Magma–tectonic interaction and the eruption of silicic

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 Kingdomb Department of Earth and Environmental Sciences, Ludwig-Maximilian University, Theresienstr. 41, 80333 Munich, Germanyc Institute of Earth Sciences “Jaume Almera,” CSIC, Luis Sole i Sabaris is/n, Barcelona, 08028, Spaind Centro de Geociencias, UNAM, Campus Juriquilla, CP 76230, Juriquilla, Querétaro, Mexico

a b s t r a c ta r t i c l e i n f o

Article history:Received 5 May 2008Received in revised form 20 April 2009Accepted 6 May 2009Available online 30 May 2009

Editor: C.P. Jaupart

Keywords:magmacrystal-liquid mushrelaxation timestrain ratevolcano–tectonicscaldera

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, irrefutableevidence 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 characterizedby i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initialfall-out. The field evidence is inconsistent with most caldera-forming deposits, which are underlain by initialfall-out indicating deposition from a sustained eruption column before the actual collapse sequence. Incontrast, the documented examples suggest early deep-level fragmentation at the onset of eruption andrepeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almostexclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptabilityand 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 ofcrystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarilyinvolves an external trigger by near-field seismicity and crustal failure. Strain rate analysis for dynamic andstatic stressing, chamber roof collapse and rapid decompression indicates that large “solid-like” silicicreservoirs may undergo catastrophic failure leading to deep-level fragmentation of batholithic magma atapproximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poormagmas or pure melt. Eruption triggers can thus include either amplified pressure transients in the liquidphase during seismic shaking of a crystal-melt mush, decompression by block subsidence or a combination ofboth. We find that the window of opportunity for the eruption of large silicic bodies may thus extent tocrystallinities 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 moltensilicate fluid and various proportions of crystals and bubbles. Accordingto Marsh (1989), increasing crystallinity (ϕ) due to the propagation ofthe solidification front transforms magma from a crystal suspension(0≤ϕ≤0.25) to a crystal-melt mush (0.25bϕb0.55) and finally to arigid crust (0.55bϕ≤1). The eruptability of magma is generally seen tobe directly dependent on its crystallinity and thus on its rheology.Increasing crystal content has two important consequences for magmarheology. Firstly, it dramatically increases effective viscosity and henceaffects its flow behaviour and secondly, it strongly affects its mechanicalproperties (Dingwell, 1997). Most explosive volcanic eruptions tapcrystal suspensions with bulk properties favorable for viscous flow,ascent and thus eruption (Woods,1995). The thresholdmagmaviscosityfor eruption is on the order of 106 Pa s (Takeuchi, 2004). Above this limit

magma tends to pond rather than erupt irrespective of the magmacomposition (Scaillet et al., 1998). With increasing crystallinity, a mushdevelops towards a rigid percolation threshold and by reaching acrystallinity exceeding0.5,magma is believed to be uneruptable (Marsh,1989; Vigneresse et al., 1996). Of course, there is evidence from effusiveeruptions that produce dome lavas with effective viscosities of well inexcess of 1010 Pa s, yet high crystallinity is attributed to late stagedecompression-driven crystallisation upon degassing within the con-duit 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; Newhalland Self,1982), the evacuationof a subsurface reservoir generally resultsin caldera collapse. Most of these eruptions are dominated byscavenging crystal-poor magma suspensions and their eruption iscontrolled by processes internal to the magmatic system, includingoverpressurisation, ring fault initiation and subsequent roof collapse(Gudmundsson, 2006). However, there are also examples of caldera-forming eruptions involving crystal-rich (ϕ around and exceeding 0.5)magmas throughout the geological record, among which are the most

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

⁎ 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

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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 ourunderstanding of magma eruptability and eruption promoters as themagmas appear to have bulk rheological properties unfavorable forevacuation from reservoirs and subsequent eruption.

