Similarities and differences in the historical records of lava dome­building volcanoes: implications for understanding magmatic processes and eruption forecasting Article 

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Sheldrake, T. E., Sparks, R. S. J., Cashman, K. V., Wadge, G. and Aspinall, W. P. (2016) Similarities and differences in the historical records of lava dome­building volcanoes: implications for understanding magmatic processes and eruption forecasting. Earth Science Reviews, 160. pp. 240­263. ISSN 0012­8252 doi: https://doi.org/10.1016/j.earscirev.2016.07.013 Available at http://centaur.reading.ac.uk/66290/ 

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Title:Similaritiesanddifferencesinthehistoricalrecordsoflavadome-1buildingvolcanoes:implicationsforunderstandingmagmaticprocesses2anderuptionforecasting.34Authors:5Sheldrake,T.E.a,*16Sparks,R.S.J.a7Cashman,K.V.a8Wadge,G.b9Aspinall,W.P.a1011aSchoolofEarthSciences,UniversityofBristol,WillsMemorialBuilding,Queen’sRoad,12Bristol,BS81RJ,UK13bDepartmentofMeteorology,UniversityofReading,Reading,RG66AL,UK1415*Thomas.sheldrake@unige.ch(correspondingauthor)16171Currentaddress:SectionofEarthandEnvironmentalSciences,UniversityofGeneva,rue18desMaraîchers13,GenevaCH-1205,Switzerland192021Abstract:22Akeyquestionforvolcanichazardassessmentistheextenttowhichinformation23

canbeexchangedbetweenvolcanoes.Thisquestionisparticularlypertinentto24

hazardforecastingfordome-buildingvolcanoes,whereeffusiveactivitymaypersist25

foryearstodecades,andmaybepunctuatedbyperiodsofrepose,andsudden26

explosiveactivity.Herewereviewhistoricaleruptiveactivityoffifteenlavadome-27

buildingvolcanoesoverthepasttwocenturies,withthegoalofcreatingahierarchy28

ofexchangeable(i.e.,similar)behaviours.Eruptivebehaviourisclassifiedusing29

empiricalobservationsthatincludepatternsofSO2flux,eruptionstyle,andmagma30

composition.Weidentifytwoeruptiveregimes:(i)anepisodicregimewhere31

eruptionsaremuchshorterthaninterveningperiodsofrepose,anddegassingis32

temporallycorrelatedwithlavaeffusion;and(ii)apersistentregimewhere33

eruptionsarecomparableinlengthtoperiodsofreposeandgasemissionsdonot34

correlatewitheruptionrates.Acorollarytothesetwoeruptiveregimesisthatthere35

arealsotwodifferenttypesofrepose:(i)inter-eruptivereposeseparatesepisodic36

eruptions,andischaracterisedbynegligiblegasemissionsand(ii)intra-eruptive37

reposeisobservedinpersistentlyactivevolcanoes,andischaracterisedby38

continuousgasemissions.Wesuggestthatthesedifferentpatternsofcanbeusedto39

inferverticalconnectivitywithinmush-dominatedmagmaticsystems.Wealsonote40

thatourrecognitionoftwodifferenttypesofreposeraisesquestionsabout41

traditionaldefinitionsofhistoricalvolcanismasapointprocess.Thisisimportant,42

becausetheontologyoferuptiveactivity(thatis,thedefinitionofvolcanicactivityin43

time)influencesbothanalysisofvolcanicdataand,byextension,interpretationsof44

magmaticprocesses.Ouranalysissuggeststhatoneidentifyingexchangeabletraits45

orbehavioursprovidesastartingpointfordevelopingrobustontologiesofvolcanic46

activity.Moreover,bylinkingeruptiveregimestoconceptualmodelsofmagmatic47

processes,weillustrateapathtowarddevelopingaconceptualframeworknotonly48

forcomparingdatabetweendifferentvolcanoesbutalsoforimprovingforecastsof49

eruptiveactivity.50

51

Keywords:Lava-domevolcanoes;Exchangeablebehaviours;Persistent;Episodic;52Magmaticprocesses;Forecasting.5354 55

1.Introduction56

57

Volcanicactivitycanbemanifestedinmanydifferentways.Fromavolcanicrisk58

perspectiveoneimportantvarietyoferuptiveactivityisextrusionoflavadomesat59

intermediateandsilicicvolcanoes.Recurrenthazardsassociatedwithdome-60

buildingactivityinclude:pyroclasticflowsandvolcanicblastsassociatedwiththe61

collapseoflavadomesandedificeinstability;fountain-fedpyroclasticflows62

associatedwithVulcaniantosub-Plinianexplosions;andcopioustephrafall.63

Worldwide,suchvolcanicactivityhasbeenresponsibleforovertwothirdsof64

volcanicfatalitiessince1600C.E.(Aukeretal.,2013).65

66

WithintheSmithsonianGlobalVolcanismProgram(GVP)databasethereare20567

recordeddome-buildingvolcanoesthathavebeenactiveintheHolocene(Siebertet68

al.,2010).Ofthese,117haveeruptedinthelastmillenniumand89haveerupted69

since1900C.E.(Ogburnetal.,2015).Historicaleruptionshavelastedmanymonths,70

yearsorevendecades(NewhallandMelson,1983;Sparks,1997;Ogburnetal.,71

2015).Overhistoricaltimescalesvolcanicactivitycanberegardedascontinuous,72

albeitfluctuating,butmayalsoincludecomplexepisodicandsometimescyclic73

fluctuationsinintensity,duration,frequencyanderuptivestyle.74

75

Lavadomeformationrequiresparticularconditions,whichsuggeststhatmagmatic76

processesatdome-buildingvolcanoeshavesharedcharacteristics.Specifically,the77

lavasofdome-buildingvolcanoeshavelowaverageeruptionrates(~10-1to10-278

km3yr-1)andhighviscosities(106to1011Pas;Yokayama,2005)thatarecommonly79

associatedwithhighgroundmasscrystallinity(Cashman,1992)and,consequently,80

substantialyieldstrength(Calderetal.,2015).Nevertheless,dome-building81

volcanoescanexhibitmarkedlydifferenteruptivehistories,includingboththe82

durationofindividualeruptiveepisodesandthepotentialforexplosiveactivity.This83

variabilityreflectsthegeneralconceptualtensionsinvolcanologywhere:(1)there84

isabeliefthatindividualvolcanoesareunique,asexhibitedbythecomplexnature85

oftheireruptiverecords,and(2)theconceptthateruptiveactivityisdrivenby86

commonmagmaticprocessesthatproducecertaineruptivestylesandvolcano87

morphologies(Cashman&Biggs,2014).88

89

Inthisreviewweidentifycharacteristicsoffifteenlava-domebuildingvolcanoes90

thataresimilar(exchangeable)orunique(notexchangeable),aswellasthosethat91

arecommononlytoasub-groupofvolcanicrecords.Involcanology,forexample,92

theconceptofexchangeablecharacteristicscanbeusedtodefinethecommontraits93

forallvolcanoes,andtoinfertheconceptualsystemthatthisdefinitionrepresents.94

Usingthisidea,thebasicexchangeablecharacteristicsofavolcanicsystem-implied95

bythedefinitionofavolcanobyBorgiaetal.(2010)-aresimplymagma,eruption,96

andedifice.Weallytothistheideathatthevolcanicsystem(andthusthe97

conceptualconstructofvolcanism)shouldbehierarchicallyorganized,suchthat98

identifyingandcharacterizingdifferenthierarchiesallowsindividualvolcanoesto99

bedistinguishedinspaceandtime(Szakács,2010).Forthisreason,wedevelopa100

hierarchyofdifferenteruptivebehavioursusingobservationsfromthehistorical101

recordsoffifteenwell-characteriseddome-buildingvolcanoes.Bycharacterising102

exchangeablebehaviourswecanassessinaccessibleelements(e.g.,themagmatic103

system)fromobservedelements(e.g.,surfacephenomena).Asimilarapproachis104

employedinmedicalsciences,whereindividuals(i.e.humans)areunique,but105

differentgroupsofhumansareknowntohavesimilarhealthtraits(Spiegelhalter,106

1986;Bestetal.,2013).107

108

Usingahierarchicalconstructforeruptivebehavioursatdome-buildingvolcanoes109

weconsidertheconceptualsystemthatcanexplainthedifferentsetsofshared110

traitsandcharacteristics.Specifically,weaskwhetherthediversityinbehaviours111

canbeexplainedbysubsystemsofthemagmaticsystem(e.g.,shallowcrustal112

reservoirs)orwhetheritrequiresamoreholisticviewofcrustalmagmatic113

processes(i.e.,atranscrustalreservoirsystemthatextendsfromthesurface114

throughthecrustandintothemantle).Thisapproachallowsustoevaluate115

emergingparadigmsforeruptiveactivitybasedonthedestabilisationand116

reorganisationofigneousmushsystems(e.g.,CashmanandGiordano,2014;117

Christopheretal.,2015),andtointerprettheroleofconnectivitywithinamagmatic118

systemonthepatternandstyleoferuptiveactivityatdome-buildingvolcanoes.119

120

Anadditionalapplicationofourstudyrelatestotheimplicationsofahierarchical121

constructontheanalysisofvolcanicdatasets.Animportantissuerelatestothe122

conceptofvolcanicactivityasapoint-processofdiscreteeventsasthisinfluences123

howmagmaticprocessesareinterpretedandhowprobabilisticforecastsaremade.124

Wealsoexaminetheimplicationsofdifferentpatternsoferuptivebehaviouron125

forecastingtheactivityofonevolcanousingobservationsfromother(perhaps126

bettercharacterized)volcanoesofthesametype.Wediscusstheissueswhen127

selectingevidencetomakeeruptiveforecastsandcontextualizethisinregardsof128

forecastingtheonsetoferuptiveactivity.129

130

2.Data131

132

Thefifteendome-buildingvolcanoesselectedforthisreviewarelistedinFigure1.133

Ourselectionisgovernedbythequalityofavailabledataandrelevantobservations,134

andguidedbytheprinciplethatourdatasetshouldcontainvolcanoesthatarewell-135

characterised,havelongrecordsofactivityandhavebeenrecentlyactive.Allfifteen136

volcanoessitinarcenvironments,anderuptmagmasthatarehydrousand137

intermediateincomposition.Asvolcanicgasemissionsareanimportantaspectof138

dome-buildingvolcanism,wealsoincludeonevolcanocharacterisedbypersistent139

gasemissionsbutnorecenteruptiveactivity.140

141

Toenablecomparisonofsimilardome-buildingbehaviour,werestrictedthe142

selectiontovolcanoesofintermediatecomposition,thusomittingdomesformedby143

theeruptionofcrystal-poorrhyolites(e.g.,Chaiten2008:Pallisteretal.,2013).To144

ensurethattheeruptiverecordsarecompleteandnotaffectedbyrecordingbiases145

(ColesandSparks,2006;Deligneetal.,2010),wereviewpatternsoferuptive146

activityonlybackto1800C.E.(Fig.2),aspriortothisdateeachoftheindividual147

eruptiverecordsisassumedtobeincomplete.However,recentadvancesinthe148

abilitytomonitorandobserveeruptiveactivity(CashmanandSparks,2013)mean149

thatmuchofthedataderivefromeruptiveactivityinthelate20thandearly21st150

centuries.Datasourcesincludeeruptiondatabases(Siebertetal.,2010;Ogburn,151

2013),peer-reviewedpublications(e.g.,journalarticles,professionalpublications),152

