9.2 Volcanic Styles & Hazards
The style of a volcanic eruption—whether it gently effuses lava flows or violently ejects pyroclastic material into the stratosphere—is fundamentally controlled by the viscosity of the magma and its volatile content. These two parameters determine whether dissolved gases can escape the ascending magma peacefully or whether they accumulate and drive explosive fragmentation. Understanding eruption styles is critical for volcanic hazard assessment and risk mitigation.
~1,500
Active Volcanoes on Land
~60/yr
Eruptions Per Year
~800 M
People Living Near Active Volcanoes
Magma Viscosity: The Master Variable
Viscosity is the single most important physical property controlling eruption dynamics. It depends on temperature, composition (especially SiO2 content), dissolved volatile content, and crystal fraction. The viscosity of natural magmas spans over ten orders of magnitude.
Arrhenius Viscosity Law
\[ \eta = \eta_0 \cdot \exp\!\left(\frac{E_a}{RT}\right) \cdot f\left(\text{SiO}_2,\; \text{H}_2\text{O},\; \phi_{\text{xt}}\right) \]
η0 = pre-exponential factor (Pa·s)
Ea = activation energy for viscous flow (~200–500 kJ/mol)
R = gas constant (8.314 J/mol·K)
T = absolute temperature (K)
φxt = crystal volume fraction (increases effective viscosity dramatically above ~40%)
| Magma Type | SiO2 (wt%) | T (°C) | η (Pa·s) | Eruption Style |
|---|---|---|---|---|
| Basalt | 45–52 | 1100–1250 | 10²–10³ | Effusive |
| Andesite | 52–63 | 950–1100 | 10³–10&sup5; | Mixed |
| Dacite | 63–69 | 850–1000 | 10&sup5;–10&sup8; | Explosive |
| Rhyolite | 69–77 | 700–900 | 10&sup6;–10&sup{12} | Highly explosive |
Water dissolved in silicate melt dramatically reduces viscosity by depolymerizing the silicate network. Adding 3 wt% H2O to rhyolite can decrease its viscosity by 3–4 orders of magnitude. However, as water exsolves during ascent (forming bubbles), the remaining melt becomes more viscous, increasing the likelihood of explosive fragmentation.
Volatiles & Fragmentation
Magmatic volatiles—principally H2O (0.1–6 wt%), CO2(50–5000 ppm), SO2, HCl, and HF—are dissolved in the melt at depth but exsolve as the magma ascends and decompresses. The solubility of water in silicate melt decreases with decreasing pressure, following an approximate square-root law:
Water Solubility
\[ X_{\text{H}_2\text{O}} \approx c \cdot \sqrt{P} \]
where XH2O is the dissolved water content (wt%), c is a composition-dependent constant, and P is pressure. At ~200 MPa (~6 km depth), basalt can dissolve ~4 wt% H2O; at atmospheric pressure, solubility drops to near zero.
Fragmentation occurs when the magma rise rate exceeds the rate at which gas bubbles can escape through the permeable magma foam. The critical threshold is reached when the volume fraction of vesicles (bubbles) exceeds approximately 75–80%, at which point the bubbly melt disintegrates into a gas-particle mixture that is ejected explosively. Low-viscosity basaltic magmas allow bubbles to rise and burst at the surface relatively gently (Strombolian activity), while high-viscosity rhyolitic magmas trap bubbles, building pressure until catastrophic fragmentation drives Plinian columns tens of kilometers into the atmosphere.
Eruption Style Classification
Hawaiian
Low-viscosity basaltic lava fountains (50–500 m high) and fluid lava flows. Lava lakes form in summit calderas. Effusion rates of 100–1000 m³/s. Minimal pyroclastic hazard beyond immediate vent area. Type example: Kīlauea, Hawai'i.
Strombolian
Rhythmic bursting of large gas bubbles at the magma surface produces discrete explosions every few seconds to minutes, ejecting incandescent cinder and bombs to heights of 100–300 m. Basaltic to basaltic-andesitic composition. Type example: Stromboli, Italy.
Vulcanian
Short-lived, violent explosions caused by rupture of a solidified plug overlying a pressurized conduit. Produces ash columns to 5–20 km, ballistic blocks, and small pyroclastic flows. Andesitic to dacitic composition. Type example: Vulcano, Italy; Sakurajima, Japan.
Plinian
Sustained eruption columns reaching 20–45 km into the stratosphere, driven by high mass eruption rates (107–109 kg/s) of gas-rich, high-viscosity magma. Produces widespread tephra fall, devastating pyroclastic density currents upon column collapse, and caldera formation. Dacitic to rhyolitic composition. Type examples: Vesuvius 79 AD, Pinatubo 1991, Tambora 1815.
Volcanic Explosivity Index (VEI)
The VEI is a logarithmic scale (0–8) that classifies eruptions by the volume of tephra ejected. Each VEI increment represents roughly a tenfold increase in erupted volume. Developed by Newhall & Self (1982), it remains the standard measure of eruption magnitude.
| VEI | Volume (km³) | Column Height | Description | Example | Frequency |
|---|---|---|---|---|---|
| 0 | <10&sup{-4} | <100 m | Non-explosive | Kīlauea | Daily |
| 2 | 10&sup{-3}–10&sup{-2} | 1–5 km | Explosive | Stromboli | Weekly |
| 4 | 10&sup{-1}–1 | 10–25 km | Cataclysmic | Eyjafjallajökull (2010) | ~yearly |
| 5 | 1–10 | >25 km | Paroxysmal | Mt. St. Helens (1980) | ~10 yr |
| 6 | 10–100 | >25 km | Colossal | Pinatubo (1991) | ~100 yr |
| 7 | 100–1000 | >25 km | Super-colossal | Tambora (1815) | ~1000 yr |
| 8 | >1000 | >25 km | Mega-colossal | Toba (74 ka), Yellowstone | ~50,000 yr |
VEI 8 “supervolcanoes” erupt >1000 km³ of dense rock equivalent (DRE). The three most notable are Yellowstone (western USA; last VEI 8 at 2.1 Ma, 640 ka), Toba (Sumatra; 74 ka, ~2800 km³ DRE), and Taupo (New Zealand; 26.5 ka Oruanui eruption, ~1170 km³ DRE). These events have global climatic impacts through sulfate aerosol injection into the stratosphere.
