Stix and de Moor Earth, Planets and Space (2018) 70:83 https://doi.org/10.1186/s40623-018-0855-z

FRONTIER LETTER Open Access Understanding and forecasting phreatic eruptions driven by magmatic degassing John Stix1*† and J. Maarten de Moor2†

Abstract This paper examines phreatic eruptions which are driven by inputs of magma and magmatic gas. We synthesize data from several signifcant phreatic systems, including two in Costa Rica (Turrialba and Poás) which are currently highly active and hazardous. We defne two endmember types of phreatic eruptions, the frst (type 1) in which a deeper hydrothermal system fed by magmatic gases is sealed and produces overpressure sufcient to drive explosive eruptions, and the second (type 2) where magmatic gases are supplied via open-vent degassing to a near-surface hydrothermal system, vaporizing liquid water which drives the phreatic eruptions. The surfcial source of type 2 erup- tions is characteristic, while the source depth of type 1 eruptions is commonly greater. Hence, type 1 eruptions tend to be more energetic than type 2 eruptions. The frst type of eruption we term “phreato-vulcanian”, and the second we term “phreato-surtseyan”. Some systems (e.g., Ruapehu, Poás) can produce both type 1 and type 2 eruptions, and all systems can undergo sealing at various timescales. We examine a number of precursory signals which appear to be important in understanding and forecasting phreatic eruptions; these include very long period events, banded tremor, and gas ratios, in particular ­H2S/SO2 and ­CO2/SO2. We propose that if these datasets are carefully integrated during a monitoring program, it may be possible to accurately forecast phreatic eruptions. Keywords: Phreatic eruptions, Magmatic inputs, Overpressure, Sealing, Vaporization, Forecasting

Introduction A critically important question during phreatic activ- Phreatic eruptions are sudden events, commonly with ity is determining if magma is involved or not. In other few if any precursors. Tey can be lethal to people close words, what is the severity of the crisis? Is a phreatic by, and they commonly precede larger magmatic erup- eruption simply a one-of event with little or no magmatic tions. Phreatic eruptions result from rapid heating and contribution? Or does it involve signifcant amounts of vaporization of fuids which are commonly situated at magmatic gas and potentially eruptible magma? Com- shallow levels beneath a volcano. By defnition, there monly the answer to this question is ambiguous. Barberi is no solid juvenile material in the eruption products et al. (1992) have pointed out that most large eruptions (although sometimes this can be difcult to determine). are preceded by phreatic activity, so it is crucial to ascer- Te fuids involved in phreatic eruptions may originate tain at an early stage if magma is involved or not. Hence, by downward percolation of meteoric fuids into hot this paper specifcally addresses magmatic inputs, the rocks or a hot conduit. Tey also may form from upward mechanisms involved, and the signs and signals indica- migration of volcanic fuids, including gases, supercriti- tive of a magmatic signature. cal fuids, and melts, into a hydrothermal system or shal- Te lack of solid juvenile material in eruptive products low aquifer. A combination of the two scenarios is also is not necessarily an indication that magma is absent at possible. shallow levels. Tis is a very difcult issue, as Cashman and Hoblitt (2004) retrospectively (i.e., > 20 years later)

*Correspondence: [email protected] recognized juvenile glass in some deposits erupted from †John Stix and J. Maarten de Moor have contributed equally to this paper Mt. St. Helens in spring 1980 prior to the climactic erup- 1 Department of Earth and Planetary Sciences, McGill University, 3450 tion on 18 May. Hence these precursory eruptions were University Street, Montreal, QC H3A 0E8, Canada Full list of author information is available at the end of the article phreatomagmatic, not phreatic in nature. On the other

© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 2 of 19

hand, Pardo et al. (2014) indicate that caution is war- phreatic eruptions to be (1) the dominance of hydrother- ranted if fresh-looking glassy material is discovered, as mally altered or lithic components in the eruptive prod- there may be few diferences between lithic and juvenile ucts and (2) the involvement of exogenous water (i.e., glassy material in many cases. steam/water not immediately exsolved from melt). Te Te purpose of this paper is to focus on magmatic lack of juvenile material in eruptive products is a clear contributions (gas, supercritical fuids, and/or melt) to indicator that magma was not directly involved in erup- phreatic eruptions. We frst examine six systems where tion, yet a minor component of fresh glassy material may signifcant insight has been obtained on phreatic activity. be accidental and does not necessary preclude an erup- Ten, we present a conceptual model of phreatic activity tion from being phreatic in nature (e.g., Christenson et al. associated with shallow magma. Te model encompasses 2010; Pardo et al. 2014). Vaporization and expansion of two types of phreatic eruptions: (1) type 1, in which mag- exogenous water is a fundamental process driving phre- matic contributions into an overlying and variably sealed atic eruptions (e.g., Rouwet et al. 2014). Exogenous water hydrothermal system result in overpressure and eventu- includes surface water such as lakes, rivers, and seawater, ally phreato-vulcanian eruptions; (2) type 2, whereby meteoric aquifers such as those found in basin fll con- the magmatic inputs vaporize confned near-surface liq- glomerates, geothermal fuids found in mature volcano- uid water, causing overpressure and eventually phreato- hosted hydrothermal systems, and acidic brines found surtseyan eruptions. We end by discussing the manner in immature magmatic–hydrothermal systems. In the by which phreatic activity can be forecast, also assess- latter systems, water derived from condensation of mag- ing the means by which magmatic contributions can be matic gas may be a signifcant or even dominant source recognized. of liquid water (e.g., Giggenbach 1992). Tus, exogenous water can include water that is principally derived from Terminology and defnitions magmatic fuids but has experienced hydrothermal pro- Phreatic eruptions are a broad class of volcanic erup- cessing such as phase changes (e.g., condensation) and tions, perhaps most easily defned in contrast to purely reaction with host rock. magmatic eruptions. Magmatic eruptions are driven by Phreatomagmatic eruptions display characteristics of processes occurring as magmas rise through the crust magmatic eruptions and phreatic eruptions. Phreato- whereby exsolution of volatiles and crystallization lead magmatic eruptions are driven by magmatic processes to overpressure and eruption. Magmatic eruptions are but also involve vaporization of exogenous water as a classifed into subcategories according to the style and contributing force (Zimanowski et al. 2015). Deposits character of magma ejection. Te classifcation scheme of from phreatomagmatic eruptions have a clear compo- magmatic eruptions is historically derived from key loca- nent of juvenile material that displays textural evidence tions or events that displayed distinct eruptive behavior, for quenching and fragmentation in response to violent i.e., hawaiian, strombolian, vulcanian, and plinian. interaction with coolant (e.g., Zimanowski et al. 2015; Phreatic eruptions are eruptions in which magmatic Alvarado et al. 2016). Eruptive products can be domi- processes are not the principal driving mechanism. nated by juvenile material or altered/lithic material. Rather, broadly defned hydrothermal processes (i.e., Eruptions in which hydrothermal processes are domi- interactions among water, rocks, and magmatic heat and nant in initiating the eruption, but which entrain passive gas) play the key role in generating phreatic eruptions. or residual magma that does not play a signifcant role in On our wet planet, explosive interactions between volca- driving the eruption, can still be considered phreatic (e.g., noes and meteoric water are extremely diverse. Phreatic Christenson et al. 2010). eruptions encompass steam-driven explosions gener- ated by magma intruding fuvial sediments and aquifers, Six signifcant phreatic systems lava or pyroclastic fows interacting with surface water, We now examine six systems which have experienced geyser-like explosions driven by depressurization of near phreatic eruptions. Tese examples have been chosen boiling-point subterranean geothermal water, and vol- because (1) the datasets are good and (2) both type 1 canic eruptions expelling hydrothermal systems formed and type 2 phreatic eruptions are illustrated. For exam- during periods of repose (e.g., Barberi et al. 1992; Rouwet ple, Ruapehu and Poás usually host acid crater lakes. et al. 2014). In contrast to magmatic eruptions, a broadly As a result, type 2 eruptions are commonly generated accepted classifcation scheme of phreatic eruptions is at these volcanoes from extensive vaporization of liq- lacking. uid water caused by magmatic input. At times, however, Tis paper specifcally focuses on phreatic eruptions at both systems undergo sealing and pressurization, some- volcanic hydrothermal systems that are fed by magmatic times resulting in type 1 eruptions and demonstrating input. We consider two defning characteristics of these that near-surface hydrothermal systems can be subject to Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 3 of 19

