1Philippine Institute of Volcanology and Seismology.
2 U.S. Geological Survey.
Sequential photographs of various locations and features of Mount Pinatubo show dramatic geomorphic changes that resulted from the eruption of June 15, 1991. Owing to the large scale of events, only a few preeruption features of outcrop scale can be shown again in posteruption view. Instead, most photographs in this collection are aerial oblique, watershed-scale views. Rapid erosion and redeposition of 1991 deposits continues as of this time of writing; revegetation is also visible in many photographs.
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Geologic change is often thought to occur so slowly that even changes in civilizations and species seem rapid in comparison. Geologic change at volcanoes can be much faster--often occurring within minutes, days, months, and years. The most rapid changes are best captured on movie film or video; changes over days to years can be captured in sequential still images taken from the same site at various times through the course of that change. This paper presents 79 photographs in 28 sequences that span from preeruption to immediately after the eruptions and continue through the first four seasons of erosion and lahar deposition.
Sequential photographs of this paper emphasize geomorphic change and complement those of other papers in this volume. Other "before and after" views in this volume include:
Collectively, photographs in this and other papers of this volume tell a story of landscape change that ranks among the most rapid in the period of modern geology. We have been privileged witnesses.
Ideally, photographs to document change are taken from a fixed tripod position by a camera with a fixed focal-length lens. Each image should be exactly the same as the previous except where change occurred. At Mount Pinatubo, harried working conditions, staff rotations, various cameras, inclement weather, the enormous scale of the event, and reliance on helicopters for overviews has made precise reoccupation of photo stations impossible. In most instances, change has been so great at outcrop scale that no sequential comparison is possible. Change has been so great that even watershed-scale features can be difficult to identify from one view to the next, but at least qualitative recognition of change is possible.
Photographs in this collection are from the three authors and several colleagues. Most are from helicopters that lacked global positioning system (GPS) navigational equipment, and time was generally too short to plot our positions on maps as we flew, so we know positions mainly from the photographs themselves. To reoccupy aerial vantage points, we sketched lines on maps for the pilots, but with frequent rotation of both scientists and pilots, and without intercoms or GPS equipment, our repeat flights were generally different from the original flights. Often, too, rain clouds or ash clouds prevented return to exactly the same place from which a previous photograph was taken.
Photographs were taken with a variety of cameras and lenses, including zoom lenses. All but two were originally shot as color slides.
After we initially arranged photographs according to geologic feature and time period, we concluded that many of the sequences showed multiple geologic features and that sequences throughout the entire study period might be more interesting than pairs that bracketed only one geologic event. We therefore rearranged the photographs, here, according to watershed. The steep preeruption edifice and its later caldera are treated as one watershed; other watersheds are named by their principal river: O'Donnell, Sacobia-Bamban, Abacan, Pasig-Potrero, Gumain, Marella-Santo Tomas, Maraunot, Balin Baquero, and Bucao, in that order (clockwise, starting from north) (fig. 1). Within each watershed, the usual order, if good sequences were found, is from the headwaters of that watershed (Mount Pinatubo) to distal parts of each alluvial fan.
Figure 1. Approximate locations and directions of view of photographs in this paper. Each number represents a sequence of related photographs; letters within each sequence indicate time progression. Dots, approximate location of photographer; arrows, direction of view.
The approximate camera site and look angle of each photograph or set of photographs is shown on figure 1. Although some captions refer to the direction of view as "to" or "from," the arrows on figure 1 are all in the direction in which the camera was pointed. Banks of rivers are named as left or right according to standard hydrologic practice, as if one were looking downstream.
Most photographs in this collection were taken before or after an event, rather than during that event. Unless otherwise specified, dates in this paper are the dates of photography, not of events. Slightly different time-date formats on photographs reflect different cameras and photographers. Times that appear on some photos are not noted in the captions.
Mount Pinatubo is photogenic, and more changes are yet to come. We encourage colleagues to try to reoccupy our photographic sites as those changes continue. Please carry a reprint of this paper with you as opportunities for flights and other visits to the area arise. Additional photographs that show change, taken previously or in the future, would be gratefully received by the authors.
We thank Val Gempis of the U.S. Air Force, T.J. Casadevall, W.E. Scott, and E.W. Wolfe of the U.S. Geological Survey, and G.P. Yumul, Jr., of the University of the Philippines for permission to use their photographs.
We also thank the Philippine Air Force, U.S. Air Force, U.S. Navy, U.S. Marine Corps, and the late Agustin Consunji of Delta Aviation for helicopter support that was so vital to tracking events of such a large scale as at Pinatubo.
