Magma–tectonic interaction and the eruption of silicic batholiths

Earth and Planetary Science Letters 284 (2009) 426–434

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Magma–tectonic interaction and the eruption of silicic batholiths J. Gottsmann a,⁎, Y. Lavallée b, J. Martí c, G. Aguirre-Díaz d a

Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom Department of Earth and Environmental Sciences, Ludwig-Maximilian University, Theresienstr. 41, 80333 Munich, Germany c Institute of Earth Sciences “Jaume Almera,” CSIC, Luis Sole i Sabaris is/n, Barcelona, 08028, Spain d Centro de Geociencias, UNAM, Campus Juriquilla, CP 76230, Juriquilla, Querétaro, Mexico b

a r t i c l e

i n f o

Article history: Received 5 May 2008 Received in revised form 20 April 2009 Accepted 6 May 2009 Available online 30 May 2009 Editor: C.P. Jaupart Keywords: magma crystal-liquid mush relaxation time strain rate volcano–tectonics caldera

a b s t r a c t Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > 106 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from Spain, Mexico, USA and the Central Andes. These silicic caldera-forming eruptions generated deposits characterized by i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initial fall-out. The field evidence is inconsistent with most caldera-forming deposits, which are underlain by initial fall-out indicating deposition from a sustained eruption column before the actual collapse sequence. In contrast, the documented examples suggest early deep-level fragmentation at the onset of eruption and repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost exclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptability and eruption initiation processes. In this paper, we present an analysis of eruption promoters from geologic, theoretical and experimental considerations. Assessing relevant dynamics and timescales for failure of crystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily involves an external trigger by near-field seismicity and crustal failure. Strain rate analysis for dynamic and static stressing, chamber roof collapse and rapid decompression indicates that large “solid-like” silicic reservoirs may undergo catastrophic failure leading to deep-level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poor magmas or pure melt. Eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal-melt mush, decompression by block subsidence or a combination of both. We find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol.% for strain rates on the order of > 10− 3 to 10− 4 s− 1. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Magma stalled in an upper-level crustal reservoir consists of molten silicate fluid and various proportions of crystals and bubbles. According to Marsh (1989), increasing crystallinity (ϕ) due to the propagation of the solidification front transforms magma from a crystal suspension (0≤ϕ ≤ 0.25) to a crystal-melt mush (0.25 b ϕ b 0.55) and finally to a rigid crust (0.55 b ϕ ≤ 1). The eruptability of magma is generally seen to be directly dependent on its crystallinity and thus on its rheology. Increasing crystal content has two important consequences for magma rheology. Firstly, it dramatically increases effective viscosity and hence affects its flow behaviour and secondly, it strongly affects its mechanical properties (Dingwell, 1997). Most explosive volcanic eruptions tap crystal suspensions with bulk properties favorable for viscous flow, ascent and thus eruption (Woods,1995). The threshold magma viscosity for eruption is on the order of 106 Pa s (Takeuchi, 2004). Above this limit ⁎ Corresponding author. E-mail address: [email protected] (J. Gottsmann). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.05.008

magma tends to pond rather than erupt irrespective of the magma composition (Scaillet et al., 1998). With increasing crystallinity, a mush develops towards a rigid percolation threshold and by reaching a crystallinity exceeding 0.5, magma is believed to be uneruptable (Marsh, 1989; Vigneresse et al., 1996). Of course, there is evidence from effusive eruptions that produce dome lavas with effective viscosities of well in excess of 1010 Pa s, yet high crystallinity is attributed to late stage decompression-driven crystallisation upon degassing within the conduit and does not reflect chamber conditions upon the onset of eruption (Sparks et al., 2000). In the case of colossal silicic explosive eruptions (≥100 km3 of magma; Volcanic Explosivity Index (VEI) ≥ 7; Newhall and Self,1982), the evacuation of a subsurface reservoir generally results in caldera collapse. Most of these eruptions are dominated by scavenging crystal-poor magma suspensions and their eruption is controlled by processes internal to the magmatic system, including overpressurisation, ring fault initiation and subsequent roof collapse (Gudmundsson, 2006). However, there are also examples of calderaforming eruptions involving crystal-rich (ϕ around and exceeding 0.5) magmas throughout the geological record, among which are the most

J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434 Table 1 List of case examples and characteristics of deposits. Case examples

Description of deposits

>>50 km3 (likely >100 km3) of crystalrich dacitic ignimbrites and lavas; crystal content of up to 60 vol.% in pumice; lithic and pumice-rich ignimbrite; primary vesicularity of pumice uncertain due to welding, but evidence for poor initial inflation; stratigraphy suggests dacites correspond to intracaldera deposits during basin development. Eocene–Oligocene ignimbrites of Durango >>200 km3 possibly >1000 km3 of State, central Sierra Madre Occidental, ignimbrites; crystal content exceeding Mexico (Aguirre-Díaz and 40 vol.% in pumice; association with Labarthe-Hernañdez, 2003) several graben systems; fissure type eruption vents; graben formation intimately related to large-scale eruption of ignimbrites (Fig. 1c); liquefaction structures in sediments immediately underlying ignimbrites along caldera margin (Fig. 1d). Cerro Panizos volcanic centre, 6.7 Ma >600 km3 DRE of two crystal-rich dacitic (Central Andes) (Ort, 1993) ignimbrites in area of normal faulting; crystal content of up to 50 vol.% in pumice; vesicularity of pumice in the lower cooling unit is less than 20 vol.%; formation of ignimbrite sheets related to onset and formation of caldera collapse evidenced by increased lithic contents in the lower unit. >200 km3 ignimbrite immediately Pagosa Peak Dacite ca. 28 Ma (San Juan Volcanic field, Basin and Range province predate eruption of Fish Canyon Tuff U.S.A. (Bachmann et al., 2000) (~5000 km3); crystal content of up to 50 vol.% in pumice. vesicularity of pumice in PPD is around 25 vol.% (at least 60% lower than pumice from FCT); angular and equant glass shards dominant; concentration of lithic fragments at base of unit indicating conduit enlargement early in the eruption; possible onset of caldera collapse related to the synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. Permo-carboniferous Prats d'Aguilo dacites (Spain) (Martí, 1996)

devastating terrestrial volcanic events: the 26 Ma Fish Canyon Tuff (Bachmann et al., 2000, 2002), the 4 Ma Atana eruption at La Pacana (Lindsay et al., 2001) and the 2.1 Ma Cerro Galan eruption(Sparks et al., 1985). Evacuation of such batholith-like reservoirs challenges our understanding of magma eruptability and eruption promoters as the magmas appear to have bulk rheological properties unfavorable for evacuation from reservoirs and subsequent eruption.

