Volcanic Hazards: Magma and Eruption Types

The nature of a volcanic eruption — whether gently effusive or violently explosive — is determined primarily by the chemistry and physical properties of the magma. Understanding magma composition is essential for explaining volcanic landforms and hazard profiles at A-Level.

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Magma Composition

The Three Main Magma Types

Property	Basaltic	Andesitic	Rhyolitic
SiO₂ content	45–52% (low)	52–63% (intermediate)	63–77% (high)
Temperature	1000–1200 °C	800–1000 °C	650–800 °C
Viscosity	Low (fluid)	Intermediate	High (very viscous)
Gas content	Low (~1–2%)	Moderate (~3–4%)	High (~4–6%+)
Eruption style	Effusive	Mixed	Explosive
Typical setting	Constructive margins, hotspots	Destructive margins	Destructive margins, calderas

Why Silica Matters

Silicon dioxide (SiO₂) polymerises within the melt, forming long molecular chains that increase viscosity. High-viscosity magma traps volcanic gases (H₂O, CO₂, SO₂), building pressure until it is released explosively. Low-viscosity basaltic magma allows gases to escape freely, producing gentler eruptions.

The Role of Volatiles

• Water vapour is the most abundant volcanic gas and is the primary driver of explosive eruptions
• Sulphur dioxide can form aerosol veils in the stratosphere, reflecting solar radiation and causing temporary global cooling
• Carbon dioxide is heavier than air and can accumulate in low-lying areas, causing asphyxiation (e.g., Lake Nyos, Cameroon, 1986 — ~1,700 deaths)

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Eruption Styles

Effusive Eruptions

• Characterised by lava flows rather than explosions
• Basaltic lava can travel long distances (tens of kilometres) due to low viscosity
• Lava types: pahoehoe (smooth, ropy surface) and aa (rough, blocky surface)
• Hazards are generally lower but lava flows destroy property and infrastructure (e.g., Kilauea, Hawaii — ongoing eruptions have destroyed over 700 homes since 1983)

Explosive Eruptions

• Associated with intermediate to high-silica magmas
• Produce tephra (volcanic fragments), ash columns, and pyroclastic density currents
• Volcanic Explosivity Index (VEI) measures eruption magnitude on a logarithmic scale from 0 to 8