FIRE and MUD Contents

Tephra Falls of the 1991 Eruptions of Mount Pinatubo

By Ma. Lynn O. Paladio-Melosantos,1 Renato U. Solidum,1 William E. Scott,2 Rowena B. Quiambao,1 Jesse V. Umbal,3 Kelvin S. Rodolfo,3 Bella S. Tubianosa,1 Perla J. Delos Reyes,1 Rosalito A. Alonso,4 and Hernulfo B. Ruelo5

1Philippine Institute of Volcanology and Seismology.

2U.S. Geological Survey.

3Department of Geological Sciences, M/C 186,University of Illinois at Chicago, 801 W. Taylor St., Chicago, IL 60607; and Philippine Institute of Volcanology and Seismology.

4National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines.

5Renison Goldfields Consolidated Exploration Pty., Limited, Australia.


ABSTRACT

Tephra falls of varying character and volume occurred between April 2 and early September 1991, from eruptions of Mount Pinatubo. From April 2 to June 12, first phreatic explosions and later ash emissions related to emplacement of a lava dome produced mostly thin and fine-grained deposits over several hundred square kilometers west and south of the vent. A brief explosive eruption on the morning of June 12 deposited about 0.014 cubic kilometers of andesitic scoria, ash, and accidental lithic fragments southwest of the volcano (layer A). Several similar events over the next 2 days, followed by numerous pyroclastic-surge-producing explosions between the afternoon of June 14 and early afternoon of June 15, emplaced a 0.2-cubic-kilometer, laminated, mostly fine-grained ash-fall deposit (layer B) over broad areas around the volcano. The wide dispersal of layer B was induced by ash clouds convecting upward from pyroclastic surges that moved radially outward about 10 kilometers from the vent and the onset of low-altitude northerly to westerly winds as a tropical storm approached the area. The most voluminous deposit of the 1991 eruption sequence is a dacitic pumice-fall deposit (layer C) that was produced by the climactic eruption during the afternoon of June 15. A densely settled area of about 2,000 square kilometers was blanketed by 10 to 25 centimeters of rain-soaked tephra; 189 people were killed by collapsing buildings, and damage to utilities and agricultural lands was extensive. Most of Luzon and a roughly 4-million-square-kilometer area of the South China Sea and Southeast Asia were affected by tephra fall. The bulk volume of layer C probably lies between 3.4 and 4.4 cubic kilometers, ranking it among the five largest of the 20th century. The climactic eruption also produced voluminous pyroclastic-flow deposits and a 2.5-kilometer-diameter caldera. Slowly diminishing ash emissions continued from several vents in the caldera for about 6 weeks following the climactic eruption and produced a fine-grained laminated tephra deposit (layer D), which has a bulk volume of about 0.2 cubic kilometer.

Grain-size analyses of samples of layer C display well-known features of plinian tephra-fall deposits as distance from the vent increases, including decrease in median grain size, decrease in maximum pumice size, and improvement in sorting. Component analyses show that pumice dominates in grain-size fractions coarser than 1 millimeter, whereas crystals dominate in finer fractions. Lithic fragments make up a few percent or less of each fraction.

Deposits of layer C typically have normal grading, which suggests that eruption intensity peaked early and then decreased until ending prior to cessation of pyroclastic-flow activity. Various lines of evidence imply that peak activity of the climactic eruption was sustained for approximately 3 hours, and a waning level of activity continued for 6 hours more.

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INTRODUCTION

All phases of the 1991 eruptions of Mount Pinatubo between April 2 and midsummer produced tephra falls of varying character and volume that climaxed with the great plinian eruption of June 15. The climactic eruption produced 5 to 6 km3 of pyroclastic-flow deposits that partly buried valleys within 12 to 16 km of the volcano (W.E. Scott and others, this volume), but tephra-fall deposits were dispersed far and wide. Ash was carried westward across the South China Sea, where trace amounts fell in parts of Vietnam, Malaysia, and Borneo (Smithsonian Institution, 1991). The magnitude of this eruption earned it a place among the largest eruptions of this century (Self and others, this volume; W.E. Scott and others, this volume).

Because the repose period of volcanoes characterized by large-magnitude eruptions is usually on the order of hundreds or even thousands of years (Simkin and Siebert, 1984), the documentation of the Pinatubo eruptions provided a rare opportunity to observe and study the physical processes attendant to this size of eruption. This paper focuses on the stratigraphy, character, distribution, and volume of tephra-fall deposits formed by the 1991 eruptions. Although tephra fall from an eruption like that of June 15 would be of enormous interest for calibrating models that relate eruption intensity to grain size and distribution of deposits (Walker, 1981; Sparks, 1986; Carey and Sigurdsson, 1986, 1989; Carey and Sparks, 1986), tephra dispersal during the climactic eruption of Mount Pinatubo was complicated by the passage of Typhoon Yunya. Furthermore, much of the tephra fell in the South China Sea, with little or no documentation.

HUMAN IMPACTS

Impacts of tephra fall are the most wide reaching among those directly resulting from explosive eruptions, as is illustrated by the Pinatubo experience with unquestionable clarity. Heavy tephra fall darkened central Luzon for most of the afternoon of June 15 during the climactic eruption. More than 30 cm of tephra-fall deposits accumulated close to the volcano, while a densely settled area of about 2,000 km2 received 10 to 25 cm. Fall deposits that were wetted by typhoon rains collapsed buildings and damaged public utilities and agricultural lands. Roof collapse accounted for 189 fatalities, or 61 percent of the total number recorded during the first 3.5 months after the eruption (Magboo and others, 1992). The estimated cost of damage to property is P10.62 billion (U.S. $400 million). The June 15 eruption itself affected about 216,000 families (National Disaster Coordinating Council, 1991). In addition, ash far from the volcano damaged aircraft (Casadevall and others, this volume) and ships.

METHODOLOGIES

Field investigations after the climactic eruption included mapping the extent of the tephra-fall deposits, measuring the thickness of individual beds or total thickness, and describing and sampling stratigraphic sections. Descriptors of grain size follow Fisher and Schmincke (1984): <0.0625 mm, fine ash; 0.0625-2 mm, coarse ash; 2-64 mm, lapilli; >64 mm, bombs. Where more detailed descriptors are necessary, we employ the Wentworth scale, for instance, medium-sand-size ash. Thickness data from more than 250 localities were used to construct isopach maps for the different eruptive events and to estimate areal extent and volume of tephra-fall deposits.

Tephra samples were collected from many localities around the volcano out to 45 km. About 20 percent of these were taken through the section from a known area so that bulk density could be determined after drying and weighing. Twelve sections of the climactic tephra-fall deposit at distances of 15 to 35 km from the vent were sampled for grain-size and component analysis. We collected 20 to 100 g of layers of fine ash and 500 to 1,000 g of layers of coarser grained tephra. Maximum pumice size was measured from a sampling area of 50x50 cm. In the laboratory, about one-quarter of each sample was manually sieved and separated into 7 to 10 fractions at 1-lc phi intervals down to 3 (0.125 mm) or 4 lc phi (0.0625 mm). The remainder was stored for future reference or study. The different fractions were weighed and examined petrographically to determine percentage frequency of pumice, crystals, and lithic fragments.

