ABSTRACT

An andesitic lava dome grew on the floor of the new caldera of Mount Pinatubo between July and October of 1992. Growth was continuous, with rates of about 8 x10⁴ cubic meters per day for the first 10 days and about 3 x10⁴ cubic meters per day thereafter until the end of October. Activity during 1993 was limited to phreatic explosions on and around the dome.

The 1992 dome is similar in composition and texture to another dome that formed in the same area during the week preceding the climactic June 15, 1991, dacitic eruption. Samples of both domes contain disequilibrium mineral assemblages and quenched mafic inclusions. These textural features, as well as chemical compositions of minerals and whole rocks, indicate that both were produced by mixing of basalt and dacite. Least squares modeling of major element compositions and differences in trace element abundances of 1991 and 1992 samples suggest that the 1992 andesite represents a new episode of magma mixing. Breakdown of cummingtonite in a sample of the 1992 dome indicates that more time elapsed between mixing and eruption in 1992 than in 1991.

In 1991, mixing of basalt and dacite and ascent of mixed andesite was followed within a week by major explosive eruptions of dacitic magma. In contrast, ascent and eruption of andesite in 1992 did not trigger explosive eruptions of dacitic magma, possibly because volatiles in the dacitic reservoir were depleted by the 1991 eruptions.

INTRODUCTION

An andesitic lava dome was extruded onto the north flank of Mount Pinatubo between June 7 and June 12, 1991, preceding the large explosive eruptions of andesite and then dacite on June 12-15. This dome was destroyed during the climactic eruption of June 15, 1991. Minor dacitic eruptions continued until early September of 1991, after which the volcano entered a period of temporary repose. Geologic evidence for dome growth late in the latest prehistoric eruptive period (Buag eruptive period; Newhall and others, this volume) suggested that further dome growth was possible.

On July 3-6, 1992, seismicity signaled renewed magma ascent, the onset of a new period of dome growth, and, potentially, more explosive eruptions. Although the topography and presumably the internal structure of the summit area were strongly modified by caldera collapse on June 15, 1991, the 1992 dome grew in the same geographic position as the 1991 dome. In addition to being erupted in the same place, available samples and observations indicate that the 1992 dome is compositionally similar to its 1991 counterpart. These similarities pose an important question: Does the 1992 dome represent a new magma mixing episode, and, if so, why has it not triggered another explosive eruption? This paper describes the growth and composition of the 1992 lava dome and attempts to answer this question.

METHODOLOGY

Difficult access and safety considerations prevented detailed measurements and detailed sampling of the 1992 dome. The following discussion is based on aerial oblique photographs and video (high and low angle) and one brief visit to the dome for sampling. Dimensions of the dome are estimated from photographs; the caldera and caldera lake were used for scale. They are approximate, as are calculated volumes. Judging from repeat trials in volume estimation by different coauthors, we believe the precision on volume estimates is +-30 percent. The samples described in this report were obtained from the dome during a brief visit by helicopter on December 4, 1992.

Seismic records are principally from station FNGZ, 9 km southeast of the dome, and are supplemented after August 9 by records from station CRAZ, newly installed on the north caldera rim, 1 km north of the dome (see Mori, White, and others, this volume; Ramos and others, this volume). SO₂ gas monitoring was by an airborne COSPEC (Correlation Spectrometer; Barringer Instruments) (Daag, Tubianosa, and others, this volume).

CHRONOLOGY OF 1992 DOME GROWTH

May to June 1992: Sporadic low-frequency harmonic tremor (average frequency, 1 Hz).

July 3: The first low-frequency volcanic earthquake during this period of seismic unrest.

July 6: Sudden increase in the number of small, shallow, low-frequency volcanic earthquakes and in low-level, low-frequency tremor, suggesting possible magma ascent. A low-frequency appearance raised immediate concern about the potential for explosive activity as well as dome growth.

July 7: Aerial reconnaissance showed a brown, turbulent plume of sediment in the caldera lake; the plume was elongate in a west-southwest to east-northeast direction. The sediment plume suggested either heating and turbulent convection of lake water or sublacustrine mud explosions.

