Why does a volcano explode instead of just oozing?

Some volcanoes pour out slow rivers of glowing lava you could almost walk beside. Others blow their whole top off in a giant bang and shoot ash miles into the sky. Same hot rock — so what makes one go BOOM? Let's dig in and find the trigger.

1What's really inside the magma

Hidden gas, and how gluey the rock is

You only need two ideas. Watch each one:

There's gas hiding inside

Magma is fizzy, like an unopened soda. Deep down it's squeezed so hard the gas stays mixed in. As it rises the squeeze drops, and the gas wants to pop out as bubbles.

The rock can be runny or gluey

Some magma flows like warm syrup; some is thick like cold honey. How gluey it is decides one big thing: can the bubbles slip out, or do they get stuck?

2Two kinds of magma

The slippery escape vs the stuck trap

Both have the same gas inside. The only difference is how gluey the rock is — and that changes everything for the bubbles:

Runny magma

The slippery escape

Thin and flowy. Bubbles zip straight up and pop free, so the gas leaves quietly.

Thick magma

The stuck trap

Gluey and stiff. Bubbles can barely budge — they get trapped, and the gas has nowhere to go.

3Your turn — feel how thickness changes the bubbles

Drag the dial and watch one blob rise

Here's one blob of magma full of gas, floating up toward the surface. Drag the thickness dial and watch what the bubbles do — and keep an eye on the pressure meter as gas tries to get out.

Pressure inside: low
RUNNYTHICK & GLUEY

4Now build a whole volcano and let it go

Make the magma thick and gluey — then let it reach the top 🌋

Same amount of gas as the runny one. You've made it thick and gluey. Guess what happens when this magma finally reaches the top — then let it rise and watch.

Guess before you let it rise

Same gas inside, but you make the magma thick and gluey. What happens at the top?