Aerial view of the Santorini caldera.
Greek volcano reawakens
9 September 2012
Since the beginning of 2011 the volcanic island group of Santorini, in the Aegean Sea off Greece, has been inflating – slowly rising and moving outwards.
This strange behaviour follows a series of very small earthquakes beneath the islands – most so small they could only be detected with sensitive seismometers. These were the first noticeable signs that the volcano could be reawakening since the seismometers were installed on the islands, around 25 years ago.
Earlier this year, UK and Greek scientists working on Santorini decided to find out what was going on. By pulling various satellite, ground and airborne survey information together the team now has a clear idea of what has been happening beneath Santorini.
'We think that molten rock, squeezing up beneath the volcano, is collecting in a magma chamber around four kilometres down,' said Oxford Volcanologist Dr David Pyle. 'This chamber has increased in size between January 2011 and April 2012 by about 10-20 million cubic metres. That's between 8 and 15 times the size of London's Olympic Stadium.'
As the magma chamber has grown, the Earth's surface has deformed and pushed out above it. The vertical movement is approaching 14cm in some places and the volcanic crater is 14cm wider now than it was at the beginning of 2011.
'Locals ... on the main island of Thera noticed the increase in earthquake activity due to the clinking of glasses in their bars.'
Funded by NERC through its Centre for Earth Observation, the team used satellite radar images to measure changes in the islands' surface features. They obtained new images captured by the ESA satellite Envisat and the German Space Agency satellite TerraSAR-X.
By comparing these newer images to some from early in 2011, graduate student Michelle Parks soon found evidence of movement in the Kameni islands, which lie in the centre of the large flooded crater and form the top part of the volcano.
Parks also talked to local people who were very aware of a change in their volcano's behaviour.
'The tour guides, who visit the volcano several times a day, would update me on changes in the strong smelling gas being released from the summit or changes in water colour around some of the island bays,' she said. 'Locals working in restaurants on the main island of Thera noticed the increase in earthquake activity due to the clinking of glasses in their bars.'
Location of recent volcanic and seismic activity on Santorini. The red line passes through locations of ancient eruptions and is probably an active fault. Black dots show earthquakes of magnitude 2 or more between January 2011 and April 2012.
The research team was joined by the crew from NERC's Airborne Research and Survey Facility (ARSF) who flew a series of passes over the volcanic islands, gathering LiDAR and Hyperspectral data. The LiDAR is a type of laser that scans the landscape and can measure changes in height, and the Hyperspectral sensors measure reflected sunlight. Together the two build a clearer picture of how the land surface is changing.
Dr Gary Llewellyn from ARSF explained. 'Instead of the three-colour wavebands a camera can pick up, we can collect tiny, tiny slices of data from the visible and near infrared spectrums at between 400 and 2500 nanometres, so we can look at specific chemicals and the way they bond together. This builds up a picture of the physical features of the volcano.'
To get an idea of size and detail of data they are collecting, one nanometre is one millionth of one millimetre.
Llewellyn added: 'Our data can also identify plants or buildings on the ground, so we can strip these out to get a clear view of what the land surface looks like.'
In collaboration with colleagues from the Greek National Technical University and the University of Athens, and with help from a group of Oxford undergraduates, the researchers re-surveyed the island-wide network of trig points – stone pillars once used as Military reference points – and also extended the local network of continuously-recording GPS (Global Positioning System) receivers which provide daily records of movement, accurate to within a few millimetres.
Juliet Biggs, a Bristol scientist specialising in satellite radar, said: 'It wasn't until we saw the changes in the GPS and the uplift on the radar images that we really knew molten rock was being injected at such a shallow level beneath the volcano.'
'Many volcanologists study the rocks produced by old eruptions to understand what's happened in the past,' she added. 'So it's exciting to use cutting-edge satellite technology to link that to what's going on in the volcanic plumbing system right now.'
Michelle Parks at Athinios Port on Thera, Santorini, with GPS equipment loaned by the NERC Geophysical Equipment Facility.
'The great thing about this volcano is that we know so much detail about its history,' said Pyle. 'Its last major explosive eruption was about 3600 years ago, which buried the islands of Santorini under metres of pumice and formed the large crater which is now flooded by the sea.'
'Over the past 2000 years there have been a few small eruptions of lava with the last one happening in 1950, more than 60 years ago,' he said. 'The Kameni islands have grown on average by about a million cubic metres a year so the amount of molten rock that has arrived beneath Santorini in the past year is equivalent to about 10-20 years' growth.'
So does this reawakening mean that another big eruption is now imminent?
Pyle thinks not – in fact the rate of earthquake activity has dropped off in the past few months.
All of the most recent eruptions on Santorini have usually involved two different sorts of magma – a dominant, silica-rich lava called dacite and a much smaller amount of a hotter, silica-poor lava known as andesite. Previous work shows that the eruptions seem to be triggered by the arrival of the andesite, which stirs up the dacite and quickly starts an eruption.
'This current episode of unrest has now lasted longer than it might have done if andesite was present so we think that the molten rock currently gathering under Santorini is dacite,' said Pyle. 'This leaves us with a better understanding of how the shallow plumbing system of the volcano works but sadly no closer to knowing for sure when it will next erupt.'
Evolution of Santorini Volcano dominated by episodic and rapid fluxes of magma from depth : Parks et al is published in Nature Geoscience
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