427

a flow deposit blurs the information on the original magma crystal content (Cas and Wright, 1993) and these deposits should not be used as evidence for crystal-rich magmas. All our examples expose primary magmatic crystal contents in pumices and thus interpretations are not based on crystal concentrations in the related ignimbrites (Fig. 3). Primary clast vesicularities are low (up to 0.3) in the case examples (Fig. 3). Welding in some deposits is a concern for assessing primary clast vesicularity and thus extra care was taken in selecting uncollapsed fragments for the assessment of primary vesicle content. Although vesicularities can vary significantly in explosive eruptive products (Houghton and Wilson, 1989) it is frequently assumed (particularly in numerical consideration of explosive volcanism) that fragmentation occurs at a vesicularity of about 77 vol.% (Sparks et al., 1997) and “fragmentation vesicularities” of pumices from Plinian eruptions are in broad agreement with the threshold value (Klug and Cashman, 1994) for “classic” explosive eruption scenarios. However, as explained above, the case examples show low degrees of primary clast vesicularity. The absence of pumice-fall deposit and hence lack of evidence for a substained (Plinian) eruption column indicates that vesiculation-induced fragmentation (see also next section) upon system decompression was of second order. The question is then as to how to tap and erupt magma, which defies the concept of eruptability. 3. Magma rheology and fragmentation 3.1. Model magma In quantifying rheology and fragmentation of magma relevant for the case examples, we assume hereforth a model dacite magma representing an averaged analogue to the investigated eruptions at the following conditions: temperature of 750 °C, water activity of 1 wt.% (below unity at 150 MPa or 5 km depth), a calc-alkaline metaluminous bulk composition a peraluminous interstitial melt phase (Table 3) and i) ϕ ≤ 0.2 (crystal suspension) and ii) ϕ = 0.55 (crystal-liquid mush). 3.2. Rheology and fragmentation of suspensions At low crystallinity, a magma behaves like a pure silicate melt and is a viscoelastic body, rheologically controlled by the shear strain rate. The melt behaves as a Newtonian fluid when deforming at low strain rate and neglecting viscous heating, the viscosity of the melt does not vary with increasing strain rate (Bagdassarov et al., 1994). As deformation approaches a critical strain rate, melt relaxation is retarded and once the melt reaches the strain rate at failure (fragmentation threshold) it undergoes transition from a liquid to a solid: the glass transition (Dingwell, 1996, 1997). This phenomenon is termed strain-rate (γ̇)-induced fragmentation and the fragmentation threshold is met as soon as

2. Field observations −1

We present field evidence from deposits generated by explosive caldera-forming eruptions of dacitic to rhyolitic magmas with ϕ of between 0.40 and 0.60 (Table 1) and eruptive volumes exceeding 100 km3 of magma (dense rock equivalent, DRE). Two cases relate to our own field investigations in the Catalan Pyrenees (Spain) and in the Sierra Madre Occidental (SMO, Mexico; (Figs. 1 and 2)). The remaining examples are based on published data (Table 1). The examples show important common characteristic features collated in Table 2, providing essential information on the dynamics of the eruption of crystalliquid mush. It is important to note that we are concerned here with pumice clasts representing juvenile samples of the fragmented magma (Figs. 1 and 3) and are as such windows into the crystallinity and vesicularity of the magma prior upon eruption. Clearly, mechanical crystal segregation and fractionation during the eruption together with concentration during the emplacement and possible reworking of

γ̇ > k
τ ml

ð1Þ

and ηs = G∞
τ ml

ð2Þ

following the Maxwell (1868) relation. Here, ηs is the melt shear viscosity and G∞ is the melt shear modulus at infinite frequency (10 ± 0.5 GPa; (Dingwell and Webb, 1990)). For a wide range of compositions, brittle magma failure occurs experimentally when γ̇ is two orders of magnitude below the critical strain rate equal to τ− 1 and thus k = 0.01. For the case of suspensions the resultant model melt shear viscosity is 108.1 Pa s after Hess and Dingwell (1996) and the magma would thus break at a strain rate of ≥1 s− 1. For the majority of explosive eruptions material failure is induced by an increase in pressure due to gas exsolution in a saturated magma

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Fig. 1. a) Surface outcrop of the Prats d'Aguilo permo-carboniferous dacitic ignimbrite (Catalan Pyrennees, Spain). Note abundance of lithic clasts and pumices rich in crystals (coin for scale). b) Close-up of crystal-rich pumice. c) Photomicrograph of collapsed and flattened pumice fragment of Prats d'Aguilo ignimbrite with crystal content of ca. 55%. Crystals are dominantly plagioclase, interstitial melt is rhyolitic in composition. Field of view is approximately 10 mm. Note crystal packing expressed as ratio of crystal diameter over mean gap width on order of 10 to 100. This criterion is employed for assessment of particle pressure during chamber agitation.

and bubble formation upon decompression as evidenced by several lines of investigations (Woods, 1995). In this scenario, pyroclastic fragmentation will occur when the time to relax an applied mechanical stress (e.g. due to bubble growth) exceeds the characteristic relaxation time (τml) of the melt phase (Papale, 1999). Quenched clasts from silicic magmas with viscosities higher than 109 Pa s, mirror the vesicularity of magma at the moment of fragmentation (Thomas et al., 1994) and we thus have to assume that primary failure (fragmentation) of the mush occurred due to processes different to the classic depressurization and bubble expansion fragmentation scenario of liquid suspensions (Barclay et al., 1995). The studied eruptions document conditions of the liquid-crystal mush and evaluating their particular rheology should allow us to inform on eruption conditions such as viscosity and critical strain rates and thus on fragmentation conditions and eruption trigger. 3.3. Rheology and fragmentation of liquid-crystal mush Several investigations have documented the drastic effect of crystals on the effective viscosity and thus the rheology of magma. Liquid-crystal mushes are known for their high effective viscosities and doctored rheology (Pinkerton and Sparks, 1978; van der Molen and Paterson, 1979). Addition of crystals to a melt impinges on the mobility of the interstitial melts and thus on the viscous response of the suspension (Einstein, 1906; Roscoe, 1952; Costa, 2005; Caricchi et al. 2007; Champallier et al., 2008; Costa et al., 2009). A fully parameterised model of magma rheology as a function of temperature, relevant water contents, chemical composition, crystal and bubble content, crystal and bubble morphology, etc is not available. However, there is unambiguous indication of a significant influence of

