Anatomy of a submarine pyroclastic flow and associated ...ocean.tongji.edu.cn/.../2014/...2008-Sedimentology.pdf · volcanic arc andesite volcanism. The current eruption began in

Anatomy of a submarine pyroclastic flow and associatedturbidity current: July 2003 dome collapse, Soufriere Hillsvolcano, Montserrat, West Indies

JESSICA TROFIMOVS, R. STEPHEN JOHN SPARKS and PETER J. TALLINGDepartment of Earth Sciences, University of Bristol, UK (E-mail: [email protected])

Associate Editor: Mike Branney

ABSTRACT

The 12 to 13 July 2003 andesite lava dome collapse at the Soufriere Hills

volcano, Montserrat, provides the first opportunity to document

comprehensively both the sub-aerial and submarine sequence of events for

an eruption. Numerous pyroclastic flows entered the ocean during the

collapse, depositing approximately 90% of the total material into the

submarine environment. During peak collapse conditions, as the main flow

penetrated the air–ocean interface, phreatic explosions were observed and a

surge cloud decoupled from the main flow body to travel 2 to 3 km over the

ocean surface before settling. The bulk of the flow was submerged and rapidly

mixed with sea water forming a water-saturated mass flow. Efficient sorting

and physical differentiation occurred within the flow before initial deposition

at 500 m water depth. The coarsest components (�60% of the total volume)

were deposited proximally from a dense granular flow, while the finer

components (�40%) were efficiently elutriated into the overlying part of the

flow, which evolved into a far-reaching turbidity current.

Keywords Granular flow, Montserrat, Soufriere Hills volcano, submarinepyroclastic flow, turbidity current.

INTRODUCTION

On 12 to 13 July 2003, the Soufriere Hills volcano,Montserrat, produced the largest ever docu-mented lava dome collapse. A sophisticatedmonitoring network detailed the sub-aerialexpression of the dome collapse (Herd et al.,2006; Voight et al., 2006). More than 210 ·106 m3 of pyroclastic material avalanched downthe Tar River Valley (Fig. 1), to be deposited intothe ocean. As the pyroclastic flow crossed theland–ocean interface, steam explosions and alarge hydrovolcanic explosion resulted (Edmonds& Herd, 2005). Until recently, nothing was knownabout the July 2003 collapse deposits or behaviourof the flow after it entered the ocean.

The inaccessibility of modern in situ submarinepyroclastic deposits is a common difficulty thathas resulted in only a small number of previousstudies. For example, Carey & Sigurdsson (1980)and Whitham (1989) detail the composition of

sub-aerially sourced pyroclasts deposited into asubmarine environment from the Roseau eruption,Dominica. Mandeville et al. (1994) describe theemplacement of pyroclastic flows into a shallowmarine environment from SCUBA dive coring ofthe 1883 Krakatau volcanic eruption. Pioneeringstudies such as these are rare for modern eruptionsand the majority of data in this field come fromancient submarine successions, which provideimportant three-dimensional deposit information.For example, Cole & DeCelles (1991) studied thetransition of an early Miocene pyroclastic flowfrom a sub-aerial to submarine environment. Theflow partially mixed with the water columnresulting in quench fragmentation. White &McPhie (1997) document a submarine ignimbritedeposit that preserves evidence of welding and,therefore, heat retention within the sub-aqueousflow. Kokelaar & Koniger (2000) detail the PittsHead Tuff, a sub-aerially sourced ignimbrite thatwas deposited into the ocean, and Kokelaar et al.

Sedimentology (2008) 55, 617–634 doi: 10.1111/j.1365-3091.2007.00914.x

� 2007 The Authors. Journal compilation � 2007 International Association of Sedimentologists 617

(2007) reconstruct the Pavey Ark ignimbritedeposit from the Scafell caldera, England, whichis the result of a pyroclastic density current thatwas deposited into a caldera lake.

There are relatively few detailed studiesof submarine pyroclastic flow deposits, thecharacteristics of which vary greatly. Interest insubmarine pyroclastic flows commonly centres onwhether a pyroclastic flow can remain gassupported and retain heat in the sub-aqueousenvironment (e.g. Sparks et al., 1980a,b; Kokelaar& Busby, 1992) or if it ingests significant amounts ofwater to form cool, water-supported density cur-rents (e.g. Whitham, 1989; Trofimovs et al., 2006).

In May 2005, a research voyage of the RRS JamesClark Ross provided an opportunity to selectivelysample the submarine deposits from the July2003 dome collapse. A Vibrocore-based drilling

programme sampled the longitudinal flow axis,together with lateral transects across the proximal,medial and distal regions of the flow deposits.Fifty-two cores were recovered for facies analysis.

This paper documents the internal architectureof the submarine dome collapse deposits, includ-ing detail on deposit morphology and volume,sedimentary structures, grain-size trends andrelationships to sea floor gradients; and discusseshow these characteristics are related to the sourcematerial, the character of the flow entering theocean, and submarine flow conditions.

GEOLOGICAL BACKGROUND

The Soufriere Hills volcano, Montserrat, providesan excellent natural laboratory for the study of

Fig. 1. Map showing the location ofMontserrat (inset), geographicalfeatures, sea floor topography andVibrocore hole numbers and loca-tions. Contours are shown in metres.

618 J. Trofimovs et al.

� 2007 The Authors. Journal compilation � 2007 International Association of Sedimentologists, Sedimentology, 55, 617–634

volcanic arc andesite volcanism. The currenteruption began in 1995, following three years ofprecursor seismic activity. A monitoring networkwas established at the beginning of the eruptionand all sub-aerial activity since 1995 has beenmeticulously recorded. Volcanic activity at theSoufriere Hills volcano is dominated by lavadome growth and collapse, with 10 major col-lapses (and numerous minor events) occurringduring the period from 1995 to May 2006 (e.g.Cole et al., 2002; Hart et al., 2004; Herd et al.,2006). By May 2007, 0Æ7 km3 of magma had beenextruded.

