Learning objectives
- Describe the main parts of a volcano and how they differ between shield volcanoes and stratovolcanoes.
- Explain how silica, temperature and crystals control magma viscosity.
- Explain why dissolved gas, together with viscosity and ascent rate, decides whether an eruption is effusive or explosive.
- Compare lava flows, ash fall, pyroclastic density currents and lahars.
Magma and lava
Magma is molten or partly molten rock below the surface, usually carrying crystals and dissolved gases. Once it erupts onto the surface it is called lava. Magma forms where the mantle or crust partly melts, mostly at plate boundaries and hotspots, as explored in thePlate Tectonics simulator. Because it is less dense than the rocks around it, magma rises and may collect in a magma chamber a few kilometres down.
Inside a volcano
- Magma chamber: a reservoir of magma, often a mush of crystals and melt rather than a big empty cave.
- Conduit: the pipe magma rises through to the main vent.
- Crater: the bowl around the summit vent. A much larger collapse after a huge eruption forms a caldera.
- Side vents and dikes: cracks that let magma erupt from the flanks, often building small cones.
- Layers: each eruption adds lava flows or tephra (ash and fragments), building the cone.
Shield volcanoes and stratovolcanoes
| Shield volcano | Stratovolcano |
|---|---|
| Broad, gentle slopes | Steep, cone-shaped |
| Built of thin, runny basalt lava flows | Alternating lava flows and explosive deposits |
| Mostly effusive eruptions | Often explosive eruptions |
| Mauna Loa, Kīlauea | Mount Fuji, Mount St. Helens, Mayon |
Many other forms exist, including cinder cones, lava domes and calderas, and a single volcano can change its behaviour over time.
What makes magma runny or sticky
Viscosity is a fluid’s resistance to flow. Magma viscosity spans more than ten orders of magnitude, from about 10² Pa·s for hot basalt (like warm syrup) to over 10¹⁰ Pa·s for cool rhyolite. Three factors matter most:
- Silica: silicate chains link up into networks, so more silica means stickier magma.
- Temperature: hotter magma flows more easily. Basalt erupts at about 1,100–1,200 °C, rhyolite at about 700–900 °C.
- Crystals and bubbles: suspended crystals make magma stiffer, like adding sand to honey.
Dissolved water also lowers viscosity. The simulator uses a simplified, illustrative relationship rather than a calibrated viscosity model.
Gas: the driver of explosions
Magma holds water, carbon dioxide and other gases dissolved under pressure, the way fizzy drinks hold CO₂. As magma rises, the pressure drops, the gas comes out of solution as bubbles, and the bubbles expand. What happens next depends on whether the gas can escape:
- In runny magma, bubbles can rise and merge, so gas escapes gently or in bursts: effusive eruptions and lava fountains.
- In viscous magma that rises quickly, bubbles are trapped. When they fill roughly three-quarters of the volume, the magma shatters (fragmentation) into ash and gas that blast out: explosive eruptions.
- Viscous magma that rises very slowly can lose its gas through connected bubbles and squeeze out as a lava dome.
Eruption style therefore depends on composition, temperature, gas content, crystals and how fast magma rises, not on any one variable.
Eruption styles
- Hawaiian: lava fountains and long lava flows from runny basalt.
- Strombolian: regular bursts of gas throwing out glowing blobs.
- Vulcanian: short, violent explosions of viscous magma.
- Plinian: sustained columns 20–40 km high, named after Pliny the Younger, who described the eruption of Vesuvius in 79 CE.
The height of an eruption column grows with the eruption rate. A widely used empirical fit is H ≈ 2.0 × Q0.241km, where Q is the volume of magma erupted per second in m³/s (Mastin et al., 2009). The Volcanic Explosivity Index (VEI) ranks eruptions from 0 to 8 by the volume of material ejected.