both crystallinity and strain rate on effective viscosity during volcanic processes. An empirical model for the non-linearity of the viscosity– crystallinity–strain rate (η–ϕ–γ̇) relationship was recently presented in Costa et al., (2009), which enables the prediction of effective viscosity as a function of both ϕ and γ̇ via : ηb ð/; γ̇ Þ =

n 1 − ð1−nÞ
erf

1+ h pffiffiffi


δ / /⁎

π/ 2/⁎ ð1 −nÞ



γ ioB/ ⁎
1 + //

ð3Þ

⁎

where x, δ, γ are empirical parameters and ϕ⁎ approximates the critical solid fraction at the onset of the exponential increase of ηb (Table 4). B is a coefficient theoretically estimated at 2.5 by Einstein (1906). Costa et al. 2009 calibrated their model using experimental results on synthetic systems with relevance to volcanism. To calculate effective viscosity of the model mush for a range of relevant strain rates, we scale fit parameters ξ, δ, γ and ϕ⁎ against γ̇ using expressions reported in Caricchi et al. (2007). Results are shown in Fig. 4. We find effective viscosity to increase by 1.5 to 3.2 orders of magnitude compared to the melt shear viscosity for ϕ = 0.55 and γ̇ values of between 10− 3 s− 1 and 10− 6 s− 1, respectively. These theoretical approximations are in broad agreement with recent experimental work on highly crystalline dry (≤0.1 wt.% water) natural magmas (Lavallée et al., 2007), which describes the strain rate γ̇ and temperature (T [°C]) dependence of the effective viscosity ηb to:   log ηb = − 0:993 + 8974 = T − 0:543
log γ̇

ð4Þ

For γ̇ of 10− 6 s− 1 to 10− 3 s− 1 at 750 °C, ηb is 1014.2 and 1012.6 Pa s, respectively. The equivalent melt shear viscosity is 1011.2 using the Hess and Dingwell (1996) model. Scaling the respective viscosities to

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Fig. 2. a) Photographs showing relationship between ignimbrites and underlying continental sediments (red beds) of the Sierra Madre Occidental, Mexico (persons for scale). Note dissected nature of deposits indicating post-caldera faulting; b) normal faulting in red beds indicates association of tectonic extension and subsequent ignimbrite emplacement (hammer for scale); and c) close-up of liquefaction structures (pen for scale) in red beds indicating seismic stressing before ignimbrite emplacement. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

a water content of 1 wt.% assuming a linear shear viscosity vs. water content relationship for isothermal conditions (a valid assumption for water contents up to 1 wt.%) yields effective viscosities between 109.5 and 1011.2 Pa s, respectively. This empirical expression assumes that near static conditions are satisfied when γ̇ = 10− 6 s− 1. However, the onset of cracking and brittle response of a mush cannot be estimated via Eqs. (1), (2), (3), and (4). When a crystal-rich magma or mush is upset, the stress decouples between the melt and crystal phases, and focuses primarily at the points of contact between crystals. The crystals are stronger than the melt and because they cannot accommodate significant deformation, they are prone to fracture (Cordonnier et al., 2009). An abundance of crystals thus argues for a rheology favoring brittle response upon perturbation compared to a crystal-poor system (Lavallée et al., 2008) and numerical calculations for fragmentation thresholds indicate an overall deepening of the fragmentation level and a decrease of vesicularity at fragmentation (Caricchi et al., 2007). To explore the influence of crystallinity on the brittle–ductile transition, we have experimentally determined the shear thinning behaviour and the onset of failure of a crystal-rich (ϕ = 0.55) magma at 940 and 980 °C — temperatures at which the interstitial melt viscosities were 108.6 and 108 Pa s, respectively (i.e., similar to the model magma's interstitial melt viscosity) following the procedure described in Lavallée et al. (2008). The near static effective viscosities were determined to be 1011.8 and 1011 Pa s, respectively, about three

orders of magnitude higher than ηs close to empirical prediction for strain rates between 10− 6 s− 1 and 10− 5 s− 1 (Fig. 5). Experiments were carried out under different applied stress increments. Brittle behaviour was indicated by acoustic emissions and the onset of failure of the crystal-liquid mush (triangles in Fig. 5) was characterized by an acceleration of energy released during successive microcracking. The samples underwent catastrophic failure at strain rates approximately 2 orders of magnitude lower than those characteristic for failure of crystal-poor magmas or melt. Catastrophic failure of the mush was observed to depend on temperature, however to a smaller extent than crystal-poor magmas. The obtained γ̇/η gradient of mush failure (Fig. 5) is shallower then that of crystal-poor magma or melt failure, suggesting a relatively stronger contribution of crystals to brittle behaviour. These findings play a pivotal role in assessing the eruption mechanism and in particular the role and timing of fragmentation in the documented cases. 4. Analysis and discussion of eruption promoters and eruption dynamics While the above kinematic relationships hold, the geological evidence excludes magma failure by catastrophic vesiculation of an overpressurised chamber. It is thus worth considering an external process that resulted in the deep-seated disruption of a highly crystalline

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Table 2 Common eruption characteristics of case examples (cf. Table 1).

Table 3 Model bulk magma and interstitial melt composition (normalised to 100% anhydrous).

