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Changing Proportions of Two Pumice Types from the June 15, 1991, Eruption of Mount Pinatubo

By Carlos Primo C. David,¹ Rosella G. Dulce,² Dymphna D. Nolasco-Javier,¹ Lawrence R. Zamoras,¹ Ferdinand T. Jumawan,¹ and Christopher G. Newhall³

1 University of the Philippines, National Institute of Geological Sciences, Diliman, Quezon City.

2 Philippine National Oil Company, Geothermal Division, Fort Bonifacio, Metro Manila.

3 U.S. Geological Survey.

ABSTRACT

Pumice erupted by Mount Pinatubo on June 15, 1991, was of two types: a white, phenocryst-rich variety and a tan phenocryst-poor variety. The volumetric proportion of phenocryst-poor pumice decreased through the course of the eruption, from about 20 percent at the base of the plinian fall unit to about 10 percent at its top. The progressive change might reflect early tapping of a hotter, deeper, phenocryst-poor part of the magma reservoir or, alternatively, early and unusually explosive tapping of the top of the magma reservoir. Abundant broken crystal fragments in the groundmass of the phenocryst-poor groundmass and preservation of the same phenocryst assemblage in the two pumice types favor the second explanation.

INTRODUCTION

Several workers have noted that June 15 eruption of Mount Pinatubo produced two distinct pumice types, one rich in phenocrysts (fig. 1) and the other poor in phenocrysts (fig. 2) (Bernard and others, this volume; Imai and others, this volume; Luhr and Melson, this volume; Pallister and others, 1992; this volume). These workers agree that the two pumice types are chemically similar dacites (64-65 wt% SiO₂) and that the principal difference between the two types is a textural difference, probably resulting from fragmentation of crystals in the phenocryst-poor type. Yet, there is also permissive evidence of some degree of crystal resorption by disequilibrium melting (Luhr and Melson, this volume; Pallister and others, this volume).

Figure 1. Phenocryst-rich pumice of June 15, 1991. A, Prominent stretching of the glassy matric of the phenocryst-rich pumice (sample CR-9, uncrossed polars, 63x). B, Phenocryst-rich pumice under crossed polars (sample CR-9, crossed polars, 25x). Vesicles in the phenocryst-rich pumice are larger and interconnected along prominent flow structures of the glassy matrix.

Figure 2. Phenocryst-poor pumice of June 15, 1991. The characteristic features of the phenocryst-poor pumice showing smaller isolated, nonelongated vesicles and a glassy matrix with generally no observable flow structure (sample A-7; A, uncrossed polars and B, crossed polars, 63x).

The dominant pumice type, which constitutes about 80-90 vol% of the lapilli and blocks observed in June 15 tephra-fall deposits, is white, contains large interconnected vesicles, and large (>3-mm-diameter) phenocrysts. The subordinate type is tan, contains small (<2-mm-diameter) spherical vesicles and has few crystals larger than 1 mm. In addition, fluidal banding appears more common in the phenocryst-rich than in the phenocryst-poor pumice. The two pumice types are readily distinguished in the field by color and by texture.

In this short paper we report the abundance of the two pumice types in several stratigraphic sections through the June 15 tephra-fall deposit, and we examine whether and how the proportions of each type changed through the course of the eruption.

SAMPLES AND METHODS

Ten samples each of phenocryst-rich and phenocryst-poor pumice were collected from the June 1991 pyroclastic-flow deposit of Mount Pinatubo, in the valley of the Sacobia River, about 500 m northwest from Gate 14 of Clark Air Base (fig. 3). Thin sections of these samples were prepared and examined to compare with the results of other workers and to verify our ability to distinguish the two pumice types in the field. Both pumice types contain similar crystal assemblages that are dominantly plagioclase and hornblende, with subordinate quartz, cummingtonite (as rims on hornblende crystals), biotite, magnetite, ilmenite, anhydrite, apatite, olivine (rimmed by hornblende), and trace zircon. Modal analyses of our thin sections are not presented here because of plucking of large grains from the phenocryst-rich pumice during section preparation and because of difficulty in optical counting the abundant, but very small, crystal fragments in the phenocryst-poor pumice.

