G3: Earthquakes & Volcanoes
Causes and features of earthquakes and volcanoes, their relationship to plate boundaries, and their impacts.
Earthquakes & Volcanoes
Causes and features of earthquakes and volcanoes, their relationship to plate boundaries, and their impacts.
Key Fact: Earthquakes occur when stress builds up at plate boundaries and is suddenly released as seismic waves
Key Fact: The focus is the point underground where the earthquake originates; the epicentre is the point on the surface directly above
Key Fact: Volcanoes form where magma reaches the surface: at divergent boundaries (fissure eruptions), subduction zones (explosive stratovolcanoes), and hotspots (shield volcanoes)
Key Fact: Effusive eruptions produce fluid basaltic lava (e.g. Iceland); explosive eruptions produce viscous andesitic/rhyolitic lava, ash and pyroclastic flows (e.g. Mt St Helens)
Key Fact: Earthquake hazards include ground shaking, liquefaction, landslides and tsunamis
Key Fact: Volcanic hazards include lava flows, pyroclastic flows, ash falls, lahars and gas emissions
📋 Key Vocabulary and Concepts
For Earthquakes & Volcanoes, you must know:
- Focus (hypocentre): The point within the Earth where an earthquake originates and seismic waves radiate from
- Epicentre: The point on the Earth's surface directly above the earthquake focus
- Pyroclastic flow: A fast-moving current of hot gas, ash and rock fragments that flows down the side of a volcano during an explosive eruption
- Lahar: A destructive mudflow of volcanic ash and water, typically triggered by heavy rain or melting snow on a volcano
- Liquefaction: When water-saturated soil behaves like a liquid during earthquake shaking, causing buildings to sink or tilt
❓ Practice Questions
Q: What is the difference between the focus and epicentre of an earthquake?
Q: Why are volcanoes at subduction zones more explosive than those at mid-ocean ridges?
Q: Name three volcanic hazards and describe each.
Q: What causes a tsunami?
Q: How does liquefaction cause damage during an earthquake?
✅ Answers
- The focus is the point underground where the earthquake originates. The epicentre is the point on the surface directly above the focus.
- Subduction zone magma is andesitic/rhyolitic with higher silica, more viscosity, and trapped gas. Pressure builds until an explosive eruption occurs. Mid-ocean ridge magma is basaltic with lower silica, less viscosity, and gas escapes easily, producing gentle effusive eruptions.
- 1) Pyroclastic flow: fast-moving current of hot gas, ash and rock. 2) Lahar: volcanic mudflow of ash mixed with water. 3) Ash fall: fine volcanic particles ejected into the atmosphere and deposited over large areas.
- A large displacement of water, typically caused by a submarine earthquake, volcanic eruption or landslide. The sudden movement of the seafloor pushes a column of water upward, generating waves that travel across the ocean at high speed.
- Earthquake shaking causes water-saturated, unconsolidated sediment to lose strength and behave like a liquid. Buildings and infrastructure can sink, tilt or collapse as the ground loses its ability to support loads.
🎯 Exam Tips
- Focus vs epicentre: focus = underground, epicentre = on the surface directly above
- Magma type determines eruption style: basaltic = gentle, andesitic/rhyolitic = explosive
- Both earthquake and volcanic hazards are examined, know at least 3 of each
- Plate boundary type determines volcano type: divergent = shield, convergent = stratovolcano
📝 Exam Technique
GCSE Geology Exam Tips — Earthquakes & Volcanoes:
1. For Earthquakes & Volcanoes questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Earthquakes & Volcanoes to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Earthquakes & Volcanoes questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Earthquakes & Volcanoes to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ The epicentre is where the earthquake starts underground ✓ The focus is the underground origin point; the epicentre is the surface point directly above it
✗ All volcanoes are equally explosive ✓ Volcano explosiveness depends on magma composition: basaltic (low silica) = gentle eruptions; andesitic/rhyolitic (high silica) = explosive eruptions
✗ Earthquakes only happen at plate boundaries ✓ While most earthquakes occur at plate boundaries, intraplate earthquakes can also occur on ancient faults within plates
✍️ Model Answer
Full-Mark Response
Explain why different types of volcano form at different plate boundaries and describe the associated hazards. [6 marks]
At divergent boundaries, basaltic magma rises from the mantle as plates separate. Basaltic magma has low silica content and low viscosity, allowing gases to escape easily. This produces effusive eruptions of fluid lava, forming shield volcanoes with gentle slopes (e.g. Iceland). Hazards are mainly lava flows and gas emissions. At convergent (subduction) boundaries, oceanic crust melts and mixes with continental material, producing andesitic or rhyolitic magma. This magma has high silica content and high viscosity, trapping gases. Pressure builds until a violent explosive eruption occurs, forming steep stratovolcanoes (e.g. Mt Fuji, Mt St Helens). Hazards include pyroclastic flows (fast, hot gas and ash currents, the most deadly volcanic hazard), ash falls (disrupting aviation and agriculture), lahars (volcanic mudflows), and directed blasts. At hotspots (e.g. Hawaii), mantle plumes produce basaltic shield volcanoes similar to divergent boundary volcanoes. The key factor controlling eruption style and hazard type is magma composition, which in turn depends on the tectonic setting.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of earthquakes & volcanoes, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of earthquakes & volcanoes to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to earthquakes & volcanoes, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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