The most voluminous collapse during the cur-rent eruption occurred on 12 to 13 July 2003.Direct observation and real-time seismic ampli-tude measurements (RSAM) facilitated a detaileddescription of the sub-aerial expression of thecollapse (Edmonds & Herd, 2005; Herd et al.,2006). The andesite lava dome had been growingfrom August 2001 to June 2003 at a variable ratenot exceeding 4 m3 sec)1 (Herd et al., 2006). On12 July 2003, the collapse began with 15 h of low-volume pyroclastic flow activity that transported10 · 106 m3 of dominantly cold talus down theTar River Valley to the ocean. This period wastermed stage 1 (of 4) by Herd et al. (2006). Stage 2involved 3 h of increased pyroclastic flow activity,producing large discrete pyroclastic flows sourcedfrom the hot lava dome. The pyroclastic flowsseparated upon entrance into the ocean whereinthe dense base was immediately submerged andthe dilute upper part of the flow passed over theocean surface for hundreds of metres beforesettling. Stages 1 and 2 involved a combined totalof 30 · 106 m3 of volcaniclastic debris, theremoval of which undermined the central part ofthe dome causing instability (Herd et al., 2006).

Stage 3 involved the catastrophic dome failure.Two hours and forty minutes of semi-continuouspyroclastic flow activity removed �170 · 106 m3

from the core of the dome and transported it intothe ocean with an average flux of 1 · 106 m3 min)1.At the peak of the collapse, lava was removed at�6to 10 · 106 m3 min)1 for 2 min (Herd et al., 2006).Pyroclastic flows descended the Tar River Valleyand crossed the Tar River Fan as thick, dense andlinearly focussed flows before entering the sea.Hydrovolcanic explosions were produced at theshoreline and concurrent dilute currents travelled2 to 3 km over the ocean surface before settling(Edmonds & Herd, 2005). Decompression of themagma chamber during stage 3 pyroclastic flowactivity produced a Vulcanian explosion (Herdet al., 2006). A 0Æ5 to 1Æ0 m tsunami formed in

response to this stage and was recorded on theshores of Guadeloupe, approximately 65 km to thesouth-east of Montserrat (Herd et al., 2006). Stage 4represents a decline in activity, when small-volume, slope-stabilizing pyroclastic flows, total-ling�10 · 106 m3 of material, occurred for severalhours after the main collapse (Herd et al., 2006).

Most of the �210 · 106 m3 of pyroclastic mate-rial from the July 2003 dome collapse wasdeposited in the ocean (�90%; Herd et al.,2006). There has been little study of the sub-aerial deposits, largely due to elevated levels ofongoing activity (May 2007). Herd et al. (2006)describe the tephra from the Vulcanian explosionand Edmonds & Herd (2005) details a hot, drypyroclastic density current that flowed 4 kminland, scaling 300 m of topography; this resultedfrom explosive interaction between pyroclasticflow material and sea water at shore. The depositcomprises a coarse-grained, fines-poor basal layerof lava clasts and carbonized vegetation, with afiner-grained, better-sorted unit overlying this.This upper unit exhibits a massive lower divisionand a stratified or cross-stratified top. Accretion-ary lapilli occur within the upper layer (Edmonds& Herd, 2005).

Detailed studies of earlier sub-aerial block-and-ash pyroclastic flow deposits from the SoufriereHills eruption (Cole et al., 2002) find a range ofdeposit morphologies from narrow (<20 m wide),small-volume lobes, commonly with bulbousfronts and well-developed levees; to thick, valleyconfined deposits with flat upper surfaces; tosheet-like deposits with flat lying front andmarginal terminations. These deposit styles areobserved in the proximal, medial and distalregions from the source lava dome, respectively(Cole et al., 2002). Generally, the material formingthe block-and-ash flow deposits is ungraded,although rare occurrences of reverse-graded basesand normally graded tops have been documented,such as in the 21 September 1997 block and ashflow deposit (Cole et al., 2002). The deposits arepoorly sorted mixtures of metre-scale blocks tofine ash. Clasts are dominantly hornblende-phy-ric and plagioclase-phyric andesite, with sub-ordinate (�1%) mafic inclusions and pumice. Thedeposits contain over 50% ash (<2 mm) including10% fine ash (<1/16 mm).

METHODS

The 9 to 18 May 2005 research voyage of the RRSJames Clark Ross used a Vibrocore system

A submarine pyroclastic flow and associated turbidity current 619


[developed by the British Geological Survey(Marine Operations), Edinburgh, UK] to sample52 sites across and along the flow axis of the 2003dome collapse deposit (Fig. 1). Medially anddistally from Montserrat, cores up to 5 m inlength were consistently recovered in waterdepths less than 2000 m. However, within thecoarse-grained proximal pyroclastic deposits onlya single 1Æ5 m core was recovered and the base ofthe deposit was not intersected. The Vibrocoresampled unconsolidated volcaniclastic materialwith clast diameters up to and including 10 cm.Real-time, onboard analysis of the Vibrocorepenetration rate against time allowed the maxi-mum penetration to be achieved in both fine-grained and coarse-grained materials. Visualobservation of the sea floor via a video cameramounted on the coring rig aided in the selectionof suitable coring sites, on shallow slopes withoutrough topography.

The recovered sediment cores were logged atscales appropriate for recording detail and sam-pled for grain-size and component analysis. Thecoarse-grained nature of the volcaniclastic mate-rial necessitated the use of nested sieve sets forgrain-size analysis. Phi (u) sieve intervals wereused between 64 and 4 mm ()6u to )2u), and half-phi intervals were used between 4 and 0Æ045 mm()1Æ5u to 4Æ5u). The cores were sampled at centi-metre-scale intervals and dried at 90 �C for 48 hbefore being gently disaggregated with a rubberpestle and mortar and dry sieved. Bulk pointcounting of a minimum of 500 grains from eachsample was used to determine component abun-dance. Point counting a minimum of 500 grains foreach phi or half-phi interval was undertaken forsamples at the top and base of the July 2003 unit, ineach of the studied cores, to ascertain the distri-bution of each grain type according to its size.