CHRONOLOGY AND DESCRIPTION OF PRECLIMACTIC TEPHRA FALLS AND DEPOSITS

The detailed chronology of the Pinatubo eruptions is discussed by other authors (Wolfe and Hoblitt; Hoblitt, Wolfe, and others; Sabit and others; all this volume); a summary of eruptive events follows that concentrates on tephra falls and their deposits. We refer to the eruptive phases of Wolfe and Hoblitt (this volume), but do not use the phases as a primary means for subdividing tephra-fall deposits because the events that define the phases (such as changes in seismicity or eruptive behavior) are not always recognizable in the tephra-fall sequence. Rather, we employ a modified version of the informal stratigraphic subdivision of tephra-fall deposits proposed by Koyaguchi and Tokuno (1993). Figure 1 shows a typical section of tephra-fall deposits southwest of the volcano and unit designations for tephra layers as used in this report. Koyaguchi (this volume) subdivides layer C into a lower, coarser grained layer C1 and an upper, finer grained layer C2.

Figure 1. Typical stratigraphic section of 1991 tephra-fall deposits southwest of Mount Pinatubo. Sketch of section located at Sitio (hamlet) Upper Kakilingan, San Marcelino (location KAK, fig. 7), about 13 km south-southwest of the volcano. Eruptive phases are those defined by Wolfe and Hoblitt (this volume).

APRIL 2

The first sign of Pinatubo's awakening in 1991 was reported by local residents in early April (early part of phase I of Wolfe and Hoblitt, this volume). Steam and ash emissions from the volcano were ascribed to phreatic explosions (Daag and others, 1991; Pinatubo Volcano Observatory Team, 1991; Punongbayan and others, 1991, this volume; Hoblitt, Wolfe, and others, this volume). Between 4 and 9 vents were active at any time. Ash clouds were observed to rise 100 to 900 m above the vents (Philippine Institute of Volcanology and Seismology, 1992). The explosion on April 2 emplaced poorly sorted debris up to 3 m thick within 100 m of the vents; the debris consisted chiefly of angular blocks to coarse ash of pink and gray hornblende andesite and dacite. Beyond the zone of coarse debris and extending out several hundred meters was a deposit of coarse ash up to a few centimeters thick (C.G. Newhall, U.S. Geological Survey, written commun., 1993). Traces of fine-grained ash reportedly reached Sitio Yamut in Barangay Burgos, Botolan, about 7 km from the vents. Most of the ash of April 2 and of emissions in the weeks following was deposited on the west-southwest flanks of the volcano, thinly covering an area of about 100 km2 (fig. 2); however, the actual distribution of tephra fall from these events is poorly known. Petrographic analyses showed the tephra to be vitric-crystal ash composed of glass, plagioclase, hornblende, magnetite, biotite, and clinopyroxene with fragments of hornblende andesite and dacite, soil, and hydrothermally altered material (A.G. Reyes, Philippine National Oil Company, written commun., 1991). Owing to the fresh appearance of the glass, Reyes suggested that the tephra was partly of magmatic origin, but the presence of pyrite also indicated a hydrothermal origin. Other investigators concluded that there was no firm evidence that the glass was a juvenile component (Wolfe and Hoblitt, this volume).

Figure 2. Outline of areas affected by tephra falls between April 2 and early June 12, 1991 (phases I through III of Wolfe and Hoblitt, this volume). Area that received tephra fall from April 2 phreatic eruption is within solid line; dashed line shows area that received light, intermittent tephra fall up until early June 12. Cross-hachured line, fissure of April 2 eruption; x, fumarole (from Wolfe and Hoblitt, this volume, fig. 6). In this and tephra-isopach maps (figs. 3, 6, and 7), small solid circle marks pre-June 15 summit of Mount Pinatubo, solid triangle marks site of June lava dome and presumed vent for explosive eruptions, and hachured line shows rim of caldera formed during climactic eruption.

MAY AND EARLY JUNE

Ash emissions intensified in late May and early June, affecting increasingly larger areas on the west side of the volcano with fine ash fall (later phase I through phase III of Wolfe and Hoblitt, this volume; Sabit and others, this volume). An explosion on June 7 that preceded growth of the June lava dome in the headwaters of the Maraunot River ejected ash to an altitude of 8 km. Succeeding ash emissions that accompanied dome growth between June 7 and the early morning of June 12 produced ash columns 2 to 5 km high. By June 12 more than 500 km2 on the west side of the volcano had received a thin coating of fine ash (fig. 2).

MORNING OF JUNE 12

A more explosive phase of activity began on June 12 at 0851 (local time), signaled by a brief subplinian eruption that sent a column to an altitude of at least 19 km (beginning of phase IV of Wolfe and Hoblitt, this volume). Tephra from this eruption, which we call layer A (Koyaguchi and Tokuno, 1993; Koyaguchi, this volume), reached the South China Sea (fig. 3; Oswalt and others, this volume, figs. 2 and 3). A normally graded bed more than 5 cm thick of gray scoriaceous lapilli and ash and lithic fragments accumulated within 10 km southwest of the vent (figs. 1 and 4B). Farther downwind, layer A is brownish-gray, crystal-rich, sand-size ash with scattered coarser clasts. Phenocrysts consist of plagioclase, hornblende, quartz, magnetite, biotite, olivine, and pyroxene (see Pallister and others and Hoblitt, Wolfe, and others, both this volume, for petrologic data). Tephra fall affected nine municipalities in the Province of Zambales with up to 1 cm of ash and covered more than 2,000 km2 of land. The bulk volume of tephra emplaced during the 0851 eruption on June 12 is about 14 million m3 (0.014 km3), as calculated from the isopach map of figure 3 by using the root-area method (table 1; fig. 5; Pyle, 1989; Fierstein and Nathenson, 1992). This bulk volume converts to a dense-rock-equivalent (DRE) or magma volume of about 4 to 6.5 million m3 (assumes bulk density of tephra-fall deposit of 0.8 to 1.2 g/cm3 and magma density of 2.6 g/cm3). For comparison, the June lava dome had a volume less than 2 million m3 (Hoblitt, Wolfe, and others, this volume).

Figure 3. Distribution of layer A, which consists of tephra-fall deposits from the June 12, 1991, eruption that began at 0851 (beginning of phase IV of Wolfe and Hoblitt, this volume). Isopachs are in centimeters; dashed line shows generalized outer limit of identifiable deposits. Measurement sites (with thickness in centimeters) and sources of data: filled circles, PHIVOLCS, USGS, Pinatubo Lahar Hazards Task Force; filled squares, Koyaguchi and Tokuno (1993); open squares, E. Listanco in Koyaguchi and Tokuno (1993).