Characteristic Description

Interpretation

Oxide

Bulk magma

Interstitial melt

Eruption volumes

Indicates abundance and eruption of a large body of magma at shallow depth Fragmentation and eruption of a pluton-like magma reservoir with rheological properties unfavorable for eruption Fragmentation not primarily caused by expanding gas phase

SiO2 TiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O P2O5

68.5 0.44 15.38 3.45 0.07 0.95 2.92 3.9 4.2 0.19

77.5 0.15 12.47 0.55 0.06 0.06 0.72 2.7 5.39 0.01

Magma crystal content

Colossal volcanic eruptions of VEI ≥ 7, involving ≥100 km3 of magma (dense rock equivalent) Pumices with crystallinity of > 0.4

Magma Poorly inflated primary pumices vesicularity with vesicularities ≤25 vol.%, no indication of low vesicularity due to phreato-magmatism Lithology Crystal-mush pumices found in deposits of and overlying deposits rich in co-magmatic (plutonic) lithics Eruption Absence of initial (sub-) Plinian dynamics phase; formation of co-ignimbritic lag breccia rich in plutonitic rocks

Regional stress field

Basin and Range-type continental extension

Conduit enlargement, erosion of conduit wall and deepening of fragmentation level to magma chamber depth Early eruption plume collapse due to high bulk density and high discharge rate; lack of supersaturated magma at reservoir top; deep initial fragmentation at reservoir depth, conduit erosion, onset of vertical caldera collapse. Formation of tectonic grabens during normal faulting

magma reservoir, given the close association of the case examples with significant regional tectonic structures. External forcing may be an effective way of driving such a system into catastrophic failure and we shall explore this possibility in the following sections. 4.1. Volcano–tectonic interaction and the ductile–brittle transition The large-scale evacuation of a batholithic body as documented in our examples could be related to active basin formation during regional extensional tectonics. There are a number of examples where basin formation is intimately related to, or accompanied by, largescale silicic volcanism (Marti, 1991; Aguirre-Díaz and McDowell, 1993; Hawkesworth et al., 1995; Breitkreuz and Kennedy, 1999; Aguirre-Díaz et al. 2008). Growth of a pluton exceeding several hundreds of cubic kilometers in volume, is likely to significantly alter the local or even regional stress field over time. Eventually, the crust will have to accommodate an increasing magmatic pressure as well as a significant thermal perturbation (Jellinek and DePaolo, 2003), both of which result in volume increase and upward doming of surrounding rock. Doming in turn results in deviatoric extensional stresses at the surface and fosters tensile failure at high topographic levels as documented by

central apical grabens on resurgent domes or in models of caldera formation (Komuro et al., 1984). Liquefaction features in continental sediments (red beds) stratigraphically immediately below the ignimbrites at the SMO (Fig. 2) demonstrate the close association of volcanism and tectonic stressing. We suggest that doming and/or active normal faulting may promote their eruption. Near-field seismicity, active extension and crustal failure represent external forces, which we discuss as possible eruption promoters next. 4.1.1. Near-field seismicity Seismic triggers of volcanic activity including large volcanic eruptions have been invoked for a number of cases recently (Lemarchand and Grasso, 2007; Linde and Sacks, 1998; Linde et al., 1994; Marzocchi, 2002), yet, there is a lack of consensus as to its importance. It is for example thought that far-field (>100 km) earthquakes generally induce strain rates too small to trigger eruptions unless the magmatic system is already close to critical instability (Manga and Brodsky, 2006). While we do not wish to enter this discussion, we feel that near-field effects are worth investigating in the context of the enigmatic nature of the case examples, particularly in the light of a recent rhyolitic eruption during active faulting along a ca. 60 km long segment of the Afar rift in 2005, which documented the close relationship between normal faulting and explosive activity (Ayalew et al., 2006; Wright et al., 2006). The following discussion is thus concerned with the near-field (above or within a few to 10 km from magmatic reservoirs) effects, that is seismicity associated with crustal extension and graben formation. Dynamic seismic stress changes for large near-field events occurring over seconds to tens of seconds are large in magnitude yet short-lived, but may induce high enough strain rates (>10− 2 s− 1) (Manga and Brodsky, 2006) for the model magma to undergo catastrophic failure, which results in the crystal-liquid mush to shatter (Fig. 5). The seismic moment (M0) associated with for example an Mw = 7 event is 4 × 1019 Nm (Mw = 2/3 [log10M0 − 9.1]). Assuming a Young's modulus of 30 GPa and fault slip of 50 to 100 m, an event of such magnitude requires down-throw of a fault area of about 4 to 80× 107 m2, which is of a conceivable scale given that the case caldera in the SMO is bound either side by a 35 km long fault system. The key problem despite seismically inducing failure is however: how to drive the mush to erupt? 4.1.2. Eruption initiation and the role of rapid decompression The deposits document that shattered mush and co-magmatic lithics are somehow funneled to and erupted at the surface in the form of widespread ignimbrite volcanism. The process of extraction from the Table 4 Model parameters (Eq. (3)) for calculation of effective viscosity for model mush for strain rates between 10− 6 and 10− 3 s− 1.

Fig. 3. Close-up photograph showing ignimbrite containing uncollapsed poorly vesicular pumice fragments (outlined) with crystallinity of ca. 0.55. Crystals of predominantly quartz, feldspar, and biotite are embedded in a scarce devitrified groundmass. Long axis of large pumice clast is 6 cm. Photograph taken by I. Petrinovic.

d g ϕ⁎ ξ

10− 6 s− 1

10− 5 s− 1

10− 4 s− 1

10− 3 s− 1

11.54 1.46 0.532 3.98E− 05

11.01 1.99 0.557 9.20E− 05

9.23 3.77 0.606 2.68E− 04

6.02 6.98 0.643 5.83E− 04

For derivation of parameters see Eqs. (6)–(9) in Caricchi et al. (2007).

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we derive ha to be on the order of 10 to 100 (Figs. 1 and 3) from the analysis of our thin sections. While crystal packing in our examples is similar to the basaltic magma considered by Davis et al., (2007), melt viscosities are orders of magnitudes higher and thus is particle pressure 3

Fig. 4. Change in effective viscosity as a function of crystallinity and strain rate (given in log units) calculated using Eq. (3) for model magma and parameters listed in Table 4. See text for details on modelling parameters. Note the predicted drastic increase of effective viscosity at ϕ ≥ 0.40.