The July 2003 unit was assumed to representthe last major episode of sedimentation in thecores. In proximal locations, the deposit could beidentified unambiguously using sea floor bathym-etry difference maps (Fig. 2).

GEOMETRY AND VOLUME OF THESUBMARINE DEPOSIT

Submarine pyroclastic debris from the July 2003dome collapse extends from the shoreline belowthe Tar River Valley for approximately 40 kmsouth-east down the Boulliante–Montserratgraben (Fig. 2A). The thickest and the mostproximal deposits have been distinguished by

successive bathymetric surveys of the Tar Riversubmarine fan in 2002 and 2005 (Deplus et al.,2001; Trofimovs et al., 2006). These surveyshighlight the distribution and geometry of thedeposits from the July 2003 dome collapse

Fig. 2. (A) Isopach map showing the thickness distri-bution of the submarine deposits of the July 2003 domecollapse event. Isopach contours are as marked inmetres and centimetres. Individual core thicknessmeasurements are given in centimetres. (B) Topo-graphic difference map highlighting the thickness ofmaterial deposited during the July 2003 dome collapseevent. Two lobate pyroclastic deposits are shown; thesingle flow-axis northern lobe and the composite-flowsouthern lobe.



(Fig. 2B). The deposits start at around 500 mbelow sea-level, at the first break in slope, wherethe gradient decreases from 37� to 9Æ5� (Fig. 3).The dome collapse flows produced two linearpyroclastic lobes up to 60 m thick (Trofimovset al., 2006), which form part of the Tar Riversubmarine fan (Fig. 2B). The southern pyroclasticlobe comprises multiple flow deposits exhibitingvaried flow axes, and represents an amalgamationof small-volume deposits from the first two stagesof the collapse (cf. Herd et al., 2006). The north-ern lobe, formed by diversion of the flow aroundthe previously emplaced southern flow deposits,comprises a single deposit axis; this correspondsto the peak collapse conditions (stage 3; Herdet al., 2006) wherein 170 · 106 m3 of pyroclasticmaterial was deposited semi-continuously. Thevolume of pyroclastic material in the two lobes,estimated for deposits thicker than 10 m, approx-imates 100 · 106 m3 (Trofimovs et al., 2006).

The proximal pyroclastic lobes exhibit steepmargins (Fig. 2B) and a sharply tapering frontregion (Fig. 4), where the deposit rapidly thinsfrom approximately 10 m to 84 cm over adistance of 500 m. This thinning occurs at asecond break in slope where the gradient furtherdecreases from 7Æ25� to 2Æ5� (Fig. 3). Downstreamfrom the proximal pyroclastic ridges, on sea floorslopes of around 1� or less, is a thin, 84 to 7 cm,sheet-like pyroclastic deposit that extends�40 km from the shore (Fig. 2A), and has avolume of �90 · 106 m3. Thus, the submarinedeposits from the July 2003 dome collapse total�190 · 106 m3; this equates to 171 · 106 m3 ofdense rock equivalent (DRE), using a measuredaverage clast density of 2000 kg m)3 and averagesubmarine sediment density (measured whendried) as 1800 kg m)3. Approximately 60% ofthe total volume is located within 6 km fromshore, in deposits thicker than 1 m dominantlycomprising the thick, linear pyroclastic lobes.Forty per cent of the material comprises the sheet-like deposit located in more distal regions.

INTERNAL ARCHITECTURE OF THESUBMARINE DEPOSIT

Stratigraphic logs from along the axis of the July2003 deposit show that it thins and fines towardsthe more distal regions (Fig. 3). A longitudinalprofile section exhibits a step-like geometry(Fig. 5), recognized in the marine cores by abruptdeposit thinning, and imaged by bathymetricsurveys and topographic parametric source

(TOPAS) shallow seismic profiles (e.g. Fig. 4).Each ‘step’ corresponds with a distinct sedimen-tary facies change. Three main facies are recogni-sed: (i) thick, metre-scale, deposits of moderatelyto poorly sorted, fines-poor pyroclastic materialcomprising metre-scale blocks to sand gradeparticles; (ii) centimetre-scale deposits of a mod-erately well to poorly sorted, fines-poor unit ofgravel and sand grade material; and (iii) a thin,centimetre-scale, moderately to well-sorted, sandand silt dominant deposit.

Facies 1: proximal, blocky pyroclastic lobes

The deposits comprising Facies 1 were emplacedwithin 5 km from the shore, forming the bulk ofthe proximal pyroclastic lobe deposits (Fig. 3).Visual analysis of the sea floor via a rig-mountedvideo camera revealed metre-scale lava blocks orblock tips on the sea bed. Vibrocoring wasdifficult within this facies as the rig had to bepositioned between the largest blocks and,consequently, only one core was recovered.Vibrocore JR123-23-V (Fig. 6) sampled the north-ernmost pyroclastic lobe from the July 2003 domecollapse event. The thick and coarse-grainednature of the deposit did not allow core penetra-tion further than 1Æ5 m.