andobservatorydataanddatabasesofvolcanicunrest(e.g.,WOVOdat;153

http://www.wovodat.org/).Detailedprofilesforthevolcanoescanbefoundinthe154

supplementarymaterial.155

156

Dataarecollatedfortwopurposes:(i)asempiricalevidenceoflong-term157

behavioursatdome-buildingvolcanoes,and(ii)asasemi-quantitativemeasureof158

theirbehaviour.Empiricalevidenceincludesobservationsofphenomenological159

behaviour,magmaticdegassing,andthebulkrockcharacteristicsoferupted160

products.Incontrasttofocussedstudiesattheindividualvolcanoes,wedonotuse161

theobservationsasdirectevidenceofspecificmagmaticprocessesorcharacteristics162

oftherespectivemagmaticsystems.Instead,weusethemonlytosubdivide163

individualvolcanoesintogroupsthatreflecttheirlong-termeruptivebehaviour.We164

thenexaminegeophysical(seismicityanddeformation)andpetrological165

observationswithingroupstocomparethebehaviourofthemagmaticsystems166

withinandbetweenvolcanogroups.167

168

2.1.Phenomenologicalbehaviour169

Dome-buildingvolcanoesexhibitarangeofeffusiveandexplosivebehaviours170

(NewhallandMelson,1983;Sparks,1997;Ogburnetal.,2015).Bydefinition,171

however,themaineruptiveactivityinvolvesprotractedlavadomeextrusion,with172

extrusivephasesthatmaylastfrommonthstomanyyears;ourreferencevolcanoes173

havealsoexperiencedperiodsofquiescenceofmonthstodecades.Duringtimesof174

activity,lavadischargeratescanbeestimatedfromground-basedandsatellite-175

basedtechniques(e.g.,Sparks,1997;vanManenetal.,2010)andusedto176

characterisetheintensityofdomegrowthphases.Theeffusionrate,togetherwith177

themagmaviscosity,determineswhetherlavamovesawayfromtheventasalava178

floworbuildseitheranever-largerdomeandtalusapronoranear-solidlavaspine179

(Wattsetal.,2002;Cashmanetal.,2008).Wherelavaaccumulatesoverthevent,the180

increaseinmagma-staticheadcreatesabackpressurethatcanresistextrusionand181

influencethelonger-termdynamicsofthemagmaticsystem(Stasiuketal.1993;182

Scandoneetal.,2007).183

184

Phasesofextrusiveactivitycanbeinterspersedwithmoreexplosiveactivity,185

includingStrombolian,Vulcanianandsub-Plinianeruptionstyles.Theintensityand186

explosivityoferuptiveactivitycanbecharacterisedusingphenomenological187

observationssuchasashcolumnheight,pyroclasticrun-out,tephrafalldeposit188

volumes,someofwhichcanserveasproxiesformagnitude,intensityandexplosion189

style(NewhallandSelf,1982).Infrequently,dome-buildingvolcanoesalsohave190

large-magnitudeexplosions,includingPlinianeruptionsandlateralblasts(Ogburn191

etal.,2015).192

193

2.2.Magmaticdegassing194

Asmagmaascendsthroughthecrust,volatilesexsolveandrisetothesurface195

(Wallace,2003;2005;Oppenheimeretal.,2011).Themostabundantvolatile196

speciesareH20andCO2.SO2,however,isthemostcommonlymonitoredvolatile197

becauseitisatracegasintheatmosphereandthusitsconcentrationcanbereadily198

measuredusingremotesensingtechniques(Roseetal.,2000;Edmondsetal.,2003;199

Galleetal.,2003;Carnetal.,2013).SO2fluxesarequantifiedusingultraviolet200

absorptionspectraandmeasuredintonnesperday(t/d);somedataforthelastfew201

decadesaresporadicallyavailableformostofthestudyvolcanoes.202

203

Priortothedevelopmentofultravioletspectroscopictechniques,gasfluxeswere204

estimatedbysamplingfumarolicgases(Giggenbach,1996).Introductionofthe205

correlationspectrometer(COSPEC)inthe1970sallowedSO2fluxmeasurements,206

althoughearlymeasurementswerepronetolargeerrors(Oppenheimeretal.,207

2011).Morerecently,fluxeshavebeenestimatedfromdifferentialoptical208

absorptionspectroscopy(DOAS;PlattandStutz,2008).Amajoradvantageofthis209

methodisthatspatiallydistributedmulti-beaminstrumentscanprovideprecise210

estimatesforplumevelocity,whichsignificantlyreducesmeasurementerrorsof211

flux(Oppenheimeretal.,2011,andreferenceswithin).Importantly,however,212

measurementsarerestrictedtosunlighthoursonlyandthequalityofgasdatafrom213

allremotesensingtechniquesdependsonmeteorologicalconditions(e.g.,low214

humidityandnoclouds).215

216

Terrestrial-basedspectroscopicmeasurementsarenotfeasibleformeasuring217

volatileemissionsinmajorexplosiveeventsbecauseabundantashmasksthe218

signals.Consequently,themassofgasreleasedduringlargeeruptioneventsis219

measuredusingsatellite-basedtechniquesandthenconvertedtofluxes(Carnand220

Prata,2010;Carnetal.,2013).221

222

2.3.Bulkrockobservations223

Inourdataset,theproductsofdomebuildingvolcanoesrangeincompositionfrom224

basalticandesite(∼52-57wt.%SiO2)todacite(∼64-69wt.%SiO2;Table1).Whilst225

itisimpossibletoobservethelong-termdynamicsofmagmaticsystemsdirectly,226

macroscopicobservationsandbulkrockanalysiscanbeusedtointerpretthe227

compositional,andpotentiallythephysical,structureofmagmaticsystems(e.g.,228

Barclayetal.,2010;Larsenetal.,2010;Coombsetal.,2013;Scottetal.,2013;229

Turneretal.,2013).230

231

Magmarheologyisamajordeterminantofphysicalbehaviour,particularlyat232

shallowdepthswhereflowatthesurfacemaybeinhibitedbyhighyieldstrength.233

Magmaisamultiphasesystemandconsequentlyitsrheologyiscomplex(Maderet234

al.,2013).Rheologyisstronglycontrolledbythecrystallinityofthemagmas,which235

istypicallyhighinintermediatearcmagmas.Crystallizationisfurtherincreasedby236

syn-ascentdecompressionanddegassingandisthusmodulatedbyeruptionrate,237

thepressureofshallowstoragepriortoeruption,thebulkcompositionofthe238

magmaandkineticfactorsassociatedwithbubbledynamics(JaupartandVergniolle,239

1989;GeschwindandRutherford,1995;NakadaandMotomura,1999;Hammeret240

al.,2000;CashmanandMcConnell,2005;Divouxetal.,2009;Wrightetal.,2012).241

Exsolvedgascanalsoleadtomarkedrheologicalvariationsasfunctionsofbubble242

sizedistributionandbubblecontent(Mangaetal.,1998;Maderetal.,2013).The243

interplaybetweenmagmaascent,decompression,gasexsolution,crystallizationand244

rheologycanleadtocomplexepisodicbehaviours(e.g.,JaupartandAllegre,1991;245

MelnikandSparks,1999;Michautetal.2013).246

247

2.4.Geophysicalobservations248

Foreachvolcanowereportcommongeophysicalobservations;forconsistency,we249

omitspecialisedobservations(e.g.,strainmeters,broadbandseismicity)madeat250

onlyoneortwovolcanoes.Geophysicalmonitoringobservationsaresusceptibleto251

spatialandtemporalbiasesassociatedwithnetworkcapacitiesandtechnological252

constraintsatvolcanoobservatories(Sparksetal.,2012).Therefore,itisimportant253

tounderstandthesebiasesandthustherobustnessandvalidityofcomparing254

records.Spatialbiasesarisefromvariationsinmonitoringcapacitiesduetoboth255

resourceavailabilityandaccessibility.Temporalbiasesareassociatedwith256

advancesintechnologythatimproveobservationthresholdsandtheprecisionof257

measurements.Thesearediscussedinmoredetailwithreferencetotheparticular258

observables.259

260

2.4.1.Seismicity261

Volcanicseismicitycanbecategorisedeitherbyitsphysicalcause,ifoccurringatthe262

surface(e.g.,rockfalls,lahars,pyroclasticflows,etc.),oritswaveformandfrequency263

