What controls volcanic hazards, and how are volcanoes monitored and managed?
The control of magma composition and gas content on eruptive style, the volcanic hazards (lava flows, pyroclastic flows, ash falls, lahars, gases, sector collapse), the monitoring of volcanoes (seismicity, ground deformation, gas, thermal), and the prediction and management of volcanic risk.
A focused WJEC and Eduqas A-Level Geology T1 answer on how magma composition and gas content control eruptive style, the range of volcanic hazards from lava flows to pyroclastic flows, lahars and ash, the methods used to monitor volcanoes, and how volcanic risk is predicted and managed.
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What this dot point is asking
This dot point applies igneous processes to hazards. WJEC wants the link from magma composition through viscosity and gas to eruptive style and hazard, the range of volcanic hazards, the monitoring methods, and how risk is predicted and managed. It builds on magma evolution (G1) and plate boundaries (F4).
The answer
What controls eruptive style
The key control is magma composition, acting through viscosity and gas:
Higher silica means higher viscosity, more trapped gas and more violent eruptions, so eruptive style and hazard are predictable from composition and tectonic setting.
Volcanic hazards
- Lava flows: destructive but usually slow (mainly from basaltic volcanoes).
- Pyroclastic flows (nuees ardentes): fast, hot clouds of gas, ash and rock; the deadliest hazard.
- Ash falls (tephra): collapse roofs, smother crops, disrupt aircraft and water.
- Lahars: volcanic mudflows when ash mixes with water or melted snow.
- Volcanic gases: toxic carbon dioxide and sulphur dioxide.
- Sector collapse: failure of an unstable flank, which can also trigger a tsunami.
Monitoring
Active volcanoes are watched for signs of rising magma:
Prediction and management
Eruptions are more predictable than earthquakes because precursors are clearer. Management uses hazard maps, exclusion zones, evacuation plans, diversion barriers for lava and lahars, and warning systems based on monitoring.
Examples in context
Mount Pinatubo (1991) was successfully monitored (seismicity, deformation, gas), allowing evacuation that saved many lives before a major explosive eruption. Mount St Helens (1980) showed flank swelling then sector collapse and a lateral blast, illustrating deformation monitoring and collapse hazard. Kilauea in Hawaii is a basaltic shield with effusive lava flows, contrasting with explosive stratovolcanoes and showing how composition controls style.
Try this
Q1. Explain why a high-silica magma tends to erupt explosively. [2 marks]
- Cue. High silica gives high viscosity that traps gas, so pressure builds until it is released explosively.
Q2. Name the deadliest volcanic hazard and describe it. [2 marks]
- Cue. Pyroclastic flows: fast, hot clouds of gas, ash and rock that sweep down the volcano.
Q3. State two methods used to monitor a volcano and what each detects. [2 marks]
- Cue. Seismometers detect earthquakes from moving magma; tiltmeters, GPS or satellite radar detect swelling of the volcano as the chamber inflates.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20206 marksExplain how the composition of magma controls the style of a volcanic eruption and the hazards produced.Show worked answer →
Link composition to viscosity and gas, then to eruptive style and hazard, because that is the chain of cause and effect.
Magma composition controls viscosity through silica content. Basaltic magma has low silica, low viscosity and lets gas escape easily, so it erupts effusively as runny lava flows (fluid, low explosivity). Rhyolitic and andesitic magma has high silica, high viscosity and traps gas, so pressure builds until it erupts explosively.
The hazards follow. Low-silica volcanoes mainly threaten with lava flows, which are destructive but usually slow enough to escape. High-silica, gas-rich volcanoes produce the most dangerous hazards: pyroclastic flows (fast, hot clouds of gas and ash), ash falls that collapse roofs and disrupt aircraft, and, with water, lahars (volcanic mudflows). Toxic gases and sector collapse add to the danger.
So high silica means high viscosity, trapped gas and explosive, deadly eruptions; low silica means low viscosity, escaping gas and effusive lava flows.
Markers reward silica controlling viscosity and gas retention, effusive basaltic lava versus explosive silicic eruptions, and the matching hazards (lava flows versus pyroclastic flows, ash and lahars).
WJEC Eduqas 20225 marksDescribe the methods used to monitor an active volcano and how they help predict an eruption.Show worked answer →
Give the monitoring methods and say what each warns of, because the marks track distinct techniques.
Seismicity: networks of seismometers detect earthquakes caused by magma moving and fracturing rock; an increase in small earthquakes, especially harmonic tremor, often precedes an eruption.
Ground deformation: tiltmeters, GPS and satellite radar (InSAR) detect swelling of the volcano as magma rises and inflates the chamber, a sign of impending eruption.
Gas emissions: measuring gases such as sulphur dioxide and carbon dioxide detects rising magma, as gas release often increases before an eruption.
Thermal and other methods: infrared and thermal imaging detect heating, and changes in groundwater temperature and chemistry can give warning.
Combining these signals lets scientists raise alert levels and order evacuation, so monitoring supports prediction even though the exact time remains uncertain.
Markers reward seismicity, ground deformation, gas monitoring and thermal methods, each linked to detecting rising magma and enabling warning and evacuation.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)