Deep underground, melted rock called magma is packed with dissolved gas, squeezed in tight by the enormous weight pressing down on it — just like the fizz dissolved in a fizzy drink with the cap still on. When the magma rises toward the surface, the weight on it drops, and the gas can no longer stay dissolved. It rushes out of the magma as bubbles, exactly like the foam that explodes out of a fizzy drink you have shaken and opened. Those bubbles expand violently and blast the magma up and out of the volcano. Thick, sticky magma traps the gas until it bursts; thin, runny magma lets it slip out gently. Build up the pressure in the simulator, then release it and watch the eruption.
Most people think a volcano erupts because the rock gets hot enough to melt and pour out. In fact heat alone makes a slow oozing flow. The violence comes from dissolved gas, which bursts out of solution as the rising magma's pressure falls, exactly like a shaken fizzy drink.
What's actually happening
From the outside an eruption looks like heat winning — the ground gets hot enough to melt and the melt comes pouring out. Heat is certainly part of it, but heat alone makes a slow, oozing lava flow, not the violent blasts that flatten forests and throw ash into the stratosphere. The thing that turns melted rock into an explosion is not the heat at all. It is gas, and the best way to understand it is sitting in your fridge: a sealed bottle of fizzy drink.
Magma deep underground is loaded with dissolved gas, mostly water vapour and carbon dioxide, held invisibly in the melt by the colossal pressure of all the rock stacked above it. A capped fizzy drink is the same trick: carbon dioxide forced into the liquid and kept there by the pressure inside the bottle, which is why it looks like plain liquid until you open it. Now let the magma rise. As it climbs toward the surface the weight pressing on it falls, and just like cracking the cap, the gas can no longer stay dissolved. It comes out of solution as bubbles, the bubbles expand fast as the pressure keeps dropping, and that expansion does the work — shredding the magma into froth and blasting it up the vent. Shake the bottle first and you get a much more violent result, which is exactly what trapped, pent-up gas does in a volcano. The simulator lets you build the gas and pressure up and then release it, so you can watch the bubbles take over and drive the eruption.
Whether the result is a gentle fountain or a catastrophic blast comes down to one property of the magma: how runny it is. Thin, runny basalt magma, low in silica, lets the gas bubble out easily, so Hawaiian volcanoes mostly fountain and flow rather than explode. Thick, sticky magma rich in silica traps the bubbles like setting glue, the pressure builds with nowhere to go, and when it finally lets go it fragments the whole mass at once. That is what happened at Mount St. Helens in 1980: gas-charged sticky magma, suddenly unroofed by a landslide, decompressed in seconds and detonated sideways with the force of a large bomb. Same fizzy-drink physics every time — only the stickiness decides whether you get a fizz or a bang.
An eruption is decompressing fizz, not just heat: gas escaping the magma drives the blast, and the magma's stickiness decides whether you get a fizz or a bang.
- 1Build up the dissolved gas and pressure with the controls while the magma sits capped — nothing happens yet, just like a closed bottle.
- 2Press Erupt to drop the pressure and watch the gas burst out of solution as bubbles that blast the magma up the vent.
- 3Switch the magma from runny to thick and erupt again — the sticky magma traps the gas and the blast comes out far more violently.
Common questions
It comes down to how runny the magma is. Thin, low-silica basalt lets gas bubble out easily in gentle fountains, while thick, sticky high-silica magma traps the bubbles until they fragment the whole mass explosively.
A landslide suddenly uncapped the gas-rich magma. The instant drop in pressure let the gas explode out laterally, flattening around 600 square kilometres of forest in minutes.
Sticky magma can solidify into a plug that seals the vent, letting pressure build with nowhere to go. When the plug finally fails the eruption is far more violent for the wait.