Hunziker, J.C.

Marini, L.

Compared to Kos , Nisyros is a very small island, how is it possible there is something interesting in that isle ?... This was thinking one of us (LM) when he was approaching Nisyros fort the first time on a small boat together with the late Prof. Giorgio Marinelli. He was full professor at the University of Pisa, from 1961 to 1962; very good teacher and very bright scientist, he promoted and led several research projects on volcanology and geothermics both in Italy and abroad, including several areas along the South Aegean volcanic arc.At that time, in the '80s, Nisyros island was the site of a geothermal exploration project aimed to generate electricity. Indeed, based on the positive indications gathered through suitable surface surveys carried out by the Institute of Geological and Mineral Exploration (IGME), under the surpervision of Giorgio Marinelli, the Greek Energy Authority (PPC) decided to drill two deep geothermal wells in the southern Lakki plain of Nisyros, near the area of the hydrothermal craters. Actually, Giorgio Marinelli was aware of the high geothermal potential of Nisyros and he also understood the dynamics of the hydrothermal eruptions that occurred in this geologically fascinating place.
After some researches carried out in the early '90s on the hydrothermal eruptions and on the chemistry of fumarolic effluents and thermal waters, we decided to work together on fluid geochemistry, enriching this subject with stable isotope geochemistry and measurements of CO2 fluxes from soil and soil temperatures in the area of the hydrothermal craters. These researches were brillianty carried out first by Tatjana Brombach, in the framework of her PhD Thesis and afterwards by Jens Fiebig as part of his post-doctoral stage both at Lausanne University.
During one of the field visits together with Claudia Principe, it became evident that knowledge of Nisyros geology was far from satisfactory. First, all the previous geological maps were prepared on a merely geolithological basis. Second, the reconstruction of tephra stratigraphy and facies analysis was uncomplete. Therefore, we charged two students of Lausanne University , Loÿc Vanderkluysen and Alain Volentik, with geological mapping and related volcanological studies under the supervision of both Claudia Principe, IGG-CNR, Pisa and one of us (JCH). This work attracted more students and we ended the work with seven persons in the field.
A further weak point in the geological work previously performed on Nisyros were the scarce and not very conclusive age determinations, so that a further PhD Thesis was assigned to Annett Büttner under the supervision of Igor Villa, University of Bern. Unfortunately, we had to find out that their investigations were not adequate for elucidating the geochronology of Nisyros volcanic rocks, which is a subject deserving further careful studies.
In 1999, we started, as a full member of the European three-year project GEOWARN (Geo- spatial warning systems Nisyros volcano, Greece . An emergency case study. IST-12310), amcollaboration of Greek, German, Italian and Swiss geoscientists. As work with Lausanne students had started two years prior to GEOWARN outlasting this european project, and mapping, geochemistry of rocks and minerals, and geochronology were not included and financed by the GEOWARN project, we decided to assemble all these data in the present volume of the Mémoires de Géologie ( Lausanne ). Partially the results have been published in different journals but the huge amount of data backing these results merit a proper documentation in this synthesis.
The major result of volcanological investigations is a new geological map in the 1:12,500 scale, which represents the pivotal subject of this monography. Thirty-six stratigragphic units were defined, including 9 pyroclastic sequences, 16 lava flow units, 7 epiclastic units, 2 lacustrine deposits, as well as as 1 debris avalanche deposit, on the basis of their physical characters, the stratigraphic position of each deposit and the presence of discontinuities, erosional surfaces, palaeosoils and epiclastic deposits within the reconstructed stratigraphic sequence. In contrast to previous maps, this new geological map is based on the use of Unconformity Bounded Stratigraphic Units (UBSU) in compliance with the guidelines of the International Subcommission on Stratigraphic Classification. UBSU are separated by discontinuities in the stratigraphic succession, which emphasise the major breaks in the evolution of the volcano and its spatial and temporal geometry. This approach greatly improves in the understanding of the geological history of the island and highlights changes in the eruptive styles as well as the localisation of the volcanic centres through time. These topics are exhaustively discussed in chapters 2 and 3.
