Apr 10, 1999 - sonic to supersonic conditions, the streamline of a jet can become tilted. The inclination of ..... the 1980 Mount St. Helens blast. Freon12 gas is of high ..... to be done on. 3-D modelling of explosive discharge from asymmetric ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B4, PAGES 7169-7181, APRIL 10, 1999
Control of crater morphology on flow path direction of Soufrire-type pyroclastic flows Mahar A. Lagmayand David M. Pyle Departmentof EarthSciences, Universityof Cambridge,Cambridge,England
Brian Dade Instituteof TheoreticalGeophysics, Universityof Cambridge,Cambridge,England
Clive Oppenheimer Departmentof Geography,Universityof Cambridge,Cambridge,England
Abstract. We presenta modelof fountaincollapsefor small-scale(Soufrigre-t3•pe) explosive eruptions thatrelatesthe asymmetryof a volcaniccrater(e.g.,thepresenceof craternotches) with theemplacement directionof p.vroclastic flows. Analysisof two-dimensional simulationsof compressible fluidsemanatingfrom asymmetricnozzlesshowsthatunder sonicto supersonic conditions, thestreamlineof ajet canbecometilted. The inclinationof the streamlineincreases with greaterslantangleof the nozzleandwith increasingexit pressure. Usingthetwo-dimensional simulations asanalogues to volcaniceruptions, we propose thatpyroclastic e.jectawithintheinnercoreof anerupting jet columncanbecome asymmetrically focusedbeforecollapsingat fountainheightsof a few hundredlnetersabove thecraterexit. The ensuingpyroclasticflows associated with fountaincollapsethusbecome directionalin characterwith flow orientationcontrolledby cratergeometryanderuptionexit pressure.The modelappliesto volcanoes with verticalconduitsandcraterto ventgeometries thatactaseffectivesonicto supersonic jet nozzles.We proposethatthe 1984 eruptionof Mayonvolcanofits thismodel. In the secondphaseof thiseruptiona prominentcraternotch imparteda southeasm'ard tilt to thebasalgasthrustregionof theeruptioncolumn. In turn, thisled to the dischargeof p)ToclasticflOWSontothe southeast flank of the volcano.
1. Introduction
eruptions usingtwo-dimensional (2-D) computational fluid dynamic simulations of compressible flows. Using the results Pyroclasticflow depositsare the productsof eruptionsof of thisanalysis, we showhowthecratermorphology playsan vastly differing intensitiesand range in scale of eruptive rolein controlling theorientation of theemergent jet volume from1012m 3[Simkin cmdSiebert, 1994]. important The large eruptionsproduceradially distributedpvroclastic flows that travel at speedsof up to 2 x 10- m s , covering 4 2 / ,, areasof up to 10 krn aroundthe vent [I,l,•lsonand I4alker, 1985]. In contrast, the small-scale eruptions produce pyroclasticflows that usuallytravelin a restricteddirection from the craterorigin. This is a featureinherentto the small volumeof the pyroclasticflows,whichleadsto its inabilityto swampall flanksof the volcanicedifice. Analysisof some
in flows that exit the craterat or abovethe speedof sound.
We thenapplythismodelto the second phaseeruption of Mayonvolcanoduringits 1984 explosive activity. This eruption generated a basalgasthrustcolumnthathada tilt towardthe southeast andfountaincollapseformedpyroclastic flows that cascadeddown the southeastflanksof the volcano.
Ouranalysis shows thatit is possible to predictthelikely direction of emplacement of pyroclastic flowsat volcanoes
asymmetric craters. small-volume (7.5x 10-•to 40 x 106m3)pyroclastic flows withprominently Theprediction of thelikelydirection of fountain collapse
/¾omthe 1974 Fuego, 1975 Ngauruhoe, 1984 and 1993 Mayon, 1988 Tokachi,and 1992CraterPeakeruptionsshows that apart from volume constraints,cratermorphologyalso affectsthe directionalcharacterof pyroclasticflows produced from fountain collapse during small-scaleSoufrifre-type eruptions(Figure1). Here we investigatethe role of craterasymmetryon the productionof directedpyroclasticflows in Soufrifre-type
formedpyroclastic flowsis essential in hazards assessment
andmitigation. Onthebasisof thefountain collapse model, pyroelastic flow hazardzonesare usuallydelineated as concentric zonesaroundthe volcano.Thistypeof pyroclastic flow hazard zonationis not necessarilyappropriatefor • 08 m3) volcanoes known toexperience small-volume (< 10-1
Soufribre-type explosive activitythat generates strongly directed pyroclastic flows.Theidentification of higherrisk zonesaroundactivevolcanoes, suchas areasunprotected by
Copyfight1999by theAmericanGeophysical Union.
topographic barriers andareasin thedirectlineof sightof a
Papernumber1998JB900105.
