Why Some Volcanoes Ooze and Others Explode

You know the two shapes. Wide and gentle, or tall and steep. You also know the line that goes with them: the shape tells you how the volcano erupts.

That is true, and it stops one step short. Something has to decide how the volcano erupts in the first place, and that same something builds the shape. It is not the size of the volcano, or how deep the magma is, or how long since the last eruption.

It is how thick the magma is. How runny or how sticky. That one property decides everything else.

Thick or runny, and what sets it

Melted rock is not one substance. It is a mixture, and one ingredient in it matters more than the rest: silica, which is the stuff glass and sand are made of.

Magma with little silica in it is properly runny — it pours. Magma with a lot of silica is more like putty, and it barely flows at all. Temperature pushes the same way: the runny sort erupts scorching hot, around 1,100 degrees Celsius, and the sticky sort is several hundred degrees cooler by the time it arrives.

Scientists have a word for how much a liquid resists flowing. They call it viscosity. Honey has high viscosity. Water has low viscosity. Magma covers a range far wider than either.

The gas is the whole story

Here is the thing that never gets mentioned when volcanoes are drawn. Magma is fizzy.

There is gas dissolved in it — mostly water and carbon dioxide — and down in the depths the weight of all that rock above squeezes it so hard the gas stays hidden inside the liquid. Exactly like an unopened bottle of something fizzy: the gas is in there, and you cannot see a single bubble.

Now open the bottle. The squeeze comes off and the gas comes out of hiding all at once.

Magma rising up a pipe is a bottle being slowly opened. The higher it gets, the less rock is above it, the less it is squeezed, and the more gas comes out of the liquid as bubbles.

Every volcano on Earth does this. What happens next is the only thing that differs, and it is decided by whether the bubbles can get out.

Runny magma: the bubbles just leave

In runny magma a bubble floats up through the liquid and out of the top, the way a bubble travels up a glass of water. Easily. Continuously. Nothing builds up.

So the gas is gone before it can do any damage, and what reaches the surface is hot liquid rock with the fizz already out of it. It pours. It runs downhill for kilometres before it cools enough to stop.

Rock that travels a long way before it stops spreads its load over an enormous area. Do that a few thousand times and you get a mountain that is wide, low and gentle — slopes of usually less than ten degrees. You could walk up one without noticing you were climbing. That is a shield volcano.

Thick magma: the bubbles are stuck

Now run the same process in magma with the consistency of putty.

The pressure drops. The bubbles still form — that part is not optional. But now they cannot rise through the liquid, because the liquid will not let anything move through it. So they stay where they are and they grow, and they push, and the pressure builds with nowhere at all to go.

Eventually the magma cannot hold. It does not crack politely. The whole thing shatters, torn apart from the inside by its own gas, and what was a sticky liquid a second earlier is blasted upward as a storm of tiny fragments.

That is the explosion. And those fragments are the ash from the first article.

What is left over then trickles out as short, thick, slow flows that stop near the top rather than running away downhill. Layer of ash, layer of stubby lava, layer of ash, layer of stubby lava — piled around the vent instead of spread out from it. Steep sides, often around thirty degrees. A composite volcano.

Same rock, two mountains

It is worth saying plainly, because it is the whole point.

Both kinds of volcano are melted rock arriving at the surface. Both have gas in them. Both drop in pressure on the way up and both grow bubbles. Nothing about the process differs.

Change one property — how easily the liquid lets things move through it — and one volcano quietly floods a landscape while the other takes its own summit off.

The next time you see the two shapes side by side in a picture, you are not looking at two kinds of mountain. You are looking at the same event twice, once where the gas got out and once where it did not.