Figure 4. Photographs of tephra-fall deposits of 1991 eruptions. A, Section on abutment of bridge across Santo Tomas River north of San Narciso, Zambales; 32 km west-southwest of vent. Layer A is 8 mm of sand-sized ash; layer B is 4 mm of mostly fine ash. Note weak normal grading of layer C and scattered coarse clasts on surface of deposit. (Photograph courtesy of J.J. Major, U.S. Geological Survey, no. R7/21, June 30, 1991.) B, Tephra-fall deposits on unimproved road about 10.5 km southwest of vent, west of Marella River. Layer A, about 4 cm thick, consists of coarse ash and fine lapilli; layer B consists of several thin layers of ash totalling 2.5 to 5 cm thick; layer C is 33 cm thick and is the thickest section of the climactic pumice-fall deposit yet found. Note normal grading overall, but 2-cm pumice lapillus in upper left; layer D consists of two 3- to 4-cm-thick beds of fine ash separated by a bed of water-reworked pumiceous ash. (Photograph by W.E. Scott, no. WES-91-14-3; July 12, 1991.) C, Tephra-fall deposits on unimproved road about 9 km southeast of vent, north side of Gumain River. Layer B is 23 cm thick and consists of numerous graded ash beds; layer C is 31 cm thick and has two zones in lower part with minor fine ash coatings. (Photograph by W.E. Scott, no. WES-91-6-9, June 28, 1991.) D, Section at mouth of Pasig River canyon about 15 km east of vent. Layer B is 10 cm thick and layer C is about 18 cm thick; note ash-rich zones that stand out owing to increased cohesiveness. (Photograph by W.E. Scott, no. WES-92-19-25, March 14, 1992.)

Figure 5. Thickness of tephra-fall deposits (layers A-D) plotted against square root of on-land isopach area. Volumes calculated from data in table 1 by using method of Fierstein and Nathenson (1992); volume shown for layer C is a minimum value calculated for a slope extrapolated from area of isopachs that close on land. Three data points that fall off curve for layer C are for on-land areas within isopachs that extend into the South China Sea (fig. 7).

Table 1. Parameters used to calculate bulk volume of tephra-fall deposits from 1991 eruptions of Mount Pinatubo.

[Multiple entries for layer C, the climactic tephra-fall deposit, represent several possible combinations of Aip1/2 and thickness of the distal isopach of figure 8 as portrayed on figure 9. Included for comparison are data for other large eruptions of the 20th century and the Taupo ultraplinian eruption of 1,820 14C yr B.P. (Fierstein and Nathenson, 1992); Quizapu volume recalculated to 9.5 km3 by Hildreth and Drake (1992), who used a three-segment distribution; Hudson data from Scasso and others (1994). To, extrapolated maximum thickness at vent; k, slope of line on a plot of logarithm of thickness versus square root of area (for distributions that define two or three straight-line segments, k is for the near-vent segment); k1 and k2, slopes of distal segment(s) for distributions that define two or three straight-line segments; Aip1/2 and Aip1/21, square root of area at the points of interception between segments of a two or three-segment distribution; Tdistal, thickness at distal isopach used for various estimates of layer C volume; -, not applicable]


Tephra layer

To (cm)

k

k1

k2

Aip1/2
(km)

Aip1/21
(km)

Volume
(km3)

Tdistal
(cm)

A

9 0.114

-

-

-

-

0.014

-

B

101 0.108

-

-

-

-

0.17

-

C

39 0.022

-

-

-

-

1.7

-

C

39 0.022 0.0083

-

119

-

3.0

-

C

39 0.022 0.0083 0.0038 119 538 3.4 0.001

C

39 0.022 0.0083 0.0021 119 538 3.9 0.005

C

39 0.022 0.0083 0.0016 119 538 4.4 0.01

D

166 0.130

-

-

-

-

0.20

-

Santa María, Guatemala, 1902

154 0.021 0.0075

-

250

-

7.8

-

Quizapu, Chile, 1932

1300 0.098 0.0067

-

48

-

9.9

-

Novarupta A, Alaska, 1912

336 0.058 0.0140

-

47

-

5.1

-

Novarupta B, Alaska, 1912

137 0.053 0.016

-

46

-

2.7

-

Taupo, New Zealand, 1,820 B.P.

197 0.022

-

-

-

-

7.8

-

Hudson, Chile, 1991

1737 0.180 0.0437 0.0066 19 56 7.6

-


LATE JUNE 12 TO EARLY AFTERNOON JUNE 15

Tephra falls produced by multiple short-lived explosive events from late on June 12 to early afternoon on June 15 deposited tephra over an ever-widening area as eruptions changed in character and the approach of Typhoon Yunya modified wind directions. Events on late June 12, morning of June 13, and early afternoon of June 14 produced narrow convecting columns that rose to >=24 km, >=24 km, and 21 km, respectively (phase IV of Wolfe and Hoblitt, this volume). Deposits of individual events have not been subdivided except in a few localities (Hoblitt, Wolfe, and others, this volume); they were probably finer grained and thinner than those of layer A but covered essentially the same area southwest of the vent. Beginning midafternoon of June 14 and continuing for almost 24 h, a series of explosions generated pyroclastic surges that moved outward more than 10 km before rising into broad convecting ash clouds (Hoblitt, Wolfe, and others, this volume; phase V of Wolfe and Hoblitt, this volume). Some ash from these clouds began drifting southeastward as low-and mid-level winds shifted to more westerly directions (see later discussion of meterorological controls on tephra distributions; Oswalt and others, this volume); much fell wet as rains of Typhoon Yunya reached the area. As the climactic eruption began, the mostly fine-grained tephra-fall deposits related to events of late June 12 to early afternoon June 15, here referred to as layer B, covered more than 4,000 km2 of Luzon; populated areas had by now received as much as 5 cm (figs. 1, 4, and 6). For the first time, tephra falls were affecting densely populated areas in sectors east of the volcano.

Figure 6. Distribution of tephra-fall deposits of layer B, which were produced by eruptions between late June 12 and early afternoon June 15, 1991 (later part of phase IV and phase V of Wolfe and Hoblitt, this volume). Isopachs are in centimeters; dashed line shows generalized outer limit of identifiable deposits. Sources of data as in figure 3.

In distal areas, layer B consists of finely to very finely laminated light-gray, grayish-brown, and tan fine ash with minor coarser particles. The lower part of the layer is typically more brown and the upper part more gray. The laminations reflect deposition from numerous ash-fall events. Accretionary lapilli and spherical to elongate vesicles indicate the presence of water during deposition of some units. Sag structures caused by impact of coarser clasts are common in the upper units. The bulk volume of layer B as calculated from the isopach map (fig. 6) is about 0.17 km3 (table 1, fig. 5), or about 0.1 km3 DRE. The total volume, including the surge deposits, is uncertain, as the thickness of surge deposits in proximal areas varies greatly in relation to local topography (Hoblitt, Wolfe, and others, this volume). These variations may not be accurately represented by the exponentially changing thickness modeled in the volume calculations.