Arboleda, R.A., and Martinez, M.L., 1992 lahars in the Pasig-Potrero River system.
Campita, N.R., Daag, A.S., Newhall, C.G., Rowe, G.L., Solidum, R., Evolution of a small caldera lake at Mount Pinatubo.
Daag, A.S., Dolan, M.T., Laguerta, E.P., Meeker, G.P., Newhall, C.G., Pallister, J.S., and Solidum, R., Growth of a postclimactic lava dome at Mount Pinatubo, July-October 1992
Delfin, F.G., Jr., Villarosa, H.G., Layugan, D.B., Clemente, V.C., Candelaria, M.R., Ruaya, J.R., Geothermal exploration of the pre-1991 Mount Pinatubo hydrothermal system.
Ewert, J.W., Lockhart, A.B., Marcial, S., and Ambubuyog, G., Ground deformation prior to the 1991 eruptions of Mount Pinatubo.
Hoblitt, R.P., Wolfe, E.W., Scott, W.E., Couchman, M.R., Pallister, J.S., and Javier, D., The preclimactic eruptions of Mount Pinatubo, June 1991
Jones, J.W., and Newhall, C.G., Preeruption and posteruption digital-terrain models of Mount Pinatubo.
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., Eruptive history of Mount Pinatubo.
Paladio-Melosantos, M.L., Solidum, R.U., Scott, W.E., Quiambao, R.B., Umbal, J.V., Rodolfo, K.S., Tubianosa, B.S., Delos Reyes, P.J., and Ruelo, H.R., this volume, Tephra falls of the 1991 eruptions of Mount Pinatubo.
Rodolfo, K.S., Umbal, J.V., Alonso, R.A., Remotigue, C.T., Paladio-Melosantos, M.L., Salvador, J.H.G., Evangelista, D., and Miller, Y., Two years of lahars on the western flank of Mount Pinatubo: Initiation, flow processes, deposits, and attendant geomorphic and hydraulic changes.
Scott, W.E., Hoblitt, R.P., Torres, R.C., Self, S, Martinez, M.L., and Nillos, T., Jr., Pyroclastic flows of the June 15, 1991, climactic eruption of Mount Pinatubo.
Torres, R.C., Self, S., and Martinez, M.L., Secondary pyroclastic flows from the June 15, 1991, ignimbrite of Mount Pinatubo.
Umbal, J.V., and Rodolfo, K.S., The 1991 lahars of southwestern Mount Pinatubo and the evolution of the lahar-dammed Mapanuepe Lake.
Wolfe, E.W., and Hoblitt, R.P., Overview of the eruptions.
Figure 2A. Preeruption Mount Pinatubo, April 16, 1991. View from the northwest, up the Maraunot River valley. The river had become acidic and silty, owing to reactivation of the hydrothermal system and phreatic explosions of April 2, 1991 (the vents of which were just out of view at left edge of photograph). Steam was from 2-week-old fumaroles on the upper north slope of the volcano. The fumarole farthest to the right (behind a jagged ridge, right of the one visible on the valley floor) would later become the site of the preclimactic lava dome extrusion of June 7-12 (Hoblitt, Wolfe, and others, this volume). Mount Negron is behind and to the right of Pinatubo. (R.S. Punongbayan)
Figure 2B. Summit caldera and lake, with partly submerged relics (rocky islets) of a dome that grew between July and October, 1992. View is from the northwest, as in figure 2A, on October 5, 1994. The diameter of the caldera averages 2.5 km, rim to rim. Low point in the caldera rim (foreground) is the truncated valley of the Maraunot River. Mount Negron is in the background. (R.S. Punongbayan)
Figure 3A. Preeruption Mount Pinatubo, as viewed from the north in late April. Grayish-tan ash and several craters from the April 2 phreatic explosion craters are visible at the left. The road led to site C of the 1988-90 geothermal drilling (lower right; also indicated on fig. 1) discussed in Delfin and others (this volume); for reference, the same road is visible in figure 2A on the ridge northeast of the Maraunot River. Major drainage at lower left is the O'Donnell River; prominent fumarole at right center was in the Maraunot drainage. (V. Gempis)
Figure 3B. Summit caldera as seen on October 4, 1991, from the north-northeast. Much of the debris from the caldera walls had been washed onto the caldera floor, which was then submerged beneath a lake fed by ground water and rain water (Campita and others, this volume). (C.G. Newhall)
Figure 4A. Preeruption Mount Pinatubo, viewed from the northeast. The April 2, 1991, phreatic explosion craters (lower right, adjacent to '91 in date stamp) and the eventual location of June 7-12 dome extrusion (beneath the ash cloud) were aligned northeast-southwest across the north face of Mount Pinatubo (map of craters in Wolfe and Hoblitt, this volume). The ridge in the foreground may have been the wall of a prehistoric caldera that was only slightly larger than that which formed in 1991. (R.P. Hoblitt, June 9, 1991)
