Notes · GeographyUK · A-Levels

Hazards

An optional physical topic analysing natural hazards - their nature, perception and management - and the tectonic, atmospheric and other processes that generate them. It covers hazard concepts and response models, plate tectonics, volcanic and seismic hazards, tropical storms and wildfires, and the evaluation of impacts and responses in places of contrasting development.

6 sections~22 min reading time3 competenciesLevel Standard 4 · Advanced 2

T·0555 / 12

Exam profile

AO1 · Understand hazard concepts, plate tectonics and the characteristics of volcanic, seismic, storm and wildfire hazardsAO2 · Apply hazard models and evaluate the effectiveness of responses in contrasting placesAO3 · Interpret hazard-distribution maps, magnitude-frequency data and impact statistics

Operators:explainanalyseassessevaluateto what extentexaminedescribe the distributioninterpret

basic level

At AS-Level the focus is on describing hazard types, plate tectonics and the impacts of hazards.

higher level

The full A-Level requires the response models, the mechanisms of each hazard, and evaluative comparison of impacts and responses across contrasting places.

Depth

Text

Contents · 6 sections

Hazards

The nature and perception of hazards#

StandardAQA 7037 3.1.5DfE GCE Geography - hazards

The Park disaster-response curve

Fig. 1Illustrative Park model: quality of life falls at the disaster, bottoms out in relief, then recovers - potentially above the original level.

A natural hazard is a natural event or process that threatens people and property; a hazard becomes a disaster only when it actually causes serious loss of life or damage that a community cannot cope with using its own resources. This distinction matters because the same physical event - an earthquake of a given magnitude, say - can be a minor incident in one place and a catastrophe in another, depending on how exposed and vulnerable the population is and how well they can respond. Hazards are grouped into geophysical (tectonic, such as earthquakes and volcanoes), atmospheric (such as tropical storms and wildfires) and hydrological (such as floods) types.

The scale of a disaster is therefore not just about the physical size of the event but about the interaction between the hazard and the human population. This is captured by the idea that risk rises with the magnitude of the hazard and the vulnerability of the people, and falls with their capacity to cope. A powerful hazard striking a wealthy, prepared, low-density population may cause few deaths, while a moderate hazard striking a poor, densely settled, unprepared population may kill thousands - which is why disasters disproportionately affect lower-income countries.

How people perceive hazards strongly shapes how they respond, and perception varies with experience, wealth, education, religion and personality. Responses range along a spectrum: fatalism (accepting the hazard as unavoidable, perhaps for religious reasons, and doing little); adaptation and adjustment (changing behaviour or building to reduce the risk); prediction and warning (using science to forecast events and evacuate); and mitigation and management (reducing the hazard's impact through engineering, planning and preparation). Wealthier and better-informed communities tend to move towards active management.

Two models organise the human response over time. The Park model (the disaster-response curve) plots the quality of life before, during and after a disaster: a period of stability, a sharp drop as the disaster strikes, a relief phase at the low point, then rehabilitation and reconstruction that may restore the community to its original state, leave it worse off, or - with good planning and investment - leave it better prepared than before. The hazard management cycle frames response as a continuous loop of preparedness, response, recovery and mitigation. Both stress that managing hazards is an ongoing process, not a single reaction.

\text{Risk} = \dfrac{\text{Hazard} \times \text{Vulnerability}}{\text{Capacity to cope}}

The risk relationship

Disaster risk rises with the magnitude of the hazard and the vulnerability of the people, and falls with their capacity to cope - which is why the same event has very different effects in different places.

Classification of natural hazards

Fig. 2Hazards are grouped by the system that generates them: geophysical, atmospheric and hydrological.

Worked example

Applying the Park model

Using the Park model, describe how the quality of life of a community changes from just before a major earthquake through to full recovery, and explain what determines the final outcome.

Before and during
Quality of life is stable at the pre-disaster level, then drops sharply as the earthquake strikes, destroying homes, services and livelihoods.
Relief and rehabilitation
It bottoms out in the relief phase (search and rescue, emergency aid), then rises through rehabilitation as services and temporary housing are restored.
Reconstruction and outcome
Reconstruction rebuilds permanently; with good planning and investment (e.g. earthquake-resistant building) the community can end up better prepared than before, but with poor recovery it may be left worse off.

Result: Quality of life dips sharply then recovers through relief, rehabilitation and reconstruction; the final level depends on the quality of the recovery and investment in resilience.

