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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
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Prokop Závada a,⁎, Zuzana Kratinová a, Vladimír Kusbach b,c, Karel Schulmann c
a Geophysical Institute, Academy of Sciences of the Czech Republic, Boční II/1401, 141 31, Prague 4, Czech Republicb Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republicc Centre de Géochimie de la Surface, EOST, Université Louis Pasteur, Strasbourg cedex, France
a b s t r a c ta r t i c l e i n f o
Article history:Received 15 January 2008Received in revised form 25 June 2008Accepted 10 July 2008Available online 16 July 2008
Viscous lava extrusions were modeled using plaster of Paris with admixed magnetite dust which served as atracer of the internal anisotropy of magnetic susceptibility fabric in model lava domes. Used analoguematerial showed pseudoplastic behavior and yield strength level proportional to increasing mixing ratio ofplaster powder and water. A series of models ranging from simple gravity flows to complex lava domesshowing combined endogenous and exogenous growth were created by intrusion of plaster into a sandbox.The similarity of model bodies is compared with natural lava domes on the basis of dynamic scaling analysis.Growth dynamics, exogenous growth and internal fabric development in natural lava domes is criticallydiscussed using the experimental results.
Lava extrusions showawide range of shapes ranging from thin flowsto axisymmetric domes, lobate domes or steep sided lava spines (Finkand Griffiths,1998). Detailedmonitoring of actively growing lava domesin the last century enabled better understanding of the couplingbetween the surface activity associatedwith development of active lavadomes (e.g. growth rates, gas flux, ground deformation and seismicity)and their deep plumbing system (Swanson et al., 1987; Nakada et al.,1995; Voight et al., 1999). An important feature of dacite and andesitedomes is the oscillation of their growth rate, transitions from effusive toexplosive activity and successive emergence of lava lobes or spines onthe surface of the dome during extrusion (Fink et al.,1990; Nakada et al.,1999; Voight et al.,1999; Cashman andMcConnell, 2005). The pulsatorylava ascent responsible for variation in growth rate of lava domes andproduction of lava lobes on their surface is explained by crystallizationand degassing driven overpressure building up in shallow levels of thecomplex feeding system (Fink and Griffiths, 1990; Sparks, 1997; Melnikand Sparks, 2005; Hale and Wadge, 2008).
A number of analogue models were constructed in order to constrain“effective viscosities” of extrusive flows (Huppert et al.,1982), influence ofyield strength (Blake, 1990) or solid crust on shapes of lava domes andtheir surface morphology (Fink and Griffiths, 1990, 1998; Griffiths andFink, 1993, 1997). In contrast, fewer works dealt with the internal strainand fabric development in extrusive domes (Buisson and Merle, 2002,2004; Talbot and Aftabi, 2004). There are also very few field studies
mapping volcanic fabrics that develop during emplacement of lavas thatcould be comparedwith the analoguemodeling results (Fink,1983; Smithand Houston, 1995; Castro et al., 2002; Závada, 2007). In addition, allanalogue models of internal strain and fabric development were doneusing Newtonian silicone putties, although natural lavas, especiallycrystal-rich dacites and andesites, behave as pseudoplastic or Binghamsuspensions of crystals and residual melt (Pinkerton and Sparks, 1978;Ventura et al., 1996; Smellie et al., 1998; Sparks et al., 2000; Saar et al.,2001). In shallow portions of lava domes, “thick” (crystal-rich) lavas alsoshow “dilatant” rheology (Sparksetal., 2000;Smith, 2002). Internal fabricsin domes composed of Bingham suspensions were not yet evaluatedbecause current techniques (superimposed strain grids or PIV— “particleimage velocimetry” (Buisson and Merle, 2002)) can not be applied tothem.
In order to evaluate the shape developmentofmodeledpseudoplasticextrusions and their internal deformation, anisotropy of magneticsusceptibility (AMS) technique was used in combination with coloringof plaster of Paris suspension (Kratinová et al., 2006). In the presentwork,we have created a sequence of models of variable “thickness” given bydifferentmixing ratio of plaster powder andwater. The “thickness” refersto relative macroscopic diffluence of plaster suspensions. “Thin” plasterexperiments resulted in low-aspect ratio (height to width) flows, while“thick” extrusions showed relatively high aspect ratio shapes and pulse-like regime of their emplacement. The latter revealed combinedendogenous and exogenous growth, which resembled the growth styleof “complex” dacite domes developed on Mt. St. Helens or Mt. Unzen(Swanson et al., 1987; Nakada et al., 1995). The internal fabric pattern ofexperimental bodies is analyzed from their vertical and horizontalsections from both the disrupted color banding and the AMS fabric. The
similarity of themodels to their natural equivalents is then considered interms of the dynamic scaling analysis. The mechanism responsible fortransition from endogenous to combined endogenous and exogenousgrowth for experimental bodies is discussed in attempt to constrain theemplacement dynamics of natural lava domes.
2. Analogue modeling using AMS
2.1. Experimental material
Plaster of Paris powder used in our experiments consists of platyparticles of hemihydrate (CaSO4 ·1/2(H2O)). Most of the particles showsizes smaller than 40 µm (inset of Fig.1). The grain size distributionwasmeasured by the laser diffraction method using the Fritsch ParticleSizer Analysette 22 (Institute of Chemical Technology in Prague).Although the aspect ratio of particles was not measured, their platyappearance observed using optical microscope can well approximateshape of plagioclase tables in dacite lavas or sanidine laths in trachytes(Závada, 2007). Slurries of plaster powder and water of increasingmixing ratios from2.2 to 2.6 (weight of plaster powder per unit weightof water) were measured using Haake Rotovisco RV 20 and ViscotesterVT 550 rheometers (Institute of Hydrodynamics, ASCR) with ledges onthe MV2 cylinder to prevent slip of the measuredmaterial on its walls.The rheograms of all measured plaster samples show similarpseudoplastic behavior and distinct transition between higherviscosities (∼5 Pa s) at low shear rates and low viscosities (∼1 Pa s)at relatively higher shear rates (Fig. 1). The shear stress level at whichthis transition occurs, approximated by the intercept of extended flatpart of the curve and vertical axis, defines the apparent yield strength(Blake, 1990). The area between rheogram curves measured atcontinuously increasing and then decreasing shear rates (hysteresisloops) corresponds to the degree of thixotropy of the measuredsample. This quantity is defined as the energy per volume needed todisrupt the framework structure of hemihydrate particles and alignthem in the shear direction (Schramm, 1994). Both values of apparentyield strength and thixotropy increase with increasing mixing ratio of
plaster (Fig. 1, Table 1). The plaster material prepared with relativelyhigher amount of plaster particles per unit volume ofwater is regardedas relatively “thicker” plaster in contrast to “thin” plaster material.