Figure 4B. Summit caldera, as seen August 1, 1991, from the northeast. The caldera formed by collapse during the June 15, 1991, climactic eruption. A small explosion had just occurred, forming the expanding ash cloud. Throughout the latter half of June and much of July, ash emission kept the caldera obscured; as continuous ash emission changed to intermittent explosions, the caldera became visible. See also, figure 1A of Campita and others, this volume. (T.J. Casadevall)
Figure 4C. Summit caldera viewed from the northeast, March 18, 1992. Row of fumaroles marks extension of the northeast-southwest-trending Sacobia lineament (figure 3 of Newhall and others, this volume) through the caldera, parallel to, and slightly south of the preeruption alignment of phreatic craters and fumaroles (fig. 4A). (R.P. Hoblitt)
Figure 5A. Mount Pinatubo, as seen from near the southwest end of the Clark Air Base runway. View is to the west, up the Sacobia valley. Mount Pinatubo is the light-colored (ash-covered) highest peak (center); prominent, darker ridges are relics of an ancestral Mount Pinatubo (Newhall and others, this volume). The highest dark ridge (right) is the relict northeastern rim of the 4x5-km-diameter Tayawan caldera, which formed in the summit of the ancestral Mount Pinatubo more than 35,000 years ago. (R.P. Hoblitt, June 14, 1991, 0722)
Figure 5B. Approximately the same view as in A, March 13, 1992. The highest peaks, light gray and stripped of vegetation, are the relict southeastern part of the preeruption Mount Pinatubo (left of center) and the previously dark ridge (Tayawan caldera rim) northeast of Pinatubo (skyline, immediately left of the siren tower). (R.P. Hoblitt)
Figure 6A. Upper O'Donnell River, late April 1991. View is from the north. Road on ridge in foreground led to geothermal drilling site C, located at right edge of photograph. (C.G. Newhall)
Figure 6B. Upper O'Donnell River, July 14, 1994. View is from the north, slightly farther from the volcano than the view in A. Low saddle on the skyline (left of center) is the ridge upon which site C was located; higher terrain that was obscured by or visible just beneath the cloud in A collapsed into the caldera. (C.G. Newhall)
Figure 7A. O'Donnell River (right) in Crow Valley, late July 1991. View is upstream, to the southwest. Circular targets and other features of a bombing range are visible on terraces of prehistoric lahar deposits. Lahars had already covered the lowest flood plain but had not yet covered higher terraces. Light-colored tephra-fall deposit partly covers all terraces, including those with the targets. Gently sloping distal pyroclastic-flow deposits of the June 15 eruption are dimly visible at the head of Crow Valley (background, right of center). (C.G. Newhall)
Figure 7B. O'Donnell River, as viewed on September 7, 1994, from above the small hill that is visible in the center foreground of A, between two circular bombing targets. All terraces in the foreground had been buried by fresh lahar deposits; several terraces in the background (unclear in this view) remained unburied. (C.G. Newhall)
Figure 8A. Fourteen-day-old undissected deposits of the Sacobia pyroclastic-flow field, east of Mount Pinatubo. View is west and upslope. Reestablishment of the drainage network had only just begun. (R.S. Punongbayan, June 29, 1991)
Figure 8B. Dissected Sacobia pyroclastic-flow deposits, June 1, 1994. Three years after emplacement, the Sacobia pyroclastic-flow deposits had a well-developed drainage system, and, locally, as much as 60% of the original deposit had been eroded. Resulting lahars had covered parts of the towns of Bamban and Concepcion in Tarlac Province, and Bacolor, Porac, Mabalacat, and Magalang in Pampanga Province. (R.S. Punongbayan)
Figure 9A. Undissected June 15, 1991, pyroclastic-flow deposits of the Sacobia pyroclastic fan, seen in this view to the east and downslope at 1043 on June 29, 1991. Mount Arayat is dimly visible in the background, left of center. (E.W. Wolfe)
Figure 9B. Extensive dissection of the June 1991 deposits of the Sacobia pyroclastic fan. View is similar to that of A, though slightly narrower; Mount Arayat is dimly visible in the center background. Clark Air Base lies between the pyroclastic fan and Mount Arayat in A and B. The lower Sacobia River, passing the north side of Clark Air Base, is at the upper left. (C.G. Newhall, August 30, 1994)
Figure 10A. Deep preeruption canyon of the Sacobia River, seen in this view to the east, downstream. Note sharp knob atop the right canyon wall, a feature that is common to B-D. Knob consists of lavas(?) of ancestral Mount Pinatubo (Newhall and others, this volume); badland topography (foreground and beyond the knob) consists of unconsolidated, prehistoric pyroclastic-flow deposits of modern Mount Pinatubo. The hamlet of Steding was at the foot of the knob; the road led upslope to geothermal sites and downslope to Angeles City. (Val Gempis, USAF, May or early June 1991)
