Volcanic Eruption Physics: Magma, Pressure & Pyroclastic Flows

Silica content decides everything

Magma is molten rock from the mantle or melted crust, and its silica (SiO₂) content is the single most important factor controlling how a volcano erupts. High-silica magma forms long polymer chains that make it extremely viscous — comparable to asphalt — trapping any gas that tries to escape. Low-silica basalt, by contrast, is closer to honey and lets gas bubbles rise and escape easily.

Type      SiO2 %   Viscosity (Pa·s)   Character
Basalt     45-52    10-100              Low-viscosity, runny
Andesite   52-63    10³-10⁵             Intermediate
Rhyolite   68-77    10⁸-10¹²            Extremely viscous

Compare: water = 10⁻³ Pa·s, honey = 10-100 Pa·s, asphalt = 10⁸ Pa·s

The champagne-bottle physics of gas exsolution

Deep underground, magma sits under enormous lithostatic pressure — at 5 km depth, roughly 130 MPa — and at these pressures water, CO₂ and SO₂ stay dissolved in the melt, exactly like CO₂ in a sealed soda bottle. As magma rises, pressure drops below the saturation point of these gases, bubbles nucleate on crystal surfaces, and grow as more gas diffuses in. In low-viscosity basalt the bubbles rise, coalesce and escape gently, producing an effusive eruption. In high-viscosity rhyolite the bubbles can't escape — they grow until the magma fragments into pyroclasts, and a decompression wave propagates down the conduit at near-sonic speed as an explosive eruption.

Effusive, Strombolian and Plinian eruptions

Hawaiian shield volcanoes like Kīlauea and Mauna Loa erupt fluid basaltic lava that advances at 1–50 km/h — the 2018 Kīlauea eruption destroyed 700 homes over three months but was rarely lethal because evacuation was possible. Strombolian eruptions produce discrete explosions every few minutes as gas slugs burst at the surface — Stromboli itself has been nearly continuously active for over 2,000 years. The most powerful and dangerous style is Plinian, named after Pliny the Elder who died observing the 79 AD eruption of Vesuvius: a continuous jet of gas and pyroclasts erupts at 100–700 m/s, building an eruption column that can reach 40 km into the stratosphere, as seen at Pinatubo in 1991 and Krakatoa in 1883.

The VEI scale and pyroclastic flows

The Volcanic Explosivity Index is logarithmic: each unit represents a tenfold increase in erupted volume, from VEI 0 (Kīlauea's effusive eruptions) up to VEI 8 "supervolcano" events like Toba roughly 74,000 years ago, which may have reduced the human population to around 10,000 individuals. The most lethal volcanic hazard is the pyroclastic density current — a mixture of hot gas (200–700°C), ash and rock fragments flowing at 100–700 km/h when an eruption column collapses or a lava dome fails. In 1902 a pyroclastic flow killed roughly 28,000 people at Saint-Pierre, Martinique, in under two minutes. Secondary hazards compound the danger: lahars (volcanic mudflows) killed 23,000 at Armero in 1985, and the 2010 Eyjafjallajökull eruption's ash cloud closed European airspace for six days.

Frequently asked questions

Try it live

Everything above runs in your browser — open Volcanic Eruption Simulator and adjust magma viscosity, gas content and pressure build rate to trigger anything from a gentle lava flow to a full explosive eruption. Nothing is installed, nothing is uploaded.