Further results of the volcanological investigations concerning tectonic and volcano-tectonic and petrology and rock geochemistry are presented in chapters 4 and 5, respectively, whereas the reconstructed eruptive history of Nisyros is dealt with in chapter 6. All together chapters 2 to 6 constitute the first part of the monography, which is devoted to the geological history of Nisyros volcano.
The second part of the monography has an applied component, namely the mitigation of the volcanic hazard both for the permanent residents of the island and for the annually over 50,000 visiting tourists mostly without awareness of the entire risk situation. It presents the results of a geophysical surveys (chapter 7) and of several geochemical studies (chapters 8, 9 and 10) which were carried out during the last fifteen years. Indeed, an impulse in geochemical investigations occurred during and after the increase in activity that took place in 1995-1998 comprising intense seismicity, with earthquakes of magnitudes between 4.1 to 5.5 on the Richter scale, ground deformations, as well as remarkable variations in the chemistry of fumarolic effluents. In spite of these considerable efforts of the scientific community that led to the publication of several papers, a thorough review and synthesis of the existing geochemical information was still lacking. Chapter 9 is aimed to fill this gap. The need for the stochastic approach in the evaluation of the fluxes of CO2 from soil and the total output of carbon dioxide is emphasized in chapter 8, thanks to the contribution of Marino Vetuschi Zuccolini, the first who understood the importance of this geostatistical technique, in spite of what other people claim. Chapter 10 is devoted to evaluate the timing of argillic alteration and advanced argillic alteration by using reaction path modeling and to infer possible consequences for debris avalanches induced by flank collapse and hydrothermal eruptions. Finally, the implications for mitigating the volcanic hazard at Nisyros drawn from all the geological, geophysical, and geochemical data presented and reviewed in this monography are discussed in Chapter 11.
We hope that our work may be of help for the people who live on this Earthly Paradise or just spend some time there. We would like to thank the population of Nisyros for their warm hospitality, that always turned our field seasons into wonderful experiences. Special thanks are due to Roberto Cioni of IGG-CNR, Pisa and Roberto Moretti of OV-INGV, Napoli for laboratory chemical analyses and field assistance, respectively. The internationally composed GEOWARN group contributed to the success of our work, many fruitful discussions helped to clear our views on our proper work and also helped with many important details. Last but not least we sincerely thank Lorenz Hurni, who allowed us to use the topographic map as base of our geology

Hunziker, Johannes C.

Marini, Luigi

Serveur académique lausannois

Mémoires de Géologie (Lausanne) Section des Sciences de la Terre Université de Lausanne BFSH-2, 1015 Lausanne, Suisse 44 This work is licensed under a Creative CommonsAttribution 4.0 International Licensehttp://creativecommons.org/licenses/by-nc-nd/4.0/least- can we really assume, that on an island degassing CO2 in the order of over 1000 tons/day the scarce 14C did not at all exchaoge with the 12C of the passing CO2 (ratio of 12/14 in the order of 1012- 10 15 ) ? In other words the 14C ages of Nisyros volcano are to be taken cum grano salis. Fission tracks on volcanic glass: In the early days of fission track dating, experts have originally used volcanic glass for dating, but with time have left this technique due to difficulties in distinction between real tracks of fission and elongated gas micro-bubbles in the glass. Today most specialists do not apply this method any more on volcanic glass or even on pumice. In order to believe in these ages one would like to see a stratigraphie succession measured and yielding successive ages, and not only 2-3 isolated determinations.K-Ar: So far no systematic work on K-Ar basbeen performed. Only whole rock measurements have been reported. In order to find out if a system is datable, minerai separates, glassy matrix as well as whole rock analyses would be required, and this on a stratigraphie profile and not on isolated samples. No such work was reported. 