Figure 10B. Early posteruption drainage from the upper part of the Sacobia pyroclastic fan, through the constriction in the midfield of this view, was into the Sacobia River (left). View is to the east, downstream. Only a small part of the upper watershed drained into the Pasig-Potrero River (right, past the constriction). (C.G. Newhall, September 1991)
Figure 10C. Drainage of the upper part of the Sacobia pyroclastic fan continued into the Sacobia River (left) through all of the 1991 and 1992 and most of the 1993 rainy seasons. As in B, only a small part of the upper watershed drained into the Pasig-Potrero River (far right). At the valley constriction, the channels of the two rivers were 600 m apart. (R.S. Punongbayan, February 18, 1993)
Figure 10D. Sacobia and Pasig-Potrero Rivers, approximately same view as in B, September 11, 1995. A secondary explosion in pyroclastic-flow deposits between the north and south forks of the Sacobia River, on October 5, 1993, generated a secondary pyroclastic flow that filled a 2-km-long segment of the Sacobia River channel and shunted flow from the upper Sacobia watershed into the channel of the Pasig-Potrero River. Subsequent deep erosion has entrenched flow into the Pasig-Potrero River (right side of valley) and greatly lessened lahar hazard in the Sacobia River valley (C.G. Newhall).
Figure 11A. An approximately 10-m-deep constriction in the lower Sacobia River at Barangay Maskup. View is downstream, June 23, 1991. Lahars during the 1991 rainy season buried the floor of the valley to a depth of about 5-6 m at Maskup; lahar deposition during 1992 and 1993 (C-F) aggraded the stream floor by an additional 6 m. (R.P. Hoblitt)
Figure 11B. Closeup view of the Maskup constriction, February 19, 1992. View is downstream; an antenna mast near the right bank transmitted data from a rain gage and tripwires to civil defense officials. About 5 to 6 m of posteruption aggradation had already occurred. (C.G. Newhall)
Figure 11C. The Department of Public Works and Highways constructed a house, near the antenna mast, for previously rain-soaked lahar observers of PHIVOLCS and nearby barangays. The house was completed in late August 1992. (C.G. Newhall)
Figure 11D. Lahars on August 29, 1992, threatened but did not destroy the observers' house; lahars on September 4-5 partly buried the house. During the worst of the September 4-5 lahars, observers moved to nearby, higher ground. Several other lahar watchpoints along banks of the Pasig-Potrero, Marella-Santo Tomas, and Bucao Rivers were also overrun or threatened and had to be relocated. (C.G. Newhall, September 5, 1992)
Figure 11E. Lahars in late 1992, and up to October 1993, completely buried the house and the lower one-third of the antenna mast. The bracket for the white rain gage is visible just above the low trees and bamboo in B. People standing atop the former location of the house are mostly PHIVOLCS observers from other Philippine volcanoes who are visiting Pinatubo to learn some of its lessons. The concrete pads in the foreground were footings for a pedestrian suspension bridge that was never completed. (C.G. Newhall)
Figure 11F. Broken wooden posts on concrete foundations of the former observers' house, exhumed shortly before September 5, 1994. The snapped stump of the antenna mast was behind the photographer. The last major lahars in this valley before this photograph was taken occurred during Typhoon Kadiang in October 1993. During that same typhoon, the Pasig-Potrero River captured most flow of the Sacobia River (fig. 10D); as a result, lahar deposition at Maskup in 1994 was exceeded by erosion and downcutting. (C.G. Newhall)
Figure 12A. Highway and railway bridges across the Bamban River, at Bamban, Tarlac, about July 17, 1991. View is upstream, to the southwest; immediately behind the bridges is the confluence of the Marimla River (directly to the rear) and the Sacobia River (joining from the left). Highway bridge spans fresh lahar deposits. Damage to the railway bridge predated 1991 and was not related to the eruption of Mount Pinatubo. (C.G. Newhall)
Figure 12B. Bamban River in mid-September, 1991, 1 month after lahars swept away the highway bridge (details in fig. 13A-C). The higher railway bridge (right of center) remains. Traffic in the lower right (beneath the date stamp) in this photograph is along the original path of a concrete road that crosses the right edge of A; debris that had covered the road was bulldozed into a makeshift levee. The same aggradation that set the stage for the demise of the Bamban highway bridge blocked the Marimla River. An impounded lake (middle background, right of center) broke out and partially drained on August 21. (C.G. Newhall)
