Hazard, risk, vulnerability, disaster

A natural hazard is a physical event that has the potential to cause loss of life, injury, damage or disruption. Notice the word "potential": an earthquake in an empty desert is a hazard event, but not a hazard in a geographical context, because a hazard is defined by its relationship with people.

• Risk — the probability of harmful consequences. Often written as risk = hazard × vulnerability ÷ capacity to cope.
• Vulnerability — the susceptibility of a population to harm. Determined by poverty, building quality, age structure, health, governance, warning systems — not by the physical event.
• Resilience / capacity to cope — the ability to absorb the shock and recover.
• Disaster — the realisation of the hazard: a hazard event that overwhelms a community's ability to cope.

Degg's model puts this in a single diagram: a hazard event is one circle, a vulnerable population is another, and a disaster occurs only in the overlap. The consequence is powerful and examinable: the same physical event produces radically different outcomes in different places, and the difference is human, not geological.

Hazard perception — why do people stay? AQA names three responses. Fatalism (acceptance: the event is unavoidable, an act of god or fate, so losses are borne). Adaptation (people believe they can reduce impact by modifying behaviour or the built environment — the rational, action-taking response). Fear (perception of risk is so strong that people move away). Perception is shaped by wealth, education, past experience, religion and the perceived benefits of staying — fertile volcanic soils, coastal fishing, cheap floodplain land.

When is it a disaster?

The Park model and the hazard management cycle

Park's disaster-response curve plots quality of life (or level of economic activity / social normality) against time. It has four phases:

• Pre-disaster — the steady state before the event. Quality of life is at its normal level.
• Disruption — a near-vertical drop as the event hits. Depth of the trough reflects the hazard's magnitude and the population's vulnerability.
• Relief — hours to days. Search and rescue, emergency medical care, shelter, food and water. Often delivered by neighbours first, then national agencies, then NGOs and international aid.
• Rehabilitation — weeks to months. Restoring services: temporary housing, reopening roads, water and power, resuming trade and schooling.
• Reconstruction — months to years. Rebuilding permanently, ideally to a higher standard than before ("building back better").

The hazard management cycle is the planning version of the same story, and it is a loop: preparedness (planning, education, drills, warning systems) → response (immediate relief) → recovery (rehabilitation and reconstruction) → mitigation (reducing the impact of the next event: building codes, land-use zoning, sea walls) → back to preparedness. The cycle's virtue is that it makes recovery the moment when future risk is reduced.

Evaluate the model, don't just draw it. Park's curve assumes a single, discrete event and a single, coherent community. It handles a slow-onset drought, a multi-hazard cascade (earthquake → tsunami → nuclear release) or a place recovering unevenly by class and neighbourhood far less well. Saying so is AO2.

Reading the Park curve

Earth structure and plate movement

From the inside out: a solid iron inner core, a liquid iron-nickel outer core (whose convection generates the magnetic field), a rocky mantle, and a thin crust. For tectonics, the useful division is mechanical rather than chemical:

• Lithosphere — rigid crust plus the uppermost mantle. This is what the plates are made of.
• Asthenosphere — the weak, ductile upper-mantle layer beneath, over which the plates move.
• Continental crust — thick, old, low density (granitic), and buoyant, so it is not subducted. Oceanic crust — thin, young, denser (basaltic), and readily subducted. That density contrast decides what happens at every convergent boundary.

What drives the motion? Mantle convection — heat from radioactive decay and residual formation heat sets up slow circulation — is the classic mechanism. Modern accounts stress two edge forces:

• Slab pull — a cold, dense subducting slab sinks under its own weight and drags the rest of the plate behind it. Generally regarded as the dominant force.
• Ridge push (gravitational sliding) — newly formed, hot lithosphere at a ridge is elevated; as it cools and thickens it slides away down the flank of the ridge.

Hot spots break the pattern: a stationary mantle plume of rising material burns through the plate far from any boundary, and as the plate migrates over it a chain of volcanic islands is built, progressively older away from the active vent. Hawaii is the standard example — proof that not all volcanism is boundary-driven.

Which plate boundary?