TEPHRA-FALL DEPOSITS OF THE CLIMACTIC ERUPTION OF JUNE 15

On the basis of seismic, barograph, and other records, the climactic eruption of June 15 began at 1342, its peak was sustained for about 3 h, and a waning level of activity continued for about 6 h more (phase VI of Wolfe and Hoblitt, this volume). The eruption column attained a maximum height of about 35 km and spread out broadly in the stratosphere, eventually reaching 250 km upwind (northeast) of the vent (Koyaguchi and Tokuno, 1993). About 7,500 km2 of Luzon was covered by more than 1 cm of tephra (fig. 7), and almost the entire 105,000-km2 island received at least a trace.

Figure 7. Distribution of tephra-fall deposits of the climatic eruption of June 15 (phase VI of Wolfe and Hoblitt, this volume), layer C, and locations of sections (triangles) sampled for grain-size and component data. KAK is location of section sketched in figure 1. Isopachs are in centimeters; sources of data as in figure 3, but open circles show total thickness of section (in centimeters), which may also include layers A and (or) B.

The tephra-fall deposit of the climactic eruption is the most extensive, and the second most voluminous, deposit of the 1991 Pinatubo eruptions. Only the pyroclastic-flow deposits emplaced during the climactic eruption are more voluminous. We refer to the entire climactic tephra-fall deposit as layer C (fig. 1), in contrast to Koyaguchi and Tokuno (1993), who arbitrarily divided the deposit into a lower, coarser grained layer C and an upper, finer grained layer D. Koyaguchi (this volume) now refers to these layers as C1 and C2, respectively. Maximum thickness measured is 33 cm at a site 10.5 km southwest of the vent (fig. 4B); the vent is thought to have been at or near the site of the June lava dome. The climactic tephra-fall deposit is typically thin or absent nearer to the volcano, owing to erosion by and (or) incorporation into moving pyroclastic flows that were being generated broadly concurrently with at least parts of the tephra fall (W.E. Scott and others, this volume).

VOLUME

The volume of the climactic tephra-fall deposit was initially poorly constrained because such a large fraction fell into the South China Sea (Scott and others, 1991; Koyaguchi and Tokuno, 1993). The on-land bulk volume enclosed within the 1-cm isopach is about 0.7 km3. The thinnest isopach that we can close with confidence is 15 cm; the thickest is 30 cm (fig. 7). Extrapolation of the trend of thickness against square-root area of the closed isopachs yields a minimum volume estimate of 1.7 km3 and a maximum thickness at the vent (T0) of 39 cm (fig. 5). The low slope of this trend compared with those of layers A, B, and D and the modest maximum thickness for a plinian deposit reflect the broad dispersal of the climactic deposit. The very high eruptive column coupled with strong winds of the tropical storm are thought to be two major contributing factors to this great dispersal. Note that the Santa María, Guatemala, eruption of 1902 and the ultraplinian Taupo, New Zealand, eruption of 1820 14C yr B.P. had similar slopes of tephra distribution (see factor k; table 1).

Tephra thicknesses measured in marine cores and estimated from sediment-trap data (Wiesner and Wang, this volume) define 2- and 1-cm isopachs, constrain the area of the 0.1-cm isopach, and further refine volume estimates. The isopachs define a second segment on a plot of log thickness versus square root of area (k1; table 1, fig. 8) that increases the total volume to 3.0 km3. But the extrapolation of the second segment to low thicknesses appears to underestimate the wide dispersal of fine-grained, thin ash in distal areas.

Figure 8. Log of thickness versus square root of area for layer C that includes proximal on-land data from Luzon (fig. 5), marine data from Weisner and Wang (this volume), and three possible distributions for distal tephra-fall deposits (dashed lines) based on two values for area of distal isopach (3.1 and 3.8 million km2; fig. 9), and three values for thickness of distal isopach (0.01, 0.05, and 0.1 mm). Numerals in boxes are calculated bulk volumes in cubic kilometers.

Figure 9. Southeast Asian distribution of distal tephra-fall deposits and edge of ash and aerosol cloud at various times. Solid lines show approximate extents of reported tephra fall. The large area trending west from the Philippines encloses an area of about 3.1 million km2; the area covering most of the Philippines represents an additional 0.7 million km2 in which minor ash fall was reported, even though much of the area is east of the major eruption cloud as viewed on satellite imagery. Long-dashed line is outline of maximum eruption-cloud extent through 1700 on June 16 from satellite data of Japan Meteorological Agency (JMA; Self and others, this volume); short-dashed line is outline of cloud extent at 0731 on June 16 from NOAA imagery (Lynch and Stephens, this volume) where it extends beyond cloud as shown in JMA imagery; dashed-dotted line shows limits of aerosol cloud on June 18 as depicted in TOMS imagery (Bluth and others, 1992) where it extends beyond limits of previous clouds. Dotted lines are axes of lobes of the ash cloud as shown on JMA imagery (Self and others, this volume). Star, Mount Pinatubo; solid line close to Pinatubo is 1-cm isopach; small filled squares, major cities without reported tephra fall; filled circles, localities that reported tephra fall. Exact sites of tephra-fall observation in states of Sabah and Sarawak, Malaysia, are not known.

Unmeasured tephra-fall deposits, described as light, were reported from several areas in Southeast Asia and the southern Philippines (fig. 9). The area of a line enclosing these sites is about 3.8 x 106 km2. Most of this area west of Pinatubo lies within regions over which the eruption cloud passed, as imaged by geostationary and polar-orbiting satellites. But many areas in the southern Philippines that reportedly received light ash fall lay south and east of this eruption cloud. If we disregard this eastern area, a 3.1 x 106-km2 area of tephra fall remains. The thickness of tephra in the distal areas was too thin to cause significant and newsworthy problems. The 4 mm of ash that fell in Metro Manila caused darkness for several hours during the late afternoon, closed the airport for several days, and created a noticeable and persistent haze as wind resuspended the ash. Because nothing approaching these conditions was reported from Southeast Asia or southern Philippine sites and because Wiesner and Wang's (this volume) data constrain the area of the 1-mm isopach to about 3 x 105 km2, we infer thicknesses substantially less than 1 mm. Furthermore, as satellite images show, the ash cloud traveled westward as several lobes (fig. 9) rather than as a broad homogeneous mass, so distal isopachs probably aren't simple curves. Therefore, we use a range of estimates of thickness--0.01 to 0.1 mm (10-100 lc mum)--and area--3.1 and 3.8 x 106 km2--for a distal isopach in order to include these many uncertainties.

Consistent with the inferred thickness range, the grain size of tephra-fall deposits in Southeast Asia was very fine. Casadevall and others (this volume) infer grain diameters were <30 lc mum along airline flight paths near Vietnam, on the basis of the lack of abrasion on aircraft windshields. A tephra sample collected in Singapore contained 1- to 10-lc mum-diameter grains of glass and crystals (C.G. Newhall, written commun., 1994). Deposits of such fine ash a few grains thick can form continuous films on smooth surfaces. We spread fine ash on glass slides at these thicknesses and created very noticeable films.