40 Ar/39 Ar: The only reliable age determination is the Kos plateau tuff (KPT) dated on Kos by Smith et al. (1996), yielding 161±1 ka. The whole age situation not being very clear, a PhD thesis of A. Büttner on the subject was started in 2000 by Prof. I. Villa in Bern. In her PhD thesis Büttner (2004) reports only 8 pumice? samples on Nisyros (3 plagioclases, 2 glasses and 3 not specified samples) -one would really like to know what exactly was analysed there, but the precise locality where the rocks have been taken was not reported. All these samples show highly discordant spectra ranging between 10 ma and negative ages. Büttner (2004) concluded, that strangely, the high temperature plagioclases are extremely poor in potassium and most of the potassium is in the glass matrix, which includes considerable amounts of parentless 40 Ar. She states, that as a consequence the age spectra of the plagioclases and glasses did not provide a reliable chronological framework. We cannot decide if these facts are due to a poor choice of the measured samples, to underestimated blank problems of the used furnace, (sample sizes range between 7 and 110 mg), or simply due to parenlless 40A.r as suggested by Bütlner (2004). One thlng however seems quite clea.r: the measured spectra have the peculiarity (hal the radiogenic 40 Ar component is mostly in the order of 0-6%. In addition the non radiogenic "atmospheric" Argon 40/36 ratio is very often in the range of 160-260 and not as generally accepted 295.5. These problems have not been addressed in the thesis. As no Ar/Ar correlation plots are reported for the Nisyros 27 samples, but instead only a Ca/K-Cl/K diagram, this anomaly most likely remained undetected. The over 400 feldspar electron microprobe analyses of the Lausanne group from all the different outcropping rocks of Nisyros, show that plagioclases normally have around 0.4-0.5 weight % of K2O, but also that in some not too rare cases, the Or component is higher, yielding up to 1.8% K2O and not as stated by Büttner (2004) lower than 0.7%. The over 100 amphibole analysis show K20 contents up to 0.7% and whole rocks show contents up to 4% K20. These rocks must contain in the micro­crystalline or glassy matrix components with high potassium contents. Here the question arises if another sample choice instead of only pumices would not have improved the results considerably. For a Swiss PhD thesis one would have expected a wider choice of samples that to our knowledge had been collected on Nisyros and also a greater number of analyses. In the original proposai of the PhD-thesis of A. Büttner there was a suggestion to increase the signal to background ratio of these most likely extremely young rocks by using a laser ablation system-this could also have changed the results. In a future study of the age relations of Nisyros volcano these points need to be clarified in order to hopefully corne to more meaningful solutions. A further approach of dating the Nisyros rocks was undertaken by Büttner (2004) with the U-Th disequilibrium method on 6 paleosols sandwiched between volcanic rocks and 3 rocks, 2 lavas and a pumice. Here also the obtained ages are highly discordant, ranging between 400 and 10 ka and orders of magnitude younger than the intercalated Ar/Ar ages. The given interpretation of open system behaviour of the paleosols and of not suitable U/Th ratios of these soils show that we are still far from solving the age problems on Nisyros. Here again we can state that in a PhD thesis a methodology newly developped in a laboratory should first be tested on a perfectly calibrated geologic section-this would be a geologists approach! Obviously not considered important. 