Cambridge Syllabus Notes

2. The Natural Environment

2.1 Earthquakes and volcanoes – causes, effects and responses

Earthquakes and volcanoes are the most visible expressions of movement within the Earth’s lithospheric plates. Understanding their causes, the hazards they produce and how societies can manage these risks is a core part of the Cambridge IGCSE Geography syllabus (Topic 4 – Tectonic hazards).

1. Structure of the Earth and the distribution of tectonic hazards

Internal structure
- Inner core – solid iron‑nickel
- Outer core – liquid iron‑nickel
- Lower mantle – hot, solid silicates
- Upper mantle (asthenosphere) – ductile rock that allows plate motion
- Crust – thin outer layer; oceanic (basaltic) and continental (granite)
- Lithosphere – crust + uppermost mantle; broken into rigid tectonic plates
Major tectonic plates (examples)
- Pacific, North American, Eurasian, African, South American, Indo‑Australian, Antarctic
Plate‑boundary types required by the syllabus
- Transform – lateral sliding (e.g., San Andreas Fault)
- Convergent – subduction – oceanic plate melts beneath another plate (e.g., Japan, Andes)
- Convergent – collision – two continental plates collide (e.g., Himalaya)
- Divergent – spreading centres (e.g., Mid‑Atlantic Ridge)
- Intraplate – earthquakes or volcanoes away from plate margins (e.g., New Madrid earthquakes, Hawaiian volcanoes)
Global distribution of tectonic hazards
- Concentrated along plate boundaries – the “Ring of Fire” around the Pacific Ocean.
- Inland seismic zones (e.g., Himalayan belt, East African Rift) and intraplate volcanic chains (e.g., Hawaii).

2. Processes and features of earthquakes

2.1 How earthquakes occur

Stress accumulation and release – tectonic forces build strain in rocks until the strength of a fault plane is exceeded, causing a sudden slip.
Fault‑type settings
- Transform – strike‑slip motion (e.g., San Andreas)
- Convergent – thrust or reverse faults in subduction zones (e.g., Japan trench)
- Convergent – collision faults in continental belts (e.g., Himalaya)
- Divergent – normal faults at spreading centres (e.g., Mid‑Atlantic Ridge)
- Intraplate – re‑activation of old crustal weaknesses (e.g., New Madrid, USA)
Human‑induced seismicity (optional) – reservoir loading, mining, geothermal projects. This is not required by the syllabus but can be included as an extension.

2.2 Earthquake parameters (syllabus requirement)

Parameter	Definition / what it measures	Typical unit
Focus (hypocentre)	Point within the Earth where rupture starts	Depth in kilometres (km)
Epicentre	Surface point directly above the focus	Geographic coordinates (latitude, longitude)
Magnitude	Energy released; measured on: Richter scale (ML) – logarithmic scale (each whole number = 10× amplitude) Moment‑magnitude scale (Mw) – based on seismic moment; now the standard for large events	M (e.g., M 7.0)
Intensity	Observed effects on people, structures and the environment	Modified Mercalli Scale (I–XII)
Seismic waves	P‑waves (compressional), S‑waves (shear), Surface waves (Rayleigh & Love)	Velocity (km s⁻¹)

2.3 Typical earthquake hazards

Ground shaking – primary cause of building collapse.
Surface rupture – visible displacement along the fault trace.
Liquefaction – loss of strength in water‑saturated soils.
Landslides & rockfalls on steep slopes.
Tsunamis – generated by under‑sea thrust earthquakes.

2.4 Quantitative case study (required)

2011 Tōhoku earthquake & tsunami (Japan)
- Magnitude 9.0 Mw, depth ≈ 29 km.
- ≈ 15 800 deaths; > 6 000 injured.
- Economic loss ≈ US$ 235 billion (≈ 10 % of Japan’s GDP).
- Triggered the Fukushima Daiichi nuclear accident.

3. Processes and features of volcanoes

3.1 How volcanoes form (syllabus requirement)

Subduction zones – oceanic plate melts, producing magma that rises (e.g., Andes volcanic arc).
Rift zones – mantle upwelling at divergent boundaries creates fissure eruptions (e.g., East African Rift).
Hotspots – mantle plumes rise independently of plate boundaries (e.g., Hawaiian Islands).
Continental collision – thickened crust can partially melt, producing volcanism in some orogenic belts.

3.2 Volcano structure (syllabus‑required features)

Vent – opening through which magma and gases escape.
Crater – bowl‑shaped depression at the summit.
Magma chamber – reservoir of molten rock beneath the volcano.
Cone types – shield, stratovolcano (composite), cinder cone.
Secondary cones – parasitic cones, lava domes.

3.3 Classification of volcanoes (active, dormant, extinct)

Category	Definition (syllabus wording)	Typical example
Active	Has erupted in historic times (last 10 000 years) and shows signs of unrest.	Mount Etna (Italy)
Dormant	No recent eruptions but could become active again.	Mount Kilimanjaro (Tanzania)
Extinct	No expected future eruptions; magma source cut off.	Mount Muir (USA) – widely cited extinct volcano

3.4 Volcanic Explosivity Index (VEI)

Logarithmic scale (0–8) that measures the volume of erupted material and column height.
VEI 6 (e.g., Pinatubo 1991) indicates a “colossal” eruption with > 10 km³ tephra.