2. Field observations

We present field evidence from deposits generated by explosivecaldera-forming eruptions of dacitic to rhyolitic magmas with ϕ ofbetween 0.40 and 0.60 (Table 1) and eruptive volumes exceeding100 km3 of magma (dense rock equivalent, DRE). Two cases relate toour own field investigations in the Catalan Pyrenees (Spain) and in theSierraMadre Occidental (SMO,Mexico; (Figs.1 and 2)). The remainingexamples are based on published data (Table 1). The examples showimportant common characteristic features collated in Table 2, provid-ing essential information on the dynamics of the eruption of crystal-liquid mush. It is important to note that we are concerned here withpumice clasts representing juvenile samples of the fragmentedmagma(Figs. 1 and 3) and are as such windows into the crystallinity andvesicularity of the magma prior upon eruption. Clearly, mechanicalcrystal segregation and fractionation during the eruption togetherwith concentration during the emplacement and possible reworkingof

a flow deposit blurs the information on the original magma crystalcontent (Cas andWright, 1993) and these deposits should not be usedas evidence for crystal-rich magmas. All our examples expose primarymagmatic crystal contents in pumices and thus interpretations are notbased 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 primaryclast vesicularity and thus extra care was taken in selectinguncollapsed fragments for the assessment of primary vesicle content.Although vesicularities can vary significantly in explosive eruptiveproducts (Houghton and Wilson, 1989) it is frequently assumed(particularly in numerical consideration of explosive volcanism) thatfragmentation occurs at a vesicularity of about 77 vol.% (Sparks et al.,1997) and “fragmentation vesicularities” of pumices from Plinianeruptions are in broad agreement with the threshold value (Klug andCashman,1994) for “classic” explosive eruption scenarios. However, asexplained above, the case examples show low degrees of primary clastvesicularity. The absence of pumice-fall deposit and hence lack ofevidence for a substained (Plinian) eruption column indicates thatvesiculation-induced fragmentation (see also next section) uponsystem decompression was of second order.

The question is then as to how to tap and erupt magma, whichdefies the concept of eruptability.

3. Magma rheology and fragmentation

3.1. Model magma

Inquantifying rheologyand fragmentationofmagmarelevant for thecase examples, we assume hereforth a model dacite magma represent-ing an averaged analogue to the investigated eruptions at the followingconditions: temperature of 750 °C, water activity of 1 wt.% (below unityat 150 MPa or 5 km depth), a calc-alkaline metaluminous bulk com-position 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 andis a viscoelastic body, rheologically controlled by the shear strain rate.The melt behaves as a Newtonian fluid when deforming at low strainrate and neglecting viscous heating, the viscosity of the melt does notvary with increasing strain rate (Bagdassarov et al., 1994). Asdeformation approaches a critical strain rate, melt relaxation isretarded and once the melt reaches the strain rate at failure(fragmentation threshold) it undergoes transition from a liquid to asolid: the glass transition (Dingwell, 1996, 1997).

This phenomenon is termed strain-rate (γ̇)-induced fragmenta-tion and the fragmentation threshold is met as soon as

γ̇> k ! τ−1ml ð1Þ

and

ηs = G∞ ! τml ð2Þ

following the Maxwell (1868) relation. Here, ηs is the melt shearviscosity and G∞ is the melt shear modulus at infinite frequency (10±0.5 GPa; (Dingwell and Webb, 1990)). For a wide range of composi-tions, brittle magma failure occurs experimentally when γ̇ is twoorders of magnitude below the critical strain rate equal to τ−1 andthus k=0.01. For the case of suspensions the resultant model meltshear viscosity is 108.1 Pa s after Hess and Dingwell (1996) and themagma would thus break at a strain rate of ≥1 s−1.

For the majority of explosive eruptions material failure is inducedby an increase in pressure due to gas exsolution in a saturated magma

Table 1List of case examples and characteristics of deposits.

Case examples Description of deposits

Permo-carboniferous Prats d'Aguilodacites (Spain) (Martí, 1996)

>>50 km3 (likely >100 km3) of crystal-rich dacitic ignimbrites and lavas; crystalcontent of up to 60 vol.% in pumice; lithicand pumice-rich ignimbrite; primaryvesicularity of pumice uncertain due towelding, but evidence for poor initialinflation; stratigraphy suggests dacitescorrespond to intracaldera depositsduring basin development.

Eocene–Oligocene ignimbrites of DurangoState, central Sierra Madre Occidental,Mexico (Aguirre-Díaz andLabarthe-Hernañdez, 2003)

>>200 km3 possibly >1000 km3 ofignimbrites; crystal content exceeding40 vol.% in pumice; association withseveral graben systems; fissure typeeruption vents; graben formationintimately related to large-scale eruptionof ignimbrites (Fig. 1c); liquefactionstructures in sediments immediatelyunderlying ignimbrites along calderamargin (Fig. 1d).