Figure 12C. Lahars of August 21 and September 1991 breached protective levees and buried Barangay Lourdes, Bamban. View is to the south-southwest. Barangay Lourdes was off the lower right corners of A and B, downstream from the Bamban bridge. By the time of this photograph (January 1992), homeowners had salvaged their galvanized iron roofing and had moved into an evacuation camp on a nearby hill or to undamaged homes of relatives and friends. Mounds of sandy lahar debris (upper part of photograph) were being added to raise the left bank levee, for (unsuccessful) sediment control. (C.G. Newhall)
Figure 13A. Ground-level view of Bamban bridge, Bamban, Tarlac, July 5, 1991. This bridge carried most of the traffic between Manila and northern Luzon. The river bed was approximately 10-12 m below the bridge deck, and in preceding weeks the channel floor and piers had been scoured by water-rich lahars. For reference, note the cuesta (asymmetric, uptilted ridge) in the background. View is upstream, to the west-southwest. (R.S. Punongbayan)
Figure 13B. Bamban bridge at risk, August 16, 1991. The lahar event that buried the hut in figure 14A left a clearance of only 1 m under the Bamban bridge. Traffic remained undisrupted. (R.S. Punongbayan)
Figure 13C. The Bamban highway bridge was lifted and swept away by a lahar on August 21, 1991. Part of the railroad bridge (fig. 12A) is now visible at right center. The reference cuesta is in full view. (R.S. Punongbayan, August 25, 1991)
Figure 14A. A house by the Sacobia-Bamban River, Bamban, Tarlac, July 23, 1991. This quaint hut was located on the left bank of the river, downstream from the Bamban bridge. (R.S. Punongbayan)
Figure 14B. Only the roof of the hut remains unburied, August 16, 1991; nearly 9 m of sediment were deposited during a single lahar event on August 15, 1991. (R.S. Punongbayan)
Figure 15A. Pyroclastic-flow deposits of June 15, 1991, filled the valley of the Sacobia River (foreground) to such a level that some of the earliest runoff and lahars (dark brown) passed through a pair of gaps in the hills (center), known informally among geologists as the "Abacan gap," and into the Abacan River (upper left). Prior to the eruption, the Abacan was fed only by its own small watershed below the Abacan gap; after the eruption, for most of the 1991 rainy season, a substantial part of the runoff from the Sacobia pyroclastic fan (upper right) flowed down the Abacan River. Downstream from the Abacan gap, pyroclastic-flow deposits in the Sacobia (lower left) blocked a tributary in which a new lake is seen here; opposite this lake is the headscarp of a secondary pyroclastic flow (Torres and others, this volume). Water from this lake might have contributed to early secondary explosions and generation of this secondary pyroclastic flow. View is to the south; all flow was from right to left. (R.P. Hoblitt, July 1, 1991)
Figure 15B. Abacan gap, October 1, 1991. View is to the east-southeast, downstream; flow in the Sacobia is from the foreground toward the left. The dark-gray top of the white pyroclastic-flow deposit is a several-meter-thick zone in which the deposit has been wetted and cooled enough to retain moisture. Erosion into pyroclastic-flow deposits led to secondary explosions (and further secondary pyroclastic flows?) and caused the headscarp shown in A to migrate upstream and outward toward the valley walls. Only a thin septum remained between the "upper Abacan" channel (above center, hugging preeruption hills) and the scalloped area within the Sacobia River watershed. The thinnest part of the septum is itself scalloped (center) and lacks the dark, moist surface layer. (C.G. Newhall)
Figure 15C. Abacan gap, March 18, 1992. View to the east shows that the "upper Abacan" (flowing from the upper right toward the center of the photograph) is on the verge of diversion into the Sacobia (lower left quadrant) and that very little remains of the original, flat surface of the June 15, 1991, pyroclastic-flow deposits. (W.E. Scott)
Figure 15D. Abacan gap, July 14, 1994. On April 4, 1992, a relatively large secondary pyroclastic flow that originated in the vicinity of the Abacan gap and flowed down the Sacobia removed the last of the septum that had kept water of the "upper Abacan" channeled out through the Abacan gap. Thereafter, runoff from the "upper Abacan" joined the Sacobia, as it had prior to the eruption. Rapid downcutting in 1992-94 reached and even incised the preeruption floor of the Sacobia in this area. Vegetation has noticeably recovered in areas where 1991 deposits were thin. (C.G. Newhall)
