Everything comes at a price: benefitting from volcanoes?

Volcanoes are not only attractive, fascinating objects. They also give fertile soil, ore deposits and cheap geothermal energy, but all this comes at a price: sometimes they kill, destroy and severely interrupt air traffic. By studying the internal chemistry of Indonesia’s subduction zone volcanoes, Professor Valentin Troll is hoping to learn just which of these dramatic landscapes can provide us with geothermal energy.

The line of volcanoes that cradles the southern coasts of the Indonesian archipelago is a truly dominating feature on the landscape of the country, and they are as perilous as they are breathtaking. But for Professor Valentin Troll and his team of scientists, they present an exciting opportunity for research into the internal chemistry of subduction zone volcanoes, and in deciphering which of these volcanoes have potential for use in geothermal energy.

“There are several reasons why this line of research is of great interest to us. From a purely scientific point of view, we wanted to see how volcanoes in subduction zones, such as Indonesia, actually work. These are the most complex volcanoes we have on Earth, with many different chemical components influencing the evolution of the magma, and therefore the gases that are expelled from the volcanoes.”

“There is also some industrial and economic interest in the research. By looking at how gases coming out of volcanoes are produced internally, we can deduce which of them are more likely to be useful for geothermal energy. Our data seems to indicate that certain types of volcanoes in certain areas are far more likely to be useful, and we can now look for particular chemical signals around other parts of Indonesia, such as Sumatra.”

A subduction zone is an area in which two tectonic plates are moving towards each other, resulting in one sliding beneath the other. In Indonesia, it is the oceanic Indo-Australian plate being forced underneath the continental Eurasian plate, and this is what causes the high level of volcanic activity in the region.

This subduction zone movement forces materials such as oceanic crust material and oceanic sediments back down into the earth’s interior, where they start to heat up. This causes many components such as water, CO2 and other gases to be released, and these fluids being driven off causes a change in the melting point of the surrounding mantle rock, at which point magma starts to form.

“So at this point there are two components to the magma: the sedimentary material that was initially taken down by the plate movement, and the material from the mantle.”

“The magma then rises up, and in doing so has to move through either an oceanic or a continental plate. This results in some of the surrounding rocks being melted, adding a third component to the final cocktail of materials that will eventually erupt from the volcano.”

“With our current research we are trying to distinguish these three source parameters, and we have some good indications that we are finally managing to do this. This is a very exciting breakthrough as it is something that we have been trying to solve for many years in this field.”

The methods that Troll and his team are applying involve the analysis of carbon and helium isotopes in the gases emitted from the volcanoes. “Helium is a noble gas, meaning that it doesn’t really combine with anything, and it is also very light, so it naturally rises up towards the atmosphere.

These properties mean that helium is a gas that can percolate through a volcano quite easily without being modified too much from any of the internal reaction processes.

By looking at the helium isotope ratio in the volcanic gases, we are able to differentiate between the signals given off by different types of volcanoes, i.e. whether or not the upper crust has had any influence on the chemical makeup of the gases emitted. The volcanoes that appear to have the strongest influence from the upper crust also seem to be the ones with the highest geothermal productivity.”

“Using this data, we are now hoping to be able to define a baseline for what subduction volcanoes do without this extra crustal input which we believe is present in many of them.”

“We have sampled volcanoes all along Java and Sumatra and now have a traverse of almost 3000km, so we can now look at how they vary with input parameters, particularly with the composition of the crust they have passed through.

“This systematic approach has yielded excellent results, allowing us to create a method that could be used at other subduction zone volcanoes elsewhere to distinguish between and quantify the various sources that go into them.”

Troll is quick to stress the dangerous nature of the area. “Because of all the fluids and volatiles involved, these are also the most hazardous volcanoes on the planet – the more gas there is in the system, the more explosive the volcanoes become.” This is a region famous for its hugely destructive volcanic explosions, including Krakatau, which in 1883 killed approximately 36,000 people when the island exploded, and Mount Tambora, which in 1815 experienced the largest eruption in recorded history, causing the legendary ‘year without a summer’ 1816.

“This is of course very relevant in terms of industrial applications; the volcanoes that are most likely to produce enough gas for geothermal energy are generally the most dangerous. So using these volcanoes for energy comes at a price!”

In terms of the future, Troll hopes to be able to extend his research in the field. “Currently we have teams working on Java and on Sumatra, and who are carrying out a lot of the post fieldwork analysis on gas and rock samples.”

“We are hoping to extend the current traverse even further along the arc of volcanoes – at the moment it is about 3000km which is excellent, but it is always instructive to have a more extensive data set, especially from a larger geographical area.”

“In the future we would really like to sample across the plate boundary that runs along where the famous zoogeographical Wallace line occurs [a line which separates Australasian and Asian faunas remarkably sharply, almost certainly because of the tectonic boundary].

There was some pilot data taken in the late 1980s that shows a dramatic change beyond the boundary due to the variation in input of subduction material. Beyond the island of Flores, it is mainly Australian continental material that is subducted, which produces a very different chemical signal and offers enormous exploration potential.”

Click here to contact Prof. Valentin T. Troll.

Published: Friday, 21st October 2011