contentiforiginatingfromwithinthecrust(e.g.,highorlow-frequencysignals;264

Chouet,1996;Neuberg,2000;McNutt,2005;ChouetandMatoza,2013).High-265

frequency(volcano-tectonic)eventshaverecognisablePandSwavefirstarrivals266

andareattributedtobrittlefracturingrelatedtoopeningofnewpathwaysforeither267

magmaormagmaticfluids(Kilburn,2003).Low-frequency(long-periodandhybrid)268

eventsareassociatedwithmovementofmagmaandmagmaticfluids(McNutt,269

2005).Seismicityismostcommonlyassociatedwitheruptiveactivitybutisalso270

observedduringperiodsofquiescence,thatis,whenavolcanoisinanon-eruptive271

state,andcanbediagnosticofincipientunrest(Phillipsonetal.,2013)orpost-272

eruptivetectonicstressrecovery(e.g.,BarkerandMalone,1991).273

274

Althoughseismicitycanbecharacterisedusingarangeofmetrics,wefocusonthe275

numberofevents(dailycounts)asthisisthemostcommonlyrecordedobservation276

acrossthevolcanoesinthedataset.Wedonotcompareabsolutenumbersofseismic277

eventsorcumulativeseismicmomentbetweenvolcanoesduetorecordingbiases278

associatedwithvariationsinnetworkcapacitiesandsensitivities(e.g.,numberof279

andtypeofinstruments).Insteadwecomparepatternsoftotalseismicityandthe280

relativefrequencyofdifferenttypesofevents,primarilylong-periodandvolcano-281

tectonicearthquakes.282

283

2.4.2.Deformation284

Theepisodicandsometimesrepetitivenatureoferuptiveactivityatmanydome-285

buildingvolcanoescommonlymanifestsastime-varyingdeformationofthecrust286

thatcanbemonitoredatthesurfaceusinggeodetictechniques.Greatvariabilityin287

instrumentationandnetworkdesigninthenear-fieldmonitoringofground288

deformation,however,makesdirectcomparisonsdifficult.Forthisreason,wefocus289

onlyonfar-fielddeformation(>5kmfromthevent).Thesedataalsoprovideuseful290

constraintsondeepermagmaticprocesses.Far-fielddeformationcanbemeasured291

bygeodeticnetworks(usingGPS),althoughthesemeasurementsrequireground-292

basedsupportandarerestrictedtoonlyafewofthevolcanoesinourdataset.On293

theotherhand,InterferometricSyntheticApertureRadar(InSAR)techniquesusing294

satellite-basedinstrumentsprovideaglobalapproachforobservingfar-field295

deformation(Biggsetal.,2014).Bycombiningobservationsfromthesetwo296

methodswecomparepatternsofdeformation(i.e.whetherthevolcanoisinflating,297

deflatingorneither)betweendifferentvolcanoesandrelatedeformationbehaviour298

toeruptiveandnon-eruptivephasesofactivity.299

300

2.5.Petrology301

Petrologicdataprovideinformationonthehomogeneityofthemagmaticsystem,302

temporalchangesofmagmacompositionandtheextenttowhicheruptiveactivityis303

influencedbytheascentofdiscretemagmabatches.Ofparticularinterestis304

evidencefortheinteractionofdifferentmagmas,whichcanoccuratarangeof305

scales.Macroscopicevidenceformagmaminglingincludesenclavesor306

compositionalbandingineruptedproducts.Microscopicdetailsofgeochemical307

interactionsprovideinformationonthenatureandtimingofminglingevents.308

Analysisofindividualcrystalsandtheirmeltinclusionsprovidesinformationon309

bothintrinsicandextrinsicproperties(e.g.,temperature,pressureandvolatile310

inventories)ofmagmastorageregions(e.g.,Nakamura,1995;Zellmeretal.,2003a;311

Dirksenetal.,2006;Humphreysetal.,2006;Costaetal.,2013).Finally,petrological312

analysesandU-seriesgeochemistrycanconstrainthetimescalesofmagmatic313

processes(e.g.,VolpeandHammond,1991;Zellmeretal.,2003b;CooperandReid,314

2008;Dossetoetal.,2008;Claiborneetal.,2010)thatcontrolandsustaineruptive315

activityatdome-buildingvolcanoes.Quantificationofgroundmasscharacteristics316

(crystallinity,crystalsizeandshape)canfurtherconstrainratesofmagmaascentto317

thesurface(e.g.,Hammeretal.,2000;Toramaruetal.2008;Wrightetal.,2012).318

319

3.Patternsoferuptiveactivityatdome-buildingvolcanoes320

321

Weidentifyinourdatasettwotypesoflong-termbehaviourdefinedbytherelative322

timeavolcanoremainsinastateoferuptionorrepose(i.e.non-eruption):(1)323

activityisepisodicwhentimescaleoferuptionismuchlessthanthetimescaleof324

repose;and(2)activityispersistent,whenthetimescaleoferuptioniscomparable325

tothatofrepose(Fig.3a).Identificationofepisodicandpersistentregimes326

representsthefirstsub-levelinourhierarchicalconstructofhistoricaldome-327

buildingvolcanism(Fig.4).328

329

Episodicandpersistentbehaviourcanbemanifestedoverdifferenttimescales(Fig.330

4)and,overtime,individualvolcanoescanshowbothtypesofbehaviour(Fig.3c).331

Overtheexaminedtimeperiodofthepast200years,forexample,manydome-332

buildingvolcanoesarecharacterisedbyepisodicbehaviour;however,withinthat333

broaddescription,somehaveremainedinapersistentregimeformultipledecades.334

Wecharacterisethesevolcanoesasbelongingtoamixedregime.Oververylong335

timeperiods,allthevolcanoesinoursamplecanbeviewedasmixed.336

337

PatternsofSO2degassingalsoprovideadditionalinsightintolong-termpatternsof338

volcanicactivity.ThelargestvolumesofSO2emissionsarealwaysassociatedwith339

majorexplosiveevents(e.g.,CarnandPrata,2010;Werneretal.,2013).Two340

patternsoflessenergeticdegassingcanbedefinedas:(1)SO2fluxthatisclosely341

correlatedwitheruptions(Fig.3b)and(2)degassingthatisnotcorrelatedwith342

eruptiveactivity(Fig.3a).Correlateddegassingiscommonatvolcanoesinan343

episodicregime;herebothgasandmagmafluxesdecreasewithtimeafteraninitial344

(oftenexplosive)maximum(Fig.5a).Poorcorrelationbetweendegassingand345

eruptiveactivity,incontrast,istypicalofpersistentactivity(Fig.5b).The346

correlationofdegassingpatternswitheruptivebehavioursuggeststhatmagmatic347

degassingconstitutesanimportantdistinctionbetweenpersistentandepisodic348

regimes(e.g.,Whelleyetal.,2015).349

350

Finally,weusedifferencesindegassingbehaviourtodistinguishtwostatesof351

repose:(1)inter-eruptivereposeseparatesepisodiceruptionsandischaracterised352

bynegligibledegassing(Fig3a;5a);and(2)intra-eruptivereposeoccursinthe353

persistentregimeandischaracterisedbysustaineddegassing(Fig3b;5b).Wealso354

identifyanon-eruptivedegassingregimetodescribedome-buildingvolcanoesthat355

remaininastateoflong-termrepose(~decades)characterisedbylowlevelsof356

persistentdegassing.357

358

3.1.Episodicregime359

Volcanoesinanepisodicregimearecharacterisedbyperiodsoferuptiveactivity360

separatedbymuchlongerperiodsofrepose.Theonsetoferuptiveepisodesis361

explosive,withhighmagmadischargerates.BothmagmadischargeratesandSO2362

fluxesdecreasewithtimeduringeruptiveperiods(e.g.,Fig.6).Duringeruptive363

periods,thelaterstagesofactivityaretypicallycharacterisedbylowextrusionrates364

andassociatedextensivesyn-eruptivecrystallisationthatcombinetoproducelava365

spines(e.g.,Wattsetal.,2002;Cashmanetal.,2008).Wedistinguishtwodifferent366

timescalesforepisodicactivityinhistoricalrecords(Fig4):(1)volcanoeswhere367

eruptiveepisodeslastseveralyears,and(2)volcanoeswhereeruptiveepisodeslast368

afewmonthsatmost.Thesetwosubgroupscanbefurtherdistinguishedbythe369

homogeneityorheterogeneityoferuptedmagmacompositions.370

371

3.1.1.Eruptiveepisodeslastingyears372

Twovolcanoesinthisreviewhaveexperiencedepisodicactivitylastingseveral373

years(Fig.2;UNZ,PEL).Inbothcases,lavacompositionsarebroadlyhomogeneous.374

Thedurationofinter-eruptiveperiodsofreposeismultipledecadesorlonger.375

376

(a) MountUnzen,Japan(UNZ),isacomplexdaciticvolcanothatlasterupted377

near-continuouslyfrom1991-1995(Fig.2).Noprevioushistoricactivityis378

knownalthoughamajorsectorcollapseeventofanolderdomeoccurredin379

1792(Uietal.,2000).Between1991and1995,thecompositionoferuptive380

productswas~65wt.%SiO2(NakadaandMotomura,1999)andtheaverage381

lavaeffusionratewas~1m3s−1,withhigherrates(∼4-6m3s−1)duringthe382

eruptiononset.Extrusionratesgenerallydiminishedwithtime,althougha383

secondarypeakwasobservedin1993(Nakadaetal.,1999;Fig.6).SO2384

fluxesaveraged137t/d,werecorrelatedwithextrusionrateanddiminished385

soonaftereruptiveactivityceased(Hirabayashietal.,1995).386

387

(b) MontPelée,Martinique(Fig.1),isanandesiticvolcanothathaserupted388

infrequently(Fig.2).Thebest-recordederuptiveactivityoccurredinthe389

earlypartofthe20thcentury,between1902-05and1929-32(Lacroix,1904;390

Perret,1937;Tanguy,1994).Duringbothperiods,lavafluxesdecreasedfrom391

>10m3s−1to∼1m3s−1(Tanguy,2004),withlaterstagescharacterisedby392

spineextrusion(Lacroix,1904;Perret,1937).Thecompositionoferuptive393

productsfromMontPeléeisquitehomogeneousat62wt.%(Fichautetal.,394

1989b;Gourgaudetal.,1989;SmithandRoobol,1990).395

396

3.1.2.Eruptiveepisodeslastingmonths397

Twovolcanoesinthisreviewhaveexperiencederuptiveepisodeslastingafew398

months(Fig.2;RED,AUG).Incontrasttothevolcanoesinthepreviousgroup,the399

durationofinter-eruptiveperiodsofreposeisseveralyearstoafewdecades.400

Additionally,lavasofdifferentcompositionareeruptedcontemporaneously.401

402

(c) MountRedoubt,USA(RED),isanandesiticvolcanothathaserupted403

intermittentlyonfourseparateoccasionssince1902(Fig.2).Themost404

recenteruptiveepisodeshavebeenin1989-90and2009,eachlastingfor405

severalmonths(MillerandChouet,1994;BullandBuurman,2013).During406

eacheruptiveepisodeeruptiveproductsrangedfrom57to63wt.%SiO2,407

withthelaterstagesinvolvingthemoresiliciclava(Nyeetal.,1994;Coombs408

etal.,2013).SO2degassingishighlycorrelatedwithperiodsoferuptive409

activity.In1989-1990,extrusionratesvariedfrom2.1to26m3s−1,with410

averagedomegrowthoccurringat~5.8m3s−1(Miller,1994).Similar411

extrusionrateswereobservedin2009(2.2-35m3s−1)althoughtheaverage412

ratewasslightlyhigherat∼9.5m3s−1(Diefenbachetal.,2013).Inbothcases413

theinitialactivitywasthemostexplosiveandtheextrusionratedeclined414

duringeruptiveactivity(Miller,1994;Diefenbachetal.,2013).Initial415

explosiveactivityin2009wasassociatedwiththelargestSO2fluxes(∼3000416

to∼17000t/d).Subsequentactivityinvolvedmorecontinuousextrusion417

withSO2fluxes≤3000t/d(Hobbsetal.,1991;Casadevalletal.,1994;Werner418

etal.,2013).Inboth1990and2009,ittookseveralyearsforSO2fluxesto419

returntoundetectablelevelsaftereruptiveactivityceased(Doukas,1995;420

Werneretal.,2013).421

422

(d) MountAugustine,USA(AUG),isanandesiticvolcanothathashadnineknown423

eruptiveepisodessince1812,withthemostrecentin1976,1986and2006424

(Fig2),eachlastingforseveralmonths(SwansonandKienle,1988;Poweret425

al.,2006;PowerandLalla,2010).Thecompositionoftheeruptedmagmahas426

rangedfrom56to64wt.%SiO2,withmoresilicicmagmapreferentially427

eruptedlaterineacheruptiveepisode(Harris,1994;Romanetal.,2006;428

Larsenetal.,2010).Duringthe2006eruptiveactivity,magmafluxesvaried429

from2to22m3s−1(Coombsetal.,2010).Notably,incontrasttoother430

volcanoesinepisodicregimes,thefinalstagesoferuptiveactivityat431

Augustinein2006werecharacterisedbyelevateddischargeratesandthe432

formationoflavaflows,althoughdischargerateswerestilllowerthanatthe433

onsetoferuptiveactivity(Coombsetal.,2010).Magmaticdegassingis434

correlatedwitheruptiveactivity,withthelargestfluxescommonly435

associatedwithexplosiveactivity(Stithetal.,1978,Roseetal.,1988;McGee436

etal.,2010).In2006,however,thehighestSO2fluxes(∼9000t/d)were437

associatedwithabriefhiatusineruptiveactivity,althoughSO2fluxeswere438

high(∼3000t/d)throughouttheeruptiveepisode(McGeeetal.,2010),andit439

took1-2yearsaftertheendoferuptiveepisodesin1986and2006forSO2440

fluxestoreturntoundetectablelevels(Symondsetal.,1990;Doukas,1995;441

McGeeetal.,2010).442

443

3.2.Persistentregime444

Weidentifyeightvolcanoesinthisreviewthathaveremainedinapersistentregime445

fordecadesorlonger.Volcanoesinapersistentregimeexhibitbroadlyconsistent446

behaviourassociatedwithstablelong-termlavafluxes.Forexample,althoughrates447

oflavaeffusionatBezymianny,Kamchatka,havevariedovertheshortterm,they448

havebeenapproximatelyconstantoverthepastseveraldecades(Fig.5).The449

eruptiveactivityofanindividualvolcanocanalsoshow‘typical’(repeatable)450

patternsofbehaviour,asillustratedbySantiaguito,Guatemala,wheretypical451