3.2.1. Age range of the volcanic succession Based on previous discussion, it is evident that the duration of volcanic activity on Nisyros is poorly constrained by few and in some cases contrasting absolute ages, as already underscored by Limburg & Varekamp (1991), Francalanci et al. (1995) and Pe­Piper & Piper (2002). Table 3.1 summarises the available age data. Since the 31-ka�old Yall pumice fall bas been found remobilized on the talus deposits inside the caldera walls (Volentik et al., 2002), but neither above the 40 a§ ., -.... ., a ., .,�� "" Ê ·.-:: 2l ., ll Chernical .,:. C "ë General description Tectonics Most representative outcrop ::s � 0 ë >, ::> C composition .: ô � >, ..6 _,,, rn .!,! 'ë �� ::s � rn ::> 0 Debris flow deposits related to hydrotherrnal eruptions; the rnatrix is clayey to sandy ,l southern part of the Lakki � gx and yellow in colour and its characteristic is the presence of a variable arnount of 30-40 N030° 0 secondary sulphur and euhedral gypsurn crystals (Marini et al., 1993) plain and Ramos area 0 pfi Post-caldera lava dornes and dome coulées including andesitic enclaves up to 30 cm in 600 N030° rhyodacitic Mt Profitis Ilias 23 � size. ., Pyroclastic deposit: the Yà!i fallout layer is found rernobilized within the talus fans 2 rhyolitic along the road to 22 ;g yaJ around the island the caldera floor Epiclastic deposit reworking up 2UPJ up Upper Pumice pyroclastic deposits: flow-w;ivy surge-flow-planar bedded surgeup to30 N030° rhyodacitic Loutrà, Pali seQuence � nlf Nilàà lava flows; a palaeosoil covers the top of the blocky surface of the flows 150 N030° rhyolitic Monastery of Aghios 21 Theologos and near NikiàplJ Epiclastic deposits cornposed of debris flow deposits that rework emb to vu,; the top is 5 and up to20 Loutrà, Avlaki -�lnedogenised Lower Pumice pyroclastic deposits: fall-flow-wavy surge-flow sequence topped by a up to20 N030°, Nl20° rhyolitic Loutrà, Pali, A vlaki 20 � lp �espread o.alaeosoil '\:! llf Lubzmià lava flow 7 N340° andesitic northern flank of Nisyros :i (above Loutra) 0 V\11 Vunàri debris avlanche deposit: rounded blocks showing jigsaw fractures within a fine 20-30N030°, N070°, northern coast of the island matrix producing a humrnocky topography N340° emb Emborià lava flows and domes with minor block and ash flow deposits, including l-150 N030°, N340° dacitic northern flank of Nisyros 19 2% of andesitic enclaves (Akimaronas Hill) "'" basalt-andesitic to ï§ blf Aghia Basilei lava flows with thick red scoriae sole and top up to25 N030° western flank ofNisyros ::s andesitic Paleocastro epiclastic deposits reworking the previous volcanic deposits, resulting in upto20 western flank of the volcano pcj the deposition of debris flow and hyper-concentrated flow in the area of Kàrdia If Lava flows related with the previous explosive unit up to 10 N030°, N070°, basalt-andesiticwidespread all around the island 17-18Nl20°,N340° xo Pyroclaslic deposits: red and black scoriae fallout, planar bedded surges, and violet up to 15 N030°, N070°' basalt-andesitic widespread all around the island 15-16 B ovroclasti.c flow Nl20°. N340° ., C .:!l "' between the Monastery of ..J ô vs,, Pyroclastic deposit: (violent strornbolian?) fallout deposit J N070° basalt-andesitic >< Kyra and the beach of Lies p al Epiclastic deposits reworking pa, resulting in debris flow and hyperconcentrated flow 20 in deep valleys in the deposits: displays flow and channel and pinch out structures eastern flank of Nisyros Table 3.2. Summary of the stratigraphy of Nisyros volcan o. 41 E g � ., E " "-" � " -5 � Chemical a·.:::� C ·2General description " Tectonics Most representative outcrop ::l � 0>. J;l composition co� C '(' � >. .-::: >....: en .