Magma–Tectonic Interaction and the Eruption of Silicic Batholiths

Home , Cerro Panizos, Fish Canyon Tuff, Silicic

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

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Earth and Planetary Science Letters

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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 b 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 article info abstract

Article history: Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > Received 5 May 2008 106 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable Received in revised form 20 April 2009 evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from Spain, Accepted 6 May 2009 Mexico, USA and the Central Andes. These silicic caldera-forming eruptions generated deposits characterized Available online 30 May 2009 by i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initial Editor: C.P. Jaupart 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 Keywords: contrast, the documented examples suggest early deep-level fragmentation at the onset of eruption and magma repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost crystal-liquid mush exclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptability relaxation time and eruption initiation processes. In this paper, we present an analysis of eruption promoters from geologic, strain rate theoretical and experimental considerations. Assessing relevant dynamics and timescales for failure of volcano–tectonics crystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily caldera 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 tends to pond rather than erupt irrespective of the magma composition (Scaillet et al., 1998). With increasing crystallinity, a mush Magma stalled in an upper-level crustal reservoir consists of molten develops towards a rigid percolation threshold and by reaching a silicate fluid and various proportions of crystals and bubbles. According crystallinity exceeding 0.5, magma is believed to be uneruptable (Marsh, to Marsh (1989), increasing crystallinity (ϕ) due to the propagation of 1989; Vigneresse et al., 1996). Of course, there is evidence from effusive the solidification front transforms magma from a crystal suspension eruptions that produce dome lavas with effective viscosities of well in (0≤ϕ≤0.25) to a crystal-melt mush (0.25bϕb0.55) and finally to a excess of 1010 Pa s, yet high crystallinity is attributed to late stage rigid crust (0.55bϕ≤1). The eruptability of magma is generally seen to decompression-driven crystallisation upon degassing within the con- be directly dependent on its crystallinity and thus on its rheology. duit and does not reflect chamber conditions upon the onset of eruption Increasing crystal content has two important consequences for magma (Sparks et al., 2000). In the case of colossal silicic explosive eruptions 3 rheology. Firstly, it dramatically increases effective viscosity and hence (≥100 km of magma; Volcanic Explosivity Index (VEI)≥7; Newhall affects its flow behaviour and secondly, it strongly affects its mechanical and Self,1982), the evacuation of a subsurface reservoir generally results properties (Dingwell, 1997). Most explosive volcanic eruptions tap in caldera collapse. Most of these eruptions are dominated by crystal suspensions with bulk properties favorable for viscous flow, scavenging crystal-poor magma suspensions and their eruption is ascent and thus eruption (Woods,1995). The threshold magma viscosity controlled by processes internal to the magmatic system, including 6 for eruption is on the order of 10 Pa s (Takeuchi, 2004). Above this limit overpressurisation, ring fault initiation and subsequent roof collapse (Gudmundsson, 2006). However, there are also examples of caldera- ⁎ Corresponding author. forming eruptions involving crystal-rich (ϕ around and exceeding 0.5) E-mail address: [email protected] (J. Gottsmann). magmas throughout the geological record, among which are the most