Extrapolation of distal segments on figure 8 from the 1-mm isopach to various thickness and area estimates define k2 values (table 1) that yield total bulk volume estimates of 3.4 to 4.4 km3. The volume added by these distal segments accounts for about 12 to 32 percent of these totals.

We have bulk-density determinations from about 20 percent of the sites at which we measured tephra thickness; we can use these to convert bulk volume to magma volume. Samples include columnar sections of layer C, as well as combined samples of layers A and B, combined samples of layers A, B, and C, and samples of layer D. Most were thoroughly wetted by rains before they were sampled, except for some samples of layer D. Samples were weighed after oven drying or sun drying; their bulk-density values are normally distributed about a mean value of 1.1 g/cm3 (fig. 10). The wide range in bulk density is the result of differences in (1) grain size that are related to distance from the vent and the varied character of layers A to D and (2) amount of compaction, owing to collection over a period of 9 months following the eruption. Because the samples represent a wide range in grain sizes and because thickness measurements were made over a substantial time period while compaction was occurring, we assume that the mean bulk-density estimate is representative of layer C. Wiesner and Wang (this volume) report an estimated bulk density of 1.1 g/cm3 for fine-grained tephra that accumulated in a submarine sediment trap. Therefore, we use a mean bulk density of 1.1 g/cm3 to convert the bulk volume range of 3.4 to 4.4 km3 for layer C to a magma volume of 1.6 to 2.0 km3 (assumes magma density of 2.4 g/cm3). Such a range suggests that the climactic tephra-fall deposit is, at most, about equal to, and may be as little as one-half, the magma volume of the climactic pyroclastic-flow deposits (2.1 to 3.3 km3; W.E. Scott and others, this volume).

Figure 10. Histogram of dry bulk densities of tephra-fall deposits, which includes 11 samples of layers A+B or B, 18 samples of layer C, 32 samples of layers B+C, and 10 samples of layer D.

Koyaguchi (this volume) estimates a magma volume of 2 to 10 km3 (4.4 to 22 km3 bulk volume if bulk density=1.1 g/cm3) was injected into the stratosphere during the climactic eruption on the basis of a fluid-dynamic model. The upper part of this range would imply a volume significantly higher than our estimate. In terms of the distribution model (fig. 8), the 2-, 1-, and 0.1-cm isopachs would have to enclose areas about 4 to 5 times larger than those determined by Wiesner and Wang (this volume). Alternatively, as inferred by Koyaguchi (this volume), there maybe a substantial fraction of very fine ash that fell at great distances and is not accounted for in our distribution model. But another 18 km3 (bulk volume) of ash would have covered an area equivalent to the entire northern hemisphere with 70 mm of ash--an event that probably could not have gone unnoticed.

GRAIN-SIZE VARIATIONS

The climactic tephra-fall deposit is typically normally graded, although some sections have an inversely graded basal portion overlain by a thicker, normally graded upper portion (figs. 1, 4A-C, and 11; table 2). Coarser grained portions of proximal sections consist of friable coarse ash and lapilli with sparse bombs. Farther away, grain size decreases, and the coarsest portion consists of coarse ash with mostly granule-size lapilli. The upper portions of all sections are ash rich, contain scattered lapilli, and typically have sufficient fine ash to make the deposit slightly cohesive. In areas close to pyroclastic-flow deposits, thin ash-rich zones occur within layer C (fig. 4D). These zones are inferred to reflect addition of fine-grained material from ash clouds of pyroclastic flows (W.E. Scott and others, this volume).

Figure 11. Grain-size distribution (in weight percent; dashed line) of layer C and frequency distribution (bar graph) of its component pumice (phenocryst rich and phenocryst poor), crystals, and lithic fragments. Sample locations shown in figure 7. Sublayers C1, C2, and so forth are numbered from base upward; samples SMZ-C and CAB-1C include the entire layer.
Figure 11--Continued.

Table 2. Statistical measures of grain size (Inman, 1952) of layer C samples, the climactic tephra-fall deposit.

[Sample codes that end in C represent composite samples of the entire deposit; codes that end in a numeral represent samples of sublayers with 1 being a basal sample. Thickness is for total deposit. Maximum pumice size was measured in a 50x50 cm area. Sample locations shown on figure 7. -, no data]


Sample

Medium
grain size (mdlc phi)


Sorting
(lc sigmaphi)

Maximum pumice
(cm)

Total
thickness
(cm)

CAB 1C

0.19 1.68 2.5 15

CAB 2C

0.20 1.58 3.9 10

SNZ C

0.71 1.24 - 18

SAZ C

1.13 1.11 2.9 16

SMZ C

0.71 1.03 - 17

PRC 1C

0.33 1.33 3.0 15

PTA 2C2

1.14 1.22 - 18

PTA 2C1

0.11 1.46 4.4 18

LLC 6C3

1.76 1.01 - 18

LLC 6C2

1.02 1.21 - 18

LLC 6C1

-0.50 1.36 2.5 18

CAP 8C3

1.41 1.14 - 20

CAP 8C2

0.28 1.35 - 20

CAP 8C1

-0.38 1.40 3.9 20

MGS 10C2

0.42 1.61 - 15

MGS 10C1

-1.11 1.93 9.8 15

PSG 11C2

-0.52 1.65 - 18

PSG 11C1

-0.58 1.55 4.1 18

KLN 12C4

1.29 1.45 - 25

KLN 12C3

1.05 1.31 - 25

KLN 12C2

0.77 1.22 - 25

KLN 12C1

0.11 1.18 - 25

Grain-size distributions of the 22 analyzed samples of layer C, which were collected between 15 and 35 km from the vent, are typically unimodal (fig. 11). A second mode in the fine-ash fraction (finer than 4 lc phi) is evident in some distributions; many would likely be expressed as a gradually fining tail, were the finer fractions analyzed. But in a few samples, the second mode approaches about 10 weight percent and is separated from the main mode by low values in the 4-lc phi fraction, which suggests fine ash additions from ash clouds of pyroclastic flows or rain flushing of fine ash from the eruption cloud. The minor coarse mode in sample MGS-10C1 is attributed to its proximity to the volcano. At such a distance, outsized pumice clasts are very common.

Median grain size ranges from fine-granule-size lapilli to medium-sand-size ash -1.1-1.8 lc phi, or 2.2-0.3 mm; figs. 4 and 11; table 2). Where several samples were collected vertically through normally graded sequences (identified as sublayers C1, C2, and so forth, from base upward), the median grain size typically fines upward by 1 to 2 lc phi units. All layer-C samples analyzed are well sorted (lc sigmalc phi=1-2), whether the entire section (total deposit thickness=10 to 18 cm) or 2 to 4 vertically arrayed subsamples (total deposit thickness=15 to 25 cm) were sampled. Sorting improves slightly (about 0.3 lc mu unit) upward in most sampled sections as median grain size decreases. Although samples represent a limited range in distance from the volcano (15 to 35 km), the Pinatubo deposit displays well several features found in many plinian tephra-fall deposits as distance from the vent increases including (1) decrease in median grain size, (2) decrease in maximum pumice size, and (3) improvement in sorting (fig. 12; table 2; Walker, 1971).