behaviourcomprises“smalltomoderateexplosionsofsteamandash,small452

pyroclasticflows...andeffusionofblockylavadomesandflows”(Scottetal.,2012).453

TypicalintermittentbehaviouratMerapi,Indonesia,incontrast,ischaracterisedby454

eruptiveactivitythatis“lowinexplosivitywithVEI-3orless...[that]involvethe455

formationofalavadome”(Ratdomopurboetal.,2013).456

457

Wedistinguishtwodifferentvariantsoflong-termpersistentbehaviour(Fig.4).458

Firstly,therearevolcanoesthathaveremainedinapersistentregimeatleastthe459

19thcentury.Thesevolcanoesproducelavaswithanapproximatelyconstantbulk460

composition.Secondly,therearevolcanoesthathaveenteredapersistentregime461

followingalongperiodofinastateofrepose.Volcanoesinthisgrouptypicallyhave462

bulkcompositionsthatshowadecreaseinSiO2contentwithtime.463

464

3.2.1.Long-termpersistentregimes465

Fourofthedome-buildingvolcanoesinthisstudyhavebeeninapersistentregime466

throughoutthe19th,20thand21stcenturies;thesevolcanoesarecharacterisedby467

frequent,intermittentphasesofdome-growth(Fig.2;MER,COL,LAS,SHI).Thestyle468

oferuptiveactivityisgenerallyconsistentthroughtimeandcharacterisedby469

definable‘typical’behaviour,exceptforrarelarge-magnitudeexplosions(Fig.2).470

Interestingly,theseexplosiveeventscommonlyinvolvemagmathatismoremafic471

thaneruptedduringtheeffusivephases.Activityateachvolcanoisdescribedin472

detailbelow.473

474

(a) Merapi,Indonesia(MER),isabasalticandesitevolcanothathasbeeninan475

eruptivestateeveryfewyearssinceatleastthe18thcentury.Eruptive476

activityischaracterisedbyminorexplosionsassociatedwiththeextrusionof477

viscouslavadomesandcouléesthatcancollapsetoformblock-and-ash478

pyroclasticflows(Voightetal.,2000).Lavaextrusionratesare479

approximatelyconstantoverhistoricalrecordsat∼0.5m3s-1(Siswowidjoyo480

etal.,1995).Persistenteffusiveactivityhasbeenpunctuatedbyatleasttwo481

majorexplosionsthathaveproducedhigh-energypyroclasticdensity482

currents(Suronoetal.,2012).Thebulkrocklavacompositionrangesfrom483

52to56wt.%SiO2(Andreastutietal.,2000;GertisserandKeller,2003)and484

showsnotemporaltrend,althoughexplosiveeventsappeartoinvolvedeeply485

sourced,volatile-richmagmas(Costaetal.,2013),whichmaybemoremafic486

(GertisserandKeller,2003).SO2degassingiscontinuouswithfluxesbetween487

50and250t/d(Humaida,2008),althoughinstantaneousfluxescanbemuch488

larger(∼10,000’st/d)duringmajorexplosiveevents(Suronoetal.,2012).489

Importantly,SO2fluxesanderuptiveactivityappeardecoupled,withSO2flux490

peaksobservedduringinter-eruptiveperiods,andsometimesassociated491

withashventing(Ratdomopurboetal.,2013).492

493

(b) Colima,Mexico(COL),isanandesitevolcanothathasbeenerupting494

intermittentlysincethe18thcentury.Periodsofintra-eruptiverepose495

normallylastontheorderofyears,althoughlongerperiodswithout496

apparenteruptiveactivityhavefollowedmajorexplosiveeventsin1818and497

1913.Theselongerperiodsofreposeprobablyinvolvedendogenousgrowth498

belowthecraterrim(Robinetal.,1991;Gonzálezetal.,2002),soweinfer499

thatColimaremainedinapersistentregimeduringpost-explosionperiods.500

Eruptiveactivityischaracterisedbylavadomeextrusion,Vulcanian501

explosionsandoccasionalblock-and-ashflows(Zobinetal.,2002).Short-502

termlavaeffusionratesvaryfrom<1to>5m3s−1(Varleyetal.,2010),but503

long-termaveragesarepoorlyconstrained.Thelavacompositionranges504

from59to62wt.%SiO2withnocleartemporaltrend(LuhrandCarmichael,505

1980;1990;Savovetal.,2008),exceptthatproductsofmajorexplosive506

eventsaremoremafic(SiO2=55-58wt.%;LuhrandCarmichael,1990;Reubi507

andBlundy,2009;Crummyetal.,2014).SO2degassingiscontinuous,with508

fluxestypicallybetween50and1000t/d(Casadevalletal.,1984;Engberg,509

2009),althoughsometimesashighas5000t/d(Taranetal.,2002;Varley510

andTaran,2003).Magmaticdegassingappearsdecoupledfromeruptive511

activity(Zobinetal.,2008),butthelargestSO2fluxesareassociatedwith512

moreexplosiveevents(Taranetal.,2002).513

514

(c) Lascar,Chile(LAS),isanandesiticvolcanothathasbeenerupting515

intermittentlyatyearlytodecadaltimescalesthroughoutmuchofitshistory.516

Lavadomegrowthhasbeenconfinedwithinalargesummitcrater.Four517

periodsofnear-continuousdomegrowthoccurredbetween1984and1993;518

eachculminatedinlavadomesubsidenceandexplosiveevents,includinga519

PlinianexplosioninApril1993(Matthewsetal.,1997).Long-termlava520

extrusionratesarepoorlyconstrainedbutarelikelytobe<0.1m3s-1521

(Matthewsetal.,1997).Since1993,activityhascomprisedepisodic522

Vulcanianexplosionsthathavedecreasedinbothintensityandfrequency;523

thelastexplosionoccurredin2007.Juvenilepyroclastsfrom1993canbe524

separatedbycompositionintotwogroups:57.6-58.7or60.4-61.4wt.%SiO2525

(Matthewsetal.,1999);similaritiestopreviouslyeruptedlavas(Deruelle,526

1985)suggestthatthemagmacompositionhasremainedconstant527

throughoutitshistory.Lascarhasexhibitedcontinuousfumarolicactivity528

(Casertano,1963;Gardeweg&Medina,1994)withrecentSO2fluxes529

sustainedbetween150and940t/d(Henneyetal.,2012,Menardetal.,530

2014).Duringmoreexplosiveactivity,fluxeshavereached2300t/d(Andres531

etal.,1991;Matheretal.,2004).SO2fluxeshaveshownanirregularpattern532

ofdegassingduringperiodsofintra-eruptivereposeandthereforeappear533

decoupledfrommagmaflux(Menardetal.,2014).534

535

(d) Shiveluch,Russia(SHI)isanandesiticvolcanothathasbeenerupting536

intermittentlysinceamajorexplosiveeventin1854.Evenpriorto1854,537

sparseobservationssuggestthatperiodsofreposelastednomorethanafew538

decades.Recentphasesoferuptiveactivityhavevariedindurationfrom539

monthstoseveralyears,andShiveluchhasbeeninanear-continuous540

eruptivestatesince2000(Belousov,1995;ZharinovandDemyanchuk,541

2008).Between1980and2007theaveragelavadischargeratewas∼0.4542

m3s−1,althoughfluxesfluctuatedconsiderably(ZharinovandDemyanchuk,543

2008).Explosiveactivityhasbeenofvariablemagnitude,withmajorPlinian544

eventsin1854and1964(Belousov,1995).Theeruptiveproductscontain545

56-62wt.%SiO2andshownotemporaltrends(Dirksenetal.,2006;546

Humphreysetal.,2006;GorbachandPortnyagin,2011).Fumarolicactivity547

hasbeensustainedthroughoutbotheruptiveactivityandintra-eruptive548

repose(Belousov,1995;Gorelchiketal.,1997;ZharinovandDemyanchuk,549

2008),butSO2fluxeshavenotbeendocumented.550

551

3.2.2.Long-durationreposeprecedingalong-termpersistentregime552

Twovolcanoesinthisstudyhaveinitiatedpersistentbehaviourafterexplosive553

eruptionsthatfollowedalongperiodinastateofrepose(∼millennia;Fig.2;SAN,554

BEZ).TheonsetofapersistentregimeatthesevolcanoesischaracterisedbyPlinian555

andlateralblastexplosions.Incontrasttothepreviousgroup,themostevolved556

pyroclastsinthisgroupareassociatedwithmajorexplosiveevents;theSiO2content557

ofsubsequentlavasdecreasessystematicallythroughtime.558

559

(e) Santiaguito(SantaMaria),Guatemala(SAN),isadomecomplexthathasbeen560

activesince1922;effusiveactivityfollowedthePlinianeruptionofitsparent561

volcano,SantaMaria,in1902(Rose,1972).Effusiveactivityhasbeennearly562

continuousatlong-termratesof~0.46m3s−1,withmarkedfluctuationsthat563

havebeenclassifiedintoeightdistinctphases(Rose,1973;Harrisetal.,564

2003;Scottetal.,2013).Eachphasehasinitiatedwithhighrates(0.5-2.1565

m3s−1)andhasbeenfollowedbylow,sustainedextrusionratesof<0.2m3s−1566

(Harrisetal.,2003;Ebmeieretal.,2012).Thelavasaredacitictosilicic567

andesiteincomposition,withSiO2contentsthathavedecreased568

systematicallyfrom∼66to∼62wt.%since1922.SO2degassingis569

continuouswithaveragefluxesbetween80and120t/d(Andresetal.,1993;570

Rodríguezetal.,2004).571

572

(f) Bezymianny,Russia(BEZ),isanandesitevolcanothathasbeenerupting573

near-continuouslytointermittentlysincealateralblastandassociatedsector574

collapsein1956(Belousovetal.,2007).Between1956and1977,eruptive575

activitywaslimitedtoperiodsofendogenouslavadomegrowthassociated576

withsustainedfumarolicactivity(Gorshkov,1959;Bogoyavlenskayaetal.,577

1985;Belousov,1996).After1977,domegrowthoccurredexogenouslyand578

includedoccasionalexplosions(vanManenetal.,2010).Morerecently,579

eruptivephaseshavedecreasedindurationandhavebecomeincreasingly580

explosive(West,2013).Thelong-termaverageextrusionratewas0.6m3s−1581

between1956and1976(Belousovetal.,2002)and1993to2008(van582

Manenetal.,2010).Since1956theeruptiveproductshavebecomesteadily583

lessevolvedwithtime,varyingfrom60.4to56.8wt.%SiO2584

(Bogoyavlenskayaetal.,1985;Turneretal.,2013).SO2degassinghasbeen585

sustained.Fluxeshavebeenmeasuredat140to280t/dduringthree586

campaignsconductedduringperiodsofloweruptiveactivity(Lopezetal.,587

2013).Thesemeasurementsarenotsufficienttoassessrelationsbetween588

degassingandmagmadischarge.589

590

3.3.Mixederuptiveregime591

Persistentandepisodicregimescanmanifestoverdifferenttimescalesatindividual592

volcanoes.Consequently,thehistoricalrecordsofsomedome-buildingvolcanoes593

exhibitpatternsoferuptiveactivitythatarecharacteristicofbothregimes:they594

exhibitpersistentbehaviouroverseveraldecadesbutarealsocharacterisedbylong595

periodsofinter-eruptiverepose.Weidentifyfourvolcanoesthatfitthiscategory596

anddefinethemas‘mixed’regimevolcanoes(Fig.2;MSH,SHV,TUN,POP).597

598

Theeruptivebehaviouratthesevolcanoesvariesmarkedly,withpersistentactivity599

overshorttimescalesbutepisodicactivityovertimescalesofdecadestocenturies600

andpersistentactivityovershortertimescales.Mixedactivityissufficientlyvaried,601

however,thatitcannotbeconsideredexchangeable.Forexample,MountStHelens602

showedpersistentactivitythroughoutmostofthe1980’swithdegassingthatwas603

wellcorrelatedtemporallywithlavaextrusion.Tungurahua,incontrast,has604

remainedinapersistentregimesince1999,withdegassingthathasbeenpoorly605

correlatedwithlavaextrusion.Acommonobservationatallofthesevolcanoes,606

however,isintermittentashventing.607

608

(a) MountSt.Helens,USA(MSH),isadaciticvolcanothathasexperiencedtwo609

eruptiveepisodesinrecenttimes:1980to1986,and2004to2008(Swanson610

andHolcomb,1990;Scottetal.,2008),followinganinter-eruptiveperiodof611

reposelasting136years(Fig.2).Eruptiveactivityin1980initiatedwith612

endogenousgrowthoftheedifice(LipmanandMullineaux,1981)thatcaused613

amajorflankcollapseaccompaniedbysub-Plinianexplosiveactivity(Voight614

etal.,1983;Glicken,1998).Thiswasfollowedbysub-PliniantoVulcanian615

explosionsinthesummerof1980thatsteadilydecreasedinmagnitudeand616

duration(ScandoneandMalone,1985).Subsequenteffusiveactivity617

transitionedbetweendiscreteandcontinuouseruptionsofvariably618

crystallinelavas(Cashman,1992).Between1980and1986,extrusionrates619

variedfrom1.4to40m3s−1,withalong-termaverageof∼0.4m3s−1620

(AndersonandFink,1990;SwansonandHolcomb,1990).Renewed621

continuouseffusionin2004occurredatratesthatdecreasedsteadilyuntil622

2008,withamaximumof<5.9m3s−1andalong-termaverageof0.1m3s−1623

(Schillingetal.,2008;Majoretal.,2009).Between1980and1986magma624

compositionswerebroadlyhomogeneousat62-64wt.%SiO2(Cashman,625

1992;Pallisteretal.,1992;Blundyetal.,2008;Pallisteretal.,2008).Lavas626

eruptedbetween2004and2008weresimilarlyhomogenousat63-65wt.%627

SiO2(Blundyetal.,2008;Pallisteretal.,2008).Duringbotheruptiveperiods,628

degassingwascontinuousandlargelycoupledwithmagmaextrusion.The629

largestSO2fluxeswereassociatedwithexplosiveactivityintheearly1980’s,630

whentheyfrequentlyexceeded1000t/d(GerlachandMcGee,1994).The631

lowestSO2fluxes(∼70t/d)wereassociatedwiththedome-buildingactivity632

in1982-86and2004-2008(GerlachandMcGee,1994;Gerlachetal.,2008).633

Followingthecessationofeacheruptiveepisode,SO2fluxesdecreased634

rapidlytonegligiblelevels.Inthe1990’s,however,detectablegasemissions635

(Gerlachetal.,2008)wereobservedconcurrentlywithelevatedshallowVT636

seismicityandexplosiveemissionsofnon-juveniletephra(Mastin,1994).637

638

(b) SoufrièreHillsVolcano,Montserrat(SHV),isanandesiticvolcanothat639

eruptedin1995followingseveralcenturiesofnoeruptiveactivity.Since640

