Science Spin November 2008

Volcanoes; how do they work?

By Sean Duke

For volcanologists the parts of the world that are by far the most interesting are those where volcanic eruptions regularly occur. These are places like Indonesia, Iceland, Japan, the Philippines, the west coast of North America, and the west coast of South America. This is where it's at, where volcanoes do their stuff, typically erupting at the boundaries between massive 'tectonic plates', where huge chunks of the Earth's crust collide, and hard rocks melt due to the temperatures and pressures generated by these huge collisions. Much is known about volcanoes, but much too is still to be learned.

Professor Gamble at UCC has two SFI-funded projects currently running, both of which involve studying volcanoes -- the chemical processes associated with the generation of molten rock, or magma, as well as how exactly eruptions occur. The big question that lots of people want answered by volcanologists is when exactly will a particular volcano erupt? In recent decades, said Prof Gamble, scientists have become far better at predicting eruptions, but there is a lot still left to understand.

Eruptions

One area of the globe that Prof Gamble has spent a lot of time in, and is a centre of volcanic activity, is Indonesia. He has a PhD student just finishing up a project that involved studying 'Anak Krakatoa', or 'Son of Krakatoa'. This is the name given to the young cone emerging from the sea, at the site of the catastrophic, 1883, Krakatoa eruption in a remote region between Sumatra and Java. The aim was to establish links between the eruption products of Anak Krakatoa and the great eruption 125 years ago.

Another important project takes place much closer to home and involves studying basaltic rocks in Co Antrim. These rocks were formed about 60 million years ago when lava flowed across the surface of Antrim, cooled and hardened into rock. The Giant's Causeway was formed during this period, and the unusual shape of the rocks there was a result of the way the basalt magma cooled. This is a cross-border study involving Prof Gamble, Dr Paul Lyle in the University of Ulster, and people based at the Geological Survey of Northern Ireland. It is now into its second year of funding.

Volcanic eruptions involving basaltic rocks, such as those that occur in Hawaii, are typically not as explosive as those involving andesite rocks, a far more viscous rock that forms slower moving magma. Andesite volcanoes occur around the Pacific 'Ring of Fire' in, or near to New Zealand, the Philippines, Japan and Indonesia, for example.

Prof Gamble is investigating the links between andesite volcanoes and what have been termed 'super volcanoes'. The typical andesite volcano might erupt once every hundred years, or perhaps even once every 25 years and produce a few cubic kilometres of magma, a small amount in volcanic terms. The ongoing eruptions on Soufriere Hills Martinique, in the French West Indies, are typical of an andesite volcano. It has been erupting for the past decade or so, but has produced just a few cubic kilometres of magma.

'Super volcanoes'

Contrast that with a super volcano. These might erupt only once every 100,000 years, but when they do erupt they can produce vast, almost unimaginable amounts of magma - thousands of cubic kilometres. These are of a size that they have cause a 'volcanic winter' or global cooling in one fell swoop. The enormous eruption that occurred at what was Mount Taupo - all that remains of it is a large lake - in New Zealand 26,500 years ago is one example of a super volcano.

The 'cargo' that magmas arising from andesite and super volcanoes are being compared by Prof Gamble and his team. The cargo is made up of crystals and rock fragments, and using micro-analytical methods, a link has been found between the two volcano types, as the composition of the magmas have been found to be similar.

This micro-analysis of magma has enabled Prof Gamble to state that super volcanoes will erupt again in the future, but when exactly, he can't say. These super eruptions could take place at many locations around the 'Ring of Fire', western USA, or western South America. If something like this occurs, everyone should be worried, said Prof Gamble, even if they are living on the other side of the world to where the eruption occurs, as the effect on global climate could be sudden and quite dramatic.

The location of the super eruption, should one occur, would have a major bearing on the effect it would have on mankind. If it occurred in the Arctic or the Antarctic, it wouldn't have a huge impact, as the nature of the spin of the Earth around its axis would mean that material would fall off and not be spread around the equator. However, if such an eruption occurred near the equator, such as happened with the Toba eruption - located in modern Sumatra - 70,000 years ago, the consequences would be serious.

There is a theory that this eruption almost wiped out mankind at the time, leaving perhaps a few thousand people alive. That's not surprising given that Toba produced 1,000 cubic kilometres of magma onto the Earth's surface. As well as the magma, tonnes of ash would have billowed into the stratosphere, along with gases and sulphates. The combination of all this eruption material in the atmosphere would have lead to the absorption of infra-red radiation from the Sun and global cooling.

Waterloo

In more recent times there have been no super volcanoes, but in 1815 there was the very large eruption that occurred in Tambora on the Indonesian island ofSumbawa. This eruption started on the 5th April that year. This was the same year as the Battle of Waterloo, which occurred in Belgium on the 18th June 1815. The impact of the volcano on climate was such that just a month after it erupted, it was reported that the summer in Europe was very, very cold, with many soldiers at Waterloo dying of exposure rather than of their wounds.

Another very large eruption, but not a super volcano, occurred in 1783 in Iceland - the 'Laki fissure eruption'. This produced 14 cubic kilometres of basaltic lava, and killed 50 per cent of Iceland's livestock and 25 per cent of its people. That was with 14 cubic kilometres, so imagine the impact of a super volcano that produces 1,000 or more cubic kilometres of material. The impact would be almost unimaginable.

Predictions

On the big question, Can geologists predict volcanoes? Prof Gamble said yes, in recent decades such predictions have become possible, but only where sophisticated monitoring equipment is in place. Even then the prediction cannot be made to the minute, but it can be made to the week, or even the day. The clue that something is up is when movement of fluid is detected underground. This indicates that magma is on the move up to the surface.

For example, the eruption of Mount Pinatubo in the Philippines in the early 1980s erupted within a day of two of what was predicted. The 'precursor activity' became clear. This pre-eruption activity can involve earthquakes - the result of rocks breaking - or the release of particular gases. Sometimes with small volcanoes -- the andesite type volcanoes -- eruptions can't be predicted. They just 'go off'. But, with the larger volcanoes, the pre-cursor activity is clear and indicates an eruption is on the way.

In Ireland, Prof Gamble is looking at igneous rocks, formed when magma cools, for more information about where the magmas came from - the Earth's mantle. He is interested in the time when the North Atlantic Ocean opened up - it wasn't always there -and during this period there was volcanism in the land now known as Ireland. The idea is to use basaltic rocks as a chemical probe to better understand what happens in the mantle, and the chemical processes going on there that lead to magma formation?

The work into assessing the link between the andesite volcanoes and explosive super volcanoes is ongoing. The samples necessary for this work have been collected, and Prof Gamble states that it is now a question of going to the laboratory and making measurements with the various types of instrumentation to further prove the link.

Benefits

There are a number of benefits to the public that can arise from their support for research into Geology and volcanism in particular, said Prof Gamble. This work can contribute to global efforts to understand more about the interior of the Earth and how volcanoes work, including the nature of the very dangerous 'super volcanoes'.

There are economic benefits too, as scientists can learn more about how ore deposits, most particularly Irish gold, have been deposited. Most of Ireland's gold that has been mined to date is of volcanic origin, and understanding more about volcanoes can help with understanding how, when and where gold might have been deposited in Ireland.

Also, understanding more about volcanoes that erupted in the past, particularly the very large volcanoes, and the super volcanoes, we can learn more about how these volcanoes made a past impact on global climate, and could do so again in the future.