Microscopic observations of plaster powder and water suspensiondroplets with small amount of magnetite revealed that gypsum crystals(CaSO4·2(H2O)) start to crystallize radially from hemihydrate platy par-ticles and entangle the magnetite grains. We did not identify anyrotations of magnetite particles that could be induced by irregulargrowth of gypsum crystals. While agitating the suspension, one can seethat further crystallization increases mutual interactions betweenadjacent clusters of solid particles prior to solidification. A retardingagent was added to the plaster powder prior to the experiments topostpone the solidification of the mixture with water (Kratinová et al.,2006). Simple shear strength tests (using a weight driven vane dippedinto the suspension) of the suspension with mixing ratio of 2.2 werecarried out to find out, whether the transformation from the weakplaster suspension (with the retarding agent) to solid is gradual or ratherabrupt and when it occurs. The suspension had constant shearingstrength at given load for the first 45 min. During the next hour theshearing strength gradually increased to complete solidification, whichprobably reflects increasing mechanical interaction and intergrowth ofgypsum crystal clusters around hemihydrate particles. Since thepreparation of plaster slurries and the experiments were done in about30 min, we can suggest that their rheology is reflected by the rheogramcurvesmeasured by the rheometer (Fig.1) andwere still formed by platyparticles of hemihydrate suspended inwater. Thegradual transformationof plaster to solid makes it a potentially attractive analogue material formodeling of progressively crystallizing magmatic systems.
In order to test whether the measured AMS fabrics frommagnetitegrains can be regarded as normal, in contrast to inverse (Rochette et al.,1999), we have characterized the magnetite domain state. Measuredhysteresis parameters of magnetite on ADE EV9 VSM (GeophysicalInstitute, Prague) and also the bulk susceptibility in relation to fre-quency of magnetic field (at 976 Hz, 3904 Hz and 15616 Hz) on MFK1Kappabridge in Agico, Inc., have clearly shown that the used magneticparticles represent multi-domain magnetite. The magnetite particleswere also analyzed for their shape. The magnetite dust was stirred inepoxy resin and oriented inmagnetic field. The BSE images of orientedthin-section cut parallel with alignment of magnetite grain chainswere vectorized in ArcView GIS and statistically evaluated using thePolyLX Matlab toolbox (Lexa et al., 2005) (http://petrol.natur.cuni.cz/~ondro/polylx:home). The magnetite particles show irregular angularshapes and reveal mean axial ratio 1.8 and average grain size 0.6 µm.The magnetite grains in the thin-section formed elongated clustersaligned parallel with the applied magnetic field, non-clustered grains
Fig.1. Rheological curves of plaster suspensions prepared with differentmixing ratios ofplaster powder and water (indicated above the curves). Area between curves measuredat increasing and decreasing shear rates defines the thixotropy of the measured sample.Inset shows cumulative frequency diagram of the sizes of measured hemihydrateparticles. Rheometry at shear rates above γ=300 for slurry with mixing ratio 2.6 wasbeyond technical possibility of the rheometer.
Table 1Properties of plaster slurries and dimensions of corresponding experimental bodies
Thixotropy value for mixing ratio of plaster slurry corresponding with experiment H4 isunderestimated with respect to other measured plaster slurries due to the technicallimits of the rheometer used. Experiment H5 was prepared with the same mixing ratioas experiment H2.
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were alignedwith their long axes parallel with the orientation of grainclusters. On the basis of these shape characteristics,we suggest that thechosen magnetic material is a good marker of flow induced fabrics.
2.2. Experimental setting and procedure
The experimental setting is similar to that used by Kratinová et al.(2006), although more powerful (10 tons maximum load) hydraulicsqueezer was used to model lava domes (Fig. 2).
Piston diameter of the squeezer is 10 cm. The squeezer load,transmitted on a squeezing board through a steel frame, forces theplaster slurry to evacuate from a steel container at the bottom of theapparatus, through a circular conduit (4 cm in diameter) to intrude a12 cm thick sand layer in a plexiglas box. Both the steel container andthe plexiglas box are rectangular and 60 cm wide.
Plaster slurry preparation, homogenization with magnetite powderand coloring were done the same way as in our previous experiments(Kratinová et al., 2006). Before the experiment, plaster was at firstsqueezed upwards to the top of the steel conduit. Then itwas buried byasand layer in the plexiglas box. Duration of experiment was measuredfrom the time when the plaster “plug” emerged above the sand layerafter further load increase. Unfortunately, the squeezer was notequipped with automatic load control and recording of imposed loadthrough time,we thereforeprovideonly themaximumloadat theendofeach run and qualitative description of extrusion evolution duringcontinuous load increase. The sand layer (with median grain size of0.2 mm) was used to model brittle to loose host rocks around lavaextrusions (e.g. represented by pyroclastic accumulations in volcanocraters). Several hours after the experiments, solidified bodies were cutalong plane of their symmetry for further documentation and AMSsampling. Although the sand around removed plaster bodies was wet,which means that some water was lost from the plaster throughcapillary action, we suggest that the seepage of water occurred mainlyafter relatively fast emplacement of the bodies within few minutes anddid not significantly alter the rheology of the suspension during theexperiment. Below, we describe the evolution of growing extrusionsduring the experiments and their sections after solidification.
2.3. Experimental runs
Over ten experimental extrusions were produced. Most of themshowed similar style of extrusive dome growth and similar internal
structure. During the experiments, appearance of a plaster plug of thesame diameter as the steel conduit was preceded by updoming andradial cracking of the sand surface above the conduit. After emergenceofthe plug above the sand, the plaster spread horizontally, after sur-mounting a 1–2 cm high mound of sand, which was thrust aside byrising plaster plug, and soon created small axisymmetric dome thatincreased in diameter. The domes grew steadily and endogenously up toabout 10 cm in their radius. Further growth was marked by certainperiods of rest in growth, although the loadwas increased continuously.At the end of each rest period, exogenous lobes of plaster emerged closeto center of thedomeandmigrated horizontally to themargin. The lobesat first rose rapidly vertically upwards along parabolic traces on thedome surface and then slightly rotated down the dip of the dome slopeas they were transported sideways. At the same time their velocity oftransport slowly decreased to rest. Striations or “groove” marks typicalfor natural lava lobes (Sparks et al., 2000;Hale andWadge, 2008) formedon the surface of each growing lobe and indicated the direction of itsmotion. The second lobemigrated in opposite direction as the first lobe.The dome slightly endogenously inflated prior to emergence of anexogenous lobe. The length of rest periods increased with domedimensions during single experiment. Experiments with relatively“thicker” plaster slurries (less dilutedwithwater) also showed relativelylonger rest periods and lobes of larger dimensions in contrast to“thinner” (more diluted) plaster experiments. Only in one experiment(H1) with the “thin” plaster of mixing ratio 2.2, the plaster broke rapidlythrough the sand to forma small fountain that quickly collapsed andwasoverlain by a viscous flow of new plaster slurry.