Typical mistakes

Treating disaster severity as purely physical - vulnerability and capacity to cope are decisive, which is why low-income countries suffer more.
Describing the Park model as a single fixed curve - recovery can leave a community better, the same, or worse off than before.

Active revision

Explain why an earthquake of a given magnitude may be a minor event in one country but a major disaster in another.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Plate tectonics and the global distribution#

StandardAQA 7037 3.1.5DfE GCE Geography - plate tectonics

A destructive (subduction) plate margin

Fig. 3At a destructive margin the denser oceanic plate subducts, forming a trench, melting to feed explosive volcanoes and generating deep earthquakes.

The distribution of earthquakes and volcanoes is explained by plate tectonics. The Earth's rigid outer shell (the lithosphere) is broken into plates that float on the mobile asthenosphere below and move a few centimetres a year. The plates are driven by the Earth's internal heat: convection currents in the mantle, and - now regarded as more important - the forces of ridge push (newly formed, cooling ocean lithosphere sliding away from the mid-ocean ridge) and slab pull (dense, old ocean lithosphere sinking at subduction zones and dragging the rest of the plate with it). Volcanoes and earthquakes cluster along the plate margins, which is why their global distribution is linear rather than random.

There are four types of plate margin. At constructive (divergent) margins the plates move apart and magma rises to form new crust, building mid-ocean ridges and rift valleys with gentle volcanic activity and shallow earthquakes. At destructive (convergent) margins an oceanic plate is subducted beneath another plate; the descending slab melts, feeding explosive volcanoes, and the friction generates powerful, deep earthquakes and ocean trenches. At collision margins two continental plates meet and neither subducts, so the crust is crumpled upward into fold mountains with major earthquakes but little volcanism. At conservative (transform) margins plates slide past each other, generating earthquakes but no volcanoes.

Not all volcanic activity occurs at plate margins. Hot spots (mantle plumes) are fixed columns of rising magma that burn through the middle of a plate; as the plate moves over the plume, a chain of volcanoes is built, as in the Hawaiian islands. Recognising that a volcano sits over a hot spot rather than a margin is a common discriminating point in the exam, and it explains volcanic activity far from any plate boundary.

The type of margin determines the character of the hazard, so identifying the margin is the key to explaining a tectonic event. Destructive margins give the most explosive volcanoes and the deepest, most powerful earthquakes and tsunamis; constructive margins give effusive eruptions and gentler earthquakes; conservative margins give earthquakes without volcanoes. The Pacific Ring of Fire, a nearly continuous belt of destructive and conservative margins around the Pacific, concentrates the majority of the world's earthquakes and explosive volcanoes and illustrates this link between margin type and hazard.

What drives plate movement

Fig. 4Internal heat drives convection, ridge push and slab pull, moving the plates and producing the four margin types.

Worked example

Identifying a margin from its hazards

A region experiences frequent, powerful, deep-focus earthquakes and explosive, andesitic volcanoes, and lies along an ocean trench. Identify the plate margin and explain the link.

Read the evidence
An ocean trench, deep and powerful earthquakes, and explosive volcanoes together point to subduction.
Identify the margin
These are the signatures of a destructive (convergent) margin, where an oceanic plate subducts beneath another plate.
Explain the link
The subducting slab melts to feed explosive, gas-rich magma (volcanoes), while friction as the slab descends stores and releases huge strain (deep, powerful earthquakes), and the descent forms the trench.

Result: It is a destructive margin: subduction produces the trench, the explosive volcanoes and the deep, powerful earthquakes.

Typical mistakes

Explaining plate movement by convection alone - ridge push and slab pull are now regarded as the main drivers.
Assuming all volcanoes lie on plate margins - hot spots produce volcanoes in the middle of plates.

Active revision

Explain why the most explosive volcanoes and the deepest earthquakes are associated with destructive plate margins.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Volcanic hazards#

StandardAQA 7037 3.1.5DfE GCE Geography - volcanic hazards

Volcanic hazards: primary and secondary

Fig. 5An eruption generates primary hazards directly and secondary hazards such as lahars that travel far beyond the volcano.

Volcanic hazards are the dangers posed by eruptions, and their nature depends on the magma. Runny, low-silica (basaltic) magma at constructive margins and hot spots produces effusive eruptions with gentle lava flows; sticky, high-silica (andesitic and rhyolitic) magma at destructive margins traps gas and produces violent, explosive eruptions. The magnitude of an eruption is measured on the Volcanic Explosivity Index (VEI), a logarithmic scale based on the volume of material ejected, so each step up represents a roughly tenfold increase.