these processes, as well as their deeper brethren. Infor- durations of infation and defation were roughly equal, mation on these six systems is summarized in Table 1. and the eruption was closely associated with the peak of infation. Superimposed on the infation was a series Nevado del Ruiz 1985 (Colombia) (type 1 phreatic of long period pulses (LPP) with a dominant period of eruption) 15–20 s. Te LPP signals increased signifcantly in mag- Nevado del Ruiz is a large andesitic stratovolcano near nitude during infation, then rapidly dissipated after the the northern end of the Northern Volcanic Zone of the infation peak. Short period tremor (SPT) with a domi- Andes. After about a year of precursory unrest, Nevado nant frequency of 0.3–0.5 s began just prior to the infa- del Ruiz erupted explosively on 13 November 1985, tion peak, then strengthened signifcantly as defation melting the summit ice cap and generating lahars which was initiated. Te SPT signals ended with the cessation killed ~ 25,000 people (Hall 1990). A smaller eruption of defation. 2 months earlier on 11 September was termed phreatic Kaneshima et al. (1996) interpret the VLPD infation at the time, based on the lack of juvenile material emit- as a gradual increase in fuid pressure due to injection of ted by the eruption (Hall 1990; Voight 1990). Distinctive heat and gas from magma at deeper levels. In this model, seismic signals, termed “banded tremor”, were observed the LPP signals are the direct manifestation of this injec- to begin several days before this phreatic eruption (Mar- tion of magmatic materials into the hydrothermal system tinelli 1990, 1991). On 5 September tremor bands were situated at shallow levels beneath the crater. Hence, both frst noted. Te bands were typically 15–20 min long, the VLPD and LPP seismicity record the increasing pres- while individual tremor cycles (tremor signal + quies- surization of the hydrothermal system as hot magmatic cent interval) had durations of ~ 90–100 min (Martinelli gases vaporize hydrothermal fuids, causing them to boil. 1990) (Fig. 1). Individual tremor bands had mean domi- At a critical overpressure exceeding the tensile strength nant frequencies of ~ 4 Hz and showed a gradual ampli- of the impermeable cap rocks sealing the top of the tude increase followed by a sudden cessation (Martinelli hydrothermal system, the rocks fail and fuids discharge 1990). and erupt by opening a conduit to the surface crater, gen- Giggenbach et al. (1990) proposed a model for the erating the SPT seismicity and allowing the hydrothermal magmatic–hydrothermal system beneath the volcano reservoir to defate. No large LPP signals are observed whereby continued crystallization of old magma releases during defation because the system is depressurizing. heat and gas to the overlying hydrothermal system. Tis hydrothermal system is situated at comparatively shal- Mount Ontake 2007 and 2014 (Japan) (type 1 phreatic low levels (probably less than 1 km deep), allowing mix- eruptions) ing to occur between magmatic fuids from below and Mount Ontake is a stratovolcano which has been the meteoric fuids percolating downward from above. Te site of numerous phreatic eruptions in both prehistoric banded tremor studied by Martinelli (1990) may repre- and historic time. In the past several decades, phre- sent periodic injections of hot, magmatically derived gas atic eruptions have occurred in 1979, 1991, 2007, and into the hydrothermal system, causing boiling to occur, 2014 (Yamaoka et al. 2016). Te 2014 eruption on 27 hence pressurization and subsequent eruption. Giggen- September was fatal to 58 people with fve more miss- bach et al. (1990) hypothesized that the eruptions were ing. Te 2007 eruption was associated with a temporally ultimately driven by release of hot gas from crystallizing interesting sequence of events. Shallow magma intru- magma (i.e., second boiling). A similar process at shorter sion occurred in December 2006–January 2007 as indi- timescales was proposed for Galeras volcano in Colom- cated by infation and volcanotectonic (VT) earthquake bia (Stix et al. 1997). swarms (Nakamichi et al. 2009). Te intrusion occurred at the same time as an increase in 3He/4He from 7.0 to Aso 1994–1995 (Japan) (type 1 phreatic eruptions) 7.2 RA in gases from Nigorigo located 4.2 km from the Aso is a large Quaternary caldera on the island of summit (Sano et al. 2015). Based on seismic and geo- Kyushu. Within the caldera is situated a smaller andesite detic measurements, the shallow magma was intruded volcano, which is active. Kaneshima et al. (1996) studied into the volcano at depths as shallow as ~ 3 km, releasing this system for a year in 1994–1995 during which a series heat and gas and pressurizing the hydrothermal system of phreatic eruptions occurred. During larger phreatic above (Nakamichi et al. 2009). A very long period (VLP) events, they observed a remarkable sequence of charac- signal was recorded on 25 January 2007 and interpreted teristic seismic events. Prior to an eruption, a very long by Nakamichi et al. (2009) as groundwater vaporiza- period displacement (VLPD) was observed for 350– tion (boiling causing infation) followed by the opening 400 s, with an initial accelerating infationary phase fol- of a crack and defation. Te crack likely did not reach lowed by an approximately linear defationary phase. Te the surface, as no eruption followed. Hence, this may Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 4 of 19 al. ( 2015 ) Sano et al. al. ( 2010 ) Christenson et al. 2015a ) ( Maeda et al. Kato ( 2015 ) et al. al. ( 2016 ) et al. Terakawa Voight ( 1990 ) Voight Kaneshima ( 1996 ) et al. Nakamichi ( 2009 ) et al. al. ( 2018 ) et al. Girona 2016a ) ( de Moor et al. 2017b ) ( de Moor et al. Sources of data Sources Martinelli ( 1990 ) into the hydrothermal the hydrothermal into system into the hydrothermal the hydrothermal into system into the hydrothermal the hydrothermal into system lowed by injection by of lowed the gas into magmatic system hydrothermal into the hydrothermal the hydrothermal into system into the hydrothermal the hydrothermal into system Injection gas of magmatic Injection gas of magmatic Magmatic eruption Injection gas of magmatic - Magma intrusion fol Injection gas of magmatic Inferred processes Inferred Injection gas of magmatic No No No Possibly No Possibly Eruption forecast? No 1 1 1? 1 1 1 Eruption type 1 25-Sep-07 27-Sep-14 13-Nov-85 Various Mid-late 2007 March 29-Oct-14 Date of eruption Date 11-Sep-85 - months days months month months months months weeks months months ~ 7 Months years to 25 s ~ 20 ~ 1 ~ 2 ~ 1–1.5 3 days 350–400 s ~ 3 ~ 5 ~ 2 ~ 3 Elapsed time frst appear - between and erup of signal ance tion 7 days ~ 3 2 2 fux fux /SO /SO 2 2 2 2 SO SO CO CO + VT swarms perature correlation Decrease in ­ Decrease Decrease in lake- Decrease tem He isotopes VLP event LP seismicity VT seismicity VLP event LP signals, tremor LP signals, VT seismicity Increase in ­ Tremor VLP events Infation Decrease in ­ Decrease LP seismicity Negative lunar-seismic lunar-seismic Negative Observed (s) signal Banded tremor Increase in ­ phreatic eruptions Summary phreatic of signals and changes prior to Ruapehu 2007 Ontake 2014 Aso 1994–1995 Aso Ontake 2007 Turrialba 2014 Turrialba 1 Table and dates Volcano Nevado del Ruiz 1985 Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 5 of 19 al. ( 2017 ) Christenson et al. al. ( 2016b ) de Moor et al. 2016b ) ( de Moor et al. ( 2017 ) Battaglia et al. Sources of data Sources 2017c ) ( de Moor et al. - - degassing into the shallow hydro the shallow into thermal system degassing degassing mal seal Increased magmatic Increased magmatic Pulses of magmatic gas of magmatic Pulses Increased magmatic Increased magmatic Inferred processes Inferred Destruction of hydrother No No No No Eruption forecast? Possibly 2 2 2 2 Eruption type 1 2012–2013 2014 (60 eruptions) Sept–Oct 2014 2017 Feb–March Date of eruption Date 2017 - weeks years weeks ~ 2 ~ 2 ~ 2 Days Months? Weeks Elapsed time frst appear - between and erup of signal ance tion 1–2 weeks 2 2 2 2 /SO /SO /SO 2 2 2 fux fux S/SO 2 2 2 H CO CO CO SO SO sions Increase in ­ Increase in ­ in ­ Decrease VT seismicity Increased spring emis - LP seismicity Defation Increased RSAM in ­ Decrease Observed (s) signal Decrease in ­ Decrease Decrease in ­ Decrease continued 2017 White Island 2012 White Rincón de la Vieja 2014, Vieja Rincón de la 1 Table Volcano and dates Volcano Poás 2014, 2017 Poás Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 6 of 19