3. Internal fabric of model extrusions
In order to understand the internal structure of model domes andconsider how it reflects their growth style, wefirst describe the patternof disrupted color banding. The latter gives a first approximation of thekinematic framework of flow, internal strain throughout the plasterbodyandfinal displacements of plastermaterial. This is complementedby detailed analysis of AMS fabric, which enables tracing of the flowinduced fabrics in 3D (three-dimensions) from orientation of homo-genously dispersed magnetite dust (Kratinová et al., 2006).
In order to trace the fabric pattern based on AMS measurementsand compare it with the internal structure of themodels characterizedby the disrupted color banding, vertical sections of models H1, H2 andH4 were drilled in hexagonal grid of 1 cm spacing. AMS fabrics inmodel H4 was also measured in one horizontal section to consider theeffect of heterogenous strain pattern induced by asymmetric empla-cement of individual plaster lobes. The oriented cylinders weremeasured with the KLY-4S Kappabridge (Jelínek et al., 1997; Pokornýet al., 2004) and the AMS data were statistically evaluated using theANISOFT package of programs (Jelínek, 1978; Hrouda et al., 1990).Vertical sections of the models are characterized by datasets withaverage 260 measurements, 409 measurements were obtained fromthe horizontal section of experiment H4. The eccentricity, shape andmean bulk susceptibility of the AMS ellipsoid can be characterized bythe following parameters (Nagata, 1961; Jelínek, 1981);
P ¼ K1=K3T ¼ 2η2−η1−η3
� �= η1−η3� �
Km ¼ K1 þ K2 þ K3ð Þ=3
where K1NK2NK3 are the principal susceptibilities, η1=lnK1, η2=lnK2,η3=lnK3. The parameter P, called the degree of the AMS fabric, indicatesthe intensity of preferred orientation of magnetite particles in plastermodels. Parameter T characterizes the shape of the AMS ellipsoid. If0bTb+1, theAMSellipsoid is oblate (themagnetic fabric is planar);T=+1means that the AMS ellipsoid is rotationally symmetric (uniaxial oblate).If −1bTb0 the AMS ellipsoid is prolate (themagnetic fabric is linear); T=−1means that theAMS ellipsoid is uniaxial prolate. IfT=0, the ellipsoid is
Fig. 2. Experimental apparatus used for analoguemodeling of lava domes. The front sideof the plexiglas box is turned 45° around its axis, its dimensions are the same as those ofthe container at the bottom of the apparatus. The inset explains descriptive termscharacterizing the experimental bodies: 1) lastly emplaced plaster lobe, 2) lateral flowmargin, 3) vent, 4) sand layer and 5) the conduit.
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neutral, on the transition between oblate and prolate. The value of meansusceptibility Km is directly proportional to the content of magnetite inthemodelmaterial and is characterized by narrow span of values around4.1×10−3 indicatinghomogenousdistributionofmagnetite in themodels(Kratinová et al., 2006).
3.1. Internal fabric pattern from disrupted color banding
Wewill nowdescribe thedimensions, surfaces and internal structurefrom vertical sections of four experimental bodies characterized by themeasured rheological curves (Fig. 1).
Experiment H1 prepared of “thin” slurry of mixing ratio 2.2resulted in flow of a pancake shape with 50 cm in diameter and aspect
Fig. 4. Photograph and traced surface pattern of shear zones of the “thin plaster” experiment H1 indicating radial shortening and circumferential stretching of the superficial andmarginal part of the extrusion. Scale bar is 10 cm.
Fig. 5. Schematic diagram of exogenous growth and lobe development in theexperiments: a) lobe 4 is crosscut by an asymmetric cylindrical shear zone that isinitiated at its walls in the deep parts of the vent (only two traces of this shear zone areobserved in vertical section); in the upper parts of the dome, themore active segment ofthis shear zone points to the saddle between lobes 3 and 4 on the dome surface, b) lobe5 rises along the shear zone to the top, displacing and rotating surrounding units.
Fig. 3. Vertical sections through the experimental bodies. Interpretation of outlines ofindividual plaster units emplaced as successive lobes is indicated for models H2, H3 andH4. Scale bar is 10 cm.
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ratio 0.16 (height to radius). The vertical cross-section shows that theflow was fed by a 6 cm wide cylindrical vent above the conduit andthat the suspension flowed over a sand mound 3 cm high around thevent to advance laterally (Fig. 3).
Theheightof theflowabove the sand layerdecreases from5 cmcloseto the vent to 3 cmclose to the lateralmargin of theflow. Color bands aresubvertical in the vent and turn almost symmetrically to subhorizontalorientationwith color stripes being decorated by fringes and decreasingin thickness towards the flow margins. The color bands that “crop out”on three places of the flow surface are locally distorted by distinctconjugate shear zones including an angle of 120° (Fig. 4).
Local inhomogeneousdistortion of theflowclose to its lateralmarginis accommodated by preferential activity of one set of shear zonesoriented at ∼45° to the trace of color banding. The kinematic inter-pretation of the shear zones reveals radial shortening and circumfer-ential stretching, whichwas reported for the top part of divergent flowsfrom other experimental studies (Blake,1990; Buisson andMerle, 2002,2004). The shear zones also include normal displacements close to theflow margin.
The sequence of experimentsH2, H3 andH4 shows increasing aspectratio (height to width) of resulting extrusions, which corresponds toincreasingmixing ratio of used plaster slurries from0.43 to 0.5 and 0.54,respectively. For “thick” extrusions, irregular morphology is typical dueto emplacement of exogenous plaster lobes. Vertical cross-sectionsshow similar pattern for all three bodies (Fig. 3). Exogenous lobes are
expressed by ‘bulges’ on the dome surface. The lastly emplaced exo-genous lobe is a toppart of a large portion of plaster that forms thedomeinterior. This lobe is demarcated by serrated color bands that form onlyfew millimeter thin vertical stripes in the vent and are up to 2 cmwideand arcuate in the central part of the dome.While the apical part of eachlobe is formedbywhite plaster, thedarkmarkermust havebeen thinnedout during the experiment and exposed the lowermost plaster layerfrom the source container. Experiment H4 reveals several vertical thinstripes of dark plaster in the vent and some of these stripes can be tracedto the bottom part of older lobes. A thin line of color plaster marks thecontact between the last and previously emplaced lobes (lobes 5 and 4,respectively in models H3 and H4).
According to the disposition of colored plaster described throughoutthe vertical sections of experiments H2, H3 and H4, we can interpret themodels tobecomposedof individualunits corresponding to successivelyemplaced lobes and outline the style of their growth (Figs. 3 and 5).