Sub-sampling of the core reveals a moderatelyto poorly sorted (1Æ12 to 2Æ22ru), angular tosub-angular breccia. Crude normal grading ispreserved within the lower part of the core, andwell-developed normal grading is preserved inthe moderately sorted (0Æ93ru), sandy top (Fig. 6).Grain-sizes range from )5u to 4Æ5u and areskewed towards the coarse-grained material withmedian clast sizes ranging from )5u to 1Æ5u.Large clasts are concentrated within distincthorizons, or chaotically distributed throughoutthe core. However, the sample comprises only thematrix between metre-scale blocks imaged on thesea floor. The sample is fines poor, containing<1 wt% silt-sized (<0Æ063 mm) particles. Approx-imately half of the total clast population arejuvenile andesite lava clasts of two types: 45% isplagioclase-phyric and hornblende-phyric with aslightly vesicular (10% to 15%) microcrystallinegroundmass, while 5% are dark grey to black,dominantly glassy clasts with low vesicularity(1% to 2%). Hydrothermally altered porphyriticandesite clasts, commonly exhibiting iron-stain-ing and (or) sulphur-staining and sulphide min-eral overgrowth, represent �20% of the clasts.Broken plagioclase, hornblende and minor pyrox-ene crystals comprise �25% of the clasts and



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dominate the finer (medium-to-fine sand) grain-size fractions. Subordinate (<1%) pyroxene andplagioclase dominant mafic clasts are observed. Abiogenic component (<3%), eroded and incorpo-rated from the substrate and comprising coral andshell fragments is randomly distributed through-out the deposit. There is little vertical variation inparticle composition (Fig. 6). The presence ofchaotically arranged centimetre-scale clasts,metre-scale blocks, coarse matrix grain-size andthe significantly thicker, lobate deposit shapedistinguish this facies from Facies 2 and 3.

Facies 2: medial, sand and gravel deposits

Deposited at the front of the northern pyroclasticlobe, approximately 5 km from the shore, is an 84

Fig. 4. Topographic parametric source (TOPAS) shal-low seismic profile exhibiting the sharply taperingmorphology of the front margin of a pyroclastic lobedeposit. Section imaged between 16�43¢69¢¢ N,62�02¢20¢¢ W and 16�43¢14¢¢ N, 62�02¢79¢¢ W (seeFig. 1).

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Fig. 5. Schematic cross-section through the July 2003 submarine deposits depicting the step-like deposit mor-phology and the three sedimentary facies observed. Grain-size distribution histograms are provided for the top andbase of the deposits, highlighting the systematic fining with distance from source. Photograph A shows the coarse-grained nature of the lava clasts (dark grey) and bioclastic sediment (white clasts) from Facies 1 (JR123-23-V, depthinterval: 148 to 153 cm). Photograph B exhibits the coarse-grained sand and granules that comprise Facies 2 (JR123-25-V, depth interval: 4 to 9 cm). Photograph C shows a fine-grained deposit from Facies 3 (JR123-7-V, depth interval:0 to 7 cm). Laminae are defined by high (light-coloured bands) and low (dark-coloured bands) abundances of bio-genic material. Weak cross-stratification is preserved in the centre of the photograph.



to 28 cm thick unit of sand with subordinategravel (Cores 24-V, 25-V and 15-V; Figs 3 and 7).The unit systematically thins and fines withdistance from source. Grain-size ranges from)0Æ5u to <4Æ5u, with median diameters between1Æ5u and 4u. This facies ranges from moderatelywell to poorly sorted (0Æ59 to 1Æ36 ru) and iscommonly massive. Rare, centimetre-scale crudestratification is defined by coarse-grained, fines-poor laminations. A normally graded top isobserved in all cores. The base of the most distalcore (site 15-V) is inversely graded; otherwisethere are no systematic vertical variations inmedian grain-size (Fig. 7). The deposit is gener-ally fines poor (<2 wt% silt-sized particles), withthe exception of the normally graded top of thedeposit, which contains between 22 and 41 wt%silt-sized particles. This facies is dominated byunaltered juvenile andesite (40% to 50%), with20% to 30% hydrothermally altered porphyriticlava dome material. An inherited bioclastic com-ponent (<2%) is distributed throughout the unit.This facies can be distinguished from Facies 3 bythe lack of fine-scale laminations and generallycoarser grain-size.

Facies 3: distal, sheet-like, sand and siltdeposits

This facies was sampled between 8 and 40 kmfrom shore, where it extends towards the south-east as a thin (centimetre-scale) sheet-like depositwithin the Boulliante–Montserrat graben. Thefacies is characterized by moderately to well-sorted (0Æ64 to 0Æ86ru), sand and silt-sized parti-cles (1Æ5u to <4Æ5u; median diameters <4u)(Fig. 8). A reversely graded base is common(Figs 3 and 9), overlain by massive and (or)millimetre-scale planar-laminated deposits. Rare,millimetre-scale cross-stratification occurs insome cores (Fig. 5C). Stratification is defined bybioclast-rich (�5% bioclasts) and bioclast-poor(�1Æ4% bioclasts) laminae. The top of the depositis normally graded and an interval with a highproportion of silt-sized particles (40 to 50 wt%)caps the unit.

The deposit forming Facies 3 thins, fines andbecomes better sorted away from source. Ahigher proportion of bioclasts (shell and coralfragments, pteropods and foraminifera) com-pared with volcanic clasts occurs in the most

Fig. 6. Grain-size and component analysis of Vibrocore JR123-23-V (Facies 1). VJDC, vesicular, juvenile dome clasts;N-VJDC, non-vesicular, juvenile dome clasts; ADC, altered dome clasts; C, crystal fragments; BC, bioclasts; M, maficclasts; n, number of particles counted.



distal regions (1% to 5%; Fig. 8). The bioclasticmaterial dominates the larger grain-size fractions(0u to 0Æ5u), whereas the juvenile and altereddome rock and crystal fragments, dominantlycomprise the fine sand to silt size fractions.Cores from sections perpendicular to the mainflow axis (Fig. 9) show the deposit becomingthinner and finer towards the lateral margins.The main flow axis, represented by the thicker,coarser grained, least sorted material, is locatedon the eastern side of the Boulliante–Montserratgraben. Only the finer edges of the flow havereached the margins of the topographic depo-centre.

