Dr Peter Baxter, Cambridge Volcanology Group
Volcanoes that erupted in the past, like Vesuvius, Tambora and Krakatoa had a massive impact on those living nearby, claiming many lives. But they can also have the surprising effect, which Ginny Smith demonstrates, of causing beautiful sunsets all over the world. But first, Cambridge volcanologist Peter Baxter explains to Chris Smith why some eruptions are so deadly, and how people can be killed...
Peter - Well, that was the big question which was asked at the eruption of Mount St. Helens, immediately afterwards when 57 people were killed in the area near the volcano. A very large amount of ash fell downwind into populated areas. I was in a team working in the United States at that time and we were contacted to go and investigate and find the reasons why people can be injured or killed in eruptions. After that point, we had no knowledge at all. To fast forward to Vesuvius, after several eruptions and being able to look and obtain more data, what we see in retrospect now in Plinyís account, what couldíve happened, and the scientific work that followed on the eruption at Mount St. Helens, on Vesuvius and the AD 79 eruption, for the first few hours when the eruption was occurring, and the pumice was falling from the sky, people in Pompeii were under a rain of pumice which was building up on the roofs of their houses and taking shelter inside, resulted in deaths caused by the roofs of the houses collapsing in on the people. Out of over a thousand bodies which have been unearthed in the excavations of Pompeii, about 400 of those were excavated from buildings where the roofs had collapsed in on the people. You can imagine that there were two or more metres of pumice and ash on top of the roofs. And so, when the roofs caved in, people would be buried in the ash and would suffocate.
Chris - So, thatís obviously one very dramatic and physical reason. What about the fact that this stuff is in the air and people are breathing in this gas and dust? Does that make a difference?
Peter - Yes and when Mount St. Helens erupted, there were people living 200 kilometres away who were in an area where the ash had fallen heavily. There was no rainfall. There was very dry area and for a whole week, the ash was being resuspended and although it wasnít pitch dark, people couldnít move, they couldnít drive or take any transport because they couldnít see where to go. We thought at that time that people would actually die potentially under this huge amount of air pollution. But in fact, it wasnít that bad. And so, in most of these ash falls, we donít expect a lot of problems except that people whoíve got lung diseases like asthma or chronic lung problems, people who have been smoking heavily during their lives, and so forth, these people will be badly affected and need to take shelter, away from ash. But in the eruption of Vesuvius, the greatest danger was the ash fall and the pumice falling on the roofs and then caving in with people inside sheltering. After about 12 hours of this eruption with ash fall, the pyroclastic flows and surges began and then we moved into a different problem for the people who were still there.
Chris - I mean, they were quite literally being cooked Ė these people Ė werenít they because youíve got gas temperatures of maybe 300 degrees, maybe up as high as weíve just heard, a thousand degrees around where there are people?
Peter - Yes. We know from the investigations that weíve done since Mount St. Helens that the temperatures at Pompeii were about 300 degrees centigrade. And so, you can imagine how hot that is and if you put your hand in boiling water for example which is 100 degrees centigrade and then you'd turn up the heat, and if this is impacting on your body, then itís a very high temperature and people die almost instantly if they're exposed to it.
Chris - I'm lucky enough to have visited Herculaneum and there are quite a few people who were taking shelter in the boatsheds which are now paradoxically many miles from the sea, owing to the fact that so much material rain down and move the coastline back, you know kilometres, didnít it? But there are all these people sheltering in these boatsheds and theyíve all got things like skulls fractures and things like that which people couldnít explain for a while until someone said, ďCould it be that actually, these people were cooking so well and so fast that their brain quite literally exploded?Ē
Peter - I mentioned that the temperature in Pompeii was 300 degrees centigrade which people were exposed to as the surges came down from the volcano. At Herculaneum, it was even hotter and we go up to 400 degrees and even 500 degrees centigrade where people were sheltering in the caves by the beach. Sheltering from the eruption but also potentially, waiting to leave by the boats. The very first search from the volcano came down into Herculaneum where the people were sheltering and the cloud went straight into the caves where they were on the beach. They were instantly killed by this intense heat which probably completely toasted the flesh away from their bodies and they were left with just skeletons and this couldíve happened almost instantaneously.
Chris - What about other things that can come out of volcanoes? Weíve heard a lot about gases and things like that. Are there any noxious gases that can literally knock people out, not just because of thermal effects but because they're just very, very poisonous?