Fig. 1 Banded tremor observed on 7 September 1985 at the Nereidas seismic station located ~ 4 km south of the summit of Nevado del Ruiz. This tremor was measured 4 days prior to a phreatic eruption on 11 September. Each tremor cycle is 15–20 min’ duration. Reprinted from the Journal of Volcanology and Geothermal Research, Volume 41, Martinelli, B., “Analysis of seismic patterns observed at Nevado del Ruiz volcano, Colombia during August–September 1985”, pages 297–314, Copyright 1990, with permission from Elsevier have been a failed eruption. Long period (LP) signals (Mori et al. 2016). Together, the lack of deformation, the and tremor were observed in February–March 2007, VT earthquake swarms, the predominantly normal VT indicating that an elevated level of pressurization was faulting, and the elevated ­SO2 fuxes suggest that a release maintained within the shallow hydrothermal system. of high-temperature gas from shallow magma occurred Te phreatic eruption took place in mid-late March (the in late August 2014, initiating the chain of events which exact date is unknown). Afterward, LP events were rare, pressurized the shallow hydrothermal system and led to although some tremor persisted. Te complete sequence the fatal eruption 1 month later. Te source of the gas is from December to March can be seen as progressive enigmatic. Te magmatic gas may have been generated pressurization of the magmatic–hydrothermal system by continued cooling and crystallization of the magma driven by shallow intrusion of magma. since 2007 (second boiling). Te gas could also be the No infation was recorded prior to the 2014 eruption product of new magma emplaced at shallow levels since (Yamaoka et al. 2016). VT seismicity was frst observed in 2007, although there is no clear evidence for this. Yama- late August 2014 about a month before the eruption, with oka et al. (2016) propose that the hydrothermal source intensifcation beginning on 6 September, while ~ 27 LP was located at 1–2 km depth. If so, this suggests that the events were noted between 8 September and the eruption hydrothermal system may have shallowed over time since on 27 September (Kato et al. 2015). Terakawa et al. (2016) the 2007 eruption when it was located at 2–3 km depth suggest that the system was pressurized for at least sev- (Nakamichi et al. 2009). eral weeks prior to the eruption causing stress changes, with local VT seismicity dominated by normal faulting Ruapehu 2007 (New Zealand) (alternating type 1 and 2 before the eruption compared to dominantly reverse phreatic eruptions) faulting afterward. A VLP event was recorded 25 s before Ruapehu is an active stratovolcano which undergoes the start of the eruption (Maeda et al. 2015a). ­SO2 fuxes periodic eruptions through a summit crater lake that of more than 2000 tons/day were recorded immedi- exhibits wide variations in temperature depending upon ately after the eruption, with 450 tons/day measured on the activity of the volcano. Type 2 phreatic and phrea- 9 October and 130–140 tons/day on 20–21 November tomagmatic eruptions commonly occur from extensive Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 7 of 19

vaporization of liquid water caused by high heat fow or a combination of both. A third VLP signal during the through the system (Christenson and Wood 1993; Sher- eruption, which was accompanied by tremor, was signif- burn et al. 1999). Te eruption of 25 September 2007 was cantly shallower at ~ 1.5 km below the crater lake (Jolly diferent; it ejected mainly lithic debris, but up to 5% of et al. 2010). the material emitted consisted of juvenile glass. Strictly speaking this was a phreatomagmatic eruption, but the Turrialba 2010–2017 (Costa Rica) (type 1 phreatic system’s behavior showed many aspects of a type 1 phre- eruptions) atic eruption, in particular its highly sealed nature prior Unrest at Turrialba volcano began in the mid 1990’s to the eruption (Christenson et al. 2010). During the year with distinct increases in seismicity and degassing after leading up to the eruption, the lake temperature declined ~ 150 years of dormancy. A change in fumarole chemistry from 25–30 °C to < 15 °C, a condition indicative of ef- showed a clear indication of reactivation with the appear- cient vent blockage and sealing beneath, preventing input ance of SO­ 2 in low temperature fumaroles in November of magmatic heat to the surface lake system. At the same 2001 (Vaselli et al. 2010). Tereafter, ­SO2 fuxes increased time, ­CO2 fuxes were both highly variable (70–660 tons/ from ~ 1 ton/day in 2002 to ~ 740 tons/day in 2008 (Mar- day) and generally declining, with a comparatively low tini et al. 2010) and peaked in mid-2009 at ~ 3500 tons/ fux of 180 tons/day 1 month before the eruption. ­SO2 day (Conde et al. 2014). Seismicity at Turrialba is best fuxes increased from near zero during January–Octo- described as highly diverse, with volcanotectonic, long ber 2016 to 34–55 tons/day in March–June 2007 before period, tornillo, and very long period events, as well declining to 13 tons/day on 23 August 2007 1 month as gliding and harmonic tremor, all observed (Martini prior to the eruption. Molar ­CO2/SO2 ratios decreased et al. 2010). Seismicity (RSAM) peaked at Turrialba in from values near 60 in late 2006 to a value of ~ 5 in 2009–2010, and the frst phreatic eruption occurred on April–May 2007 before increasing to ~ 20 soon before 5 January 2010, which opened a jetting fumarole vent. the eruption. Te magmatic ­CO2/SO2 ratios of 5 and Tereafter, RSAM and ­SO2 fux both decreased to more measurable ­SO2 emissions are indicative of input of mag- moderate levels, although remaining well above back- matic gases into the system, although Christenson et al. ground levels for ~ 1.5 years (Fig. 2). Another phreatic (2010) suggest that the sulfur could have been remobi- eruption occurred in January 2012, which opened a sec- lized from accumulations within the hydrothermal sys- ond jetting fumarole vent. Te vents simultaneously emit- tem which had previously sealed it. Te decreasing lake ted ash in May 2013. A new period of increased activity temperatures, increasing CO­ 2/SO2 ratios, and low CO­ 2 began on 29 October 2014 with an energetic blast and and ­SO2 fuxes in August 2007 immediately prior to the sustained ash eruptions thereafter, and another strong eruption clearly indicate a well-sealed system (Christen- blast occurred on 9 December 2014. Distinctive bombs son et al. 2010). Girona et al. (2018) have demonstrated of silicifed polymict breccia were erupted during these a signifcant, negative lunar-seismic correlation at this events, and were interpreted as components of the sealed time caused by low permeabilities (~ 10−10 m2), con- conduct (de Moor et al. 2016a). Tis activity was accom- sistent with the sealing hypothesis. After the eruption, panied by increased RSAM and ­SO2 fux, which contin- lake temperatures climbed to 32–38 °C in late 2007 and ued during the following months of quiescence in terms early 2008. At times ­CO2 and SO­ 2 fuxes, respectively, of ash emissions (de Moor et al. 2016a). A new period of exceeded 2000 and 500 tons/day, while the CO­ 2/SO2 ratio eruptive activity occurred in March–April 2015, with less was maintained at a level near 5 indicating magmatic energetic eruptions but sustained ash emissions that had conditions. In summary, the 25 September 2007 eruption signifcant societal impact, blanketing the capital city of altered the hydrothermal system beneath the crater lake San José with ash and disrupting fights. Low-energy ash from one that was nearly fully sealed beforehand to one emissions occurred in October 2015, and sporadic semi- afterward that was nearly fully open, allowing essentially continuous periods of ash emissions have been occurring pure magmatic gases to fow to the surface (Christenson through 2016 and 2017. et al. 2010). Te activity at Turrialba can be described as a slow re- Several minutes prior to the eruption and during the awakening process, with vent-opening eruptions becom- eruption itself, a series of VLP, VT, and tremor signals ing generally more frequent with time (Fig. 2). Eruptive were recorded. Two pre-eruptive VLP events appear products were initially characterized as phreatic in char- to have been sourced at 3–7 km depth, accompanied acter, but a small proportion of fresh glassy material was by VT events sourced at ~ 4 km and shallow tremor at later recognized (Alvarado et al. 2016). However, the 1–2 km. According to Jolly et al. (2010), it is unclear composition of this allegedly juvenile material was very whether the VLP signals are the result of fuid pressuri- similar to the composition of scoria from the 1855–1856 zation, depressurization of the system by the VT events, magmatic eruption, thus calling into question whether Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 8 of 19