Comparison of submarine and sub-aerialblock-and-ash deposits

The submarine pyroclastic deposits offshore fromMontserrat exhibit an increase in the degree ofsorting from the proximal to the distal environ-ments (Fig. 10A). Comparison with sub-aerialblock-and-ash deposits from the Soufriere Hillsvolcano (Cole et al., 2002) shows that the mostpoorly sorted submarine deposits (Facies 1)exhibit better sorting than their sub-aerial coun-terparts.

The total weight per cent of fine ash(<0Æ063 mm) preserved within the submarine

Fig. 7. Grain-size and component analysis of Vibrocores JR123-24-V, JR123-25-V and JR123-15-V, which compriseFacies 2 of the July 2003 dome collapse deposits. Abbreviations as for Fig. 6.



deposits (�2 wt%) lies within the lower part ofthe range covered by the sub-aerial depositsmeasured by Cole et al. (2002) (Fig. 10B). How-ever, when the samples are subdivided torepresent the proximal, medial and distal faciesobserved, there is an obvious partitioning of theash content within the submarine deposits; theproximal facies contains <1 wt% fine ash, far lessthan the average ash content for either the sub-aerial pyroclastic flows or the total grain popula-tion of the submarine deposit. The majority of thefine ash has been deposited in the distal facies,predominantly within Facies 3 (Fig. 10B).

DISCUSSION AND INTERPRETATIONS

The sub-aerial expression of the July 2003 lavadome collapse has been well-documented (Ed-monds & Herd, 2005; Herd et al., 2006). For thefirst time, the submarine sequence of events canbe included in the reconstruction of the collapsechronology (Fig. 11). Depositional processes

recorded by Facies 1, 2 and 3 are analysed below.The term ‘granular flow’ is used to denote a flowin which grain-to-grain interactions dominateparticle support (Iverson, 1997; Iverson &Vallance, 2001). The term ‘turbidity current’ isused to denote a more dilute flow in which fluidturbulence is the dominant support mechanism(Kneller & Buckee, 2000; Mulder & Alexander,2001), and in which the deposit accumulatesprogressively (cf. Branney & Kokelaar, 1992).

Deposition process for Facies 1: high particleconcentration granular flow

Imaging and sampling of the July 2003 domecollapse deposits reveal that, as the pyroclasticflows entered the ocean, they remained linearlyfocussed, depositing steep-sided lobes of coarse-grained pyroclastic material. This planformdeposit geometry characterizes dense granularflows with high sediment concentrations, asobserved in the field and in experimental flows(Johnson, 1970; Iverson, 1997; Major, 1997;

Fig. 8. Grain-size and component analysis of Vibrocores JR123-10-V and JR123-5-V, which comprise Facies 3 of theJuly 2003 dome, collapse deposits. Abbreviations as for Fig. 6.



Fig. 9. Topographic profiles and correlative stratigraphic logs for two transects located perpendicular to the mainflow axis (see inset map for section locations). Deposits thin and fine upslope towards the margins of the Boulliante–Montserrat graben.



Iverson & Vallance, 2001). The flows lackedcohesive fine-grained sediment. Frictional inter-locking of clasts may have caused the flows to‘freeze’ abruptly at their margins, generating thick,steep-sided lobes. Visual analysis (by a rig-mounted video camera) and a single core withinthese deposits suggest that the deposits are poorlygraded and contain randomly distributed outsizeclasts; this is the result of high sediment concen-trations suppressing differential settling of thelarge and small particles. Horizons of outsizeclasts may record emplacement by waxing andwaning of the flow during continuous, progressivedeposition. The finer-grained upper part of thedeposit has well-developed normal grading. Thisfinal episode of deposition occurred from a moredilute flow phase, during which large and smallparticles were able to segregate during deposition.

Facies 1 deposits are morphologically similar tothe sub-aerial pyroclastic flow deposits on Mont-serrat (cf. Cole et al., 2002). The submarine

deposits, however, contain a much smaller frac-tion of fine particles (<2 wt% of <0Æ063 mmparticles; Fig. 10); this contrasts markedly withthe poorly sorted deposits on land that contain>50% ash (<2 mm) including 10% fine ash(<0Æ063 mm; Cole et al., 2002). Efficient removalof the finest particles must have occurred under-water or as the pyroclastic flow entered the ocean.Mechanisms for this removal of the fine particlesare discussed below. The ubiquitous inheritedbioclastic material distributed throughout theFacies 1 deposit demonstrates that the flow wascapable of submarine erosion and that efficientvertical mixing occurred as the flow moved alongthe ocean floor.

Deposition process of Facies 2

The deposits immediately in front of the marginsof the thick granular flow lobes comprise pre-dominantly of massive sand, with subordinategravel and silt (Facies 2). The origin of massivesubmarine sand deposits has been the subject ofconsiderable debate (Kneller & Buckee, 2000;Shanmugam, 2000). It has been proposed thatthese deposits accumulate progressively from atemporally steady and relatively dilute turbiditycurrent (Kneller & Branney, 1995); or that suchintervals are emplaced from dense non-cohesivegranular flows (Shanmugam, 2000). Randomlyarranged gravel clasts in Core 24-V favour enmasse emplacement by a dense flow, whichsuppresses differential settling of outsize clasts.Occurrence of gravel clasts along discretehorizons suggests layer-by-layer accumulationfrom a more dilute flow, as observed in the moredistal cores (25-V and 15-V; Fig. 3). Basal inversegrading in Core 15-V could result from kineticsieving within a dense granular flow (Legros,2002). However, inverse grading could also begenerated by a waxing turbidity current. Thislatter origin seems more likely as inverse gradingalso occurs in the adjacent Facies 3, which moreclearly results from turbidity current deposition.The thickness of Facies 2 and Facies 3 deposits issimilar, also favouring the view that Facies 2progressively aggraded from a turbidity current.The strong normal grading in the upper part ofFacies 2 is attributable to deposition from awaning turbidity current. A lack of planar orcross-lamination in this upper normally gradedinterval does not preclude deposition from aturbidity current. Such structures are commonlysuppressed in turbidites when grain-sizes aresufficiently coarse (Kuenen & Humbert, 1969)