1995ithasexhibitedintermittentactivitywithfivephasesoferuptive641

activitylastingseveralmonthstoyears(Youngetal.,1998;Sparksand642

Young,2002;Wadgeetal.,2010;2014),withthelastphaseendingin2010.643

Theeruptiveactivityhasincludedlavadomeextrusion,block-and-ashflows644

andVulcanianexplosions;periodsofreposehavebeencharacterisedbyash645

ventingandcontinuousdegassing(Wadgeetal.,2014).Thetime-averaged646

lavaextrusionhasbeen3m3s−1,althoughratesexceeding10m3s−1have647

characterisedsomephasesofdomeextrusion(Wadgeetal.,2010;Wadgeet648

al.,2014).TheSiO2contentofhistoricallyeruptedproductshasvariedfrom649

58to62wt.%(Murphyetal.,2000;Zellmeretal.,2003b;Barclayetal.,2010;650

Christopheretal.,2014).TheaverageSO2emissionratefrom1995to2010651

was∼530t/d(Christopheretal.,2010)andlargelydecoupledfromeruptive652

activity(Christopheretal.,2010;Edmondsetal.,2010;Christopheretal.,653

2015).SoufrièreHillsVolcanocontinuestodegasat~430t/d(Christopher654

etal.,2015).Duringperiodsofintra-eruptiverepose,peaksindegassingof655

severalthousandt/dhavebeenassociatedwithburstsinseismicity(VTs)656

andaresometimesaccompaniedbyashventing(Coleetal.,2014).657

658

(c) Tungurahua,Ecuador(TUN),eruptedin1999following81yearsofno659

eruptiveactivity.Slowlavaextrusionandfrequentexplosiveactivityduring660

phasesoferuptiveactivityhavelimitedlavadomegrowth.Between1999661

and2006Tungurahuaalternatedbetweenexplosive(Strombolianto662

Vulcanian)eruptionsandrelativelyquietperiodsdominatedbyashventing663

andfumarolicactivity.ThemostexplosiveactivityoccurredduringJulyand664

August2006(Arellanoetal.,2008),afterwhichactivityreturnedtofrequent665

low-intensityStrombolianexplosions(Steffkeetal.,2010).Whilstthemagma666

supplyratehasvariedovertimescalesofmonths(Wrightetal.,2012),the667

long-termemissionrateofashhasbeenapproximatelyconstantat>0.2668

m3s−1,andpossibly>0.4m3s−1(LePennecetal.,2012).Theeruptiveproducts669

havecompositionsof56-59wt.%SiO2andshownosystematicvariationwith670

timeoreruptivestyle(Samaniegoetal.,2011),exceptthatmajorexplosive671

eventsin1866and2006haveincludedaminordaciticcomponent672

(Samaniegoetal.,2011).Between1999and2006,SO2fluxesvariedfrom673

severalhundredtothousandsoft/d;degassinghasbeenlargelydecoupled674

fromeruptiveactivity(Arellanoetal.,2008),althoughsince2006dailySO2675

fluxeshavedecreasedandappeartobebettercorrelatedwitheruptive676

activity.677

678

(d) Popocatépetl,Mexico(POP),hasexperiencedseveralperiodsoferuptive679

activityinthe20thcentury.Mostrecently,eruptiveactivitywasrenewedin680

1994andhasinvolvedrepeatedperiodsofdomegrowththathave681

culminatedinexplosiveeruptionsanddomecollapse.Extrusionrateshave682

rangedfrom0.5to4.1m3s−1duringdome-growthin1996and1997;the683

long-termaveragehasbeen0.24m3s−1(Delgado-Granadosetal.,2001).Prior684

to1995,Popocatépetllasteruptedbetween1920and1927(Delgado-685

Granadosetal.,2001)followedbyseveraldecadesofminordegassingand686

ashventing(Brennan,2007).Pyroclastseruptedbetween1996and1998687

rangedinbulkcompositionfrom∼59to64wt.%SiO2(Athanasopoulos,688

1997;StraubandMartin-DelPozzo,2001),withallcompositionserupted689

contemporaneously(Witteretal.,2005).In1994,averageSO2fluxeswere690

severalthousandt/d.SimilarlyhighSO2fluxes(30,000-50,000t/d)marked691

explosiveactivitybetween1996and1998(Goffetal.,1998;Delgado-692

Granadosetal.,2001).DOASmeasurementsoftheplumein2006providean693

averagefluxof2450t/d,withlargedailyvariationsnotalwaysassociated694

witheruptiveactivity(Grutteretal.,2008).695

6963.4.Non-eruptivedegassingregime697

Atvolcanoesthathaveremainedinapersistentregimethroughoutthe20thand21st698

centuries(section3.1.1),fumarolicactivitymaybesustainedduringperiodsof699

reposelastingyearsorevendecades(e.g.,Lascar;Gardeweg&Medina,1994).One700

volcanoinourdatabasehasnoteruptedduringthe20thand21stcenturiesbuthas701

exhibitedsustainedandpersistentdegassingofSO2.702

703

(a) Kudryavy(Moyorodake/Medvezhia),Russia(KUD),isabasalticandesite704

volcanothathasbeeninapersistentstateofhightemperaturefumarolic705

degassingandphreaticactivitysinceitslastmagmaticeruptionin1883706

(Fischeretal.,1998;Korzhinskyetal.,2002).Theonlymeasurementscome707

fromasinglecampaignin1995,whichmeasuredSO2fluxesof73±15t/d708

(Fischeretal.,1998).709

7104.Magmaticbehaviourinpersistentandepisodicregimes711

712

Geochemicalanalysisoferuptedproductsandgeophysicalobservationscanprovide713

semi-empiricalevidencefordifferentmagmaticprocesses.Wesummarisethese714

dataforthefifteendome-buildingvolcanoes,withaparticularfocusonsystematic715

variationsinthebehaviourofvolcanoesinthedifferentregimes.716

717

4.1.Interactionofmagmas718

Evidenceofmixingandminglingbetweendifferentbatchesofmagmaareobserved719

inall14volcanoesinourdatabasethathaveeruptedinthe20thcentury(Table2720

andreferencestherein).Differentmagmabatchestypicallyvaryincomposition,721

althoughinteractionsarealsoobservedbetweenmagmasormeltsthataresimilar722

incompositionbutdifferintemperatureandcrystallinity(CashmanandBlundy,723

2013;Costaetal.,2013;Trolletal.,2013).Evidenceformagmainteractionover724

shorttimescales(daystoyears)isubiquitousandincludes:(1)disequilibrium725

mineralassemblages;(2)disequilibriabetweenmineralassemblagesandmatrix726

glass;and(3)phenocrystzoning(Table2).Zoningpatterns,inparticular,provide727

evidencethatmagmamixingissustainedoverarangeoftimes.Discretemagma728

mixingeventsmaybeassociatedwithsingleexplosiveevents(Pallisteretal.,2008;729

Samaniegoetal.,2011;Scottetal.,2013)orindividualphasesofeffusiveactivity730

lastingmonths(Dirksenetal.,2006).Frequentandnear-continuousmagmamixing731

mayaccompanysustainedlavaeffusion(Nakamura,1995;Barclayetal.,2010;732

Turneretal.,2013).733

734

Thedegreeofmixingrangesfromcontemporaneouseruptionofdifferentmagma735

compositionstotheeruptionoflavasthatarehomogeneousinbulkcompositionbut736

heterogeneousonathinsectionscale.Evidenceforincompletemixingincludes737

bandedlavaorpumice,ormaficenclavesinmoresilicichostlavas.Where738

incompletemixingisobserved,historicalactivitytendstobeepisodicwith739

moderatetolongperiodsofinter-eruptiverepose.Persistentactivity,incontrast,740

tendstoproducehomogeneouslavas;hereevidenceformagmamixingispreserved741

onlyatthemicro-scale,inmeltinclusions,disequilibriummineralassemblages,742

polymodalmineralcompositions,andphenocrystzonation(Table2).743

744

4.2.Geophysicalobservations745

4.2.1.Seismicity746

Similarpatternsofseismicityareobservedacrossallthevolcanoesinthisreview,747

withnoapparentcorrelationwitheruptiveregime.Mostvolcanicearthquakesoccur748

priortoandduringeruptiveactivity.Renewederuptiveactivityisgenerally749

precededbyelevatedVTseismicity,withelevatedLPseismicityimmediatelyprior750

toeruptioninitiation.LevelsofLPseismicityarehighestatvolcanoesinpersistent751

regimeswheredegassingratesarehigh(e.g.,Lascar,Popocatépetl;Aschetal.,752

1996).Hybridevents(LPseismicitywithclearP&Swavearrivals)arecommonly753

associatedwithdome-growth(e.g.,Milleretal.,1998;Umakoshietal.,2008).754

755

Onceavolcanohasremainedinastateofreposeformorethanafewmonths,the756

levelofseismicitydecreases,althoughepisodicincreasesinVTseismicityare757

commonandareoftenassociatedwithelevateddegassingandashventing(Mastin,758

1994;Ratdomopurboetal.,2013;Budi-Santosoetal.,2013;Sernageomin,2013;759

Coleetal.,2014).Seismiccrisescanoccurduringinter-eruptiverepose;thesemay760

lastforseveralmonthstoseveralyearswithmultiplefeltearthquakesandno761

eruptionofmagma(JapanMeteorologicalAgency,1996,Youngetal.,1998).762

763

4.2.2.Deformation764

Geodeticmeasurementsoffar-fielddeformationaremorecommonatvolcanoesin765

anepisodicregimethanatthoseinapersistentregime(Table3),althoughthis766

apparentcorrelationcouldbecoincidental,sincemanyofthevolcanoesinour767

datasetthatexhibitepisodicbehaviourarelocatedindevelopedcountries,which768

tendtohavewell-establishedmonitoringandresearchcapabilities(e.g.,USAand769

Japan).Alternatively,volcanoesinapersistentregimemaylackfar-field770

observationsbecauseonlynear-fieldobservationsarerequiredforshort-term771

forecasting.Atepisodicvolcanoes,periodsofreposemayshowinflation,whereas772

deflationisprimarilyassociatedwithphasesofdomegrowth(Table3).The773

timescalesofinflationvaryfromyears(e.g.,Augustine,Redoubt;Cervellietal.,2010;774

Grapenthinetal.,2013a)todecades(e.g.,Augustine,Unzen;Kohnoetal.,2008;Lee775

etal.,2010).SoufrièreHillsVolcano,whichhasremainedinapersistentregime776

since1995,alsoexhibitscyclesoffar-fieldinflationanddeflationcoincidentwith777

eruptiveandnon-eruptivecyclesofmonthstoyears(Odbertetal.,2014a).Where778

persistentbehaviourincludesshortphasesoflavaeffusionandexplosiveeruption779

(e.g.,Bezymianny,Merapi,Colima),InSARmeasurementssuggestnegligiblefar-field780

deformation(Chaussardetal.,2013;Grapenthinetal.,2013b).781

782

5.Conceptualmagmaticmodelsfordome-buildingvolcanism783

784

Theinterpretationofmagmaticprocessesandtheirrelationtovolcanismrequiresa785

conceptualmodelforvolcanicactivity.Fromthisperspective,understandingthe786

geometryofpre-eruptivemagmastorageiscritical.Awidespread,butnotuniversal,787

observationaboutdome-buildingvolcanoesisthatmagmaissuppliedfromstorage788

regionsintheshallowcrust(Table5andreferencestherein),whichhasstimulated789

modelsoferuptiveactivitymodulatedbyshallowmagmachambers(Gourgaudet790

al.,1989;Murphyetal.,2000;Moraetal.,2002;Humphreysetal.,2008;Robergeet791

al.,2009;Larsenetal.,2010;Samaniegoetal.,2011;Shcherbakovetal.,2011;792

Coombsetal.,2013;Turneretal.,2013).Thereisalsoevidence,however,fordeeper793

levelsofmagmastorage,includingmid-tolowercrustalearthquakesassociated794

withvolcanism(McNutt,2005;Poweretal.,2013),deepsourcesofdeformation795

(PritchardandSimons,2002;Elsworthetal.,2008),anddeepsourcesofgas(Troll796

etal.,2013;Hautmannetal.,2014;Christopheretal.2015).Petrologicaland797

geochemicaldatahelptoquantifytheimportanceofdeepigneousprocesses798

(Hildreth,2004;Trolletal.,2013;Edmondsetal.,2014),includingmineral799

assemblagesthatrecordmultiplecrystallisationdepths(Matthewsetal.,1994;800

Marteletal.,1998;Scottetal.,2012;CashmanandBlundy,2013;Turneretal.,801

2013)andgeochronologyevidenceforlongcrustalresidencetimes(Volpeand802

Hammond,1991;Zellmeretal.,2003b;CooperandReid,2008;Dossetoetal.,2008;803

Claiborneetal.,2010).Finally,tomographicimagesofarcvolcanoessuggestmagma804

storageoccursatdifferentdepthsthroughoutthecrust(e.g.,Koulakovetal.,2013).805

806

Hereweplacegeochemicalandgeophysicalevidencefortranscrustalmagmatic807

systemsinthecontextofourcategorisationoftemporalvariationsinthehistorical808

recordsoflavadome-buildingvolcanoes.Specifically,weaddressthequestionofthe809

extenttowhichobservedregimesareconsistentwithnon-linearprocesses810

associatedwithashallowmagmachamber,orwhethertheyrequireinvolvementof811

verticallyextensivecrustalprocesses.Importantly,ouraimisnottoattributethe812

behaviourofanindividualvolcanooreruptiveeventtoeitherparadigm,butinstead813

toinvestigatetheextenttowhichdifferenteruptiveregimesmayreflect814

fundamentallydifferentsubsurfaceconditions,atleastwithregardtotheextentand815

connectivityofindividualmagmalenses.Weconcludethatwhilststorageofmagma816

intheuppercrustexertsanimportantcontrolonwhenandwhateruptiveactivity817

occurs,overhistoricaltimescalesdifferentpatternsofvolcanismcanbebetter818

ascribedtoaconceptualmodelbasedoncomplexbehavioursofverticallyextensive819

magmastorageregions.820

821

5.1.Shallowchamberparadigm822

Acommonmodelforeruptiveactivityatdome-buildingvolcanoesisashallowmelt-823

dominatedmagmachamberthatisreplenishedfromdepthandperiodically824

dischargesmagma(Fig.7).Inthisparadigm,intrusionofmaficmagmafromdepthis825

assumedtotriggertheeruptionofshallowmagmabodies(Gourgaudetal.,1989;826

Murphyetal.,2000;Moraetal.,2002;Humphreysetal.,2008;Robergeetal.,2009;827

Larsenetal.,2010;Samaniegoetal.,2011;Shcherbakovetal.,2011;Coombsetal.,828

