INTRODUCTIONSupervolcanoes erupt SiO2-rich (silicic) magmas. Judging bythe association of supervolcanoes with collapse calderas,the reservoirs containing the magma must have roofs thatreach upper-crustal depths. Given the potentially devastat-ing effects of supereruptions, we are indeed fortunate thatsupervolcanoes erupt as infrequently as they do. But infre-quent eruptions have an insidious aspect, as notorioussupervolcanoes like Yellowstone (western USA) and Toba(Sumatra, Indonesia) teach us: longer periods of volcanoquiescence, or repose, are often followed by larger eruptions(FIG. 1A). In fact, it has long been recognized that there is abroad correlation between repose time and the volume oferupted magma (Smith 1979). This could mean that super-volcano systems are continuously fed by magma and thatthe magma reservoirs (i.e. subsurface domains containingsilicate liquid) grow quiescently until something triggers aneruption. If so, the repose times for supervolcanoes wouldmean that magma accumulates within these reservoirs forseveral hundred thousand years or more.

Long periods of magma accumulation have important con-sequences for the supervolcano system. First, silicic magmabodies as large as those responsible for supereruptionswould undergo appreciable crystallization in even a hundredthousand years unless there were intermittent input frommagmas that are more mafic (higher in Mg and Fe, similarto basalt) and therefore hotter than the silicic magmas.Second, based on the correlation between repose time anderuptive volume, an average rate of magma accumulationof about 1 km3 per thousand years can be estimated (Smith1979). Even though supervolcanoes erupt infrequently, wecould expect to find evidence for at least a handful of 100 km3

and larger magma bodies globally.No such chambers have been con-vincingly detected. Do supervolcanoreservoirs accumulate gradually andpersist in a nearly solid state, only totransform into chambers withvoluminous eruptible magma whenconditions are right? Or, do super-volcano reservoirs accumulate muchmore rapidly than the volume–repose relationship suggests? Morequantitative constraints on theduration of magma accumulationare needed if we are to understandhow long—and what—it takes toproduce a supereruption.

THE FIRST ATTEMPTS TO DATE THE BIRTHOF A SUPERVOLCANO RESERVOIR Geologists routinely make use of radioactive decay to measuregeologic time. Silicate liquids contain radioactive atomsthat can be incorporated into crystals as the minerals grow,and these radioactive parents will transform, over time, intoradiogenic daughter atoms. The relative proportions ofradioactive and radiogenic atoms, which are typically meas-ured on a mass spectrometer, can be used to determine theage of the material in which they are contained. Most min-erals and volcanic glasses contain only a small concentra-tion of radioactive and radiogenic atoms. Furthermore, ifthey are to be used for dating crystallization, radioactiveand radiogenic atoms cannot migrate into or out of crystalsheld under magmatic conditions. Only certain radioac-tive–radiogenic isotope pairs, e.g. 87Rb–87Sr, 238U–206Pb,and 238U–230Th, in certain minerals have the high rates ofradioactive decay and the relative immobility required todistinguish the duration of magma storage from the analyt-ical uncertainty related to an eruption age.

In a series of ground-breaking studies, scientists working onthe youthful Bishop Tuff, eastern California, found that87Rb–87Sr mineral and glass ages were as much as 1.7 Myolder than a well-established 760 ka 40Ar/39Ar eruption agefor the Bishop Tuff (FIG. 1B; Halliday et al. 1989; Christensenand DePaolo 1993; van den Bogaard and Schirnick 1995;Christensen and Halliday 1996). Age similarities betweenminerals from the Bishop Tuff and the 0.8–1.2 Ma GlassMountain rhyolite domes that lie just outside the calderacreated by eruption of the Bishop Tuff support a model inwhich the same silicic magma body was tapped repeatedly.Radiometric ages thus support volcanological evidence thateruption of the Bishop Tuff cataclysmically drained amagma chamber that had otherwise been accumulatingpassively beneath Long Valley for somewhere between ahalf and over one million years. Nonetheless, importantquestions about the significance of these results remained.