Cerro Panizos volcanic centre, 6.7 Ma(Central Andes) (Ort, 1993)

>600 km3 DRE of two crystal-rich daciticignimbrites in area of normal faulting;crystal content of up to 50 vol.% inpumice; vesicularity of pumice in thelower cooling unit is less than 20 vol.%;formation of ignimbrite sheets related toonset and formation of caldera collapseevidenced by increased lithic contents inthe lower unit.

Pagosa Peak Dacite ca. 28 Ma (San JuanVolcanic field, Basin and Range provinceU.S.A. (Bachmann et al., 2000)

>200 km3 ignimbrite immediatelypredate eruption of Fish Canyon Tuff(~5000 km3); crystal content of up to50 vol.% in pumice. vesicularity of pumicein PPD is around 25 vol.% (at least 60%lower than pumice from FCT); angular andequant glass shards dominant;concentration of lithic fragments at baseof unit indicating conduit enlargementearly in the eruption; possible onset ofcaldera collapse related to thesynchronous disruption of the southernmargin of the Fish Canyon magmachamber by block faulting.

427J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434

and bubble formation upon decompression as evidenced by severallines of investigations (Woods, 1995). In this scenario, pyroclasticfragmentation will occur when the time to relax an appliedmechanical stress (e.g. due to bubble growth) exceeds the character-istic relaxation time (τml) of the melt phase (Papale, 1999).

Quenched clasts from silicic magmas with viscosities higher than109 Pa s, mirror the vesicularity of magma at the moment offragmentation (Thomas et al., 1994) and we thus have to assumethat primary failure (fragmentation) of the mush occurred due toprocesses different to the classic depressurization and bubbleexpansion fragmentation scenario of liquid suspensions (Barclayet al., 1995). The studied eruptions document conditions of theliquid-crystal mush and evaluating their particular rheology shouldallow us to inform on eruption conditions such as viscosity and criticalstrain rates and thus on fragmentation conditions and eruptiontrigger.

3.3. Rheology and fragmentation of liquid-crystal mush

Several investigations have documented the drastic effect ofcrystals on the effective viscosity and thus the rheology of magma.Liquid-crystal mushes are known for their high effective viscositiesand doctored rheology (Pinkerton and Sparks, 1978; van der Molenand Paterson, 1979). Addition of crystals to a melt impinges on themobility of the interstitial melts and thus on the viscous response ofthe suspension (Einstein, 1906; Roscoe, 1952; Costa, 2005; Caricchiet al. 2007; Champallier et al., 2008; Costa et al., 2009). A fullyparameterised model of magma rheology as a function of tempera-ture, relevant water contents, chemical composition, crystal andbubble 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 volcanicprocesses. An empirical model for the non-linearity of the viscosity–crystallinity–strain rate (η–ϕ–γ ̇) relationship was recently presentedin Costa et al., (2009), which enables the prediction of effectiveviscosity as a function of both ϕ and γ̇ via :

ηb /;γ̇ð Þ =1 + /

/⁎

! "δ

1− 1−nð Þ ! erfffiffiffiπ

p/

2/⁎ 1−nð Þ! 1 + /

/⁎

! "! "γh in oB/⁎ð3Þ

where x, δ, γ are empirical parameters and ϕ⁎ approximates thecritical 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 experimentalresults on synthetic systems with relevance to volcanism. To calculateeffective viscosity of the model mush for a range of relevant strainrates, we scale fit parameters ξ, δ, γ and ϕ⁎ against γ̇ using expressionsreported in Caricchi et al. (2007). Results are shown in Fig. 4. We findeffective viscosity to increase by 1.5 to 3.2 orders of magnitudecompared to the melt shear viscosity for ϕ=0.55 and γ̇ values ofbetween 10−3 s−1 and 10−6 s−1, respectively.

These theoretical approximations are in broad agreement withrecent experimental work on highly crystalline dry (≤0.1 wt.% water)natural magmas (Lavallée et al., 2007), which describes the strain rateγ̇ and temperature (T [°C]) dependence of the effective viscosity ηb to:

log ηb$ %

= − 0:993 + 8974= T − 0:543 ! logγ̇ 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 theHess and Dingwell (1996) model. Scaling the respective viscosities to

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 (coinfor 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 aredominantly 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 gapwidth on order of 10 to 100. This criterion is employed for assessment of particle pressure during chamber agitation.

428 J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434

a water content of 1 wt.% assuming a linear shear viscosity vs. watercontent relationship for isothermal conditions (a valid assumption forwater contents up to 1 wt.%) yields effective viscosities between 109.5

and 1011.2 Pa s, respectively. This empirical expression assumes thatnear static conditions are satisfied when γ̇=10−6 s−1.