3.5 Typical volcanic hazards

Lava flows – destroy infrastructure but usually move slowly enough for evacuation.
Pyroclastic flows – hot, fast‑moving mixtures of gas and fragments; extremely lethal.
Ash fall – affects air quality, agriculture and can cause roof collapse.
Volcanic gases – SO₂, CO₂, H₂S; cause respiratory problems and can influence climate.
Lahars – volcanic mudflows triggered by rain or ice melt.
Secondary flooding – from crater‑lake breach or rapid ice melt.

3.6 Quantitative case study (required)

1991 Mount Pinatubo eruption (Philippines)
- VEI 6; ash column reached 35 km.
- ≈ 800 deaths (mainly from lahars).
- Global temperature fell ~0.5 °C for ~2 years due to sulphur‑aerosol injection.
- US $ 800 million in agricultural losses; US $ 1 billion in infrastructure damage.

4. Why people live in high‑risk zones (syllabus requirement)

Fertile volcanic soils support intensive agriculture.
Rich mineral deposits (e.g., copper, gold) in volcanic and orogenic belts.
Geothermal energy potential.
Cultural, historical or religious significance of particular volcanoes or fault zones.
Coastal economies that benefit from ports and fisheries despite tsunami risk.

5. Managing the impacts of tectonic hazards

5.1 Monitoring and prediction (primary response)

Seismograph networks – locate earthquakes and determine magnitude in real time.
GPS, InSAR and tilt‑meter data – detect ground deformation signalling volcanic unrest.
Tsunami warning systems – sea‑level gauges, DART buoys, rapid alert dissemination.
Volcanic gas monitoring – SO₂ flux measured by spectrometers (e.g., DOAS).

5.2 Engineering protection measures

Earthquake‑resistant construction – reinforced concrete, steel frames, base isolation, shear walls.
Lahar diversion channels, check‑dams and sediment traps.
Coastal sea‑walls, tsunami‑evacuation platforms and mangrove restoration.
Retrofitting of existing vulnerable buildings (e.g., adding steel braces).

5.3 Planning and community preparedness (land‑use strategies)

Hazard zoning – restrict development on active fault traces, tsunami‑prone coastal strips and volcanic exclusion zones.
Building codes that incorporate local seismic design criteria and volcanic‑hazard considerations.
Public education – “Drop, Cover, Hold” drills, volcano warning‑level systems, clear evacuation route signage.
Emergency kits, designated shelters and robust communication plans.

5.4 Sustainability evaluation of management techniques

Sea‑walls – protect coastal communities but can:
- Alter natural sediment transport, leading to beach erosion downstream.
- Increase flood risk in adjacent low‑lying areas.
Large dams for flood or lahar control – provide water storage but may:
- Displace communities and wildlife.
- Increase downstream flood risk if overtopped.
Geothermal exploitation – renewable energy source but can induce seismicity if fluid injection is poorly managed.
Re‑forestation of slopes – reduces landslide risk and enhances biodiversity when native species are used; however, planting non‑native species may harm local ecosystems.

6. Comparison of earthquakes and volcanoes

Aspect	Earthquakes	Volcanoes
Primary cause	Sudden slip along a fault due to stress release.	Magma ascent from mantle or crust.
Typical plate‑boundary settings	Transform, convergent (subduction & collision), divergent, intraplate.	Convergent (subduction), divergent (rift), hotspot, continental collision.
Key hazards	Ground shaking, surface rupture, liquefaction, landslides, tsunamis.	Lava flows, pyroclastic flows, ash fall, volcanic gases, lahars, secondary flooding.
Predictability	Short‑term prediction limited; long‑term hazard maps possible.	Short‑term monitoring (seismicity, deformation, gas) improves forecasts, but precise eruption timing remains difficult.
Main management approaches	Monitoring, earthquake‑resistant design, land‑use planning, public drills.	Monitoring, exclusion zones, evacuation plans, engineering barriers (e.g., lahar channels), public awareness.

7. Suggested diagrams for revision

Cross‑section of a subduction zone showing:
- Earthquake focal depth, volcanic arc, magma chamber, and trench.
World map highlighting the distribution of earthquakes (red circles) and volcanoes (black triangles) along plate boundaries – the “Ring of Fire”.
Diagram of a stratovolcano with labelled vent, crater, magma chamber, lava dome and typical eruption products (ash plume, pyroclastic flow).
Illustration of seismic wave propagation (P‑wave, S‑wave, surface waves) from focus to surface, showing the first‑motion polarity used to locate the epicentre.
VEI scale diagram linking eruption column height and tephra volume.

2.1 Earthquakes and volcanoes: Describe causes, effects and responses to tectonic hazards.