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Geochemistry and Geophysics of Active Volcanic Lakes: an Introduction

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Geochemistry and Geophysics of Active Volcanic Lakes: an Introduction Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 Geochemistry and geophysics of active volcanic lakes: 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, Japan 4Department of Biological, Geological and Environmental Sciences, University of Bologna, Bologna, Italy *Correspondence: [email protected] Gold Open Access: This article is published under the terms of the CC-BY 3.0 license Volcanoes sometimes host a lake at the Earth’s from the magma (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 lahars, which can travel up to ration (Pasternack & Varekamp 1997; Rouwet & tens of kilometres down the slope of the volcano 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, 437,1–8. First published online April 20, 2017, 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 23, 2021 2 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 Cameroon, 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 crater lake. 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 23, 2021 INTRODUCTION 3 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, Vanuatu by K. Ne´meth; Ruapehu, New Zealand by V. Chiarini; Kawah Ijen, Indonesia by D. Rouwet; El Chicho´n, Mexico by D. Rouwet; Nyos, Cameroon by D. Rouwet; and Tristan da Cunha 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
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