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Explosive Volcanism on Venus Transient Volcanic Explosions As a Mechanism for Localized Pyroclast Dispersal

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Explosive Volcanism on Venus Transient Volcanic Explosions As a Mechanism for Localized Pyroclast Dispersal View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Lancaster E-Prints JOURNAL OF GEOPHYSICALRESEARCH, VOL. 100,NO. El2, PAGES26,327-26,338, DECEMBER 25, 1995 Explosivevolcanism on Venus:Transient volcanic explosions as a mechanismfor localizedpyroclast dispersal SarahA. Fagentsland Lionel Wilson EnvironmentalScience Division, Institute of Environmentaland Biological Sciences, Lancaster University, Lancaster, England Abstract.It is proposedthat transient volcanic explosions of the vulcanian type may provide a mechanismfor thegeneration and dispersal of pyroclasticmaterial on Venus. The influence of the Venusianhigh atmospheric pressure environment implies that continuous discharge plinian erup- tiveactivity is relativelyuncommon: the tendency for suppression of exsolution and expansion of magmaticgases favors effusive eruptions. However, it maybe possible for explosive activity to occur,in a fashionanalogous to vulcanianeruptions on Earth, as a resultof theaccumulation of hot,pressurized gas under a coherentrock "lid". The explosion may be initiated by the failure of thisretaining caprock, causing the catastrophic release of thehigh-pressure gas, which expands outof thevent driving the fragmented caprock material ahead of it anddisplacing the surrounding atmosphere.On Earththe driving gas may originate either from vaporization of groundwater or fromdegassing of a stalledmagma body in thenear-surface crust, whereas on Venus, where the presenceof crustalstores of volatilecompounds isuncertain, the latter option only is favored: prolongeddegassing may lead to an accumulation of gas sufficient toinitiate an explosion. This paperpresents the results of a numericalmodel describing the explosion process under boundary conditionsrepresenting the Venusian physical environment. This involves treatments of the accel- erationof thedriving gas, caprock and displaced atmospheric gas out of thevent and the subse- quentmotions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmentedcaprock. In thisway, predictions of the eruption velocities and of theresulting distri- butionof (large)solid ejecta can be obtained for likely conditions on Venus. Deposits of large blockydebris are predicted to rangeup to a maximumdistance of theorder of 1 kmfrom the vent onVenus, compared with distances of severalkilometers commonly attained by ejecta from tran- sientexplosions on Earth. More typical blocky deposits may extend for only a fewhundred meters,which implies that they would not be detected in theMagellan radar data. However, the possiblepresence of associatedpyroclastic flow and fine-grained ashfall deposits may constitute aidsto the identificationof sitesof vulcanianeruptions on Venus. 1. Introduction the formationof pyroclasticflows is morelikely [Sugitaand Matsui, 1993; Thomhill, 1993]. It may be expectedtherefore that The veryhigh atmospheric pressure on Venus(ranging from steadyexplosive (plinian or Hawaiian)activity on Venusis -10 MPa to -5 MPa over the -10 km rangeof planetaryeleva- relativelyuncommon, with effusiveactivity dominating. Indeed, tion) will tendto suppress,or at leastreduce, the exsolutionand analysesof the huge amountof high-resolutionradar data expansionof volatilescontained in ascendingmagmas [Wood, returnedby the Magellanmission to Venushave demonstrated 1979; Wilson and Head, 1983; Head and Wilson, 1986]. the widespreadoccurrence of vastvolcanic plains and immense Uncommonlyhigh (by terrestrialstandards) total magmatic lava flows [e.g., Head et al., 1991], with relatively few volatilecontents are requiredfor subsurfacemagma fragmenta- indicationsof pyroclasticactivity [Head et al., 1991;Guest et al., tion (and hence the initiation of explosiveactivity in which a 1992; Iranov, 1992; Moore et al., 1992; Wenrichand Greeley, steadydischarge of gasand pyroclasts takes place) to occurat all 1992]. Nevertheless,intermittent, transient explosive eruptions [Garvin et al., 1982;Head and Wilson,1986]. It is by no means arepossible at quitemodest magmatic volatile contents on Venus clear that these volatile contentsare achievable in magmas on if gasconcentration occurs in a slowlyascending or stationary Venus. Furthermore, exsolved volatile contentsin excessof intrudedmagma. The resultmay be Strombolianactivity in low- severalweight percent, together with favorablecombinations of viscosity,mafic magmas[Garvin et al., 1982] or vulcanian high ventaltitude and eruption temperature, are necessaryfor activityin moreviscous melts [Fagents, 1994]. eruptionsfeeding high, convectingeruption clouds to be Fagentsand Wilson[1993] developeda modelfor transient