Volcanic Hazards

Volcanic eruptions are among the most spectacular and destructive natural hazards on Earth. Approximately 1,500 potentially active volcanoes exist worldwide, with around 50–70 erupting in any given year. Over 800 million people live within 100 km of an active volcano (Brown et al., 2015). This lesson examines volcanic processes, hazard types and detailed case studies at A-Level standard.

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Distribution of Volcanoes

The global distribution of volcanoes is closely linked to tectonic plate boundaries:

Location	Percentage of Active Volcanoes	Examples
Convergent boundaries (subduction zones)	~80%	Pacific Ring of Fire — Andes, Cascades, Japan, Philippines, Indonesia
Divergent boundaries	~15%	Mid-Atlantic Ridge (Iceland), East African Rift (Nyiragongo, Erta Ale)
Hot spots (intraplate)	~5%	Hawaii, Yellowstone, Réunion, Galápagos

Key Definition: The Pacific Ring of Fire is a horseshoe-shaped zone of intense seismic and volcanic activity stretching approximately 40,000 km around the margins of the Pacific Ocean. It contains about 75% of the world's active volcanoes and generates approximately 90% of earthquakes.

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Types of Volcano

Volcanoes are classified by their shape, composition and eruption style:

Shield Volcanoes

Feature	Details
Shape	Broad, gently sloping dome with a large base; slope angles typically 2–10°
Composition	Basaltic lava (low silica, ~50% SiO₂); low viscosity; high temperature (~1,100–1,200°C)
Eruption style	Effusive — lava flows gently from the vent and travels long distances (up to 50 km)
Explosivity	Low — gases escape easily from the fluid lava
Examples	Mauna Loa (Hawaii) — the world's largest active volcano by volume (~75,000 km³); Kilauea (Hawaii); Skjaldbreiður (Iceland)
Associated hazards	Lava flows (slow-moving, allowing evacuation); volcanic fog (vog); rarely pyroclastic

Composite (Stratovolcanoes)

Feature	Details
Shape	Steep-sided, symmetrical cone; slope angles typically 25–35°; built of alternating layers of lava and pyroclastic material
Composition	Andesitic to rhyolitic lava (intermediate-high silica, 55–75% SiO₂); high viscosity; lower temperature (~800–1,000°C)
Eruption style	Explosive — viscous lava traps gases, which build pressure until violent eruption
Explosivity	High — pyroclastic flows, ash columns, lahars, debris avalanches
Examples	Mount St. Helens (USA), Mount Pinatubo (Philippines), Mount Vesuvius (Italy), Mount Fuji (Japan), Nevado del Ruiz (Colombia)
Associated hazards	Pyroclastic flows, lahars, tephra fall, lava flows, volcanic bombs, sector collapse

Cinder Cones

Small, steep-sided cones built from tephra (volcanic fragments). Typically < 300 m high. Often occur on the flanks of larger volcanoes. Example: Parícutin, Mexico (emerged in a cornfield in 1943 and grew to 424 m in its first year).

Calderas

Very large volcanic depressions formed by the collapse of a magma chamber after a massive eruption. Yellowstone Caldera (USA) measures 72 × 55 km and last erupted catastrophically ~640,000 years ago. These represent supervolcanic systems.

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Volcanic Hazard Types

graph TD
    A["Volcanic Eruption"] --> B["Primary Hazards<br/>(directly from eruption)"]
    A --> C["Secondary Hazards<br/>(triggered by eruption)"]
    B --> D["Lava flows"]
    B --> E["Pyroclastic flows"]
    B --> F["Tephra / ash fall"]
    B --> G["Volcanic gases"]
    B --> H["Volcanic bombs"]
    C --> I["Lahars"]
    C --> J["Tsunamis"]
    C --> K["Landslides / debris avalanches"]
    C --> L["Jökulhlaups (glacial outburst floods)"]
    C --> M["Acid rain"]

Lava Flows

• Basaltic lava (shield volcanoes): low viscosity, travels fast (up to 60 km/h on steep slopes), but typically 1–10 km/h on gentle slopes
• Andesitic/rhyolitic lava (stratovolcanoes): high viscosity, travels slowly (metres to tens of metres per day), often forms lava domes
• Lava flows rarely kill people directly (they are usually slow enough to evacuate from) but destroy buildings, infrastructure and agricultural land
• Pāhoehoe (smooth, ropy basaltic lava) and ʻaʻā (rough, clinkery basaltic lava) are the two main surface textures

Pyroclastic Flows (Nuées Ardentes)

Key Definition: A pyroclastic flow is a fast-moving current of superheated gas, ash, and rock fragments (collectively called pyroclastic density currents) that races down the flanks of a volcano at speeds of 100–700 km/h and temperatures of 200–700°C.

Pyroclastic flows are the most lethal volcanic hazard. They are virtually impossible to outrun, and survival within the flow is effectively zero. They were famously described after the 1902 eruption of Mount Pelée, Martinique, which killed approximately 29,000 people in the town of Saint-Pierre — the deadliest volcanic disaster of the 20th century.

Tephra and Ash Fall

Tephra is the collective term for all material ejected into the air by a volcanic eruption:

Size Classification	Diameter	Term
Fine ash	< 2 mm	Volcanic ash
Lapilli	2–64 mm	Lapilli
Blocks and bombs	> 64 mm	Volcanic bombs (molten when ejected), blocks (solid when ejected)

Ash fall can:

• Collapse roofs when accumulated (wet ash is especially heavy — 1 m of wet ash can weigh over 1,000 kg/m²)
• Contaminate water supplies
• Destroy crops and livestock
• Disrupt aviation (volcanic ash can melt inside jet engines and re-solidify on turbine blades)
• Cause respiratory problems (fine silica particles cause silicosis)

Lahars (Volcanic Mudflows)

Key Definition: A lahar is a destructive mudflow composed of volcanic debris and water that travels down river valleys on the flanks of a volcano, often at speeds of 20–60 km/h.

Lahars are triggered by:

• Eruptions melting summit glaciers or snow
• Heavy rainfall on loose volcanic deposits
• Volcanic debris damming rivers, then the dam failing
• Eruptions into crater lakes

Case Study: Nevado del Ruiz, Colombia (1985) The eruption of Nevado del Ruiz on 13 November 1985 was relatively small (VEI 3), but pyroclastic flows melted approximately 10% of the summit ice cap. The resulting lahars travelled at up to 60 km/h down river valleys, burying the town of Armero (population ~29,000) under 5 m of mud. Approximately 23,000 people died. The tragedy was compounded by:

• Hazard maps had been produced showing Armero in the lahar danger zone, but were not effectively communicated
• Warnings were issued but contradicted by local officials who told residents to remain calm
• There was no evacuation plan despite known risk
• The town had been destroyed by a lahar from the same volcano in 1845

Volcanic Gases

Volcanoes emit gases including water vapour (H₂O, ~60%), carbon dioxide (CO₂, ~25%), sulphur dioxide (SO₂, ~10%), hydrogen sulphide (H₂S), hydrogen fluoride (HF) and hydrogen chloride (HCl).

Lake Nyos disaster (1986): CO₂ that had accumulated in the deep waters of a volcanic crater lake in Cameroon was suddenly released (a limnic eruption), creating a dense cloud of CO₂ that flowed downhill and asphyxiated approximately 1,746 people and 3,500 livestock within a 25 km radius.

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The Volcanic Explosivity Index (VEI)