2013;Turneretal.,2013).Theconceptofmafictriggersderivesprimarilyfrom829

near-ubiquitousevidenceformagmamixing(Table2).Intrudingmaficmagmaalso830

providesanexplanationforobservationsofexcessSO2(thatis,emissionofSO2in831

excessofamountsdissolvedintheeruptedmagma;Andresetal.,1991;Wallace,832

2003;Shinohara,2008;Christopheretal.,2010;WallaceandEdmonds,2011),as833

SO2ismuchmoresolubleinmaficmagmasthatinsilicicmagmas(Wallace,2005).834

Petrologicevidenceforshallowmagmastoragecomesfromsaturationpressures835

recordedinmeltinclusions,aswellasphaseassemblagesconsistentwithstorage836

pressures≤200MPa(e.g.,MooreandCarmichael,1998;BlundyandCashman,837

2001;Couchetal.,2001).838

839

Themodulatingeffectofshallowmagmaticsystemsoneruptiveprocessesis840

supportedbygeophysicaldata.Deflationduringeruptiveperiodscanberelatedto841

magmadischargefromupper-ormid-crustalmagmachambers(Nishietal.,1999;842

Elsworthetal.,2008;Cervellietal.,2010;Mattiolietal.,2010;Grapenthinetal.,843

2013a).Furthermore,mostseismicityassociatedwithunrestanderuptiveactivityis844

restrictedtodepthsof<10kilometres(RatdomopurboandPoupinet,2000;Moran845

etal.,2008;PowerandLalla,2010;Thelanetal.,2010;Petrosinoetal.,2011).846

Seismicityiscommonlyinferredtorecordthestresseffectsoftheformationof847

magmatransportpathways(Kilburn,2003;Scandoneetal.,2007)andriseof848

magmaticfluidsfromshallowmagmachambers(Neuberg,2000;McNutt,2005;849

ChouetandMatoza,2013).Shallowseismicityisalsoassociatedwithshallow850

magmaintrusion(Moranetal.,2011),pressurisationandpre-eruptiveinflation.851

852

Patternsofrechargehavebeenusedtoexplainpulsatoryandcyclicbehaviour853

(MelnikandSparks,1999;Barminetal.,2002).Indeeditislikelythatvolcanismis854

modulated,jointly,bydifferentpartsofthevolcanicsystem,includingshallow855

magmachambers.However,becausethemechanismforreplenishmentinthe856

shallowchamberparadigmispoorlyunderstood,itcannotcompletelyexplainthe857

hierarchyofcommonbehavioursandsimilarpatternsandstylesoferuptiveactivity.858

859

5.2.Transcrustaldestabilisation860

Ashallowmagmachambercanbeenvisagedastheuppermanifestationofamuch861

largertranscrustalsystem(Marsh,2000;Cañón-TapiaandWalker,2004),which862

mayextendthroughoutthecrustandevenintothemantle(Fig.8).Sucha863

conceptualmodelimpliesthatmechanismsforunrestanderuptionmayinvolve864

morecomplexprocessesthandiscreteintrusions.Specifically,magmaticsystems865

canbeviewedascomprisingextensivebodiesofcrystal-richmagma(mush)with866

interspersedlensesofmeltandmagmaticfluidsthatareformedbyrepeated867

intrusionofmaficmeltsfromthemantle(Solanoetal.,2012;Connollyand868

Podladchikov,2013;Christopheretal.2015).Fromthisperspective,meltandfluid869

layersaresusceptibletodestabilisation,andreorganisationoftheselayersmay870

provideatriggerforeruptiveactivityinmafic(Tarasewiczetal.,2012;Neaveetal.,871

2013)andlargecalderasystems(CashmanandGiordano,2014).Similarly,872

transcrustalprocessescanexplainapparentlyanomalousactivityinsomedome-873

buildingvolcanoes(Christopheretal.,2015),whilstalsoprovidingasourceofdeep874

magmaandmagmaticfluids.Keyistheconceptofthemeta-stabilityoftranscrustal875

magmaticsystemsanddestabilisationeventsthatinvolveeitherallorpartofthe876

melt-bearingregion(Fig.8a,b),withorwithoutcontemporaneouseruptiveactivity877

(Fig.8c).878

879

Temporalandspatialvariationsinthesusceptibilityofverticallyextensive880

magmaticsystemstodestabilisationcanalsoexplainlong-termpatternsoferuptive881

activityatdome-buildingvolcanoes.Firstwereturntothequestionofmafic882

eruptiontriggers,particularlyasevidencedbyvaryingintensitiesof883

magmamixingintheeruptiveproducts.Mixinghaslongbeenusedtodescribethe884

homogenisationoftwomelts,asmanifestedinlineartwo-elementgeochemical885

diagrams.Mixing,however,isincreasinglyviewedasinvolvingcomplexinteractions886

betweenmeltsandcrystalmushes(Blundyetal.,2008;Humphreysetal.,2009;887

CashmanandBlundy,2013).Fromthisperspective,theroleofmixingasaprimary888

mechanismoferuptiontriggeringislessclear.Infact,mixingmaybeaneffect,as889

muchasacause,oferuptiveactivity,particularlyiftriggeredinitiallyby890

destabilisationofthemagmaticsystem.Destabilisationcouldoccurfromthebottom891

up,withdeepleveldisturbancespropagatingintotheuppercrust(e.g.,Christopher892

etal.,2015).Alternativelydestabilisationcouldpropagatedownward,drivenbya893

downwardpropagatingdecompressionwavecausedbyearlyeruptiveactivity(e.g.,894

Tarasewiczetal.,2012).Ineithercase,destabilisationofacomplexmagmatic895

systemcanforceinteractionamongmeltlensesandinterveningcrystalmushzones896

(e.g.,CashmanandGiordano,2014).897

898

Anotherimportantaspectofdome-buildingvolcanoesinhydrousarcsystemrelates899

totheevolutionandmigrationofvolatiles.Fractionationofdeeplysourcedarc900

basalts(Annenetal.,2006)cancausesulphursaturationofmoreevolvedfelsic901

meltsinthemiddleandlowercrust(Wallace,2005).Thisoccursbecause,although902

sulphurishighlysolubleinbasalticmelts,itismuchlesssolubleinfelsicmelts903

(Lesneetal.,2011).Asaconsequence,SO2degassingcanstartdeepwithinthecrust,904

wellbelowlevelsofshallowmagmastorage.ThesameistrueofCO2,wherestrong905

pressure-dependencemaypromoteCO2exsolutionthroughoutthecrust(e.g.,906

Blundyetal.,2010).Differentvolatileelementscanthereforebefractionatedand907

storedindependentlyatmultiplecrustallevelsduringinter-eruptiveperiodsof908

repose.Separationofvolatilesfromtheirparentalmagmasduringtheseperiodsof909

reposecanexplainboththeexcessSO2degassinganddecouplingofgasandmagma910

fluxesobservedindome-buildingvolcanoesinthepersistentregime.Ascentof911

magmaticfluidsfromdepthcanalsoexplaindecouplingofshallowseismicityfrom912

eruptiveactivity(Moran,1994;Romanetal.,2004;Gironaetal.,2014;Hautmannet913

al.,2014,Christopheretal.,2015).Similarly,deep(20to40km),longperiod914

earthquakesinarcscanbeexplainedbyexsolutionandmigrationofinsolublegases915

likeCO2(McNutt,2005;Nicholsetal.,2011).Finally,independentriseofmagmatic916

fluidsmaycausethesurfacedeformationobservedatpassivelydegassingvolcanoes917

(Gironaetal.,2014),andcanhelptoexplainvaryingtimescalesoffar-fieldinflation918

atdome-buildingvolcanoes.919

920

5.3.Persistentdome-buildingbehaviour921

Thepersistentregimecombinespulsatoryphasesofeffusiveeruptionand922

homogeneousmagmacompositionswithsustained,anddecoupled,degassing923

(section3.1.1),andistypicalof‘open’systembehaviour(e.g.,Chaussardetal.,924

2013).Theseobservationsappeartorequireadynamicallyconnected,through-925

goingmagmaticsystemtosustainapersistentregime,especiallyoverlong926

timescales.Largeexplosiveeruptionsinthesesystemsinvolvemagmathatismore927

mafic(deeper,morevolatile-rich)thanthatproducedduringeffusiveactivity.928

Transitionsbetweenpersistentshallow-seatedeffusivebehaviourandintermittent929

deep-seatedexplosionsthussuggestthatmagmaticsystemsatthesevolcanoesare930

verticallyextensiveand(transiently)dynamicallyconnected,atleasttomid-crustal931

levels(Fig.8a).Moregenerally,rapidtransportofdeep,maficandvolatile-rich932

magmasiscommonlyinvokedforparoxysmaleventsatopen-systembasaltic933

volcanoes(e.g.,Métrichetal.,2010;Sidesetal.,2014).934

935

Eruptiveactivityatasecondgroupofvolcanoesinthepersistentregime(section936

3.1.2)reactivatedwithmajorexplosiveeventsthatfollowedlongperiodsofinter-937

eruptiverepose.Inthesevolcanoes,theexplosivelyeruptedmagmaismoreevolved938

thansubsequentextrusivelavas,whichshowgradualdecreasesinSiO2withtime.939

Progressivevariationinthecompositionoferuptedproductscanbeexplainedbya940

verticallyextensiveandconnectedmagmaticsystem,althoughamoretraditional941

zonedmagmachambermodel(e.g.,Scottetal.,2013)cannotbeexcludedonthe942

basisofthesecharacteristicsalone.Mostimportantfromavolcanichazards943

perspective,however,arethecompositionalhomogeneityandpaucityofmafic944

enclaves(Scottetal.,2013;Turneretal.,2013)thatcharacteriseactivity.This945

suggeststhatthesepersistentlyactivevolcanoeshaverelativelystablemagmatic946

systemsthatarelesssusceptibletolarge-scaledestabilisationthanduringinter-947

eruptiveperiodsofrepose.948

949

Theobservationthatexplosiveeruptionsmaybeeithermoreorlessevolvedthan950

magmaeruptedeffusivelyfromthesamesystemprovidesinsightintoexplosive951

eruptiontriggers.‘Top-down’destabilisationisobservedincasesofedificecollapse952

followingeitheralongdurationinastateofinter-eruptiverepose(Bezymianny,953

Santiaguito,MountSt.Helens)orsustainedeffusiveactivityanddomegrowth954

(Lascar).Top-downtriggeringtapsevolvedmagmafromhighinthecrust.‘Bottom-955

up’destabilisation,incontrast,explainsexplosiveeventsthatappeartobetriggered956

bytherapidriseofdeep-derivedmagmas(Merapi,Colima,Shiveluch).957

958

Persistenteruptiveregimesrequirethatthemagmaticsystemis‘open’,orvertically959

connected.Undertheseconditions,eruptiveactivitymaybeneitherstrictly‘top960

down’nor‘bottomup’butinsteadreflecttheintrinsicinstabilityofcomplex961

magmaticsystems.Onemechanismofinstabilityrelatestothebehaviourofcrystal-962

meltsuspensions,whichsegregatetoformseparatelayersofmeltand/orvolatiles.963

Wesuggestthatthese(unstable)layerscanreorganiserapidlytotriggerabrupt964

changesineruptionpatterns.Layerdestabilisationmayoccurbecauseofexternal965

triggers,suchasregionaltectonicsoreruptionsofneighbouringvolcanoes(e.g.,966

Walteretal.,2007;DelaCruz-Reynaetal.,2010;Biggsetal.,2016).Alternatively,967

passivevolatilereleaseduringastateofreposemaycausethepressuredistribution968

sufficientlytocausereplenishmentofmagmafromdepth(Gironaetal.,2015).Such969

mechanismsarenotrestrictedtodome-buildingvolcanoes,andhavebeenobserved970

atbasalticarcsystemsthatareverticallywell-connectedandexhibitcomplex971

feedbackmechanismsformagmadischarge(e.g.,Stromboli;Ripepeetal.,2015).972

973

5.4.Episodicdome-buildingbehaviour974

Dome-buildingvolcanoesthatshowepisodicbehaviourarecharacterisedby975

diminishingeruptionrateswithtimeandcorrelationsbetweenlavaextrusionand976

volatileemission.Bothcharacteristicsareindicativeofclosedsystembehaviour,977

whichlikelyreflectstheformationandascentofdiscretemagmabatches.Inmanyof978

thesevolcanoes,however,thereisevidencefortheinteractionofdifferentmelts979

(Table3),whicharguesagainstdiscretemeltbatches.Infact,volcanoesinan980

episodicregimethateruptfrequently(e.g.,Augustine,Redoubt)eruptawiderange981

ofcompositionsduringanyindividualeruption.Thissuggeststhatsmallmelt982

batchesevolveindependentlyandinteractonlyduringeruptions(e.g.,Romanetal.,983

2006).Morehomogeneousmagmacompositionsproducedbyvolcanoesthaterupt984

lessfrequently(e.g.,MontPelée,Unzen),incontrast,suggeststhatmagmamixing985

mayoccurpriorto,aswellasduring,eruptiveepisodes(Browneetal.,2006).986

987

Amagmaticmodelbasedontheshallowchamberparadigmsuggeststhatifmagmas988

aregeneratedataconstantrateatdepth,thenthedurationavolcanoremainsina989

stateofreposewillcontrolthevolumeofmagmacomponents(volatiles,melt,and990

crystalmush)thatcanaccumulate;thistime-dependentvolumemay,inturn,991

influencethedurationavolcanoremainsinaneruptivestate.Incontrast,underthe992

transcrustalparadigm,variationsinfrequencyanddurationoferuptiveepisodes993

couldreflectpatternsofdestabilisationwithinthedeepersystem.Stabilitymaybe994

controlledbyphysicalproperties,suchasthesizeofmagmaticsystems,or995

fundamentalparameterssuchasthefluxofmagmaatdepth(Caricchietal.,2014).996

997

5.5.Large-magnitudeexplosiveeruptions998

Thedynamicnatureoferuptiveactivityatdome-buildingvolcanoessuggeststhat999

pastbehaviourislikelytoinfluencestabilityofthemagmaticsystem,andfuture1000

patternsoferuptiveactivity.Forexample,edificecollapseassociatedwithlarge1001

magnitudeexplosionsisknowntoreducestoragepressures(Pinel&Albino,2013)1002

andenabletheeruptionofdenser,moremaficmagmas,whichwouldotherwisestall1003

atshallowdepths(Pinel&Jaupart,2000;2005).Indeed,volcanoesinourdataset1004

