IB Geography Option D: Geophysical hazards

What Option D covers

Option D starts with geophysical systems: plate tectonics, from divergent, convergent and conservative boundaries to subduction, hotspots and the forces that move plates, such as slab pull; shield and composite volcanoes; earthquakes, with their focus and epicentre; and mass movement, such as rotational slides. Geophysical hazard risks: the hazards themselves, from pyroclastic flows and lahars to liquefaction and tsunamis, and how they are measured, with the moment magnitude scale, the Modified Mercalli scale and the Volcanic Explosivity Index. Hazard risk and vulnerability: why a hazard becomes a disaster, why risk depends on vulnerability and capacity as well as on the hazard, and how people perceive it. Future resilience and adaptation: hazard mapping and land-use zoning, retrofitting, and recovery after a disaster, with the Park model.

Where students slip

• Magnitude and intensity are different scales. The moment magnitude scale measures the energy an earthquake releases, so each earthquake has one value, and each whole number up is about 32 times more energy. The Modified Mercalli scale, from I to XII, measures the effects observed at a place, so one earthquake has many values.
• A pyroclastic flow is an avalanche of hot gas, ash and rock racing down a volcano during an eruption. A lahar is a mudflow of volcanic ash and water, and it can strike far down a valley, long after the eruption, when heavy rain or melting ice remobilises the ash.
• A hazard is not a disaster in itself. It becomes one when its impacts exceed a community's ability to cope with its own resources, which is why risk is the hazard multiplied by vulnerability and divided by capacity.