Peter - It wasnít a major factor in the Vesuvius eruption but in other instances, it can be important. And particularly in volcanoes which can just emit gas only and the gases like sulphur dioxide from a volcano can be very irritating to a population living downwind. We had an interesting eruption in Iceland which began in August last year, the Holuhraun eruption where the fissure opened and this was a lava eruption, so not like the Vesuvius eruption weíre talking about which is an explosive one, and here, we have very fluid lava running out of a fissure in the centre of Iceland, at the same time, a very high discharge of gas, mainly sulphur dioxide. The levels of sulphur dioxide in the air as the plume swept around Iceland depending upon the wind direction were high enough really to trigger asthma attacks and people who suffered from asthma or really cause a lot of problems in people with chronic lung disease. But the Icelanders have this very good resistant houses to weather wind and cold because obviously, much of the year, they live in very cold conditions. They were given warnings where the wind was blowing, where the plume would go, and people with these problems could stay inside and be absolutely protected.
Chris - Itís good to hear. Peter Baxter, thank you very much.
Ginny - Now in this show, weíve looked at the devastating effects volcanoes can have on those living nearby. But large eruptions like Tambora can even cause changes on the other side of the world. 1816 is known as the year without a summer because of the huge volumes of ash produced by Tambora. This formed a layer in the atmosphere, reflecting away the sunís energy and causing a reduction in global temperatures for around 1 degree which is enough to cause crop failures and widespread famine. But this eruption also caught some strange and beautiful effects which I'm going to demonstrate to you now. So, weíre going to do a little experiment here and I have two versions of it. I have one thatís going to show you guys really well today and I have another version that you could try at home. So, I'm going to need someone to come in and help me out with the one that you can try at home. What's your name?
Jillen - Jillen .
Ginny - Okay, so what I've got here is I've got a pint glass. I've got a bowl of water and I've got a jug and I've got in this bottle some milk. So, what I'm going to do is just pour the water into the jug for you because it might spill. And then all you need to do is fill up the pint glass with the water from the jug for me. Can you do that? Now, I'm going to add a dash of milk and it really doesnít need to be very much, just a tiny amount.
Chris - Wow! When you said tiny, that really is tiny, Ginny. Thatís literally a few drips.
Ginny - Yeah, it really is, just a few drips. You can see that white kind of swirling around. So, what's that done to the liquid?
Jillen - The water is sort of white because of the milk.
Ginny - Great, exactly. So, weíve only added a tiny bit but itís gone all kind of cloudy, hasnít it? Now, what weíre going to do is weíre going to take this torch. I'm going to hold up the glass and shine the light from the bottom. What colour does the milk look?
Jillen - White.
Ginny - Can you see any other colours in there at all?
Female - Purple.
Ginny - Can anyone see anything?
Male - Blue.
Ginny - Okay, so when I shine the light from the bottom, it makes the milk look sort of bluey, slightly purpley. If you look down the top, what colour does the light look?
Jillen - Orange.
Ginny - It looks orange, exactly. Now, thatís interesting. If I pour some of this milky water out, have a look again. What colour does the light look now?
Jillen - Yellow?
Ginny - Yellow, so itís changed colour. All I've done is poured out a bit of the milky water so you're looking through less of the water. Iíll pour out a bit more and what colour does it look now?
Jillen - White.
Ginny - White. So, by changing the amount of milky water weíre shining the light through, weíre changing the colour of the light.
Chris - Why did that work?
Ginny - What happened there is the milk is doing something really clever. Itís doing something we call Rayleigh scattering. What that means is that when the light hits it, it bounces off in all sorts of directions. So, light is a wave and if you imagine a wave in water, if it hits something, it can bounce back. And sometimes it can bounce back at different angles. The milk in the water is doing that to the light. Itís scattering it. Now, because light is a wave, it comes in different wavelengths. So, red light has a long wavelength and blue light has a short wavelength thatís the kind of scale that goes all the way through the rainbow. Short wavelengths scatter better. So, when you're looking at the milk with the light underneath and you're looking at it from the side, what you're seeing is the light that gets scattered out and that looks blue. And thatís why the milky water looked blue. But when you're looking down the top, you're seeing the light thatís travelled through the water and that is whatever is left once the blue has been scattered out.
Chris - Why does the blue get thrown out the side then?
Ginny - Because itís bumping into these particles of milk and because short wavelengths scatter more, thatís the one that bounces back.
Chris - The red comes right through because itís less scattered.
Ginny - Exactly, yes. So, when the light has to travel through all the milk, there's a really good chance that most of the blue will have scattered out. So, what you get left with is red. When there's only a little bit of milk to travel through, itís less likely that the blue has been scattered out. So, you get a mixture of blue and red which gives you orange then yellow. And then when there's hardly any milk at all, you get white.