Fig. 10. (A) Median grain-size diameter (Mdu) versussorting (ru) for the marine sediment samples, separatedinto Facies 1, 2 and 3. Data from Cole et al. (2002) arepresented showing the range for sub-aerial block-and-ash flow deposits at the Soufriere Hills volcano. (B)Median grain-size diameter (Mdu) versus wt% of fineash (<0Æ063 mm) for the marine sediment samples,separated into Facies 1, 2 and 3. Data from Cole et al.(2002) are presented to show the ash content of thesub-aerial block-and-ash flows. The total grain-sizepopulation for the July 2003 dome collapse deposit ispresented.



and when sediment fallout rates are rapid (Arnott& Hand, 1989).

Deposition process of Facies 3

Facies 3 displays planar lamination and occa-sional ripple cross-lamination, typical of Boumadivisions b, c and d (Figs 3, 5C and 9). Thedeposit is much more extensive and tabular thanFacies 1 and the well-developed vertical grading,together with tractional features, indicate layer-by-layer deposition from a turbidity current.

The lack of internal grain-size discontinuitiessuggests that there was no influence from currentsreflected by the basin margins (cf. Pickering et al.,1992), which is somewhat surprising given thepresence of steep topographic boundaries withinthe studied area. The deposit extends laterallyacross sea floor topography with up to 300 m ofvertical relief (Figs 3 and 9), indicating a com-bined record of the vertical thickness of the flow

and its ability to run up-slope (Muck &Underwood, 1990).

Gradient and run-out distance

The proximal pyroclastic lobes (Facies 1) weredeposited on gradients of 9Æ5� to 7Æ25�. Thesegradients are comparable to slopes upon whichcoarse-grained sub-aerial pyroclastic flows termi-nate. For example, at El Chichon, Mexico, coarse-grained lithic breccias are deposited after breaksin slope on gradients between 11� and 3� (Macıaset al., 1998). Lithic-rich pyroclastic flows termi-nate on slopes up to 14� at Lascar, Chile, whereasthe pumice-rich flows deposit on slopes of <4�(Sparks et al., 1997). Thin, pumice-rich pyroclas-tic flows at Pinatubo, Philippines, terminate onlesser slopes, between 0Æ5� and 2� (Scott et al.,1996). The submarine granular flows that formedFacies 1 appear to have had a similar mobility togas-supported sub-aerial pyroclastic flows of

Fig. 11. Reconstruction of the sub-aerial and submarine events involved in the July 2003 dome collapse at theSoufriere Hills volcano.



similar volumes. The finer-grained submarinematerial at Montserrat (Facies 2 and 3) is depos-ited on gradients between 2Æ5� and 0Æ2�.

Turbidity currents in general can travel formuch longer distances (>>100 km) than sub-aerial pyroclastic flows, and are capable of flow-ing across lower gradients (<0Æ2�; Elmore et al.,1979; Piper et al., 1999; Rothwell et al., 2000;Wynn et al., 2002). Turbidites with similar vol-umes to the July 2003 dome collapse depositextend for over 100 km in some ancient turbiditesequences (Talling et al., 2007). The turbiditycurrent generated by this dome collapse isunusual in that it contained almost no fine-grained cohesive sediment. Laboratory experi-ments indicate that the presence of a smallfraction of fine-grained sediment can substan-tially increase the run-out distance of a turbiditycurrent (Gladstone et al., 1998). However, thesenon-channelized fines-poor turbidity currents offMontserrat were still capable of flowing for morethan 30 km, across the sea floor with a gradient of<1Æ0� (Fig. 3). Therefore, the lack of fine-grainedsediment does not appear to have reduced theflow mobility significantly.

Origin of inverse grading in Facies 2 and 3

Turbidite Facies 2 and 3 display inverse gradingat their base. The thickness of this inverselygraded interval is variable, ranging from 2 to10 cm. Basal inverse grading is uncommon inturbidites, although it is observed on rare occa-sions in deposits from varied settings (Mulderet al., 2001; Mutti et al., 2002; Nakajima, 2006).

A number of mechanisms can generate basalinverse grading. Kinetic sieving may lead toinversely graded deposits from thick granularflows (Legros, 2002). However, as discussed pre-viously, Facies 2 and 3 do not represent deposi-tion from such a thick granular flow and thismechanism is therefore discounted. Turbiditycurrents can generate dense traction carpets ofsediment near their base (Hiscott, 1994; Sohn,1997). It has been proposed that these tractioncarpets produce multiple, thin, inversely gradedintervals with abrupt tops (Hiscott, 1994). Asmooth upward change from fine silt to sand-sized particles is observed in the majority of theJuly 2003 dome collapse deposits, with no abruptbreaks in grain-size. This change suggests that theinverse grading did not originate from tractioncarpets. Rapid suspended load fallout may trapfiner particles such that decreasing fallout ratesgenerate inverse grading (Sylvester & Lowe,

2004). However, this process does not lead toinverse grading of coarsest fraction of the grain-size distribution, as is observed in the July 2003dome collapse turbidite.