reservoir requires momentum, for example, in the form of a sudden release of energy due to decompression of a pressurised magmatic system, e.g., due to static stress change during active extension. The presence of juvenile ash and pumice clasts indicates that the magmatic volatile content at the initiation of eruption was above atmospheric equilibrium conditions and both volatile exsolution and fragmentation must have occurred somewhere between the magma chamber and the vent. However, the eruption did not develop a sustained (Plinian) eruption column documented by the absence of initial airfall deposits. We thus suspect that volatile exsolution in an overpressurised magma at depth did not play the central role in fuelling the eruptions, which is in agreement with the documented low vesicularity of pumices. In fact, typical large-scale silicic eruptions are fuelled by overpressurisation and the kinetic energy stored in bubbles, whereby upon decompression, fragmentation of the bubble walls generates typical cuspate shapes of bubble-wall glass shards. In the case of the Pagosa Peak dacite for example, glass shards have a contrasting angular and equant shape (see Fig. 6 in Bachmann et al. 2000) and it appears that, rather than the melt, crystals suffered catastrophic fragmentation. Expansion of melt inclusions trapped in crystals may have contributed to the effective fragmentation of the mush as the high crystallinity fostered the abundance of more volatile inclusions than in the typical (crystal-poor) silicic explosive eruption. The energy stored in such inclusions may provide additional thrust to an otherwise volatile undersaturated magma. Catastrophic system destabilisation may thus be promoted by the physico-chemical characteristics of the mush itself, in the form of a network of hard crystals and we now explore two possible scenarios. (i) Crystal-liquid mush destabilisation was investigated in a recent study by Davis et al. (2007), who provide the theoretical concept for excitation of a silicic magma chamber by passing seismic waves. The main conclusion from that study is that crystal-rich magma with ϕ > 0.5 is particularly prone to undergo destabilisation due to seismic agitation leading to an increase in particle pressure. Following Gundogdu et al., (2003) the particle pressure, resulting from the interactions between adjacent crystals in the magma (Davis et al., 2007), scales with both fluid (melt) viscosity ηs and crystal packing ha, expressed as the ratio of crystal diameter over mean spacing width (Torquato, 1995). a 6/ð2 − /Þ = h ð1−/Þ3

ð5Þ

Assuming that crystal packing in the Mexican and Catalan crystal-rich pumices is indicative for chamber crystal packing,

(proportional to η2s following (Davis et al., 2007)). Seismic agitation of the silicic mush thus results in an increase in particle pressure, yet near-instantaneous magma contraction and simultaneous melt depressurization, shown by Davis et al. (2007) to be on the order of 106 Pa s− 1. The effect of decompression is catastrophic in evolved silicic chambers compared to mafic systems due to the general inability of “stronger” melts to accommodate high strain rates ductilely (Angell, 1991). More importantly though, as shown in Fig. 5, a liquid-crystal mush would fail at 2 orders of magnitude lower strain rates than predicted for a crystal-poor magma. In order to relax the induced decompression stresses, the melt undergoes the glass transition and as a consequence fails in a brittle manner causing fragmentation of the material. (ii) A probably even more effective way for catastrophic failure of crystal mush is large-scale decompression by crustal failure above the magma reservoir. Using the model equation of Spieler et al. (2004) for the fragmentation threshold ΔPf upon decompression as a function of porosity (θ): ΔPf =

σm θ

ð6Þ

where effective tensile strength σm = 1 MPa, we calculate pressure drops of between 10 and 2.5 MPa for porosities of between 0.1 and 0.4. With experimental decompression rates of 1–100 GPa/s, fragmentation occurs at strain rates of about 102 s− 1 and higher. Crustal on loading and down-throw of roof rock of about 100 m is required to attain the decompression stresses in nature, which appears a reasonable scale and would fit the

Fig. 5. Viscosity vs. strain rate dependence showing the ductile–brittle transition of model magma. At low crystallinities (ϕ ≤ 0.20) effective viscosity is approximated by melt shear viscosity (square; calculated using Hess and Dingwell (1996) and data in Table 3). Under stress, the melt approaches (red broken arrow) the Maxwell glass transition (bold line) and undergoes catastrophic failure at a critical strain rate marking the at fragmentation threshold (Eq. (1)). The predicted effective viscosity of the model crystal-liquid mush with crystallinity of 0.55 is orders of magnitudes higher (see Fig. 4) and highly strain-ratedependent (diamonds). Eq. (4) can be used to evaluate the shear thinning paths of a deforming mush. Dotted lines characterize the experimentally determined shear thinning and failure behaviour. A linear fit to the onsets is extrapolated (-.-) to extend to a wider range of conditions. The γ̇/η gradient of mush failure is shallower then that of crystal-poor magma or melt failure, suggesting a relatively stronger contribution of crystals to brittle behaviour. The predicted viscosity–strain rate path using Eq. (3) (diamonds) is broadly consistent with the experimentally derived paths and predicts catastrophic failure of the model mush at strain rates of ≳ 10− 3 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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requirements of an Mw = 7 event as shown above. While experimental strain rates are likely to be higher than in nature, we find that 4 orders of magnitude lower strain rates (order of one hundredth of a second for decompression) would suffice to drive the magma into brittle failure. These threshold decompression rates are also on the order of magnitude calculated for melt film collapse (Davis et al. 2007). As a consequence, decompression by either melt collapse or crustal failure/unloading will result in strain rates matching conditions of magma failure identified in Fig. 5. Another noteworthy phenomenon is that crustal failure may directly affect the magma chamber by propagating faults through a crystal-liquid mush. Essentially, crustal failure and block subsidence will stress the magmatic system. If the deviatoric stress is sufficient, the crystal-liquid mush will locally deform at high strain rate and undergo the catastrophic ductile–brittle transition, resulting in a fault zone. Fig. 6 shows results from an experiment, whereby applying a deviatoric stress of 70 MPa, the crystal-liquid mush sample of section 3.2 (at T = 980 °C, interstitial melt viscosity matching our model system and ϕ = 0.55) fails at strain rates of 10− 2 s− 1 (Fig. 6a). Microcracks rapidly grow through the crystals and the melt, and coalesce until macroscopic failure of the magma (Lavallée et al., 2008) and in-situ fault formation (Fig. 6b). The combination of seismic stressing, normal faulting during active crustal extension and block subsidence appear feasible promoters of

large-scale eruption of crystal mush for the documented cases. A particular role may be attributed to the imposed pressure transients during continual or periodic seismicity, which may result in nonequilibrium volatile exsolution, and bubble nucleation which may drive the system towards a critical state, at which large-scale faulting and block subsidence lead to catastrophic system failure. Disruption of the upper mushy portion of a huge reservoir by block subsidence is likely to induce physico-chemical changes in its deeper parts (Kennedy et al., 2008) and may hence trigger further eruptions as shown for example in the succession of the Pagosa Peak Dacite and Fish Canyon Tuff eruptions (Table 1). 4.2. Incompatibility of timescales We propose that the eruption of crystal-liquid mush in the case examples was triggered by the incompatibility of the timescale of magma relaxation (τmg) with the timescales of interstitial melt (τml) relaxation, dynamic seismic stressing (τs), block subsidence (τb) and rapid decompression (τΔP), and resultant strain rates, whereby: τmg > τml >> τs  τ b  τΔP

ð7Þ

and thus γ̇ ΔP  γ̇ b  γ̇ s >> γ̇ ml > γ̇ mg :

ð8Þ

We showed that the relationship in Eq. (8) can be quantified as 2 −1

10 s

Fig. 6. Catastrophic failure of a crystal-liquid mush. a) Uniaxial deformation of a magma with ϕ = 0.55 at a temperature of 980 °C and a deviatoric stress of 70 MPa (see Lavallée et al., 2008 for details on experimental set-up). Once the applied stress is reached, the strain rate exceeds 10− 2 s− 1 and the brittle regime prevails: microscopic cracks grow inside the mush and link up, causing a decrease in monitored deviatoric stress and an increase in strain rate. Macroscopic failure occurs after 2.4 s when the applied stress drops and the strain rate abruptly accelerated. b) Photograph of the concurrent macroscopic fracture developed inside the mush. Sample height is ca. 6 cm.