The direct (primary) hazards are those of the eruption itself. Lava flows destroy everything in their path but usually move slowly enough to escape; pyroclastic flows - fast, superheated avalanches of gas, ash and rock - are the deadliest, racing downslope at great speed and incinerating everything; tephra (ash and rock fragments) is thrown into the air and falls over a wide area, collapsing roofs and disrupting agriculture and aviation; and volcanic gases can be toxic and contribute to climate effects.

The indirect (secondary) hazards can be even more damaging. Lahars are mudflows formed when ash mixes with water (from rain, or from melting snow and ice on the volcano) and rush down valleys with devastating force. Volcanic activity can also trigger flooding (from melted ice), landslides, and - where large eruptions inject ash and sulphur into the atmosphere - short-term climate cooling. Because these secondary hazards travel far from the volcano, they widen the zone of risk well beyond the immediate slopes.

The impacts and responses follow the pattern set out in the hazard concepts. Impacts are social (deaths, injuries, displacement), economic (destruction of property, farmland and infrastructure, loss of tourism) and environmental (destruction of ecosystems, ash deposition), and they hit poorer, less-prepared communities hardest. Responses centre on prediction and preparation, because eruptions can often be forecast: monitoring ground deformation, gas emissions and seismic activity allows evacuation, while hazard mapping, land-use zoning and diverting lava reduce the impact. Evaluating why an eruption of a given size caused very different losses in two places is the key higher-level task.

Worked example

Explaining the deadliness of a secondary hazard

A snow-capped volcano erupts and, although the eruption itself is moderate, a town in a valley many kilometres away is destroyed. Explain the hazard responsible.

The trigger
The eruption melts the snow and ice on the volcano and mixes the meltwater with ash and loose debris.
The flow
This forms a lahar - a fast, dense volcanic mudflow - which is funnelled down the river valleys away from the volcano.
The impact
The lahar travels far beyond the crater and can bury a distant town with little warning, which is why a moderate eruption can be so deadly downstream.

Result: A lahar - meltwater mixed with ash - flowed down a valley and destroyed a distant town, showing how secondary hazards extend the danger zone.

Typical mistakes

Assuming lava flows are the main killer - pyroclastic flows and lahars cause far more deaths.
Treating the VEI as a linear scale - it is logarithmic, so each step is roughly a tenfold increase in ejected material.

Active revision

Explain why the impacts of a volcanic eruption of a given magnitude can differ greatly between a developed and a developing country.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Seismic hazards#

AdvancedAQA 7037 3.1.5DfE GCE Geography - seismic hazards

The seismic hazard chain

Fig. 6Stress released on a fault sends out seismic waves whose shaking triggers a range of secondary hazards.

Earthquakes occur where stress built up as plates move is suddenly released along a fault. The point underground where the rupture begins is the focus (or hypocentre); the point on the surface directly above it is the epicentre, where shaking is usually strongest. The energy travels outward as seismic waves: fast P (primary) waves, slower S (secondary) waves, and the surface waves that do most of the damage. A shallow focus generally produces more destructive surface shaking than a deep one of the same size.

Two kinds of scale describe an earthquake, and confusing them is a common error. Magnitude measures the energy released and is now given by the moment magnitude scale (which superseded the Richter scale for large events); it is logarithmic, so each whole number is about 32 times more energy. Intensity measures the effects at a place and is given by the modified Mercalli scale (I to XII), based on observed shaking and damage; intensity falls with distance from the epicentre, so one earthquake has a single magnitude but many intensities.

Much of the damage comes from secondary hazards triggered by the shaking. Liquefaction occurs where violent shaking turns saturated, loose sediment into a fluid, so buildings sink or tilt; landslides are shaken loose on steep slopes; fires break out where gas and power lines rupture; and, most dangerously, a submarine earthquake that displaces the sea floor can generate a tsunami - a fast, low wave in the open ocean that rises into a devastating surge as it reaches shallow water. These secondary hazards often cause more casualties than the shaking itself.

Because earthquakes cannot yet be reliably predicted, management relies on the three Ps: prediction (limited - monitoring and probability forecasting rather than precise warning), protection (earthquake-resistant building design, retrofitting, and land-use planning to avoid the most vulnerable ground) and preparation (education, drills, warning systems and emergency planning). The stark contrast in death tolls between well-prepared, wealthy regions and poorer, densely settled ones for earthquakes of similar magnitude is the clearest evidence that vulnerability and capacity to cope, not magnitude alone, determine the scale of the disaster.