The inverse grading appears to record longitu-dinal changes in grain-size from the front to therear of the flow. Longitudinal grading can form byincorporation of eroded fine-grained sedimentinto the front of the flow (Mulder et al., 2001),or as a result of coarser particles being transportedat slower rates than fine particles, such that thecoarser grains progressively lag behind the headof the flow (Hand, 1997). However, changes in theflux of sediment entering the ocean are the mostprobable cause for basal inverse grading. Thepeak of the July 2003 dome collapse involvedsemi-continuous pyroclastic flow activity for2 h:40 min (phase 3; Herd et al., 2006). Duringthis time, the removal of pyroclastic materialwaxed to a peak of 6 to 10 · 106 m3 min)1 beforewaning into small-scale slope stabilising flows.This waxing and waning of a continuous sedi-ment flux into the ocean appears to be recordedwithin the turbidite, which exhibits an inverselygraded base and normally graded top.

Deposit volumes

The total submarine deposit volume obtainedfrom the pre-collapse and post-collapse sea floorsurveys and sampling (171 · 106 m3 DRE) issomewhat less than the sub-aerial volume quotedfor the collapse (210 · 106 m3; Herd et al., 2006).The submarine volume is a minimum estimate, asthe surveying vessel could not operate at waterdepths <300 m. However, the main differencebetween the estimates probably results from theloss of the finest ash fraction (<0Æ063 mm). Thisfraction is predominantly missing from the sub-marine deposits, with the exception of a propor-tion maintained in the most distal sandy turbiditefacies. Cole et al. (2002) record that sub-aerialpyroclastic flow deposits on Montserrat typicallycontain around 10% fine ash, which would be�20 · 106 m3 of fine ash within the July 2003dome collapse flows. During transport 15% of theash is typically elutriated into the dilute overrid-ing cloud (Cole et al., 2002; Horwell et al., 2003)which, in the July 2003 eruption, was observed todecouple from the body of the pyroclastic flow asit entered the ocean and flowed over the watersurface for several kilometres before settling(Edmonds & Herd, 2005). The remaining finescomponent that entered the ocean within themain flow was also efficiently separated from the



coarser material and this study envisages thatmost of the <0Æ063 mm particles were transportedout of the studied area by prevailing currentstowards the west.

Mechanism for efficient removal of fines

The lack of fines seen within the submarinedeposits of the 2003 collapse is unusual whencompared with sub-aerial block-and-ash flows;how were the fines removed so efficiently? Elu-triation of fines occurs within sub-aerial gas-supported flows. However, this elutriation pro-cess is relatively inefficient, as sub-aerial depositstypically contain a higher proportion of finematerial throughout (Fig. 10B). Thorough mixingof hot pyroclasts with water results in rapid heatexchange and steam production. Numeroussmall-scale steam explosions at the shorelineprovide evidence for this, together with the largehydrovolcanic explosion that corresponded withthe main phase of dome collapse (Edmonds &Herd, 2005). The passage of steam through thesubmarine flow, and into the overlying watercolumn, is the most likely mechanism for efficientremoval of fine material from the proximal part ofthe flow. The most proximal core recovered (core23-V) implies that this process occurred in waterdepths of less than 500 m.

Submarine flow: gas or water supported?

A fundamental question is whether sub-aeriallysourced pyroclastic flows entering the oceanremain as hot, gas-supported flows or rapidlyingest a large volume of water becoming water-supported mass flows? Distal Facies 2 and 3 recorddeposition from cold, water-supported flows, asthe deposits resemble turbidites seen in non-volcanic settings. For comparison, a number ofexamples of volcaniclastic turbidite deposits thatoriginated from primary pyroclastic density cur-rents entering the ocean have been identifiedwithin the rock record (e.g. Dali Ash, Wright &Mutti, 1981; Roseau tephra, Carey & Sigurdsson,1980; Whitham, 1989). More ambiguous is theemplacement temperature and support mech-anism for the granular flow deposits of Facies 1.Facies 1 displays vertical grading and bioclastsrecord evidence of submarine erosion and verticalmixing within the flow. These features areobserved in the deposits from sub-aerial hot,gas-supported flows and (or) cold water-supportedflows; therefore, they are not diagnostic of flowtemperature or support mechanism.

Sparks et al. (1980a,b) suggest that pyroclasticflows with a density greater than water will beable to make a smooth transition into a sub-aqueous environment whilst maintaining theirintegrity as hot gas-supported flows in moderatewater depths due to steam production and anincrease in ambient pressure. Mandeville et al.(1994) using thermo-remnant magnetism (TRM)techniques estimate an emplacement temperatureof 475 to 550 �C for pyroclastic flow materialsourced from the 1883 eruption of Krakatau,which was deposited �10 km from shore in water£40 m deep. This deposition suggests that theflow continued as gas-supported during and afterthe transition from a sub-aerial to submarineenvironment. Kokelaar & Koniger (2000) docu-ment deposits from the Pitts Head Tuff, interpret-ing from these that a large-volume pyroclasticflow entered the ocean (‡50 m deep) withoutresulting in significant mixing with the water,thereby remaining a gas-supported flow andretaining its heat (>580 �C); this resulted in sub-aqueous welding of the internal part of theignimbrite upon compaction by burial from aprogressively aggrading current. The non-weldedbasal region is interpreted as record mixing of theleading edge of the current with the watercolumn, and the upper part of the ignimbritewas sub-aerially emplaced (Kokelaar & Koniger,2000). White & McPhie (1997) describe sub-aqueous welding of ignimbrites within theTyndall Group, western Tasmania, where uncom-monly the welded deposit was emplaced in waterdepths interpreted as ‡200 m.

Whitham (1989) reported that pyroclastic flowsfrom the Roseau eruption in Dominica mixedthoroughly with the ambient water after enteringthe ocean, based on an increase in the crystal tolithic ratio in the submarine deposits comparedwith their sub-aerial counterparts. Whitham(1989) hypothesises that this indicates quenchfragmentation of the hot pyroclasts as they mixedwith the water and is suggestive of a water-supported flow. These deposits were sampled atapproximately 3000 m water depth. Cole &DeCelles (1991) document the transition of apyroclastic flow deposit from a sub-aerial tosubmarine environment several hundred metresdeep. The sub-aerial deposits are massive, withlocalized basal inverse grading, oxidized andpartially welded, whereas the submarine depositshave been differentiated according to clast den-sity, and exhibit quench fragmented and perlitictextures and preserve vesicles and gas segregationpipes within the deposit matrix. Cole & DeCelles



(1991) interpret from this that the submarinepyroclastic flow partially mixed with the waterresulting in quench fragmentation of the hotparticles. This process may have preceded acomplete transition to a cool, water-supporteddebris flow.