Tap a feature or process, then tap the boundary type it belongs to. Every tectonic question in the paper starts with getting this right.

Magma chemistry decides everything

The single most powerful explanatory line in volcanology at A-level: silica content controls viscosity, viscosity controls gas escape, and gas escape controls explosivity.

• Shield volcanoes (constructive boundaries and hot spots) erupt basaltic magma: low silica, low viscosity, gas escapes easily. Result — gentle effusive eruptions, fast-moving but rarely lethal lava flows, broad gentle cones.
• Composite (strato-) volcanoes (destructive boundaries) erupt andesitic/rhyolitic magma contaminated by melted crust and water: high silica, high viscosity, gas is trapped until pressure fails. Result — violently explosive eruptions, steep layered cones.

The secondary hazards kill far more people than lava:

• Pyroclastic flow — a ground-hugging current of superheated gas, ash and rock travelling at high speed. Essentially unsurvivable; the reason evacuation, not shelter, is the response.
• Tephra / ash fall — collapses roofs under wet ash loading, contaminates water, ruins crops, grounds aviation, and causes respiratory harm.
• Lahar — volcanic mud/debris flow: ash mobilised by rain, or by an ice-cap melting during an eruption. It follows valleys, and can strike long after the eruption itself.
• Jökulhlaup — a glacial outburst flood released when a sub-glacial eruption melts an ice cap (an Icelandic hazard in particular).
• Gas emissions — CO₂, SO₂, H₂S, HF. CO₂ is denser than air and can pool in hollows, asphyxiating silently.

The VEI (Volcanic Explosivity Index) runs 0–8 and is based on erupted volume, plume height and duration. Like the earthquake scales it is logarithmic — each step is roughly a tenfold increase in erupted volume — and, crucially, it measures the physical event, not the impact. A modest VEI eruption in a densely populated valley can cause far greater loss than a large one in an ocean.

Why is one volcano explosive?

Earthquakes: focus, waves and magnitude

Stress builds where plates lock; when friction is overcome the rock ruptures and stored elastic strain energy is released as seismic waves. The rupture begins at the focus (hypocentre); the point on the surface directly above it is the epicentre. Shallow-focus quakes do most damage; deep-focus quakes occur along the subducting slab (the Benioff zone) and attenuate before reaching the surface.

• P waves (primary) — fastest, compressional, travel through solids and liquids. First to arrive, least damaging.
• S waves (secondary) — slower, transverse, cannot travel through liquid. More damaging.
• L waves (Love/Rayleigh surface waves) — slowest, confined to the surface, largest amplitude — the main cause of destruction.

Measuring it. Moment magnitude (M_w) measures the energy released at source and has largely replaced Richter for large events (Richter saturates above about M7). It is logarithmic: each whole step is roughly a tenfold increase in ground-shaking amplitude and about a 31.6-fold increase in energy. The Modified Mercalli scale instead records intensity — observed shaking and damage — on a I–XII scale, so a single earthquake has one magnitude but many intensities.

• Ground shaking — the primary hazard; damage depends on duration, frequency and building resonance.
• Liquefaction — saturated, unconsolidated sediment loses strength under shaking and behaves as a fluid, so buildings sink or tilt intact. A key reason coastal and reclaimed land amplifies loss.
• Landslides and avalanches — shaking destabilises slopes; often the dominant killer in mountainous terrain.
• Tsunami — vertical displacement of the sea floor (typically at a subduction zone) displaces the water column. In deep ocean the wave is long, fast and low; as it shoals it slows, shortens and its amplitude rises dramatically.

Your turn — a logarithmic scale

Your turn — energy, not amplitude

Amplitude and energy are not the same thing. Each whole step on the moment magnitude scale releases about 31.6× more energy (because 31.6 ≈ 10^1.5).

Tropical storms: formation, structure, impact

Called hurricanes, typhoons or cyclones depending on the ocean basin — the physics is identical. AQA wants the formation conditions stated precisely:

• Sea-surface temperature of roughly 26–27°C or more, through a depth of about 50–60 m — the ocean is the fuel, supplying latent heat as water evaporates.
• Latitude roughly 5–20° from the equator. The Coriolis force is zero at the equator, so the system cannot begin to spin — which is why tropical storms never form on the equator itself.
• Low vertical wind shear — strong shear tears the developing vertical structure apart.
• An existing low-level convergence/disturbance to trigger uplift, and unstable, humid air aloft.