Volcanic Hazards
Pyroclastic Density Currents (PDCs)
Ground-hugging flows of hot gas, ash, and rock fragments traveling at 100–700 km/h with temperatures of 200–700°C. PDCs are the deadliest volcanic hazard: they annihilated Pompeii in 79 AD, killed 29,000 at Mont Pelée in 1902, and devastated the north flank of Mt. St. Helens in 1980. PDCs form by column collapse (when the eruption column becomes too dense to remain buoyant) or by dome collapse on steep volcanic slopes.
Lahars (Volcanic Mudflows)
Rapidly flowing mixtures of volcanic debris and water that travel down valleys at 20–60 km/h. They can be triggered by eruptions melting summit glaciers, by heavy rainfall on fresh ash deposits, or by lake breakouts. Lahars killed 23,000 people at Nevado del Ruiz, Colombia in 1985 when the Armero lahar buried the town under 5 m of mud. Lahars can travel >100 km from the volcano.
Tephra Fall & Ballistics
Airborne volcanic fragments range from fine ash (<2 mm) to large ballistic blocks (>64 mm). Tephra fall can collapse roofs (loads >10 kPa), disrupt aviation (silicate glass melts in jet turbines at ~1100°C), contaminate water supplies, and cause respiratory illness. The 2010 Eyjafjallajökull eruption disrupted European air traffic for weeks despite being only VEI 4.
Volcanic Edifice Classification
Shield Volcano
Broad, gently sloping edifice (slopes 2–10°) built by successive flows of low-viscosity basaltic lava. Enormous volumes: Mauna Loa contains ~75,000 km³ of basalt. Summit calderas form by magma reservoir drainage. Examples: Mauna Loa, Mauna Kea, Fernándina (Galapagos).
Stratovolcano (Composite)
Steep-sided cone (slopes 20–35°) built by alternating layers of lava flows, pyroclastic deposits, and lahars. Andesitic to dacitic composition. These are the classic arc volcanoes and the most hazardous. Examples: Mt. Fuji, Mt. Rainier, Cotopaxi, Vesuvius.
Caldera
Large collapse depression (1–80 km diameter) formed when the roof of a shallow magma reservoir collapses during a large eruption. Ring fractures feed post-caldera volcanism. Examples: Yellowstone (72 × 55 km), Crater Lake (Mt. Mazama), Santorini, Taupo.
Cinder Cone & Maar
Cinder cones: Small (<300 m high), steep-sided cones of scoria built by Strombolian eruptions. Short-lived, often monogenetic. Example: Parícutin, Mexico.
Maars: Low-relief craters formed by phreatomagmatic explosions (magma-water interaction). Surrounded by a low tephra ring. Example: Eifel district, Germany.
Magma Chamber Dynamics
The traditional view of magma chambers as large, liquid-filled cavities beneath volcanoes has been replaced by the crystal mush model. Geophysical imaging (seismic tomography, magnetotellurics) consistently shows that sub-volcanic reservoirs contain only 5–30% melt distributed within a crystalline framework. Key features of the modern understanding include:
- Mush reservoirs: Large volumes (10s–1000s km³) of crystal-rich material (50–70% crystals) at near-solidus temperatures. Melt is present in interstitial pockets.
- Melt segregation: Eruptible magma (melt fraction >40%) accumulates in lenses within the mush, driven by compaction, reactive infiltration, and gas-driven filter pressing. These lenses may assemble rapidly before eruptions.
- Recharge-driven eruptions: Injection of hot, volatile-rich mafic magma into a cooler silicic reservoir can remobilize the mush, mix compositions, and trigger eruptions. Many large eruptions show evidence of magma mixing in the form of banded pumice and disequilibrium mineral assemblages.
- Timescales: Crystal residence times from diffusion chronometry (Fe–Mg in olivine, Ba in sanidine) indicate that mush storage lasts 10³–10&sup5; years, while melt accumulation and eruption triggering can occur within months to decades.
Effusion Rate & Eruption Style
The mass eruption rate (MER) determines eruption intensity and column height:
\[ H \approx 0.24 \cdot \dot{M}^{\,0.25} \]
where H is the column height in km and Ṁ is the mass eruption rate in kg/s (Sparks et al., 1997). A Plinian eruption with Ṁ = 108 kg/s produces a column ~24 km high. Higher effusion rates with low-viscosity magma produce longer, more extensive lava flows rather than taller columns.
Key Takeaways
- Viscosity (η) spans 10 orders of magnitude from basalt to rhyolite, controlling eruption style
- H2O (up to 6 wt%) is the dominant volatile; its exsolution drives explosive fragmentation
- Fragmentation occurs when vesicularity exceeds ~75–80% and gas cannot escape fast enough
- The VEI scale (0–8) is logarithmic; VEI 8 supervolcanoes erupt >1000 km³ DRE
- Pyroclastic density currents (100–700 km/h) are the deadliest volcanic hazard
- Modern magma chamber models invoke crystal mush with small melt-rich lenses, not large liquid bodies
- Column height H ∝ Ṁ0.25: mass eruption rate governs eruption intensity