Table 1 a flow deposit blurs the information on the original magma crystal List of case examples and characteristics of deposits. content (Cas and Wright, 1993) and these deposits should not be used Case examples Description of deposits as evidence for crystal-rich magmas. All our examples expose primary Permo-carboniferous Prats d'Aguilo >>50 km3 (likely >100 km3) of crystal- magmatic crystal contents in pumices and thus interpretations are not dacites (Spain) (Martí, 1996) rich dacitic ignimbrites and lavas; crystal based on crystal concentrations in the related ignimbrites (Fig. 3). content of up to 60 vol.% in pumice; lithic Primary clast vesicularities are low (up to 0.3) in the case examples and pumice-rich ignimbrite; primary (Fig. 3). Welding in some deposits is a concern for assessing primary vesicularity of pumice uncertain due to welding, but evidence for poor initial clast vesicularity and thus extra care was taken in selecting inflation; stratigraphy suggests dacites uncollapsed fragments for the assessment of primary vesicle content. correspond to intracaldera deposits Although vesicularities can vary significantly in explosive eruptive during basin development. products (Houghton and Wilson, 1989) it is frequently assumed Eocene–Oligocene ignimbrites of Durango >>200 km3 possibly >1000 km3 of (particularly in numerical consideration of explosive volcanism) that State, central Sierra Madre Occidental, ignimbrites; crystal content exceeding Mexico (Aguirre-Díaz and 40 vol.% in pumice; association with fragmentation occurs at a vesicularity of about 77 vol.% (Sparks et al., Labarthe-Hernañdez, 2003) several graben systems; fissure type 1997) and “fragmentation vesicularities” of pumices from Plinian eruption vents; graben formation eruptions are in broad agreement with the threshold value (Klug and intimately related to large-scale eruption Cashman, 1994) for “classic” explosive eruption scenarios. However, as of ignimbrites (Fig. 1c); liquefaction structures in sediments immediately explained above, the case examples show low degrees of primary clast underlying ignimbrites along caldera vesicularity. The absence of pumice-fall deposit and hence lack of margin (Fig. 1d). evidence for a substained (Plinian) eruption column indicates that 3 Cerro Panizos volcanic centre, 6.7 Ma >600 km DRE of two crystal-rich dacitic vesiculation-induced fragmentation (see also next section) upon (Central Andes) (Ort, 1993) ignimbrites in area of normal faulting; system decompression was of second order. crystal content of up to 50 vol.% in pumice; vesicularity of pumice in the The question is then as to how to tap and erupt magma, which lower cooling unit is less than 20 vol.%; defies the concept of eruptability. formation of ignimbrite sheets related to onset and formation of caldera collapse 3. Magma rheology and fragmentation evidenced by increased lithic contents in the lower unit. Pagosa Peak Dacite ca. 28 Ma (San Juan >200 km3 ignimbrite immediately 3.1. Model magma Volcanic field, Basin and Range province predate eruption of Fish Canyon Tuff 3 U.S.A. (Bachmann et al., 2000) (~5000 km ); crystal content of up to In quantifying rheology and fragmentation of magma relevant for the 50 vol.% in pumice. vesicularity of pumice case examples, we assume hereforth a model dacite magma represent- in PPD is around 25 vol.% (at least 60% lower than pumice from FCT); angular and ing an averaged analogue to the investigated eruptions at the following equant glass shards dominant; conditions: temperature of 750 °C, water activity of 1 wt.% (below unity concentration of lithic fragments at base at 150 MPa or 5 km depth), a calc-alkaline metaluminous bulk com- of unit indicating conduit enlargement position a peraluminous interstitial melt phase (Table 3) and i) ϕ≤0.2 early in the eruption; possible onset of caldera collapse related to the (crystal suspension) and ii) ϕ=0.55 (crystal-liquid mush). synchronous disruption of the southern margin of the Fish Canyon magma 3.2. Rheology and fragmentation of suspensions chamber by block faulting. 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 devastating terrestrial volcanic events: the 26 Ma Fish Canyon Tuff rate and neglecting viscous heating, the viscosity of the melt does not (Bachmann et al., 2000, 2002), the 4 Ma Atana eruption at La Pacana vary with increasing strain rate (Bagdassarov et al., 1994). As (Lindsay et al., 2001) and the 2.1 Ma Cerro Galan eruption(Sparks et al., deformation approaches a critical strain rate, melt relaxation is 1985). Evacuation of such batholith-like reservoirs challenges our retarded and once the melt reaches the strain rate at failure understanding of magma eruptability and eruption promoters as the (fragmentation threshold) it undergoes transition from a liquid to a magmas appear to have bulk rheological properties unfavorable for solid: the glass transition (Dingwell, 1996, 1997). evacuation from reservoirs and subsequent eruption. This phenomenon is termed strain-rate (γ̇)-induced fragmentation and the fragmentation threshold is met as soon as 2. Field observations 1 γ̇ > k τ− 1 We present field evidence from deposits generated by explosive Á ml ð Þ caldera-forming eruptions of dacitic to rhyolitic magmas with ϕ of and between 0.40 and 0.60 (Table 1) and eruptive volumes exceeding 100 km3 of magma (dense rock equivalent, DRE). Two cases relate to ηs = G∞ τml 2 our own field investigations in the Catalan Pyrenees (Spain) and in the Á ð Þ

