VolcanicDegassing

Santorini

Flooding the Santorini Caldera

Flooding the Santorini Caldera

The flooded caldera of Santorini volcano holds many secrets, buried beneath the ash and pumice of its last great eruption. In the Late Bronze Age, about 3600 years ago, an explosive eruption several times larger that of Krakatoa, 1883, wreaked devastation across this thriving island.

View, looking North, of the northern part of the Santorini caldera. The island of Thersia (left) is separated from the island of Thera (right) by a channel, that is now thought to have formed at the end of the Late Bronze Age eruption.

View, looking North, of the northern part of the Santorini caldera. The island of Therasia (left) is separated from the island of Thera (right) by a channel, that is now thought to have formed at the end of the Late Bronze Age eruption.

A great trading port, Akrotiri, was buried under metres of pumice; preserving for future generations a snapshot of Late Cycladic urban life.

Akrotiri ruins

Bronze Age street scene – Akrotiri, Santorini. Precursory earthquakes damaged the town. But there was time for the residents to clear up rubble (foreground), and rescue furniture (those are casts of bed frames in the street), before the town was buried in pumice.

Once the eruption was over and the dust had, literally, settled the geography of Santorini would have looked much like it is today: a group of three islands, forming a ring around a caldera occupied by the sea. The central ‘Kameni’ islands would not have begun to emerge for another thousand years, or more.

But when did the sea flood the caldera, and what were the consequences? It has long been suspected that the Santorini eruption triggered tsunamis, implicated in Late Bronze Age destruction along exposed coastlines around the Aegean. Observations of the Krakatoa eruption in August 1883 include harrowing accounts of the great sea waves that swept across low-lying areas of Java, Sumatra as this eruption reached a climax. These tsunami were triggered either by the convulsions of the sea-floor, as the island of Rakata collapsed into the newly-forming caldera; or, as now seems more likely, by the displacements of colossal volumes of water as the pyroclastic currents raced into the sea.

Krakatoa - before and after images.

Krakatoa – before and after: notice how much of the island of Rakata (labelled Krakatau) has disappeared in the eruption; and the large shoals of pumice that formed new islands north-west of Krakatau (Proceedings of the Royal Geographical Society).

But could tsunami have been triggered by the Santorini eruption; and how? Until now it has been thought that collapse of the caldera could have triggered a tsunami; but only if the caldera was already flooded by the sea. A growing body of geological and archaeological evidence suggests that the crater of Santorini volcano was not open to the sea, as it is today, at least at the beginning of the Late Bronze Age eruption. Large blocks of stromatolites, an algal concretion, and chicken-wire gypsum that were caught up in the eruption debris point to the presence of a muddy lake in the northern part of Santorini. Before the eruption, this would probably have been an area of hot, bubbling mud-pools, fed by volcanic heat and fluids from depth. Among the wall-paintings from Akrotiri is one, the flotilla fresco, that shows a city on a hill, with a river running by. Are these features that might have been found inside the caldera of the volcano?

Metre-wide block of stromatolite, thrown out during the LBA eruption

Metre-wide block of stromatolite, thrown out during the LBA eruption, now in the entrance to the Heliotopos hotel.

In a new paper in Nature Communications, we present some new evidence that may help to resolve some of these questions. High-resolution mapping of the seafloor shows the presence of a remarkable scour at the northern end of the deep channel that separates the islands of Therasia and Thera, below the town of Oia. This feature can be seen on older sea-floor maps; but not in such detail. Using sound-waves to look at the structures that this channel cuts through shows that the channel didn’t form until most of the material thrown out in the eruption had already been deposited. The shape of the scour will be familiar to anyone who has built dams of sand on a wet beach, and then let it fail. This shape shows that the channel formed quickly, and that the sea broke through and poured down a great waterfall into the caldera, but only after the eruption was over. Any tsunami waves that formed during the eruption must have been triggered by the entry of the pyroclastic currents into the sea.

Image of the northern 'breach' in the Santorini caldera, and the scour and channel that cut through it.

Image of the northern ‘breach’ in the Santorini caldera, and the scour and channel that cut through it. From Nomikou et al., 2016.