Structure. Air spirals inward, rises in the eyewall — a ring of towering cumulonimbus where the strongest winds and heaviest rain occur — and diverges at altitude. Air subsides in the eye, which is therefore calm, warm and often cloud-free, with the lowest surface pressure. Outside the eyewall, spiral rain bands feed the system. Latent heat released by condensation is the energy source, so the storm intensifies over warm water and decays over land or cool water when that source is cut.

• Saffir–Simpson scale — categories 1–5, based on sustained wind speed. Note the limitation: it says nothing about rainfall or surge, yet water, not wind, causes most deaths.
• Storm surge — the dominant killer. Low central pressure allows the sea to rise, and onshore winds pile water against the coast; the effect is amplified by a shallow, funnel-shaped coast and by coincidence with high tide.
• High winds — structural damage, flying debris, downed power lines. Inland flooding from extreme rainfall — often the largest footprint of damage, and the reason storms remain lethal after landfall.

Why not on the equator?

Fire: conditions, causes and a paradox

A wildfire needs the fire triangle — fuel, oxygen and heat — but the geography lies in what makes fuel available and fire spread:

• Vegetation type — fine, dry, resinous or oil-rich fuels (eucalypt, chaparral, conifer, grass) ignite readily and carry fire fast. Fuel load and continuity matter as much as species.
• Fuel moisture content — the master variable. Drought, heatwave and low humidity dry the fuel; a wet spring that grows abundant grass followed by a hot dry summer that cures it is the classic dangerous sequence.
• Wind — supplies oxygen, bends flames forward, pre-heats the fuel ahead, and carries embers ("spotting") that start new fires far beyond the front. Hot dry downslope winds are notorious.
• Relief — fire travels faster uphill, because rising heat pre-heats and dries the fuel above the flame front; slope also channels wind through valleys.
• Ignition — natural (lightning, rarely volcanic activity) but the great majority are human: power lines, machinery, campfires, discarded cigarettes, arson.

The fire-suppression paradox. Many ecosystems are fire-adapted: periodic low-intensity fire clears understorey, releases nutrients and triggers germination in serotinous species. Decades of aggressive suppression allow fuel to accumulate, so when a fire finally escapes control it burns at far higher intensity than the ecosystem — or the community within it — has ever experienced. Suppression can therefore increase long-run hazard. This is why prescribed burning and fuel-load management are now central to fire policy, and it is a gift of an evaluative point in an essay.

Vulnerability is concentrated at the wildland–urban interface, where housing pushes into flammable vegetation. Management therefore mixes prediction (fire-danger indices from weather and fuel moisture), protection (defensible space, ember-proof construction, firebreaks), preparedness (evacuation plans, "leave early or stay and defend" doctrine, community education) and adaptation (land-use planning that stops building in the highest-risk zones at all).

Name that hazard process

Tap a description on the left, then the process it defines. Using the precise term — and then explaining the mechanism — is what lifts an answer.

Prediction, protection, preparedness, adaptation

AQA asks you to apply the same four-way framework to every hazard — and, crucially, to notice that it works better for some hazards than others.

• Prediction. Volcanoes: generally good — seismic swarms, ground deformation, gas emission and thermal anomalies give days-to-weeks warning. Storms: good and improving — satellite tracking gives days. Wildfires: conditions can be forecast, ignition cannot. Earthquakes: the honest answer is that short-term prediction remains unachievable; only long-term probabilistic forecasting (seismic gaps, recurrence intervals) is possible. This asymmetry is a superb evaluative axis.
• Protection. Engineering the environment and the built form: aseismic design (cross-bracing, base isolation, damped counterweights), sea walls and surge barriers, lava diversion channels and cooling, firebreaks and defensible space, cyclone shelters.
• Preparedness. The human software: warning and evacuation systems, drills and education, emergency stockpiles, land-use zoning, insurance, hazard mapping.
• Adaptation / mitigation. Longer-run adjustment: relocating settlement away from the highest-risk zone, changing crops and livelihoods, restoring mangroves and coastal wetlands to absorb surge, prescribed burning to cut fuel loads.