E L E M E N T S , V O L . 4 , P P . 2 3 – 2 8 FEBRUARY 2008

Mary R. Reid*

* Department of GeologyNorthern Arizona UniversityFlagstaff, AZ 86011-4099, USAE-mail: mary.reid@nau.edu

How Long Does It Taketo Supersize an Eruption?

23

Along-recognized correlation between the volume of major eruptions andthe time interval between them suggests that magma may accumulatefor about a million years before a supereruption. However, radiometric

ages and time-dependent phenomena like crystal growth and compositionalhomogenization show that the duration of supervolcano magma accumulationcould be significantly shorter than this. Crystals in supervolcano magmas mayhave protracted growth histories and may grow from chemically different hostsas crystallization progresses. Semisolid crystal mushes rather than liquid-richmagma chambers may be the prevalent state of supervolcano feeder systemsand should be the focus of geophysical studies aimed at predicting futuresupereruptions.

KEYWORDS: magmatic processes, geochronology, explosive volcanism, calderas, kinetics

DOI: 10.2113/GSELEMENTS.4.1.23

Could isotopic differences between minerals and their hostmagmas have arisen from effects other than the duration ofmagma storage? What if, for example, foreign material, fromsurrounding rock incorporated in the magma, were inadver-tently part of mineral separates analyzed? Unfortunately, fewsupervolcanoes have compositions suitable for using 87Rb–87Srdating to test the generality of the Bishop Tuff results.

TINY ZIRCON WEIGHS INThe search for direct evidence about the duration of build-upto a supervolcano eruption began in earnest with the pub-lication of the studies of the Long Valley caldera systemmentioned above. Zircon held particular promise in this effortbecause it is a common, if relatively minor, mineral con-stituent of silicic magmas. As zircon grows, it incorporatesuranium (U), a radioactive element, but excludes lead (Pb),an element with isotopes that are the radiogenic daughtersof U. Once isolated in zircon’s crystal structure, both U andany Pb formed by later U decay are highly immobile, evenat the temperatures of silicic liquids. This means that whenzircon crystallizes, the radiometric clock is set, and it is notcontinuously reset by movement (diffusion) of the isotopesthrough the crystal structure at magmatic temperatures.

Zircon has another important advantage when it comes tounderstanding the pre-eruption history of magmas. Using amass spectrometer that is specially designed so that U andPb can be mined directly from individual zircon grains byan accelerated beam of ions in an ion microprobe (see alsoDavidson et al. 2007), workers can determine specific agesfor small portions of individual zircon grains (FIG. 2).Alternatively, zircon can be chemically digested and ana-lyzed by thermal ionization mass spectrometry, a methodthat yields more-precise ages but with a loss of spatial reso-lution. For either method, the amount of 206Pb produced bythe decay of 238U in zircon can permit time intervals of<100,000 years to be resolved.

2424E L E M E N T S FEBRUARY 2008

Examples of zircon dating by ion microprobe analysis.(A) Cathodoluminescence image showing growth-

related oscillatory zoning in a zircon crystal from the Whakamaru erup-tion, New Zealand, as revealed by polishing (Brown and Fletcher 1999).Circles show locations of zones excavated by the ion beam (usually30–40 micrometers across) and 238U–206Pb ages associated with them.(B) Ages obtained by letting an ion probe beam excavate continuouslyfrom the rim towards the core (depth-profiling) of a zircon from theYoungest Toba Tuff. The width of each box gives the average age anduncertainty (1 sigma) associated with the indicated depth interval. Theages indicated by the lighter shaded boxes are derived from 238U–230Thdata; those shown by the darker shaded boxes are derived from238U–206Pb data. 206Pb*/238U (top axis) signifies the ratio between theradiogenic daughter (206Pb*) and the radioactive parent (238U) in thecase of no initial disruption of the decay series (see FIG. 3).