Prior to emergence of lobe 5 in models H3 and H4, thick serratedcolor bands in the left part of lobe 5 were connected with thick colorbands at the bottomof lobe4 (Fig. 5a). Lobe 5 then formedby transectionof lobe 4 by a shear zone, which was initiated at the walls of lobe 4 indeeper parts of the vent zone probably just above the conduit. Becausethe shear zone bounding the new lobe has irregular cylindrical shape in3D, only two traces of this shear zonecanbe identified invertical section.Propagation of this shear zone accommodates ascent of lobe 5, whichslightly inflated laterally in the interior of the dome and formed a bulge
Fig. 6. Interpretation of internal structure of the model H4 from vertical and two horizontal sections. Note a discrete shear zone emanating from boundary of lobe 5 displacingsurrounding units in Section 2. Numerous conjugate shear zones with small displacements disrupt the color stripes in marginal parts of horizontal sections.
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Author's personal copy
“popping up” on the dome surface (Fig. 5a,b). Rotation of lobes (e.g.clockwise rotation for lobe number 5 in domes H3 and H4) is explainedby easier slip along one side of the shear zone (its left segment tran-secting lobe 4) and relatively higher degree of coupling on the oppositeside of this cylindrical shear zone. The degree of coupling is given by thevolume and number of already emplaced units on respective side of thedome, therefore ascent and clockwise rotation of lobe 5 requires pushanddeformationonlyof units 3 and1, in contrast tounits 1, 2 and4 in theopposite direction. The interpretation about initiation of shear zones indeep parts of the vent zone is in agreement with numerical modelingresults of stress accumulation and shear band formation responsible forinitiation of exogenous lobes in the walls of upper parts of conduitsfeeding lava domes (Hale and Wadge, 2008). Our results show that thegrowth of exogenous lobes is asymmetrical and their ascent is partlyprescribed by the fabrics of already emplaced units as will be furtherdiscussed below. Motion along newly formed shear zones also causesdeformation in the surrounding lobes, which is reflected by “smeared”dark marker in lobe 4 along its new boundary with lobe number 5(models H3 and H4, Fig. 3).
Model H2 can be separated into 4 units (number 1 is a remnant ofendogenous dome; 2, 3, 4 are partly exogenous lobes), models H3 andH4are formedby5units (2–5partlyexogenous)(Fig. 3). Interpretation ofboundaries between successively emplaced plaster units is also shownin two horizontal sections throughmodel H4 (Fig. 6), where one can seethat emplacement of lobe 5 was accommodated by a narrow trans-current shear zone emanating from boundary of lobe 5 towards thelateral margin, displacing the thin banding in surrounding units (units1–3). In addition, close observation of horizontal sections of model H4(Fig. 5) revealed conjugate shear sets disrupting the banding especially
in the marginal parts of the model. These shears include acute anglesfacing to the vent area and show displacements smaller than 1 cm.
3.2. AMS fabric in model domes
The AMS fabric is presented in four figures for each model bodywith traces of magnetic foliations on vertical sections (planesperpendicular to K3 mean susceptibility direction), plunge directionwith contours of plunge angle from the vertical section for magneticlineations (K1 directions) and contour diagrams of shape T and thedegree of AMS parameter P, respectively. Magnetic foliation planes aremostly perpendicular to the vertical model sections. Model H4 is alsocharacterized by AMS fabric pattern in one horizontal section, which ispresented in separate figure and includes contour diagrams ofmagnetic foliation plane dip angles and plunge angles of magneticlineations together with combined 3D contour diagrams of P and Tparameters in horizontal and vertical sections.
Magnetic fabrics inmodel H1 (Fig. 7) can be divided into five zones.The vent is marked by low intensity oblate fabrics (P=1.07–1.18,
T=0.58–0.96) with subhorizontal foliations and lineations. The linea-tions are parallel with the vertical section. The zone above the ventshows similar orientation and shape of magnetic fabric as the vent, butits degree of AMS is relatively high (P=1.292–1.3). The walls of the ventreveal vertical foliations and lineationswith neutral AMSellipsoid (T∼0)and the degree of anisotropy ranges between 1.21 and 1.26. In the upperpart of lateralflows,magnetic lineations are parallel to themargin of thebody in plan-view.Magnetic foliations characterize an imbricated fabricpattern at the top and bottom of the flow, which is consistent with thetransport of material from the vent area to lateral margin as it was
Fig. 7. AMS pattern of model H1: a) traces of magnetic foliation planes superposed on the model section, b) plunge directions of magnetic lineations with contours of their plungeangle from the vertical section, c) contour diagram of shape parameter T, d) contour diagram of the degree of AMS parameter P.
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identified in natural lava flow (Ventura et al., 1996). The T parameter inthe upper part of lateral flow progressively decreases with the radiusfrom0.55 to −0.55 and P parameter reaches relatively high values of 1.26to 1.29. Shape of fabrics continuously changes from prolate to oblatewith depth of the lateral flow of model H1. The narrow basal parts oflateral flows characterized by imbricated magnetic foliation planes areassociated with magnetic lineations plunging shallowly towards thevent. In this domain, the parameters of shape T and degree of fabricanisotropy P range between 0.39 to 0.96 and 1.21 to 1.29, respectively.
Similar division into zones of characteristic fabric as for the “thin”flowH1 can be used for ‘lobate domes‘H2 and H4, although the fabricsreach different intensities of AMS and their pattern is morecomplicated in their central part (Figs. 8 and 9).
The domain of relatively low intensity fabric in ‘lobate domes’ isconfined in a narrow zone at the bottom of lastly emplaced lobe andmarks the transition zone from prolate vertical fabrics of T=−0.4–0above the feeding conduit towards oblate fabrics of T between 0.8 and 1in central part of the lobe. The oblate fabrics in this part are associatedwith the maximum values of AMS parameter P attained in the models.Maximum P parameter values in this zone are slightly higher for the“thicker”model H4 with P=1.32–1.36 in contrast to values P=1.28–1.32inmodelH2. Similardifference ofAMS intensity between bothmodels isperceptible throughout the whole dome. Magnetic foliations show“plug” like imbrication in the last lobe. This holds also for thesurrounding units (lobes 2,3 in H2 and lobes 3,4 in H4), where the“plug” like fabric pattern is rotatedwith respect to the central lobe and isslightly asymmetrical. The domains in the upper parts of lateral flowsmarked by magnetic lineations being parallel with the margin of thebodies in plan-view (of moderate to high plunge angles from verticalsections) optically increase in volumetric proportion in the extrusions
with higher mixing ratio of the plaster slurry. This is obvious whencomparing AMS patterns in models H2 and H4 (Figs. 8 and 9). Marginalparts of both models show neutral shape of AMS ellipsoids. Boundarybetween lobes 3 and 4 in model H2 and lobes 4 and 5 in model H4 ismarked bymagnetic lineations subparallel to this boundary and parallelwith the vertical section. This boundary also divides the units showingthe highest angular difference between their fabric orientations(reaching almost 90° in model H4). A local maximum of high valuesof P parameter marks the upper part of lobe 4 in model H4 (Fig. 9).Horizontal section through model H4 at the level coinciding withSection 2 in Fig. 6 reveals that the strike ofmagnetic foliations is parallelwith the margin of the body in plan-view and their dip angle valuesform symmetric domains with respect to the centre of the dome(Fig. 10). The foliations have steep dip angles at the margin of the domeand in the domain at the walls and in a 4 cmwide belt surrounding thelastly emplaced lobe. The centre of lobe 5 is formed by fabrics withshallow dip angles that dip to the margins of the dome. The foliations inthe region between the lastly emplaced lobe and the marginal domainwith steep fabrics is formed by subhorizontal fabrics that dip regularlytowards the centre of the dome. The magnetic lineations show mostlyshallow plunge angles up to 20° fromhorizontal and are subparallel withstrike of magnetic foliations. Only in the walls of lobe 5 and mainly closeto the contact with lobe 4, the plunge angle of magnetic lineationsexceeds values of 60° and their plunge directions are perpendicular to theboundarywith lobe4. The contourdiagramsof shapeof theAMSellipsoidreveals that the strongly oblate fabrics dominate the central part of thedome and encompass mainly the interior of lastly emplaced lobes. Themarginal parts are characterized by neutral and locally prolate fabricellipsoids. The diagram of P parameter shows more scattered patternwith high intensity AMS fabrics (P=1.28–1.33) dominating the interior of
Fig. 8. AMSpatternofmodelH2. Boundariesof individual exogenous lobes are indicated as solidblack lines, a) traces ofmagnetic foliationplanes superposedon themodel section, b) plungedirections of magnetic lineations with contours of their plunge angle fromvertical section, c) contour diagram of shape parameter T, d) contour diagram of the degree of AMS parameter P.