July 9: Explosions from beneath the shallow caldera lake (possibly beginning on July 8) built a small, roughly 50-m-diameter tuff cone above lake level by 0830 on July 9 (Lieutenant Scheidegger and Major Caldwell, U.S. Marine Corps, oral commun., 1992). By the time we observed the island at 1400 on the same day (fig. 1), it was circular, approximately 70 to 100 m in diameter, and 5 m high. Small phreatic explosions from a 10- to 20-m-wide vent on the north-northwest side of the cone were shooting fountains of steam and mud 50 to 200 m into the air at intervals of several minutes. Steaming from fumaroles around the south shore of the caldera lake was notably less than it had been before July. Nighttime observations by Captain C. Owens (U.S. Marine Corps, oral commun., 1992) from 1900 to 2100 showed that explosions were continuing. The products were not hot enough to appear bright when viewed through night-vision goggles.

Figure 1. Small tuff cone being formed on July 9, 1992. View is from the northwest.

July 10: The tuff ring had grown to 100 to 150 m in diameter and 5 to 10 m high. The size and frequency of explosions might have decreased; only one of two observation flights saw explosions, which were smaller than those seen on July 9. Video footage from that flight showed interbedded light and dark layers in the tuff cone, possibly representing an alternation of juvenile and accidental material. Steaming along the south shore of the caldera lake that had noticeably diminished on July 9 returned to precrisis (pre-July) levels. Seismicity was limited to a few high-frequency volcanic earthquakes.

July 11-13: Observations were interrupted by passage of Typhoon Eli (local name, Konsing). Occasional low-frequency harmonic tremor.

July 14: A new dome was sighted during a flight at 1000; seismicity indicates that it may have begun to grow late on July 13 (Ramos and others, this volume). A steep-sided jumble of jagged, light-brown, ash-dusted blocks occupied what had been a water-filled vent for phreatic explosions and was steaming profusely. When first sighted, the dome was about 50 m in diameter and 5 to 10 m high and had a volume of between 10⁴ and 10⁵ m³ (table 1).

July 15: Jagged spines developed on the dome, and portions of the uplifted tuff cone slumped into the lake (fig. 2).

Figure 2. Early dome growth, July 15, 1992. Dome had been growing for 1 or 2 days and was shouldering aside (and burying) unconsolidated debris of the tuff cone. View is from the north.

Table 1. Growth of the 1992 lava dome.

[Dimensions in meters; volume in cubic meters. Estimation errors are 30%]

---

Date	North-South	East-West	Height	Volume
July 14	50	50	5-10	104-105
July 16	100	100	30	0.2x106
July 24	200	200	50	0.8x106
August 5	200	200	50	1x106
August 26	300	300	50-100	<2x106
September 2	300	350	50-100	2x106
September 22-26	300	400	70-100	>2x106
September 28	300	400	100	3x106
October 28-30	350	450	150(?)	4x106

---

 

July 16: Two days after it was first sighted, the dome had roughly doubled in diameter (now about 100 m), grown to a height of about 30 m, and occupied about 60 percent of the 150-m-diameter island. Steep sides and jagged spines suggested a dacitic composition, though later sampling showed only andesite.

July 23: The tuff cone was now completely covered (or shed) by the new dome, and a talus slope was developing on the north side of the dome.

July 24: The dome had grown to 200 m in diameter and 50 m high and had two 25-m-wide lobes of lava on its west side.

July 28: The dome had become structurally complex, with numerous fractures and fumaroles. The most profuse steaming was along a north-south fracture cutting at least the northern half of the dome (fig. 3). Blocks of talus several meters in diameter lay on the lake floor, partly submerged, immediately east, north, and southwest of the dome.

Figure 3. Expanded dome, July 28, 1992, as seen from the north. Note individual boulders lying partially submerged in shallow water around the dome.

July 30: The dome appeared to have flattened, due to lateral spreading.