Chris - So, with that really big long tube youíve now got here, you're able to effectively do this many, many times over what youíve got in the glass. So, you should get maximum scattering and therefore, you should see very red at the top and very blue at the bottom.
Ginny - Hopefully, weíre going to effectively see a sunset. So, the Rayleigh scattering is the same reason that we see blue in the sky. Because the sunlight is hitting the atmosphere,itís scattering and we see what's being scattered and thatís blue. The colours that you see when looking directly at the light, they relate to a sunset. So, I've got some more milky water here. This one, I actually use milk powder and thatís going to behave more like the particles of ash in our volcano. So, what we had in the glass is effectively a normal sunset. When the sun is high in the sky, the light from it has to pass through a bit of atmosphere to get to us but not huge amounts. So, some of the blue is scattered and what you get left with is a kind of yellow orange sun. At sunset, the sun is low on the horizon so the light has to pass through more atmosphere before it gets to our eyes, more blue is scattered out, and weíre left with a beautiful glowing red sun. What lots of people noted is that in years with big volcanoes like Tambora, sunsets became more vibrant, beautiful colours. There are some beautiful paintings and poems written in the year 1816 that scientists now think are based on this effect. So, I've got my water with milk powder which is going to represent ash and I'm going to pour it into this tall tube. I'm going to turn on the light at the bottom of the tube. What can you see?
Jillen - It kind of like changing shades as you go up the cylinder. So, it starts off as a sort of whitey colour and then it changes as you look up the tube so the yellowy colour then an orangey colour.
Ginny - So, when you're looking through all of that atmosphere with all those particles in, you get a really dramatic effect of this scattering and you see some really beautiful sunsets.
Chris - Ginny Smith, thank you very much. Any questions?
Male - Which eruption was the biggest?
Male - Itís actually not an easy thing to answer because volcanoes that erupted billions of years ago, we donít have much evidence left to know how big they were. The biggest one that you'll find is an eruption known as the Fish Canyon Tuff eruption 4 or 5 million years ago. We have a magnitude scale which is a bit like the Richter scale for earthquakes and thatís about a magnitude 9. Compare that to the Vesuvius eruption that weíve been talking about, thatís about a thousand times bigger. The last really big eruption in more recent times was that of the Toba volcano in Sumatra that had a magnitude of about 8.8 which is equivalent to something like 3,000 or 4,000 cubic kilometres of magma. And that was only about 74,000 years ago at a time when our human ancestors were around and there's been a lot of debate as to what effect that eruption might have had on our ancestors at the time.
Amelia - My name is Amelia and I would like to know where were that tectonic plates on the Earth?
Chris - Marian, tectonics.
Marian - Tectonic plates, there's an awful lot of them. Along the centre of the Atlantic, itís splitting apart. So, youíve got one plate on the west side which is the American plate and then youíve got the European and African plates on the east. So, Africa has got its own plate. Europe is made of a mishmash of lots of small plates that are all sort of sliding past each other. The biggest plate of all is probably that of the Pacific. The Pacific Ocean, almost all of it is a single plate thatís moving towards Japan at the moment and away from America.
Graihagh - Given today that so many people still live on the mountainside of Vesuvius, what happens if there is an eruption? Is there some sort of evacuation process thatís in place today?
Chris - PeterÖ
Peter - So, there is a national plan, but if we knew when it was going to erupt, everything would be dead easy because you could just get people out of the way. But we have this huge uncertainty of even if itís showing very serious activity, whether in fact it is going to move into full eruption or not, but the danger is so great that there will be a tendency to evacuate many, many people in advance. But thatís the crucial thing, is knowing whether you can get people out in time, if you delay the evacuation call. Of course, hundreds of thousands of people won't like being moved to another part of Italy if the eruption doesnít actually follow.
Jillen - Do we always have an earthquake before an eruption starts like Pliny described?
Marian Holness [Earth Sciences, Cambridge] - Yes, I think you do. The Holuhraun eruption in Iceland was preceded by a great swarm of earthquakes. Iceland has got an enormous number seismometers scattered around it. And as soon as the first one recorded the first sort of set of earthquakes, all the geophysicists at Cambridge University got terribly excited and jumped on a plane with even more seismometers and they scattered them all around the area and they could actually see where this magma was moving by tracking where the earthquakes are in real time, and that was terribly exciting. So, they could tell pretty much when that eruption was going to happen.
Would you count death as a longterm effect?
Its the unpredictability of volcanoes that make them so, ermmmm, unpredictable.
Short memories; rich soil. Ophiolite, Wed, 6th May 2015
When visiting Hawaii, I heard about the volcano that won a war.