Figure 12. Plot of median grain size (Mdlc phi) and sorting (lc sigmalc phi) of layer C against distance from vent. For sections that were sampled in sublayers, only the basal (C1) sample is plotted.

COMPONENT ANALYSES

The frequency of pumice, crystals, and lithic fragments in the climactic pumice-fall deposit was determined by point counting each size fraction of 18 of the 22 grain-size samples (fig. 11) in order to characterize the deposit. In most samples, the number of clasts in -3-lc phi (8 mm) and coarser fractions is small (a few to several tens); therefore, the frequency percentages in these fractions have high uncertainties and are not considered in the following discussion.

Two varieties of pumice occur in layer C--a white, phenocryst-rich type and a buff to gray, phenocryst-poor type--and were differentiated in the component analyses. They vary only in appearance and are chemically similar (Pallister and others, 1992, this volume). As grain size decreases below 0 lc phi (1 mm), pumice is fragmented into glass and crystals, and discrimination of the two pumice types becomes difficult. Most fine-grained glass is counted here as phenocryst-rich pumice. Phenocryst-rich pumice is the dominant component in fractions coarser than 0 lc phi (fig. 11). The frequency of phenocryst-poor pumice is subordinate to the phenocryst-rich variety, typically <10 percent of the pumice content of a given size fraction. A few sites show a slight decrease in the frequency of phenocryst-poor pumice upwards through the deposit, but such changes are typically not significant at a 2-lc sigma level of uncertainty.

David and others (this volume) also determined the proportion of the two pumice types in layer C east of the volcano. Their results are similar to ours, but they report a greater range in the content of phenocryst-poor pumice, 5 to 25 percent. Their study showed a decreasing amount of phenocryst-poor pumice upward in the deposit, which suggests its content decreased as the eruption progressed. However, as mentioned above, layer C fines upward, and the decreasing-upward amount of phenocryst-poor pumice may also be attributed to nonrecognition in the upper, finer-grained portion.

Crystals occur in the 0-lc phi fraction of all samples and increase in frequency in progressively finer fractions (fig. 11). They comprise as little as 1 percent of the total components in the 0-lc phi fraction to greater than 90 percent in 3-lc phi (0.25 mm) fraction. Plagioclase and hornblende are the most common phases, while quartz, biotite, magnetite, olivine, and anhydrite are present in minor amounts. The felsic minerals almost wholly comprise the crystal component of the 0-lc phifraction; an increasing amount of the mafic minerals was observed in finer fractions.

Lithic fragments are found in minor amounts (fig. 11). In size fractions with a statistically significant number of clasts, frequency of lithics ranges from <1 to 13 percent. Average frequency of lithics in a given size fraction for all samples averages 2.5 to 5 percent. No definitive trends are apparent in their distribution with respect to distance from source (15 to 35 km), position in the section, or grain-size fraction. The lithic components consist of red to brown pre-1991 hornblende andesite in various stages of alteration, and dark-gray, angular fragments of pre-1991 and early-June-1991 andesite.

INFERENCES REGARDING DYNAMICS OF JUNE 15 CLIMACTIC ERUPTION

The grain size and grading of the climactic tephra-fall deposit provide insight into eruption processes and dynamics. The nearly ubiquitous normal grain-size distribution (unimodal) of the samples indicates a uniform mode of transport--fallout from a turbulent cloud--throughout the climactic eruption. Of the 22 samples analyzed, only one proximal sample showed a minor secondary coarse mode.

The normal grading, or mostly normal grading with a thin inversely graded base, that characterizes the deposit in all sectors likely reflects an initially rapid increase of eruption intensity. Coarse particles attained their maximum dispersal early in the eruption; the small amount of fines in basal samples is consistent with an initially high eruption intensity (great column height) and resulting effective winnowing of finer particles by wind. Following this reasoning, the gradual decrease in grain size upward would indicate a decrease in eruption intensity, although coarse particles do occur throughout the deposit, albeit sporadically (figs. 4A and B). Observations of seismicity and atmospheric disturbances during the eruption have been interpreted by most workers as indicating an initial 3-h period of intense activity starting at about 1340 followed by about 6 h of markedly lower and slowly decreasing activity (Power and others, Tahira and others, Wolfe and Hoblitt, W.E. Scott and others, all this volume). Two alternative correlations of this eruptive behavior with variations in the tephra-fall deposit are possible: (1) the coarser, basal part of layer C represents the 3-h peak, and the upper part represents all or part of the 6-h period of declining activity or (2) all of layer C accumulated during the 3-h peak, after which the fine ash of layer D began to accumulate.

Several observations lend support to the second option or a variation thereof:

  1. Koyaguchi and Tokuno (1993) show from satellite imagery that the stratospheric cloud expanded upwind (northeast) until 1940, 6 h after the climactic eruption began. They considered this support for 5 h of intense activity, but Koyaguchi (this volume) concludes that this upwind expansion could have continued after its source eruption had ceased, and he thinks a 3-h duration is more likely.

  2. Eyewitnesses from the Pinatubo Volcano Observatory (J.W. Ewert and A.B. Lockhart, U.S. Geological Survey, oral commun., 1994) left Clark Air Base about 1500 and drove slowly eastward 15 km to Pampanga Agricultural College, arriving about 1630. They recall that the coarsest grained phase of the tephra fall had largely stopped by the time they left the base, that during their drive the tephra fall consisted chiefly of sand-size material with minor mud, and that on the morning of June 16 their vehicle had little tephra on the windshield, whereas those of vehicles that had been at the college throughout the afternoon of June 15 had several centimeters. These recollections support the interpretation that the part of the eruption that produced layer C was in the initial 3-h period.

  3. W.E. Scott and others (this volume) interpret the appearance of ashy zones in the middle and upper parts of layer C near the margins of the pyroclastic-flow deposits as reflecting the onset of substantial production of pyroclastic flows and attendant ash clouds. This effect may also have contributed to the increase in ash content upward in more distal sections of tephra-fall deposits. The production of pyroclastic flows ceased during caldera formation, and only postclimactic layer D overlies the pyroclastic-flow deposits. W.E. Scott and others (this volume) marshall evidence that suggests but is not conclusive that caldera formation occurred 3 to 5 h after the beginning of the climactic eruption. No doubt, most of layer C accumulated during the 3-h peak, but deposition may have continued through all or part of the 6-h period of declining activity.

