Extreme geology

How do volcanoes and earthquakes occur?
01 December 2020

Interview with 

Jess Johnson, UEA


A volcano in New Zealand


Volcano expert Jess Johnson explains to Katie Haylor how extreme geological events like earthquakes and volcanoes happen. But first, Jess relates these events to the earth's tectonic plates, off the back of an interview from our Earth on the Move show from Hannah Sophia Davies from the University of Lisbon...

Hannah - There's plates being made and destroyed. So where the ocean is being made is at a mid-ocean Ridge. The big mountain range you see in the Atlantic, on maps of the Atlantic, that's where plate is being produced. And then where plate is destroyed, is in the subduction zone. So you see that in the Pacific ring of fire, this big ring of subduction zones where ocean plate sinks back into the earth and destroys it. As the plate sinks into the mantle, it pulls plates along with it. And so essentially the continents float around and they're being pulled and pushed by the ocean plate as it's being made and destroyed. And that's essentially how the plates move.

Katie - That was Hannah Sophia Davies from the University of Lisbon, from our Earth on the Move show, giving us a bit of a Geology 101. Jess, how do these dramatic events like volcanoes and earthquakes relate to the concept of Earth's plates?

Jess - As we've just heard, the Earth has a thin crust on the outside, which is what we live on. And that crust is split up into plates, and we call these tectonic plates, and they move around on the surface of the Earth. They do move slowly, but in some places they're moving apart. Some places they're moving together, and some places they're moving side to side. At the boundaries between the plates, that's where we're likely to get earthquakes and volcanoes. So where they move apart, pressure is released from the material underneath, and as it moves to fill the gap, that's where we get volcanoes. Where the plates move together, one plate often gets pushed beneath the other one, and the underlying plate releases water as it's squeezed. And the water travels up through the overlying plate, melts the material and in turn that causes volcanoes there. That third type of boundary though, where the plates move side to side, we don't often get volcanoes, but we do get large earthquakes. Earthquakes are usually caused when there's a relative movement between two plates, and that causes the rock to crack. They can be any size, they can be really tiny, that we won't even feel, but the big earthquakes that we hear about happen at these destructive plate boundaries, the subduction zones that you just heard about, and these are called megathrusts.

Katie - Where are these? Where in the world are we talking about?

Jess - They happen at the boundaries between plates. We heard in that clip, the Ring of Fire. So that goes all the way around the Pacific, because the Pacific plate is bounded by all of these other plates, that are kind of moving relative to that plate. Through the middle of the Atlantic, there's a plate boundary. There's a sort of newish one coming through Africa. But the UK is quite safe in the middle of one of these plates. We don't get very many earthquakes and we don't have any volcanoes. However, earthquakes can happen everywhere. It's not only at the plate boundaries that they happen. And often when earthquakes happen away from these plate boundaries, it's caused by old stresses. So stresses from where we used to be on a plate boundary, or even when the glaciers that used to cover the UK have melted, the plate is kind of rebounding slowly, and the stresses as that's moving, can cause small earthquakes.

Katie - So how good are scientists at predicting where a quake might happen, and what the impact is likely to be?

Jess - We're not very good at predicting earthquakes. They don't give us much warning. We can tell quite a lot. We can tell where an earthquake is likely to happen. Over long periods, we can tell which areas are likely to have more earthquakes. And we can say how big the biggest earthquakes are likely to be. But the actual trigger for large earthquakes is a bit of a random process. So telling exactly when an earthquake will occur is impossible, at the moment. We do model the impact of large earthquakes though. So we can tell where earthquakes are likely to happen, just not when, and we can tell from past earthquakes, we can look at current plate movement, and then we can assess what the shaking is likely to be. And we have to look at other things, like where people live, what sort of buildings they're in, to look at the impact that an earthquake might have.

Katie - What about volcanoes? Because I mean, I assume it's pretty obvious where a volcano is.

Jess - Yeah. Most of the time. Yeah. We're a bit better at forecasting volcanic eruptions, mainly because they give us more warning. We know where volcanoes are. We measure gases coming out of the ground. We measure deformation of the ground, when the magma reservoirs pressurise. And we also measure small earthquakes that are caused by magma breaking through the rock as it makes its way to the surface. So all of these clues tell us when a volcano is getting ready to erupt. Of course we need lots of monitoring equipment on the ground to gather all of those data, but even then it might do something unexpected, about half of the time when a volcano looks like it's about to erupt, it doesn't.

Katie - What is your research about at the moment?

Jess - My main research uses those small earthquakes around volcanoes to track the stresses in the rocks and fluids under the ground. At the moment, I'm working with data from the eruption in Hawaii in 2018, that people may have heard about. There were over 50,000 smaller earthquakes over about three months that accompanied that eruption. I take the earthquake waves, I measure them, and earthquake waves can actually be polarised in the same way that light can be. So because all rocks have these microscopic cracks in them. If you put pressure on a rock, the cracks, some of them close, some of them open, and they kind of all line up. And that means that earthquake waves will travel faster in one direction than the other, and that polarises the earthquake wave. So I use that polarisation to map the pressures and the fluids underground. I'm also involved in some other projects that work with communities, that live with natural hazards every day, to mitigate and reduce the risks that they live with. Currently working with people in Dominica in the Caribbean, and the idea is to create sustainable solutions to monitoring the hazards and reducing the risk.


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