FIGURE 2

(A) Graph schematically illustrating the relationshipbetween average repose interval and eruption size. The

center of each volcano symbol corresponds to the repose interval anderupted volume of a supervolcano; the sizes of the symbols are propor-tional to the erupted volume. Eruptions shown are associated with inter-mediate to silicic caldera-related systems that have experienced multi-ple episodes of eruptions. The graph is based on Trial and Spera (1990),except where estimates have been revised by White et al. (2006). Unla-belled volcanoes are, in order of increasing repose interval, Taupo (post-Oruanui), Valley of Ten Thousand Smokes, Zaragosa, and TopapahSprings. (B) Pre-eruption crystallization ages for early- and late-eruptedportions of the Bishop Tuff calculated from the differences betweencrystallization and eruption ages. Ages obtained from 87Rb–87Sr and238U –206Pb radiometric dating as reported by Halliday et al. (1989),Christensen and DePaolo (1993), Christensen and Halliday (1996), andSimon and Reid (2005). Abbreviations: EBT: Early Bishop Tuff; LBT: LateBishop Tuff; MIB: melt inclusion–bearing (relatively Rb-rich and Sr-poormagmatic liquids were trapped in quartz as they grew).

FIGURE 1

A

A

B

B

Ages obtained from growth zones within supervolcano zir-con grains show that most crystallized within 350 ky oferuption (Reid 2003; Turner and Costa 2007). Zircon in theLava Creek Tuff (Yellowstone) and Bishop Tuff, for exam-ple, grew mostly <100 ky before their respective eruption(Bindeman et al. 2001; Simon and Reid 2005). In other zir-con grains, such as those from Whakamaru, New Zealand(FIG. 2), core-to-rim age variations within individual grainsprovide evidence of zircon growth in the 200–400 ky inter-val leading up to eruption. Crystallization ages of400–800 ky before eruption have been documented (e.g. LaPacana caldera, Chile, and the Youngest Toba Tuff,Indonesia) but are relatively rare (Reid 2003; Vazquez2004). Zircon crystals that give pre-eruption ages still olderthan this, i.e. millions to billions of years older than erup-tion, have been identified, but these are inferred to bexenocrysts (crystals that were derived from older rocks for-eign to the magmatic system into which they were incor-porated). One general conclusion is that, whereas theearliest pre-eruption ages are found in some of the largestsupervolcanoes, there is otherwise no general correlationbetween eruption size and the age distribution of zirconcrystals.

In some young supervolcanoes, zircon may have crystal-lized so recently that the tiny quantities of Pb produced by238U decay cannot be confidently distinguished from thefew Pb atoms that were incorporated during crystal growth.For these volcanoes, it is possible to exploit the fact that thedecay of 238U to 206Pb involves a series of intermediate radi-ogenic daughters that are themselves radioactive (see alsoTurner and Costa 2007 and Watson 2007). One of thelongest-lived of the U-series radioactive daughters is 230Th,which has a half-life of 75,000 years. As it grows, zirconpreferentially incorporates U relative to Th, and this causesan initial disequilibrium in which the rate of 230Th produc-tion is greater than the amount of 230Th that is lost throughdecay (FIG. 3B). This results in a net accumulation of 230Thuntil its decay rate matches its production rate. The attain-ment of this steady-state or “secular” equilibrium condition

requires ~350 ky. A crystallization age for zircon crystalsthat are <350 ky old can thus be obtained by making use ofthe time-dependent return of the 230Th–238U ratio to secu-lar equilibrium. Allanite, another minor phase in silicicmagmas, is also a candidate for U–Th disequilibrium dating.An epidote-group mineral rich in thorium and rare earthelements, allanite strongly prefers 230Th to 238U (FIG. 3C).Without a steady supply of radiogenic 230Th from 238Udecay, there is a time-dependent decrease in 230Th.238U–230Th ages for individual zircon and allanite crystalscan be obtained using a high- resolution ion microprobe,affording the same spatial resolution as for U–Pb dating.