Figure 3. Location map for June 15 coarse tephra samples. Scale is approximately 1:250,000.

Samples of tephra-fall deposit, for component analysis, were taken at seven sites that are roughly 25 km east and northeast of Mount Pinatubo's crater. Sampling 6 months after the eruption, we encountered some difficulty in finding complete and undisturbed sections of the June 15 coarse tephra deposits. At each site, the coarse tephra layer was divided into three stratigraphic intervals: bottom (L1), middle (L2), and top (L3). In some cases where the June 15 tephra layer was less than 3.5 cm thick, the section is only divided into two intervals, L1 and L2 (fig. 4). Approximately 200 pumice fragments >2 mm in diameter were counted per sample layer.

Figure 4. A, Generalized tephra section, as seen in Castillejos, Zambales, southwest of Pinatubo (not sampled; shown here for reference). B, Generalized tephra section for Floridablanca, Porac, and Clark Air Base. C, Generalized tephra section for Bamban, Capas, Mabalacat, and Telebastagen. All materials are pumiceous ash and lapilli, with variations in grain size as shown.

PROPORTIONS OF TWO PUMICE TYPES

The dominant pumice type in the June 15 coarse tephra layer is the phenocryst-rich type (table 1). It comprises 80-90% of the coarse tephra deposits, while the phenocryst-poor type comprises the remaining 10-20%. In sections less than 3.5 cm thick, the phenocryst-poor percentage is greater in the bottom interval L1, than in the top interval L2. In sections having three sampled intervals (Floridablanca, Porac, and Clark), the phenocryst-poor percentage at the bottom interval (L1) is greater than in the topmost layer (L3), and the middle layer (L2) contains varying proportions of the two types.

We think these results are representative of tephra fall on all sides of the volcano, including down the principal axis (southwest from Pinatubo). Field counts of pumice types by R.P. Hoblitt (USGS, written commun., 1992) and subsequent, similar counts from tephra sampled in San Antonio, Zambales (southwest of Pinatubo), over several time increments during the climactic eruption (K. Rodolfo and C. Arcilla, Univ. of Illinois, Chicago, written commun., 1993) both gave results consistent with ours. However, for completeness, we note an inconsistency with the results of an early, impromptu field count of pumice types by several of us at Dalanaoan (south-southwest of Pinatubo). That initial survey, done without size grading or washing of pumices, suggested the opposite sense of change, in which the phenocryst-poor variety increased upward in the section, from 10% near the base to 21% near the top of the coarse plinian pumice-fall deposit. If subsequent investigations bear out this inconsistency, we would have to conclude that local wind patterns were a factor and that the tephra southwest of the volcano is not strictly contemporaneous with that east of the volcano.

Table 1. Relative proportions of phenocryst-rich and phenocryst-poor pumice, June 15, 1991, tephra fall.

¹ Pumice counts (N) were normalized to 100% before relative proportions (%) were counted. Lithics were excluded from percent calculation.

DISCUSSION

We explore three hypotheses for the origin of the two pumice types. The first two hypothesis call on a magma reservoir that was vertically zoned and tapped in such a way as to produce a dominant (and increasing) percentage of the phenocryst-rich type through the course of the eruption. At Pinatubo, we have no evidence of gross compositional zonation, but we can imagine some differences in temperature or volatile content of the magma that could have resulted in different textures of pumice. Higher temperature would inhibit crystallization (and enhance resorption of crystals formed at lower temperature) and thus would result in fewer phenocrysts. However, we can rule out a region of sustained higher temperature, because similar Fe-Ti oxide temperatures have been determined for the two pumice types (Pallister and others, this volume; Rutherford and others, this volume; Bernard and others, this volume; Luhr and Melson, this volume; Imai and others, this volume). This relation argues strongly against thermal zoning.