This new evidence, along with new ‘cosmic ray exposure’ age measurements showing that the cliffs of the northern part of Santorini are ancient and formed before the Late Bronze Age, may strengthen arguments that the pre-eruption interior of Santorini island was not filled with seawater as it is today. It doesn’t, however, have any direct connection to the events that led to the collapse of the major Late Cycladic culture on Crete, the Minoan, a few decades later. Disruption caused by the loss of a major trading hub and short-term damage caused by fallout of ash across the islands of the eastern Aegean might well have contributed to the eventual decline and fall of Minoan culture; but the later destruction of the Cretan palaces must have another cause. Earthquake, anyone?

Selected references 

Heiken G, McCoy F, 1984, Caldera development during the Minoan eruption, Thira, Cyclades, Greece. Journal of Geophysical Research 89, 8441–8462

Manning SW et al., 2006, Chronology for the Aegean Late Bronze Age 1700-1400 B.C.,  Science 312, 565-569

Nomikou P et al., 2016, Post-eruptive flooding of Santorini caldera and implications for tsunami generation, Nature Communications 7: 13332, 8 November [open access]

Novikova T et al., 2011, Modelling of tsunami generated by the giant Late Bronze Age eruption of Thera, South Aegean Sea, Greece. Geophys J Int 186, 665-680.

Paris R, 2015, Source mechanisms of volcanic tsunamis, Phil Trans Roy Soc London A 373: 20140380

Pichler H, Schiering W, 1977, The Thera eruption and Late Minoan-IB destructions on Crete, Nature 267, 819 – 822

Acknowledgements

Thanks to NERC for funding for my work on Santorini; the organisers of NEMO 2016 and Heliotopos Conferences for a recent visit to Santorini; to Christos Doumas, Lefteris Zorzos, Clairy Palyvou and Maya Efstathiou for introductions to Akrotiri and the Late Bronze Age, and Floyd McCoy for discussions.

Growth of the Kameni Islands Volcano, Santorini, Greece

Growth of the Kameni Islands Volcano, Santorini, Greece
Santorini

Surface morphology of the Kameni islands, Santorini, Greece, based on new submarine and surface mapping, published by Nomikou et al. (2014).

A new paper, published in the journal GeoResJ, reveals the intricate details of the volcanic Kameni islands that lie in the flooded caldera of Santorini, Greece. The Kameni islands started growing shortly after the explosive eruption that formed much of the present day caldera. For the past 3500 years or so these islands have grown in pulses, with each new eruption adding more material to the edifice. In this new paper, we have brought together high-resolution imagery of the seafloor with a digital elevation model of the parts of the islands that emerge above sealevel, and have used this to reconstruct the piecemeal growth of these islands from an analysis of their surface shape. Much more remains to be done, but the fascinating part of the work for me was the dawning recognition of just how little we know about the lifecycle of submarine volcanoes, and how much of the volcanic history of Santorini remains underwater, and essentially untouched.

Interpretation of the growth of the Kameni islands, Santorini, Greece, over the past 2000 years.

Interpretation of the growth of the Kameni islands, Santorini, Greece, over the past 2000 years. From Nomikou et al., 2014 (Supplementary dataset 2).

Acknowledgements

Funding for this project came from agencies in Greece, the United Kingdom and the United States. Submarine multibeam data were collected from R/V AEGAEO, of the Hellenic Centre for Marine Research (HCMR), in 2001 and 2006, with support from the National Science Foundation. Onshore data were collected during a Natural Environment Research Council Airborne Remote Sensing Facility (ARSF) campaign to the eastern Mediterranean in May 2012, with additional support from the National Centre for Earth Observation (NCEO) and COMET.

Reference

P Nomikou, MM Parks, D Papanikolaou, DM Pyle, TA Mather, S Carey, AB Watts, M Paulatto, ML Kalnins, I Livanos, K Bejelou, E Simou, I Perros, 2014, The emergence and growth of a submarine volcano: The Kameni islands, Santorini (Greece), GeoResJ 1–2, 8–18. [Open Access]

Dataset

LiDAR data from the NERC ARSF Campaign EU-12-12-137 to Santorini on figshare

Related posts

The Kameni islands, Santorini, Greece

Santorini: a volcano in remission

The Kameni islands, Santorini, Greece

A glimpse of the spectacular Kameni or ‘burnt’ islands of Santorini, Greece from the air reveals in intricate detail the overlapping lava flows, explosion craters and fields of volcanic ash from which the islands have been built in successive eruptions over the past 2000 years, and more.

