What are the hazards of a volcanic eruption, and how do we forecast and mitigate them?
Volcanic hazards: the hazards of an eruption (lava flows, pyroclastic flows, ash falls, lahars, volcanic gases and sector collapse) and how they relate to magma type and the Volcanic Explosivity Index; the methods of monitoring a volcano (seismicity, ground deformation, gas emissions and thermal anomalies); the use of hazard maps, exclusion zones and evacuation to mitigate risk; the comparison with earthquakes in terms of predictability.
A focused answer to the OCR H414 dot point on volcanic hazards. Covers the hazards of an eruption (lava, pyroclastic flows, ash, lahars, gases, sector collapse) and their link to magma type and explosivity, the monitoring methods (seismicity, ground deformation, gas, thermal), hazard maps, exclusion zones and evacuation, and how volcanoes compare with earthquakes for predictability.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
OCR wants you to describe the hazards of a volcanic eruption and how they relate to magma type and explosivity, to describe the methods of monitoring a volcano, to explain the use of hazard maps, exclusion zones and evacuation, and to compare volcanoes with earthquakes for predictability.
The answer
Volcanic hazards and their link to magma type
The hazards depend on the magma type and so the explosivity (measured by the Volcanic Explosivity Index, VEI):
- Lava flows. Mainly from basaltic (effusive) volcanoes; usually slow enough to escape, so they threaten property more than life.
- Pyroclastic flows. From explosive (andesitic and rhyolitic) volcanoes; fast, hot mixtures of gas and fragments that incinerate and bury everything in their path. The most lethal hazard.
- Ash falls. Collapse roofs, ruin crops, damage lungs and stop aircraft engines; spread widely from explosive eruptions.
- Lahars. Volcanic mudflows formed when ash mixes with water (rain or melted snow and ice); fast and far-reaching.
- Volcanic gases. Carbon dioxide and sulfur dioxide can suffocate or poison.
- Sector collapse. Part of the volcano's flank fails catastrophically.
Viscous, gas-rich (andesitic and rhyolitic) magma erupts explosively, producing the deadliest hazards (pyroclastic flows, ash, lahars); runny basaltic magma erupts gently, mainly producing lava flows.
Monitoring a volcano
Unlike earthquakes, volcanoes usually give warning signs that can be monitored:
- Seismicity. Increasing small earthquakes show magma moving and fracturing rock.
- Ground deformation. Tiltmeters, GPS and satellite radar (InSAR) detect the surface swelling (inflating) as magma accumulates.
- Gas emissions. Rising sulfur dioxide and carbon dioxide can signal magma nearing the surface.
- Thermal anomalies. Increasing heat (detected by satellite) shows magma rising.
Mitigation
- Hazard maps show which areas are at risk from each hazard, guiding land-use planning.
- Exclusion zones keep people out of the most dangerous areas.
- Evacuation moves people away before an eruption, once monitoring indicates rising risk.
Comparison with earthquakes
Volcanoes are generally more predictable than earthquakes: because magma movement produces measurable precursors (seismicity, deformation, gas, heat), monitoring can often forecast an eruption days to weeks ahead and trigger evacuation. Earthquakes, by contrast, strike with essentially no short-term warning, so the defence is mitigation rather than forecasting.
Examples in context
Example 1. Pyroclastic flows as the deadliest hazard. Explosive eruptions of andesitic stratovolcanoes have repeatedly produced pyroclastic flows that destroyed towns within minutes, showing why magma type and explosivity dominate the hazard.
Example 2. Successful evacuation from monitoring. Where rising seismicity, ground inflation and gas emissions were detected in time, authorities have evacuated tens of thousands of people before major eruptions, saving many lives and illustrating effective forecasting and mitigation.
Try this
Q1. Name the most lethal volcanic hazard and explain why it is so dangerous. [2 marks]
- Cue. Pyroclastic flows: fast, hot mixtures of gas and fragments that incinerate and bury everything in their path, giving little chance of escape.
Q2. Describe one method of monitoring a volcano and what it reveals. [2 marks]
- Cue. For example ground deformation (tiltmeters, GPS, InSAR): inflation of the volcano shows magma accumulating and pressure building beneath, warning of an eruption.
Q3. Explain why volcanoes are generally more predictable than earthquakes. [2 marks]
- Cue. Magma movement produces measurable precursors (seismicity, ground inflation, gas, heat) that can be monitored to forecast an eruption, whereas earthquakes give essentially no short-term warning.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/02 20196 marksExplain why an andesitic stratovolcano poses a greater range of life-threatening hazards than a basaltic shield volcano, referring to magma type and explosivity.Show worked answer →
A level-of-response answer; link magma type to explosivity to hazards.