August 5-9: The dome appeared to have a central vent, which was later seen to be a spreading center with a north-south axis. Smooth faces of fresh, viscous lava formed where lava had risen and peeled apart along a north-south fissure and had not yet been broken by endogenous growth. The dome as a whole was also slightly elongated north to south, with an average diameter of about 200 m and a height of about 50 m. On August 5, the amplitude of low-frequency volcanic earthquakes began to increase; earthquake traces were emergent, with indistinct P-wave arrivals at FNGZ. A new seismic station (CRAZ) was installed on the north caldera rim on August 9 to get a closer look at this seismicity.

August 10-11: Uplift of the north side of the dome and adjacent lake bottom, probably a result of sublacustrine endogenous dome growth, had created a 50-m-wide "beach" on which several previously submerged "marker blocks" were recognized (fig. 4). The northern half of the dome was higher than the southern half.

Figure 4. Dome on August 10, 1992, with an expanding northern lobe, including a small beach, and what appears to be a craterlike feature above the original vent. View is from the northeast.

August 12 and subsequent days: Intense rain raised the lake level and submerged the "beach" on the north side of the island, as well as other shorelines of the caldera lake.

August 15: A new family of shallow, low-frequency volcanic earthquakes appeared, with a dominant frequency of 1.25 Hz. P-wave arrivals were emergent but recognizable.

August 24: Another family of low-frequency volcanic earthquakes appeared, with a dominant frequency of 2 Hz. P-wave arrivals were emergent but recognizable.

August 26: Growth had shifted to a new, eastern lobe. Uplift on the east side of the dome had created a new, 50-m-wide eastern "beach." The dome continued to grow and by now had 3 peaks (north, southwest, and east) and a small topographic low (crater?) within the southwestern high. North-south and east-west dimensions were both about 300 m; the maximum height was 50 to 100 m.

August 27-September 2: Heavy rains caused the lake level to rise. However, continued uplift of the eastern lobe of the dome kept the eastern "beach" above lake level. On or before August 29, westward aggradation of an apron of debris that had washed from the eastern wall of the caldera, combined with uplift of the dome itself, joined the island to the eastern lake shore. On August 30, a new family of low-frequency volcanic earthquakes began, and these were of larger amplitude than any earlier in the 1992 unrest. Dominant frequency was 1.25 Hz.

September 2: The eastern lobe continued to grow, increasing the east-west diameter to about 350 m and the volume to about 2x10⁶ m³. The eastern shore of the lake also continued to aggrade, so the eastern lobe of the dome was surrounded by a debris apron, which sloped gently toward the lake.

September 13: The appearance of yet another family of low-frequency volcanic earthquakes of small magnitude was recorded only at the CRAZ station.

September 18: The eastern lobe was areally smaller but steeper and taller than the northern lobe. Most steaming was from vents in the southwestern area.

September 22-26: The dome continued to grow slowly and now measured about 300 m north-south (unchanged) and about 400 m east-west (increased). The highest point on the dome was on the southeastern lobe, about 70 to 100 m above lake level; two high areas on the northern lobe were about 50 m above the lake.

September 28: Horizontal dimensions of the dome were unchanged: the height appeared to have increased slightly.

October 1-10: No significant changes were noted.

October 15: Renewed uplift of the northern lobe caused several huge blocks to spall off and tumble to the north foot of that lobe. Uplift accompanied an increase in small earthquakes, recorded at CRAZ.

October 23: Seismicity increased sharply; low-frequency volcanic earthquakes were occurring at a rate of 1 per minute (see fig. 8).

October 28-30: A fresh spine of dark lava protruded 30 to 50 m above the north-central lobe. The south-central part of the dome also expanded (fig. 5). Total volume of dome was now about 4 x10⁶ m³.

Figure 5. Dome on November 21, 1992, essentially unchanged since late October. The spine on the northern lobe formed partly by extrusion during October 1992; it was later accentuated by collapse of surrounding material. The area southwest of the central vent has been built up slightly, and the eastern lobe is also crumbling, leaving a fragile core. Samples described in this report were collected along the upper right (north and northeast) sides of the dome. View is from the southeast.

