Geological hazards and geological maps
This domain tests hazard and risk concepts, the main Andalusian hazards, and the reading of geological and topographic maps. Map questions reward careful scale and symbol work.
Hazards
- Seismic hazard is highest in the Betic Cordillera and the Gulf of Cadiz; magnitude measures energy released on a logarithmic scale, intensity measures effects at a site.
- Flooding affects the Guadalquivir plain and Mediterranean ramblas; return period expresses average recurrence, not regularity.
- Landslides occur on steep Betic slopes in weak marls after heavy rain; liquefaction affects saturated sands during earthquakes.
- Subsidence and collapse occur in karst terrains (Antequera, Sorbas) and where aquifers are overexploited.
- Coastal erosion and tsunamis affect the Gulf of Cadiz; Cabo de Gata volcanism is inactive.
Risk
Risk = hazard x exposure x vulnerability. Mitigation usually reduces vulnerability or exposure, not the hazard itself.
Geological maps
- Check scale, legend, contour interval and orientation before interpreting anything.
- Strike and dip symbols show direction and angle of dip; fault barbs mark the downthrown side; fold symbols mark anticlines and synclines.
- The rule of V's relates outcrop pattern to topography and dip.
- Vertical exaggeration = horizontal scale divided by vertical scale.
Worked example
On a 1:50,000 map, two contour lines 20 m apart plot 4 mm apart. What is the slope? Ground distance = 4 mm x 50,000 = 200,000 mm = 200 m. Slope = 20/200 = 0.10, or 10%, about 5.7 degrees. The reasoning is scale conversion first, gradient second.
Traps that cost marks
Magnitude and intensity are swapped; risk is equated with hazard; scale conversions slip by factors of ten; dip direction is confused with dip angle; a 100-year return period is read as "once every 100 years exactly".
Key points
- Risk combines hazard, exposure and vulnerability.
- Magnitude is logarithmic energy; intensity is local effect.
- Fault barbs mark the downthrown block.
- Convert map millimetres to ground metres before gradients.
- Return period is a probability, not a schedule.