Worked example

Magnitude versus intensity

An earthquake is reported as moment magnitude 7.0, but two towns record modified Mercalli intensities of IX and V respectively. Explain how one earthquake can have a single magnitude but two intensities.

Magnitude
Magnitude measures the total energy released at the source, so the earthquake has one value - 7.0 - regardless of where it is felt.
Intensity
Intensity measures the shaking and damage experienced at a place, which depends on distance from the epicentre, the local geology and the buildings.
The explanation
The town nearer the epicentre (or on soft ground) suffers stronger shaking (intensity IX), while the more distant town feels much less (intensity V) - so one magnitude produces many intensities.

Result: Magnitude is a single measure of energy at the source; intensity varies with location, so the same earthquake is felt at different intensities in different places.

Typical mistakes

Confusing focus (underground rupture point) with epicentre (the surface point above it).
Treating the magnitude scale as linear - it is logarithmic, so a one-point rise is about 32 times more energy.

Active revision

'The secondary hazards of an earthquake are more significant than the shaking itself.' Assess this statement.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Storm hazards: tropical cyclones#

StandardAQA 7037 3.1.5DfE GCE Geography - tropical storms

Structure of a tropical cyclone

Fig. 7A calm eye of descending air is ringed by the eyewall of strongest winds; warm moist air flows in at the surface and out at the top.

Tropical cyclones (called hurricanes or typhoons in different oceans) are intense low-pressure systems that form over warm tropical seas. They require several conditions together: a sea-surface temperature above about 27 degrees Celsius to supply heat and moisture, an ocean depth of at least 50-60 metres, a location roughly 5-20 degrees from the equator (so that the Coriolis effect can start the system spinning, but not on the equator itself where it is zero), low wind shear so the storm is not torn apart, and converging, rising air. This is why their distribution is confined to particular tropical ocean basins and seasons.

The storm is powered by latent heat. Warm ocean water evaporates and the moist air rises, cools and condenses, releasing latent heat that warms the surrounding air, causing it to rise further and drawing in more warm, moist air at the surface - a self-sustaining engine that intensifies as long as the storm stays over warm water. It weakens rapidly once it moves over land or cooler water, because its energy supply is cut off. Its structure is distinctive: a calm central eye of descending air, surrounded by the eyewall of the tallest clouds and strongest winds, with spiralling rain bands beyond.

The magnitude of a tropical cyclone is measured on the Saffir-Simpson scale, categories 1 to 5, based on sustained wind speed. The hazards are threefold: extreme winds that destroy buildings and infrastructure; torrential rain that causes river and flash flooding and landslides; and, most deadly, the storm surge - a dome of seawater pushed ashore by the winds and low pressure, which floods low-lying coasts and causes the majority of deaths. As with other hazards, impacts fall hardest on poorer, densely settled, low-lying coasts with weak defences.

Responses combine prediction, protection and preparation. Because cyclones can be tracked by satellite, forecasting and warning are relatively effective, allowing evacuation - the single most important life-saving measure. Protection includes building codes, flood defences, sea walls and the preservation of natural buffers such as mangroves, while preparation involves shelters, education and emergency planning. A warming ocean is expected to make the most intense storms more powerful and to raise the baseline sea level on which surges act, so evaluating how far management can keep pace with a changing hazard is an increasingly important judgement.

The Saffir-Simpson scale

Fig. 8The five Saffir-Simpson categories, defined by minimum sustained wind speed (km/h).

Worked example

Why a cyclone weakens over land

A tropical cyclone maintains its strength for days over the ocean but weakens rapidly within hours of making landfall. Explain why.

The energy source
The storm is powered by latent heat released as water evaporated from warm ocean water condenses; the warm sea is its fuel.
Landfall
Over land there is no warm ocean to supply evaporation and moisture, so the supply of latent heat is cut off.
The result
Without its energy source, and with increased friction over the rougher land surface, the storm's central pressure rises and its winds weaken rapidly.

Result: The cyclone weakens because landfall removes the warm-ocean moisture that supplies its latent-heat engine, and land friction slows it.

Typical mistakes

Saying cyclones form on the equator - they need the Coriolis effect, which is zero at the equator, so they form 5-20 degrees away.
Underrating the storm surge - it, not the wind, causes the majority of deaths in most tropical cyclones.