October 31: Sharp decrease in seismicity, and the apparent end of 1992 dome growth.

February 1993: Shallow seismicity occurred beneath the caldera, and numerous phreatic explosion pits formed in the alluvial apron north, east, and south of the dome. The greatest concentration of explosion pits was near the southeastern corner of the caldera floor, about 400 m north of a long-standing line of fumaroles that apparently lies along a regional fault. No change was noticed in the dome itself.

March-July 1993: Episodic tremor took place, possibly associated with gas venting or hydrothermal explosions from the dome or the caldera floor. The dome itself appears not to have grown since October 1992 (J. Sincioco, PHIVOLCS, oral commun., 1993).

SIZE, SHAPE, AND VOLUME THROUGH TIME

Changes in the size, shape, and volume of the dome that took place between July and October of 1992 are summarized in figures 6A-F and 7. The most notable feature is that the locus of growth shifted several times, ultimately forming two high lobes (north and east) and a third south-central lobe that rises above the original vent. Uplifting of "beaches," first on the north and later on the east, suggests that the north and east extrusions were not simply lateral breakouts from the central lobe but, rather, a sequel to intrusions and endogenous growth beneath those northern and eastern lobes.

Figure 6. Sketches of the dome growth, July­October 1992. A, July 9, initial tuff cone. B, July 14, early dome set within tuff cone. C, Late July-early August, northward expansion of dome. D, Late August, eastward expansion of the dome and notable westward aggradation of the nearby shoreline, after heavy rains. E, Late September, minor additional growth of dome, but remarkable aggradation of lakeshore, partly surrounding the dome. F, Renewed growth of the northern lobe, with some growth in the southwest portion also possible, followed by crumbling of outer portions, leaving a prominent spine on the northern lobe. Steep topography is the inner caldera wall; dashed line is the shoreline of the caldera lake. Topographic base from Jones and Newhall (this volume).

The dome grew continuously at roughly 0.1x10⁶ m³/d for the first few days, 0.08x10⁶ m³ for the next week, and then 0.03-0.04x10⁶ m³/d thereafter until growth abruptly ended in late October (fig. 7). Similar rates of dome growth were observed at Santiaguito dome in Guatemala following the major plinian eruption of its parent volcano, Santa María, in 1902 (Rose, 1972, 1973), at Mount St. Helens, U.S.A., following its plinian eruption in 1980 (Swanson and Holcomb, 1989), at Bezymianny, Kamchatka Peninsula, following its major eruption in 1956 (Kirsanov, 1979), and at Unzen Volcano, Japan, in 1991 and early 1992 (Nakada, 1992b). However, all four of these other domes grew for a longer time than the Pinatubo dome, so they are much larger (Santiaguito, 1x10⁹ m³ in 70 years of growth, Rose, 1987; Mount St. Helens, 7.4x10⁷ m³ in 6 years of growth, Swanson and Holcomb, 1989; Bezymianny, 1.6x10⁹ m³ in the first 27 years of growth, Kirsanov, 1979; Unzen, 8.2x10⁷ m³ of lava extrusion in the first 9 months of growth, of which about half later collapsed and produced pyroclastic flows, Nakada, 1992b). Dome growth at Mount Lamington, Papua New Guinea, was even faster during a 1.4-yr period after its major eruption of 1951 (1.0x10⁹ m³ in 1.4 years of growth, Taylor, 1958; Swanson and others, 1987).

Figure 7. Cumulative dome volume, July­October 1992. An initial period of rapid growth (steep slope) was followed by slower growth until extrusion abruptly stopped in late October.

SURFACE FEATURES

Much growth of the 1992 dome was exogenous, and surface features reflected extrusion of viscous lava that spread bilaterally from its vent on top of the dome. Known as "spreading centers" (Swanson and others, 1987), "crease structures" (Anderson and Fink, 1992), "cleavage canals" (Omori, 1916), and "petal structures" (Nakada, 1992a,b), such structures have conspicuously smooth, nonscoriaceous surfaces where lava peeled apart as it spread.

