What is a volcano?

What exactly is a volcano?
26 June 2018

Interview with 

Dr Jessica Johnson - University of east Anglia


Volcanoes have been in the news a lot recently, with eruptions in Hawaii and Guatemala; so this week we’re taking a look at the impact that volcanoes and eruptions can have on the environment, technology and our lives. Chris Smith spoke to volcanologist Jessica Johnson from University of East Anglia for a quick 101; what actually is a volcano? 

Jess - A volcano happens because there is magma under the ground. Magma is made up of molten rock and gas and small crystals of rock, and that magma is going to be less dense than the surrounding crust, the surrounding rock, and because it’s less dense it tries to get to the surface. Once it gets to the surface it erupts as lava, as an explosion of gas, ash, and that’s what we call an eruption of a volcano.

Chris - What powers a volcano? Where does it get this heat from?

Jess - The thing that’s actually causing the magma, the melting of the rock,t could be due to decompression. Where the rock is coming to the surface it has less pressure on top of it and that causes it to melt. Or it could be a chemical reaction with the addition of water. When you add water to a rock it lowers the melting point and that might melt the rock. Or it might be a bit hotter than the surrounding area and that might also melt the rock.

Chris - Apart from the hot rock that we can see, what else is issuing that we can’t?

Jess - Mostly gases. There are lots of different gases that come out of the magma at different depths as well. The gases that we usually associate with the eruptions are carbon dioxide, sulphur dioxide, steam. And then if it’s an explosive eruption you might have tiny particles of the lava, which we call ash as well so that would also be caught up in the plume.

Chris - How do those gases get in there in the first place?

Jess - The gases are dissolved in the rocks and as the rocks melt and become less pressurised as they rise through the crust, different types of gases will come out of solution at different depths. So by looking at the gases we can tell how deep the magma is.

Chris - Why do volcanoes happen where they happen?

Jess - The Earth has a thin crust on the outside, which is what we live on. And that crust is split up into plates, which we call “tectonic plates” and they move around on the surface of the Earth. They move slowly, but some places they’re moving apart, some places they’re moving together, and some places they’re moving side to side. At places where they move toward each other or apart from each other, that’s where we’re likely to get volcanoes.

Chris - What triggers them to go off?

Jess - That’s a very good question. We don’t know exactly what causes an eruption to start when it does, but we do know that there are some triggers; for example if there’s a new batch of magma that gets pushed into the magma reservoir, that can cause a chemical reaction. Ultimately, gas is what drives most eruptions. If there is gas trapped under the ground then it will try to get out and that’s what causes an eruption.

Chris - How good are we at predicting or forecasting when an eruption might happen?

Jess - We’re okay. We’re getting better. We monitor volcanoes, particularly volcanoes that are near populated areas with seismographs, which measure motion of the ground. And so what we’re looking for there are small earthquakes because when the magma’s pushing its way through the rock, it will crack the rock and cause lots of small earthquakes so if we can monitor where those earthquakes are we can tell where the magma’s going. That magma might also deform the surface of the ground and so we can monitor that with satellites.

Chris - You can see that can you?

Jess - Yes you can. Absolutely. In fact, in Hawaii right now the surface of the caldera, because this magma has been withdrawn from the summit it’s subsided by tens of metres.

Chris - Really. So you can actually physically measure the ground buckling and changing shape...

Jess - Absolutely.

Chris - … as the magma moves in and out?

Jess - Yes, absolutely.

Chris - And those movements are then used to make predictions?

Jess - Yeah.

Chris - Anything else you can look for? What about gases coming out because if you’ve got magma and things moving, presumably the gas composition could change and you could look at that?

Jess - Yes, absolutely. As I said before, because the different gases come out of solution at different depths we can look at how much gas, which gas, and where those gases are coming out to tell us where the magma’s moving.

Chris - And what happened in Hawaii? Is that comparable with what happened in Guatemala?

Jess - Not really. It’s the same that there is magma underground that is trying to get out but they’re two very different volcanoes. In Hawaii, the magma is a lot runnier and a lot hotter; that allows the gas to escape and it allows the lava, once it’s out of the ground, to continue to be runny which means that we don’t get so many explosions.

Whereas in Guatemala, it’s a different type of volcano caused by water being dissolved into the rock and that’s what’s causing the magma and so there is more gas, the lava’s very sticky and that’s why the gas can’t escape, and pressure builds up and that’s why we get a big explosion. This sort of activity, volcanoes are quite local processes. There aren’t any connections between these two volcanoes.


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