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three lastly emplaced lobes (lobes 3, 4 and 5). The rest of the fabricsaround the center is dominated by lower intensity fabrics (P=1.14–1.28)and it decreases in value towards the dome margins.
4. Dynamic scaling of the experiments
Our experimental lava domes can be regarded as realisticequivalents to the natural domes only if criteria of dynamic scalingare satisfied (Hubbert, 1937; Ramberg, 1981; Koyi, 1997). Experimentsare always simplification of nature, which is given by the physicalproperties of the analogue materials used, which mostly do not scaleup perfectly with properties of their originals. In our models, effect ofsurface cooling of natural lava domes creating a solid crust isneglected. We will discuss this limitation below. For present we willconsider that shapes of natural lava domes reflect solely the effect ofyield strength of the lava forming the dome (Blake, 1990; Griffiths andFink, 1993). Yield strength of an isothermal Bingham plastic can becalculated as in approach of Blake (1990) and Griffiths and Fink (1997)who used the following formula after Nye (1952):
τ0 ¼ H2ρgC2R
; ð1Þ
where τ0 is the yield strength of the material forming the natural (orexperimental) dome, H is height, ρ is density of the extrudedmaterial,R is radius, g is the acceleration due to gravity. C is constant of valueffiffiffi2
pin original Nye's work (Nye, 1952). Blake derived another value of
this constant (C=1.76) to fit themeasured true yield strength to shapes
h(r) of experimental domes predicted by Nye's analytical solution foraxisymmetric dome in static balance:
h2 ¼ 2h0 R−rð Þ; ð2Þ
where h0=τ0 /ρg, and r is the radial coordinate.This relationship does not depend on the effusion rate (Blake, 1990).
The shape of our experimental dome extruded on horizontal linoleumsurface (modelH5, Fig.11) is best predictedby the shape of dome in staticbalance (Eq. (2)) using yield strength calculated with Nye's constant.
Shape of dome calculated using constant derived by Blake (1990)underestimates the dome height. This might be due to higher shearstress given by the coupling of the plaster extrusions on the base inour experiment than in the experiments of Blake (1990). We thereforeuse the Nye's constant for calculation of yield strength of experimentalmodels and natural lava domes for the purpose of dynamic scaling.
Values of yield strength calculated using both Nye's and Blake'sconstants (Eq. (1)) for the plaster models of density 2300 kg/m3 arehigher for ‘lobate domes’ and smaller for experiment H1 than the valuesof “apparent yield strength” measured by the rheometer (Table 1).Higher predicted values of yield strength for ‘lobate domes’ could becaused by the mechanism of piling of the successive lobes around thevent area, because the formula by Nye (1952) (Eq. (2)) accounts forhomogeneous spreading of the material. In addition, higher friction onthe walls of plaster extrusions given by presence of sand and themechanical interaction of plaster and sand producing the moundsshould also slowdown the lateral spreading of plaster. In contrast, lowerpredicted values of yield strength for experimentH1probably reflect the
Fig. 9. AMSpatternofmodelH4. Boundariesof individual exogenous lobes are indicated as solidblack lines, a) traces ofmagnetic foliationplanes superposedon themodel section, b) plungedirections of magnetic lineations with contours of their plunge angle fromvertical section, c) contour diagram of shape parameter T, d) contour diagram of the degree of AMS parameter P.
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fact that the flow did not attain the static balance between yield stressand buoyancy, because it was emplaced relatively rapidly after suddenburst of the material due to load overpressure. To consider whichviscosity valuewas dominant during spreading of plaster in themodels,we calculate the Bingham number (Blake, 1990):
B ¼ τ0HηU
;
where η is the viscosity corresponding to the higher shear rates at theflat parts of rheological curves (“Bingham plateaus”) and U is the lateraladvance velocity of thedomemargin. This quantity is given byR/t,wheret is time. Only for values of Bingham number much lower than one canbe the flows regarded as Newtonian, characterized by the flat parts ofcurves in Fig. 1. In our case, high value of this parameter for all modelsshows that the flow is dominated by the higher viscosities characteristicfor low shear rates. Anyway, measured variation of viscosity within oneorder of magnitude plays negligible role during dynamic scaling.
To compare the scaling relationships between the model andoriginal, we only need to know the time necessary for formation ofnatural extrusive domes, their final dimensions and viscosities, yieldstrengths and densities of the materials that form them. On the basis ofsimilar aspect ratios, we have selected the Obsidian Dome rhyolite lavaflow (CA, USA) (Castro et al., 2002) as an original to comparewithmodelH1 andMt. St. Helens dome (WA, USA) and Mt. Unzen dome (Japan) formodels H3 and H4, respectively. Parameters for the selected extrusionsare listed in Table 2.
Only final dimensions of the bodies at given growth period areconsidered. For Mt. St. Helens, the final shape at the end of the 1980–1986 growth period (Swanson and Holcomb, 1990) is considered. Mt.