For the 530 km3 Oruanui eruption of New Zealand, U–Thdisequilibrium zircon ages range up to ~150 ky before erup-tion but cluster at 12 and 70 ky before eruption (Charlier etal. 2005). For the 2800 km3 Youngest Toba Tuff, U–Th dis-equilibrium zircon ages range from essentially identical toeruption (74 ka) to indistinguishable from steady-stateequilibrium (i.e. ≥350 ka). Depth-profiling (FIG. 2) of someToba zircon grains using a combination of U–Th and U–Pbdating shows a progressive increase in age with distancefrom the rim. Individual allanite grains from the same tuffrange in age from that of eruption at their rims to as muchas 150 ky before eruption at their cores (Vazquez and Reid2004). The rim-to-core chemical variations in individualallanite grains are complicated and, together with the ageresults, suggest that they crystallized in a variety of chemi-cal environments as magma accumulated for more than100,000 years.

MORE WAYS TO ESTIMATE THE DURATIONOF MAGMA STORAGE Other methods besides those involving radioactive decaycan provide insights into the timescales for accumulation ofmagmas in the reservoirs beneath supervolcanoes. Theserely mainly on the time dependence of chemical reactions,such as phase changes and chemical equilibration. Reactionrates depend on the temperature and chemical compositionof a system, which means that the timescales we determine

25E L E M E N T S FEBRUARY 2008

Cartoon illustrating the decay relationships between 238Uand 230Th. Both 238U and 230Th are radioactive but 230Th

has a much shorter half-life. The shorter-lived daughter isotopes between230Th and 238U are not illustrated here because their effects are negligible.(A) Representation of a system at secular equilibrium. The rate of 230Thaddition from 238U decay is matched by the rate of 230Th decay (secularequilibrium). (B) Representation of the evolution of a system in which230Th has been preferentially lost or excluded relative to 238U. Initially,230Th addition from 238U decay is greater than the loss of 230Th due to

decay. The rate of 230Th decay rises in proportion to the amount ofaccumulated 230Th until it matches the rate of 230Th addition. This sce-nario applies to zircon, which tends to exclude Th from its crystal struc-ture as it grows. (C) Representation of the evolution of a system in which238U has been preferentially lost or excluded relative to 230Th. Becausethere is essentially no source of 230Th, 230Th remaining in the system pro-gressively decays away. This scenario applies to allanite, which tends toexclude U from its crystal structure as it grows.

FIGURE 3

B

A

C

26E L E M E N T S FEBRUARY 2008

from kinetic phenomena are subject to certain key assump-tions. To date, kinetic methods have been less widelyapplied to supervolcanoes than radiometric dating.

How large a crystal will become depends on the rate andduration of its growth and whether growth ceases beforeeruption because the crystal abuts other crystals. Studies ofcrystal sizes in silicic magmas have focused mainly onquartz and zircon. Based on an estimated linear growth rateof ~10-15 cm/s, 50–150 µm zircon grains would have grownfor tens of thousands to a few hundred thousand years(Bindeman 2003; Reid 2003), durations comparable tothose estimated from radiometric ages for zircon. Using areported linear growth rate of at least 10-11 cm/s, a maxi-mum crystal size of 0.3 cm for quartz (Bindeman 2003)would be attained in no more than a thousand years. Ifquartz crystals could grow in a liquid for as long as a mil-lion years without abutting other crystals, they would reach0.3–3 meters across! Crystals this size are essentiallyunknown except in pegmatites, showing that there must bea limit to how big a crystal can become before it is effec-tively isolated from liquid.

Minerals, like the Youngest Toba Tuff allanite noted above,can sometimes exhibit complex compositional zoning pro-files. Minerals tend to homogenize, and so compositionalheterogeneities will be dissipated by diffusion, in a time-dependent fashion. The time required for a crystal to gofrom having a particular zoning profile to having a more“relaxed” zoning profile (FIG. 4) provides a measure of thetime elapsed since the crystal (or a specific portion withinit) grew. The main information that is required, besides thecharacteristics of a crystal’s heterogeneity, is the rate(s) ofdiffusion of the element(s) of interest and some means ofestimating the initial heterogeneity of a crystal. Diffusionrates are strongly temperature dependent, and so are thechemical relaxation ages derived from them. For example,this approach yields no more than ~100 ky of storage at T >800°C for plagioclase feldspars in the Bishop Tuff (Andersonet al. 2000; Morgan and Blake 2006). If storage were mainlyin a semisolid state at temperatures of <750°C, the durationcould, on the other hand, be almost indefinite.