Air photo mosaic of the Kameni island of Santorini, based on images taken during a 2004 NERC Airborne Research and Survey Facility campaign

Air photo mosaic of the Kameni island of Santorini, based on images taken during a NERC Airborne Research and Survey Facility campaign in 2004, and published later in an open access paper (Pyle and Elliott, 2006). A high resolution (340 Mb) version of this image is now available from figshare.

Of course, what we can see from the air is just the literal ‘tip’ of the present-day volcano which has grown up within the flooded caldera of Santorini since the last major explosive eruption, the Minoan eruption of ca. 1600 BC. Historical records and accounts from as far back as the Greek geographer Strabo, suggest that there have been at least ten eruptions in and around the Kameni islands since 197 BC. It is quite likely that there have been more that either weren’t noticed (because they were underwater), or that have been forgotten about with the passage of time. The present day the Kameni islands have a volume of about 3 cubic kilometres (of lava), measured from the sea-floor, and must have grown up at an average rate of about 1 million cubic metres per year since the Minoan eruption.

Data sources.

The high resolution version of the composite aerial photograph of the Kameni islands is available to download from figshare,
http://dx.doi.org/10.6084/m9.figshare.928563

Link to the original paper: DM Pyle and  JR Elliott, 2006, Quantitative morphology, recent evolution and future activity of the Kameni islands volcano, Santorini, Greece, Geosphere 2 (5), 253-268  [Open Access]

Related web pages and posts.

A blog post from August 2013 – ‘Santorini: a volcano in remission

Some web pages introducing the volcanic history of the Kameni islands.

One year of volcanicdegassing

One year of volcanicdegassing

One year has passed since I first wrote a post for this occasional blog. Now, 12 months, 22 posts and 7500 page views later, here’s a quick look back. For me, this has been a way of using some of my back catalogue of field photographs, of fleshing out a bit of context around papers I have been working on, and adding a little commentary on some more topical aspects. I am pleased with the results so far, and will aim to keep it going for a little longer. In the meantime, thank you for reading and sharing the posts!

Sunset behind Mt Lokon, Sulawesi, Indonesia

Sunset behind Mt Lokon, Sulawesi, Indonesia

Top three posts of the past year.
A tribute to Barry Dawson
Sea-floor spreading: magmatism in the Afar, Ethiopia
An episode of volcanic unrest beneath Santorini, Greece

Santorini: a volcano in remission?

Santorini: a volcano in remission?

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.

SantoriniCraters

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

An update on Santorini

As you may have heard by now, Santorini volcano has recently been showing some unrest. Of course, it has only just come to the attention of the media, some of which have taken things a little further than can be justified.  But for those of us involved in the work, this is a story which has taken rather longer to piece together.

In my own case, the story started 26 years ago this week, when I first stepped onto Santorini during the first few days of my PhD research. Santorini was hot, dusty and felt rather exotic as it was my first taste of Greece. These steps launched me into my research career in volcanology and culminated, as I thought at the time, with the publication of the ‘Santorini Memoir‘, which summarised our reconstruction of the volcanic history of the islands over the past 600,000 years or so. But Santorini is such an iconic place to visit, and the geology is just so well laid out, that there always were reasons to return. So when a new PhD student, Michelle Parks, arrived looking for a project that would involve both ‘remote sensing’ (in this case, satellite observation) with field work on a volcano, Santorini was the obvious choice for a study of how volcanoes behave in between eruptions. In Santorini’s case, the last very small eruption was in 1950, but there was a rich record of carefully observed eruptions stretching back to 1707: a gold mine of information from which  we could tease out ideas about how it might behave in the future. When we started work in early 2010, there had been no sign at all of any life in the volcano for as long as anyone could remember.

Three years on, and that has now all changed: the volcano has just had its first ‘sharp intake of breath‘ since the last eruption, with the arrival of a fresh pulse of molten rock into the shallow crust, four or five kilometers beneath the volcano. In the time it took for the scientific paper to pass through the peer review system the rumble of tiny earthquakes which heralded this period of unrest has quietened down, and the volcano seems to be returning to slumber for just a little longer. This time, though, everyone is watching.

References.

Hooper, A., 2012, Volcanology: a volcano’s sharp intake of breath. Nature Geoscience 5, 686–687, doi:10.1038/ngeo1584

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

 

Air photo of the northern part of Nea Kameni, the youngest of the volcanic islands of Santorini. Taken in May 2012 by the NERC Airborne Research and Survey team.