What hazards do volcanoes pose, and how are they monitored and predicted?
Volcanic activity ranges from gentle effusive eruptions of runny basaltic lava to violent explosive eruptions of viscous silica-rich magma; the hazards include lava flows, ash falls, pyroclastic flows, lahars (mudflows) and toxic gases; volcanoes can be monitored using seismometers (earthquake swarms), ground deformation (tilt and bulging), gas emissions and rising temperatures, so eruptions are more predictable than earthquakes, and risk is reduced by monitoring, hazard mapping, exclusion zones and evacuation.
A focused answer to the Eduqas GCSE Geology statement on volcanic hazards. Covers effusive versus explosive eruptions and what controls them, the hazards (lava, ash, pyroclastic flows, lahars, gases), the warning signs used to monitor and predict eruptions, and how risk is reduced.
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What this dot point is asking
Eduqas wants you to explain why volcanic activity ranges from gentle effusive eruptions of runny basaltic lava to violent explosive eruptions of viscous silica-rich magma, to list the hazards (lava flows, ash falls, pyroclastic flows, lahars and toxic gases), and to describe how volcanoes are monitored (seismometers, ground deformation, gas emissions, temperature) so that eruptions are more predictable than earthquakes. You also need to explain how risk is reduced by monitoring, hazard mapping, exclusion zones and evacuation. The recurring high-mark question links magma composition to eruption style.
The answer
What controls the eruption style
The single biggest control is the silica content of the magma, because it sets the viscosity (how runny or thick the magma is) and therefore how easily gas can escape:
- Effusive (gentle) eruptions. Basaltic magma is low in silica, so it is runny (low viscosity) and gas bubbles out easily. The lava flows out steadily, building broad, gently sloping shield volcanoes. These eruptions are relatively safe.
- Explosive (violent) eruptions. Andesitic or rhyolitic magma is silica-rich, so it is thick (high viscosity) and gas cannot escape. Pressure builds until the magma is blasted apart into ash and pumice. These build steep composite (strato-) volcanoes and are far more dangerous.
So low silica means runny and gentle; high silica means viscous and explosive.
The hazards
Volcanic hazards differ between the two styles, but a major eruption can bring several:
- Lava flows. Destroy everything in their path but usually move slowly enough for people to escape (effusive volcanoes).
- Ash falls. Clouds of fine ash that collapse roofs, smother crops, contaminate water and ground aircraft, sometimes far downwind.
- Pyroclastic flows. Fast, scorching avalanches of hot gas, ash and rock that race downslope and kill almost instantly (explosive volcanoes). The deadliest hazard.
- Lahars. Mudflows formed when ash mixes with water (rain or melted snow and ice), flowing down valleys and burying towns.
- Toxic gases. Carbon dioxide, sulphur dioxide and others that can suffocate or poison and harm air quality.
Monitoring and prediction
Crucially, a volcano usually gives warning signs as magma rises over days or weeks, so eruptions are more predictable than earthquakes. The main monitoring methods are:
- Seismometers. Detect earthquake swarms as magma forces its way upward and cracks the rock.
- Ground deformation. Tiltmeters and surveying detect the volcano bulging or tilting as the magma chamber inflates.
- Gas emissions. Sensors detect rising or changing gases (such as sulphur dioxide) as magma nears the surface.
- Temperature. Rising ground, fumarole or crater-lake temperatures signal magma approaching.
Because these precursors build up, monitoring can warn of an approaching eruption, which is why volcanoes are far more predictable than earthquakes (which strike with no reliable short-term warning).
Reducing the risk
Risk is reduced by combining monitoring with planning:
- Monitoring to detect the warning signs and raise the alarm.
- Hazard mapping to show which areas lava flows, pyroclastic flows and lahars would reach.
- Exclusion zones to keep people out of the most dangerous areas.
- Evacuation of threatened areas once warning signs appear, the main life-saver.
Examples in context
Example 1. Hawaiian shield volcanoes. Hawaii's runny basaltic lava builds vast, gently sloping shields and erupts effusively, so lava flows are the main hazard and can often be diverted or simply avoided.
Example 2. A composite volcano above a city. A steep, silica-rich composite volcano near a populated valley threatens pyroclastic flows and lahars. Continuous monitoring and a rehearsed evacuation plan are the main defences when the warning signs appear.
Try this
Q1. State what makes a magma erupt explosively rather than gently. [1 mark]
- Cue. A high silica content, which makes the magma viscous (thick) so trapped gas cannot escape and pressure builds.
Q2. Name two hazards of an explosive eruption. [2 marks]
- Cue. Any two of: pyroclastic flows; ash falls; lahars (mudflows); toxic gases.
Q3. Give one warning sign that monitoring a volcano can detect before an eruption. [1 mark]
- Cue. Any one of: swarms of small earthquakes; ground bulging or tilting; rising gas emissions; rising temperatures.
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 20196 marksExplain why some volcanoes erupt gently while others erupt explosively, and describe two hazards associated with explosive eruptions.Show worked answer →
Link magma composition to eruption style, then give two explosive hazards.
