IB Geography D.2.1 the Distribution of Geophysical Hazards Notes

Geophysical Hazards Form Uneven Global Patterns

1. Global belts reflect the geometry of plate margins, but the width and density of each belt vary with fault orientation, convergence rate and the age of the lithosphere involved.
2. Event catalogues are more complete for recent decades and densely monitored regions, so apparent clusters partly reflect where instruments and historical records are strongest.
3. Spatial concentration does not equal equal risk because settlement density, construction quality and preparedness vary greatly along the same tectonic belt.
4. An earthquake occurs when stored strain is released by sudden movement along a fault.
5. Earthquakes can occur anywhere, but most follow plate-boundary zones where relative plate motion concentrates strain.
6. The circum-Pacific seismic belt contains about 81% of the world's largest earthquakes because it includes many active subduction zones.
7. The Alpide belt runs from Indonesia through the Himalayas and Mediterranean and contains about 17% of the world's largest earthquakes.
8. The mid-ocean ridge system produces frequent shallow earthquakes where oceanic plates separate.

1. A distribution map shows where past events cluster, but it cannot state the exact time or magnitude of the next event.
2. Map pattern: Subduction produces broad seismic belts whose earthquake foci deepen inland, while mid-ocean ridges and transform faults produce narrower belts of shallow earthquakes.

Volcanoes Follow Melting Zones but Hotspots Break the Pattern

1. Arc position usually lies inland from an ocean trench because the descending slab must reach sufficient depth to release water into the mantle wedge.
2. Ridge volcanism is mostly submarine because the global mid-ocean ridge system lies beneath the oceans, making much of Earth's volcanism less visible to people.
3. Hotspot tracks form age-progressive chains where a moving plate passes over a persistent magma source, although not every intraplate volcano fits a simple fixed-plume model.
4. A volcano is an opening through which magma, gases and fragmented rock reach Earth's surface.
5. Most active volcanoes lie near plate boundaries because plate motion creates conditions for mantle rock to melt.
6. At subduction zones, water released from the descending slab lowers the melting temperature of the mantle above it.
7. At divergent margins, rising mantle melts as pressure falls and feeds mainly basaltic volcanism.
8. A hotspot is a long-lived source of magma beneath a moving plate, so it can form a volcanic chain away from a boundary.
9. The Pacific Ring of Fire contains volcanic arcs, while Hawaii shows that intraplate volcanism is a real exception.

Mass Movements Follow Susceptible Slopes and Trigger Zones

1. Susceptibility identifies terrain capable of failure from slope, material and drainage, while a trigger such as rainfall or shaking determines when movement may begin.
2. Regional clustering occurs where steep relief overlaps with monsoon rainfall, tropical cyclones, active faults or rapid road construction.
3. Local variation remains important because two neighbouring slopes can differ in rock structure, vegetation, groundwater and human modification.
4. Mass movement is the downslope movement of rock, debris or soil under gravity.
5. Steep relief raises the downslope force, while weak or fractured material lowers resistance to movement.
6. Heavy or prolonged rainfall adds water, raises pore-water pressure and can turn shallow slides into fast debris flows.
7. Earthquake shaking can remove the remaining resistance on unstable slopes and trigger many landslides at once.
8. Young mountain belts such as the Himalayas and Andes combine steep slopes, active faults and fractured rock.
9. Humid tropical and monsoon regions add intense rainfall to steep terrain, so landslide risk can rise during storms.
10. Road cutting, deforestation, mining and poorly planned construction can create local landslide hotspots within a wider susceptible region.

Distribution Maps Need More than One Layer

1. Hazard layers should represent probability or intensity for a defined process rather than combining earthquakes, eruptions and landslides into one undifferentiated surface.
2. Risk overlays add exposure and vulnerability, such as population, buildings, evacuation access and critical infrastructure, to the physical hazard pattern.
3. Resolution affects decisions because a national map can show broad belts while a local map is needed to distinguish individual slopes, faults or lahar channels.
4. A hazard map shows the probability or expected intensity of a hazardous process across space.
5. Earthquake and volcano maps should be read with plate boundaries, fault type, focal depth and local geology.
6. Mass-movement maps should combine slope angle, rock or soil strength, drainage, land cover and known triggers.
7. Exposure data must then locate people and assets within the hazard zone before the map can support risk management.
8. A place outside the main global belts may have low event frequency but still face serious losses if preparedness is weak.