- Magma type and explosivity
- An andesitic stratovolcano has viscous, gas-rich magma that traps gas until it erupts explosively (a high Volcanic Explosivity Index). A basaltic shield volcano has runny magma from which gas escapes easily, so it erupts gently (effusively).
- Hazards of the stratovolcano
- Explosive eruptions produce the most lethal hazards: pyroclastic flows (fast, hot mixtures of gas and fragments that incinerate and bury everything in their path), heavy ash falls (which collapse roofs, ruin crops and damage lungs and engines), lahars (volcanic mudflows when ash mixes with water or melted snow), and dangerous volcanic gases. Sector collapse can also occur.
- Hazards of the shield volcano
- A basaltic shield volcano mainly produces lava flows, which are slow enough that people can usually move out of the way, so they threaten property more than life.
- Conclusion
- Because the andesitic stratovolcano erupts explosively, it produces fast, far-reaching and deadly hazards (especially pyroclastic flows and ash), whereas the basaltic shield volcano's gentle lava flows are far less lethal.
Top-band answers link viscous gas-rich magma to explosive eruptions and to the specific deadly hazards, contrasting with the effusive, mainly property-threatening basaltic case.
OCR H414/01 20204 marksDescribe two methods used to monitor a volcano and explain what each can reveal about an impending eruption.Show worked answer →
Choose two monitoring methods and link each to a warning sign.
Seismic monitoring. Seismometers detect the small earthquakes caused by magma moving and fracturing rock beneath the volcano. An increase in the number and intensity of these earthquakes can indicate that magma is rising towards the surface, warning of a possible eruption.
Ground deformation. Tiltmeters, GPS and satellite radar (InSAR) measure swelling of the volcano's surface. Inflation (the ground bulging) shows that magma is accumulating in the chamber beneath, building pressure, which can precede an eruption; deflation may follow an eruption.
(Other valid methods: measuring gas emissions, for example rising sulfur dioxide, and detecting thermal anomalies.) Markers reward two methods with a correct interpretation of each (rising seismicity and ground inflation both indicating magma movement and rising pressure).
Related dot points
- Volcanism: the control of magma composition (silica content), viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions and andesitic or rhyolitic explosive eruptions; volcanic products (lava, tephra, pyroclastic flows and gases); volcanic landforms (shield volcanoes, stratovolcanoes, calderas and fissures); the link between volcanism and plate setting.
A focused answer to the OCR H414 dot point on volcanism. Covers how silica content, viscosity and gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, volcanic products (lava, tephra, pyroclastic flows, gases), the landforms (shield, stratovolcano, caldera, fissure), and the link to plate setting.
- Earthquake hazards: the primary and secondary hazards (ground shaking, surface rupture, liquefaction, landslides and tsunamis); the distinction between hazard, vulnerability, exposure and risk; the factors that determine the impact of an earthquake (magnitude, depth, ground conditions, population density, building design and preparedness); monitoring and mitigation (building codes, land-use planning, early-warning systems and education); the limits of earthquake prediction.
A focused answer to the OCR H414 dot point on earthquake hazards. Covers primary and secondary hazards (shaking, surface rupture, liquefaction, landslides, tsunamis), the distinction between hazard, vulnerability, exposure and risk, the factors controlling impact, monitoring and mitigation, and the limits of earthquake prediction.
- Mass movement: the types of slope failure (rockfall, translational and rotational slides, slumps and debris flows); the balance of driving and resisting forces on a slope; the factors that trigger failure (slope angle, rock and soil type, water content, discontinuities, weathering, earthquakes and human activity); the recognition of warning signs; the engineering methods used to stabilise slopes and reduce risk.
A focused answer to the OCR H414 dot point on mass movement. Covers the types of slope failure (rockfall, translational and rotational slides, slumps, debris flows), the balance of driving and resisting forces, the triggers of failure (slope angle, rock type, water, discontinuities, earthquakes, human activity), warning signs, and the engineering methods used to stabilise slopes.
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
- Engineering geology: the engineering properties of rocks and soils (strength, jointing and discontinuities, weathering and the behaviour of clays, sands and gravels); the purpose and methods of site investigation (desk study, boreholes, trial pits and core logging); the ground conditions that cause problems for foundations (weak or compressible soils, swelling clays, solution cavities in limestone, made ground and high groundwater); the role of foundations and the ground model.
A focused answer to the OCR H414 dot point on engineering geology. Covers the engineering properties of rocks and soils, the purpose and methods of site investigation (desk study, boreholes, trial pits, core logging), the ground conditions that cause foundation problems (weak or swelling soils, solution cavities, made ground, groundwater), and the role of foundations and the ground model.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)