However, the onset of cracking and brittle response of a mushcannot be estimated via Eqs. (1), (2), (3), and (4). When a crystal-richmagma or mush is upset, the stress decouples between the melt andcrystal phases, and focuses primarily at the points of contact betweencrystals. The crystals are stronger than the melt and because theycannot accommodate significant deformation, they are prone tofracture (Cordonnier et al., 2009). An abundance of crystals thusargues for a rheology favoring brittle response upon perturbationcompared to a crystal-poor system (Lavallée et al., 2008) andnumerical calculations for fragmentation thresholds indicate anoverall deepening of the fragmentation level and a decrease ofvesicularity at fragmentation (Caricchi et al., 2007).

To explore the influence of crystallinity on the brittle–ductiletransition, we have experimentally determined the shear thinningbehaviour and the onset of failure of a crystal-rich (ϕ=0.55) magmaat 940 and 980 °C — temperatures at which the interstitial meltviscosities were 108.6 and 108 Pa s, respectively (i.e., similar to themodel magma's interstitial melt viscosity) following the proceduredescribed in Lavallée et al. (2008). The near static effective viscositieswere determined to be 1011.8 and 1011 Pa s, respectively, about three

orders of magnitude higher than ηs close to empirical prediction forstrain rates between 10−6 s−1 and 10−5 s−1 (Fig. 5).

Experiments were carried out under different applied stressincrements. Brittle behaviour was indicated by acoustic emissionsand the onset of failure of the crystal-liquid mush (triangles in Fig. 5)was characterized by an acceleration of energy released duringsuccessive microcracking. The samples underwent catastrophic failureat strain rates approximately 2 orders of magnitude lower than thosecharacteristic for failure of crystal-poor magmas or melt. Catastrophicfailure of the mush was observed to depend on temperature, howeverto a smaller extent than crystal-poor magmas.

The obtained γ ̇/η gradient of mush failure (Fig. 5) is shallowerthen that of crystal-poor magma or melt failure, suggesting arelatively stronger contribution of crystals to brittle behaviour.

These findings play a pivotal role in assessing the eruptionmechanism and in particular the role and timing of fragmentation inthe documented cases.

4. Analysis and discussion of eruption promoters anderuption dynamics

While the above kinematic relationships hold, the geologicalevidence excludes magma failure by catastrophic vesiculation of anoverpressurised chamber. It is thus worth considering an externalprocess that resulted in thedeep-seateddisruptionof ahighlycrystalline

Fig. 2. a) Photographs showing relationship between ignimbrites and underlying continental sediments (red beds) of the Sierra Madre Occidental, Mexico (persons for scale). Notedissected 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 thereferences to colour in this figure legend, the reader is referred to the web version of this article.)


magma reservoir, given the close association of the case examples withsignificant regional tectonic structures. External forcing may be aneffective way of driving such a system into catastrophic failure and weshall 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 inour examples could be related to active basin formation duringregional extensional tectonics. There are a number of examples wherebasin formation is intimately related to, or accompanied by, large-scale silicic volcanism (Marti, 1991; Aguirre-Díaz andMcDowell, 1993;Hawkesworth et al., 1995; Breitkreuz and Kennedy,1999; Aguirre-Díazet al. 2008). Growth of a pluton exceeding several hundreds of cubickilometers in volume, is likely to significantly alter the local or evenregional stress field over time. Eventually, the crust will have toaccommodate an increasingmagmatic pressure as well as a significantthermal perturbation (Jellinek and DePaolo, 2003), both of whichresult in volume increase and upward doming of surrounding rock.Doming in turn results in deviatoric extensional stresses at the surfaceand fosters tensile failure at high topographic levels as documented by

central apical grabens on resurgent domes or in models of calderaformation (Komuro et al., 1984). Liquefaction features in continentalsediments (red beds) stratigraphically immediately below theignimbrites at the SMO (Fig. 2) demonstrate the close association ofvolcanism and tectonic stressing. We suggest that doming and/oractive normal faulting may promote their eruption. Near-fieldseismicity, active extension and crustal failure represent externalforces, which we discuss as possible eruption promoters next.