this material was truly juvenile or remobilized from the However, the initial ash samples highlight the intrinsic previous magmatic system/products (either shallow difculty of determining whether eruptions should be quenched dikes, near-surface vesiculated material, or termed “phreatic” or “phreatomagmatic”. residual shallow magma; de Moor et al. 2016a). Te “juve- Te eruptions at Turrialba are a prime example of nile” component from the 2014–2015 eruptions displays the type of phreatic or phreatomagmatic eruptions that quench features consistent with interaction with liquid accompany volcano activation after a prolonged period water, as well as subtle chemical alteration of clast sur- of dormancy. Te slow ramping up of the activity at Tur- faces (Alvarado et al. 2016). Bombs from the 29 October rialba allows detailed study of the eruptions. High-fre- 2014 eruption included blocks of silicifed hydrothermal quency gas monitoring during this period highlights two breccia, providing evidence that the system was hydro- key aspects of the activity (Fig. 3): (1) the occurrence of thermally sealed, or at least partially, since ­SO2 fuxes magma intrusion events identifed through high ­CO2/ were still signifcant in the months beforehand (de Moor SO2 precursors to the eruptive activity, and (2) expulsion et al. 2016a). Later eruptive products from 2015 to 2017 of the hydrothermal system and opening of magmatic contained true juvenile material (Alvarado et al. 2016; conduits identifed through the disappearance of ­H2S (de Rizzo et al. 2016), and spatter bombs deformed at impact Moor et al. 2016a). Tus, Turrialba-type “phreatic” erup- were observed in the crater (de Moor et al. 2017b). tions are accurately described as vent-opening eruptions

Fig. 2 Turrialba monitoring data for 2008–2017 showing cumulative real-time seismic amplitude measurement (RSAM), SO­ 2 emissions, and erup- tions during this period. Note the episodic RSAM behavior, the steady output of SO­ 2, and the signifcant increase in eruptive activity beginning in 2015. Periods of low ­SO2 fux before the 29 October 2014 eruption suggest partial sealing of the system beforehand. [Reproduced with permission from Conde et al. (2014) and de Moor et al. (2016a, 2017b)] Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 9 of 19

in response to new magma injections that disrupt and shown by the gas data (de Moor et al. 2016a). Monitoring eventually expel the hydrothermal system. of ­CO2/SO2 provided clear deep precursors to eruptions, whereas monitoring of ­H2S/SO2 ratios was particularly Poás 2006–2017 (Costa Rica) (alternating type 1 and 2 insightful in terms of understanding the shallower pro- phreatic eruptions) cesses involved. At Turrialba, it was also difcult during Phreatic eruptions at Poás are distinct in character from the initial eruptions (2014–2015) to distinguish between the vent-opening eruptions at Turrialba. Phreatic erup- juvenile and lithic glass (de Moor et al. 2016a; Alvarado tions occur through a hyper-acid crater lake. Tus, et al. 2016). interaction with external water is obvious in contrast to Both Ruapehu and Poás produce type 2 phreatic erup- eruptions at Turrialba. Phreatic bursts were very com- tions at times of high heat fow from vaporization of liq- mon during 2006–2016, with hundreds recorded seis- uid water. Yet both systems are capable of generating type mically per year. Many smaller “gas bursts” (eruption 1 eruptions. At Ruapehu, there are multiple lines of evi- columns less than 10 m above the lake surface) also dence suggesting that the shallow hydrothermal system occurred but were not detectable seismically. Eruptions was well sealed prior to the 25 September 2007 erup- are characterized by a “cockscomb” ejection of lake water, tion (Christenson et al. 2010), with the eruption breaking lithics, sediment ± minor juvenile material that is erupted the seal and forming a direct conduit to the surface for from the lake to heights ranging from 1 m to more than magmatic gas release. At Poás, a similar progression was 500 m, accompanied by dilute lateral base surges of associated with the April 2017 eruptions (de Moor et al. steam. In April 2017, a more explosive phreatomagmatic 2017c). eruption changed the system dramatically and the lake Sealing and unsealing is common and occurs repeat- was completely vaporized, revealing two high-fux jetting edly at various timescales in phreatic systems. Sealing fumarolic vents that presumably were the sources of the can be a progressive process occurring over several years, eruptions in the period 2012–2016. but it also may occur rapidly, sometimes at timescales Te variations in magmatic gas fux to the lake appear as short as days or weeks. A key challenge is to identify to drive the systematic short-term variations in CO­ 2/SO2 the sealing process and its rate of formation in real time, observed at Poás (de Moor et al. 2016b). Essentially, sul- unambiguously distinguishing this process from a transi- fur chemistry drives these variations. At low temperature tion into repose. and higher pH (low gas fux), sulfur scrubbing reactions A model for magmatically driven phreatic result in molar CO­ 2/SO2 ratios typically greater than ~ 0.7. At high temperature and low pH (high gas fux), the eruptions scrubbing reactions in the lake are less efcient, resulting Magmatic–hydrothermal interactions in lower CO­ 2/SO2 below 0.4; these values are similar to It is clear that many if not most phreatic eruptions are ratios observed in the high-temperature magmatic gases the result of interaction between magma emplaced at at the Poás dome fumaroles. Tus, when gas input to the comparatively shallow crustal levels (0–5 km) and an lake decreases, the ­CO2/SO2 ratio increases and phre- aquifer or hydrothermal system above the magma. Te atic eruptions occur (de Moor et al. 2016b). Long-term hydrothermal system thus acts as a bufer, receiving high- variations at Poás follow similar principles, but CO­ 2/ temperature magmatic input (gas, supercritical fuids, SO2 varies to much larger degrees over several orders of melt) at its base, and releasing fuids and rock at its top magnitude. through a conduit system which reaches the surface (see de Moor et al. 2016a; Giggenbach et al. 1990; Kaneshima Synthesis of observations et al. 1996). In the cases of Nevado del Ruiz and Aso, the geophysi- For these types of systems, there are three common cal data support a model of magmatic fuid injection into elements: magmatic gas input, hydrothermal sealing, a shallow hydrothermal system. At Ruiz, banded tremor and vaporization of liquid water. Hydrothermal sealing was observed for 1 week before the 11 September 1985 provides the means by which a hydrothermal system can eruption. At Aso, VLPD and LPP events were recorded. pressurize. Sealing can occur in all hydrothermal systems Tese signals imply pressurization of the hydrothermal ranging from deep (i.e., several km) to near-surface. Te system. At Ontake, magma intrusion occurred in late sealing process appears to occur over a range of time- 2006 and early 2007 followed by a phreatic eruption, scales from fast (e.g., days) to slow (e.g., months to years), while in 2014 an accumulation of magmatic gas led to and this element of time is clearly signifcant in terms of another phreatic eruption. Turrialba is notable both for when an eruption occurs. If one can identify when and its extended awakening process and its evolution from why the sealing process is occurring, this may be a means “hydrothermal” behavior to “magmatic” behavior, as to identify a system which is undergoing pressurization Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 10 of 19