Unzen dome is characterized by dimensions at the end of its 1990–1995 growth period (Nakada and Motomura, 1999). Yield strengths ofselected lava bodies are calculated using Eq. (1). Both laboratory andnatural flows are slow, with Reynolds numbers well below 1 (Merle,1998), therefore inertial forces can be neglected in the scaling analysis(Hubbert, 1937). To test, if dynamic similarity was attained in themodels, scaling ratios for yield strength σ=δλ and viscosity ψ=δλτ arecalculated, where δ is the ratio of densities, λ ratio of lengths and τ
Fig. 10. AMS pattern of horizontal section throughmodel H4 coinciding with Section 2 in Fig. 6. Boundaries between individual exogenous lobes are indicated as solid black lines,a) traces of magnetic foliation planes superposed on the contour diagram of their dip angle, b) plunge directions of magnetic lineations with contours of plunge angle fromhorizontal section, c) combined contour diagram of shape parameter T from vertical and horizontal section of model H4, d) combined contour diagram of shape parameter P fromvertical and horizontal section of model H4.
Table 2List of parameter values for three lava domes selected as “originals” to be comparedwith experimental bodies
Density of lava (kg/m3) 2000 [3] 2300 [7] 2400 [10]Crystallinity (vol.%) 5 [1] 40 [8] 55 [10]
Final dimensions are indicated for Obsidian Dome. Final dimensions at the end ofrespective growth periods are indicated for Mt. St. Helens (1980–1986) and Mt. Unzendomes (1990–1995). [1] → Castro et al. (2002), [2] → Merle (1998), [3] → Fink (1980),[4] → Swanson and Holcomb (1990), [5] → Fink and Griffiths (1990), [6] → Griffithsand Fink (1993), [7]→Hoblitt and Harmon (1993), [8]→ Cashman and Taggart (1983),[9]→ Nakada et al. (1995), [10]→ Nakada and Motomura (1999), [11]→ Blake (1990),[12] → Nye (1952).
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ratio of times necessary for formation of the model and original,respectively (Table 3) (Hubbert, 1937).
Model H1 can be considered as a convenient analogue of ObsidianDome rhyolite flow, although yield strength of the model materialshould be an order ofmagnitude smaller. Model H3 shows two orders ofmagnitudehigher viscosities thanappropriate to scale correctlywithMt.St. Helens dome, although its yield strength fits exactly. In contrast, thecalculated value of viscosity in the model H4 compared with Mt. Unzenshould be 2 orders of magnitude lower and its yield strength of 270 Pahigher than expected to scalewith original lava dome. In conclusion,wecan suggest that both ‘lobate domes’ are good approximation of dacitedomes possessing yield strength of Mt. St. Helens dome emplaced in atime range of 5 years with viscosities of 1010 Pa s, which is half-way onthe viscosity range between dacites of Mt. St. Helens and Mt. Unzen.Difference of two magnitudes of viscosity is however relatively per-missible range, having inmind that lava viscosity estimates are based oncounterbalancing the effect of water and crystal contents (Lejeune andRichet, 1995; Dingwell et al., 1996; Nakada and Motomura, 1999).Rhyolite melts range in viscosities from 106 Pa s to 1012 Pa s due todecreasingwater content (Nakada andMotomura,1999; Giordano et al.,2004; Yokoyama, 2005) and viscosity of highly crystalline andesite ofSouffriére hills, Montserrat was suggested to be 7 orders of magnitudehigher than its crystal-free hydrous equivalent (Sparks et al., 2000).Furthermore, viscosity and yield strength change considerably withtime during degassing driven crystallization in upper parts of thefeeding conduits below growing lava domes (Melnik and Sparks, 1999).The influence of crystals of different shapes and size distributions oneffective viscosities of lavas at higher concentrations is notwell-foundedand is being estimated indirectly (Melnik and Sparks, 1999). We canbypass the “effective viscosity”problemgivenby the amountof crystals byscaling down the viscosity of a crystal-free hydrous melt of 106 Pa s fromMt. St. Helens (Murase et al., 1985) to viscosity of water (10−3 Pa s) inplaster suspensions. This approach gives the same order of magnitude forviscosity of H4 experiment as a model of Unzen dome and one order ofmagnitude difference for model H3 as an equivalent to Mt. St. Helensdacite dome. Contents of crystals in both dacite domes of (42–50%) thencorrespondwellwith volumeof hemihydrate particles (47%) in themodel.However, their size scales up to 10 cm big phenocrysts in the dacite lava.
5. Discussion
The experimental bodies created by squeezing the plaster of Paris intoa sandbox using a hydraulic squeezer seem to reproduce many featurestypical for natural lava domes. Models of lava bodies correspond in shapeto extensive rhyolite flows for the “thin” slurries, while experimentsprepared with “thick” plaster slurries produce lobate domes resemblingin shape and evolution complex dacite domes like on Mt. St. Helens or
Unzen. Similarity with Unzen dome applies only for its second growthepisode during the 1990–1995 growth period that showed combinedendogenous and exogenous growth (Nakada et al., 1999).
We assume that the cooling of crystal-poor obsidian lava does notgreatly affect the manner of its viscous spreading due to its relativelyfast emplacement. Regarding the crystal-rich lavas, it was suggestedthat for andesite lava at Soufriére hill volcano, heat conduction fromthe surface of extrusive flows is negligible (Sparks et al., 2000) andthat stiffening of the lava is caused by degassing driven crystallizationin the conduit and the dome itself. It is thus reasonable to assume thatisothermal experiments with suspensions of different solid particleconcentrations are a good first-order approximation of such systemsgoverned by complex kinetic processes. Experimental extrusions withsuspensions of different solid particle concentrations are comparablewith extrusion style of lavas with different degree of crystallinity.Crystal-rich lavas typically form domes with more pronouncedexogenous lobes and grow at slower rates than lavas with smallerproportion of crystals (Hale and Wadge, 2008).
5.1. Internal kinematics of flow in model extrusions
The internal fabric distribution of the “thin” experiment H1 reflectsthe kinematic framework in axisymmetric lava domes (also recognizedbyCastro et al. (2002) in obsidian lavaflow) previously simulated usingdifferent experimental techniques (Buisson and Merle, 2002, 2004;Merle, 1998). In the upper part of divergent flows, circumferentialstretching produces lineations that are parallel with the frontalmarginof the flow in plan-view. The kinematic analysis of conjugate shearzones on the surface of experiment H1 is also consistent with thisinterpretation. On the base of lateral flow, radial stretching producesimbricated magnetic lineations and foliations that dip towards thevent (Buisson and Merle, 2002, 2004; Merle, 1998).
Internal strain pattern as obtained by Buisson and Merle (2004)accounts for Newtonian rheology and constant extrusion rate of thematerial. In contrast to Newtonian extrusions that continued to spreadafter the extrusion stopped, in our experiments with pseudoplasticplaster slurries, domes extruded on sand surface retained their finalshapes. Consequently, the topology of the domains of characteristic 3Dstrain throughout the domes (Buisson and Merle, 2002, 2004) isstrongly modified due to the rheology of analogue material used.
