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Geochemistry and geophysics of active volcanic : an introduction

CORENTIN CAUDRON1,2*, TAKESHI OHBA3 & BRUNO CAPACCIONI4 1Laboratoire de Volcanologie, G-Time, DGES, Universite´ Libre de Bruxelles, Brussels, Belgium 2Seismology and Gravimetry Department, Royal Observatory of Belgium, Uccle, Belgium 3Tokai University, School of Science, Department of Chemistry, Kanagawa, 4Department of Biological, Geological and Environmental Sciences, University of Bologna, Bologna, *Correspondence: [email protected]

Gold Open Access: This article is published under the terms of the CC-BY 3.0 license

Volcanoes sometimes host a at the Earth’s from the (Montanaro et al. 2016). These surface. These lakes are the surface expressions of eruptions, sometimes termed steam-driven erup- a reservoir, often called a hydrothermal system, tions (Montanaro et al. 2016), are often more vio- in highly fractured, permeable and porous media lent than magmatic eruptions and can have where fluids circulate (Fig. 1). The existence of a ejection velocities .130 m s21 (Rouwet & Morris- volcanic lake depends on a balance between: (1) a sey 2015); they may culminate as phreato-magmatic seal at the bottom of the lake to prevent water seep- eruptions. Both types of eruption can be accompa- age; (2) abundant meteorological precipitation; (3) a nied by base surges after column collapse, tsunamis sustained input of volcanic fluids; and (4) a limited or seiches (Mastin & Witter 2000). They can also heat input to avoid drying out of the lake by evapo- generate destructive , which can travel up to ration (Pasternack & Varekamp 1997; Rouwet & tens of kilometres down the slope of the Tassi 2011). (Manville 2015). Another spectacular and well- When these conditions are met, volcanic lakes studied hazard is limnic gas bursts releasing large become excellent monitoring targets. First, they amounts of CO2 (Kusakabe 2015). An indirect integrate the heat flux discharged by an underlying impact is the prolonged release of acidic gas species magma body. An increase in magmatic activity can (HC1, SO2 and HF). Water contamination and wall therefore be detected remotely or directly by prob- rock failure after seepage also occur in the direct ing the lake temperature. Second, they condense vicinity of volcanic lake environments (van Hins- some volcanic gases, leading to a mixture of dis- berg et al. 2010; Delmelle et al. 2015). solved chemical compounds. These solutes are pre- This volume brings together scientific papers served in solution for a long time compared with the on volcanic lakes, including studies of their struc- gases emitted into the atmosphere through fuma- ture, hydrogeological modelling, long-term multi- roles. Because they trap volcanic heat and gases, they disciplinary monitoring and a number of innovative are excellent tools that can provide additional infor- methods of sampling, data acquisition and in situ or mation about the status of a volcano and the hazards laboratory-based experiments. Several papers chal- related to a volcanic lake (Rouwet & Tassi 2011; lenge long-established paradigms and introduce Manville 2015; Rouwet & Morrissey 2015). new concepts and terminologies. This collection of Depending on their volume, volcanic lakes can, papers is a useful reference for researchers dealing however, filter and delay the surface expressions with volcanic lakes and more generally with hydro- of volcanic unrest. Despite only 8% of reported thermal systems, phreatic/hydrothermal eruptions eruptions worldwide having occurred in a subaqu- and wet volcanoes. eous setting, they have caused 20% of fatalities (Mastin & Witter 2000). One of the most dramatic hazards is a phreatic eruption, which arises from History and state of the art the sudden input of fluids and energy from a mag- matic source into a more superficial aquifer (Rouwet In the history of volcanology, knowledge about a & Morrissey 2015). Hydrothermal explosions specific type of phenomena has often undergone involve water close to its boiling temperature and an abrupt acceleration only after a catastrophic are also generated at shallow depths, but are not trig- event (D. Rouwet, https://iavcei-cvl.org/). Investi- gered by an input of mass or energy derived directly gations on volcanic lakes made dramatic progress

From:Ohba, T., Capaccioni,B.&Caudron, C. (eds) 2017. Geochemistry and Geophysics of Active Volcanic Lakes. Geological Society, London, Special Publications, 437, https://doi.org/10.1144/SP437.18 # 2017 The Author(s). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

