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To the Editor Detailed analysis of volcanic materials1 allows the study of eruptions that have not been witnessed directly. Rotella etal.2 undertake the challenge of interpreting submarine volcaniclastic deposits around Macauley Volcano, part of the Kermadec Arc in the southwest Pacic Ocean. They propose a Tangaroan eruption style, based on the textural characteristics of dredged pumice clasts. However, we argue that the analytical methodology provides an inadequatebasis for the identication of a new eruption style.

The style of an eruption is determined based on a suite of parameters that dene the duration, mass eruption rate and degassing behaviour of the eruption3. To dene eruptive style, a single event must

be isolated in the volcanic stratigraphy. Additionally, to link vesicle textures observed within individual clasts with magma-degassing behaviour, it must be shown either that all the samples representa single magma type, or that a variety of magma types are associated with the same distinctive vesicle texture. Otherwise, variations in vesicle textures caused by magma degassing cannot be isolated from those resulting from variations in major element content, volatile compositionand temperature. The samples used by Rotella etal.2,4 were dredged along seven transects, each hundreds of metres in length. They probably originate from dierent eruptions, as demonstrated by the large range in chemical composition and isotopic signatures5. Moreover, clast density varies

widely and inconsistently with composition5.

Because individual eruptive events cannot be distinguished, and the textural and geochemical variations are relatively large, the degassing histories of clasts within the deposit cannot be attributed to a single style, much less underpin a previously unrecognized eruption type.

Rotella etal.2 also cite bimodal clast-density distributions as evidence fora unique eruption style. However, the distributions are obtained by aggregating data from samples collected from multiple transects, thus probably from separate eruption events. Such data stacking produces a composite picture thatmasks the styles of individual events. To demonstrate the consequences of this procedure, we apply the same treatmentto pyroclast-density data from two well-characterized, subaerial explosive sequences. The magmas have homogeneous bulk compositions and each sequence is composed of discrete eruptive units that exhibit a range of eruptive styles (Fig.1a,b). The bimodal density distributions inour stacked data are similar to thoseof the Macauley samples2, illustratingthe failure of this stacking procedure to recover the degassing history of a nuanced eruptive sequence.

Finally, Rotella etal.2 analysed in detaila single gradient clast, interpreted to represent the contact between the quenched rim and expanded interior of a pumice clast. They suggest the gradient clast isthe remnant of a distinct parcel, or blebof magmatic foam that detached fromthe volcanic conduit, but we present an alternative interpretation (Supplementary Information). The bubble number densities (BNDs) calculated for this clast span an order of magnitude, similar to values inferred for highly explosive, high mass eruption rate (MER > 106kgs1) subaerial events68 (Fig.1c). However, becauseBNDs depend on water concentration and diusivity, the availability of nucleation substrates and magma temperature9,

the values are too scattered to allow discrimination of MER. Instead, BNDs can be used to assess eruption style through calculation9 of magma decompression rate, dP/dt (Fig.1d). Calculated dP/dt for the Macauley pumice varies from about 4to 35MPas1. These ascent rates impose high levels of shear strain10 and should promote

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Figure 1 | Pyroclast density and bubble number density interpretations. a,b, Stacked density distribution of pyroclasts from the ad181 Taupo (New Zealand)5 (a) and 122bc Etna (Sicily)6 (b) eruptions, after merging all eruptive units. Pie charts of erupted volumes per unit show that merging distributions disregards their relative volumetric contribution. c, Bubble number densities within pyroclasts arenot correlated with eruption rate8. d, However, they can be used to calculate decompression rates assuming either homogeneous or heterogeneous nucleation (controlled by ). Decompression rates calculated for the Macauley magmas range between about 4to 35MPa s1 (orange-to-purple gradient box), typical of explosive eruptions8. Model parameters for calculations in d are detailed in the Supplementary Information.

802 NATURE GEOSCIENCE | VOL 6 | OCTOBER 2013 | www.nature.com/naturegeoscience

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magma fragmentation in the conduit. Rapid magma ascent is also consistent with the absence of microlite crystallization in the Macauley magmas2. The high BNDs therefore do not support the low-to-intermediate magma discharge rates that would be consistent with bleb detachment2.

Rather, the Macauley data seem to preserve evidence of an explosive style, consistent with recognized styles of submarine pumice eruptions11.

References

1. Kano, K. Geophys. Monogr. AGU 140, 213229 (2003).2. Rotella, M.D., Wilson, C.J.N., Barker, S.J. & Wright, I.C. Nature Geosci. 6, 129132 (2013).

3. Walker, G.P.L. Geol. Rundsch. 62, 431446 (1973).4. Barker, S.J., Rotella, M.D., Wilson, C.J. N., Wright, I.C. & Wysoczanski, R.J. Bull. Volcanol. 74, 14251443 (2012).

5. Barker, S.J. etal. J.Petrol. 54, 351391 (2013).6. Houghton, B.F. etal. J.Volcanol. Geotherm. Res. 195, 3147 (2010).7. Sable, J.E., Houghton, B.F., Del Carlo, P. & Coltelli, M. J.Volcanol. Geotherm. Res. 158, 333354 (2006).8. Alfano, F., Bonadonna, C., Gurioli, L. Bull. Volcanol. 74, 20952108 (2012).

9. Toramaru, A. J.Volcanol. Geotherm. Res. 154, 303316 (2006).10. Namiki, A. & Manga, M. J.Geophys. Res. 111, B11208 (2006).11. Allen, S.R. & McPhie, J. Geology 37, 639642 (2009).