In the “thin” experiment (H1), the vent domain shows a plug-flowprofile induced by vertical ascent of the plaster, which is defined byattitude of the magnetic foliations with flat central part and thin zoneof imbricated fabrics at the walls. Similar narrow zone of imbricatedfabrics at the base of lateral flows associated with radial stretching(Buisson and Merle, 2004) contrasts with relatively larger volume ofthis domain in silicon putty experiments (Buisson and Merle, 2004).
Extrusions with “thick plaster” suspensions revealed a complexinternal fabric distribution resulting from successive emplacement ofplaster lobes. Each emplaced lobe shows characteristic “plug-flow”
profile defined by alignment of magnetic foliations. This profile widensfrom the narrow “stem” of the lobe in the vent to the interior of eachlobe. Newly emplaced lobes clearly overprint fabrics in the surroundingplaster units. Viscous drag induced by subvertical ascent of new lobesalong shear zones produces lineations aligned parallel with this shearzone and in the direction of plaster lobe motion. This is clearlydemonstrated by AMS fabric captured in vertical sections of ‘lobate’domes (Figs. 8 and 9). In contrast, horizontal section through lower partof the ‘lobate’ dome H4 reveals lineations aligned subhorizontally andparallel with boundaries of newly emplaced lobes and thereforedominant circumferential stretching at this level (Fig. 10). The contourdiagramsof P parameter revealed that the intense fabrics that develop inthe interior of new lobes are preservedmainly in upper parts of the olderlobes (note partial conservation of fabrics in lobe 4 of model H4 (Figs. 9and 10)). This means that the lobes are to some extent passivelytransported “en-masse” due to emplacement of newer lobes. Rotation of
Table 3Scaling ratios between the model and original, respectively, for experimental andnatural lava domes
Model H1 H3 H4
Original Obsidian Dome Mt. St. Helens Unzen
Model ratios (Hubbert, 1937)λ (size) 2.0×10−4 4.0×10−4 3.7×10−4
Values of viscosities and yield strengths for correctly dynamically scaled experiments,reflecting evolution of corresponding lava domes, are indicated to be compared withcorresponding properties of plaster suspensions in Table 1.
110 P. Závada et al. / Tectonophysics 466 (2009) 101–113
the lobes along shear zones produces fabric discordanceswith respect toadjacent lobes. Lateral advance of the lobes also induces localizeddeformation in its surroundings, which is manifested by formation of ashear zone extending from the boundary of lobe 5 to the margin inmodel H4 in the direction of lobe motion and numerous small shearzones disrupting the thin banding (Fig. 6).
The consequence of the suggested scheme of exogenous lobegrowth implies that new lobes initiate at the vent due to formation ofnew shear zone at the walls of previously emplaced lobes, which aremarked by steep fabrics. The propagating shear zone further transectspreviously emplaced lobe subparallel with its internal fabrics toaccommodate rise of new plaster portion. The steep fabrics in the ventzone and orientation of fabrics in surrounding units thus seem tophysically prescribe the preferential pathway for new emerging lobes.The margins of the vent zone are finally formed by several parallelshear zones, representing walls of successively emplaced new lobesand all reveal vertical fabrics. It should be noted that the newly formedlobes are confined by a single shear zone of asymmetric cylindricalshape in 3D (Watts et al., 2002), although in vertical section, we cansee only two traces of this curved cylindrical plane.
In contrast to previously published experimental results oninternal fabric development in lava domes (Buisson and Merle,2002, 2004), our models reveal a distinct zone of fabric transpositionin the vent of the extrusions above the feeding conduit. In this domain,subvertical prolate fabrics generated in the conduit are transposed byradially diverging flow in the vent into subhorizontal oblate fabricsinduced byweight of the already emplaced plaster material above thiszone (Kratinová et al., 2006). This transposition is also reflected bythickening of the vertical dark bands and diffusion of their boundaries.We suggest that this transposition of fabrics occurs at a certain level,where vertical channel confining the flow increases in width leadingto progressive flow divergence. The critical level and volume of thetransposition domain is governed by internal pressure and rheology ofthe extruding material, weight of the overburden and strength of thehost material. While in the “thin” model (H1), this domain en-compasses the major part of the vent, in “thick” extrusions (H3 andH4), it is confined to a narrow zone in the lastly emplaced lobe.Numerical models of fabric development in salt diapirs driven up-
wards by buoyancy suggest that this transposition zone occursdirectly below the upper surface of diapiric head (Cruden, 1990),which contrasts with presented data from forceful extrusions ofoverpressured analogue material in our experiments.
5.2. Mechanism of lobe formation
5.2.1. Exogenous growthIn comparison with previous models of dome extrusions using
pseudoplastic materials (Blake, 1990; Griffiths and Fink, 1997; Finkand Griffiths, 1998), our models of “lobate domes” reproduce thepulse-like ascent of relatively large exogenous and smooth lava lobesabove the surface of a circular dome in plan-view (Swanson et al.,1987). Experiments with pseudoplastic slurries at constant effusionrate at low or no cooling-induced solidification on the surface did notshow emplacement of lobes (or spines) (Blake, 1990; Griffiths andFink, 1993). Only in kaoline+wax models of Griffiths and Fink (1993),formation of spines was generated by relatively rapid cooling of modelmaterial and/or relatively slow effusion rate. However, individualportions of material in “lobate” to “spiny” regime of Griffiths and Fink(1993) were angular in shape, and produced a dome of star-fishoutline from top-view (Griffiths and Fink, 1993, 1997). This extrusionstyle greatly differs from our evidence of lobe formation. What causesthe transition from mostly endogenous to combined endogenous andexogenous growth of model domes in our isothermal slurryexperiments?
5.2.2. Growth dynamics in natural domesFor andesite dome at Soufriere Hills Volcano, Montserrat, where
cooling from the surfacewas considered as unimportant in controllinggrowth, pulsatory growth rate was attributed to the “slip-stick”mechanism (Voight et al., 1999; Sparks et al., 2000). This mechanismreflects the cyclic change of two states: 1) ascent of “thick” crystallinemush plug of andesite lava is inhibited by high shear stresses onconduit walls; 2) lava ascent is resumed due to elastic deformation ofthe conduit walls due to increased overpressures in the lava below theplug, which decreases shear stresses resisting the ascent of the plug.This is reflected by periodic inflation and deflation of the whole
Fig. 11. Correlation of dome shapes with height to width profiles characteristic for static balance of yield stress and buoyancy (Nye, 1952). Note the close proximity of H5 dome shapewith Nye's solution (a) and less irregular surface outline and less clearly defined “lobes” than in experiments extruded on sand, where exogenous lobes “pop-up” above the profilegiven by Nye's solution (b).
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volcanic edifice (Voight et al., 1999). The “slip-stick”mechanism couldbe responsible for unstable flow of plaster in our experiments, whererising plaster material in the vent can induce elastic deformation ofsand.