4.1.1. Near-field seismicitySeismic triggers of volcanic activity including large volcanic erup-

tions have been invoked for a number of cases recently (Lemarchandand 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 forexample thought that far-field (>100 km) earthquakes generally inducestrain rates too small to trigger eruptions unless themagmatic system isalready close to critical instability (Manga andBrodsky, 2006).Whilewedo not wish to enter this discussion, we feel that near-field effects areworth investigating in the context of the enigmatic nature of the caseexamples, particularly in the light of a recent rhyolitic eruption duringactive faulting along a ca. 60 km long segment of the Afar rift in 2005,which documented the close relationship between normal faulting andexplosive activity (Ayalew et al., 2006; Wright et al., 2006). Thefollowing discussion is thus concerned with the near-field (above orwithin a few to 10 km from magmatic reservoirs) effects, that isseismicity associated with crustal extension and graben formation.Dynamic seismic stress changes for large near-field events occurringover seconds to tens of seconds are large in magnitude yet short-lived,but may induce high enough strain rates (>10−2 s−1) (Manga andBrodsky, 2006) for the model magma to undergo catastrophic failure,which results in the crystal-liquid mush to shatter (Fig. 5). The seismicmoment (M0) associated with for example an Mw=7 event is4×1019 Nm (Mw=2/3 [log10M0−9.1]). Assuming a Young's modulusof 30 GPa and fault slip of 50 to 100 m, an event of such magnituderequires down-throwof a fault area of about 4 to 80×107m2,which is ofa conceivable scale given that the case caldera in theSMO isboundeitherside by a 35 km long fault system. The key problem despite seismicallyinducing failure is however: how to drive the mush to erupt?

4.1.2. Eruption initiation and the role of rapid decompressionThe deposits document that shattered mush and co-magmatic lithics

are somehow funneled to and erupted at the surface in the form ofwidespread ignimbrite volcanism. The process of extraction from the

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

Oxide Bulk magma Interstitial melt

SiO2 68.5 77.5TiO2 0.44 0.15Al2O3 15.38 12.47FeOtot 3.45 0.55MnO 0.07 0.06MgO 0.95 0.06CaO 2.92 0.72Na2O 3.9 2.7K2O 4.2 5.39P2O5 0.19 0.01

Table 4Model parameters (Eq. (3)) for calculation of effective viscosity for model mush forstrain 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 6.02g 1.46 1.99 3.77 6.98ϕ⁎ 0.532 0.557 0.606 0.643ξ 3.98E−05 9.20E−05 2.68E−04 5.83E−04

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

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

Characteristic Description Interpretation

Eruptionvolumes

Colossal volcanic eruptions of VEI≥7, involving ≥100 km3 ofmagma (dense rock equivalent)

Indicates abundance and eruptionof a large body of magma atshallow depth

Magmacrystalcontent

Pumices with crystallinity of >0.4 Fragmentation and eruption of apluton-like magma reservoir withrheological properties unfavorablefor eruption

Magmavesicularity

Poorly inflated primary pumiceswith vesicularities ≤25 vol.%, noindication of low vesicularity dueto phreato-magmatism

Fragmentation not primarilycaused by expanding gas phase

Lithology Crystal-mush pumices found indeposits of and overlying depositsrich in co-magmatic (plutonic)lithics

Conduit enlargement, erosion ofconduit wall and deepening offragmentation level to magmachamber depth

Eruptiondynamics

Absence of initial (sub-) Plinianphase; formation of co-ignimbriticlag breccia rich in plutonitic rocks

Early eruption plume collapse dueto high bulk density and highdischarge rate; lack ofsupersaturated magma atreservoir top; deep initialfragmentation at reservoir depth,conduit erosion, onset of verticalcaldera collapse.

Regionalstress field

Basin and Range-type continentalextension

Formation of tectonic grabensduring normal faulting

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


reservoir requires momentum, for example, in the form of a suddenrelease of energy due to decompression of a pressurised magmaticsystem, e.g., due to static stress change during active extension. Thepresence of juvenile ash and pumice clasts indicates that the magmaticvolatile content at the initiation of eruption was above atmosphericequilibrium conditions and both volatile exsolution and fragmentationmust have occurred somewhere between the magma chamber and thevent. However, the eruption did not develop a sustained (Plinian)eruption columndocumented by the absence of initial airfall deposits.Wethus suspect that volatile exsolution in an overpressurised magma atdepth did not play the central role in fuelling the eruptions, which is inagreement with the documented low vesicularity of pumices. In fact,typical large-scale silicic eruptions are fuelled by overpressurisation andthe kinetic energy stored in bubbles, whereby upon decompression,fragmentation of the bubble walls generates typical cuspate shapes ofbubble-wall glass shards. In the case of the Pagosa Peak dacite forexample, glass shards have a contrasting angular and equant shape (seeFig. 6 in Bachmann et al. 2000) and it appears that, rather than the melt,crystals suffered catastrophic fragmentation. Expansionofmelt inclusionstrapped in crystalsmay have contributed to the effective fragmentation ofthemush as the high crystallinity fostered the abundance ofmore volatileinclusions than in the typical (crystal-poor) silicic explosive eruption. Theenergy stored in such inclusions may provide additional thrust to anotherwise volatile undersaturated magma.