POSTCLIMACTIC TEPHRA FALL

As the climactic eruption waned during the evening of June 15 and for about 6 weeks following, ash plumes that rose as high as 18 to 20 km billowed from one or more vents on the caldera floor (first half of phase VII of Wolfe and Hoblitt, this volume). Ash also was derived from secondary pyroclastic flows (Torres and others, this volume) and secondary explosions generated by interaction of surface and ground water with the still-hot pyroclastic-flow deposits. The deposits of layer D represented by the isopachs in figure 13 exclude sediments that we infer as having been emplaced by secondary processes. We identify these secondary deposits chiefly on the basis of crossbedding, which is characteristic of surges driven by secondary explosions, large content of clasts derived from pre-1991 deposits that underlay the pyroclastic-flow deposits, and a coarse-grained (very coarse ash to bombs) component derived from the pyroclastic-flow deposits. The secondary emplacement processes also produced distal fine-grained ash that, because it can't be differentiated from ash-fall deposits derived from the vent, is included in the measurements in figure 13. Therefore, the calculated volume of layer D, 0.2 km3 bulk, or about 0.1 km3 DRE (fig. 5; table 1), should be regarded as a maximum value for postclimactic ash erupted from caldera vents.

Figure 13. Distribution of tephra layer D, deposits of ash falls that immediately followed the climactic eruption and continued for about 6 weeks (first half of phase VII of Wolfe and Hoblitt, this volume). Isopachs in centimeters; dashed line shows generalized outer limit of identifiable deposits. Sources of data as in figure 3.

The tephra of layer D is dominantly gray fine-grained ash (figs. 4B and D; W.E. Scott and others, this volume, fig. 17). Accretionary lapilli formed of fine ash are common throughout the layer, as are zones of vesicular ash formed by rainfall on dry powdery ash. Layer D is laminated to thinly bedded; in proximal areas where it approaches 1 m thick, individual beds rarely exceed 1 cm and are more typically a few millimeters thick. Southwesterly and east-northeasterly winds dominated during this period of ash emission, as is evident from the distribution shown in figure 13. Although most of layer D accumulated in sparsely settled areas and areas already devastated by the climactic eruption, some densely settled areas received up to 10 cm of ash. Fine ash falling continuously for several days at a time and being constantly resuspended by wind and vehicles made life miserable for many residents in northern Pampanga and southern Tarlac Provinces.

METEOROLOGICAL CONTROLS ON DISTRIBUTION OF TEPHRA-FALL DEPOSITS

The Asian monsoon imparts seasonal variation to the wind regime in central Luzon, which is dominated by the northeasterly to easterly trade winds. From June through September, a southwesterly monsoonal flow affects the lower troposphere, while easterly winds continue in the upper troposphere, as well as in the stratosphere, which lies above about 17 km (Pettersen, 1969). These dominant wind directions were used to delimit two main tephra-fall zones in the hazard map constructed in May and June 1991 (Punongbayan and others, this volume). At the time of increasing unrest in May and June, the southwesterly flow was anticipated to begin at any time. Meteorological data for assessing likely tephra-fall patterns during the 1991 eruptions were provided by weather-forecast offices at Cubi Point Naval Air Station and Clark Air Base (fig. 14). There was, however, a gap in the data on June 14-15 when the two stations were unable to take measurements of winds aloft. The only available wind information for this time period is the 24-h wind forecast.

Figure 14. Wind data and wind forecasts for June 10 to 15, 1991 (Weather Office, U.S. Naval Air Station, Cubi Point, written commun., 1992). 24-h wind forecast maps of Southeast Asia for June 15, 1991, at (A) 400 mbar, about 7 km, and (B) 300 mbar, about 9 km in altitude. Bar indicates wind direction; barb is upwind. Full barb represents wind speed of 10 knots (18.5 km/h); half barb, 5 knots (9.3 km/h); triangle, 50 knots (92.6 km/h). C, Summary of observed wind directions at different altitudes above Cubi Point between June 10 and early June 14, 1991, and forecast winds for June 15 based on model for approaching typhoon. Symbols as in A and B.

Distribution of tephra-fall deposits from April to early June, and the tephra-fall deposit of June 12, layer A (figs. 2 and 3), conformed well to the hazard zone depicted for easterly winds. Weather radar at Cubi Point and satellite images showed the tephra cloud of June 12 drifting in a generally west-southwest direction (Oswalt and others, this volume, figs. 2-5), consistent with observed wind directions (fig. 14C). Likewise, layer D, the fine-grained ash-fall deposit of the weeks following the climactic eruption, was distributed both west to southwest and northeast of the vent (fig. 13). A southwesterly flow from June 17 to 25 transported much of the ash in the northeast-trending lobe of layer D that affected populated areas in northern Pampanga and southern Tarlac Provinces.

In contrast, the distribution of layer B is very broad and extends much farther to the south (fig. 6), which reflects both changing eruption style and winds. The narrow convecting eruption columns of the first few events that contributed to layer B rose to altitudes of more than 24 km and deposited tephra in much the same fallout zone as layer A. The change to surge-producing explosions on June 14-15 produced lower (typically <20 km), broader (>=10 km radius) eruption clouds (Hoblitt, Wolfe, and others, this volume) that dispersed tephra more widely around the volcano. Concurrently, winds below 5 km were shifting to more northerly and northwesterly directions as the typhoon approached (figs. 14A and C), which directed much fallout to the south and east.

The magnitude and distribution pattern of the tephra-fall deposit of the climactic eruption, layer C, did not conform to the predictions portrayed on the hazard map for two reasons. First, the magnitude of the tephra fall was underestimated, as field studies had found little evidence of tephra-fall deposits in Pinatubo's record of past eruptions (Punongbayan and others, this volume). Rapid erosion of the climactic 1991 fall deposit, coupled with its essential absence near the volcano (W.E. Scott and others, this volume), probably explains the meager evidence of past tephra-fall events. More importantly, the eye of the typhoon, which had been reduced to a tropical storm shortly after making landfall, was passing just 75 km north of Pinatubo during the climactic eruption (Oswalt and others, this volume, fig. 7). Its passage influenced the tephra-fall distribution by introducing complications to the normal wind pattern as shown in the wind forecast for June 15 (figs. 14A and B). The forecast shows typical cyclonic winds associated with the storm (counterclockwise rotation) extending over central Luzon from the surface up to about 7,000 m; at 9,000 m and above, the prevailing wind over central Luzon was easterly to northeasterly. Therefore, the portion of the tephra cloud within the effective range of the cyclonic winds was below 9,000 m, between one-third to one-fourth of the 34-km-high eruption column. These cyclonic winds were responsible for deposition of tephra farther south and east than would have occurred under more typical wind conditions.

Another important influence on the distribution of layer C was the great distance >=250 km) that the mushrooming top of the eruption column flowed radially outward as a density current in the stratosphere (Koyaguchi and Tokuno, 1993; Koyaguchi, this volume). This broad stratospheric plume, in combination with the low-altitude cyclonic circulation, dispersed fine ash fallout over broad areas north, east, and south of the volcano, as well as far downwind (southwest; fig. 9). The major downwind part of the tephra and aerosol cloud displays three axes of dispersal, west-, west-southwest-, and southwest-trending. Also plotted on figure 9 are localities that received fine ash fall, as reported by observers from the Philippine Institute of Volcanology and Seismology, but stand east and south of the cloud's path. Low-level cyclonic winds of the passing typhoon were probably responsible for transporting a small amount of ash to these areas, which extend over much of the Philippine archipelago.