Sierra Madre Occidental (SMO, Mexico; (Figs.1 and 2)). The remaining following the Maxwell (1868) relation. Here, ηs is the melt shear ﬁ examples are based on published data (Table 1). The examples show viscosity and G∞ is the melt shear modulus at in nite frequency (10± important common characteristic features collated in Table 2, provid- 0.5 GPa; (Dingwell and Webb, 1990)). For a wide range of composi- ing essential information on the dynamics of the eruption of crystal- tions, brittle magma failure occurs experimentally when γ̇ is two liquid mush. It is important to note that we are concerned here with orders of magnitude below the critical strain rate equal to τ−1 and pumice clasts representing juvenile samples of the fragmented magma thus k=0.01. For the case of suspensions the resultant model melt (Figs. 1 and 3) and are as such windows into the crystallinity and shear viscosity is 108.1 Pa s after Hess and Dingwell (1996) and the 1 vesicularity of the magma prior upon eruption. Clearly, mechanical magma would thus break at a strain rate of ≥1s− . crystal segregation and fractionation during the eruption together For the majority of explosive eruptions material failure is induced with concentration during the emplacement and possible reworking of by an increase in pressure due to gas exsolution in a saturated magma 428 J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434

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 both crystallinity and strain rate on effective viscosity during volcanic lines of investigations (Woods, 1995). In this scenario, pyroclastic processes. An empirical model for the non-linearity of the viscosity– fragmentation will occur when the time to relax an applied crystallinity–strain rate (η–ϕ–γ̇) relationship was recently presented mechanical stress (e.g. due to bubble growth) exceeds the character- in Costa et al., (2009), which enables the prediction of effective istic relaxation time (τml) of the melt phase (Papale, 1999). viscosity as a function of both ϕ and γ̇ via : Quenched clasts from silicic magmas with viscosities higher than δ 109 Pa s, mirror the vesicularity of magma at the moment of 1+ / ; = /⁎ 3 fragmentation (Thomas et al., 1994) and we thus have to assume ηb / γ̇ γ B/ ðÞ pπ/ / ⁎ ð Þ 1 − 1−n erf 1+ that primary failure (fragmentation) of the mush occurred due to 2/⁎ 1−n /⁎ ð Þ Á ðÞffiffiffi Á processes different to the classic depressurization and bubble nohi expansion fragmentation scenario of liquid suspensions (Barclay where x, δ, γ are empirical parameters and ϕ⁎ approximates the et al., 1995). The studied eruptions document conditions of the critical solid fraction at the onset of the exponential increase of ηb liquid-crystal mush and evaluating their particular rheology should (Table 4). B is a coefficient theoretically estimated at 2.5 by Einstein allow us to inform on eruption conditions such as viscosity and critical (1906). Costa et al. 2009 calibrated their model using experimental strain rates and thus on fragmentation conditions and eruption results on synthetic systems with relevance to volcanism. To calculate trigger. 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 3.3. Rheology and fragmentation of liquid-crystal mush 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 Several investigations have documented the drastic effect of compared to the melt shear viscosity for ϕ=0.55 and γ̇ values of crystals on the effective viscosity and thus the rheology of magma. between 10− 3 s− 1 and 10− 6 s− 1, respectively. Liquid-crystal mushes are known for their high effective viscosities These theoretical approximations are in broad agreement with and doctored rheology (Pinkerton and Sparks, 1978; van der Molen recent experimental work on highly crystalline dry (≤0.1 wt.% water) and Paterson, 1979). Addition of crystals to a melt impinges on the natural magmas (Lavallée et al., 2007), which describes the strain rate mobility of the interstitial melts and thus on the viscous response of γ̇ and temperature (T [°C]) dependence of the effective viscosity ηb to: the suspension (Einstein, 1906; Roscoe, 1952; Costa, 2005; Caricchi et al. 2007; Champallier et al., 2008; Costa et al., 2009). A fully log ηb = − 0:993 + 8974 = T − 0:543 log γ̇ 4 Á ð Þ parameterised model of magma rheology as a function of tempera- ÀÁ − 6 − 1 − 3 − 1 14.2 12.6 ture, relevant water contents, chemical composition, crystal and For γ̇ of 10 s to 10 s at 750 °C, ηb is 10 and 10 Pa s, bubble content, crystal and bubble morphology, etc is not available. respectively. The equivalent melt shear viscosity is 1011.2 using the However, there is unambiguous indication of a significant influence of Hess and Dingwell (1996) model. Scaling the respective viscosities to J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434 429

