Processes involved in volcano formation

Earthquake and Volcanic Hazards – Processes Involved in Volcano Formation

Learning Objective (AO1)

Explain the geological processes that generate volcanoes, describe how these processes vary with tectonic setting, evaluate the associated hazards, and discuss monitoring and mitigation strategies – in line with Cambridge AS & A Level Geography (9696) Paper 3.

1. Global Distribution of Volcanoes & Their Tectonic Setting

• Ring of Fire – Continuous belt of active volcanoes and earthquakes around the Pacific Ocean. Setting: Subduction (convergent) zones where an oceanic plate sinks beneath another plate.
• Mid‑Ocean Ridges – Linear volcanic systems marking divergent plate boundaries (e.g., Mid‑Atlantic Ridge). Setting: Oceanic crust is pulled apart, causing decompression melting.
• Hotspot Chains – Intraplate volcanic chains such as the Hawaiian Islands, Galápagos, and the Deccan Traps. Setting: Mantle plumes (upwelling of hot material) produce localized melting.
• Continental Rift Zones – Examples include the East African Rift. Setting: Continental lithosphere is stretched, leading to decompression melting of the underlying mantle.

2. Generation of Magma – Three Primary Melting Mechanisms (AO1)

1. Decompression Melting
   • Occurs when mantle material rises beneath divergent boundaries or mantle plumes.
   • Pressure drops faster than temperature, crossing the solidus and producing predominantly basaltic magma.
   • Plate‑boundary link: Divergent (mid‑ocean ridges, continental rifts).
2. Flux (Water‑Rich) Melting
   • Water‑rich fluids released from a subducting slab infiltrate the over‑lying mantle wedge.
   • Volatiles lower the peridotite solidus, generating magmas that range from basaltic to rhyolitic.
   • Plate‑boundary link: Convergent (subduction‑zone volcanic arcs).
3. Heat‑Transfer (Contact) Melting
   • Direct heating of crustal rocks by upwelling hot mantle or by a large magma body.
   • Can melt continental crust, producing felsic (andesitic to rhyolitic) magmas.
   • Plate‑boundary link: Intraplate hotspots and the lower crust beneath continental arcs.

3. Magma Evolution – From Source to Surface (AO1)

While ascending, magma chemically evolves through three main processes:

• Fractional Crystallisation – Early‑forming minerals (e.g., olivine, pyroxene) crystallise and settle out, enriching the residual melt in silica. Bowen’s Reaction Series illustrates the progressive change: basalt → andesite → dacite → rhyolite.
• Assimilation – Incorporation of surrounding country rock, often adding silica and trace elements.
• Mixing – Co‑mingling of two or more magma batches with differing temperature, composition or volatile content.

The final magma composition (basaltic, andesitic, dacitic, rhyolitic) controls viscosity and eruption style.

4. Ascent of Magma (AO1)

• Buoyancy – Magma is less dense than the surrounding solid rock.
• Gas Exsolution – Decompression causes dissolved H₂O, CO₂, SO₂ to form bubbles, raising internal pressure.
• Tectonic Stress & Fracturing – Plate motions create fractures that magma exploits, forming:
  • Dyke: vertical, sheet‑like intrusion that transports magma upward.
  • Sill: horizontal intrusion that spreads laterally.

Quantitative Example – Buoyant Force

For a basaltic magma body of volume V = 1 km³ (1 × 10⁹ m³), magma density ρ_magma = 2600 kg m⁻³, surrounding rock density ρ_rock = 3000 kg m⁻³, and g = 9.81 m s⁻²:

\[ F_b = (\rho_{rock} - \rho_{magma}) \, g \, V = (3000 - 2600) \times 9.81 \times 1 \times 10^{9} = 3.9 \times 10^{12}\; \text{N} \]

A positive buoyant force of ≈ 3.9 × 10¹² N is sufficient to fracture overlying rock and initiate an eruption.

5. Eruption Mechanisms & Styles (AO1 & AO2)

Eruption Type	Magma Composition (SiO₂ %)	Viscosity (relative)	VEI Classification*	Primary Hazards
Effusive	Basaltic (45‑52 %)	Low	VEI 0‑2 (tephra volume < 10⁶ m³)	Lava flows, fire fountains, limited ash
Explosive (Plinian / Vulcanian)	Andesitic‑to‑Rhyolitic (55‑77 %)	High	VEI 3‑7 (tephra volume 10⁶‑10⁹ m³)	Pyroclastic density currents, extensive ashfall, volcanic bombs, lahars
Phreatic / Phreatomagmatic	Any magma that interacts with external water	Variable	VEI 1‑5 (steam‑driven explosions)	Steam blasts, base‑surge ash, localized lahars

*VEI (Volcanic Explosivity Index) is based on the volume of tephra ejected, eruption column height and eruption duration.

6. Tectonic Settings – Dominant Melting Mechanism & Typical Magma Types (AO1)

Tectonic Setting	Dominant Melting Mechanism	Typical Magma Composition	Common Volcano Type
Divergent (mid‑ocean ridges, continental rifts)	Decompression melting	Basaltic (low SiO₂)	Shield / fissure eruptions
Convergent (subduction‑zone arcs)	Flux melting in mantle wedge	Range: basalt → andesite → dacite → rhyolite	Stratovolcanoes (composite)
Intraplate (hotspots)	Combination of heat‑transfer & decompression melting	Basaltic to andesitic; occasional felsic in large igneous provinces	Shield volcanoes, volcanic islands, flood basalts

7. Earthquake–Volcano Coupling (AO2)

• Triggering of Magma by Tectonic Earthquakes
  • Large subduction‑zone earthquakes can alter stress fields, opening new pathways for magma ascent (e.g., 1995 Kobe earthquake preceding renewed activity at Mount Unzen, Japan).
• Volcanic Seismic Signals Used for Forecasting
  • Volcanic tremor: Continuous, low‑frequency vibration indicating sustained magma movement.
  • Long‑period (LP) events: Low‑frequency earthquakes linked to gas‑rich magma rising.
  • VT (volcano‑tectonic) earthquakes: High‑frequency events caused by rock fracturing around magma.
• Illustrative Cases
  • 2010 Eyjafjallajökull (Iceland) – increased VT earthquakes and LP events preceded the explosive phase that disrupted European aviation.
  • 2008 Mount St‑Helens (USA) – dome growth monitored by seismicity and deformation; a sudden LP swarm signalled the 2008 dome collapse.