SUMMARY

Tephra falls of changing character and volume occurred between April 2 and midsummer during the 1991 eruptions of Mount Pinatubo. These changes reflect the evolving eruptive behavior that built to a high-intensity plinian eruption and produced voluminous pyroclastic-flow deposits and a small caldera.

  1. From April 2 to June 12, first phreatic explosions and later ash emissions related to emplacement of a lava dome produced mostly thin and fine-grained tephra-fall deposits that covered several hundred square kilometers west and south of the vent.

  2. A brief explosive eruption on the morning of June 12 deposited about 14 million m3 of andesitic scoria, ash, and accidental lithic fragments southwest of the volcano (layer A). This event initiated a series of short-lived eruptions that led up to the climactic eruption.

  3. Several events similar to that of the morning of June 12 occurred over the next 2 days, but each produced ejecta of smaller volume and finer grain size than layer A. These were followed by numerous pyroclastic-surge-producing eruptions between the afternoon of June 14 and early afternoon of June 15 (Hoblitt, Wolfe, and others, this volume). Together these events emplaced about 0.17 km3 of laminated, mostly fine-grained ash-fall deposits (layer B) over broad areas around the volcano. The wide dispersal of layer B was induced by ash clouds convecting upward from the pyroclastic surges that moved radially outward >=10 km from the vent, and by the onset of low-altitude northerly to westerly winds as a tropical storm approached the area.

  4. The most voluminous fall deposit of the 1991 eruption sequence is a 3.4- to 4.4-km3 (bulk) dacitic pumice-fall deposit (layer C) that was produced by the climactic eruption during the afternoon of June 15. This volume probably ranks fifth among 20th century tephra-fall deposits. The climactic eruption also emplaced 5 to 6 km3 (bulk) of pumiceous pyroclastic-flow deposits and ended with formation of a 2.5-km-wide caldera (W.E. Scott and others, this volume). Most of Luzon and a 3- to 4-million-km2 area of the South China Sea and Southeast Asia were affected by tephra fall.

  5. Grain-size analyses of samples of layer C display well known features of plinian tephra-fall deposits as distance from the vent increases, including decrease in median grain size, decrease in maximum pumice size, and improvement in sorting.

  6. Component analyses show that pumice dominates in grain-size fractions coarser than 1 mm, whereas crystals dominate in finer fractions. Lithic fragments make up a few percent or less of each fraction.

  7. The thickness and distribution of layer C was not well forecast in the initial hazard assessment because (1) tephra-fall deposits of past eruptions were not well preserved, as evidenced by the rapid erosion of layer C; (2) the eruption column reached a very high altitude (35 km) and mushroomed out widely in the stratosphere, even in the upwind direction (about 250 km from the vent), which helped to create a broad distribution; and (3) the coincidental passage of Typhoon (later tropical storm) Yunya to the northeast of the volcano caused a shift in low- and middle-tropospheric winds so that tephra was transported farther south than forecast.

  8. Deposits of layer C typically have normal grading, which suggests that eruption intensity peaked early and then decreased until ending prior to cessation of pyroclastic-flow activity. Various lines of evidence imply that layer C was deposited in 3 to perhaps as much as 9 h.

  9. Slowly diminishing ash emissions continued from several vents in the caldera for about 6 weeks following the climactic eruption and produced a fine-grained laminated tephra-fall deposit (layer D) that has a bulk volume of about 0.2 km3.

ACKNOWLEDGMENTS

We thank the numerous people who contributed field measurements, descriptions, and samples of tephra-fall deposits, including colleagues at the Philippine Institute of Volcanology and Seismology, Pinatubo Lahar Hazards Taskforce, Mines and Geosciences Bureau, National Institute of Geological Sciences at the University of the Philippines, and U.S. Geological Survey. Personnel at the weather stations at the U.S. Naval Air Station at Cubi Point and Clark Air Base shared weather data and observations; Dr. Emmanuel Anglo and Dr. Jorge de las Alas of the Institute of Meteorology and Oceanography at the University of the Philippines also provided climate information. The manuscript benefited greatly from reviews by Steve Carey, Takehiro Koyaguchi, Chris Newhall, and Ed Wolfe.

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Pyle, D.M., 1989, The thickness, volume, and grainsize of tephra fall deposits: Bulletin of Volcanology, v. 51, p. 1-15.

Sabit, J.P., Pigtain, R.C., and de la Cruz, E.G., this volume, The west-side story: Observations of the 1991 Mount Pinatubo eruptions from the west.

Self, S., Zhao, J-X., Holasek, R.E., Torres, R.C., and King, A.J., this volume, The atmospheric impact of the 1991 Mount Pinatubo eruption.

Scasso, R.A., Corbella, Hugo, and Tiberi, Pedro, 1994, Sedimentological analysis of the tephra from the 12-15 August 1991 eruption of Hudson volcano: Bulletin of Volcanology, v. 56, p. 121-132.

Scott, W.E., Hoblitt, R.P., Daligdig, J.A., Besana, G., and Tubianosa, B.S., 1991, 15 June 1991 pyroclastic deposits at Mount Pinatubo, Philippines [abs.]: Eos, Transactions, American Geophysical Union, v. 72, p. 61-62.

Scott, W.E., Hoblitt, R.P., Torres, R.C., Self, S, Martinez, M.L., and Nillos, T., Jr., this volume, Pyroclastic flows of the June 15, 1991, climactic eruption of Mount Pinatubo.

Simkin, T., and Siebert, L., 1984, Explosive eruptions in space and time: Duration, intervals, and a comparison of the world's active volcanic belts, in Geophysics Study Committee and others, Explosive volcanism: Inception, evolution, and hazards: Washington, D.C., National Academy Press, p. 110-121.

Smithsonian Institution, 1991, Pinatubo: Bulletin of the Global Volcanism Network, v. 16, no. 5, p. 2-8.

Sparks, R.S.J., 1986, The dimensions and dynamics of volcanic eruption columns: Bulletin of Volcanology, v. 48, p. 3-15.

Torres, R.C., Self, S., and Martinez, M.L., this volume, Secondary pyroclastic flows from the June 15, 1991, ignimbrite of Mount Pinatubo.

Walker, G.P.L., 1971, Grainsize characteristics of pyroclastic deposits: Journal of Geology, v. 79, p. 696-714.

------1981, Generation and dispersal of fine ash and dust by volcanic eruptions: Journal of Volcanology and Geothermal Research, v. 11, p. 81-92.

Williams, S.N., and Self, Stephen, 1983, The October 1902 plinian eruption of Santa María volcano, Guatemala: Journal of Volcanology and Geothermal Research, v. 16, p. 33-56.

Wolfe, E.W. and Hoblitt, R.P., this volume, Overview of the eruptions.

FIRE and MUD Contents

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Last updated 06.11.99