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 orders of magnitude higher than ηs close to empirical prediction for content relationship for isothermal conditions (a valid assumption for strain rates between 10− 6 s− 1 and 10− 5 s− 1 (Fig. 5). water contents up to 1 wt.%) yields effective viscosities between 109.5 Experiments were carried out under different applied stress and 1011.2 Pa s, respectively. This empirical expression assumes that increments. Brittle behaviour was indicated by acoustic emissions near static conditions are satisfied when γ̇=10− 6 s− 1. and the onset of failure of the crystal-liquid mush (triangles in Fig. 5) However, the onset of cracking and brittle response of a mush was characterized by an acceleration of energy released during cannot be estimated via Eqs. (1), (2), (3), and (4). When a crystal-rich successive microcracking. The samples underwent catastrophic failure magma or mush is upset, the stress decouples between the melt and at strain rates approximately 2 orders of magnitude lower than those crystal phases, and focuses primarily at the points of contact between characteristic for failure of crystal-poor magmas or melt. Catastrophic crystals. The crystals are stronger than the melt and because they failure of the mush was observed to depend on temperature, however cannot accommodate significant deformation, they are prone to to a smaller extent than crystal-poor magmas. fracture (Cordonnier et al., 2009). An abundance of crystals thus The obtained γ̇/η gradient of mush failure (Fig. 5) is shallower argues for a rheology favoring brittle response upon perturbation then that of crystal-poor magma or melt failure, suggesting a compared to a crystal-poor system (Lavallée et al., 2008) and relatively stronger contribution of crystals to brittle behaviour. numerical calculations for fragmentation thresholds indicate an These findings play a pivotal role in assessing the eruption overall deepening of the fragmentation level and a decrease of mechanism and in particular the role and timing of fragmentation in vesicularity at fragmentation (Caricchi et al., 2007). the documented cases. To explore the influence of crystallinity on the brittle–ductile transition, we have experimentally determined the shear thinning 4. Analysis and discussion of eruption promoters and behaviour and the onset of failure of a crystal-rich (ϕ=0.55) magma eruption dynamics 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 While the above kinematic relationships hold, the geological model magma's interstitial melt viscosity) following the procedure evidence excludes magma failure by catastrophic vesiculation of an described in Lavallée et al. (2008). The near static effective viscosities overpressurised chamber. It is thus worth considering an external were determined to be 1011.8 and 1011 Pa s, respectively, about three process that resulted in the deep-seated disruption of a highly crystalline 430 J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434

Table 2 Table 3 Common eruption characteristics of case examples (cf. Table 1). Model bulk magma and interstitial melt composition (normalised to 100% anhydrous).

Characteristic Description Interpretation Oxide Bulk magma Interstitial melt

Eruption Colossal volcanic eruptions of VEI Indicates abundance and eruption SiO2 68.5 77.5 3 volumes ≥7, involving ≥100 km of of a large body of magma at TiO2 0.44 0.15 magma (dense rock equivalent) shallow depth Al2O3 15.38 12.47

Magma Pumices with crystallinity of >0.4 Fragmentation and eruption of a FeOtot 3.45 0.55 crystal pluton-like magma reservoir with MnO 0.07 0.06 content rheological properties unfavorable MgO 0.95 0.06 for eruption CaO 2.92 0.72