8. Impacts of Volcanic Hazards (AO2 & AO3)

Impact assessment follows the exposure / vulnerability framework.

• Lava Flows – Destroy homes, roads and agricultural land. Impact severity rises with high population density and lack of land‑use planning.
• Pyroclastic Density Currents (PDCs) – High‑speed, lethal flows; risk amplified in steep valleys and densely built‑up urban areas.
• Ashfall – Reduces visibility (aviation hazard), contaminates water, damages machinery, and causes respiratory problems. Severity depends on wind direction, settlement pattern and building design.
• Volcanic Gases
  • SO₂ → sulfate aerosols → acid rain and short‑term climate cooling.
  • CO₂ → asphyxiation in low‑lying depressions.
• Secondary Hazards – Lahars, landslides, and volcanic tsunamis. Their impact is greatest where communities occupy river valleys or coastal slopes.

9. Management, Monitoring & Mitigation Strategies (AO3)

Strategy	Hard / Soft Engineering	Examples	Evaluation Criteria (Cost, Sustainability, Community Acceptance, Effectiveness)
Land‑use Planning	Soft	Zoning away from high‑risk zones; restricting settlement on steep slopes; buffer zones around known lahar pathways.	Low cost, high sustainability, depends on enforcement and public awareness.
Lahar Diversion Channels & Check‑dams	Hard	Channels at Mount St‑Helens (USA) and Mount Pinatubo (Philippines); check‑dams on the slopes of Soufrière Hills (Montserrat).	Effective when maintained; high initial capital cost; may alter natural drainage – requires community consultation.
Early‑Warning & Monitoring Systems	Soft	Seismic networks, gas‑emission stations, GPS deformation monitoring, satellite thermal imaging (e.g., Icelandic Volcano Monitoring System, USGS Volcano Observatory Network).	Provides crucial evacuation time; reliability hinges on data interpretation and communication protocols.
Public Education & Evacuation Planning	Soft	Community drills, multilingual hazard maps, mobile‑app alerts, school curricula.	Improves resilience; requires ongoing funding and stakeholder engagement.
GIS & Remote‑Sensing for Hazard Mapping	Soft	Digital elevation models for lahar flow‑path modelling; satellite‑derived ash‑cloud tracking.	High analytical power; initial software/training costs, but enhances long‑term risk management.

10. Detailed Case Study – 1991 Mount Pinatubo (Philippines)

• Setting: Convergent subduction zone (Philippine Sea Plate beneath the Eurasian Plate). Flux melting in the mantle wedge produced andesitic‑to‑dacitic magma.
• Prediction & Monitoring (AO2)
  • Increased shallow seismicity, rapid ground deformation (GPS & tilt‑meter), and a sharp rise in SO₂ emissions detected by satellite.
  • USGS and PHIVOLCS established a dense seismic‑gas network, providing clear precursory signals.
• Eruption Details (AO1)
  • Plinian eruption on 15 June 1991 – VEI 6 (tephra volume ≈ 10 km³, eruption column ≈ 35 km high).
  • Released ~20 Mt of SO₂ into the stratosphere, causing a global temperature drop of ~0.5 °C in 1992‑93.
• Hazards Produced (AO2)
  • Pyroclastic flows travelled up to 25 km from the vent.
  • Ashfall > 1 m in some locations – roof collapses, aviation shutdown, respiratory issues.
  • Post‑eruption lahars persisted for years, intensified by monsoon rains.
• Management & Mitigation (AO3)
  • Early‑warning and evacuation of ~200 000 people saved thousands of lives.
  • Construction of check‑dams, diversion channels and re‑forestation programmes reduced downstream lahar damage.
  • Long‑term GIS‑based lahar‑risk maps are still used for land‑use planning.
• Evaluation (AO3)
  • Successes: Rapid evacuation, effective communication, and sustained engineering works.
  • Limitations: High cost of infrastructure, need for continual maintenance, and residual risk from unexpected heavy rainfall.
  • Lesson: Integrated approaches (monitoring + hard engineering + community engagement) provide the most resilient mitigation.

11. Summary (AO1)

1. Generation of magma by decompression, flux, or heat‑transfer melting, each linked to a specific tectonic setting.
2. Chemical evolution through fractional crystallisation, assimilation, and mixing (illustrated by Bowen’s Reaction Series).
3. Ascent driven by buoyancy, gas exsolution, and tectonic fracturing** (dykes & sills).
4. Eruption style determined by magma composition, viscosity and volatile content – classified by VEI and associated hazards.
5. Tectonic environment dictates dominant melting mechanism and typical volcano type (shield, stratovolcano, fissure).
6. Hazards (lava, PDCs, ash, gases, lahars) impact societies according to exposure and vulnerability.
7. Effective risk reduction combines land‑use planning, engineering works, robust monitoring, early‑warning, GIS‑based hazard mapping, and community education.