Figure 16A. View to the southeast, down the north (Timbo) fork of the Pasig-Potrero River, to its confluence with the south (Papatak) fork. The lightest-colored material in the valley floor is a several-meter-thick pyroclastic-flow deposit from June 15, 1991. Slightly darker in color material is from early lahars across the 1991 pyroclastic-flow surface. Hills with columnar joints exposed along the valley walls are of the >35-ka, semiwelded Inararo pyroclastic-flow deposit (Newhall and others, this volume). Terraces in the background are underlain by post-Inararo lahar deposits. (V. Gempis, July 1991)
Figure 16B. Closeup view down the constricted section of the Timbo fork (right-center in A). By the time of this photograph (September 19, 1991), erosion by energetic, high-discharge lahars had cut through about 5(?) m of June pyroclastic-flow deposits and about 15 m into pre-1991 stream sediments. A concrete "sabo dam" that had been constructed a decade earlier, to trap sediments before they could reach and fill lowland channels, had been undercut and left hanging. The exact location of this sabo dam in A is at the small L-shaped extension of bedrock into the constricted channel; the right abutment of the sabo dam is against the long leg of the L, as can be seen in B and C. On figure 1, the sabo dam is near the tip of the arrow for sequence 16. (C.G. Newhall)
Figure 16C. Same view as in B on November 21, 1992. Erosion had cut about 5 m deeper than in B. (C.G. Newhall)
Figure 16D. Same view as in A on September 30, 1994. Rapid lahar deposition in August and September 1994, as well as deposition from a secondary pyroclastic flow, refilled the Timbo valley and reburied the sabo dam. Much of this rapid deposition was accomplished by continuous lahars with discharges of only 10 m3/s or less, related to reappearance of springs along a several-kilometer-long upstream reach, like springs that fed the river in preeruption time. The level of fill was about 15 m higher than in 1991; the L-shaped extension disappeared, as did the lowest two terraces of the background. (C.G. Newhall)
Figure 17A. Pyroclastic-flow deposits of June 15, 1991, in the valleys of the north (Timbo) and south (Papatak) forks of the Pasig-Potrero River. View is to the south, across the Timbo (foreground) to the larger Papatak (center); flow was from right to left. Deposits in the Papatak valley blocked a tributary (center), and a small lake had already begun to form behind this blockage by June 22. The lake grew larger through the ensuing rainy season and, on September 7, 1991, overtopped and breached its dam. The resulting large lahar killed several people downstream. A nearly identical series of events occurred in 1992. A secondary pyroclastic flow blocked the same tributary on July 13, and, on August 29, 1992, breaching caused 9 h of serious lahars downstream. Additional photographs of 1992 changes are in Arboleda and Martinez (this volume). (R.P. Hoblitt, June 22, 1991)
Figure 17B. Incipient formation of yet another lake in the same tributary as the lake in A, at the foot of Mount Cutuno, July 5, 1994. The principal impounding agent in 1994 was lahar deposition, though some secondary pyroclastic flows might also have contributed. (C.G. Newhall)
Figure 17C. Full development of the impounded lake, same location, August 30, 1994. (C.G. Newhall)
Figure 17D. Drained, eroded floor of the impounded lake, which broke out on the night of September 22, 1994, after moderate rainfall. Approximately 25 people were killed, mostly in Barangay Manibaug Pasig (fig. 18D). (C.G. Newhall)
Figure 18A. Pasig-Potrero River at barangay Mancatian, Porac, Pampanga, August 31, 1991. View is to the southwest; Mount Pinatubo's summit is about 22 km upslope (to the right). At this time, the channel of the river was relatively deep, and most lahar deposition was occurring downstream of Mancatian. The widespread light-gray areas are ash-fall deposits, mostly from the climactic eruption of June 15, 1991; brown strips are intrachannel and overbank lahar deposits. For reference, note red-roofed church near the top center of the picture (left side of road), the prominent bend of the road to the right after it crosses the river, and a factory complex (large dark buildings) on the left side of the road and the left (near) bank of the river. The last, known locally as "San Miguel," processed pumiceous sand from Mount Pinatubo for glass bottles. The distance from the glass factory to the church is 1.0 km. (R.S. Punongbayan)