Fig. 3 High-resolution gas monitoring at Turrialba in 2014 and 2015. Note the increases in CO­ 2/ST prior to ash emission events. Note water-rich gas plumes in phases 1–3 (photos). The disappearance of ­H2S in early March 2015 marks the rapid transition from hydrothermally infuenced to purely magmatic degassing, interpreted to refect expulsion of the hydrothermal system and conduit opening. Reproduced with permission from de Moor et al. (2016a) and used courtesy of the American Geophysical Union

and may erupt. Finally, all hydrothermal systems will magmatic gas into the hydrothermal envelope has two vaporize when magmatic fuids are injected into them. consequences. First, magmatic water condenses; second, For deeper hydrothermal systems, the process of vapori- liquid hydrothermal water is vaporized. Both processes zation will cause overpressure and may be manifested by occur at high temperatures; under such conditions, host geophysical signals such as banded tremor, LP, and VLP rocks undergo alteration while sulfur precipitates as mag- events. For shallow or near-surface systems including matic gas condenses (Christenson et al. 2010). Te end those with crater lakes, the vaporization process will be result is the development of a seal. Such conditions are extensive and the volcano’s response potentially rapid in conducive to phreatic eruptions. A key question is the terms of phreatic activity. sealing rate, i.e., how fast can a seal develop? Both wet and dry volcanoes have the potential to pro- Te ability of the hydrothermal system to trap and duce phreatic eruptions. Te abundance of surface and absorb magmatic heat and gas depends on four main fac- near-surface water on wet volcanoes results in a large tors. First, the size of the hydrothermal system in terms volume expansion as liquid water is vaporized by magma of its areal extent and thickness is important. Small, thin and/or magmatic gas. Tis mechanism drives phreatic aquifers cannot take up signifcant magmatic inputs; the eruptions. For comparatively dry volcanoes hosting a magmatic gases should be able to efciently pass through hydrothermal system, the injection of magma and/or the aquifer to the surface, while also drying the aquifer in Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 11 of 19

the process. By contrast, a well-developed hydrothermal magmatic gases arrive. If the upper seal is strong, over- system as seen at Nevado del Ruiz should have signif- pressure will build in the hydrothermal system. A deep cantly greater bufering capacity. Second, the depth of the hydrothermal system can generate more overpressure hydrothermal system should play a fundamental role in than a shallow system due to the greater lithostatic over- the energy and explosivity of phreatic eruptions, as shal- burden on the system. Te degree of alteration also plays low hydrothermal seals should fail more easily under less a role in determining the strength and extent of the seal. overpressure. Tird, the size of the magmatic system will At a critical level of overpressure, the upper seal will rup- play a signifcant role. A single, small-volume intrusion ture, allowing gases and lithic debris to be transported of magma will have signifcantly less impact than either a through a conduit system and erupted at the surface. larger volume of magma or a series of magma intrusions. If the overpressure does not reach this critical level, an Related to this is the depth at which the intruding magma eruption will not occur, although there may be seismic stops moving upward. A magma that stalls at 5 km depth and gas signals (e.g., long period events, elevated H­ 2S/ should have less dramatic efects than one which reaches SO2, low gas fuxes) indicating that the system is sealed 1 km depth, in which case the efects will be much more and overpressured. Hazards associated with these erup- strongly felt. For example, diferences in depth may be tions are ejection of ballistics to signifcant distances the reason why some eruptions emit juvenile material (> 1 km; e.g., Ontake), small pyroclastic fows and surges, (shallow magma), while others do not (deeper magma). and emission of fne ash that can travel signifcant dis- Fourth, whether the magma is dynamic or static is an tance with impacts on air trafc (e.g., Turrialba). important element. A magma that is ascending, decom- pressing, and releasing gas will generate signifcant Type 2 systems efects that vary in space and time as the rising magma Te hyperacid crater lake at Poás has emitted signifcant approaches the hydrothermal interface. Tis was likely amounts of SO­ 2 and other magmatic gases (de Moor the case at Turrialba in 2009–2015. Conversely, a magma et al. 2016b). Such behavior has been recognized at other which is static will be cooling, crystallizing, and releas- crater lake systems as well (Shinohara et al. 2015; Tam- ing gas in some sort of steady-state fashion. Te overly- burello et al. 2015). De Moor et al. (2016b) showed that ing impacts and efects should be likewise steady-state in gases emitted from Poás crater lake vary systematically, large part. Such a scenario may occur at Aso and Ontake. with a decrease in the ­CO2/SO2 ratio preceding phreatic We now present two conceptual models for type 1 and eruptions. Tey present a model in which transient varia- type 2 phreatic systems. A key common element for both tions in magmatic gas fux (i.e., power supply) to the lake types is that they receive hot magmatic fuids (gases) result in decreasing ­CO2/SO2 prior to eruptions and also from deeper levels. Key diferences are that type 1 sys- drive phreatic eruptions (Fig. 5). By this model, phreatic tems are commonly deeper and seal themselves, allowing eruptions occur during periods when the magmatic input pressurization to develop, while type 2 systems appear to to the lake is actively increasing, and do not occur when be more open and shallow in nature. Type 1 systems typi- the input is decreasing. Te key consideration is whether cally exhibit vulcanian eruptive activity, while type 2 sys- the vapor–liquid boundary in the sublimnic hydrother- tems are associated with surtseyan activity. However, we mal system is rising or falling. If the boundary is rising, note that some systems, notably Ruapehu and Poás, can confned liquid water is vaporized, driving phreatic erup- exhibit both type 1 and type 2 behavior at various times. tions. If the boundary is falling, infltration of hydro- thermal fuids into the sublimnic zone loads the system Type 1 systems for the next eruption. Many volcanoes hosting shal- Figure 4 shows a schematic view of a type 1 phreatic sys- low hydrothermal systems with abundant surface water tem. Te hydrothermal system is underlain by a magma may display these types of phreatic eruptions, including body which periodically releases gases upward into the Rincón de la Vieja (Costa Rica), Kawah Ijen (Indonesia), hydrothermal system. Te magmatic gases are released White Island and Ruapehu (New Zealand), and Copahue by intrusion, crystallization, or a combination of the two (Argentina). Hazards associated with these types of erup- processes. A magma body enclosed by a solidifed cara- tions include ejection of crater lakes and generation of pace provides a partial seal which ruptures and releases lahars (e.g., Kawah Ijen) and wet pyroclastic surges (e.g., gas when magmatic overpressure exceeds the tensile Ruapehu). Ballistics and ash emissions typically have less strength of the carapace. If the carapace is absent or impact compared to type 1 eruptions due to the very wet poorly developed, magmatic gases can escape continu- nature of type 2 eruptions, which tends to restrict the ally. Te hydrothermal system also may be sealed at its aerial extent of the fner eruptive products. base and/or top. Te bottom seal can rupture when Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 12 of 19

