The power of volcanoes

There are many local gods associated with volcanoes around the world. A very famous example is Pele in the Hawaiian Islands. What's really beautiful about the legend of Pele is that she arrives at the Hawaiian Islands, moves from the older land to the newer land and is based in the present day to reside in the Halema'uma'u crater, which is on Kilauea, the most active of the Hawaiian volcanoes. The legend of Pele follows the actual geology in terms of the Hawaiian Islands getting younger as you move towards Kilauea. It's a lovely example of the way that legend can, in some ways, resonate with what we know scientifically.

Our understanding of volcanoes as a species has really been an incredible journey. In Roman times they had experience of volcanoes; Vesuvius wiped out Pompeii and Herculaneum in AD 79.

Understanding volcanoes has been woven together with understanding the structure of the Earth. Most of us spend our lives walking the surface of the planet with no real direct experience, unless we go into mines, of what goes on beneath our feet. We have to piece evidence together. We learn a lot about the Earth's internal structure from seismic waves. When a big earthquake occurs, it can cause enormous damage and be incredibly tragic, but it also gives us really interesting information; we can look at the way the different waves travel through the Earth. In the simplest case, some waves are similar to sound waves. We also have waves that more resemble a Mexican wave going around a stadium. The sound or pressure waves can travel through solids and through liquids, whereas the other waves can travel only through solids, because they need to have the atoms pulled back in. By looking at the way these waves travel through the Earth, we're able to understand more about the layers of the Earth.

We understand that we first have the crust, which is solid, which is what we live on and what we're most familiar with. Then, we have a layer called the mantle, which is also solid but actually creeps very slowly, at about the same speed as your fingernails grow, despite being solid. Beneath that, we have the liquid outer core and then the solid inner core. All this amazing information has come to us through studying the signals we can see at the surface and sometimes the rocks that come to the surface. The key is that to get magmas, to get liquid rock or lavas, special things need to happen inside the Earth that melt the mantle and create these liquid magmas.

Plinian eruption columns are a really fascinating phenomenon. They're named after the account of the AD 79 eruption of Vesuvius by Pliny the Younger – the eruption that wiped out Pompeii and Herculaneum. Pliny the Younger and his uncle, Pliny the Elder, were staying around the Bay of Naples and were able to see the events unfolding. Pliny the Elder set off on a rescue mission to the people affected by the eruption and subsequently died, potentially of asphyxiation associated with asthma, or possibly a heart attack brought on by the volcanic fumes during the rescue mission. Pliny the Younger then wrote two letters describing what happened and describing this phenomenon, which is like an umbrella pine: It has a strong trunk and then spreads out at the top. We now understand much more about the science of this: When a volcanic explosion drives such an eruption, there is a ballistic path, similar to rocks being fired out of a gun or a cannon; they're fired upwards but are sufficiently heavy that they would not get very high if that was the only factor.

Lava flows come in all sorts of different types, but often the ones that make the most spectacular pictures are these so-called basaltic flows: they are quite fluid and flow relatively freely. They have a low viscosity, in more technical terms. They can be very liquid flows, or quite blocky flows that we call "ʻaʻā", a Hawaiian word – or they can go in lobes and inflate themselves, which we call "pāhoehoe" flows – another Hawaiian word. Standing next to a lava flow is an incredible experience. You can smell the volcanic gases. There may be a mixture of a rotten-egg or burnt-matches smell; those are the different sulfur gases. There might be a clinking sound, which is the way the flow sounds when it is moving along. If there are explosions, there might be vibrations coming through the ground as well. Of course, close to the flow, it's incredibly hot, about 1,000°C. At night, the whole landscape will be black with red breakouts where the crust of the lava is broken.

Volcanoes are a manifestation of Earth's inner energy. Inside the Earth, it's very hot. Some of that heat is actually primeval heat retained from when the Earth was formed, when it accreted, when it came together from the different building blocks. Some of that heat is from the radioactive decay of different atoms and different isotopes within the Earth over geological time.

Massive volcanic eruptions like Krakatoa in 1883 have the ability to blast enormous quantities of material into the stratosphere. The stratosphere is the layer of the atmosphere above the troposphere, where the ozone layer is. One of the key things about the stratosphere is that once things reach it, it is much more difficult to get them down. It doesn't circulate in the way the troposphere does, and it doesn't have weather to rain things out, so things get stuck in the stratosphere for much longer periods. After a big eruption like Krakatoa, sulfur dioxide remains in the stratosphere; it is the gas that smells a bit like burnt matches. It chemically changes; it oxidizes over a period of a few years to form a fine aerosol veil of very small particles. What happens is the stratosphere becomes hazy. It creates a hazy day, a misty day, but that means that it reflects light in different ways.

After the Krakatoa eruption, the Royal Society pulled together lots of accounts of the atmospheric phenomenon and the pressure-wave measurements as well. Krakatoa is interesting, because it was in many ways the first global volcanic eruption. It was the first time telegraph cables were laid under the ocean, so it was the first time that news had the potential to travel around the planet essentially at the speed of electrical signals. People knew about the Krakatoa eruption very rapidly. There was an enormous and tragic loss of life locally in Indonesia, but people were afterward on the lookout for changes in the atmosphere and had barometers in different places around the globe. They were able to see the pressure wave traveling around the planet, possibly up to about seven times. People were on high alert about Krakatoa.

The Tambora eruption happened in 1815 in Indonesia, and the year after the Tambora eruption is sometimes referred to in more modern times as the year without a summer. Plenty of evidence from the time shows that people realized they were having very atypical weather. There was snowfall in New England; there was a big disruption to harvests in Europe; and there are accounts of rioting in France. In Switzerland, people were forced to eat moss and their pets. The weather was terrible.

Sometimes people think of volcanoes as being very destructive, but one thing that really inspires me about volcanoes is how important they have also been for the development and maintenance of our beautiful, habitable planet. As far as we know, this planet is unique in the universe. It may turn out not to be the case, but what we understand now, in terms of hosting life, and for complex life like ourselves to evolve, we have needed a degree of stability of the climate, and volcanoes play a key role in that. In our atmosphere, we have carbon dioxide. We have learned a lot about that recently, and we know about its negative effects, but we do need some carbon dioxide in our atmosphere; otherwise, the planet would be incredibly cold.

I think we can learn a lot from volcanoes in terms of the current climate crisis and uncertainty. If we think about the natural cycle of carbon-dioxide emissions, the rate at which volcanoes are putting carbon dioxide into our atmosphere at the moment gives us a benchmark that we can compare ourselves to, and the evidence suggests humankind is emitting carbon dioxide at about 60 times the rate of the world’s volcanoes.