maintained.Otherwise the collapseof the eruptioncolumn and vulcanianexplosive eruptions on Earthbased on the scenarioin whichmagma intrudes close to the surfaceof theplanet but fails 1Now at Centerfor Earthand Planetary Studies, National Air and to erupt(most likely because the rise rate is sufficientlyslow that SpaceMuseum, Smithsonian Institution, Washington, D.C. excessivecooling intervenes). Exsolving volatiles may accumu- late at the top of sucha magmacolumn, possibly as a foambut Copyright1995 by theAmerican Geophysical Union. later, if the foam collapses[Jaupart and Vergniolle,1989], as a Papernumber 95JE03202. gas pocket. This magmaticgas may effectively be trapped O148-0227/95/95JE-03202505.00 beneatha rigid "lid" if the pathwayto the surfaceis sealed 26,327 26,328 FAGENTS AND WILSON: EXPLOSIVE VOLCANISM ON VENUS (possibly by magma invading any near-surface fractures). 1992]. Whether this high-viscositymorphology is indicativeof a Alternatively, if potentially volatile compoundsexist within the moresilicic magma, or is simplya cooling-inducedfeature, or the country rocks, these may be evaporated and trapped as high- effect of a highly inflated bubble-richmagma, remains open to pressuregases if their path is not connectedefficiently to the debate [McKenzie et al., 1992; Pavri et al., 1992; Sakimotoand surface.The failure of part of the lid overlying an accumulation Zuber, 1993]. of high-pressuregases leads to the expansionof locally released Considerationof volatile solubilitieswith respectto the high gasesand the accelerationof the overlying rocks. Furthermore, atmosphericpressure on Venus causesdoubts as to whether local decompressionleads to the propagationof an expansion steadyexplosive activity can occur at all. For typical magmatic wave into the surroundings,and this can triggerfailure of moreof volatile contents,the high pressurewill act to inhibit or at least the lid, leadingquickly to the catastrophicdisruption of all of the reducethe exsolutionand expansionof magmaticgases [Wood, pressurizedregion in a vulcanianstyle explosion[Wilson, 1980]. 1979; Garvin et al., 1982; Wilson and Head, 1983; Head and The Fagents and Wilson [1993] model incorporatesa detailed Wilson,1986]. In the shallowlithosphere, lithostatic pressures are treatment of the mutual aerodynamic interactions between greaterthan on the Earth as a result of the high atmosphericsur- volcanicejecta and the gasflow field aroundthe eruptivesite and face pressure.As a result,the pressuresat which magmafrag- thusrepresents a significantimprovement over previousefforts to mentationtakes place (when the magmaticgas bubble volume model the dispersalof coarse volcanic material. Analysesof a fractionexceeds some critical value of-•0.75 [Sparks,1978]) are number of terrestrial eruptionshave shown that, by comparing always reached at shallower depths on Venus [Garvin et al., computed travel distances of large clasts ejected in such 1982;Head and Wilson,1986] and muchlarger volatile contents explosionswith their observedpositions in the field, it is possible are requiredif magmafragmentation is to take place at all. Table to constrainthe rangesof initial conditionsthat would explainthe 1 givesthe minimumtotal H20 andCO 2 contentsrequired for observeddispersal of ejecta[Fagents and Wilson,1993; Fagents, disruption of a basaltic magma at various vent altitudes.For 1994]. Geologicallyplausible values of theseparameters (excess rhyolitic magmas,Head and Wilson [1986] show that exsolved gas pressure,gas mass fraction, and sourceregion radius) are volatile contentsof severalweight percentare neededfor plinian obtained.Having testedthis model with terrestrialcase studies, it eruptioncolumns to form. Clearly, the minimumvolatile contents now seems appropriate, in view of the questions raised by requiredfor the disruptionof Venusianmagmas (i.e., to produce Magellan about Venus still being volcanically active, to employ continuousexplosive eruptions) lie aboveor at the upperend of the model to predict the extents of eruptive deposits from the range of typical magmatic volatile contentsof terrestrial transient volcanic explosions on Venus, given plausible magmas[Basaltic Volcanism Study Project, 1981]. preemptionconditions. Even if a magmaon Venus containssufficient volatiles for the A number of candidate sites for explosive vc•le•ni•m have been identified on Venus [e.g., Head et al., 1991, 1992; Ivanov, will also reducethe amountof gas expansion.This reducesthe 1992; Moore et al., 1992; Wenrichand Greeley,1992; Campbell, energyavailable to drive the eruption,which is a functionof the 1994; Campbelland Rogers;1994], with evidencefor quitelocal- ratio of the initial (preexpansion)and final (atmospheric)gas ized activity in somelocations [e.g., Bulmer, 1994]. The nature pressures,relative to that on Earth. The velocity reachedby the and modesof formation of thesedeposits
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