There are circumstances under which the isotopes of certainelements in crystals have relative abundances that are dif-ferent from those expected for crystals at equilibrium withtheir host liquids. These isotopic differences will adjust overtime so that the minerals and the liquid attain equilibriumwith each other. For example, the 18O/16O ratios in quartzand feldspar in the Bandelier Tuff of Valles caldera, NewMexico, are not in equilibrium with the 18O/16O ratio in thehost rhyolite glass (Wolff et al. 2002). The relatively rapiddiffusion of oxygen in feldspar and magmatic liquid meansthat this oxygen isotope disequilibrium would have beenlost if the feldspars were in contact with the liquid for morethan a few hundred years at magmatic temperatures. Thisand the distinct initial isotopic composition of the crystalssuggest that, rather than being continuously in contactwith magma, the crystals may have been scoured from theroof of the magma chamber during eruption and onlybriefly reimmersed in liquid.

TIMESCALES OF SUPERVOLCANO MAGMAACCUMULATION IN REVIEWSince the first half of the 20th century, a tenet of igneouspetrology has been to use the chemical characteristics ofminerals and their associated glasses as windows into whathappens in magma plumbing systems. Minerals like quartzand zircon are expected to crystallize from the silicic mag-mas that characterize supervolcanoes. In most cases, thesesilicic magmas form in part by modification of more mafic

precursors. Processes include separation of more silicic liq-uid from crystals during cooling of parental magmas andpartial melting of the plutonic equivalent of basalt (e.g.Huppert and Sparks 1988; Bachmann and Bergantz 2004,2008 this issue). Consequently, a first-order assumption isthat the ages of minerals like quartz and zircon tell us whena magma evolves to compositional characteristics broadlylike those of the magma that ultimately erupts. But appear-ances can be deceiving. In some cases, radiometric agesreveal that some crystals are truly xenocrystic; in othercases, experimental work shows that crystals cannot havebeen in equilibrium with their hosts. Thus, we need to becareful before asserting that any particular mineral crystal-lized from its host magma.

Illustration of chemical relaxation in a compositionallyzoned mineral. (A) The distributions of two elements, X

and Y, initially have stepwise variations in concentrations (absolute con-centrations are not important). (B) and (C) Over time, the steep con-centration gradients become more subdued as material diffuses acrossthe gradients. The faster diffusion rate of X compared to Y results in pro-gressive differences in their zoning profiles, and these differences can beused to determine how much time has elapsed since crystallization pro-duced the original zoning profiles. An example is diffusion of bariumand strontium in feldspar: both partition into the same lattice site, butSr, being smaller, can migrate faster than Ba over the same composi-tional gradient. Modified after Morgan and Blake (2006).

FIGURE 4

B

A

C

27E L E M E N T S FEBRUARY 2008

In trying to understand the significance ofcrystal ages, it is also important to thinkabout crystal nurseries and what happenswhen crystals “come of age.” If crystalsnucleate and grow mainly on the marginsof the magma reservoir, they become iso-lated from the liquid as heat is extracted from the reservoir,and the boundary of the immobile, crystal-rich zoneadvances inward (FIG. 5). If crystals nucleate and growmainly suspended in liquid, large crystals of minerals thatare denser than the liquid around them will tend to settlethrough and be removed from the liquid. Either way, anindividual crystal is a transient feature of the magma reservoirsince it cannot grow for too long and/or become too big beforeit is removed from the liquid.

If one thinks about a magma reservoir as more dynamicthan a passively inflating, balloon-like body, additionalconsiderations come into play. Magma fluxes into a reservoircan entrain not only xenocrysts but also crystals from thepartially to completely solidified roof, floor, and walls of thereservoir, as inferred for Bandelier Tuff quartz and feldspar.If reentrained crystals are prevented from further growth bybeing erupted or by settling to the chamber floor, diffu-sional reequilibration may be limited and young rims onminerals may be absent. Even where age distributions sug-gest protracted immersion of crystals in liquid, growth mayhave been stepwise rather than continuous. The result isthat many supervolcano “phenocrysts” (presumed nativecrystals) may not actually spend much of their time sus-pended in and surrounded by large volumes of liquid!