Outside the spreading centers, the dome's surface was initially scoriaceous and showed evidence of individual lava-flow lobes. Later endogenous growth created blocky, fracture-crossed surfaces surrounded by talus. Small spines, each roughly 10 m high, formed from time to time along the edges of spreading centers, only to collapse during continued spreading or during the next period of endogenous growth. The tallest and longest lived spine rose about 50 m above the rest of the northern lobe; it was formed partly by extrusion during October 1992 and later accentuated by collapse of surrounding material (fig. 5).

DOME GROWTH, SEISMICITY, AND SO₂ EMISSION

During and slightly after the period of fastest dome growth (until middle August 1992), SO₂ discharge averaged 580+-360 t/d (Daag, Tubianosa, and others, this volume). In contrast, the few measurements for late August-early November, when dome growth was slower, averaged 270+-40 t/d (same reference).

Most of the earthquakes before and during dome growth appeared to be low-frequency events and are shown as such on figure 8. However, close inspection of CRAZ records shows that many had a high-frequency onset followed by a lower frequency coda. These may be high-frequency, brittle-fracture events at shallow depth, with direct arrivals followed by lower frequency, ground-filtered surface waves. Alternatively, both the high- and the low-frequency contents may have been source effects of fracturing in the brittle carapace of a mushy dome. Tremor that occurred shortly before and shortly after the dome began to grow had a dominant frequency of 1 Hz at the FNGZ station but was associated with steam venting and phreatic explosions. Again, path and source effects are hard to distinguish. Part of the low-frequency appearance may be a real source effect; part may be the result of attenuation of higher frequency components with travel distance, even to the closest station, CRAZ, 1 km from the dome. Details of seismicity during dome growth are given by Ramos and others (this volume).

Figure 8. Earthquake counts and SO2 discharge versus time, through dome growth, July­October 1992.

During the period of rapid dome growth and high SO₂ discharge (late July through early August), earthquake counts were relatively low; during the period of slow dome growth and lower SO₂ discharge, earthquake counts were relatively high (fig. 8). Although the correlation is imperfect in day-to-day detail, we think that the change from high SO₂ discharge, low seismicity, and rapid dome growth to low SO₂ discharge, high seismicity, and slow dome growth reflects progressive solidification of magma in the shallow part of the conduit and within the dome itself. Solidification may have resulted from early volatile loss, and solidification may also have trapped remaining volatiles. Thus, during the late stage of 1992 dome growth, gas-depleted, increasingly viscous magma had difficulty breaking new paths to the surface.

PETROLOGY AND CHEMISTRY

The 1992 dome consists mainly of andesitic lava that is host to numerous inclusions. The three samples described here represent the host andesite (CN12492-1h), abundant basaltic andesite inclusions (CN12492-1i), and sparse dacite inclusions (CN12492-2). The 1992 dome andesite and the mafic inclusion samples are similar in composition, mineral assemblage, and texture to andesite and mafic inclusions from the lava dome that grew in the head of the Maraunot River valley during the week preceding the climactic June 15, 1991, eruption (Pallister and others, this volume). The dacite inclusion sample is similar in composition to dacite erupted during the climactic eruption of June 15, 1991. However, subtle differences in composition and texture suggest that the 1992 dome represents a new mixing event rather than left over hybrid magma from 1991.

DISCUSSION

The 1992 dome is similar to the dome that formed just before the major explosive eruptions of 1991. Both domes are composed mainly of andesite that formed by mixing of basalt and dacite. However, in sharp contrast to events of 1991, extrusion of the 1992 dome was not followed by significant explosive activity.

Is the 1992 andesite left-over mixed magma from the same batch that was first erupted on June 7, 1991, or does it represent a new episode of magma mixing in 1992? If new mixing has occurred, the dacitic magma reservoir is still being replenished by mafic magma, and a strong possibility of further eruptions remains. On the other hand, if the 1992 andesite is part of the 1991 batch that was stored in the reservoir or conduit system, its eruption would not indicate replenishment and accompanying disturbance of the main dacite reservoir at depth.