1 −1

> 10 s

0 −1

> 10 s

>> 10

−2 −1

s

> 10

−3

− 10

−4 −1

s

ð9Þ

Certainly, also the eruption of both volatile- and crystal-rich magmas may be promoted by crustal failure and may explain, for example, the enigmatic eruptions of for example the 2.1 Ma Cerro Gallan ignimbrite (Sparks et al., 1985), and the 4 Ma Atana eruption (Lindsay et al., 2001). The latter study concludes on petrological grounds the need for an external trigger of the eruption. In search for an alternative “relatively” fast in-situ process, such as gas perlocation (Bachmann and Bergantz, 2006), which may eventually drive a large crystal-rich batholitic system at near-solidus temperatures into eruptive conditions we find this process to take on the order of 105 years and thus around 10 to 12 orders of magnitude slower than the proposed processes of crustal failure and strain rateinduced fragmentation. While “in-situ” processes such as magma rejuvenation or reheating may undoubtedly result in a thermodynamic instability of the reservoir and may hence initiate eruption of an oversaturated cap magma, all our geological evidence (Table 2) is inconsistent with such a scenario. We therefore suggest that the investigated eruptions were initiated by local volcano–tectonic interaction whereby reservoir agitation resulted from local faulting events and fragmentation was caused by (perhaps multiple) catastrophic ductile to brittle transition(s) at reservoir depths. Undoubtedly, the dynamics discussed above may only be achievable during a specific time–temperature–viscosity window of opportunity of crustal magma reservoir evolution. A higher temperature (c.f., a higher abundance of melt and lower crystallinity) and thus lower effective viscosity will increase the system's capability to viscously relax strains induced by crustal failure. Ensuing eruptions would thus tap crystal suspensions as is the case in the overwhelming majority of explosive silicic eruptions. A lower temperatures (c.f., higher crystallinity) and thus higher bulk and melt viscosities will eventually lock the system preventing eruption and promoting the formation of plutons. Based on the geological evidence and rheological considerations presented herein, we propose the eruptability of high-level silicic magma with chamber crystallinity exceeding 50 vol.% and water activity below unity at pressures and temperatures relevant to chamber conditions at strain rates on the order of >10− 3 and perhaps down to 10− 4 s− 1.

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5. Summary and conclusions We present a conceptual framework to explain the origin of enigmatic volcanic deposits related to the collosal explosive eruption of (supposingly uneruptable) crystal-rich silicic magmas. Relevant processes include catastrophic magma–tectonic interaction resulting from seismic agitation and roof rock failure during crustal extension which represent the trigger for the evacuation of a batholithic magma reservoir and caldera formation. Amplified pressure transients in the liquid phase during seismic shaking of a crystal-melt mush as well as dynamic stresses due to large-scale faulting may drive high-level granitoids towards a critical state (Davis et al., 2007; Linde et al., 1994), at which large-scale faulting and graben formation eventually trigger the mush to erupt (Fig. 7). We show that deep-level fragmentation of batholithic magma occurs at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poor magmas or pure melt. In our framework of eruption triggering, the inherent physical properties of crystal-liquid mush and the principle incompatibility of timescales governing system relaxation and stress accommodation during seismic agitation, crustal failure and melt depressurization, result in a catastrophic perturbation of a high-level silicic magmatic system. Our proposed framework involves (i) the deep-seated failure of the magma reservoir during active crustal

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faulting (as evidenced by high abundance of syn-plutonitic lithics in co-ignimbritic lag breccia), (ii) rapid decompression (as evidenced by large-scale evacuation of a pluton-like body), (iii) widespread ignimbrite deposition and (iv) roof collapse and caldera formation. While we do not intend to promote seismicity as an ubiquitous trigger for large volcanic eruptions, we find that the enigmatic nature of the case examples warrants the exploration of alternative eruption promoters. Colossal volcanic eruptions are extremely rare events and even fewer may have been induced in the way brought forward here. Nevertheless, as shown here, there appears to be a window of opportunity for catastrophic system failure for mature batholithic bodies, if system perturbation occurs on timescales and at effective viscosities indicated above. It may be that this kinematic window of opportunity represents the last thermodynamic condition facilitating eruption in the evolution of a batholithic body. If neither condition is met, the body will remain untapped forming a pluton unless it is partially re-melted at a later date. Recent events in the Afar area may serve as an example of the intimate association between continental extension, magmatism and volcanism (Wright et al., 2006) and this link is clearly worth further investigation. For example, large concentric ground deformation anomalies (up to 70 km in diameter) in the Central Andes are interpreted to result from the growth of sizable magma bodies at mid-

Fig. 7. a) Illustration showing the proposed scenario (I–IV) for large-scale evacuation of crystal mush from a thermally zoned magma reservoir to explain eruptive evolution of documented case studies. Dark colors indicate relatively cool, crystal-rich magma (mush) below a solidification front, light colors indicate hot crystal suspension). b) Relationship between magma relaxation time (τmg), effective shear viscosity (ηb), strain rate (γ̇) and temperature (T) of the system (color coding as in (a), x indicates conditions for crystal mush during stages I–IV. I) Extensional tectonics facilitating generation and stalling of large evolved silicic magma reservoirs with upper-level mush at T–τmg–γ̇–ηb conditions indicated by 1. II) Active faulting creates near-field seismicity. Small to intermediate sized events create dynamic and static stresses that weaken the crust and induce strain rates that magma can relax viscoelastically (reversible path 1 → 2 → 1) in (b). Imposed pressure transients during seismic agitation, may lead to decompression vesiculation (Davies et al., 2007) driving the system towards a critical state. III) Strain rates of instant elastic seismic energy release by a large near-field earthquake (≥ M7) during normal faulting cannot be relaxed leading to catastrophic failure and breaking of magma by undergoing the ductile–brittle transition (path 1 → 3). IV) Fragmentation is induced by decompression caused by melt excitation, onset of block faulting and the widening of existing or the opening of new conduits.