&> -� �C""'"' � 0 Pahia Ammos pyroclastic deposits: light grey surges, proximal fallout and minor 80 N070°, N340° andesitic to dacitic between the beaches of 17 pa pyroclastic flow, with a number of intercalated palaeosoils Lies and Pahia Ammos If' Light grey lava flow 10 N070°, N340° dacitic between the beaches of Lies and Pahia Ammos a]f Argos lava: dark grey lava flows and domes with minor block and ash flow deposits; upto60 N030°, N070°, rhyolitic southern caldera walls below 14 reddish banding and perlitic textures N120° Monastery of Stàvros "' Stàvros pyroclastic deposits: show a surge-flow sequence (note the Jack of airfall 30 N030°, N070° basalt-andesitic southern caldera walls below "' ., SV 13 C deposit) Monastery of Stàvros " 0 ;::J 1f6 Lava flow with a thick sole of red scoriae up to 40 N030°, N070° andesitic north-eastern part of Nisyros 12 xlfb Lava flow with the peculiarity of incorporating up to 30% of enclaves (mainly up to25 N030°, N070° dacitic north-eastern part of Nisyros 11 andesi tic blebs) IF Lava flow up to40 N030°, N070°, andesitic north-eastern part of Nisyros 10 N340° Epiclastic deposit reworking ms 2 northern and north-eastem 9 ms; caldera walls (below Emborio) Melisseri pyroclastic deposits: fall-flow-fall-surge-flow succession; magmatic fraction 35 N030° andesitic northern caldera walls 8 " ms is composed of pinkish red pumices and scoriae below Emborio � north-eastem caldera walls (ubiquitous "t:l If' Scoria-rich lava flows 70 N030° basalt-andesitic 7 0 around the caldera) "Lacustrine deposits: sarne deposits as fl1 but the sequence ends with deposition of ., en ..J s� 5 north-eastern caldera walls subaqueus lapilli fallout and fine sand-waved surges � " Ir' Lava flow emplaced on the top of the lacustrine beds, resulting in Joad cast structures -" within Ûle water-saturated sediments 5 N030° basalt-andesitic north-eastern caldera walls 5 � Lacustrine deposits: varved deposits of dominant volcanic input wiili few intercalations li; fl, of black, Mn-rich layers 5 north-eastern caldera walls 4 kb Lakkl B pyroclastic deposits: show a surge-fall-flow-surge-flow sequence; same 60 N030° basalt-andesitic eastern caldera walls magmatic fraction as ka � Ir2 Lava flow 40 N030°, N070° andesitic north-eastern caldera walls 3 ., Lakkl A pyroclastic deposits: show a fall-flow-surge-fall succession; the magmatic � ka 30 N030° basalt-andesitic north-eastern caldera walls 2 � fraction is black, poorlv vesiculated and crvstal-ooor (cox and fds). tr• Lava flow; strongly tectonised in some places 35 N030° basalt-andesitic north-eastern caldera floor 1 ,., ho. Epiclastic deposits reworking and interbedded within ho.; intense hydrothermal 10 north-western coast ,;:: alteration in some places ., � ho. Holaki hyaloclastites and pillow lavas 40-50N030° basalt-andesitic Monastery of Aghios Spiliani Table 3.2. continued 100 6. Nisyros' volcanic evolution: the growth of a strato-volcanoLoye Vanderkluysen(l), Alain Volentik(l), Claudia Principe<2>, Johannes C. Hunziker<1>, Jean Hernandez<1> (Il Institut de Minéralogie et Géochimie, BFSH-2, Université, CH-1015 Lausanne, Switzerland. <2l Istituto di Geoscienze e Georisorse, Area della Ricerca CNR-Pisa San Cataldo Via G. Moruzzi 1, 1-56124 (Pisa), ltalyAbstract - Field evidence clearly shows that both today's cone-like morphology is the result of the migration of volcanic centres along major tectonic trends and the volcano's morphology was periodically reorganized by collapse and faulting episodes. After the formation of a submarine paleo-edifice (which went through an immersion-and-emersion episode), a new subaerial cone was built eastwards by multiple effusive and explosive eruptions. These lead to the formation of a summit depression in which a Jake settled. After the Melisserl eruption, activity drifted to the North-East, and the South of the island. After this stage, a widespread activity occurred throughout the island, giving birth to monogenetic vents scattered along Nisyros' main tectonic trends. Effusive activity followed in the North-West and North-East, and the formation of dacitic dames triggered the collapse of the northern volcano's flank. The Lower Pumice eruption lead to caldera collapse at the island's centre, whereas the Upper Pumice eruption was secluded to the flank collapse depression to the North. The caldera was later partly filled by dacitic dames, and it has been the main locus of hydrothermal activity in recent times. 