Racing to record the ashes
8 May 2009
It pays off to be at the right place at the right time. A team of Oxford scientists travelled to South America just three weeks after the Chaitén volcano erupted in Chile, and returned with a wealth of data on how eruptions scatter ashes.
Sebastian Watt (right) and colleague Robert Martin collect ashes from the Chaitén eruption.
Only six months after the event, most of the information was already washed away by rain and wind.
The Chaitén volcano eruption, in the Southern Andes of Chile, started on 2 May 2008. Over the next six days, the volcano released about 160 million tonnes of ash, deposited mostly across the border with Argentina. It was the largest explosive eruption of the last 17 years and Chaitén's in several thousand years.
Sebastian Watt from the University of Oxford is researching past eruptions in this region of southern Chile for a PhD in volcanology, examining ash layers preserved in the soil profiles. The Chaitén eruption could not have come at a better time for Watt's work. 'The event is a modern example of the older eruptions I am studying', he says.
To make the most of the opportunity, Watt and his supervisors Dr Tamsin Mather and Professor David Pyle applied for a Natural Environment Research Council urgency grant and quickly set up an expedition to map the ash scattered by the eruption and study its effects on the local environment.
'This was an exciting opportunity because it's very unusual to have an eruption of this scale depositing most of it's ash on land. Normally ash falls on the sea and the information is lost,' says Mather.
The team worked with Argentinean scientists and travelled thousands of kilometres to collect data from 227 sites. 'At each stop we measured ash thickness and collected samples to determine grain size and chemical composition,' explains Watt. Back in the lab, the team built an ash thickness map and used it to estimate the total volume of ash released by the volcano.
In January 2009, Watt returned to the eruption area and in some areas the ash had virtually disappeared. 'The area around a volcano immediately after an eruption is like a crime scene where the evidence can quickly be destroyed by the elements,' says Pyle.
The amount of lost information shows how difficult it is to reconstruct past events from geological records. Most of the evidence disappears very quickly, especially the fine ashes that can be washed away by rain and wind. It's very likely that the strength and magnitude of historical volcanic eruptions is underestimated, argues the report, which is published in the Journal of Geophysical Research.
This is an important finding because volcanologists use old ash deposits to estimate the size of past events and to assess future volcanic risks. By recording the Chaitén eruption so soon after it started it was possible to show that a single one-week event produced a complex ash deposit.
'This is a fantastic example of what we're able to achieve if we can react quickly to events,' says Mather.
Watt, S. F. L., D. M. Pyle, T. A. Mather, R. S. Martin, and N. E. Matthews (2009), Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaitén, Chile, J. Geophys. Res., 114, B04207, doi:10.1029/2008JB006219.
Keywords:
Geology,
Hazards,
South America,
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