Fig. 4 Model for type 1 phreatic systems and eruptions. A shallow magma body releases gas by intrusion and/or crystallization. The gases are trans- ported upward through a series of cracks, intersecting the hydrothermal system above. If the hydrothermal system is sealed at its top, the system will become pressurized from the addition of hot magmatic gases. Such conditions promote phreatic eruptions

Forecasting phreatic eruptions inform hazard assessment. We envision volcanoes prone To our knowledge, no phreatic eruption has been for- to phreatic eruptions being monitored using multiple mally and accurately forecast as such or in terms of parameters (geophysical and geochemical) feeding into its size and timing. However, the technical tools and automated probabilistic calculations forecasting the understanding of phreatic eruptions driven by mag- likelihood of eruption within a given time period. Here, matic gas should allow quantitative assessment that can we examine parameters by which to forecast phreatic Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 13 of 19

eruptive activity. Because these eruptions are commonly involve signifcant magma movement, will be more likely small and sudden events, precursory signals may be sub- to detect subtle changes that do occur prior to explosions tle or absent (Maeda et al. 2015b). In our opinion, the if instruments are deployed closer to the conduit than application of broadband seismicity and gas ratios ofer would normally be the case for volcanoes prone to mag- the most useful and cost-efective means for forecasting. matic eruptions. If these two types of data can be integrated on a real-time or near real-time basis, new insight may be gained in Very long period seismicity terms of our ability to forecast phreatic eruptions. Other Te very long period signals discussed above appear to techniques such as deformation may also be helpful, have common behavior in terms of an infation–defation although many phreatic eruptions appear to occur with mechanism. Te infationary phase is ascribed to pres- no signifcant precursory infation, which would suggest surization, commonly from injection of hot magmatic no pressure buildup and a type 2 eruption mechanism. gas into a cooler hydrothermal reservoir system causing However, the lack of recognized deformation may be a boiling. Te defation results from evacuation of the res- result of the shallow deformation source in the hydro- ervoir as the seal ruptures from excessive overpressure, thermal system or upper conduit. Monitoring of systems allowing fuid discharge upward through a propagating prone to phreatic eruptions, which do not necessarily crack system.

Fig. 5 Model for type 2 phreatic eruptions at Poás. Increasing magmatic gas input into the lake raises the vapor–liquid boundary, resulting in vaporization of confned liquid water, generating volume change, pressurization, and eruption. Compared to type 1 phreatic systems, here the con- duit is more open with a shallower magma system. Type 2 phreatic eruptions are also very common at Rincon de la Vieja and other hyperacid crater lake systems. The eruptive style is more akin to surtseyan eruptions than vulcanian eruptions which are more related to type 1 phreatic activity Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 14 of 19

Fig. 6 Monitoring of gas compositions can provide crucial insight into the infuence of the magmatic system. Generally, as a volcanic system reacti-

vates, the gas emissions will evolve from ­CO2-rich to SO­ 2-rich. This plot shows proposed generalized felds for the characterization of hydrothermal- magmatic gas emissions, based on recent studies of Central American volcanoes (Aiuppa et al. 2014; de Moor et al. 2016a, b, 2017a). The exact boundary lines would vary with tectonic setting and other factors (Aiuppa et al. 2017), but are shown here with key gas ratio values considered appropriate for Central American volcanoes

At Aso VLP signals were of long duration (350–400 s) of clear precursors could be due to progressive sealing of and shallowly sourced (1–1.5 km deep) (Kaneshima the shallow hydrothermal system. et al. 1996). At Ontake in 2007, a VLP event located at While these data indicate that VLP events can originate ~ 2.4 km was observed 2 months before the eruption at diferent depths, their occurrence also appears to be an (Nakamichi et al. 2009). By contrast, in 2014, a VLP event indication of pressurization–depressurization sequences was recorded only 25 s before the eruption and was shal- leading to phreatic eruptions. Teir recognition is thus lowly sourced (< 1 km) (Maeda et al. 2015a). At Ruap- an important tool in assessing the probability of a future ehu a series of two VLP events, VT earthquakes, and eruption, although predicting such eruptive events can- tremor were observed several minutes prior to eruption. not clearly be done using the VLP data alone. Te VLP events were thought to originate at deep levels (3–7 km) (Jolly et al. 2010). Banded tremor Mayon volcano in the Philippines periodically experi- Banded tremor is an unusual seismic signal character- ences phreatic eruptions without clear precursors (Cat- ized by periods of tremor interspersed with periods of ane and Mirabueno 2001; Maeda et al. 2015b). During quiescence. Te duration of both the tremor and the a phreatic eruption on 7 May 2013, a VLP event was quiescent periods are sometimes constant, producing recognized and interpreted as a subhorizontal tensile a striking pattern on a seismogram, as can be seen in crack slightly ofset from the crater at a depth of ~ 240 m Fig. 1 (Martinelli 1990, 1991). Te occurrence of banded (Maeda et al. 2015b). Tese authors suggest that the lack tremor commonly precedes explosive eruptive activity Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 15 of 19

