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Volcanic eruptions are the most visually dramatic of all tectonic hazards, ranging from gentle lava flows to catastrophic explosive events that can affect global climate. This lesson examines the processes that generate volcanic activity, the different types of magma and eruption styles, the variety of volcanic landforms, and the primary and secondary hazards associated with volcanism. This content supports Edexcel A-Level Geography Enquiry Questions 1 and 2 (EQ1 and EQ2).
Volcanic eruptions begin with the generation of magma — molten rock beneath the Earth's surface. Magma forms when solid rock in the mantle or lower crust partially melts. Three main processes trigger melting:
As rock in the mantle rises toward the surface (at mid-ocean ridges or above mantle plumes), the decrease in pressure lowers the melting point. The rock's temperature does not need to increase — it simply crosses the solidus (the temperature-pressure curve below which rock is entirely solid) due to reduced pressure. This produces basaltic magma, which is low in silica and has low viscosity.
At destructive plate boundaries, water released from the subducting oceanic slab (from hydrated minerals such as serpentinite) infiltrates the overlying mantle wedge. This water acts as a flux, lowering the melting point of the mantle rock by 200–400°C. The resulting magma is typically andesitic, intermediate in silica content and viscosity.
Hot basaltic magma rising through continental crust can transfer enough heat to melt the surrounding (granitic) rock, generating rhyolitic magma — high in silica, high in viscosity, and extremely gas-rich. This process is particularly important at continental hotspots (e.g., Yellowstone).
The characteristics of magma fundamentally determine eruption style, volcanic landform and hazard type.
| Property | Basaltic | Andesitic | Rhyolitic |
|---|---|---|---|
| Silica content | 45–52% (low) | 52–63% (intermediate) | 63–78% (high) |
| Temperature | 1,000–1,200°C | 800–1,000°C | 650–900°C |
| Viscosity | Low (flows easily) | Intermediate | Very high (thick, sticky) |
| Gas content | Low (1–2%) | Moderate (2–4%) | High (4–7%) |
| Eruption style | Effusive (lava flows) | Mixed (effusive + explosive) | Explosive (pyroclastic) |
| Tectonic setting | Divergent boundaries, hotspots | Subduction zones | Continental hotspots, subduction |
| Typical landform | Shield volcano, lava plateau | Composite (stratovolcano) | Caldera, lava dome |
The relationship between silica content and viscosity is critical: higher silica content creates more polymerised molecular structures, making the magma thicker and less able to flow. High viscosity traps dissolved gases, building pressure that leads to explosive eruptions.
Exam Tip: Examiners reward candidates who explain the causal mechanism linking silica content to eruption style. Simply stating "basaltic magma is runny and rhyolitic is sticky" is insufficient at A-Level. Explain that silica molecules form polymer chains that resist flow, and that high viscosity traps volatiles (H₂O, CO₂, SO₂), causing pressure build-up and explosive fragmentation of magma.
| Feature | Detail |
|---|---|
| Shape | Broad, gently sloping dome (slope angle ~2–10°) |
| Size | Very large — Mauna Loa (Hawaii) has a volume of ~75,000 km³ |
| Magma type | Basaltic (low viscosity) |
| Eruption style | Effusive — lava flows, fire fountains, lava lakes |
| Hazard level | Generally low — lava flows are slow-moving and predictable |
| Examples | Mauna Loa, Kilauea (Hawaii); Erta Ale (Ethiopia) |
Mauna Loa, measured from its base on the ocean floor, rises approximately 9,170 m — taller than Mount Everest. Its gently sloping shape results from the low viscosity of basaltic lava, which flows long distances before solidifying.
| Feature | Detail |
|---|---|
| Shape | Steep-sided, symmetrical cone (slope angle ~25–35°) |
| Size | Medium to large — typically 2,000–4,000+ m high |
| Magma type | Andesitic to dacitic (intermediate to high viscosity) |
| Eruption style | Explosive — pyroclastic flows, tephra, lahars; interspersed with lava flows |
| Hazard level | Very high — explosive eruptions with multiple hazard types |
| Examples | Mt. St. Helens (USA), Mt. Pinatubo (Philippines), Vesuvius (Italy), Merapi (Indonesia) |
Composite volcanoes are built from alternating layers of lava flows and pyroclastic material (hence "strato-volcano"). They are the most dangerous type of volcano because their intermediate-to-high viscosity magma traps gases, leading to violent explosive eruptions, and their steep slopes channel pyroclastic flows and lahars into populated valleys.
| Feature | Detail |
|---|---|
| Shape | Large, roughly circular depression |
| Size | Can exceed 100 km in diameter |
| Formation | Collapse of the ground surface into an emptied magma chamber following a catastrophic eruption |
| Magma type | Rhyolitic (extremely high silica, very explosive) |
| Eruption style | Ultra-explosive (super-eruptions ejecting > 1,000 km³ of material) |
| Examples | Yellowstone (USA), Toba (Indonesia), Campi Flegrei (Italy) |
The Volcanic Explosivity Index (VEI) measures the magnitude of volcanic eruptions on a scale from 0 to 8:
| VEI | Ejecta Volume | Description | Example | Frequency |
|---|---|---|---|---|
| 0 | < 10,000 m³ | Non-explosive | Kilauea (continuous) | Daily |
| 2 | > 1,000,000 m³ | Explosive | Stromboli (frequently) | Weekly |
| 4 | > 0.1 km³ | Cataclysmic | Eyjafjallajökull (2010) | ~yearly |
| 5 | > 1 km³ | Paroxysmal | Mt. St. Helens (1980) | ~every decade |
| 6 | > 10 km³ | Colossal | Pinatubo (1991) | ~every century |
| 7 | > 100 km³ | Super-colossal | Tambora (1815) | ~every millennium |
| 8 | > 1,000 km³ | Ultra-Plinian | Toba (~74,000 yrs ago) | ~every 50,000 yrs |
Molten rock reaching the surface and flowing downslope. The hazard depends on viscosity, effusion rate, topography and volume:
The most lethal volcanic hazard. A pyroclastic flow is a fast-moving current of hot gas, volcanic ash and rock fragments that travels down the flanks of a volcano at speeds of 100–700 km/h with temperatures of 200–700°C.
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