Magma Poorly inflated primary pumices Fragmentation not primarily Na2O 3.9 2.7 vesicularity with vesicularities ≤25 vol.%, no caused by expanding gas phase K2O 4.2 5.39 indication of low vesicularity due P2O5 0.19 0.01 to phreato-magmatism Lithology Crystal-mush pumices found in Conduit enlargement, erosion of deposits of and overlying deposits conduit wall and deepening of central apical grabens on resurgent domes or in models of caldera rich in co-magmatic (plutonic) fragmentation level to magma formation (Komuro et al., 1984). Liquefaction features in continental lithics chamber depth sediments (red beds) stratigraphically immediately below the Eruption Absence of initial (sub-) Plinian Early eruption plume collapse due dynamics phase; formation of co-ignimbritic to high bulk density and high ignimbrites at the SMO (Fig. 2) demonstrate the close association of lag breccia rich in plutonitic rocks discharge rate; lack of volcanism and tectonic stressing. We suggest that doming and/or supersaturated magma at active normal faulting may promote their eruption. Near-field reservoir top; deep initial seismicity, active extension and crustal failure represent external fragmentation at reservoir depth, forces, which we discuss as possible eruption promoters next. conduit erosion, onset of vertical caldera collapse. Regional Basin and Range-type continental Formation of tectonic grabens 4.1.1. Near-field seismicity stress field extension during normal faulting 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, magma reservoir, given the close association of the case examples with 2002), yet, there is a lack of consensus as to its importance. It is for significant regional tectonic structures. External forcing may be an example thought that far-field (>100 km) earthquakes generally induce effective way of driving such a system into catastrophic failure and we strain rates too small to trigger eruptions unless the magmatic system is shall explore this possibility in the following sections. 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 4.1. Volcano–tectonic interaction and the ductile–brittle transition worth investigating in the context of the enigmatic nature of the case examples, particularly in the light of a recent rhyolitic eruption during The large-scale evacuation of a batholithic body as documented in active faulting along a ca. 60 km long segment of the Afar rift in 2005, our examples could be related to active basin formation during which documented the close relationship between normal faulting and regional extensional tectonics. There are a number of examples where explosive activity (Ayalew et al., 2006; Wright et al., 2006). The basin formation is intimately related to, or accompanied by, large- following discussion is thus concerned with the near-field (above or scale silicic volcanism (Marti, 1991; Aguirre-Díaz and McDowell, 1993; within a few to 10 km from magmatic reservoirs) effects, that is Hawkesworth et al.,1995; Breitkreuz and Kennedy,1999; Aguirre-Díaz seismicity associated with crustal extension and graben formation. et al. 2008). Growth of a pluton exceeding several hundreds of cubic Dynamic seismic stress changes for large near-field events occurring kilometers in volume, is likely to significantly alter the local or even over seconds to tens of seconds are large in magnitude yet short-lived, regional stress field over time. Eventually, the crust will have to but may induce high enough strain rates (>10−2 s−1)(Manga and accommodate an increasing magmatic pressure as well as a significant Brodsky, 2006) for the model magma to undergo catastrophic failure, thermal perturbation (Jellinek and DePaolo, 2003), both of which which results in the crystal-liquid mush to shatter (Fig. 5). The seismic result in volume increase and upward doming of surrounding rock. moment (M0) associated with for example an Mw =7 event is 19 Doming in turn results in deviatoric extensional stresses at the surface 4×10 Nm (Mw =2/3 [log10M0 −9.1]). Assuming a Young's modulus and fosters tensile failure at high topographic levels as documented by 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.

10− 6 s− 1 10− 5 s− 1 10− 4 s− 1 10− 3 s− 1 d 11.54 11.01 9.23 6.02 g 1.46 1.99 3.77 6.98 Fig. 3. Close-up photograph showing ignimbrite containing uncollapsed poorly ϕ 0.532 0.557 0.606 0.643 vesicular pumice fragments (outlined) with crystallinity of ca. 0.55. Crystals of ⁎ ξ 3.98E−05 9.20E−05 2.68E−04 5.83E−04 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. For derivation of parameters see Eqs. (6)–(9) in Caricchi et al. (2007). J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434 431