Catastrophic system destabilisation may thus be promoted by thephysico-chemical characteristics of the mush itself, in the form of anetwork of hard crystals and we now explore two possible scenarios.

(i) Crystal-liquid mush destabilisationwas investigated in a recentstudy by Davis et al. (2007), who provide the theoreticalconcept for excitation of a silicic magma chamber by passingseismic waves. The main conclusion from that study is thatcrystal-richmagmawithϕ>0.5 is particularly prone to undergodestabilisation due to seismic agitation leading to an increase inparticle pressure. FollowingGundogdu et al., (2003) the particlepressure, resulting from the interactions between adjacentcrystals in themagma (Davis et al., 2007), scales with both fluid(melt) viscosity ηs and crystal packing a

h, expressed as the ratio

of crystal diameter over mean spacing width (Torquato, 1995).

ah

=6/ 2− /ð Þ1−/ð Þ3

ð5Þ

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

we derive ahto 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 basalticmagma considered by Davis et al., (2007), melt viscosities areorders of magnitudes higher and thus is particle pressure

(proportional toη32s following (Davis et al., 2007)). Seismic agitation

of the silicic mush thus results in an increase in particle pressure,yet near-instantaneousmagma contraction and simultaneousmeltdepressurization, shownbyDavis et al. (2007) to be on the order of106 Pa s−1. The effect of decompression is catastrophic in evolvedsilicic chambers compared to mafic systems due to the generalinability of “stronger” melts to accommodate high strain ratesductilely (Angell, 1991). More importantly though, as shown inFig. 5, a liquid-crystal mush would fail at 2 orders of magnitudelower strain rates thanpredicted for a crystal-poormagma. In orderto relax the induced decompression stresses, the melt undergoesthe glass transition and as a consequence fails in a brittle mannercausing fragmentation of the material.

(ii) A probably even more effective way for catastrophic failure ofcrystal mush is large-scale decompression by crustal failureabove the magma reservoir. Using the model equation ofSpieler et al. (2004) for the fragmentation threshold ΔPf upondecompression as a function of porosity (θ):

ΔPf =σm

θð6Þ

where effective tensile strength σm=1 MPa, we calculatepressure drops of between 10 and 2.5 MPa for porosities ofbetween 0.1 and 0.4. With experimental decompression rates of1–100GPa/s, fragmentationoccurs at strain rates of about 102 s−1

and higher. Crustal on loading and down-throw of roof rock ofabout 100 m is required to attain the decompression stresses innature, which appears a reasonable scale and would fit the

Fig. 4. Change in effective viscosity as a function of crystallinity and strain rate (given inlog 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 ofeffective viscosity at ϕ≥0.40.

Fig. 5. Viscosity vs. strain rate dependence showing the ductile–brittle transition ofmodelmagma. At low crystallinities (ϕ≤0.20) effective viscosity is approximated by melt shearviscosity (square; calculated using Hess and Dingwell (1996) and data in Table 3). Understress, the melt approaches (red broken arrow) the Maxwell glass transition (bold line)and undergoes catastrophic failure at a critical strain rate marking the at fragmentationthreshold (Eq. (1)). The predicted effective viscosity of themodel crystal-liquidmushwithcrystallinity of 0.55 is orders of magnitudes higher (see Fig. 4) and highly strain-rate-dependent (diamonds). Eq. (4) can be used to evaluate the shear thinning paths of adeformingmush. Dotted lines characterize the experimentally determined shear thinningand failure behaviour. A linear fit to the onsets is extrapolated (-.-) to extend to a widerrange of conditions. The γ̇/η gradient ofmush failure is shallower then that of crystal-poormagma or melt failure, suggesting a relatively stronger contribution of crystals to brittlebehaviour. The predicted viscosity–strain rate path using Eq. (3) (diamonds) is broadlyconsistent with the experimentally derived paths and predicts catastrophic failure of themodelmush at strain rates of≳10−3 s. (For interpretationof the references to colour in thisfigure legend, the reader is referred to the web version of this article.)


requirements of an Mw=7 event as shown above. Whileexperimental strain rates are likely to be higher than in nature,wefind that 4 orders ofmagnitude lower strain rates (orderof onehundredth of a second for decompression) would suffice to drivethe magma into brittle failure. These threshold decompressionrates are also on the order of magnitude calculated for melt filmcollapse (Davis et al. 2007).