Pyroclastic flows from sub-aqueous vents of theOhanapecosh Formation, Washington (Fiske,1963), and the Tokiwa Formation, Japan (Fiske &Matsuda, 1964) rapidly mixed with water to pro-duce cool, water-supported sediment gravity flowsthat evolved into turbidity currents. Whereas,Kokelaar & Busby (1992) record welding of glassshards and pumice in tuff deposits from a sub-aqueous explosive eruption; indicative of hot, gas-supported transport and deposition with limitedincorporation of ambient water into the flow.

Experimental modelling of pyroclastic flowsentering the ocean suggests that the flow mixesthoroughly and turbulently with the water form-ing phreatic explosions at the shoreline at tem-peratures greater than 200 �C (Freundt, 2003).Freundt released cool (�100 �C) ash, pumice andlithic pyroclastic flows down a 26� ramp to flowinto a standing body of water. The majority of thematerial was immediately submerged and turbu-lently mixed with the water to become stratifiedaccording to density and grain-size. Coarser anddenser blocks were deposited close to source dueto a rapid decrease in momentum as the flowcontacts the water. The finer particles mix withthe water column in the upper portion of the flowto form turbidity currents. Simultaneously, alofted ash cloud separated from the flow to travelover the water surface. The deposits producedfrom these ‘cool’ experimental runs preserve airpockets trapped in the sediment, indicating onlypartial mixing with the water (Freundt, 2003). Hot(�350 �C) pyroclastic flows also resulted in rapidand thorough mixing with the water column.These runs produced significant phreatic explo-sions, which had the effect of expelling finematerial into the atmosphere and accelerating thespeed of the ash cloud that flowed over the watersurface. The submarine deposits of the ‘hot’experiments produced proximal normally graded,poorly sorted lobes. Medially, two deposit layerswere observed. A basal coarse-grained, normallygraded, fines-depleted layer and an upper fine-grained, normally graded, poorly sorted layer.Freundt (2003) attributes the deposition of thelower layer to dense material rapidly depositedfrom high sediment concentration sedimentplumes, whereas the upper layer was depositedfrom a turbidity current.

McLeod et al. (1999) used saline solution,silicon carbide (SiC) and plastic particles inmethanol to simulate a sub-aerial flow enteringthe ocean. Low-density contrasts between theflow mixture and the water results in significantwater incorporation and the formation of aturbulent cloud that collapses to produce adensity current. At high-density contrasts thesub-aerially sourced flow continues into thesubmarine environment without mixing withthe water column. Intermediate density contrastsare highly influenced by internal density stratifi-cation within the sub-aerial flow; this results inthe formation of a dense submarine flow, inconjunction with a buoyant gravity current overthe water surface (McLeod et al., 1999).

The observations in this study at Montserrat areconsistent with the water-supported flow models(Whitham, 1989; Cole & DeCelles, 1991; Freundt,2003). Phreatic explosions were recorded as thepyroclastic material crossed the shoreline(Edmonds & Herd, 2005), implying rapid andviolent interaction between the hot pyroclastsand the water. Such rapid heat exchange andsteam production would cause considerable mix-ing leading to sorting of the clasts within the flow.However, none of the previously well-docu-mented examples of pyroclastic flows crossing ashoreline has preserved evidence for such large-scale hydrovolcanic activity (e.g. Cole & DeCelles,1991; Carey et al., 1996; Mandeville et al., 1996).This evidence could imply either a lack ofpreservation in the rock record or the rarity oflittoral explosions, such as that recorded at theSoufriere Hills volcano.

CONCLUSIONS

Approximately 210 · 106 m3 of pyroclastic mate-rial entered the ocean during the 12 to 13 July2003 dome collapse at the Soufriere Hills vol-cano, Montserrat. Upon crossing the shoreline,the flows rapidly ingested water promoted viaphreatic activity, and mixed with the watercolumn prior to deposition at 500 m water depth.Thus, the original particulate mixtures wereefficiently sorted and physically differentiatedaccording to density and grain-size in water-ladensubmarine mass flows.

A loss of momentum at a break in thesubmarine slope caused the largest and mostdense blocks to settle quickly from the base of theflow. The thick, lobe-like deposit morphology ofthese coarse-grained deposits mimics the proxi-



mal linear flow lobes observed on land. The finerparticles were elutriated into the upper part of theflow-forming granular sediment density currentsthat evolved into turbidity currents with furthersediment deposition and water incorporation.The coarse-grained deposits comprise 60% ofthe total volume, whereas the finer turbiditefacies makes up 40%. This result broadly corre-lates with the original block versus ash propor-tions (�50% ash) in the source pyroclastic flows,taking into account the loss of some of the finestash fraction to the ocean currents.

ACKNOWLEDGEMENTS

This work was supported by the NERC grantNER/A/S/2002/00963. The authors thank theBritish Geological Survey for their technicalsupport and expertise and the captain, crew andscientific team on the RRS James Clark Ross fortheir invaluable assistance. Thorough reviewsfrom A. Freundt and P. Kokelaar greatly improvedthis manuscript.

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Manuscript received 26 July 2006; revision accepted30 August 2007



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Anatomy of a submarine pyroclastic flow and associated ...ocean.tongji.edu.cn/.../2014/...2008-Sedimentology.pdf · volcanic arc andesite volcanism. The current eruption began in