we derive a h 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 2 (proportional to ηs 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 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. the glass transition and as a consequence fails in a brittle manner See text for details on modelling parameters. Note the predicted drastic increase of causing fragmentation of the material. effective viscosity at ϕ≥0.40. (ii) A probably even more effective way for catastrophic failure of crystal mush is large-scale decompression by crustal failure reservoir requires momentum, for example, in the form of a sudden above the magma reservoir. Using the model equation of release of energy due to decompression of a pressurised magmatic Spieler et al. (2004) for the fragmentation threshold ΔPf upon system, e.g., due to static stress change during active extension. The decompression as a function of porosity (θ): presence of juvenile ash and pumice clasts indicates that the magmatic σ volatile content at the initiation of eruption was above atmospheric ΔP = m 6 f θ ð Þ 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) where effective tensile strength σm =1 MPa, we calculate eruption column documented by the absence of initial airfall deposits. We pressure drops of between 10 and 2.5 MPa for porosities of thus suspect that volatile exsolution in an overpressurised magma at between 0.1 and 0.4. With experimental decompression rates of 2 1 depth did not play the central role in fuelling the eruptions, which is in 1–100 GPa/s, fragmentation occurs at strain rates of about 10 s− agreement with the documented low vesicularity of pumices. In fact, and higher. Crustal on loading and down-throw of roof rock of typical large-scale silicic eruptions are fuelled by overpressurisation and about 100 m is required to attain the decompression stresses in the kinetic energy stored in bubbles, whereby upon decompression, nature, which appears a reasonable scale and would fit the 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 Fig. 5. Viscosity vs. strain rate dependence showing the ductile–brittle transition of model destabilisation due to seismic agitation leading to an increase in magma. At low crystallinities (ϕ≤0.20) effective viscosity is approximated by melt shear particle pressure. Following Gundogdu et al., (2003) the particle viscosity (square; calculated using Hess and Dingwell (1996) and data in Table 3). Under pressure, resulting from the interactions between adjacent stress, the melt approaches (red broken arrow) the Maxwell glass transition (bold line) fl and undergoes catastrophic failure at a critical strain rate marking the at fragmentation crystals in the magma (Davis et al., 2007), scales with both uid threshold (Eq. (1)). The predicted effective viscosity of the model crystal-liquid mush with a (melt) viscosity ηs and crystal packing h, expressed as the ratio crystallinity of 0.55 is orders of magnitudes higher (see Fig. 4) and highly strain-rate- of crystal diameter over mean spacing width (Torquato, 1995). dependent (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 a 6/ 2 − / range of conditions. The γ/̇ η gradient of mush failure is shallower then that of crystal-poor = ðÞ3 5 h 1−/ ð Þ magma or melt failure, suggesting a relatively stronger contribution of crystals to brittle ðÞ 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 Assuming that crystal packing in the Mexican and Catalan −3 model mush at strain rates of ≳ 10 s. (For interpretation of the references to colour in this crystal-rich pumices is indicative for chamber crystal packing, figure legend, the reader is referred to the web version of this article.) 432 J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434

requirements of an Mw =7 event as shown above. While large-scale eruption of crystal mush for the documented cases. A experimental strain rates are likely to be higher than in nature, particular role may be attributed to the imposed pressure transients we find that 4 orders of magnitude lower strain rates (order of one during continual or periodic seismicity, which may result in non- hundredth of a second for decompression) would suffice to drive equilibrium volatile exsolution, and bubble nucleation which may the magma into brittle failure. These threshold decompression drive the system towards a critical state, at which large-scale faulting rates are also on the order of magnitude calculated for melt film and block subsidence lead to catastrophic system failure. Disruption of collapse (Davis et al. 2007). the upper mushy portion of a huge reservoir by block subsidence is likely to induce physico-chemical changes in its deeper parts As a consequence, decompression by either melt collapse or crustal (Kennedy et al., 2008) and may hence trigger further eruptions as failure/unloading will result in strain rates matching conditions of shown for example in the succession of the Pagosa Peak Dacite and magma failure identified in Fig. 5. Another noteworthy phenomenon Fish Canyon Tuff eruptions (Table 1). is that crustal failure may directly affect the magma chamber by propagating faults through a crystal-liquid mush. Essentially, crustal 4.2. Incompatibility of timescales failure and block subsidence will stress the magmatic system. If the deviatoric stress is sufficient, the crystal-liquid mush will locally We propose that the eruption of crystal-liquid mush in the case deform at high strain rate and undergo the catastrophic ductile–brittle examples was triggered by the incompatibility of the timescale of transition, resulting in a fault zone. Fig. 6 shows results from an magma relaxation (τmg) with the timescales of interstitial melt (τml) experiment, whereby applying a deviatoric stress of 70 MPa, the relaxation, dynamic seismic stressing (τs), block subsidence (τb) and crystal-liquid mush sample of section 3.2 (at T=980 °C, interstitial rapid decompression (τΔP), and resultant strain rates, whereby: 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 τmg > τml >> τs τb τΔP 7 crystals and the melt, and coalesce until macroscopic failure of the ð Þ magma (Lavallée et al., 2008) and in-situ fault formation (Fig. 6b). and thus The combination of seismic stressing, normal faulting during active crustal extension and block subsidence appear feasible promoters of γ̇ γ̇ γ̇ >> γ̇ > γ̇ : 8 ΔP b s ml mg ð Þ

