Tag Archives: fieldwork

Santorini: a volcano in remission?

7 Aug

In January 2011, Santorini volcano in Greece began to show the first subtle signs of stirring after many decades of quiet – or at least many decades without detectable activity. This presented an exceptional opportunity to track the behaviour of a very well-studied volcano at the start of a phase of ‘unrest’. Although it may seem counter-intuitive, volcanologists don’t really have a terribly good idea of how volcanoes behave in the long intervals between eruption. Most of the time, resources are devoted to studying volcanoes that are about to erupt, are already erupting, or that have recently erupted, rather than the slumbering volcanoes that might be thought to pose rather less of an immediate hazard. In the case of Santorini, the signs that the volcano might be awakening that we saw in early 2011 presented a scientific chance not to be missed. With urgency funding from NERC (although we did have to explain what the urgency was, without an eruption having happened) and support from our Greek collaborators, we were able to mobilise quickly and make the most of the opportunity to observe and measure while the episode of ‘unrest’ unfolded. Now, two and half years on, the stirring has subsided, and Santorini seems to be settling back into another period of quiet slumber. With the benefit of this hindsight, we can now take a look back over the ‘pulse’ of unrest, and begin to think about what this tells us about how the volcano works.

At the beginning, in early 2011, the first signs of something stirring came from the tiny earthquakes that began to be detected beneath the centre of the volcano. Shortly afterwards, we were also able to see the signs of ground movement from both satellite and ground-based instruments, as the volcano began to swell. Measurements and modelling of this swelling both pointed strongly to the root cause of the unrest being the arrival of molten rock, or magma, about 4 kilometres  beneath the volcano, at a point somewhere beneath the northern part of Santorini’s sea-filled caldera.

Santorini Vertical Deformation Model

Vertical deformation of Santorini during the period of unrest in 2011 – 2012, determined by Michelle Parks (University of Oxford) from measurements of the deformation field across the islands. The deformation is best explained by the intrusion of magma about 4 km below the red dot.

Over the course of the next 12 – 15 months (until about March – April 2012), ten to fifteen million cubic metres of molten rock slowly squeezed into this subterranean reservoir at depth, while we watched our instruments trace out the gradual changes at the surface. Over the same period we were also able to detect subtle changes in the gases leaking out of the summit craters of the Kameni islands; the young volcanic islands in the centre of the caldera. The Kameni islands are almost barren, formed from the overlapping fields of lava erupted over the course of a series of eruptions during the past 2000 years and more. You can get a sense of this from the aerial photographs captured by the NERC-funded aircraft that surveyed the islands in May 2012.

Right in the centre of the younger of these islands, Nea Kameni, the tourist trails circle around the shallow craters formed during eruptions over the past century. Although there is very little visible evidence, apart from a couple of small steamy vents, this summit area is gradually leaking carbon dioxide and other volcanic gases to the atmosphere. The concentrations of these gases are too low to be measured remotely (from satellites, or automated spectrometers), and instead have to be measured directly during field campaigns.

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Aerial view of the summit area of Nea Kameni, Santorini, Greece, showing the tourist trails (in grey – look for the people) that run around the edges of the Agios Giorgios craters. Photo taken by the NERC Airborne Research and Survey aircraft on flight EU12-12, May 2012.

We were interested in measuring the carbon dioxide that is escaping out of the soil, as this is one of the gases that we expect to be released from magmas as they rise up through the Earth’s crust. Carbon dioxide is quite easy to measure, because it has a couple of strong absorption bands in the infra-red, and there are several tailor-made instruments available that can make these sorts of measurements routinely. Most ‘soil gas flux’ instruments are based on the ‘accumulation chamber’ method, a technique adapted for volcanic applications in the early 1990’s. This involves measuring the rate at which carbon dioxide seeps out of the soil into a small volume chamber, resting on the ground surface.

Soil gas measurement using an accumulation chamber, with a PP systems chamber and portable gas analyser.

Soil gas measurement system using a PP systems accumulation chamber and portable gas analyser. The accumulation chamber sits on a collar, pressed into the soil. This picture is from a field setting on a volcano in Ethiopia.

In the field set up that we adopted on Santorini, Michelle Parks was also able to collect small fractions of the soil gas for carbon isotope analysis in parallel with the measurements she was making of the soil gas flux itself.

LiCOR soil gas accumulation system, ready for deployment. Courtesy of Michelle Parks.

LiCOR soil gas accumulation system on Santorini, ready for deployment. Courtesy of Michelle Parks.