- Why gentle (effusive) eruptions happen
- Basaltic magma is low in silica, so it is runny (low viscosity), and gas escapes easily. The lava flows out steadily, building gently sloping shield volcanoes. Eruptions are effusive and relatively safe to approach.
- Why explosive eruptions happen
- Silica-rich (rhyolitic or andesitic) magma is thick (high viscosity), so gas cannot escape easily. Pressure builds until the magma is blasted apart violently, fragmenting it into ash and pumice. These eruptions are explosive and far more dangerous.
- Two explosive hazards
- Pyroclastic flows: fast, scorching avalanches of hot gas, ash and rock that flow downslope and kill almost instantly. Ash falls: clouds of fine ash that collapse roofs, ruin crops, contaminate water and ground aircraft. (Lahars and toxic gases are also valid.)
Markers reward linking low silica to runny effusive lava and high silica to viscous explosive eruptions (because gas escape depends on viscosity), plus two valid explosive hazards described.
Eduqas 20225 marksDescribe four methods used to monitor a volcano and explain why eruptions can usually be predicted more successfully than earthquakes.Show worked answer →
Give four monitoring methods, then contrast volcano and earthquake predictability.
Four methods. Seismometers detect swarms of small earthquakes as magma forces its way upward. Tiltmeters and surveying detect ground deformation (the volcano bulging or tilting as the magma chamber inflates). Gas sensors detect rising or changing gas emissions (for example sulphur dioxide) as magma nears the surface. Temperature measurements detect rising ground or crater-lake temperatures.
Why eruptions are more predictable. A volcano usually gives clear, building warning signs over days or weeks (more earthquakes, bulging, more gas, higher temperatures) as magma rises, so monitoring can detect an approaching eruption. Earthquakes, by contrast, strike suddenly with no reliable short-term warning, so the exact time and place cannot be predicted.
Markers reward four valid monitoring methods and the explanation that volcanoes give detectable precursors as magma rises, whereas earthquakes do not.
Related dot points
- Earthquakes are caused by the sudden release of stress along faults, mainly at plate margins; they radiate seismic waves (P-waves and S-waves) whose arrival times locate the epicentre and whose amplitude measures magnitude; the hazards include ground shaking, building collapse, tsunamis, fires and landslides; the risk is reduced by hazard mapping, building design and emergency planning, but precise short-term prediction remains impossible, so forecasting relies on probability from past records.
A focused answer to the Eduqas GCSE Geology statement on earthquake hazards. Covers how earthquakes are caused by stress release on faults, the P-waves and S-waves used to locate the epicentre and measure magnitude, the hazards, and how risk is reduced when precise prediction is impossible.
- Mass movement is the downslope movement of rock and soil under gravity; it includes slow creep, slides, slumps, flows and rockfalls; failure is triggered when the driving force (gravity, increased by steep slopes, heavy rain, loading and undercutting) exceeds the resisting force (friction and cohesion, reduced by water and weak or weathered rock); the risk is reduced by improving drainage, reducing slope angle, building retaining structures and avoiding building on unstable ground.
A focused answer to the Eduqas GCSE Geology statement on mass movement. Covers the types of mass movement, why slopes fail (the balance of driving and resisting forces and the role of water), the triggers, and how landslide risk is reduced by drainage, regrading, retaining structures and avoidance.
- The Earth's outer layer is divided into tectonic plates that move slowly over the mantle, driven by convection; the evidence for plate tectonics includes the fit of the continents, matching fossils and rock sequences across oceans, and the symmetrical magnetic stripes of the sea floor; plates meet at constructive (divergent), destructive (convergent) and conservative (transform) margins, each with characteristic earthquakes, volcanoes and landforms.
A focused answer to the Eduqas GCSE Geology statement on plate tectonics. Covers tectonic plates and the convection that drives them, the evidence (continental fit, matching fossils and rocks, magnetic stripes and sea-floor spreading), and the three types of plate margin with their earthquakes, volcanoes and landforms.
- Igneous rocks form by the crystallisation of magma or lava; cooling rate controls crystal size (slow cooling at depth gives coarse-grained intrusive rocks such as granite, fast cooling at the surface gives fine-grained extrusive rocks such as basalt); rocks are classified by crystal size and by silica content (felsic, intermediate, mafic); minerals also crystallise from hydrothermal fluids to form veins.
A focused answer to the Eduqas GCSE Geology statement on igneous rocks. Covers how magma and lava crystallise, how cooling rate controls crystal size (intrusive granite versus extrusive basalt), classification by silica content (felsic to mafic), and the crystallisation of minerals from hydrothermal fluids in veins.
- Engineering geology assesses the ground before construction: foundations, tunnels, dams and reservoirs must suit the rock and soil present; geologists check the strength and stability of rock, the presence of faults, the slope stability, the permeability of the ground (for a reservoir to hold water or a tunnel to stay dry), and the hazards of weak, soluble or swelling materials; poor ground investigation can lead to subsidence, leakage, collapse or failure, so a site investigation is carried out first.
A focused answer to the Eduqas GCSE Geology statement on engineering geology. Covers why the ground must be assessed before construction, the factors checked (rock strength, faults, slope stability, permeability, weak or soluble materials), and how poor investigation leads to subsidence, leakage or collapse.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)