C. CAUDRON ET AL.

Fig. 1. Schematic cross-section of a typical high-activity volcanic lake and its main features. after the 21 August 1986 degassing event at Lake and (6) no activity. The essential subdivision Nyos in , which killed more than 1800 between the two first classes and the other four relies people. At that time, it became clear that volcanic on the balance between input and output fluxes. The lakes may constitute a danger in themselves, based high-activity acidic lakes are further sub-divided on their limnological characteristics and type of depending on their total dissolved solutes (TDS) interactions with the volcanic system. This has content and temperature: class 3a, hot, TDS ¼ resulted in new investigations and monitoring 150–250 g l21, T ¼ 35–458C; class 3b, cool, actions to mitigate the risk inherent in the existence TDS ¼ 40–150 g 121, T ¼ 20–358C. The medium of a . activity lakes are less acidic and often oxidized In the 1990s, the volcanology community real- (class 4a). Some reduced acid–saline lakes exist ized that monitoring volcanic lakes could provide (class 4b). Low activity lakes basically consist of additional information about changes in the activity steam-heated pools (class 5a) or CO2-dominated of degassing volcanoes and hydrothermal systems. lakes (class 5b). Dissolved CO2 can reach the sur- Research started and improved the forecasting of face in shallow lakes (class 5bl) or remain trapped volcanic eruptions. By the end of the century, differ- in the hypolimnion forming a stratified lake, also ent disciplines – including volcanology, geochem- called a Nyos-type lake (class 5b2). Varekamp istry, hydrology, limnology, microbiology and et al. (2000) introduced two physical–chemical economic geology – were being used to study inter- classifications based on the sulphate + chloride con- twined aspects of volcanic lakes. centrations and pH values and the percentage resid- Since the 2000s, the community studying these ual acidity. ‘blue windows’ (Christenson et al. 2015) became Another physical classification is based on the broader, with further geophysical and modelling outlet dynamics: closed lakes, without surface out- efforts. Volcanic lakes are now not only investigated lets with a variable volume; and open lakes with using state of the art thermal and chemical an outlet and an overall constant volume (Varekamp approaches, but also through geophysical surveys, 2015). biological analyses, numerical modelling and multi- One crucial aspect concerns the residence time disciplinary efforts. These 30-year long efforts have (RT), which is defined by the lake volume and the led to a better understanding of these environments input (or output) fluxes (RT ¼ V/Qinput); the larger and a more comprehensive classification. the lake volume and the lower the input fluxes, the There are currently 474 lakes listed in the new longer the RT will be (months to years). Smaller database of volcanic lakes, VOLADA (Rouwet et al. lakes affected by a high fluid input will result in a 2014), grouped into six classes (Fig. 2) based on short RT (weeks to months (Varekamp 2003; their volcanic activity (following Pasternack & Var- Taran & Rouwet 2008; Rouwet et al. 2014). The ekamp 1997): (1) erupting; (2) peak activity; (3) RT therefore determines the lake’s sensitivity to high activity; (4) medium activity; (5) low activity; potential changes caused by external processes and Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION

Fig. 2. Classification of volcanic lakes. The different classes of volcanic lakes, simplified from Pasternack & Varekamp (1997) and modified from Rouwet et al. (2014), are illustrated with pictures of typical lakes. From top to bottom, these are: Voui, by K. Ne´meth; Ruapehu, by V. Chiarini; Kawah , by D. Rouwet; El Chicho´n, Mexico by D. Rouwet; Nyos, Cameroon by D. Rouwet; and by A. Hicks.

the frequency of monitoring required (Rouwet et al. volcano-influenced groundwater) is at least as 2014). Recent efforts are further challenging and important as the gaseous flux. It is therefore of par- investigating this crucial aspect, which should amount importance to consider the aqueous flux to drive monitoring strategies (Shinohara et al. 2015; estimate the impact of volcanoes on their environ- Tamburello et al. 2015; De Moor et al. 2016; ment and the contribution of volcano-hydrothermal Caudron et al. 2017). Other studies are presented systems to global cycling. in this volume and are now briefly introduced. Gunawan et al. (2016) report the results from the highly focused international wet volcano work- shop that took place at Kawah Ijen in September This volume 2014. Wet volcanoes are volcanoes hosting crater Acidic lakes lakes and/or extensive hydrothermal systems. The study provides a detailed description of the different Several papers concern the Kawah Ijen volcanic technologies deployed during fieldwork. Measure- lake (East , Indonesia). This lake is emblematic ments include seismic and acoustic recordings, and is the largest hyper-acidic volcanic lake on in situ and remote thermal measurements of both Earth. The volcano-hydrothermal system was stud- high-temperature fumaroles and lake water and also ied from a geochemical perspective in the 1990s multi-gas analyser, differential optical absorption and the early 2000s (Delmelle & Bernard 1994; spectroscopy and laser spectroscopy measurements Delmelle et al. 2000; Takano et al. 2004). Caudron from instruments deployed around and within the et al. (2015a, b, 2017) provide further insights into crater. All the data and results are combined to build its volcanic activity using geophysical instruments a conceptual model of the Kawah Ijen volcano. This combined with volcanic lake parameters. study highlights the similarity of the Kawah Ijen Van Hinsberg et al. (2015) constrain the flux volcano and Ruapehu volcanoes (New Zealand) emitted by the volcano to the environment for and improves our understanding of wet volcanoes. most elements of the periodic table. The composi- Further insights into the Kawah Ijen volcano tional signature of emissions is similar to the are provided by Caudron et al. (2016), who present major volcanic emitters, but the element fluxes the first comprehensive geophysical investigation. are smaller. Importantly, the aqueous flux (i.e. The hydrogeological system surrounding the Kawah the seepage of volcano-hydrothermal fluids and Ijen crater lake consists of three different and Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