As well as measuring carbon dioxide, we also measured concentrations of the short-lived radioactive gas, radon-222 in the soil gas. Radon is a naturally-occurring radionuclide, which decays by alpha-decay. Radon can be measured using ‘passive’ detectors made of a special plastic (manufactured by TASL), that records the tracks left by the alpha particles that are released from the radon atoms as they decay. After exposure to the soil gas environment for a few days, the plastic detectors are etched to reveal the tracks, ready for counting and calculation of the radon gas concentration. Together, these measurements of carbon dioxide emission rate; of carbon dioxide concentration; of carbon isotopic composition, and the radon concentration – allowed us to tease apart the different sources of carbon dioxide that come together to form the ‘soil gas’. In particular, we distinguish the carbon dioxide produced by bacteria in the soil, from that produced deeper inside the volcanic system; and we can also distinguish between carbon dioxide that has recently escaped from a degassing body of magma, and the carbon dioxide released by reactions between the hot, intruding magma and the limestone rock that forms a part of the ancient basement to the volcano. Our new measurements show that after the intrusion of magma began in early 2011, the pattern of soil-gas carbon dioxide changed, as new gas percolated into and through the shallow parts of the volcano towards the surface, before escaping. This gas pulse has now passed through the system, and all of the signs now suggest that the volcanic system beneath Santorini is returning to a quiet state. We will, though, all be keeping a watchful eye.

Update: February 2015.

Santorini volcano remains in remission, and the episode of unrest has passed. The story of the past 20 years of satellite-observation of the slow ups-and-downs of the volcano has now been documented in another recent paper by Michelle Parks (Parks et al., 2015).

Further reading (non technical): http://santorini.earth.ox.ac.uk

Selected further reading (technical): a selection of the papers that describe some of the features of unrest on Santorini since 2011.

Foumelis, M. et al., 2013, Monitoring Santorini volcano (Greece) breathing from space, GEOPHYSICAL JOURNAL INTERNATIONAL Volume: 193 Issue: 1 Pages: 161-170 DOI: 10.1093/gji/ggs135

Lagios, E et al., 2013, SqueeSAR (TM) and GPS ground deformation monitoring of Santorini Volcano (1992-2012): Tectonic implications, TECTONOPHYSICS 594, 38-59 doi 10.1016/j.tecto.2013.03.012

Newman, AV et al., 2012, Recent geodetic unrest at Santorini Caldera, Greece  GEOPHYSICAL RESEARCH LETTERS 39, L06309 DOI: 10.1029/2012GL051286

Papoutsis, I., et al., 2013, Mapping inflation at Santorini volcano, Greece, using GPS and InSAR GEOPHYSICAL RESEARCH LETTERS 40, 267-272 DOI: 10.1029/2012GL054137

Papageorgiou, E. et al., 2012,  Long-and Short-Term Deformation Monitoring of Santorini Volcano: Unrest Evidence by DInSAR Analysis  IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING 5, 1531-1537 DOI: 10.1109/JSTARS.2012.2198871

Parks, MM et al., 2012, Evolution of Santorini Volcano dominated by episodic and rapid fluxes of melt from depth, NATURE GEOSCIENCE  5, 749-754 DOI: 10.1038/NGEO1562

Parks, MM et al., 2015, From quiescence to unrest – 20 years of satellite geodetic measurements at Santorini volcano, Greece. Journal of Geophysical Research (Solid Earth), doi:10.1002/2014JB011540

Tassi, F., et al., 2013, Geochemical and isotopic changes in the fumarolic and submerged gas discharges during the 2011-2012 unrest at Santorini caldera (Greece) BULLETIN OF VOLCANOLOGY 75, 711 DOI: 10.1007/s00445-013-0711-8

M.M. Parks, S. Caliro, G. Chiodini, D.M. Pyle, T.A. Mather, K. Berlo, M. Edmonds, J. Biggs, P. Nomikou, & C. Raptakis (2013). Distinguishing contributions to diffuse CO2 emissions in volcanic areas from magmatic degassing and thermal decarbonation using soil gas 222Rn-delta13C systematics: application to Santorini volcano, Greece Earth and Planetary Science Letters, 377-378, 180-190 DOI: 10.1016/j.epsl.2013.06.046

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Chaiten: anniversary of an eruption

1 May

May 1st marks the anniversary of the start of the first historical eruption of Chaiten, a small volcano in southern Chile, in 2008. A lot has been written on the eruption elsewhere, starting with Erik Klemetti’s eruptions blog which first reported on the event at the time. This is an opportunity to share some field photos, which I took during field visits to Chaiten in 2009. At the time of the eruption, Chaiten was not well known,  but it was recognised to be an old dome of obsidian lava, last thought to have erupted about ten thousand years previously. In fact, we now know that Chaiten has a long history of explosive eruptions of  rhyolite magma, and is probably one of the most prolific producers of rhyolite in southern Chile.

The snapshots illustrate some of the transient consequences of explosive, ash-rich eruptions for both people, and the environment; and some of the excitement of  trying to read the deposits before they have been washed away. Enjoy!

Further reading: a special issue of the Open Access journal ‘Andean Geology‘ on the Chaiten eruption was published in May 2013. This issue contains a number of papers that describe the 2008 eruption and its consequences, and others that reconstruct the past history of this volcano.