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crustal depth (Pritchard and Simons, 2002) and while this area is also prone to great earthquakes and crustal extension, future catastrophic volcano–tectonic interaction may take place in that region. In conclusion, we encourage further detailed analysis of large-scale crystal-liquid mush eruptions in order to further test the hypotheses on near-field magma–tectonic interaction put forward here. Acknowledgments We thank A. Rust, A. Costa and D. Dingwell for valuable discussions on an earlier draft of the manuscript. Editor C. Jaupart and three reviewers provided constructive comments, which helped improve the paper. The work was finalised while JG was supported as visiting Chair by LMUexcellent funds of the Research Chair in Experimental Volcanology (D. B. Dingwell) at the LMU of Munich. The authors acknowledge funding by a Royal Society University Research Fellowship (JG), a CSIC–Royal Society International Joint Project (JM and JG), CONACYT (P46005) and UNAM (Mexico)–CSIC (Spain) interchange/ sabbatical scheme grants (GAD and JM), MICINN grant PR2008-0207 (JM), and Deutsche Forschungsgemeinschaft (grant: LA 2651/1-1) and Desjardins Foundation (both YL).

References Aguirre-Díaz, G., Labarthe-Hernañdez, G., 2003. Fissure ignimbrites: fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with basin and range faulting. Geology 31 (9), 773–776. Aguirre-Díaz, G.J., McDowell, F.W., 1993. Nature and timing of faulting and synextensional magmatism in the southern basin and range, central-eastern Durango, Mexico. Bull. Geol. Soc. Am. 105, 1435–1444. Aguirre-Díaz, G.J., Labarthe-Hernández, G., Tristán-González, M., Nieto-Obregón, J., Gutiérrez-Palomares, I., 2008. Ignimbrite flare-up and graben-calderas of the SMO, Mexico. In: Martí, J., Gottsmann, J. (Eds.), Caldera Volcanism. In: Developments in Volcanology, vol. 10. Elsevier, New York, pp. 143–180. Angell, C.A., 1991. Relaxation in liquids, polymers and plastic crystals-strong/fragile patterns and problems. J. Non-Cryst. Solids 131–133, 13–31. Ayalew, D., et al., 2006. The Da'ure' eruption at the Boyna Volcanic Complex during the September 2005 Afar extension episode: volcanology. AGU Fall Meeting, San Francisco, pp. T33E–02. Bachmann, O., Bergantz, G.W., 2006. Gas percolation in upper-crustal silicic crystal mushes as a mechanism for upward heat advection and rejuvenation of nearsolidus magma bodies. J. Volcanol. Geotherm. Res. 149, 85–102. Bachmann, O., Dungan, M.A., Lipman, P.W., 2000. Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado. J. Volcanol. Geotherm. Res. 98 (1–4), 153–171. Bachmann, O., Dungan, M.A., Lipman, P.W., 2002. The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: rejuvenation and eruption of an upper-crustal batholith. J. Petrology 43 (8), 1469–1503. Bagdassarov, N., Dingwell, D.B., Webb, S.L., 1994. Viscoelasticity of crystal- and bubblebearing rhyolite melts. Phys. Earth Planet. Inter. 83, 83–99. Barclay, J., Riley, D., Sparks, R.S.J., 1995. Analytical models for bubble growth during decompression of high viscosity magmas. Bull. Volcanol. 57, 422–431. Breitkreuz, C., Kennedy, A., 1999. Magmatic flare-up at the Carboniferous/Permian boundary in the NE German Basin revealed by SHRIMP zircon ages. Tectonophysics 302 (3–4), 307–326. Cas, R., Wright, J., 1993. Volcanic Successions. Capman and Hall, London. 528 pp. Caricchi, L., Burlini, L., Ulmer, L., Gerya, T., Vassalli, M., Papale, P., 2007. Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics. Earth Planet. Sci. Lett. 264, 402–419. Champallier, R., Bystricky, M., Arbaret, L., 2008. Experimental investigation of magma rheology at 300 MPa: From pure hydrous melt to 76 vol.% of crystals. Earth Planet. Sci. Lett. 267 (3-4), 571–583. Cordonnier, B., Hess, K.-U., Lavallee, Y., Dingwell, D.B., 2009. Rheological properties of dome lavas: case study of Unzen volcano. Earth Planet Sci Lett. 279, 263–272. doi:10.1016/j.epsl.2009.01.014. Costa, A., 2005. Viscosity of high crystal content melts: dependence on solid fraction. Geophys. Res. Lett. 32. doi:10.1029/2005GL024303. Costa, A., Caricchi, L., Bagdassarov, N., 2009. A model for the rheology of particle-bearing suspensions and partially molten rocks. Geochem. Geophys. Geosyst. 10, Q03010. doi:10.1029/2008GC002138. Davis, M., Koenders, M.A., Petford, N., 2007. Vibro-agitation of chambered magma. J. Volcanol. Geotherm. Res. 167 (1–4), 24–36. Dingwell, D.B., 1996. Volcanic dilemma: flow or blow? Science 273, 1054–1055. Dingwell, D.B., 1997. The brittle–ductile transition in high-level granitic magmas material constraints. J. Petrol. 38, 1635–1644. Dingwell, D.B., Webb, S.L., 1990. Relaxation in silicate melts. Eur. J. Mineral. 2, 427–449.