We’ve seen that radiometric ages and kinetic constraintsfrom crystal growth and diffusional relaxation show that thecrystals contained in supervolcano magmas can form sev-eral tens of thousands to hundreds of thousands of years beforeeruption. While this age information cannot uniquely dis-tinguish among the various scenarios of magma reservoirdynamics and evolution, two observations are particularlynoteworthy: (1) the ages obtained for the crystals are generallyless than those expected from gradual magma accumulation,and (2) there is no clear correlation between crystal ages andvolume of magma erupted. Thus, the crystal populations insupervolcano magmas may document only the latest stage ina protracted gestation period during which crystallizationand melt segregation, melting, and/or mixing of precursormagmas occurred. If so, the repose interval between eruptionscould represent a critical staging period that sets in motionthe ultimate development of a supervolcano magma chamber.

Another consideration for supervolcano magmas is thatthey must be liquid rich at the time of eruption, but the ageinformation that we have for them comes chiefly from theircrystal cargo. Rather than being one large magma chamberwith crystals suspended in liquid throughout, supervolcanomagma reservoirs may be mainly a mushy to semisolid crystalmatrix that encloses smaller domains of crystal-poor liquid(FIG. 5). The liquid-dominated portion that is ultimatelyerupted may be derived from the crystal–liquid mush by,for example, gravitational collapse (think of squeezing liq-uid from a sponge), melting by the addition of hotter mag-mas and their gases, or growth of gas bubbles that force poreliquid out of the mush (Sisson and Bacon 1999; Bachmannand Bergantz 2004, 2006). The crystals in the erupting magma,according to this scenario, are simply crystals scoured fromthe mush by the segregating liquid. Ages obtained fromcrystals that are scavenged randomly could still accuratelyreflect how long it took to “supersize” the eruption (FIG. 5).On the other hand, ages obtained from younger crystalsthat are scavenged preferentially because older ones are moreisolated from liquid could be significantly less than thetime required to form a supervolcano magma. Longevity ofthe reservoir may also be underestimated because early crys-tals can dissolve when heat is introduced by fresh magmareplenishments. Whatever the case—and this could differ fromone supervolcano to the next—it is clear that magmas arepresent in the subvolcanic system well before eruption.Geophysical efforts aimed at detecting very large but liquid-poor reservoirs that include distributed rather than largeballoon-like magma chambers hold promise for identifyingsupervolcano magmas well before they erupt.

ACKNOWLEDGMENTSThe expert editorial handling of Calvin Miller and David Warkand reviews by Charlie Bacon, Kurt Hollocher, Ian Parsons,Lily Claiborne, and Ashley Bromley significantly improvedthis paper. I am indebted to Jorge Vazquez, Justin Simon, KariCooper, and Jennifer Garrison for the countless synergisticinteractions that contributed to the ideas presented here.The support of EAR-0538309 is gratefully acknowledged. .

Cartoon illustrating scenarios forcrystal age variations within super-

volcano magma reservoirs and their eruption prod-ucts. Red lines bound regions from which magmasare derived. Age variations are represented by vari-ations in crystal shading (age zonation within crys-tals is not included). (A) Homogeneous magmareservoir that evolves by progressive magma addi-tions and crystallization. Crystal ages in the super-volcano (τ) represent the duration of magma reser-voir accumulation (τ). (B) Progressive inwardcrystallization of reservoir with oldest crystals closestto the chamber margins. Only the youngest crys-tals are suspended in the liquid or entrained fromthe margins of the reservoir during eruption.Supervolcano crystal ages represent only theyoungest stage in the evolution of the magmareservoir. (C) Repeated intrusion, partial to com-plete solidification, and entrainment of crystalsfrom mush and wall rock intrusions during eruptionresult in a heterogeneous crystal age population.Crystal ages represent the overall duration of thesupervolcano magma reservoir more completelythan in scenario B but perhaps not as completely asin scenario A.

FIGURE 5

BA C

28E L E M E N T S FEBRUARY 2008

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