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Ash and leaf litter

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Prints in the ash

Impressions

Impressions in ash

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Ash in the undergrowth

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“Chaiten will not die”

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“We want to return to Chaiten, our little town”

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Wood shavings

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Chaiten bay, choked with pumice

Approaching Chaiten

Approaching Chaiten

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Survey spot

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Field volcanology

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Evening glow

Acknowledgements: funding for fieldwork on Chaiten and elsewhere in southern Chile was provided by grants from NERC and the British Council. Field collaborators included Fabrizio Alfano, Constanza Bonadonna, Chuck Connor, Laura Connor and Seb Watt.

Further reading:

JJ Major and LA Lara, 2013, Overview of Chaiten volcano, Chile, and its 2008-2009 eruption, Andean Geology 40 (2), 196-215. [Open Access]

SFL Watt et al., 2009, Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaiten, Chile, Journal of Geophysical Research 114 (B04207).

SFL Watt et al., 2013, Evidence of mid- to late-Holocene explosive rhyolitic eruptions from Chaitén Volcano, Chile,  Andean Geology 40 (2), 216-226. [Open Access]

Field report: Pumice

12 Jan

One of the most rewarding parts of fieldwork on volcanoes is when the parts start to fit together, and hunches turn into firmer ideas. When piecing together ancient volcanic eruptions, the process often starts with the discovery of the trace of a new deposit in a road cut section. This might be something as simple as the appearance of a scruffy yellow or orange band that catches our eye as we pass by (as, for example, in the photo below from an earlier blog post).

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit - the orange layer - in a road cut.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit – the orange layer – in a road cut.

A quick clean up of the surface with a trowel , and running the sample through your fingers, is usually enough to tell whether this really is a pumice deposit – and then the hunt is on. Where did it come from? How big was the eruption? How old is the deposit? Where does it fit into the record of eruptions we already know about?

We can usually get fairly good answers to most of these questions from the field, but to understand why we need to know a little bit about pumice. Everybody knows what pumice is: it’s that frothy rock that floats on water, and many people might even have a small lump of it in a corner of their bathroom. But to the volcanologist, pumice is the grail: a gobbet of magma, frozen in mid-flight that captures in its essence the story of the eruption: where the molten rock came from, how it matured before the eruption began, and perhaps even how the eruption started.

A block of pumice. This pumice sample had a chequered history even before it was erupted: it was broken apart while rising up through the plumbing system, and shortly afterwards healed up again, leaving the broken pieces of a pinkish pumice welded to the main white pumice.

A block of pumice. This pumice sample had a chequered history even before it was erupted. It broke apart, and then welded back together, as it rose up through the volcanic conduit towards the surface. The break separates the pink from the white areas. Coin is about 3 cm across.

Most explosive eruptions produce pumice, and it is this that gets carried high up into the atmosphere in the eruption plume and blown by the atmospheric winds, before raining down onto the surface below to form a pumice fall deposit.

Photo of pumice deposit

Pumice fall deposit, near volcan Mocho Choshuenco

These sorts of deposit are remarkably similar from volcano to volcano the world over. Typically, they are made up of angular fragments of white, cream, yellow or grey pumice that just pile on top of each other where they fell. Pumice blocks can range in size up to about 20 – 30 cm across, but are more usually centimetre to millimetre sized. And because these deposits are formed from strong eruption plumes – similar to that described by Pliny the Younger at Vesuvius in AD 79 – the pumice deposit covers the landscape like a carpet, gradually getting thinner and finer away from the volcano. We can use these changes in thickness and fragment size to work out the size and strength of the eruption; and we can also use the map of the deposit to work out not only where the eruption began, but also the wind direction at the time.

But we can also go further. Pumice is mainly volcanic glass: the molten rock, flash frozen as it erupted. This freezing process often catches the processes that actually cause the eruption in the act. Whether it is the expanding bubbles of volcanic gases that have caused the magma to froth in the first place,

Large bubble in a pumice block, formed as several smaller bubbles coalesced. Bubble is about 3 cm across.

Large bubble in a pumice block, formed as several smaller bubbles coalesced. Bubble is about 3 cm across.

or whether it is the droplet of once-hot basalt magma that might have stirred up the eruption in the first place.

Frozen droplet of basalt, on the edge of a large gas bubble, trapped within a larger block of rhyolite pumice. Did the arrival of some hot basalt into the rhyolite magma chamber cause the eruption?

Frozen droplet of dark-grey basalt, on the edge of a large gas bubble, trapped within a larger block of cream coloured rhyolite pumice. Did the arrival of some hot basalt into the rhyolite magma chamber cause the eruption? Bubble is 2 cm across.

The clues to both why an eruption happened, and its consequences at the time, can all be extracted from the buried remains of that ancient carpet of pumice.

After a day of successful pumice-hunting in the field here in Chile, we have returned to base camp (well, it’s actually a very well appointed wooden cabin) laden with samples, and many more questions to think about.