Active revision

Explain the conditions necessary for a tropical cyclone to form and why it weakens once it makes landfall.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

Wildfires and comparative case studies#

AdvancedAQA 7037 3.1.5DfE GCE Geography - wildfires and case studies

The fire triangle

Fig. 9A wildfire needs fuel, oxygen and heat together; removing any one - the basis of firefighting - stops the fire.

Wildfires are uncontrolled fires in areas of combustible vegetation, and they need three things together - fuel, oxygen and heat (an ignition source) - often summarised as the fire triangle. The conditions that favour intense wildfires are therefore vegetation that provides plenty of dry fuel (such as forests, scrub and grassland), a climate with a wet season to grow the fuel followed by a dry, hot season to dry it out, and strong winds to spread the flames. Ignition may be natural (lightning) or, more commonly, human (accidents, arson, discarded cigarettes, power lines).

The behaviour and spread of a wildfire depend on the fuel, the weather and the terrain. Dry, fine fuel ignites and spreads fastest; high temperatures, low humidity and drought prime the landscape; and strong winds drive the fire and carry embers ahead of the front. Fire spreads uphill faster than downhill because the flames preheat the slope above. These controls explain why some regions - such as those with a Mediterranean-type climate - experience a regular and intensifying fire season.

The impacts are social, economic and environmental. Wildfires kill and injure people, destroy homes and infrastructure, and produce smoke that damages health over wide areas; they impose huge firefighting and rebuilding costs and disrupt economies; and they destroy habitats and release large amounts of stored carbon, though some ecosystems are fire-adapted and depend on periodic fire to regenerate. Responses centre on prevention (fuel management, controlled burning, public education, planning restrictions), prediction (fire-danger forecasting) and response (firefighting, evacuation and warning systems).

The synoptic skill the specification rewards is comparison: setting two contrasting hazard events side by side to evaluate why their impacts and the effectiveness of their management differed. The recurring lesson is that the physical magnitude of an event is only part of the story - the level of development, the density and vulnerability of the population, the quality of governance, prediction and preparation, and the resources available for response together determine whether a hazard becomes a manageable emergency or a catastrophe. A strong comparative answer isolates these human factors and reaches a supported judgement rather than simply narrating two events.

Conditions and controls on wildfires

Fig. 10Dry fuel, hot dry weather, wind and an ignition source combine to start and spread a wildfire.

Worked example

Comparing responses in contrasting places

Two countries of contrasting wealth experience earthquakes of similar magnitude, but one records far more deaths. Explain how the human geography, rather than the magnitude, accounts for the difference.

Equal physical magnitude
Since the magnitudes are similar, the energy released is comparable, so the difference in deaths cannot be explained by the physical event alone.
Vulnerability and preparation
The wealthier country has earthquake-resistant buildings, enforced building codes, warning systems, drills and well-resourced emergency services, reducing casualties.
The judgement
The poorer country's weaker buildings, denser and more vulnerable population and limited capacity to respond mean the same magnitude causes far greater loss - so development and preparation, not magnitude, explain the contrast.

Result: With magnitude held equal, the difference in deaths is explained by the affected areas' vulnerability, building quality, preparation and capacity to respond - human, not physical, factors.

Typical mistakes

Listing wildfire impacts without linking them to the conditions (fuel, weather, wind) that governed the event.
In comparisons, narrating two events instead of analysing why development, vulnerability and preparation made their impacts differ.

Active revision

'The impact of a hazard depends more on the level of development of the affected area than on the magnitude of the event.' To what extent do you agree, using two contrasting examples?

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for geography (Department for Education) · AQA A-level Geography 7037 specification (AQA)

References & sources

Sources

Department for Education

GCE AS and A level subject content for geography

AQA

AQA A-level Geography 7037 specification

Hazards

6 sections~22 min reading time3 competenciesLevel Standard 4 · Advanced 2

Hazards

Key points

Applying the Park model

Key points

Identifying a margin from its hazards

Key points

Explaining the deadliness of a secondary hazard

Key points

Magnitude versus intensity

Key points

Why a cyclone weakens over land

Key points

Comparing responses in contrasting places

Sources

Hazards

Key points

Applying the Park model

Key points

Identifying a margin from its hazards

Key points

Explaining the deadliness of a secondary hazard

Key points

Magnitude versus intensity

Key points

Why a cyclone weakens over land

Key points

Comparing responses in contrasting places

Sources