Sea level influences volcanic eruptions

We normally think of volcanoes as unchangeable - we couldn't stop one erupting if we tried, and we have tried! - but it turns out that some environmental factors like high sea level can prevent volcanoes from erupting. Eva Higginbotham reports...

Eva - If you live in the UK, you may not think that often about the risks of volcanic eruptions, but the 800 million people around the globe who live within a hundred kilometres of an active volcano surely do. Others like geologist Chris Satow from Oxford Brookes University just really likes studying them.

Chris - The question we were trying to answer was "What effects does sea level change have on volcanic activity, in particular at volcanic ocean islands?" And we were able to discover that low sea levels are the times when the majority of eruptions are concentrated and at high sea levels, the high sea level itself has the effect of reducing the likelihood of eruptions. And we used the volcanic island of Santorini as our case study.

Eva - Scientists had previously seen that in places where there had been large ice sheets in the past covering volcanic areas, like Iceland for example, there had been a big increase in volcanic eruptions when that ice had been removed.

Chris - So it got us thinking that perhaps the same effect might occur in the oceans, but of course in the oceans, it's not ice that's being removed and added. It's water that's being removed and added as the sea level has risen and fallen over many tens and hundreds of thousands of years. And the sea level goes up and down globally by quite a substantial amount. So the difference between the sea level today and the sea level during the last ice age is around a hundred meters. That's a huge change in the sea level. That's essentially saying that the English Channel would be entirely dry and you could walk from England to France at that time. That's a huge amount of mass to remove from the surface of the Earth or the top of a volcano. And that gave us an idea that the sea level change might have some influence on the activity of the volcano.

Eva - But how do they know when eruptions happened anyway? David Pyle is another volcano enthusiastic this time from the University of Oxford.

David - So Santorini is a fantastic example of a volcano that shows what we'd call a layer cake. You can see different layers of rock making up the cliffs, and some of those layers of rock, they weather in different ways, they crumbled in different ways. They've got different colours and different patterns on the surfaces. And as a geologist, what we do is essentially work our way down through those layers.

Eva - By analysing these layers of pumice and ash and solid grey lava, geologists can come up with a relative time or stratigraphy of when eruptions took place, which is what they did for Santorini. The next step was comparing that timeline with the sea level record.

Chris - By taking samples of sediments from the bottom of the oceans, and these come in huge tubes - cores as we call them - which might be even up to a kilometre in length. And we know that the mud that's higher up in that core is very young. And the further down you go, the further back you go in time. And if you can process them, if you can split it up into its constituent parts, you start to find fossils and most useful fossils for us to work out what the sea level was, are fossils of these creatures called 'foraminifera', or 'forams' if you're on first name terms with them.

伊娃-有孔虫这些微小的海洋生物ho live almost everywhere on Earth, and they produce these shelves using the materials from the seawater they live in, including oxygen. Like all elements, oxygen comes in different varieties, called isotopes. And luckily for scientists, the ratio of various oxygen isotopes in the foram shells reflect what the sea level was when they were alive.

Chris - So these little creatures are recording what the sea level has been at various points in the past into the composition of their shells. And then we can extract them from their fossilised mud, analyse them and look back through time at how the sea level has changed over hundreds of thousands of years.

Eva - Chris, David and their colleagues then used computer modeling to marry up the data on Santorini's eruption history with the established sea level record and found that, indeed, during times of low sea level, there were more eruptions and times of high sea level, there were fewer. So does this mean, contrary to what we normally think, that having high sea levels like we do now is kind of a good thing?

Chris - The truth is we don't quite know what's going to happen to all the magma that's being stored underneath the volcano, if it's not being erupted. So it could be of course, that the magma gets stored and then more magma gets added and more gets added and then over time. So it could be that in fact, you build up a much larger volume of magma underneath the volcano, which then in the future could results in a much larger eruption than would otherwise have been anticipated

Eva - Interestingly though because the sea level rises all over the world at the same time, does this suggest that in some ways, all the underwater volcanoes around the world are sort of linked?

David - If you allow the system to persist for long enough, then indeed you'd expect that volcanoes would come into sync or there'd be some sort of synchronisation or synchronicity between volcanic systems that are physically independent, but they're all kind of responding to the same external processes.

Eva - So just as everyone around the planet is likely to be affected by the changing climate due to global warming, it seems volcanoes may be going to experience a similar phenomenon.