Volcanic hazards
| Hazard | What it is and how it moves |
|---|---|
| Lava flows | Molten rock following valleys, usually slowly enough to walk away from, destroying everything in its path |
| Ash fall | Rock fragments carried downwind, collapsing roofs and grounding aircraft hundreds of kilometres away |
| Pyroclastic density currents | Hot avalanches of gas, ash and rock, often faster than 100 km/h; the deadliest hazard |
| Lahars | Volcanic mudflows of water and debris rushing down river valleys, even long after an eruption |
Terrain shapes every hazard. In the simulator, lava runs downhill along valleys; pyroclastic currents are modelled with an “energy cone”, reaching any ground lower than a line sloping down from above the summit, so ridges can block them; lahars follow river channels; and ash spreads downwind.
Important: these are educational visualisations, not real-time forecasts or evacuation tools. Always follow official warnings from local authorities and volcano observatories.
Why it matters
- About 800 million people live within 100 km of an active volcano.
- Volcano observatories watch earthquakes, ground swelling and gas output to warn of eruptions.
- Hazard maps guide land use, evacuation routes and lahar warning systems.
- Volcanic ash clouds are tracked to keep aircraft safe; large eruptions can cool the global climate for a year or two.
- Volcanic soils are fertile, and geothermal heat provides clean energy.
Interesting facts
- The 1991 eruption of Mount Pinatubo sent a column about 35 km high and cooled global temperatures by about 0.5 °C.
- In 1985, lahars from Nevado del Ruiz in Colombia buried the town of Armero, killing more than 20,000 people.
- Kīlauea’s 2018 eruption poured out about a cubic kilometre of lava and destroyed over 700 homes.
- Mauna Loa is the largest active volcano on Earth by volume.
Common misconceptions
- “Lava is the biggest danger.” Pyroclastic currents, lahars and ash usually cause far more deaths and damage.
- “All volcanoes look and behave the same.” Shield volcanoes and stratovolcanoes differ in shape, magma and eruption style.
- “Magma chambers are giant underground lakes of liquid.” Many are mostly crystal mush with pockets of melt.
- “Explosiveness depends only on gas.” Viscosity, temperature, crystals and ascent rate all matter.
What the simulator simplifies
The 3D models are generic. The viscosity relationship is simplified and illustrative. Water solubility uses the square-root law S ≈ 0.42·√P wt% (P in MPa), which suits rhyolite better than basalt, and gas expansion assumes an ideal gas at magma temperature. Gas escape is judged by comparing the rise speed of 10 cm coalesced bubbles (Stokes’ law) with the magma’s ascent speed in an assumed conduit, plus a fixed slow-ascent threshold for outgassing; fragmentation is taken at a gas fraction of 0.75. Chamber pressure simply builds to 20 MPa before each eruption. The hazard map uses an invented landscape: lava follows the steepest path downhill for up to 9 km, pyroclastic currents use an energy cone (H/L = 0.35) from 300 m above the summit, lahars follow the steepest path from four valley heads, and ash thickness falls off exponentially downwind. Real hazard assessment uses detailed terrain, eruption scenarios and expert judgement.
Important terms
- Magma
- Molten or partly molten rock beneath the surface.
- Lava
- Magma that has erupted onto the surface.
- Viscosity
- A fluid’s resistance to flow.
- Fragmentation
- The shattering of bubbly magma into gas and ash fragments in an explosive eruption.
- Effusive eruption
- An eruption in which lava flows out rather than being blasted out.
- Pyroclastic density current
- A fast, hot, ground-hugging flow of gas and volcanic fragments.
- Lahar
- A volcanic mudflow of water, ash and debris.
Key takeaways
- Magma is underground; lava is magma at the surface.
- Silica, temperature and crystals control viscosity: basalt is runny, rhyolite is sticky.
- Dissolved gas expands as magma rises; trapped gas in viscous magma makes eruptions explosive.
- Shield volcanoes are built by runny lava; stratovolcanoes by alternating lava and explosive deposits.
- Lava, ash, pyroclastic currents and lahars move differently and are all shaped by the landscape.