Figure 18B. Fanhead of deposition had moved upstream to Mancatian by November 14, 1992. To prevent the spread of lahars, levees were built along both the right and left banks of the original channel (center), and deposition over the right levee, just downstream (left) of the village and four long chicken coops, had reached the right edge of a pre-1991 flood bypass (light green swath without houses, bounded on its far side by a small road, passing right to left across the upper part of photograph). The main road crosses the flood bypass on a low bridge about 100 m on the near side of the red-roofed church. (R.S. Punongbayan)
Figure 18C. The central part of Mancatian (near the four chicken coops) and the northeastern part of Mancatian (near the glass factory) were buried by lahars in late 1993. The right-bank levee upstream from Mancatian arrested the spread of lahar toward the church, but the left-bank levee was breached upstream from the barangay. The glass factory, at the bend in the road, is now surrounded by the lahar field. In the foreground is Barangay Manibaug Pasig of Porac town. (R.S. Punongbayan, February 26, 1994)
Figure 18D. Continued deposition, and continued raising of the levees, created a situation in which the Pasig-Potrero River was now building an elevated flood plain between levees, about 15 m above the surrounding countryside. Small-scale breaches of the right-bank levee in 1994 resulted in partial burial of the church and near total burial of nearby houses and a school (see also E). Larger breaches of the left-bank levee, on and before September 22, 1994, buried most of Barangay Manibaug Pasig (foreground). Breaches of the left-bank levee also directed the large, lake-breakout lahar of September 22 into Bacolor (fig. 20 A,B) (R.S. Punongbayan, October 1, 1994)
Figure 18E. The Mancatian-to-Manibaug Pasig crossing, as viewed from southwest to northeast (opposite to that of A-D). The church is buried to its front awning; a side building is buried to its roof. Just northeast of the church, at the location of the former right-bank levee, the road climbs up onto the elevated flood plain of the Pasig-Potrero, as described for D. Remains of the glass factory (circle) are visible just in front of the active channel. (C.G. Newhall, September 30, 1994)
Figure 18F. A second-generation, 10-m-tall buried telephone pole (the first generation was completely buried). Remains of the glass factory are in the background. The budding scientist in the photograph was spray painting marks at 2-m elevations on third-generation posts, such as that in the background. (C.G. Newhall, August 13, 1994)
Figure 19A. Town of Bacolor (foreground), the raised Gapan-Olongapo highway (right to left), and lahar-threatened barangays Santa Barbara, Parulog, and San Antonio, all of Bacolor. The levee-bounded Pasig-Potrero River formerly flowed off the left edge of the photograph. However, owing to several upstream breaches in its left-bank levee in 1991, lahars and stream flow covered a wide fan-shaped area east of the intended channel (virtually the entire field of view). By the time of this photograph, residents of Santa Barbara, San Antonio, and Parulog had begun to rebuild rice paddies and fishponds on top of 1991 lahar deposits. Breaches of the right bank in 1992 created the large gray area west (left) of the intended channel (covering Barangays Mitla, Balas, and several others), on which little recovery had been attempted. New lahar outbreaks in July 1994 had begun to encroach at the top right. (C.G. Newhall, July 27, 1994)
Figure 19B. Lahars from September 1994 through and after the date of this photograph (September 6, 1995) reburied Bacolor to depths of 5 m in the town proper and >10 m in some outlying villages. (C.G. Newhall)
Figure 20A. Lot for sale, Barangay Parulog, Bacolor, August 21, 1994. View is to the northwest, from the highway and across the prominent, flooded field, right center of figure 19A. (R.S. Punongbayan)
Figure 20B. Same lot as in A, but no longer for sale. Buried to depth of 0.5 to 1 m by lahar of September 22, 1994. Hundreds of square kilometers of land like this are being buried by distal overbank flows (here, 30 km from Pinatubo's summit). Houses on stilts can survive; fields are left temporarily unusable. (R.S. Punongbayan, January 15, 1995)
Figure 21A. Abandoned meander of the Gumain River, May 28, 1991. (R.S. Punongbayan)
