What are the hazards of a volcanic eruption, and how are volcanoes monitored?
Volcanic hazards and monitoring: the primary and secondary hazards of volcanic eruptions (lava flows, pyroclastic flows, tephra and ash fall, lahars, gases, sector collapse); the control of magma composition on eruption style and hazard; and the monitoring and prediction methods (seismicity, ground deformation, gas emissions, thermal and historical records) used to forecast eruptions.
A focused answer to the Eduqas Geology geohazards statement on volcanoes. Covers primary and secondary volcanic hazards, how magma composition controls eruption style and hazard, and the monitoring methods (seismicity, ground deformation, gas, thermal) used to forecast eruptions and reduce risk.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
Part of the Geohazards theme in Component 3. Eduqas wants you to set out the primary and secondary hazards of eruptions (lava, pyroclastic flows, tephra and ash, lahars, gases, sector collapse), to explain how magma composition controls eruption style and hazard, and to describe the monitoring and prediction methods used to forecast eruptions. It applies the volcanic science (silica, viscosity and eruption style) to the human consequences.
The answer
The volcanic hazards
Volcanic hazards range from the merely destructive to the rapidly lethal:
- Lava flows: destroy property but are usually slow enough to escape; the main hazard of effusive basaltic volcanoes.
- Pyroclastic flows (nuees ardentes): fast-moving (over 100 km per hour), hot (hundreds of degrees) ground-hugging clouds of gas, ash and rock fragments. They are the most lethal volcanic hazard because they are too fast to outrun and incinerate everything in their path.
- Tephra and ash fall: fragments thrown out and falling downwind; ash collapses roofs, contaminates water, grounds aircraft and damages lungs and crops.
- Lahars: volcanic mudflows of ash mixed with water (from rain, melted snow or a crater lake) that race down valleys, burying settlements far from the volcano.
- Volcanic gases: carbon dioxide, sulphur dioxide and others can asphyxiate or poison, and contribute to climate effects.
- Sector collapse and debris avalanches: part of the volcano's flank fails catastrophically, and can trigger a tsunami if it enters the sea.
Many of the deadliest hazards (pyroclastic flows, lahars) are secondary consequences of an explosive eruption rather than the lava itself.
Magma composition controls the hazard
The hazard profile follows directly from the magma composition, exactly as for eruption style:
- Basaltic (low silica) magma is runny (low viscosity); gas escapes easily, so eruptions are effusive and the main hazard is lava (low casualties).
- Andesitic and rhyolitic (high silica) magma is viscous; gas is trapped, so eruptions are explosive, producing the lethal pyroclastic flows, heavy tephra and lahars.
So the same factors that build a shield volcano (gentle) or a stratovolcano (steep) also set whether the volcano is mainly a property hazard or a mass-casualty hazard.
Monitoring and prediction
Volcanoes give warning signs as magma rises, so eruptions can often be forecast (unlike earthquakes):
- Seismicity: rising magma fractures rock, producing increasing numbers of small earthquakes and harmonic tremor.
- Ground deformation: the volcano inflates (swells) as magma accumulates, measured by tiltmeters, GPS and satellite InSAR; it deflates after eruption.
- Gas emissions: increasing sulphur dioxide and carbon dioxide as magma nears the surface and degasses.
- Thermal monitoring: rising heat detected by thermal cameras and satellites.
- Historical records: past eruptions give the likely style and recurrence interval.
Combining several signals (rising seismicity plus inflation plus more gas) gives the best forecast, allowing evacuation, the single most effective mitigation. Hazard mapping (of pyroclastic-flow and lahar paths) and land-use planning reduce exposure.
Examples in context
Example 1. Pyroclastic flows at a stratovolcano. Andesitic stratovolcanoes repeatedly demonstrate that the deadliest events are pyroclastic flows and lahars, not the lava, which is why their hazard zones are mapped and evacuated.
Example 2. Inflation before eruption. Satellite InSAR detecting centimetres of ground swelling over months has provided the warning that magma is accumulating, allowing authorities to raise alert levels and prepare evacuations.
Try this
Q1. State the most lethal volcanic hazard and explain why. [2 marks]
- Cue. Pyroclastic flows: they are fast (over 100 km per hour), hot and ground-hugging, so they cannot be outrun and incinerate everything in their path.
Q2. Explain why an andesitic volcano is more hazardous than a basaltic one. [2 marks]
- Cue. Andesitic magma is high in silica, so it is viscous and traps gas, erupting explosively with pyroclastic flows and lahars; basaltic magma is runny and erupts effusively, mainly producing escapable lava.
Q3. Name two volcano monitoring methods and what each detects. [2 marks]
- Cue. Any two: seismicity (earthquakes and tremor from rising magma); ground deformation (inflation as magma accumulates); gas emissions (rising sulphur dioxide and carbon dioxide); thermal monitoring (rising heat).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20206 marksExplain how the composition of the magma controls the eruption style and the main hazards of a volcano, comparing a basaltic shield volcano with an andesitic stratovolcano.Show worked answer →
A levels-of-response answer; link composition to style, then to hazard, for each type.