wheretheonsetoferuptiveactivityinvolvededificecollapsemaywellhaveshown1005

differentlong-termpatternsoferuptiveactivityiftheonsetoferuptiveactivityhad1006

beeneffusive.Conversely,whereedificecollapseoccurredafteralongdurationina1007

stateofrepose(~millenia),persistentactivityappearstolastformanydecades(e.g.1008

Bezymianny,Santiaguito;Fig.2).Removaloftheedificeduringtheselarge1009

magnitudeeventsthusappearstodestabilisethesystem(Pinel&Albino,2013).1010

1011

AdifferentsituationoccurredatMountSt.Helensin1980,wheretheinitial1012

explosiveeruptionwasrelatedtoedificecollapse,butthepriorreposeintervalwas1013

onlyslightlymorethanacentury.Inthiscase,persistentbehaviourcontinuedfor1014

onlysixyears.Itisnoteworthythatthevolcanoreactivatedbetween2004-20081015

(Fig.2)aftertwointerveningepisodesofinferredrechargefromdeeperinthe1016

system(Moran,1994;Musumecietal.,2002).Thelimitedpersistentactivityof1017

MountSt.HelenscomparedtoBezymiannyandSantiaguitomaybesimplyaresult1018

ofshorterinter-eruptiverepose,whichcouldlimittheaccumulationoferuptible1019

magma.Alternatively,itmayberelatedtothedaciticcompositionofmagmaat1020

MountSt.Helens,comparedtotheandesiticmagmasofBezymiannyand1021

Santiaguito.1022

1023

6.Conceptualisingvolcanismintime1024

1025

Recordsoferuptiveactivityinformourunderstandingofmagmaticprocessesand1026

arecommonlythebasisforforecastsoferuptiveactivity.Traditionally,volcanismis1027

conceptualisedasaseriesofdiscreteeruptions(Siebertetal.,2010)thatare1028

characterisedbymeasureablepropertiessuchasmagnitude,duration,intensityand1029

eruptivestyle(Mercalli,1907;NewhallandSelf,1982;Pyle,2000).Theintervals1030

betweeneruptionsareusuallyreferredtoasreposeperiodsandatthesetimesthe1031

volcanoiscommonlyinterpretedtobeinadormantstate.Thisontologyofvolcanic1032

activityasapointprocessstemsfromgeologicalrecordsthatcompriseapunctuated1033

seriesofdistinctdeposits,andhistoricalrecordsthatarebiasedtowardsoccasional1034

memorable,andgenerallyexplosive,individualevents(SzakácsandCañón-Tapia,1035

2010).1036

1037

Adifferentperspectiveemergesfromouranalysisoflong-termeruptivebehaviours1038

atfifteenwell-studieddome-buildingvolcanoes.Insteadofidentifyingdiscrete1039

eruptions,wesuggestthatperiodsoferuptiveactivitybeclassifiedinthecontextof1040

theeruptivehistory.Forexample,attwodifferentvolcanoes,periodsofdome1041

extrusionmayhavesimilarlavavolumes,ratesofextrusion,andduration,butcan1042

occurinverydifferentsituations(e.g.,asperiodofepisodicactivityoraphaseof1043

lavaextrusioninapersistentregime).Includingtimeasakeyparameterhighlights1044

theshortcomingsofviewingvolcanoesasinonlyeitheran“eruptive”or“non-1045

eruptive”state.Critically,thisontologyofvolcanicactivityshouldinfluence1046

interpretationofbothvolcanicdataandinferredmagmaticprocesses.1047

1048

Theevidencefordifferentstatesofreposeprovidedbyourcasestudiessuggests1049

thatlavadome-buildingvolcanoescanbecharacterisedbythree,ratherthantwo,1050

states:(i)astateofdormancywithoutabnormalgeochemicalorgeophysicalsignals1051

(inter-eruptive);(ii)anactivestateinwhichmagmaiserupted;and(iii)astateof1052

unrestwhereperturbationsinthesystematdepthcausemarkedandmeasurable1053

departuresfromabackground(dormant)state(intra-eruptive).Historicalrecords1054

allowvolcanoclassificationbyone,twoorallthreeofthesestates.Overgeological1055

timescales,weassumeallvolcanoesexperienceperiodsofdormancyorinter-1056

eruptivereposeperiods.Intra-eruptivereposeperiodscanbemoredifficultto1057

identify,andpresentthegreatestchallengesforvolcanichazardassessment.1058

1059

Inter-eruptivereposeoccursatvolcanoesthatshowepisodicbehaviour,meaning1060

thattheyconformmorecloselytothetraditionalinterpretationofvolcanismasa1061

sequenceofdiscreteeruptions.Thedurationofinter-eruptivereposecanvaryfrom1062

manyyears(e.g.,Augustine,Redoubt)tocenturies(e.g.,MountUnzen),butinall1063

casesthevolcanoisdeemedtobeinadormantstatebetweeneruptiveperiods.1064

Volcanoesclassifiedasdormantcanmoveintotheunreststatewithincreasesin1065

geophysical(e.g.,seismicity,anddeformation)andfumarolicactivity.Forexample,1066

priorto1992,SoufrièreHillsVolcanohadbeeninadormantstateforover3501067

years,buthadmovedintoastateofunrestin1896-97,1933-37and1966-67,as1068

evidencedbyelevatedfumarolicactivityandintermittentseismiccrises(Shepherd1069

etal.,1971;Odbertetal.,2014b).Similarseismiccriseswerealsoobserved1070

throughoutthe20thcenturyatMtUnzenpriortoeruptiononsetin1991(Japan1071

MeteorologicalAgency,1996).1072

1073

Intra-eruptivereposeisobservedatvolcanoesinapersistentregimewhere1074

intervalsbetweenpulsesoferuptiveactivitycanlastformonthstoyearsoreven1075

decades,especiallyfollowingmajorexplosiveevents(e.g.,Bezymianny,Colima,1076

Lascar,Santiaguito).Atthesevolcanoes,however,periodsofreposeare1077

characterisedbysustaineddegassing,intermittentseismicityandashventing,allof1078

whichindicatemagmaticunrestthatisnotconsistentwithdormancy.Importantly,1079

unrestundertheseconditionsdoesnotimplyimminenteruptiveactivity,as1080

observedintheexampleofKudryavywhereapersistentstateofhightemperature1081

fumarolicdegassingandphreaticactivityisinferredsinceitslastmagmaticeruption1082

in1883(Fischeretal.,1998;Korzhinskyetal.,2002).1083

1084

Bycharacterisingexchangeabletraitsofvolcanicbehaviour,wedemonstratethat1085

thecasehistoriesinthisreviewchallengethedepictionofvolcanismasapoint1086

processintime,andraisequestionsaboutwhatitmeanstosaythatavolcanois1087

dormantandhowtoviewperiodsofnon-eruptivevolcanicunrest.Importantly,1088

severalofourcasestudyvolcanoesshowunrestsignalsthataregreatlyelevated1089

aftereruptiveactivity,incomparisontounrestsignalswhenavolcanoisinaperiod1090

oflongerdormancy(e.g.,Merapi,Lascar,Bezymianny).Forthisreason,wesuggest1091

thatthestateofunrestbeusedtoclassifyvolcanicactivity,withthecaveatthatitis1092

importanttorecognisewhenthedistinctionbetweenunrestanddormancyis1093

determinedbyachangeindetectionthresholdsandnotbytruechangesinthestate1094

ofamagmaticsystem.1095

1096

Theconceptualisationoferuptionsasdiscreteeventshasbeen,andstillis,1097

fundamentaltovolcanoclassification,volcanodatabases,dataselectionin1098

probabilisticforecastsandtheinterpretationofmagmaticprocesses.TheGVP1099

database(Siebertetal.,2010)istheonlycomprehensiveglobalcompilationof1100

activevolcanoes,andiswidelyusedtocharacterisevolcanism,inform1101

interpretationsofvolcanicprocessesandprovideevidenceforeruptiveforecasts.1102

Thecatalogueispredicated,however,onviewingvolcanismasanalternationoftwo1103

differentevents,reposeperiodanderuption.TheGVPfurtherdefinesreposeasany1104

cessationineruptiveactivitythatexceeds3months.Thisdefinitionworkswellfor1105

someofourcasestudies(e.g.,Augustine,Redoubt),butisproblematicforvolcanoes1106

showingprolongedintermittentactivity(e.g.,Bezymianny,MountSt.Helens,1107

Merapi,SoufrièreHillsVolcano).Morecritically,theGVPdatabasestructuredoes1108

notrecordinformationthatisusefulforbothcharacterisingandinterpretingstates1109

oferuptionandunrest.1110

1111

7.Informationexchangeabilityinforecastingvolcanicactivity1112

1113

Inrecentdecadesprobabilisticmethodshavebecomeestablishedastheprincipal1114

approachtoforecastingvolcanicactivity.Importantly,theycancaptureboth1115

aleatoryandepistemicuncertaintiesandincludemultiplestrandsofevidenceand1116

differentkindsofdata(e.g.,NewhallandHoblitt,2002;Aspinalletal.,2003;1117

Marzocchietal.,2004;SparksandAspinall,2004;Nerietal.,2008;Sobradeloetal.,1118

2013;AspinallandWoo,2014;Hincksetal.,2014;SobradeloandMartí,2015).1119

Probabilisticapproaches,however,havehighlightedspecificchallengesassociated1120

witheruptiveforecastsatdome-buildingvolcanoes.Themostacuteproblemrelates1121

toalackofdata,especiallyatvolcanoeswithinfrequenteruptiveactivityinepisodic1122

regimes.Theissueofsparsedata,however,canalsomanifestatvolcanoesina1123

persistentregime,whenforecastingalongperiodofdormancy.Consequently,an1124

importantquestioninvolcanologyiswhetherobservationsfromanumberofwell-1125

studiedvolcanoescanbeusedtoreduceuncertaintyassociatedwithalackofdataat1126

anindividualvolcano.Thisisespeciallypertinentwiththedevelopmentofglobal1127

databases(e.g.,SmithsonianGVP;LaMEVE;WovoDAT)andglobalapproachesto1128

datacollection(e.g.,Biggsetal.,2014;Carnetal.,2016).1129

1130

Importantly,theprincipleofusingobservationsfrommultiplevolcanoesrequires1131

anassumptionofinformationordataexchangeability(e.g.,Bebbington,2014;1132

Sheldrake,2014).FromaBayesianperspective,exchangeabilityrequiresa1133

(subjective)levelofsimilarity,butimportantly,doesnotrequirethebehavioursof1134

theobjectstobeidentical(Bernado,1996;Gelmanetal.,2013).Hence,similar1135

behavioursandtraitsbasedonphenomenologicalobservationsidentifiedinthis1136

reviewcouldbeabasisforassumptionsofexchangeability.1137

1138

7.1.Approachestoassumingexchangeability1139

Oneapproachtotheproblemoflimiteddataisthroughexpertjudgement(Aspinall1140

andCooke,2013),whereexperiencedscientistsassesskeyparametersand1141

likelihoodsoffutureeventsbasedupontheirownknowledge,experienceand1142

judgements.Inprinciple,theexpertsshouldalsoestimatetheuncertaintyoftheir1143

likelihoodassessment(Aspinall,2010).Issuesofexchangeabledataarisewhen1144

comparisonswithothervolcanoesenterintothesediscussions,atleastinformally.1145

Inmanyvolcanoemergencies,forexample,suchassessmentsareadhocand1146

executedlargelythroughunstructureddiscussionwithinavolcanoobservatory1147

team.Theseeffortscanbeimprovedbyformalisedmethodsforpoolingexpert1148

judgements,asillustratedbyhazardassessmentsforSoufrièreHillsVolcano(Wadge1149

andAspinall,2014).Importantly,theexperienceofanexpertinpreviousvolcanic1150

criseswilllikelyinfluencetheirviews.Thisillustratesamajordisadvantageinthe1151

informalapproach,wherethebasisforassessmentmaybeanecdotalandbiased1152

towardspreviouslywitnesseddiscreteevents.Moreover,eventhemostexperienced1153

volcanologistisunlikelytohavewitnessedmorethanahandfuloferuptiveevents,1154

sothesecomparisonswarrantamorerigorousapproachtoidentifyingappropriate1155

analoguevolcanoesandtowhatextentcomparisonsarejustified.1156

1157

Broadclassificationsforvolcano‘type’basedoncharacteristicssuchasmorphology1158

(Rittmann,1962;Siebertetal.,2010)oreruptivestyle(e.g.,Hawaiian,Strombolian,1159

Peléean,VulcanianandPlinian;Bullard,1962)provideanaturalframeworkfor1160

assumptionsofexchangeability.However,astheanalysisinthisreviewhas1161

outlined,thehistoricalrecordsofdome-buildingvolcanoesareonlypartially1162

exchangeable.Thus,whilstexchangeabilitymaybeassumedbasedonvolcano‘type’1163

(e.g.,lava-domebuilding),thelimitationsandsourcesofaleatoryuncertaintyof1164

probabilisticforecaststhatarisefromthisassumptionmustbeaddressedby1165

identifyingboththeunderlyingconceptualmodelandthecommonprocessthat1166

togetherformthebasisforexchangeability.Itisequallyimportanttorecognisekey1167

differenceswhenapplyingexchangeability.Thisisevidentinacladisticsanalysisof1168

Japanesearcvolcanoes(Honeetal.,2007)thatidentifiedthreebroadvolcanotypes1169

groupedbycomposition,eruptiveproductsandmorphologicalcharacteristics.1170

Differencesarealsoidentifiedinastudyofmagnitude-frequencyrelationsthat1171

treatsseparatelyclosed-andopen-ventstratovolcanoes(Whelleyetal.,2015).1172

1173

7.2.Volcanicunrest1174

Theconceptofexchangeabilitycanbeusedtointerpretvolcanicunrest,whichisan1175

almostaubiquitousprecursortovolcanicactivity.Signsofunrestaretypically1176

monitoredusinggeodetic,geophysicalandgeochemicalsurveys(e.g.,Swansonetal.,1177

1983;Sparks,2003;Sandrietal.,2004;Jaquetetal.,2006;ChouetandMatoza,1178

2013).Critically,thesemonitoringdataareusedtoinfermagmaticprocesses(e.g.,1179

Voight,1988;Kilburn,2003;Smithetal.,2007;Lavalléeetal.,2008),anapproach1180

thatrequiresimplicit,ifnotexplicit,comparisonswithunrestfrompreviousactivity.1181

