IB Geography HL — Geophysical hazard risks

Key Idea: Topic 10.2 is about the hazards geophysical events create, and why the same event hits two places so differently. It pulls together two micros: 10.2.1 — earthquakes & tsunamis: energy released along a fault shakes the ground (the primary hazard) and triggers secondary hazards — tsunamis, landslides, liquefaction, fires. Magnitude and frequency vary by plate margin (subduction = the biggest quakes + tsunamis). 10.2.2 — volcanic & mass-movement hazards: lava, pyroclastic flows, lahars and ashfall from volcanoes, and rockfalls, landslides and debris flows moving downslope under gravity — fast, unwarned events kill, slow ones mostly wreck property. This is Option D content, examined on Paper 1. SL students answer two options, HL students answer three — the same questions at both levels. Each option is a data-response read off a hazard map or distribution diagram plus a structured question, ending in a [10] Examine essay marked on markbands.

🌍 10.2.1 — Earthquake and tsunami hazards

An earthquake releases energy along a fault: the focus is the rupture point underground, the epicentre is the surface point directly above it. Magnitude measures the energy released; intensity measures the felt shaking, which falls with distance from the epicentre. The shaking is the primary hazard; it triggers secondary hazards — and a tsunami forms when an offshore quake suddenly displaces the seabed and water column.

Tip: For a hazard map or event profile, read the key or scale first. Identify the most-affected zone, State an exact value off the key, Estimate a distance on the map scale, then describe the trend with distance. Always quote the units.

🌋 10.2.2 — Volcanic and mass-movement hazards

Volcanic hazards — lava flows, pyroclastic flows (fast, super-hot clouds of gas and ash), lahars (volcanic mudflows) and ashfall (tephra) — and mass movements — rockfalls, landslides, debris flows, slumps — are downslope, gravity-driven threats, often triggered by eruptions or earthquakes. Speed, not size, often decides the death toll: slow hazards (most lava, slumps) wreck property but let people escape; fast, unwarned ones (pyroclastic flows, lahars, sudden landslides) cause the mass casualties.

Example: Nevado del Ruiz, Colombia (1985) — a small eruption melted summit ice and sent lahars down the valleys; Armero was buried and about 23,000 died, despite warnings not acted on. Eyjafjallajokull, Iceland (2010) — a moderate eruption killed nobody, but its ash cloud grounded European flights for days: a huge economic impact with almost no death toll. Haiti (2010) vs Tohoku, Japan (2011) — the smaller Haiti quake killed 200,000+ in a poor, weakly built city, while better-prepared Japan limited its quake-and-landslide deaths: vulnerability, not just geophysics, sets the toll.

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