Contrasting levels of development. The physical event does not discriminate; the outcome does. Higher-income countries generally suffer greater absolute economic losses (higher-value assets) but far fewer deaths — enforced building codes, functioning warning systems, insurance and rapid emergency services. Lower-income and emerging countries typically suffer higher mortality and a much larger relative economic loss, driven by informal housing, unenforced codes, weak warning coverage and dependence on aid. Beware the lazy version of this: governance and enforcement, not GDP alone, do the work — a wealthy state with corrupt building inspection can perform very badly, and a poorer state with an effective cyclone-shelter and warning programme can perform very well.

Where prediction fails

A multi-hazardous place, and a specific event

AQA requires two case-study types. Learn them as arguments, not as fact-lists.

1. A multi-hazardous place — somewhere exposed to several hazards at once, to illustrate the human response to risk. Japan, the Philippines, Iceland and California are the usual choices. The structure that scores:

• Why is it multi-hazardous? Tie each hazard to a physical cause — e.g. a place on a subduction margin gets earthquakes and composite volcanism and tsunami; a tropical maritime location adds typhoons; steep relief and heavy rain add landslides and lahars.
• Why do people still live there? Fertile volcanic soils, geothermal energy, mineral resources, coastal trade, ports, tourism, inertia and place attachment — the benefits are real and continuous, the hazard is episodic.
• How has society responded? Perception (fatalism vs adaptation), then the four Ps, then the cost of them.
• Cascading hazards: the strongest material. A single trigger can set off a chain — an offshore megathrust earthquake causes a tsunami, which floods low-lying industrial or nuclear infrastructure, which produces a technological hazard on top of the natural one. Park's single-event curve struggles with this.

2. A specific hazard event — to illustrate its nature, impacts and responses. Structure it around primary vs secondary impacts, social / economic / environmental / political effects, and short-term relief vs long-term reconstruction, then evaluate against the Park curve: did this place get back to normal, or better than before, or worse?

Statistics: be honest. Examiners reward a precise mechanism far more than a half-remembered number, and a confidently wrong death toll or magnitude damages your credibility. Quote a figure only if you are certain of it; otherwise write "of the order of", "widely reported as", or simply describe the scale qualitatively — "losses fell overwhelmingly on informal housing on unconsolidated coastal sediment, where liquefaction was severe" is worth more than an invented percentage.

Writing the evaluative essay

Hazards essays are almost always comparative or conditional: "To what extent is the impact of a hazard determined by the level of development of the country affected?" or "Assess the effectiveness of hazard management strategies." A reliable approach:

• Unpack the command and the key term. "Impact" — deaths? economic loss? long-term development? Those three answers point in different directions, and saying so in the introduction is instant AO2.
• Build a conceptual spine. Use Degg (hazard × vulnerability), Park (recovery trajectory) and the management cycle explicitly, as tools of argument — not as decoration to be defined and abandoned.
• Argue in blocks: claim → mechanism → evidence → counterweight. Every paragraph should contain a "but".
• Distinguish scale and timescale. An event can be catastrophic locally and trivial nationally; a country can recover economically in two years while a neighbourhood does not recover in twenty.
• Conclude with judgement, on a condition. Not "there are many factors". Say to what extent and on what it depends.

Try it: "Assess the extent to which the impacts of a tectonic hazard are determined by a country's level of economic development." Three blocks — (1) development buys protection: enforced aseismic codes, warning systems, emergency services → far lower mortality; (2) but it also concentrates high-value assets, so absolute economic loss is often higher in richer countries; (3) and development is a crude proxy — governance, enforcement, corruption, hazard-specific preparedness and the physical characteristics of the event (magnitude, depth, time of day, ground conditions) all cut across it. Judgement: development strongly predicts mortality but poorly predicts economic loss, and its effect is mediated by governance rather than wealth as such.

Sharpening the judgement

The big ideas to know

That is the core of AQA 3.1.5. Press Finish to see your score.