Additional informationSupplementary information accompanies this paper on http://www.nature.com/naturegeoscience

Web End =www.nature.com/naturegeoscience .

Thomas Shea*, Julia Hammer and Emily First Geology and Geophysics, University of Hawaii, SOEST, Honolulu, Hawaii 96822, USA. *e-mail: mailto:[email protected]

Web End [email protected]

Authors reply Shea etal. raise three issues pertaining to our work1. First, they argue that our pyroclasts were potentially from dierent eruptions or magma types with dierent degassing histories. However, we do not require the Tangaroan pyroclasts to be from a single eruption; indeed, we propose that this style can apply to magmas of diverse compositions for eruptionsat submarine volcanoes worldwide. At Macauley Volcano, glass chemistriesfor Tangaroan dredged pyroclasts are dacitic1 and the clasts lack microlites, indicating a common history without signicant degassing2. Furthermore, we do not claim that all dredged pyroclasts are Tangaroan in origin3. Some high-density microlite-bearing clasts, for example, have contrasting textures interpreted to reect dome-forming eruptions3.

Second, Shea etal. argue that our stacked density data provide a misleading representation of the density distributions for individual eruption events. However, as discussed in ref.3, irrespective of whether the data are derived from individual stratigraphic levels, single or multiple eruption sequences or dredge hauls, the pyroclast density characteristics fromthe four volcanoes we have studied are consistent within and distinctive between the volcanoes and eruptive settings. We chose only one representative Tangaroan clast for detailed discussion, but descriptions and analyses of more clasts are presented elsewhere4. The stacking of density data presented in Fig.1 from Shea etal. is misleading. The Taupo eruption density bimodality (Fig.1a) is caused by data from diering eruption styles, recognisable from

textural characteristics. The high-density mode represents microlite-rich, degassed clasts from phreatoplinian (Unit4) and sub-lacustrine eusive (Unit7) phases, whereas their low-density mode is causedby microlite-poor, highly explosive plinian eruptions5. In contrast, the Tangaroan-style pyroclasts we have studied span both density modes. Individual fragments are linked by the density values and textures across the gradient clasts1.

Third, Shea etal. compare our data to selected data from pyroclasts with diering magma compositions and crystal contents (Fig.1c). They assert that the Tangaroan discharge rate was equally high and the activity explosive. This comparison is misplaced because explosively erupted magmas with similar compositions to ours show higher bubble number density (BND) values. For example, the Mount StHelens eruption in Washington in 1980 generated clasts with BND values of 8.2108cm3 (ref.6) and the Mount Mazama eruption in Oregon around 7,700years ago generated clasts with BND values of 6.0109cm3 (ref.7). In contrast to Shea etal., we conclude that BND values from natural pyroclasts are oen higher than those obtained through experiments8 or numerical simulations9. The equations9 on which Shea etal. construct their comparative argument are based on a single, homogenous nucleation event that produces bubbles with a narrow size range. Such conditions are more easily replicated in experimental simulations. Natural pyroclasts, however, may result from multiple nucleation events or continuous nucleation before fragmentation (for example, ref.10 and references therein).

Comparison between the denser, quenched rims of the Tangaroan clasts from Macauley Volcano and subaerially erupted pyroclasts with similar chemistries and crystal contents taken from Raoul Volcano (alsopart of the Kermadec Arc in the southwest Pacic Ocean) shows that the BND valuesof the Tangaroan clast rims are signicantly lower than the BND values of 2.6109 to 1.91010cm3 measured for the Raoul clasts that were erupted in explosive events4. We therefore conclude that when relevant data are compared on an equal basis, our proposal for the Tangaroan eruption style remains fully justied and open to further application.

References

1. Rotella, M.D., Wilson, C.J. N., Barker, S.J. & Wright, I.C. Nature Geosci. 6, 129132 (2013).

2. Blundy, J. & Cashman, K.V. Geology 33, 793796 (2005).3. Barker, S.J., Rotella, M.D., Wilson, C.J. N., Wright, I.C. & Wysoczanski, R.J. Bull. Volcanol. 74, 14251443 (2012).

4. Rotella, M.D. Ph.D. thesis, Victoria Univ. Wellington (2013); http://researcharchive.vuw.ac.nz/handle/10063/2729

Web End =http://researcharchive.vuw.ac.nz/handle/10063/2729

5. Houghton, B.F. etal. J.Volcanol. Geotherm. Res. 195, 3147 (2010).6. Klug, C. & Cashman, K.V. Geology 22, 468472 (1994).7. Klug, C., Cashman, K.V. & Bacon, C.R. Bull. Volcanol. 64, 486501 (2002).

8. Mangan, M. & Sisson, T. Earth Planet. Sci. Lett. 183, 441455 (2000).

9. Toramaru, A.J.Volcanol. Geotherm. Res. 154, 303316 (2006).10. Hamada, M., Laporte, D., Cluzel, N., Koga, K.T. & Kawamoto, T. Bull. Volcanol. 72, 735746 (2010).

Melissa D.Rotella1*, Colin J.N.Wilson1, Simon J.Barker1 and Ian C.Wright2

1School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington 6140, New Zealand,

2National Oceanography Centre, University of Southampton Waterfront Campus, Southampton SO14 3ZH, UK.*e-mail: mailto:[email protected]

Web End [email protected]

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