Alternatively, extreme volatile concentration at the top of the reservoir could have inhibited phenocryst development, and thus the earliest tapped magma (from the top of the reservoir) would have produced a relatively high percentage of phenocryst-poor pumice. Circumstantial evidence of volatile zonation includes the exceptionally high volatile content of erupted magma (Gerlach and others, this volume) and the fact that the erupted magma represents only a relatively small percentage of the whole Pinatubo magma reservoir (Mori, Eberhart-Phillips, and Harlow, this volume; Pallister and others, this volume). However, this hypothesis is not consistent with the presence of the same phenocryst mineral assemblage in both types, including late-grown cummingtonite, which indicates that both pumice types had initially crystallized to the same extent, nor does it explain the fragmental textures in the phenocryst-poor pumice.

Our third and favored explanation is that the magma reservoir could have been more or less homogeneous until processes associated with the eruption itself changed the texture of some of the magma and produced two different pumice types (Pallister and others, this volume; Bernard and others, this volume). Unusually abundant, broken crystal fragments in the phenocryst-poor pumice suggest that unusually strong explosions might have produced that pumice type while more sustained ejection produced the phenocryst-rich pumice. In this scenario, the phenocryst-rich pumice is a vesicular equivalent of the reservoir dacite, and the phenocryst-poor pumice is a severely shocked derivative of the phenocryst-rich magma.

Our petrographic examination cannot resolve any significant difference other than the smaller average grain size and greater abundance of small crystal fragments in the phenocryst-poor variety. Thus, tentatively, we accept fragmentation as the simplest explanation for differences between the two pumice types. However, we cannot rule out the possibility that extreme volatile concentration near the top of the reservoir might also have caused some resorption and thus enhanced the effect of fragmentation on crystal size reduction.

CONCLUSION

The phenocryst-rich type, always dominant, increased in abundance through the course of the eruption. One scenario for this change, consistent with but unproven by our data, is that the uppermost, most volatile rich magma was erupted most explosively and a greater percentage of the phenocrysts were fragmented. Then, as magma with the highest volatile concentrations was depleted, less powerful explosions erupted the remaining magma with correspondingly less fragmentation of phenocrysts.

ACKNOWLEDGMENTS

We thank Richard Hoblitt and John Pallister for constructive reviews and John Pallister for further assistance during the revision process.

REFERENCE CITED

Bernard, A., Knittel, U., Weber, B., Weis, D., Albrecht, A., Hattori, K., Klein, J., and Oles, D., this volume, Petrology and geochemistry of the 1991 eruption products of Mount Pinatubo.

Gerlach, T.M., Westrich, H.R., and Symonds, R.B., this volume, Preeruption vapor in magma of the climactic Mount Pinatubo eruption: Source of the giant stratospheric sulfur dioxide cloud.

Imai, A., Listanco, E.L, and Fujii, T., this volume, Highly oxidized and sulfur-rich dacitic magma of Mount Pinatubo: Implication for metallogenesis of porphyry copper mineralization in the Western Luzon arc.

Luhr, J.F., and Melson, W.G., this volume, Mineral and glass compositions in June 15, 1991, pumices: Evidence for dynamic disequilibrium in the dacite of Mount Pinatubo.

Pallister, J.S., Hoblitt, R.P., Meeker, G.P., Knight, R.J., and Siems, D.F., this volume, Magma mixing at Mount Pinatubo: Petrographic and chemical evidence from the 1991 deposits.

Mori, J., Eberhart-Phillips, D., and Harlow, D.H., this volume, Three-dimensional velocity structure at Mount Pinatubo, Philippines: Resolving magma bodies and earthquakes hypocenters.

Pallister, J.S., Hoblitt, R.P., and Reyes, A.G., 1992, A basalt trigger for the 1991 eruptions of Pinatubo volcano?: Nature, v. 356, p. 426-428.

Rutherford, M.J., and Devine, J.D., this volume, Preeruption pressure-temperature conditions and volatiles in the 1991 dacitic magma of Mount Pinatubo.

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Last updated 06.10.99