C. CAUDRON ET AL. independent aquifers. The volcanic lake is surpris- Rouwet & Iorio (2016) describe two seemingly ingly heterogeneous, as illustrated by spectacular unrelated phenomena: clays floating on the surface subaqueous degassing. These results provide new of the El Chicho´n crater lake resembling Sumi- fundamental knowledge on the hydrothermal sys- nagashi patterns, the traditional paper marbling tem of Kawah Ijen volcano, which may be useful technique. Clays floating on the lake are subject to in volcanic surveillance programmes. Marangoni flow when the lake becomes very shal- Poa´s volcano () hosts another hyper- low (a few centimetres) near the lake shore. The flat acidic crater lake of great importance and is one of lake bottom and the dynamic alternation between the most dynamic wet volcanoes. After 12 years of retreat and incurrence of the lake shore result in quiescence, phreatic activities resumed in March precipitation of the clay patterns, preserving the 2006 and continue until the day of writing. Rouwet Suminagashi-like designs in the lake sediments. et al. (2016) describe and analyse the geochemical The recognition of similar patterns in a sedimentary fluctuations during the period 2006–10. No system- record could indicate the presence of a shallow sedi- atic relationship between geochemical concentra- mentary environment in the past, possibly pointing tions and phreatic activity exists and a dynamic to a heated crater lake. fluid recycling between the lake and the underlying magmatic–hydrothermal system is suggested. The Neutral lakes authors also report phenomena such as the outgas- sing of chlorine from the lake surface, describe the Six papers deal with lakes Nyos and Monoun (Cam- Caliente as an ‘open fumarole’ and provide eroon), the iconic volcanic lakes where limnic erup- a stimulating new paradigm: the water chemistry tions, driven by the outgassing of CO2, occurred in reflects a transient and fast process of gas flushing 1986 and 1984, respectively. through the lake, rather than slow fluid inputs/out- Kozono et al. (2016) developed a numerical puts. It therefore discredits water chemistry as an model of CO2 degassing from the lake waters dedi- efficient monitoring tool within the classic monitor- cated to the problem of CO2 saturation at mid-depths ing frequency. that could lead to a limnic eruption. They modelled Capaccioni et al. (2016) challenge the assump- the effects of changes in the CO2 profiles due to the tion that HCl behaves conservatively when dis- entrance of CO2-unsaturated fluids from the lake solved in extremely acidic water. Their laboratory bottom. They emphasize the future possibility of a experiments highlight HCl degassing from limnic eruption caused by violent degassing at extremely acidic waters. Two main outcomes arise mid-depth. Their model allows an estimate of CO2 from this study: (1) Cl2 does not behave conserva- concentrations at the lake bottom from the heights tively once dissolved in extremely acidic systems; of fountains observed at the lake surface. and (2) the presence of very acidic gas species in Ohba et al. (2015) studied the annual variability fumaroles can be associated with a feeding system of the total amount of CO2 stored within Lake dominated by liquid water. Nyos since 2011. They point out the effects of the Twenty years of geochemical and temperature installation of the degassing pipes that led to a data provide insights into the volcano-hydrothermal rapid decrease in the amount of CO2 stored within system of volcano () and reveal . fundamental changes in the system preceding or Saiki et al. (2016) analyse the vertical sound accompanying magmatic and phreatic eruptions speed profiles (see Sanemasa et al. 2015 for meth- (Agusto et al. 2016). The integration of geophysical odology) measured at lakes Nyos and Monoun in data allows the development of a conceptual model 2012 and 2014. They found a significant linear cor- of this volcanic system and the identification of pre- relation between the total CO2 concentrations and cursory signals for eruptive activity. the vertical sound speed in the lake water. By per- By using an interesting combination of water forming multi-point measurements, they observed chemistry and echo-sounding techniques, Herna´n- horizontal variations at both lakes. Their results dez et al. (2017) investigated the less acidic (c. pH show a stable vertical stratification at Lake Nyos with 2–3), but dangerous, Taal volcanic lake (Philip- a decrease in the total CO2 from 2012 to 2014 and pines). They report changes in the chemical compo- distinct CO2 concentrations at Lake Monoun due sition and increases in CO2 emissions associated to the presence of three different basins. An increase with volcano-seismic unrest in April 2010–June in CO2 was further detected in the main basin from 2011, attributed to deep fluid injections. They inter- 2012 to 2014. pret the inflationary periods as due to the faulting of Sanemasa et al. (2015) present a new method an impermeable cap rock sealing the deeper hydro- of measuring dissolved CO2 concentrations using thermal reservoir in response to degassing and con- sound speed and electrical conductivity. It has vective movements in the underlying Taal magma been tested at lakes Nyos and Monoun and may chamber. improve the forecasting of future limnic eruptions Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION as a