(Martinelli 1990; Gresta et al. 1996; McKee et al. 1981). shallow levels appears to exert a signifcant infuence on Hence, it is an important precursory signal. the overlying hydrothermal or groundwater system. Te banded tremor described at Nevado del Ruiz was clearly signifcant in terms of short-term forecasting. For Gas emissions the 11 September 1985 phreatic eruption, banded tremor Gas emission monitoring has high potential as an erup- was frst recorded on 5 September 1 week beforehand tion forecasting tool for phreatic eruptions. Tis is based (Martinelli 1990). For the devastating 13 November mag- on the fact that hydrothermal and magmatic gases are matic eruption 2 months later, Voight (1990) reported vastly diferent in character; thus, small changes in a that 3 days of continuous tremor commenced on 10 volcano’s hydrothermal system result in signifcant and November before the eruption, although it is unclear readily detectable changes in gas composition (de Moor if this tremor was banded. Very similar banded seismic et al. 2016a). Te delivery of heat from magmatic systems signals were observed at Mt. Etna, Italy, in March–May to overlying hydrothermal systems essentially occurs 1987 associated with two phreatic eruptions on 8 April through the upward migration of high-temperature mag- and 17 April (Gresta et al. 1996). Banded tremor was frst matic fuids. Tus, the fundamental process responsible recorded on 1 April, 1 week before the frst eruption. for driving phreatic eruptions should also be quantif- Individual tremor bands lasted 25–35 min and quiescent able and measurable through the gases emitted. However, spacings between bands 140–160 min, for total cycle the lack of gas emission can be equally important, as durations of 165–195 min. During this 1-week interval, this could signify the formation of a hydrothermal seal, tremor amplitudes increased progressively to the time of resulting in accumulation of gas and pressure in the eruption, then disappeared temporarily before resuming hydrothermal system and ultimately leading to phreatic on 13 April, 4 days before the second eruption. eruptions (e.g., Christenson et al. 2010). Distinguishing In contrast to these short-term eruption indicators, between a transition to quiescence and a transition to banded tremor at Karkar volcano, Papua New Guinea, sealing with ongoing gas input is a key issue requiring was signifcantly longer-lived during unrest in 1978– integrated assessment of multiple monitoring parameters 1979. McKee et al. (1981) suggest that a small body of such as deformation and seismicity. Additionally, sealing magma intruded to shallow levels in 1977–1978, as mani- could conceivably result in pressurization without erup- fested by elevated seismicity, incandescence to 1000 °C, tion, potentially shutting of magmatic input from below. and widespread fumarolic activity. Banded tremor began Te feld of volcanic gas monitoring is experiencing in July 1978, 6 months prior to the initiation of phreatic rapid technological advances. New methods for in situ explosive activity in January 1979. Te tremor strength- and remote measurements of gas fux and gas compo- ened appreciably in late August and was associated with sition have recently been developed (e.g., Aiuppa et al. increased gas emissions from the craters. Tremor ampli- 2005; Shinohara 2005; McGonigle et al. 2008; Kern et al. tudes peaked in late October and declined thereafter. In 2015). Hydrothermal–magmatic systems that typically late January 1979, the color of the gas emissions changed produce phreatic eruptions are rather challenging for from white and blue to dense white vapor. A second monitoring when exclusively using gas fuxes (e.g., SO­ 2 important decline in tremor amplitude occurred in early fuxes; Galle et al. 2010), as the emissions from these February, while LP events began to be recorded in mid- systems tend to be low compared to more magmatic February at rates of 15–40 events/day. Taken together, systems. Tis is primarily due to the fact that systems these changes in early 1979 may signal sealing and/or with high gas and heat fuxes cannot easily establish and pressurization of the shallow hydrothermal system. Te maintain hydrothermal systems because meteoric water most signifcant eruption, which was phreatic in nature is rapidly boiled of (Pasternack and Varekamp 1997). By containing no juvenile material, occurred on 8 March contrast, volcanoes with inherently lower magmatic gas 1979. output can form extensive hydrothermal systems, which Te Nevado del Ruiz and Mt. Etna examples illustrate interact with magmatic volatiles introduced from below. that banded tremor can serve as a short-term precursor Tese reactions remove reactive volatiles from the mag- to phreatic explosive activity. Te Karkar activity dem- matic gas phase (Symonds et al. 2001). Sulfur dioxide is onstrates that occurrence of banded tremor can also the primary gas species used for remote monitoring of extend over an appreciable time period. Nevertheless, the gas fux, and is also a reactive gas, forming sulfuric acid tremor at Karkar was clearly associated with explosive via a process known as “scrubbing”. Two fundamental activity which itself extended over 8 months (January– dissociation reactions dominate this process (e.g., Kusak- August 1979). For this type of phreatic activity observed abe et al. 2000): at these and other volcanoes, the presence of magma at Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 16 of 19

4SO + 4H O → H S + 3H SO 1. 2(g) 2 (l) 2 (g) 2 4(aq) hydrothermal-magmatic gases, and gases with minor H­ 2S ­(H S/SO < 0.5) are considered dominantly magmatic, + = o + − + + 2 2 2. 3SO2(g,aq) 2H2O(l) S(s,l) 2HSO4(aq) 2H(aq) without signifcant hydrothermal infuence. Extremely sulfur-rich gases with ­CO2/SO2 < 0.2 are not easily explained by purely magmatic degassing processes and Reaction 1 produces H­ 2S, another gas useful for moni- probably have additional sulfur contributed to the gas toring, and sulfuric acid. Reaction 2 does not produce a phase by remobilization or combustion of stored hydro- gas species, but instead native sulfur and sulfuric acid. thermal sulfur (e.g., Giggenbach 1987). Tis may be the Deeper hydrothermal systems associated with more case at Poás where ­CO2/SO2 ratios of < 0.1 were observed explosive phreatic eruptions tend to be associated with following phreatomagmatic eruptions in April–May 2017 ­H2S (e.g., Turrialba and Ontake; de Moor et al. 2016a; (de Moor et al. 2017c), and also at Ruapehu prior to the Mori et al. 2016), whereas shallower hydrothermal sys- September 2007 eruption when CO­ 2/SO2 declined from tems fed by air-saturated water (more oxidizing condi- 60 to 5 (Christenson et al. 2010). Within the magmatic tions) generate eruptions which can be ­H2S-poor (e.g., and hydrothermal-magmatic felds, we make a distinc- Poás; de Moor et al. 2016b). Te sulfur chemistry at these tion between deep ­CO2-rich gases and shallow ­SO2-rich latter systems is probably dominated by reaction 2, but gases. ­CO2 is less soluble in melt than sulfur and thus both reactions can be active at the same time, or may exsolves at higher pressure. As a magma rises through vary in relative signifcance with time, space, and volcanic the crust, the frst gases to reach the surface are therefore activity. rich in CO­ 2 (e.g., Giggenbach 1996). As magma contin- Carbon dioxide is another very important gas species ues to rise and then pond at shallow levels, the gases will in these systems because it is abundant and readily meas- become richer in SO­ 2 as magma reaches lower pressure urable, and behaves very diferently to sulfur gases. Under conditions allowing S to exsolve from the melt. acidic conditions found in high enthalpy hydrothermal It is important to recognize that the classifcation in systems, ­CO2 is essentially inert and is not removed from Fig. 6 is proposed as a generalization useful for near real- the gas phase by interactions with hydrothermal liquids time monitoring. In dynamic hydrothermal-magmatic (e.g., Christenson et al. 2010). Carbon dioxide is not read- systems, distinguishing between gases derived from ily measurable using remote techniques (e.g., Schwand- deep and shallow magmatic degassing that have also ner et al. 2017), requiring in situ measurements. Difuse been afected by hydrothermal processes such as scrub- CO­ 2 degassing using accumulation chamber methods bing (producing ­H2S at the expense of ­SO2), oxidation can be useful to monitor (e.g., Chiodini et al. 2005; Notsu or remobilization of sulfur species, and mixing between et al. 2006; Epiard et al. 2017), although time-consuming gas sources, can be extremely difcult without additional surveys are required in often dangerous conditions. information (e.g., isotopic or trace gas data; Giggenbach Multiple Gas Analyzer Systems (Multi-GAS) provide 1987; de Moor et al. 2016a) that is not readily available a highly efective, semi-continuous, real-time and in situ during volcanic crises. method for measuring the key gas species ­(CO2, ­SO2, Concluding remarks ­H2S; Shinohara 2005; Aiuppa et al. 2005). Gas mixing ratios ­(CO2/SO2 and H­ 2S/SO2) are calculated using linear For type 1 phreatic systems, we propose that the likeli- regression through concentration data points measured hood of a phreatic eruption increases when the upper at a rate of 1–10 Hz. Tese calculations can be done in parts of the shallow hydrothermal system become sealed, real-time. Typically, permanent Multi-GAS stations are accompanied by continued magmatic input from below programmed to analyze 4 times every 24 h, for a period in the form of high-temperature gas, supercritical fuids, of 30 min during each analytical session. and/or melt. Magma intruded at shallow levels releases Figure 6 shows an interpretive triangular ­CO2–SO2– large amounts of gas due to decompression (frst boiling). H2S diagram of hydrothermal–magmatic gases, with Once emplaced in this comparatively shallow and cold felds useful for assessing the state of activity in volcanic environment, the magma solidifes by crystallization and systems prone to phreatic eruptions. Low temperature further gas release (second boiling). For type 2 phreatic hydrothermal gases typical of volcanoes in a dormant systems, we propose that the combined efects of shallow state lack ­SO2 and fall along the ­CO2–H2S axis. As a magmatic gas input and vaporization of the liquid-domi- volcano reactivates, SO­ 2 becomes a signifcant compo- nated hydrothermal system (typically below a crater lake) nent of gas emissions, characterized by detectable mag- drive phreatic activity. matic input (e.g., Vaselli et al. 2010). Gases with ­H2S/ Both banded tremor and VLP seismic signals appear SO2 in the range of 0.5–10 are considered to have a to be reliable indicators of pressurization of the shal- signifcant magmatic component and are classifed as low hydrothermal system, although the timescales of Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 17 of 19

