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Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.5 Hazards — vulcanicity: the nature of vulcanicity and its relationship to plate tectonics; the forms of volcanic hazard, the spatial distribution, magnitude, frequency, regularity and predictability of volcanic events; and the contrasting impacts (primary/secondary, social/economic/environmental). This depth lesson develops the physical controls on eruption style — magma chemistry, viscosity, gas content and the eruption mechanics that flow from them — that determine which hazards a volcano produces. It builds directly on the boundary processes of the plate-tectonics lesson (§3.1.5), draws on the systems framing of §3.1.1 (the volcano as an energy-and-matter system transferring heat and material from mantle to surface), and links forward to the human-impact and vulnerability content of §3.2. Assessment objectives: AO1 (magma chemistry, eruption mechanics, hazard types), AO2 (applying chemistry to predict hazard at a named volcano) and AO3 (manipulating and evaluating VEI, viscosity and gas-content data).
The controlling idea is that a volcano's hazard profile is set, before it ever erupts, by the chemistry of its magma. Silica content, dissolved gas and temperature together determine viscosity; viscosity determines whether gases escape gently or build to explosive failure; and that single split — effusive versus explosive — cascades into everything that follows, from landform to death toll. Reading magma chemistry is therefore the master skill of volcanic geography, and it feeds straight into the Hazard term of the risk equation introduced in lesson 1.
It is useful to begin with where magma comes from, because the three magma types are not arbitrary chemical recipes but the predictable products of three different melting processes at three different tectonic settings. The mantle is solid almost everywhere; it melts only when one of three things happens. Decompression melting occurs where hot mantle rises and the confining pressure drops (its melting point falls faster than its temperature), producing low-silica basalt at constructive margins and hotspots. Flux melting occurs at subduction zones, where water driven off the descending slab lowers the melting point of the overlying mantle wedge, generating wetter, gas-charged magma that then evolves toward intermediate and silicic compositions. Heat-transfer (or crustal) melting occurs where rising basaltic magma ponds at the base of, or within, thick continental crust and partially melts it, producing the most silica-rich rhyolitic magmas — the fuel of caldera systems. So magma chemistry, viscosity, eruption style and hazard type all trace back, ultimately, to the plate-boundary processes of lesson 1: the tectonic setting determines the melting mechanism, which determines the chemistry, which determines the hazard. Holding that whole chain in view is what turns a descriptive account of volcanoes into the explanatory, synoptic geography AQA is looking for.
As with every hazard in this course, magma chemistry only sets the Hazard term. Whether a given eruption becomes a disaster still depends on Vulnerability (who lives on the volcano's flanks and in its valleys) and Capacity (whether they are monitored, warned and evacuated). The next lesson's case studies make this explicit; here the task is to understand the physics thoroughly enough to predict, from chemistry alone, what kind of hazard a volcano will pose — which is the first step in managing it.
| Property | Basaltic | Andesitic | Rhyolitic |
|---|---|---|---|
| SiO₂ content | 45–52% (low) | 52–63% (intermediate) | 63–77% (high) |
| Temperature | 1,000–1,200 °C | 800–1,000 °C | 650–800 °C |
| Viscosity | Low (very fluid) | Intermediate | High (very viscous) |
| Dissolved gas | Low (~0.5–2%) | Moderate (~3–4%) | High (~4–7%+) |
| Eruption style | Effusive | Mixed | Explosive |
| Typical setting | Constructive margins; hotspots | Destructive margins (subduction) | Destructive margins; calderas |
| Typical VEI | 0–2 | 2–4 | 4–8 |
Silicon and oxygen form silica tetrahedra (SiO₄) in the melt. As silica content rises, these tetrahedra polymerise — linking into long chains and networks that physically obstruct flow. This raises viscosity by orders of magnitude: basaltic magma at ~10²–10³ Pa·s flows like warm honey, whereas rhyolitic magma at ~10⁶–10⁹ Pa·s barely flows at all.