As a consequence, decompression by eithermelt collapse or crustalfailure/unloading will result in strain rates matching conditions ofmagma failure identified in Fig. 5. Another noteworthy phenomenonis that crustal failure may directly affect the magma chamber bypropagating faults through a crystal-liquid mush. Essentially, crustalfailure and block subsidence will stress the magmatic system. If thedeviatoric stress is sufficient, the crystal-liquid mush will locallydeform at high strain rate and undergo the catastrophic ductile–brittletransition, resulting in a fault zone. Fig. 6 shows results from anexperiment, whereby applying a deviatoric stress of 70 MPa, thecrystal-liquid mush sample of section 3.2 (at T=980 °C, interstitialmelt viscosity matching our model system and ϕ=0.55) fails at strainrates of 10−2 s−1 (Fig. 6a). Microcracks rapidly grow through thecrystals and the melt, and coalesce until macroscopic failure of themagma (Lavallée et al., 2008) and in-situ fault formation (Fig. 6b).

The combination of seismic stressing, normal faulting during activecrustal extension and block subsidence appear feasible promoters of

large-scale eruption of crystal mush for the documented cases. Aparticular role may be attributed to the imposed pressure transientsduring continual or periodic seismicity, which may result in non-equilibrium volatile exsolution, and bubble nucleation which maydrive the system towards a critical state, at which large-scale faultingand block subsidence lead to catastrophic system failure. Disruption ofthe upper mushy portion of a huge reservoir by block subsidence islikely to induce physico-chemical changes in its deeper parts(Kennedy et al., 2008) and may hence trigger further eruptions asshown for example in the succession of the Pagosa Peak Dacite andFish Canyon Tuff eruptions (Table 1).

4.2. Incompatibility of timescales

We propose that the eruption of crystal-liquid mush in the caseexamples was triggered by the incompatibility of the timescale ofmagma relaxation (τmg) with the timescales of interstitial melt (τml)relaxation, dynamic seismic stressing (τs), block subsidence (τb) andrapid decompression (τΔP), and resultant strain rates, whereby:

τmg > τml >> τs $ τb $ τΔP ð7Þ

and thus

γ̇ΔP $ γ̇b $ γ̇s >> γ̇ml > γ̇mg : ð8Þ

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

102s−1> 101s−1

> 100s−1>> 10−2s−1

> 10−3 − 10−4s−1 ð9Þ

Certainly, also the eruption of both volatile- and crystal-richmagmas may be promoted by crustal failure and may explain, forexample, the enigmatic eruptions of for example the 2.1 Ma CerroGallan ignimbrite (Sparks et al., 1985), and the 4 Ma Atana eruption(Lindsay et al., 2001). The latter study concludes on petrologicalgrounds the need for an external trigger of the eruption.

In search for an alternative “relatively” fast in-situ process, such asgas perlocation (Bachmann and Bergantz, 2006), which may even-tually drive a large crystal-rich batholitic system at near-solidustemperatures into eruptive conditions we find this process to take onthe order of 105 years and thus around 10 to 12 orders of magnitudeslower than the proposed processes of crustal failure and strain rate-induced fragmentation. While “in-situ” processes such as magmarejuvenation or reheating may undoubtedly result in a thermody-namic instability of the reservoir andmay hence initiate eruption of anoversaturated cap magma, all our geological evidence (Table 2) isinconsistent with such a scenario. We therefore suggest that theinvestigated eruptions were initiated by local volcano–tectonicinteraction whereby reservoir agitation resulted from local faultingevents and fragmentation was caused by (perhaps multiple) cata-strophic ductile to brittle transition(s) at reservoir depths.

