Environmental Geology and Natural Hazards: Volcanic, Seismic, Slope, Coastal and Fluvial Risks
This domain assesses whether you can separate hazard, vulnerability, exposure and risk, then analyse a specific process and propose mitigation. Balearic examples include seismic shaking and tsunamis, coastal erosion, slope instability and torrential floods.
Core definitions
- Hazard: probability of a damaging event of a given magnitude in a period and area.
- Vulnerability: propensity of exposed elements to be damaged.
- Exposure: people, assets and activities in the hazard zone.
- Risk is commonly expressed as hazard × vulnerability × exposure (expected loss). Reducing exposure and vulnerability is the practical lever.
Seismic hazard
Distinguish magnitude (energy released, one value per earthquake) from intensity (local effects, EMS-98 scale). Hypocentre versus epicentre. The Balearics have low to moderate seismicity, linked to Africa-Eurasia convergence and to nearby Algerian margin faults; the 2003 Boumerdès earthquake produced a small tsunami reaching Balearic coasts.
Volcanic hazard (general)
Viscosity and gas content control eruptive style: fluid basaltic lava versus explosive rhyolitic eruptions producing pyroclastic flows, ash and lahars. No active volcanism exists in the Balearics, but the concepts are examinable.
Slope, coastal and fluvial hazards
- Slope: falls, slides and flows, triggered by rainfall, earthquakes and undercutting; the factor of safety decreases with pore pressure.
- Coastal: storm waves, longshore drift, dune loss, sea-level rise and beach narrowing. Hard defences often shift erosion downdrift.
- Fluvial: flash floods in rambles and torrents, where a dry channel can carry a debris-laden flow within minutes.
Worked example
On 9 October 2018, more than 200 mm of rain fell in a few hours on the Sant Llorenç area of eastern Mallorca. A small, normally dry torrent drained a steep, partly urbanised catchment. The lag time was very short because of impermeable surfaces and channel encroachment, so the peak arrived with little warning and caused fatalities. The lesson: hazard was extreme rainfall, but risk was amplified by exposure on the floodplain and by a short lag time.
Traps that cost marks
- Using magnitude and intensity interchangeably.
- Saying a 100-year return period means the event occurs every 100 years.
- Confusing hazard with risk; a hazard in an empty area has no risk.
- Believing hard coastal defences eliminate erosion rather than displace it.