Einstein, A., 1906. A new determination of the molecular dimensions. Annalen der Physik 19, 289–306. Gudmundsson, A., 2006. How local stresses control magma-chamber ruptures, dyke injections, and eruptions in composite volcanoes. Earth Sci. Rev. 79 (1–2), 1–31. Gundogdu, O., Koenders, M.A., Wakeman, R.J., Wu, P., 2003. Permeation through a bed on a vibrating medium: theory and experimental results. Chem. Eng. Sci. 58, 1703–1713. Hawkesworth, C., et al.,1995. Calc-alkaline magnetism, lithospheric thinning and extension in the basin and range. J. Geophys. Res., B, Solid Earth Planets 100 (6), 10,271–10,286. Hess, K.-U., Dingwell, D.B., 1996. Viscosities of hydrous leucogranitic melts: a nonArrhenian model. Am. Mineral. 81 (9–10), 1297–1300. Houghton, B.F., Wilson, C.J.N., 1989. A vesicularity index for pyroclastic deposits. Bull. Volcanol. 51, 451–462. Jellinek, A.M., DePaolo, D.J., 2003. A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions. Bull. Volcanol. 65 (5), 363–381. Kennedy, B.M., Jellinek, A.M., Stix, J., 2008. Coupled caldera subsidence and stirring inferred from analogue models. Nat. Geosci. 1, 385–389. Klug, C., Cashman, K.V., 1994. Vesiculation of May 18, 1980, Mount St. Helens magma. Geology 22 (5), 468–472. Komuro, H., Fujita, Y., Kodama, K., 1984. Numerical models on the formation mechanisms of collapse basins during the Green Tuff orogenesis of Japan. Bull. Volcanol. 47, 649–666. Lavallée, Y., Hess, K.U., Cordonnier, B., Dingwell, D.B., 2007. A non-Newtonian rheological law for highly-crystalline dome lavas. Geology 35, 843–846. Lavallée, Y., Meredith, P.G., Dingwell, D.B., Hess, K.U., Wassermann, J., Cordonnier, B., Gerik, A., Kruhl, J.H., 2008. Seismogenic lavas and explosive eruption forecasting. Nature 453, 507–510. Lemarchand, N., Grasso, J.-R., 2007. Interactions between earthquakes and volcano activity. Geophys. Res. Lett. 34, L24303. doi:10.1029/2007GL031438. Linde, A.T., Sacks, I.S., 1998. Triggering of volcanic eruptions. Nature 395 (6705), 888–890. Linde, A.T., Sacks, I.S., Johnston, M.J.S., Hill, D.P., Bilham, R.G., 1994. Increased pressure from rising bubbles as a mechanism for remotely triggered seismicity. Nature 371, 408–410. Lindsay, J.M., et al., 2001. Magmatic evolution of the La Pacana Caldera System, Central Andes, Chile: compositional variation of two cogenetic, large-volume felsic ignimbrites. J. Petrology 42 (3), 459–486. Manga, M., Brodsky, E.E., 2006. Seismic triggering of eruptions in the far field: volcanoes and geysers. Annu. Rev. Earth Planet. Sci. 34, 263–292. Marsh, B.D., 1989. Magma chambers. Annu. Rev. Earth Planet. Sci. 17, 439–472. Martí, J., 1991. Caldera-like structures related to Permo-Carboniferous volcanism of the Catalan Pyrenees (NE Spain). J. Volcanol. Geother. Res. 45, 173–186. Martí, J., 1996. Genesis of crystal-rich volcaniclastic facies in the Permian red beds of the Central Pyrenees (NE Spain). Sediment. Geol. 106, 1–19. Marzocchi, W., 2002. Remote seismic influence on large explosive eruptions. J. Geophys. Res. 107, 2018. doi:10.1029/2001JB000307. Maxwell, C., 1868. On the dynamical theory of gases. Philos. Mag. 35, 129–145. Newhall, C.G., Self, S., 1982. The Volcanic Explosivity Index (VEI) an estimate of explosive magnitude for historical volcanism. J. Geophys. Res. 87 (C2), 1231–1238. Ort, M.H., 1993. Eruptive processes and caldera formation in a nested downsag-collapse caldera; Cerro Panizos, central Andes Mountains. J. Volcanol. Geotherm. Res. 56 (3), 221–252. Papale, P., 1999. Strain-induced magma fragmentation in explosive eruptions. Nature 397, 425–428. Pinkerton, H., Sparks, R.S.J., 1978. Field-measurements of rheology of lava. Nature 276, 383–385. Pritchard, M.E., Simons, M., 2002. A satellite geodetic survey of large-scale deformation of volcanic centres in the central Andes. Nature 418, 167–170. Roscoe, R., 1952. The viscosity of suspensions of rigid spheres. Br. J. Appl. Phys. 3, 267–269. Scaillet, B., Holtz, F., Pichavant, M.,1998. Phase equilibrium constraints on the viscosity of silicic magmas, 1, volcanic–plutonic comparison. J. Geophys. Res. 103 (B11), 27,257–27,266. Sparks, R.S.J., Lejuene, A.M., Watts, R.B., Barclay, J., Young, S.R., 2000. Control on the emplacement of the andesite lava dome of the Soufriere Hills volcano, Montserrat by degassing-induced crystallization. Terra Nova 12, 14–20. Sparks, R.J.S., Bursik, M.I., Carey, S., Gilbert, J.S., Glaze, L.S., Sigurdsson, H., Woods, A.W., 1997. Volcanic Plumes. John Wiley and Sons, New York. Sparks, R.J.S., Francis, P., Hamer, R.D., Pankhurst, R.J., ÓCallaghan, L.O., Thorpe, R.S., Page, R., 1985. Ignimbrites of the Cerro Galan Caldera, NW Argentina. J. Volcanol. Geotherm. Res. 24, 205–248. Spieler, O., et al., 2004. The fragmentation threshold of pyroclastic rocks. Earth Planet. Sci. Lett. 226 (1–2), 139–148. Takeuchi, S., 2004. Precursory dike propagation control of viscous magma eruptions. Geology 32 (11), 1001–1004. Thomas, N., Jaupart, C., Verniolle, S., 1994. On the vesicularity of pumice. J. Geophys. ResSolid Earth 99 (B8), 15,633–15,644. Torquato, S., 1995. Nearest-neighbor statistics for packings of hard spheres and disks. Phys. Rev. E 51 (4), 3170. van der Molen, I., Paterson, M.S., 1979. Experimental deformation of partially-melted granite. Contrib. Mineral. Petrol. 70, 299–318. Vigneresse, J.L., Barbey, P., Cunny, M., 1996. Rheological transitions during partial melting and crystallization with application to felsic magma segregation and transfer. J. Petrol. 37 (6), 1579–1600. Woods, A.W., 1995. The dynamics of explosive volcanic eruptions. Rev. Geophys. 33 (4), 495–530. Wright, T.J., Ebinger, C., Biggs, J., Ayele, A., Yirgu, G., Keir, D., Stork, A., 2006. Magmamaintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature 442 (7100), 291–294.