result of the great spatial and temporal resolu- Multi-disciplinary efforts are becoming increas- tions compared with traditional techniques. ingly popular, as shown in this volume. They are Yoshida et al. (2015) describe the results of the essential in characterizing a volcano-hydrothermal installation of a new deep water removal system, system. The recent progress made in instrument operated by a pumping system powered by solar and storage capabilities has allowed sampling at a energy, following the loss of capability of the previ- much better time resolution. For example, multi- ous systems due to reduced CO2 partial pressures. gas analysis or thermal imagery can provide time Deep anoxic conditions at Lake Nyos cause the series with a sampling frequency of a few seconds dissolution of iron as Fe2+ and the precipitation of (e.g. Gunawan et al. 2016), approaching the sam- siderite. The water transported upwards by degas- pling rate of most common monitoring instruments, sing pipes undergoes a rapid increase in redox such as seismometers or the global positioning potential, causing the precipitation of iron as system. This allows the sensing of new processes Fe(OH)3 and leading to the surface water becoming by integrating a wide range of multi-disciplinary reddish. Ozawa et al. (2016) investigate iron disso- data. It will therefore improve our understand- lution/precipitation using in situ experiments at ing and forecasting of volcano-hydrothermal sys- Lake Nyos and laboratory analyses. They describe tems. However, major questions need to be the dissolution process for siderite at depths answered before declaring the usefulness of these ,50 m and precipitation at greater depths. By techniques in volcano monitoring. Probabilistic applying interferometric techniques, they highlight methods are also becoming increasingly popular the change in the rate of siderite precipitation in the field of volcanic lake studies and could facil- along the vertical profile, revealing a high increase itate unrest and eruption forecasting (e.g. Tonini close to the lake bottom. et al. 2016). Zimmer et al. (2015) present another innovative Recent studies have also questioned the tradi- analytical method based on a gas membrane sensor tional monitoring strategies at active crater lakes. to quantify the concentrations of CO2 and CH4 gases The concept of residence time is the key to elaborat- in water columns from volcanic lakes. They tested ing an efficient volcano monitoring scheme. The this new set-up at the Monticchio Grande and Pic- chemical and thermal homogeneity of a lake over colo (Mt Vulture, Italy) and Pavin (Massif Central, time also need to be assessed to obtain representa- ) lakes and successfully compare the results tive measurements and valuable monitoring data, with the more conventional single-hose method. even at the most dynamic and active systems (Rou- Other neutral lakes were investigated through wet et al. 2016; Caudron et al. 2017). This is also two studies. Melia´n et al. (2016) report the first crucial in the correct interpretation of remote tem- detailed study of volcanic lakes of Sa˜o Miguel perature data from forward-looking infrared or island, Sete Cidades, Fogo and , located in satellite imagery. the archipelago (). By combining a A burst of new studies and projects are dealing floating accumulation chamber and echo-sounding with active crater lakes following the 2014 Ontake measurements, they detected low surface CO2 phreatic eruption, which caused the deaths of more degassing, but dense and moderate subaqueous than 50 climbers. The filtering and delaying capac- emissions. Strong dissolution occurs in the water ity of volcanoes hosting well-developed hydrother- column due to the high pH. mal systems make them very hard to monitor, but Lefkowitz et al. (2016) present the general geo- the number of variables that can be monitored chemical features of the unique twin crater lakes on make them attractive to better understand the gener- , USA. The lakes are ation of these eruptions. Active crater lakes also both carbonate-rich, but are different in chemical condense fluids rising from below. They are there- composition, morphology and sediment composi- fore very sensitive to sudden changes in pressure tion, although separated by only a narrow volcanic (fluid injections), which increase the likelihood of ridge. shows the features of a terminal a steam-driven eruption (Manville 2015). Sealing lake with a gaseous geothermal input, whereas Pau- has been proposed as the driving force of some phre- lina Lake has subaqueous hot springs and an outlet. atic eruptions that have occurred at several different Sudden CO2 degassing presents a volcanic hazard to volcanic lakes (Christenson et al. 2010; Agusto be considered in East Lake, as well as the presence et al. 2016; Caudron et al. 2016; Rouwet et al. of highly toxic components. 2016). Several experiments are characterizing the influence of the type and degree of alteration or the host rock lithology on the generation of phreatic Future perspectives eruptions (e.g. Mayer et al. 2015) and the partition- ing of energy into thermal, mechanical or seismic This Special Publication comes at a challenging and energy, the fragmentation energy associated with exciting time for the volcanic lake community. these volcanic events (e.g. Montanaro et al. 2016). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