The slightly more siliceous composition of the 1992 samples, our inability to produce the 1992 compositions by two-component mixing involving 1991 components, and the distinct trace element patterns of the 1992 samples indicate that they are not unerupted aliquots of the magma that formed the June 7-12, 1991, andesite dome and the June 12, 1991, andesite scoria. Instead, they represent the products of a new mixing event.

Unreacted cummingtonite rims on hornblende, and unhomogenized magnetite-ilmenite pairs, suggest that less than 4 days elapsed between the final stages of magma mixing and eruption in 1991 (Rutherford and others, 1993; Rutherford and Devine, this volume). Conversely, the breakdown of cummingtonite rims observed in the 1992 andesite sample indicates that more than 4 days elapsed between mixing and eruption in 1992.

We envision that some of the "1992" dacitic magma rose from the main reservoir (>8 km depth) and reached depths where hornblende was not stable (<4 km, Rutherford and Devine, this volume). Ascent took place sometime after June 15, 1991, and before July 1992. The dacite body cooled around its margins and developed a microcrystalline groundmass. Then, in mid-1992, magnesian olivine basalt (Mg#=68) intruded the >8-km-deep dacitic reservoir and mixed first with a small amount of similar "1992" dacitic magma to form basaltic andesite and then with a larger volume of the same "1992" dacitic magma to form andesitic magma. Days or perhaps weeks later, the andesitic magma rose toward the surface, picked up the xenolith that is our only surviving sample of "1992" dacite, and was erupted to form the 1992 lava dome. Preservation of Ti-zonation in magnetite (fig. 9H) indicates that the xenolith was included and heated by the andesitic magma for only a brief period before the eruption.

The lack of another explosive dacite eruption in 1992-93, despite evidence for renewed replenishment and magma mixing, suggests that eruption from the large residual dacite reservoir (Mori, Eberhardt-Phillips, and Harlow, this volume) is no longer easily triggered. The climactic eruption of June 15, 1991, certainly depleted volatiles in the dacitic reservoir, perhaps to such a low level that large eruptions will be unlikely until volatiles can reconcentrate in the upper parts of that reservoir. Alternatively, the mixing episode in 1992 may have been too small to trigger an explosive eruption from the dacitic reservoir. The total volume of andesitic magma erupted in 1991 (June 7-12 dome and June 12 tephra) was between 10⁷ and 10⁸ m³; the volume of andesitic magma erupted to form the 1992 dome and left in a hypothetical conduit beneath that dome was of the same order of magnitude. Thus, we suspect that volatile depletion in the dacitic reservoir is preventing further large eruptions, but we cannot rule out that a larger intrusion of basalt than occurred in 1992 could still trigger a large eruption from the dacitic reservoir.

ACKNOWLEDGMENTS

We are grateful to the Philippine Air Force and the U.S. Marine Corps for photographic air support. Various colleagues kindly shared their photos, video, and sketches. We are also grateful to Ms. Tina Monzon-Palma (Channel 5, Manila) and the late Captain Agustin Consunji for helicopter access to the crater to sample the 1992 lava dome. Don Swanson and Jon Fink provided helpful reviews.

REFERENCES CITED

Andersen, D.J., and Lindsley, D.H., 1988, Internally consistent solution models for Fe-Mg-Mn-Ti oxides: Fe-Ti oxides: American Mineralogist, v. 73, p. 714-726.

Anderson, S.W., and Fink, J.H., 1992, Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows: Geological Society of America Bulletin, v. 104, p. 615-625.

Daag, A.S., Tubianosa, B.S., Newhall, C.G., Tuñgol, N.M, Javier, D., Dolan, M.T., Delos Reyes, P.J., Arboleda, R.A., Martinez, M.L., and Regalado, M.T.M., this volume, Monitoring sulfur dioxide emission at Mount Pinatubo.

Jones, J.W., and Newhall, C.G., this volume, Preeruption and posteruption digital-terrain models of Mount Pinatubo.