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

2 − 1 1 − 1 0 − 1 − 2 − 1 − 3 − 4 − 1 10 s > 10 s > 10 s >> 10 s > 10 − 10 s 9 ð Þ 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 rate- induced 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

Fig. 6. Catastrophic failure of a crystal-liquid mush. a) Uniaxial deformation of a magma thus higher bulk and melt viscosities will eventually lock the system with ϕ=0.55 at a temperature of 980 °C and a deviatoric stress of 70 MPa (see Lavallée preventing eruption and promoting the formation of plutons. Based on et al., 2008 for details on experimental set-up). Once the applied stress is reached, the the geological evidence and rheological considerations presented 2 1 strain rate exceeds 10− s− and the brittle regime prevails: microscopic cracks grow herein, we propose the eruptability of high-level silicic magma with inside the mush and link up, causing a decrease in monitored deviatoric stress and an chamber crystallinity exceeding 50 vol.% and water activity below unity 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 at pressures and temperatures relevant to chamber conditions at strain 3 4 1 macroscopic fracture developed inside the mush. Sample height is ca. 6 cm. rates on the order of >10− and perhaps down to 10− s− . J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434 433

5. Summary and conclusions faulting (as evidenced by high abundance of syn-plutonitic lithics in co-ignimbritic lag breccia), (ii) rapid decompression (as evidenced by We present a conceptual framework to explain the origin of large-scale evacuation of a pluton-like body), (iii) widespread enigmatic volcanic deposits related to the collosal explosive eruption ignimbrite deposition and (iv) roof collapse and caldera formation. of (supposingly uneruptable) crystal-rich silicic magmas. Relevant While we do not intend to promote seismicity as an ubiquitous trigger processes include catastrophic magma–tectonic interaction resulting for large volcanic eruptions, we ﬁnd that the enigmatic nature of the from seismic agitation and roof rock failure during crustal extension case examples warrants the exploration of alternative eruption which represent the trigger for the evacuation of a batholithic magma promoters. Colossal volcanic eruptions are extremely rare events and reservoir and caldera formation. Ampliﬁed pressure transients in the even fewer may have been induced in the way brought forward here. liquid phase during seismic shaking of a crystal-melt mush as well as Nevertheless, as shown here, there appears to be a window of dynamic stresses due to large-scale faulting may drive high-level opportunity for catastrophic system failure for mature batholithic granitoids towards a critical state (Davis et al., 2007; Linde et al.,1994), bodies, if system perturbation occurs on timescales and at effective at which large-scale faulting and graben formation eventually trigger viscosities indicated above. It may be that this kinematic window of the mush to erupt (Fig. 7). We show that deep-level fragmentation of opportunity represents the last thermodynamic condition facilitating batholithic magma occurs at approximately 2 orders of magnitude eruption in the evolution of a batholithic body. If neither condition is lower strain rates than those characteristic for failure of crystal-poor met, the body will remain untapped forming a pluton unless it is magmas or pure melt. In our framework of eruption triggering, the partially re-melted at a later date. inherent physical properties of crystal-liquid mush and the principle Recent events in the Afar area may serve as an example of the incompatibility of timescales governing system relaxation and stress intimate association between continental extension, magmatism and accommodation during seismic agitation, crustal failure and melt volcanism (Wright et al., 2006) and this link is clearly worth further depressurization, result in a catastrophic perturbation of a high-level investigation. For example, large concentric ground deformation silicic magmatic system. Our proposed framework involves (i) the anomalies (up to 70 km in diameter) in the Central Andes are deep-seated failure of the magma reservoir during active crustal 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. 434 J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434 crustal depth (Pritchard and Simons, 2002) and while this area is also Einstein, A., 1906. A new determination of the molecular dimensions. Annalen der Physik 19, 289–306. prone to great earthquakes and crustal extension, future catastrophic Gudmundsson, A., 2006. 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