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This volume shows that the risk at lakes Nyos Christenson, B., Reyes, A., Young, R., Moebis, A., Sherburn Cole-Baker Britten and Monoun has been reduced by eliminating CO2 , S., ,J.& , K. 2010. gas dissolved in deep water by ‘self-gas lift’ as a Cyclic processes and factors leading to phreatic symptomatic treatment. However, the removal of eruption events: insights from the 25 September 2007 gas by this method is not perfect, because self-gas eruption through Ruapehu Crater Lake, New Zealand. Journal of Volcanology and Geothermal Research, lift does not operate when the dissolved gas concen- 191, 15–32. tration falls below a certain threshold, preventing Christenson, B., Xemeth, K., Rouwet, D., Tassi, F., the complete artificial degassing of gas-loaded Vandemeulebrouck, J., Varekamp, J.C. 2015. Vol- lakes. Such a situation is occurring in Lake Monoun. canic lakes. In: Rouwet, D., Christenson, B., On Lake Nyos, self-gas lift continues in 2016, but Tassi,F.&Vandemeulebrouck, J. (eds) Volcanic will stop within 10 years. Thereafter, the dissolved Lakes. Springer, Berlin, 1–20. gas concentration in the deep water may rise again Delmelle,P.&Bernard, A. 1994. Geochemistry, min- and observations of the deep waters of the lake eralogy, and chemical modeling of the acid crater Geochimica will be needed. Local research institutions are lake of Kawah Ijen Volcano, Indonesia. et Cosmochimica Acta, 58, 2445–2460. responsible for the monitoring of this situation, Delmelle, P., Bernard, A., Kusakabe, M., Fischer, co-operating with researchers from other countries. T.P. & Takano, B. 2000. Geochemistry of the magmatic-hydrothermal system of Kawah Ijen vol- We thank Dmitri Rouwet for reviewing this paper, sharing cano, East Java, Indonesia. Journal of Volcanology his thoughts and providing the classification figure. We and Geothermal Research, 97, 31–53. also thank P.T. Leat for his sound and positive review. Delmelle, P., Henley, R.W., Opfergelt, S., Detienne, C. Caudron is funded through a Charge´ de Recherches M. 2015. Summit acid crater lakes and flank instability FNRS postdoctoral grant. in composite volcanoes. In: Rouwet, D., Christen- son, B., Tassi,F.&Vandemeulebrouck, J. (eds) Volcanic Lakes. Springer, Berlin, 289–305. References De Moor, J., Aiuppa,A.et al. 2016. Short-period volca- nic gas precursors to phreatic eruptions: insights from Agusto, M.R., Caselli,A.et al. 2016. The crater lake of Poa´s Volcano, Costa Rica. Earth and Planetary Sci- Copahue volcano (Argentina): geochemical and ther- ence Letters, 442, 218–227. mal changes between 1995 and 2015. In: Ohba, T., Gunawan, H., Caudron,C.et al. 2016. New insights Capaccioni,B.&Caudron, C. (eds) Geochemistry into Kawah Ijen’s volcanic system from the wet vol- and Geophysics of Active Volcanic Lakes. Geological cano workshop experiment. In: Ohba, T., Capaccioni, Society, London, Special Publications, 437. First pub- B. & Caudron, C. 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