Kirsanov, I.T., 1979, Extrusive eruptions on Bezymianny volcano in 1965-1974 and their geologic effect: Moscow, Akademia Nauk SSSR, Sibirskoye Otdeleniye, Institut Geologii i Geofiziki, Nauka, p. 50-69. [translated by D.B. Vitaliano.]

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.

Mori, J., White, R.A., Harlow, D.H., Okubo, P., Power, J.A., Hoblitt, R.P., Laguerta, E.P., Lanuza, L., and Bautista, B.C., this volume, Volcanic earthquakes following the 1991 climactic eruption of Mount Pinatubo, Philippines: Strong seismicity during a waning eruption.

Nakada, S., 1992a, Photographic records of eruption products at Unzen Volcano during May 1991-May 1992, in Yanagi, T., Okada, H., and Ohta, K., eds., Unzen Volcano: The 1990-1992 Eruption: Fukuoka, Japan, The Nishinippon and Kyushu University Press, p. 12-20.

-----1992b, Lava domes and pyroclastic flows of the 1991-1992 eruption at Unzen Volcano, in Yanagi, T., Okada, H., and Ohta, K., eds., Unzen Volcano: The 1990-1992 Eruption, The Nishinippon and Kyushu University Press, Fukuoka, p. 56-66.

Newhall, C.G., Daag, A.S., Delfin, F.G., Jr., Hoblitt, R.P., McGeehin, J., Pallister, J.S., Regalado, M.T.M., Rubin, M., Tamayo, R.A., Jr., Tubianosa, B., and Umbal, J.V., this volume, Eruptive history of Mount Pinatubo.

Omori, F., 1916, The Sakura-jima eruptions and earthquakes: Tokyo, Bulletin of the Imperial Earthquake Investigation Committee, v. 8, p. 181-321.

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.

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.

Ramos, E.G., Laguerta, E.P., and Hamburger, M.W., this volume, Seismicity and magmatic resurgence at Mount Pinatubo in 1992.

Rose, W.I., Jr., 1972, Santiaguito volcanic dome, Guatemala: Geological Society of America Bulletin, v. 83, p. 1413-1434.

------1973, Pattern and mechanism of volcanic activity at the Santiaguito volcanic dome: Bulletin Volcanologique, v. 37, p. 73-94.

------1987, Volcanic activity at Santiaguito volcano, 1976-1984, in Fink, J.H., ed., The emplacement of silicic domes and lava flows: Geological Society of America Special Paper 212, p. 17-27.

Rutherford, M.J., Baker, Leslie, and Pallister, J.S., 1993 Petrologic constraints on timing of magmatic processes in the 1991 Pinatubo volcanic system [abs.]: Eos, Transactions, American Geophysical Union, v. 74, p. 671.

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

Rutherford, M.J., and Hill, P.M., 1993, Magma ascent rates from amphibole breakdown: An experimental study applied to the 1980-1986 Mount St. Helens eruptions: Journal of Geophysical Research, v. 98, p. 19667-19685.

Stormer, J.C., 1983, The effects of recalculation on estimates of temperature and oxygen fugacity from analyses of multi-component iron-titanium oxides: American Mineralogist, v. 68, p. 586-594.

Swanson, D.A., Dzurisin, D., Holcomb, R.T., Iwatsubo, E.Y., Chadwick, W.W., Jr., Casadevall, T.J., Ewert, J.W., and Heliker, C.C., 1987, Growth of the lava dome at Mount St. Helens, Washington (USA), 1981-1983, in Fink, J.H., ed., The emplacement of silicic domes and lava flows: Geological Society of America Special Paper 212, p. 1-16.

Swanson, D.A., and Holcomb, R.T., 1989, Regularities in growth of the Mount St. Helens dacite dome, 1980-1986, in Fink, J.H., ed., Lava flows and domes: Emplacement mechanisms and hazard implications: New York, Springer-Verlag, IAVCEI Proceedings in Volcanology No. 2, p. 3-24.

Taylor, G.A.M., 1958, The 1951 eruption of Mount Lamington, Papua: Australian Bureau of Mineral Resources Bulletin, v. 38, 117 p.