There is another mechanism that could be responsible for theobserved dome evolution. According to the theoretical analysis of lavaflow dynamics fed by a single magma chamber by Stasiuk and Jaupart(1997), the superficial lava flow passes through a distinct transition ofgrowth regimes from emplacement of thick voluminous flows toemplacement of small volume batches of lava of increasing aspectratio. This transition corresponds to the state, when pressure due toweight of a lava column above the vent reaches equilibrium with thechamber overpressure. It was suggested that a transition between thetwo regimes can occur, when “coupling” between surface lava flowand deep plumbing system is enhanced by either the formation ofsolid crust due to cooling or by the yield strength of lava impeding thespreading, although these variables are not implemented in theanalysis (Stasiuk and Jaupart, 1997).
5.2.3. Transition of growth regimes in the experimentsWe suggest that growth in our experiments could pass through the
same transition of growth regimes as indicated by Stasiuk and Jaupart(1997), when equilibrium of pressure in the vent controlled by thesqueezer and weight of the plaster above the vent was attained. Thisstage could be reflected by the temporarily waning effusion rate.Additional pressure increase from this time induces emplacement ofthe first plaster lobe, which disrupts the dome's symmetry. In otherwords, the dome fails to spread symmetricaly and its weight causesbackpressure opposing rise of new material. Because plaster suspen-sion is pseudoplastic, stress accumulation at the top part of theconduit at increased overpressure induces formation of cylindricalshear zone bounding the first exogenous lobe (Hale andWadge, 2008).This moment marks the transition from endogenous to exogenousgrowth regimes.
The role of “coupling” (Stasiuk and Jaupart, 1997) or “elastic conduitwalls” (Voight et al., 1999) is more clear from comparison of experimentH5 (Fig. 11), where plaster was extruded from the steel conduit on flatlinoleum surface and shows aspect ratio 0.37, in contrast to equivalentplaster slurry experiment H2 of aspect ratio 0.43 extruded on sand.Experiment H5 is marked by higher amount of smaller successivelyemplaced portions of plaster and less irregular surface than model H2.This can be due to easy horizontal spreading on the smooth surface,reducing the shear stress on base of lateral flows and therefore reduced“coupling” above the vent in terms of the model of Stasiuk and Jaupart(1997). Another option are the inelastic walls of the steel conduit notallowing the “slip-stick”mechanism.ModelH5 shows closeproximity tothe extrusion surfacedefinedby static equilibrium (Eq. (2)) in contrast tothe models extruded on sand, where plaster lobes “pop-up” above thissurface. This effectmight alsoexplain,why is thegrowthofMt. St.Helensdome better approximated by BCS (buoyancy-crust strength) regimerather than BPI (buoyancy-plastic interior) of Griffiths and Fink (1993)and Fink and Bridges (1995).
5.3. Implications of analogue modeling for dynamics of complex domes
Exogenous lava lobes on Mt. St. Helens dome were interpreted as“outbreaks” of more liquid lava due to brittle failure of solid carapace,which extended as the dome endogenously inflated by a new pulse ofmagma (Fink and Griffiths, 1990). In our experiments, the exogenousoutgrowths are only surface expressions of large lobes that compriselarge volume of dome's interior. The degree of pressure build upbetween emplacement of following lobes seems to be controlled byinternal fabrics of the dome, since shear zones bounding new lobestransect previously emplaced lobes subparallel with their internalfabrics. For domes formed by “thick”magmas like crystal-rich dacites,where lobes can be generated the sameway as in our experiments, the
crust should not hamper subvertical ascent of individual lobes,because stresses in the crust induced by rise of lobes should be ap-plied mainly subparallel to columnar joint planes that affect the crust(DeGraff and Aydin, 1987).
The proposed mechanism of lobe formation causes the lobes tobecome emplaced in opposite direction than previously emplacedlobes and explains distribution of successively emplaced lobes on thesurface of Mt. St. Helens dome during its 1980–1986 growth period(Swanson et al., 1987). Activity of shear zones controlling exogenousgrowth could also explain seismic signals showing normal faultingwithin the dome (Nakada andMotomura, 1999). Endogenous inflationprior to emplacement of lobes is caused by extension and inflation ofthe material around the vertically moving exogenous lobes. Thecomparison of models extruded on smooth surface and into deform-able sand (Fig. 11) suggest that material of finite strength (like sand inour experiments) confining the vent can control the dome emplace-ment dynamics. This is supported by the fact that growth of lobes ofMt. St. Helens dome was preceded by thrusting of the crater floor(monthly reports on the activity of volcanoes, Smithsonian Institution(1980–2006), http://www.volcano.si.edu).
We can conclude that our experimental technique provided a newinteresting insight into the interior of lava domes, mechanism of theirevolution and their internal fabric development, although growth ofnatural lava domes is governed by complex kinetic processes (degassingdriven crystallization and stiffening of the lava) (Sparks et al., 2000). It isclear that the transition between endogenous to exogenous growth isprovided by the rheological properties of the extruding material(pseudoplastic with yield strength) and also by the mechanical proper-ties of the host rock environment.
New experiments should be conducted at controlled rates ofoverpressure build up, with slurries of different “thickness” andvariable elastic properties of the surroundingmaterial to constrain thecontrolling mechanisms governing the “pulsatory” growth of theextrusions.
6. Conclusions
Experimental results of fabric development using AMS in modellava domes using plaster of Paris have shown similar results for “low-aspect ratio” lava domes or “gravity flows” for dilute suspensions asmodels with Newtonian materials (Merle, 1998; Buisson and Merle,2002, 2004). Extrusions of “thicker” suspensions resulted in morecomplex domeevolution similar to natural crystal-rich lava domes thatwas not encountered in previous experiments on dome growth usingBingham or pseudoplastic slurries. This behavior is attributed eitherdue to a transition in growth regimes, when weight of the domecolumn is equilibrated with the buoyancy of the rising material(Stasiuk and Jaupart, 1997) or due to “slip-stick”mechanism reflectingelastic deformation of the vent walls (Voight et al., 1999). New lavalobes in the experiments form by “decapitation” of older lobes alongasymmetric cylindrical shear zones bounding the new lobes.
Acknowledgements
This work would not be possible without material support from PetrJakeš. This work was financed from the grants of the Czech ScienceFoundation (GAČR), No. 205/03/0204 and the “Junior grants” of theGrantagency of the Czech Academy of Sciences (GAAV), No. B301110703 andNo. B300120702. Prof. František Hrouda is thanked for consultations andfor providing us the measurements on the MFK1 Kappabridge device inAgico, Inc.We thank toDr. E. Petrovský for using theADEEV9VSMdeviceon the Geophysical Institute (ASCR), to Dr. Petr Štern for carrying out therheometry of suspensions, to Dr. Willi Pabst for measurement of particlesizes and to HSR-Rudolf firm for construction of the apparatus.Comments of two anonymous reviewers have helped to improve thefirst version of the manuscript.
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