1182

Thesimplestapproachtocomparingvolcanicunrestamongvolcanoesistoconsider1183

allsignalsofunrestasweaklyexchangeable,withvariationsintheduration,pattern1184

andoccurrencetheresultofaleatoryuncertainty,reflectingthenaturalvariabilityof1185

volcanicsystems.Astrongerassumptionofexchangeabilitycomparessignsof1186

unrestbetweenvolcanoesofaspecifictype(e.g.,Phillipsonetal.,2013),withthe1187

underlyingassumptionthatdifferenttypesofvolcanoesshouldbehaveinsimilar1188

ways.Ourworkshows,however,thatevenparticularvolcano‘types’canvary1189

greatlyinbehaviour.Inparticular,wehaveshownthatintra-reposeunrestofa1190

volcanoinapersistentregimemayreflectaverydifferentstateofactivitythan1191

inter-reposeunrestintheepisodicregime,whichmayheraldtheonsetofexplosive1192

activity.Inthisway,ourcategorizationoferuptiveactivityatdome-building1193

volcanoesasepisodic(closed-system)orpersistent(open-system)couldhelpto1194

furtherrefineclassificationsofunrest,particularlywithregardtotheproblemof1195

distinguishingbetweennon-eruptiveunrestandunrestrelatedtoreawakeningofa1196

volcanoinrepose(e.g.,Phillipsonetal.,2013).Furthermore,byattemptingto1197

understanddifferencesinepisodicandpersistentbehaviourintermsofmagmatic1198

processes,thisprovidesanopportunitytointerpretpatternsofvolcanicunrestin1199

termsofthesemagmaticprocesses,ratherthanpurelytheoutcomeoferuptive1200

activity(e.g.,Hincksetal.,2014).1201

1202

8.Conclusions1203

1204

Wehaveshownthatdome-buildingvolcanoesshowtwofundamentallydifferent1205

patternsoferuptivebehavioursthatwetermepisodicandpersistent.Episodic1206

behaviourischaracterisedbydiscreteepisodescomprisinganexplosiveonset1207

followedbyeffusionanddomeformation.Inthisregime,explosivelyerupted1208

magmamayhavemoreevolvedcompositionsthanlater-eruptedlava.Excessgas1209

emissionsmaybeobservedduringexplosiveactivity,butSO2fluxesarecorrelated1210

withtheeruptionoflavaanddiminishtonegligiblelevelsfollowingtheendofeach1211

eruptiveepisode.Persistentbehaviour,incontrast,ischaracterisedbyfrequent1212

(~yearly)phasesoferuptiveactivityandsustainedgasfluxesduringperiodsof1213

intra-eruptiverepose.Eruptedmaterialisoftencompositionallyhomogeneous,1214

exceptduringexplosive(paroxysmal)eruptions,whichofteninvolvedeep,more1215

primitive,magmacompositions.Alternatively,atvolcanoesthathavenoterupted1216

foralongtime(~millenia),largeexplosivePlinianeruptionscanbefollowedby1217

persistentbehaviourwherelavacompositionsbecomelessevolvedwithtime.1218

Importantly,allvolcanicactivityisepisodicifviewedoversufficientlylongtimes.1219

1220

Weexplainthevarietyofepisodicandpersistentbehaviourthroughthelensof1221

verticallyextensivemagmaticsystems,wheretheextentofconnectivitywithinthe1222

systemdictatesepisodicorpersistentbehaviour(e.g.,Christopheretal.,2015).1223

Importantly,open-systembehaviourinvolvestransient,dynamicallytriggered1224

magmatransferfromdepthbutcontinuousgastransferthroughthesystem.1225

Episodicbehaviour,incontrast,recordseruptionandgaslossfromamagmabatch1226

thatisquicklyisolatedfromdeeper(mid-crustal)reservoir.Aninterestingquestion1227

relatestotheimportanceofvolatilesandvolatile-richmeltsindeterminingthe1228

stabilityofamagmaticsystem,particularlytransitionsbetweenepisodicand1229

persistentregimes,anderuptiontriggeringinepisodicregimes(e.g.,Borisovaetal.,1230

2014;Christopheratal.,2015;Gironaetal.,2015).1231

1232

Fromahazardforecastingperspective,our15casestudiesshowthatdome-building1233

volcanicactivitycannotbecharacterisedbyapointprocess.Thisobservation1234

highlightsakeyontologicalissueforvolcanology.Discreteeruptiveeventscan1235

appearsimilarinnatureinbothanepisodicandpersistentregime,butare1236

associatedwithdifferentstatesofreposeandlong-termbehaviour.Therefore,when1237

analysingvolcanicdata,andinterpretingmagmaticprocesses,itisimportantto1238

characteriseeruptiveactivityinthecontextofthelonger-termbehaviourofa1239

volcanicsystem.Wehaveshownthatgasdata,inparticular,mayhelpto1240

discriminatebetweeninter-andintra-eruptiverepose.Alsoimportantarepatterns1241

ofseismicity,whichprovideinformationonthedepthandvolumeofmagmastorage1242

(e.g.,WhiteandMcCausland,2016).1243

1244

Alsoimportantforhazardforecastingisdevelopingamethodtodeterminehow1245

monitoringdatafromwell-observedvolcanoescanbeusedtoinform1246

interpretationsofmonitoringdatafromperiodsofunrestatless-studiedvolcanoes.1247

Suchanapproachisfeasible,butrequiresanunderstandingoftheextenttowhich1248

themonitoringdatacanbeconsideredexchangeable.Wesuggestthat1249

exchangeabilitycanbeformalisedbyassessingtemporalpatternsinvolcanic1250

phenomena(especiallyrelativepatternsoferuption,degassingandrepose),evenif1251

thedatasetshavedifferentspatialandtemporaldata.Fromatheoreticalstandpoint,1252

linkingassumptionsofexchangeability(e.g.,episodicvs.persistent)toconceptual1253

modelsofvolcanicsystems(e.g.,closedvs.open)providesamechanismtointerpret1254

monitoringdatausingaframeworkofmagmaticprocesses.1255

1256

Importantly,theapproachemployedinthisreviewcannotbeusedtoidentify1257

uniquemagmaticprocessesatindividualvolcanoes,andinthatsensecannotreplace1258

‘in-depth’studiesofindividualvolcanicsystems.However,itprovidesaconceptual1259

frameworkforinterpretingcommonprocessesatdome-buildingvolcanoes.Froma1260

broaderperspective,ourworkdemonstratesthevalueofconstructingahierarchical1261

frameworkforvolcanicactivitybasedonexchangeablebehaviours.Wesuggestthat1262

thisapproachcouldbeextendedtovolcanoeswithothertypesofcharacteristic1263

activity,andthusprovidesaholisticapproachtoanalysingglobalvolcanicrecords.1264

1265

Acknowledgements1266

ManythankstoProf.JontyRougierintheSchoolofMathematics,Universityof1267Bristol,whoprovidedadviceonthedefinitionandapplicationofexchangeability,1268bothinageneralcontextandmorespecificallyatvolcanoes.12691270Wealsothanktwoanonymousreviewerswhoserevisionsandsuggestionshelped1271usmoreclearlyexplainthemethodologythathasbeenused,andclarifyspecific1272aspectsofthediscussion.12731274TESandRSJSweresupportedbyaEuropeanResearchGrant,Voldies.WPAwas1275supportedinpartbytheNaturalEnvironmentResearchCouncilthroughthe1276ConsortiumonRiskintheEnvironment:Diagnostics,Integration,Benchmarking,1277LearningandElicitation(CREDIBLE;NE/J017450/1).KVCwassupportedbythe1278AXAResearchFundandaRoyalSocietyWolfsonMeritAward.127912801281References:1282

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221322142215FigureCaptions:221622172218(1.5column)2219Figure1:Locationsofthe15dome-buildingvolcanoesinthisstudy:(a)Augustine;2220(b)Bezymianny;(c)Colima;(d)Kudryavy;(e)Lascar;(f)Merapi;(g)MountSt.2221Helens;(h)MontPelée;(i)Popocatépetl;(j)Redoubt;(k)Santiaguito;(l)Shiveluch;2222(m)SoufrièreHillsVolcano;(n)Tungurahua;(o)MountUnzen.Theyareallfoundin2223subductionsettings:eitheroceanic-continentaloroceanic-oceanicboundaries.22242225(1.5column)2226Figure2:Binaryplotsindicatingwhether(magmatic)eruptiveactivity(ash2227explosionsandlavadomegrowth)wasrecordingineachyearsince1800C.E,at2228eachofthe15volcanoesinthisstudy.Importantly,theredbarsdonotequateto2229continuouseruptiveactivity,butinsteadaremeanttoindicatethevariationinlong-2230termpatternsoferuptiveactivity.LabelsareMER-Merapi;LAS–Lascar;COL–2231Colima;SHI–Shiveluch;SAN–Santiaguito;BEZ–Bezymianny;POP–Popocatépetl;2232TUN–Tungurahua;SHV-SoufrièreHillsVolcano;HEL-MountSt.Helens;AUG–2233Augustine;RED–Redoubt;UNZ–Unzen;PEL–Pelée;KUD–Kudryavy.Volcanoes2234withthemostpersistentbehaviourarefoundtowardsthetopofthefigure,andwe2235havehighlightedissueswithspecificallyidentifyingapersistentregimeinolder2236records.TherecordofvolcanicactivityisbasedupontheSmithsoniandatabase2237(SiebertandSimkin,2002),andreferencesspecifictoeachvolcanothatcanbe2238foundinsection3andthesupplementarymaterial.22392240(Singlecolumn)2241Figure3:Representativecartoonsforthetwodifferenteruptiveregimesthatare2242identifiedinthisreview;(a)Episodicbehaviour,wherethedurationavolcano2243remainsinaneruptivestateisproportionallymuchshorterthatthedurationit2244remainsinnon-eruptivestate.Degassingistemporallycorrelatedwitheruptive2245activity,andtheregimeischaracterisedbyperiodsofnoeruptiveinwhich2246degassingisnegligible,whichwedefineasinter-eruptiverepose;(b)Persistent2247

behaviour,wherethedurationavolcanoremainsinaneruptivestateis2248proportionallysimilartothedurationitremainsinnon-eruptivestate.Degassingis2249notnecessarilytemporallycorrelatedwitheruptiveactivity,andtheregimeis2250characterisedbyperiodsofnoeruptiveinwhichdegassingiscontinuousand2251sustained,whichwedefineasintra-eruptiverepose.(c)Athirdmixedregimeis2252characterisedtoidentifyhowavolcanocanexhibitbothepisodicandpersistent2253behaviourinitseruptiverecord.22542255(Doublecolumn)2256Figure4:Ahierarchicalconstructforhistoricaleruptiveactivityatdome-building2257volcanoes.Thefirstsub-levelofthisconstructidentifiesthetwodifferent2258behaviours,episodicandpersistent.Thesecondsub-levelofthisconstructidentifies2259twodifferentstylesofepisodicandpersistentbehaviourthatareobservedin2260historicalrecords,overidenticaltimescales(i.e.betweenpointsaandb).Key2261characteristicsforeachbehaviourareidentifiedintheboxesbeloweachcartoon.226222632264(Doublecolumn)2265Figure5:(a)EpisodicbehaviouratAugustinebetween1970and2008,consistingof2266foureruptiveepisodeslastingmonths(redlinesrepresentonsets),adaptedfrom2267PowerandLalla,(2010).SO2degassing(orange)istemporallycorrelatedwiththe2268eruptiveepisodes,asindicatedbythedatafromMcGeeetal.,(2010),overlaidonthe2269lowerchart.Blackbarsrepresentseismicity,whichiselevatedpriorandduring2270eruptiveepisodes;(b)PersistentbehavioratMerapibetween1990and2006,with2271severalphasesofdomegrowth(bluebars)andassociatedexplosions(bluevertical2272arrows),adaptedfromRatdomopurboetal.,(2013).SO2degassing(orange)is2273temporallyuncorrelatedwitheruptiveactivity,asobservedbytheoverlaiddata2274between1992and1998.Seismicityiscorrelatedwithphasesoferuptiveactivity,as2275indicatedbythevariationinthecumulativeseismicenergy(redline).22762277(Singlecolumn)2278Figure6:Estimatedeffusionrate(bluedots)atUnzenbetween1990-1995,from2279Nakadaetal.(1999).ThisisanexampleofasingleeruptiveepisodeatUnzenthat2280lasted5yearsbetween1990-1995(Fig.2).Thelatterstagesoftheeruptiveepisode2281arecharacterisedbycrystal-richlavasandloweffusionrates.Duringtheeruptive2282episode,however,thereareperiodicincreasesineffusionrate,suchasin1993.22832284(Singlecolumn)2285Figure7:Estimatedextrusionrates(bluedots)for23phasesofdomegrowthat2286Bezymiannyvolcanobetween1993and2008,fromvanManenetal.,(2010).This2287patternofactivityisanexampleofapersistentregime,inwhichfrequentperiodsof2288dome-growthoccur,withaconsistentlong-termextrusionrate.However,the2289intensityandfrequencyofphasesofdomegrowthcanvary.Thereddashedline2290indicatesthecumulativeextrudedvolume,inwhichperiodsofdomegrowthand2291reposecanbeobserved.22922293

2294(Singlecolumn)2295Figure8:Exampleofaconceptualmodelforeruptiveactivityassociatedwiththe2296shallowchamberparadigmatLaSoufrière,Guadeloupe,adaptedfromHincksetal.2297(2014),wheregeophysicalandgeochemicalobservationsatthesurfaceare2298interpretedintermsofshallowcrustalmagmaticprocesses.22992300(Singlecolumn)2301Figure9:Schematicfortheinteractionofmeltlayersinatranscrustalmagmatic2302systematlavadome-buildingvolcanoes.Possiblescenariosforeruptiveactivityand2303volcanicunrest;(a)completedestabilisationofthetran-scrustalsystem,involving2304deeplysourcedmaficmeltsthatprovidevolatilesandheat,resultinginmajor2305explosiveactivity;(b)partialdestabilisationofthetranscrustalsysteminvolving2306magmastoredinshallowcrustalregionsresultingineffusiveandminorexplosive2307activity;(c)partialdestabilisationofthemagmaticsystemresultinginvolcanic2308unrestbutnoteruptiveactivity.Importantly,thisisinnowayatruerepresentation2309ofthestructureanddimensionsofmagmaticsystemsatlavadome-building2310volcanoesastheyarefoundinsubductionzones.Indeed,perpendiculartotectonic2311platemarginsthearcwidthsofactivevolcanismaregenerallyverynarrow(~5km2312orless).23132314231523162317231823192320232123222323

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