energy, minerals and tourism, and in earthquake zones because of jobs, family and the low frequency of large events.
10. Risks can be reduced by monitoring volcanoes (seismometers, tiltmeters, gas readings), earthquake-resistant buildings (cross-bracing, deep foundations, shock absorbers), hazard mapping, evacuation plans and drills.

📖 Key terms

Tectonic plate

A large section of the Earth's crust and upper mantle that moves slowly.

Subduction

The sinking of a denser oceanic plate beneath another plate at a destructive boundary.

Focus

The point underground where an earthquake starts.

Epicentre

The point on the Earth's surface directly above the focus.

Magma

Molten rock below the Earth's surface; it is called lava when it reaches the surface.

Pyroclastic flow

A fast-moving current of very hot gas, ash and rock from an erupting volcano.

Tsunami

A series of large sea waves caused by an earthquake or eruption under the sea.

Lahar

A mudflow of volcanic ash and water.