Why does a volcano explode instead of just oozing?
After you watchWhy does a volcano explode instead of just oozing?
The short answer
A volcano explodes when thick, gluey magma traps the gas dissolved inside it. As the magma rises, that gas tries to bubble out, but gluey magma won't let the bubbles escape, so pressure builds until the whole top bursts. Runny magma lets the gas slip out the whole way up, so it just oozes and flows instead of exploding.
Try this next
- What if you kept the magma runny but added way more gas to it? Push the gas amount up while leaving the magma thin. Predict first: does more gas finally make a runny volcano explode, or does the thin magma still let it all slip out?
- What if the magma rose much faster toward the surface? Imagine speeding up how fast the magma climbs. Predict whether fast-rising bubbles have enough time to escape, or get trapped like they do in gluey magma.
The whole story
How it works
Magma deep underground has gas (mostly steam and carbon dioxide) dissolved in it, like the fizz hiding in an unopened soda. As magma rises toward the surface, the squeeze on it drops and that gas turns into bubbles. In thin, runny magma the bubbles rise and escape easily, so the gas leaves quietly and the magma just oozes out. In thick, gluey magma the bubbles get stuck and can't escape. The trapped gas keeps expanding and the pressure climbs until it blasts the magma apart into ash and rock fragments — an explosion.
What people get wrong
People often think volcanoes explode because they get too hot, or simply because there is more lava. Heat and amount are not the trigger. Two volcanoes can be the same temperature; the one that explodes is the one whose thick, gluey magma trapped its gas. The runny one lets the same gas escape, so it only oozes.
The catch
Runny volcanoes are calm and rarely explode, but their lava can flow for months and slowly bury roads, forests, and valleys. Thick volcanoes mostly sit quiet for years because gluey magma plugs them up, but when the trapped gas finally wins they erupt all at once, throwing ash miles into the sky, which makes them far more sudden and dangerous.
Questions kids ask
Do volcanoes explode because they get too hot?
No. Two volcanoes can be the same temperature and only one explodes. The real trigger is trapped gas. Thick, gluey magma won't let the gas bubbles escape, so pressure builds until it bursts. Runny magma lets the same gas slip out, so it just oozes.
Where does the gas in magma come from?
The gas, mostly steam (water) and carbon dioxide, is dissolved right inside the magma deep underground, where it is squeezed under huge pressure, like fizz in an unopened soda. As the magma rises and the squeeze drops, that gas turns into bubbles and tries to get out.
Why does some magma ooze instead of exploding?
Runny magma is thin and flowy, so gas bubbles rise straight up and pop free the whole way to the surface. The gas leaves quietly and the pressure never builds, so the magma simply oozes out and flows down the sides, like the volcanoes in Hawaii.
What makes magma thick or runny?
Mostly how much silica it contains. Magma with a lot of silica is thick and gluey and traps gas, leading to explosive eruptions. Magma with little silica is runny and lets gas escape, leading to calm, oozing eruptions. Temperature and water content matter too.
Talk about it
- Before we run it, guess: which one explodes — the thick gluey magma or the thin runny magma? Why do you think so?
- Where in everyday life have you felt pressure build up until something popped or sprayed?
- If you were a scientist, what would you measure to warn a town that a volcano might blow?
For grown-ups
The controlling variable is magma viscosity, set mainly by silica content along with temperature and water. Low-silica basalt is runny, so as magma rises and confining pressure drops, dissolved volatiles (mostly H2O and CO2) exsolve into bubbles that escape easily, giving an effusive eruption. High-silica andesite and rhyolite are far more viscous, trapping the bubbles; the expanding gas builds overpressure until it fragments the magma into ash and pumice, an explosive eruption. The trigger is decompression plus gas, gated by viscosity, not by how hot the rock is.
Keep going
What else makes you wonder?
- If gas needs to escape to keep a volcano calm, what happens to all that ash and gas once it's blasted miles up into the sky?
- Could we ever tell ahead of time whether a volcano holds thick magma or runny magma before it erupts?
- What other things in nature build up pressure quietly until they suddenly let go all at once?