craternotch,is animportant stepin developing moreeffective
0148-0227/99/1998JB900105509.00
volcano hazards models. 7169
7170
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Figure. l. Preemption contour mapsof volcanoes withtheirrespective pyroelastic flowdeposit distribution. In all thesecases theeruptive eventis described tohaveat somepointgenerated fountain collapse pyroelastic flows.Thepreemption cratertopography is unwrapped, andthecorresponding histogram of all pyroelastic flowdeposits formedduringtheentireeruptive eventsequence is plotted.Thehistogram represents theranoutlength of thepyroelastic flowsat 5øintervals. Notethecorrespondence of lowercrater elevation andflow distribution exceptfortheCraterPeakpyroelastic flowdeposits. (a) VoleanFuegoandits 1974pyroelastic flowdeposits (Theoutline ofthecraterof VoleanFuegoisinferred fromcraternotchdepths andpositions as described by Davieset al. [1978].),(b) Ngaumhoe andits 1975pyroelastic flowdeposits [NairnandSelf, 1978],(c) Tokachi volcano withits 1988pyroelastic flowdeposits [Katsui et al., 1990),(d) Mayonandits 1993pyroelastic flowdeposits [Ui, andCatane, 1993],and(e) CraterPeakandits 1992pyroelastic flow deposits [Milleret al., 1992].(A histogram couldnotbemadeonthisexample dueto thefactthatthe pyroelastic flowsdidnotflowradially awayfromthecrater.Instead, we indicate witharrows wherethe pyroelastic flowsoriginate.)
2. Theoretical
Model
Severalmodelshavebeendevelopedthat describemagma ascentwithina conduit[e.g., Wilsonet al., 1980;Burestiand Casarosa, 1989; Giberti and Wilson, 1990; Jaupart and All•gre, 1991; Dobran, 1992]. In all of thesemodelsthe eruptingmixture is treated as a dusty gas definedas a
multiphasefluid composed of gases,liquid droplets,and hot pyroclasts that are in thermalequilibrium. This dustygas approximates a perfectgaswhosedensityis equalto the fluid mixtureand whosepressureis equalto the pressureof the gaseousphase [Kieffer, 1984]. This eruptingmixture is predicted to exit the conduitat a ventwith sonicvelocityand pressures thatareequalto or greaterthanatmospheric [Woods
LAGMAY
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ET AL.' FLOW
PATH DIRECTION
OF PYROCLASTIC
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7171
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Figure 1. (continued)
and Bower, 1995]. Above the vent the mixture may volcanic eruptions.In sections 2.1 and2.2 we describe our decompress freely into the atmosphere or enterinto a crater, studyonjets that emergefrom asymmetricnozzlesand apply whereit can accelerateto supersonic velocitiesand emergeas it to modelvolcanicjet behavior. a momentum-driven jet. As this jet entrainsand heatsthe surrounding air, themixturemaybecomebuoyantandriseasa 2.1. Model Description thermalplume [Wilson,1976]. If mixing is incomplete,the To simulate sonic vent dischargeduring explosive inner core of the eruption column eventually loses its volcanism,we model a convergent nozzlewhere a highmomentum, collapses, and generatespyroelastic flows pressure fluid from a reservoircan discharge into a chamber [Woods,1995]. with atmospheric pressure(Figure2). This construction is a While the generalprinciplesof fountaincollapseare now Computational FluidDynamic(CFD) geometry analogue to a well understood [Sparkseta/., 1997],little attentionhasbeen laboratory shocktube,whichseveralworkershavesuggested paidto theeffectsof asymme• in thecraterregion.It is well [Bennett,1971, 1974;McGetchinand Ulrich, 1973;KiefJ•r, known, however,from the aerodynamics literaturethat the 1984]canbe usedto modelfluid discharge duringexplosive geometryof the nozzle can play an importantrole in volcanism. In the laboratory shocktube experiments, gas controllingthe behaviorof emerging jets that are supersonic flowsfromthehigh-pressure reservoir througha duetandinto [e.g.,Hokenson,1986]. Specifically, the streamline of thejet a low-pressure chamber.If thetubehasconverging geometry., may be tilted away from the vertical. We explorethis an initially subsonicfluid flow canaccelerateand attainsonic phenomenonand its specific applicationsto explosive velocityat theshocktube'snarrowest portionor throat[Ferri,
7172
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PATH DIRECTION
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whereit disrupts intoa mixtureof pyroclasts andreleased gas is calledthefragmentation surface[Wilsonet al., 1980]. If the reservoirpressure(i.e., pressureat fragmentation level) is more than about double the atmosphericpressure,as is frequentlythe case,the flow will acceleratethroughthe narrowing volcanicconduituntil it discharges fromthe vent with somcvelocity[I