Undoubtedly, the dynamics discussed above may only be achievableduring a specific time–temperature–viscosity window of opportunity ofcrustal magma reservoir evolution. A higher temperature (c.f., a higherabundance of melt and lower crystallinity) and thus lower effectiveviscosity will increase the system's capability to viscously relax strainsinduced by crustal failure. Ensuing eruptions would thus tap crystalsuspensions as is the case in the overwhelming majority of explosivesilicic eruptions. A lower temperatures (c.f., higher crystallinity) andthus higher bulk and melt viscosities will eventually lock the systempreventing eruption and promoting the formation of plutons. Based onthe geological evidence and rheological considerations presentedherein, we propose the eruptability of high-level silicic magma withchamber crystallinity exceeding 50 vol.% and water activity below unityat pressures and temperatures relevant to chamber conditions at strainrates on the order of >10−3 and perhaps down to 10−4 s−1.

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


5. Summary and conclusions

We present a conceptual framework to explain the origin ofenigmatic volcanic deposits related to the collosal explosive eruptionof (supposingly uneruptable) crystal-rich silicic magmas. Relevantprocesses include catastrophic magma–tectonic interaction resultingfrom seismic agitation and roof rock failure during crustal extensionwhich represent the trigger for the evacuation of a batholithic magmareservoir and caldera formation. Amplified pressure transients in theliquid phase during seismic shaking of a crystal-melt mush as well asdynamic stresses due to large-scale faulting may drive high-levelgranitoids towards a critical state (Davis et al., 2007; Linde et al.,1994),at which large-scale faulting and graben formation eventually triggerthe mush to erupt (Fig. 7). We show that deep-level fragmentation ofbatholithic magma occurs at approximately 2 orders of magnitudelower strain rates than those characteristic for failure of crystal-poormagmas or pure melt. In our framework of eruption triggering, theinherent physical properties of crystal-liquid mush and the principleincompatibility of timescales governing system relaxation and stressaccommodation during seismic agitation, crustal failure and meltdepressurization, result in a catastrophic perturbation of a high-levelsilicic magmatic system. Our proposed framework involves (i) thedeep-seated failure of the magma reservoir during active crustal

faulting (as evidenced by high abundance of syn-plutonitic lithics inco-ignimbritic lag breccia), (ii) rapid decompression (as evidenced bylarge-scale evacuation of a pluton-like body), (iii) widespreadignimbrite deposition and (iv) roof collapse and caldera formation.While we do not intend to promote seismicity as an ubiquitous triggerfor large volcanic eruptions, we find that the enigmatic nature of thecase examples warrants the exploration of alternative eruptionpromoters. Colossal volcanic eruptions are extremely rare events andeven fewer may have been induced in the way brought forward here.Nevertheless, as shown here, there appears to be a window ofopportunity for catastrophic system failure for mature batholithicbodies, if system perturbation occurs on timescales and at effectiveviscosities indicated above. It may be that this kinematic window ofopportunity represents the last thermodynamic condition facilitatingeruption in the evolution of a batholithic body. If neither condition ismet, the body will remain untapped forming a pluton unless it ispartially re-melted at a later date.

Recent events in the Afar area may serve as an example of theintimate association between continental extension, magmatism andvolcanism (Wright et al., 2006) and this link is clearly worth furtherinvestigation. For example, large concentric ground deformationanomalies (up to 70 km in diameter) in the Central Andes areinterpreted 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 ofdocumented case studies. Dark colors indicate relatively cool, crystal-rich magma (mush) below a solidification front, light colors indicate hot crystal suspension). b) Relationshipbetween magma relaxation time (τmg), effective shear viscosity (ηb), strain rate (γ ̇) and temperature (T) of the system (color coding as in (a), x indicates conditions for crystal mushduring 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 by1. 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 canrelax viscoelastically (reversible path 1 → 2 → 1) in (b). Imposed pressure transients during seismic agitation, may lead to decompression vesiculation (Davies et al., 2007) drivingthe 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 tocatastrophic 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.


crustal depth (Pritchard and Simons, 2002) and while this area is alsoprone to great earthquakes and crustal extension, future catastrophicvolcano–tectonic interaction may take place in that region. In conclu-sion, we encourage further detailed analysis of large-scale crystal-liquidmush eruptions in order to further test the hypotheses on near-fieldmagma–tectonic interaction put forward here.

Acknowledgments

We thank A. Rust, A. Costa and D. Dingwell for valuable discussionson an earlier draft of the manuscript. Editor C. Jaupart and threereviewers provided constructive comments, which helped improvethe paper. The work was finalised while JG was supported as visitingChair by LMUexcellent funds of the Research Chair in ExperimentalVolcanology (D. B. Dingwell) at the LMU of Munich. The authorsacknowledge funding by a Royal Society University Research Fellow-ship (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).

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