How Volcanoes Work: Magma, Pressure & Eruption Physics

Melting rock that isn't supposed to melt

The mantle is mostly solid peridotite, kept solid by immense pressure despite temperatures of 1300-1500°C. Three mechanisms disturb that balance enough to produce melt: decompression melting at mid-ocean ridges, where rising mantle rock decompresses faster than it cools; flux melting at subduction zones, where water released from a sinking slab lowers the surrounding rock's melting point; and heat transfer from mantle plumes beneath hotspots like Hawaii and Iceland. The resulting magma's silica content decides everything downstream — basaltic magma (~50% SiO₂) is hot and runny, rhyolitic magma (~75% SiO₂) is viscous and gas-rich.

Buoyancy drives the ascent

Magma is less dense than the solid rock around it, so it rises the way oil rises through water. As it climbs, falling pressure lets dissolved gases — H₂O, CO₂, SO₂ — come out of solution and form bubbles, a process called vesiculation. At 8 km depth, basaltic melt can hold about 4 wt% water; near the surface that solubility drops to a fraction of a percent, so exsolution accelerates sharply on the final approach — the critical step that separates a quiet eruption from an explosive one.

ΔP/Δz = (ρ_rock − ρ_melt) · g ≈ (3300 − 2600) × 9.8 ≈ 6860 Pa/m
η_basalt  ≈ 10²–10³ Pa·s   (honey-like at ~1200°C)
η_rhyolite ≈ 10⁶–10¹² Pa·s  (near-solid at surface conditions)

Four ways a volcano can erupt

Eruption style is a spectrum set by magma viscosity and gas content. Hawaiian eruptions effuse low-viscosity basalt as lava fountains and flows, gas escaping gradually (Kīlauea). Strombolian eruptions burst intermittently as large gas pockets rise and pop, hurling lava bombs up to 300 m. Vulcanian eruptions are short violent blasts that blow out a solidified plug sealing the vent. Plinian eruptions — named for Pliny the Younger's account of Vesuvius in 79 CE — sustain a jet of ash and pumice reaching 40-50 km into the stratosphere, with mass discharge rates exceeding 10⁸ kg/s.

Column height (Morton-Taylor-Turner plume equation):
H ≈ k · Q^(1/4)      k ≈ 0.24 km/(kg/s)^(1/4)
Q = 10⁸ kg/s  →  H ≈ 0.24 × (10⁸)^0.25 ≈ 24 km

Lava that flows and rock that flies

Lava behaves as a Bingham plastic — it only flows once shear stress exceeds a yield threshold set by suspended crystals and vesicles. Slow, low-viscosity lava forms smooth ropy pāhoehoe; faster, thicker lava shears its own crust into sharp ʻaʻā rubble. When a Plinian column collapses, the hot ash-and-gas mixture becomes denser than air and races downhill as a pyroclastic density current — up to 700 km/h, 300-800°C, and responsible for most volcanic deaths in history, including the 79 CE Pompeii surge.

Frequently asked questions

Try it live

Everything above runs in your browser — open Volcano Eruption and raise chamber pressure and magma viscosity to decide whether the volcano oozes lava or blasts out an ash column. Nothing is installed, nothing is uploaded.