pressurization may be variable. Banded tremor pressuri- processes, which likely vary between diferent systems zation timescales vary from days (e.g., Nevado del Ruiz from decades to days, and few high-frequency interdis- and Etna; see Martinelli 1990; Gresta et al. 1996) to sig- ciplinary datasets (including both geochemical and geo- nifcantly longer (e.g., Karkar; see McKee et al. 1981). physical monitoring) are available for phreatic eruptions. VLP timescales appear to be short, on the order of min- Chemical and physical changes may occur very slowly, as utes before an eruption occurs, although they can also in the case of Ontake volcano, or may occur very quickly occur without eruption, e.g., Ontake 2007 (Nakamichi on the scale of days at more open system volcanoes such et al. 2009), which could represent a failed eruption. Te as Poás. Chemical thresholds related to temperature, additional presence of long period seismic signals is also pressure, pH, and redox conditions in magmatic–hydro- clearly signifcant, indicating increased or continuing thermal fuids clearly play fundamental roles in seal for- pressurization and most importantly an increased prob- mation (e.g., Christenson et al. 2010; Rodriguez and Van ability of eruption, as was seen at Karkar in February– Bergen 2017). Physical changes in hydrological systems March 1979 and at Ontake in September 2014. and volcano plumbing systems may cause rapid pre- We suggest that gas ratios may be able to play a key cipitation of hydrothermal minerals, leading to sudden role in forecasting magmatically driven phreatic erup- sealing. By contrast, slow chemical evolution of hydro- tions. At Poás, the association of decreasing ­CO2/SO2 thermal systems over time may gradually lead systems and phreatic eruptive activity in April–May 2017 was a toward a state of sealing. At White Island and Ruapehu clear indication that magma was intruding to very shal- in New Zealand, condensation of magmatic vapor and low levels, in the process evaporating the hydrothermal precipitation of elemental sulfur and associated miner- system. At Rincón de la Vieja, another volcano in Costa als in pore spaces of host rocks appear to exert a fun- Rica with a crater lake, phreatic eruptions in 2014 were damental control on seal formation (Christenson et al. associated with lowered CO­ 2/SO2 and ­H2S/SO2 (de Moor 2010, 2017). Rapid sealing is favored by (1) an interface et al. 2016b). Recent high-frequency Multi-GAS time between hot magmatic vapor and cold water causing sul- series data show that individual phreatic eruptions are fur precipitation and (2) pore space which is sufcient to associated with pulses of H­ 2S-poor magmatic gas (Batt- accommodate and store the sulfur which is precipitated aglia et al. 2017). (Christenson et al. 2017). In the opposite case, increases in CO­ 2/SO2 and H­ 2S/ Tree key elements provide a basis for understanding SO2 and low gas fuxes for type 1 systems could indi- magmatically driven phreatic eruptions: (1) recognizing cate a process of progressive sealing of the hydrothermal magmatic involvement and magmatic input at an early system, or scrubbing without sealing. However, if these stage of unrest, (2) identifying the timing, location, and changes are accompanied by anomalous low-frequency rate of hydrothermal sealing, and (3) understanding the seismic signals indicating continued magmatic gas input conditions which control vaporization of liquid water. to the hydrothermal system, the system is likely under- Recognition of these three elements could help forecast going pressurization. In some cases, gas ratios could pro- and mitigate phreatic eruptions. As importantly, such vide the only information that a system is sealing, e.g., recognition could be invaluable in the case of an eruption Mayon volcano where no clear precursors were observed sequence which begins with phreatic eruptions and then before two phreatic eruptions (Catane and Mirabueno transitions into larger and more signifcant magmatic 2001; Maeda et al. 2015b). For type 2 systems, increased eruptions. CO­ 2/SO2 and ­H2S/SO2 could indicate an expansion of Authors’ contributions the liquid-dominated hydrothermal system from infltra- The two authors contributed equally to the paper in terms of intellectual input tion of lake water and groundwater. Characterization of and writing. Both authors read and approved the fnal manuscript. the extent and depth of hydrothermal systems at volca- Author details noes prone to phreatic eruptions (e.g., magnetotelluric 1 Department of Earth and Planetary Sciences, McGill University, 3450 Uni- methods) and assessment of conduit conditions repre- versity Street, Montreal, QC H3A 0E8, Canada. 2 Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI), Universidad Nacional, AP 2386‑3000, sent a fruitful approach for assessing the potential of vol- Heredia, Costa Rica. cano-hydrothermal systems to produce type 1 or type 2 phreatic eruptions, which have distinct and diferent Acknowledgements We are grateful to Taryn Lopez and one anonymous reviewer whose insightful implications for hazard assessment. and thoughtful comments and suggestions allowed us to make substantial Te process of hydrothermal sealing plays a direct role improvements in the science and presentation of this paper. Javier Pacheco in determining the explosivity of phreatic eruptions. and Geofroy Avard are thanked for Turrialba data shown in Fig. 2. Understanding and recognizing this sealing process is Competing interests a key direction for research and monitoring eforts. In The authors declare that they have no competing interests. particular, little is known about the timescales of these Stix and de Moor Earth, Planets and Space (2018) 70:83 Page 18 of 19

Availability of data and materials de Moor JM, Aiuppa A, Pacheco J, Avard G, Kern C, Liuzzo M, Martínez M, Not applicable. Giudice G, Fischer TP (2016b) Short-period volcanic gas precursors to phreatic eruptions: insights from Poás volcano, Costa Rica. Earth Planet Funding Sci Lett 442:218–227. https​://doi.org/10.1016/j.epsl.2016.02.056 This work was supported by Discovery and Accelerator grants to JS from the de Moor JM, Kern C, Avard G, Muller C, Aiuppa A, Saballos A, Ibarra M, Protti Natural Sciences and Engineering Research Council of Canada, and funding to M, La Femina P, Fischer TP (2017a) A new sulfur and carbon degas- MdM from Costa Rican Ley Transitorio 8933 and the Deep Carbon Observatory sing inventory for the Southern Central American Volcanic Arc: the Biology Meets Subfunction project. importance of accurate time series datasets and implications for global volatile budgets. Geochem Geophys Geosyst 18:4437–4468. https​://doi. org/10.1002/2017G​C0071​41 Publisher’s Note de Moor JM, Muller C, Pacheco J, Mora M, Avard G, Aiuppa A, Alvarado A, Kern Springer Nature remains neutral with regard to jurisdictional claims in pub- C, Kelly P, Pesicek J, Wright H, Pallister J, Marso J (2017b) The slow awaken- lished maps and institutional afliations. ing of Turrialba volcano: An increasing challenge for eruption forecasts. IAVCEI General Assembly, Portland, Oregon, USA, 14–18 August 2017 Received: 21 December 2017 Accepted: 1 May 2018 de Moor JM, Aiuppa A, Alan A, Avard G, Cascante M, Corrales E, D´Arcy F, Diaz JA, Fischer TP, Rüdiger J, Stix J (2017c) A sulfur trigger for the 2017 phreatomagmatic eruption of Poás volcano, Costa Rica? Insights from MultiGAS and drone-based gas monitoring. 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