Viscosity, in turn, controls gas escape. The same volatiles dissolved at depth (water, CO₂, SO₂) exsolve into bubbles as magma rises and pressure falls (decompression). In low-viscosity basalt, bubbles rise and escape freely — gas leaks out, pressure stays low, the eruption is gentle. In high-viscosity rhyolite, bubbles are trapped; gas pressure builds until it exceeds the strength of the magma, which then fragments catastrophically into ash and pumice — an explosive eruption. This is the central causal chain of the whole topic:
flowchart TD
A["High SiO2 content"] --> B["Silica polymerisation"]
B --> C["High viscosity"]
C --> D["Gas bubbles trapped"]
D --> E["Pressure build-up"]
E --> F["Explosive fragmentation: ash, pumice, pyroclastic flows"]
G["Low SiO2 content"] --> H["Low viscosity"]
H --> I["Gas escapes freely"]
I --> J["Effusive lava flows"]
It is worth adding that magma temperature reinforces the silica control on viscosity rather than opposing it. Basaltic magma is not only silica-poor but also hotter (1,000–1,200 °C), and higher temperature makes a melt more fluid, so the hottest magmas are also the runniest — a double reason basalt erupts effusively. Silicic magmas are both silica-rich and cooler (650–800 °C), so high silica and low temperature both push viscosity up, which is why rhyolite is so extraordinarily viscous (up to a billion times more so than basalt) and so prone to explosive behaviour. Temperature, silica content and gas content therefore act together, all pointing the same way, which is why the three magma types form such a clean spectrum from hot, fluid, gas-poor, effusive basalt to cool, viscous, gas-rich, explosive rhyolite.
The mechanism by which gas drives an explosive eruption deserves to be set out step by step, because it is the heart of the topic and a frequent six-mark target. At depth, volatiles are dissolved in the melt under high confining pressure. As magma ascends, pressure falls and the gases reach saturation and begin to exsolve into bubbles (a process called vesiculation). In fluid basalt the bubbles rise and burst at the surface harmlessly, sometimes producing fire-fountaining. In viscous silicic magma the bubbles cannot escape; they grow and the magma foams. As pressure continues to drop, the bubbles expand explosively, and when the gas volume fraction exceeds roughly 75–80% the foam fragments — the continuous liquid disintegrates into a high-speed spray of gas, glassy ash and pumice clasts. This fragmentation surface is, in effect, the moment a body of magma becomes a volcanic explosion. The energy released is enormous: the sudden conversion of dissolved chemical-and-pressure energy into kinetic energy can drive an eruption column upward at speeds exceeding 100 m/s and inject material tens of kilometres into the stratosphere. Understanding fragmentation explains why gas content (not just silica) matters — a relatively silica-poor but extremely gas-rich magma can still erupt explosively, while a degassed silicic magma may quietly extrude a lava dome.
The general rule, however, holds: because effusive lava is slow (typically walking pace or less) and follows topography predictably, it is the one major volcanic hazard from which people can usually be evacuated in time, so effusive eruptions overwhelmingly threaten property and land rather than life. The economic loss can still be enormous — buried towns, farmland and infrastructure — but the death toll is typically low. This is the single clearest illustration of the lesson's central theme: hazard type, set by magma chemistry, largely determines whether a volcano is primarily a threat to lives or to livelihoods.
Intermediate-to-high-silica magmas fragment into tephra (collective term for airborne ejecta: ash <2 mm, lapilli 2–64 mm, blocks/bombs >64 mm), drive towering eruption columns, and generate pyroclastic density currents. Magnitude is standardised by the Volcanic Explosivity Index (VEI), a logarithmic scale of erupted volume from 0 to 8 — each step represents roughly a tenfold increase in erupted tephra volume.
| VEI | Description | Plume height | Tephra volume | Example |
|---|---|---|---|---|
| 0 | Effusive | < 100 m | < 10⁴ m³ | Kīlauea (continuous) |
| 2 | Explosive | 1–5 km | > 10⁶ m³ | Stromboli |
| 3 | Severe | 3–15 km | > 10⁷ m³ | Nevado del Ruiz (1985) |
| 4 | Cataclysmic | 10–25 km | > 0.1 km³ | Eyjafjallajökull (2010) |
| 5 | Paroxysmal | > 25 km | > 1 km³ | Mount St Helens (1980) |
| 6 | Colossal | > 25 km | > 10 km³ | Pinatubo (1991) |
| 7 | Super-colossal | > 25 km | > 100 km³ | Tambora (1815) |
| 8 | Mega-colossal | > 25 km | > 1,000 km³ | Yellowstone (~640 ka) |
Because the scale is logarithmic, a VEI 6 (Pinatubo) erupts ~10 times more material than a VEI 5 (St Helens) and ~100 times more than a VEI 4 (Eyjafjallajökull) — a point that becomes a quantitative skills exercise below.
A useful discipline is to separate primary hazards — those produced directly by the eruption itself (lava, PDCs, tephra fall, gas) — from secondary hazards, which are triggered by, but follow on from, the eruption (lahars, jökulhlaups, landslides, tsunami, climatic cooling). The distinction is examinable in its own right and is also analytically important, because the secondary hazards are frequently the bigger killers and persist long after the eruption has ended.
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