- Basaltic shield volcano
- Basaltic magma is low in silica, so it has low viscosity and dissolved gas escapes easily. Eruptions are gentle (effusive), dominated by fast but rarely lethal lava flows that build a broad, gently sloping shield. The main hazard is property loss from lava, with few deaths because the flows are slow enough to escape.
- Andesitic stratovolcano
- Andesitic (and rhyolitic) magma is high in silica, so it is very viscous and traps gas; pressure builds until the eruption is violent (explosive). The hazards are far more lethal: pyroclastic flows (fast, hot clouds of gas and ash), heavy tephra and ash fall, lahars (volcanic mudflows) and sector collapse. These build steep stratovolcanoes and cause most volcanic deaths.
- The link
- Low silica means low viscosity, easy gas escape and effusive, lava-dominated, lower-hazard eruptions; high silica means high viscosity, trapped gas, explosive eruptions and the lethal pyroclastic and lahar hazards.
Top-band answers connect silica content to viscosity and gas escape, then to the contrasting eruption styles and their characteristic hazards for each volcano type.
Eduqas 20215 marksDescribe three methods used to monitor a volcano and explain what each can reveal about an impending eruption.Show worked answer →
Give three distinct methods and their interpretation.
- Seismicity
- Networks of seismometers detect earthquakes caused by magma forcing its way upwards and fracturing rock. An increase in the number and a change in the character of these earthquakes (especially harmonic tremor) often precedes an eruption.
- Ground deformation
- Tiltmeters, GPS and satellite (InSAR) measurements detect swelling (inflation) of the volcano as magma accumulates beneath it, and deflation after an eruption. Rapid inflation warns of magma rising.
- Gas emissions
- Measuring gases such as sulphur dioxide and carbon dioxide (from the ground or by remote sensing) can show changes as magma approaches the surface and degasses, signalling rising magma.
- Thermal and other
- Thermal imaging detects rising heat, and historical eruption records give the likely behaviour and recurrence.
- Use
- Combining these (rising seismicity, inflation and increasing gas) gives the best forecast of an impending eruption.
Markers reward three genuinely different methods (seismicity, deformation, gas, thermal) each linked correctly to what it reveals about rising magma.
Related dot points
- Volcanic activity and eruption styles: the control of silica content, viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions (shield volcanoes and fissures) and andesitic or rhyolitic explosive eruptions (stratovolcanoes, pyroclastic flows and calderas); the volcanic products (lava, tephra and pyroclastic material); the link between eruption style and plate setting.
A focused answer to the Eduqas Geology statement on volcanic activity. Covers how silica content, viscosity and dissolved gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, the products (lava, tephra and pyroclastic material), the landforms (shield, stratovolcano, caldera and fissure), and the link to plate setting.
- Earthquake hazards, risk and mitigation: the primary and secondary hazards of earthquakes (ground shaking, liquefaction, landslides, tsunami, fire); the distinction between hazard, vulnerability, exposure and risk; the factors controlling the severity of impact; and the prediction, monitoring and mitigation strategies (building design, hazard mapping, early warning and planning).
A focused answer to the Eduqas Geology geohazards statement on earthquakes. Covers primary and secondary earthquake hazards, the distinction between hazard, vulnerability, exposure and risk, the controls on severity, and the prediction, monitoring and mitigation strategies that reduce impact.
- Mass movement and landslide hazards: the types of mass movement (rockfall, slide, slump, flow and creep); the factors controlling slope stability (slope angle, rock and soil strength, water, bedding orientation, vegetation and undercutting); the triggers of slope failure; and the engineering and planning methods used to reduce landslide hazards.
A focused answer to the Eduqas Geology geohazards statement on mass movement. Covers the types of mass movement, the factors controlling slope stability, the triggers of slope failure, and the engineering and planning methods used to reduce landslide hazards.
- Plate margins and their features: the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-ocean island arcs, ocean-continent margins and continent-continent collision); the Benioff zone, subduction and decompression melting; the diagnostic rocks, structures, earthquakes and volcanoes of each margin type.
A focused answer to the Eduqas Geology statement on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-ocean, ocean-continent and continent-continent sub-types, the Benioff zone, subduction and decompression melting, and the diagnostic rocks, structures, earthquakes and volcanoes that identify each margin in the exam.
- Igneous rock classification and textures: the classification of igneous rocks by silica content and composition (ultramafic peridotite, mafic basalt and gabbro, intermediate andesite and diorite, felsic rhyolite and granite) and by grain size and cooling rate (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic textures); and the relationship between cooling rate and crystal size.
A focused answer to the Eduqas Geology statement on igneous rock classification. Covers the compositional series from ultramafic peridotite through mafic basalt and gabbro and intermediate andesite and diorite to felsic rhyolite and granite, the link between cooling rate and crystal size, and the named textures (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic).
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)