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Ash shows past eruptions ‘underestimated’

5 May 2009

A study into ash fallout from the biggest volcanic eruption in almost 20 years has shown that the impact of past eruptions is likely to have been significantly underestimated as so much of the evidence quickly disappears, Oxford University scientists report.
 
The study focuses on the Chaitén volcano in southern Chile that began to erupt explosively on 2 May 2008. For six days afterwards the volcano pumped huge volumes of ash high into the atmosphere before its activity began to decline to a low intensity eruption still going on today.
 
With emergency funding from the UK’s Natural Environment Research Council, a team of scientists from the University of Oxford was quickly dispatched to map out the distribution of ash from the eruption and to study its impacts on the local environment, in collaboration with Argentinian scientists.
 
A first report of their findings is published in the Journal of Geophysical Research.
 
‘The area around a volcano immediately after an eruption is like a crime scene where the evidence can quickly be destroyed by the elements,’ said Professor David Pyle of Oxford University’s Department of Earth Sciences. ‘Ash deposited on land will rapidly be removed by rain or wind, while ash deposited out to sea is only accessible by collecting core samples of the sea-floor sediment. This makes it extremely difficult for volcanologists to accurately reconstruct a past eruption from the available evidence and say how much fine ash was deposited, and over what area, during an eruption.’
 
The team’s work on Chaitén has shown that the several millimetres thickness of ash deposited across Argentina have been lost from wide areas – of at least 50,000 square kilometers – in only nine months. A geologist attempting to map the region affected by ash fallout now would significantly underestimate the size of the area affected, and as a result would underestimate the size of the eruption and the amount of ash erupted.
 
‘By using satellite imagery to guide us, we were able to map the ash fallout across Argentina to a thickness of less than one tenth of a millimetre,’ said Sebastian Watt, a PhD student in Oxford’s Department of Earth Sciences, who led the team. ‘We collected samples from over 220 sites across an area of 100,000 square kilometres and with these field data and samples we were able to make the first scientific assessment of the size and impact of the eruption.’
 
The Chaitén eruption had immediate social and economic impacts across Patagonia (southern Chile and Argentina), with more than 5,000 people evacuated from settlements up to 75 km from the volcano, and extensive ash deposition leading to regional disruption of agriculture and aviation. The volcano was not routinely monitored, and there was no recognised warning before the eruption started.
 
Sebastian Watt said: ‘Chaitén is still erupting right now, although it is currently in a less explosive phase. Our previous work has shown that eruptions from this volcano have the potential to be much larger in size making careful monitoring of the present eruption a priority.’
 
For further information contact Dr Tamsin Mather on +44 (0)1865 282125 email Tamsin.Mather@earth.ox.ac.uk or Professor David Pyle on +44 (0)1865 272048 email David.Pyle@earth.ox.ac.uk Images of volcanic activity at Chaiten can be downloaded at http://www.ox.ac.uk/media/news_releases_for_journalists/090505.html
 
Alternatively, contact the University of Oxford Press Office on +44 (0)1865 283877 or email press.office@admin.ox.ac.uk 

Notes to Editors

  • The research is funded by the Natural Environment Research Council through an urgency grant, the National Centre for Earth Observation and a research studentship to Sebastian Watt.
  • The main investigators involved in the project are Sebastian Watt (research student), Dr Tamsin Mather (RCUK Academic Fellow) and Professor David Pyle (University Lecturer).
  • A report of the work, entitled ‘Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaitén, Chile’ will be published in the Journal of Geophysical Research – Solid Earth: View an online version: http://www.agu.org/journals/jb/jb0904/2008JB006219/
  • The Chaitén eruption is the largest eruption since the August 1991 eruption of Hudson, Chile. It is only the fourth major ash fall event to have been mapped in such detail in the past 30 years. Ash fallout on the ground corresponded closely to areas identified with satellite imagery, allowing us to identify the time sequence of deposition of the different ash units. This new information provides a once in a decade opportunity to test models of ash dispersal and deposition, and will have a significant impact in terms of improving models of ash fall hazards.
  • With an erupted ash volume of 0.2 cubic kilometres (enough to fill the new Wembley stadium two hundred times over), this was the largest volcanic eruption globally for nearly 20 years, and the largest explosive eruption of rhyolite (a very viscous and highly explosive type of lava) for 90 years. The area affected by ash fallout (an area about 8 times the size of Wales covered with more than 0.1 millimetres of ash) was similar in scale to that affected by the May 1980 eruption of Mt St Helens in the USA. 

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