Tectonic hazards: plate tectonics, earthquakes and volcanoes

The Earth's surface is broken into about 15 large lithospheric plates that float on the semi-molten asthenosphere. Heat from the core drives convection currents in the mantle, dragging plates apart, together or alongside one another. Almost all earthquakes and volcanoes occur at plate margins.

The four plate boundaries

1. Constructive (divergent) margin. Plates move apart, magma rises, new oceanic crust forms. Frequent shallow earthquakes (low magnitude) and gentle, effusive basaltic volcanism. Example: the Mid-Atlantic Ridge running through Iceland — the Eyjafjallajökull eruption (2010) disrupted European aviation for a week.

2. Destructive (convergent) margin. Plates push together. Where oceanic meets continental crust, the denser oceanic plate is forced down (subduction) and melts, fuelling explosive andesitic/rhyolitic volcanism and powerful earthquakes. Example: the Nazca Plate subducting under South America builds the Andes and Mount St Helens-style stratovolcanoes. The 2011 Tōhoku earthquake (M9.1) was a megathrust on the Pacific–North American boundary.

3. Collision margin. Where two continental plates meet, neither subducts — instead they crumple upwards, building fold mountains (Himalayas, Alps) and producing strong earthquakes but no volcanism. Example: the 2015 Nepal earthquake (M7.8) killed nearly 9 000.

4. Conservative (transform) margin. Plates slide past each other. No volcanoes, but immense stress builds and releases as massive earthquakes. Example: the San Andreas Fault — Pacific Plate moves north-west relative to the North American Plate. The 1989 Loma Prieta and 1906 San Francisco earthquakes both occurred here.

Why earthquakes happen

Friction locks the plates. As convection drags them, strain energy accumulates in the rock. When the strain exceeds the friction, the rocks suddenly slip — releasing the energy as seismic waves. The point inside the Earth where the slip starts is the focus; the point on the surface directly above is the epicentre. Magnitude is measured on the moment magnitude scale (logarithmic — each whole number is ~32 times more energy released).

Why volcanoes erupt

At constructive margins, the mantle rises by decompression melting to form thin runny basalt — eruptions are gentle. At destructive margins, water from the subducting plate lowers the melting point of the overlying mantle wedge; the magma is thick, gas-rich, and silica-rich. Trapped gas builds pressure until the volcano blasts open, producing pyroclastic flows, ash columns and lahars.

A few volcanoes occur at hotspots — stationary plumes of hot mantle (Hawaii, Yellowstone) — well away from any plate boundary.

Contrasting case studies — examiner gold

You must know two recent earthquakes, one in an HIC and one in an LIC/NEE, and compare primary effects, secondary effects, immediate responses and long-term responses. A standard pairing:

• HIC: 2011 Tōhoku, Japan, M9.1. Primary: 19 750 deaths (mostly from the tsunami), 130 000 buildings destroyed. Secondary: Fukushima nuclear accident, $235 billion damage. Immediate response: rapid evacuation due to early-warning sirens; 100 000 SDF troops deployed within days. Long-term: $13 billion sea wall, restored infrastructure within 5 years.
• LIC: 2010 Haiti, M7.0. Primary: ~160 000 deaths, 250 000 homes destroyed. Secondary: cholera outbreak (10 000 deaths), 1.5 million homeless. Immediate response: slow — government collapsed, port damaged; international aid took days. Long-term: weak — many still in tent cities a decade later.

The contrast shows the crucial role of wealth, governance and preparedness.

Reducing risk

• Monitoring — seismometers, ground-deformation GPS, gas spectrometers (volcanoes).
• Prediction — useful for volcanoes (warning signs over weeks); virtually impossible for earthquakes.
• Protection — earthquake-resistant buildings (cross-bracing, base isolators, deep foundations); evacuation routes; sea walls.
• Planning — hazard maps, exclusion zones around active volcanoes, public education and drills.

Examiner tip

When asked to compare two earthquakes, never just describe them in turn. Use linking phrases: "Whereas in Japan…", "In contrast…", "Both events…". Marks come from explicit comparison.