6.1. Introduction The geology of Nisyros has been described by previous authors (e.g. Davis, 1967; Keller, 1971; Di Paola, 1974; Vougioukalakis, 1984, 1993, 1998; Bohla & Keller, 1987; Keller et al., 1990; Limburg & Varekamp, 1991; Papanikolaou et al., 1991; St. Seymour & Vlassopoulos, 1992; Francalanci et al., 1995) as resulting from a succession of effusive and explosive volcanic eruptions, ranging in composition from basaltic andesites to rhyolites and emitted from a central vent. In the literature, the island's evolution has generally been divided in 4 major stages. Stage 1 is characterised by submarine activity, which likely pre-dates, at least partially, the eruption of the Kos Plateau Tuff (Keller, 1971; Rehren, 1988; Keller et al., 1990), which took place 161 ka B.P. (Smith et al., 1996). Stage 2 consists of the growth of the main andesitic strato-cone. The two eruptions of the Lower and Upper pumices and the effusion of large volumes of rhyolites (the Nikià lava) characterise stage 3 of Nisyros' activity. Finally, stage 4 comprises the caldera collapse, resulting from these pumice eruptions, and the subsequent emplacement of the dacitic domes of Prophitis Bias, filling the western two thirds of the present day caldera. Based on newly collected data and together with the findings of Volentik et al. (2002), the volcanic history that we draw differs with the general accepted view on various accounts. In the following reconstruction, we show that the present, perfectly conic shaped morphology of this "classical" strato-volcano topped by a summit caldera was caused by the migration of the eruptive vents along the main tectonic trends and by the occurrence of repeated collapses and faulting episodes. 6.2. Volcanic Evolution The first volcanic events on Nisyros (Fig. 6.1-A and 6.2-A) gave birth to a submarine volcano. It consisted of lava pillows, now outcropping on the north-western shore of the island (Fig. 2.5 in Volentik et al. (a), this volume). There is full agreement in the literature that this pillow lava sequence lies at the base of the Nisyros volcanic stratigraphie succession. This submarine sequence is interrupted by an emersion stage testified by the erosion of the pillow lavas and their re-deposition in debris flow facies. The presence of reworked rounded purnices at the top of the submarine sequence, attributed by Keller (1971) to the KPT eruption that crops out in the nearby island of Kos (Allen & Cas, 2001), implies that the pillow lava sequence formed prior to 161 ka (the age of the KPT eruption; Smith et al., 1996). After this sub­aerial episode, an important tectonic event might have brought this edifice back below sea level, where submarine lava em1ss10n resumed. Alternatively, an inflation-deflation mechanism linked to the KPT magma chamber and the ernission of this ignimbrite can be hypothesized. As a matter of fact, the occurrence of a sub-aerial nàlua Nort nakia SoutMonasteyof Stàvros 60 SiO2 w9% 70 Sechanca andesite aga! Andesil Omast (1887 rater) Oacite 0mast 1573 rater)Beac b Irachvandesite ea atered by Trachyandesite (> B0g m'd') Data from Rhyolite lArea of interse CO; Mu 20) Legend Hydrothermal Tephrniphonalite 12 -Landsl |TAS. Chemical classification of volcanic deposits Cnncen: low andhyper Debns avalanche d pasi depos Ash low Pyroclast1c block low depost Lihic-ag breccad posiPyrocastic fo Enclave-dch lava flowLiholacies Geologcal sketch map Quamy (2003) and oal onieachet a et al(2003) Themal torg Fumarole Aeolopcal secton Talus ta Lava lo Dike erred emisson c on center e O) Nec A Tuf cong Scona cone ferred buned m of the unsn cal caldera n calder Se a nm Hydrothemal nhscs Fracdure Symbols Legend PA Scale 1 12500 1 o 2 (1) Institut de Minéralogie et A GEOLOGICAL MAP OF NISYROS VOLCANO (GREECE) Polybotes (1671 cralers area km dccd Géochimie, Unversitb de Lausanne Switzerland-. (2) Istituto di Geosoenze Geornsorse, CNR di Pisa Alain C. M. Volentik, Claudla Principe, Lojc Vanderkluysen"and Johan es C. Hunziker" 

The geology, geochemistry and evolution of Nisyros volcano (Greece). Implications for the volcanic hazards

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