Figure 21B. Abandoned meander of the Gumain River, November 9, 1991. Now partly filled by lahars. Basa Air Base in the middle distance. (R.S. Punongbayan)
Figure 22A. Preeruption Marella River, June 3, 1991. View is to the northeast. Prominent hill in the center of the photograph rose above the fan of prehistoric pyroclastic debris. The flat-topped deposit that snaked its way down from the volcano (at least as far as the prominent hill) was a young lithic pyroclastic-flow deposit, probably from late in the 500-y-B.P. Buag eruptions. (R.P. Hoblitt)
Figure 22B. Posteruption Marella River, June 22, 1991. View is to the northeast. Same hill as in A appeared as an island (kipuka) in a sea of 1991 pyroclastic-flow deposit, the surface of which was undissected. (R.P. Hoblitt)
Figure 22C. Deeply dissected, medium-dark gray 1991 pyroclastic-flow deposits of the Marella River. The 1991 pyroclastic flows filled the narrow preeruption canyon (right of center in A) and deposited an additional 50-100 m on top of the preeruption, flat-surfaced, 500-year-old deposits (center in A). Erosion has removed well in excess of 100 m of 1991 deposit from the east edge of the prominent hill and might have reached to the preeruption canyon floor. Light-colored deposits in the main Marella channel (right) and in a lesser channel (left) are post-1991 lahar deposits. (C.G. Newhall, September 7, 1994)
Figure 23A. The Mapanuepe River, which does not head on Mount Pinatubo, flowed west to its confluence with the Marella River, in the background, which flowed from right to left. The hill near the confluence is Mount Bagang. Heavy tephra fall in this area lent a gray color, but, except on June 14-15, no significant lahars had yet occurred. View is to the west. (C.G. Newhall, mid- to late-July, 1991).
Figure 23B. Mapanuepe Lake formed when lahars of the Marella River dammed the Mapanuepe River. The new lake submerged several villages, including one (Barangay Pili) in the foreground, left. Although sediment continues to aggrade (and prograde) the Marella-Mapanuepe delta, lake level was stabilized in late 1992 at approximately the same level seen here, by excavation of a trench through bedrock of the dark green, low peninsula that is directly in front of Mount Bagang and immediately south (left) of the confluence and impoundment. (C.G. Newhall, September 19, 1991)
Figure 24A. Preeruption view southeast up the Maraunot River. The prominent steam cloud was issuing from a fumarole in the headwaters of the Maraunot River. (R.P. Hoblitt, June 6, 1991)
Figure 24B. Valley of the upper Maraunot River, June 14, 1991. Fresh pyroclastic-flow deposit of the June 12 or June 13 eruption (Hoblitt, Wolfe, and others, this volume) partially fills the valley. The remaining part of the June 7-12 dome is steaming in the background. (R.P. Hoblitt)
Figure 24C. Posteruption view southeast up the Maraunot River. Most of what remain of the preeruption edifice is now hidden behind hills (old domes?) northwest of Mount Pinatubo's summit. The flat surface is essentially that of 1991 pyroclastic-flow deposits; the stratified veneer on that surface consists of fall deposits from postclimactic ash emission (layer D of Paladio-Melosantos and others, this volume) and fall and surge deposits from nearby secondary explosions. Group is a graduate class in volcanology from the University of the Philippines, accompanied by a group of Aetas and nuns from LAKAS (Lubos na Alyansa ng mga Katutubong Ayta sa Sambales, or Negrito People's Alliance of Zambales). The LAKAS group formerly lived in this area. (G.P. Yumul, Jr., February 22, 1992)
Figure 25A. Maraunot River (foreground), flowing into the Balin Baquero River (middle background) and thence into the Bucao River (distant background) and the South China Sea, April 16, 1991. Most of the area in the foreground consisted of dissected pyroclastic-flow deposits from previous eruptions. (R.S. Punongbayan)
Figure 25B. Similar view as in A, on July 20, 1991, showing subdued topography in which prehistoric deposits were partly but not wholly buried beneath 1991 pyroclastic-flow deposits. Postclimactic tephra-fall deposits (layer D of Paladio-Melosantos and others, this volume) smoothly mantle the surface and are incised by shallow, incipient rills. (C.G. Newhall)
Figure 25C. Detail of an area near the prominent steaming in B. View is to the west-northwest and shows the confluence of the Maraunot River (prominent, unfilled channel at right) with the Balin Baquero River (top, flowing from left to right). Voluminous pyroclastic-flow deposits of June 15, 1991, had filled the main valley of the Maraunot River upslope from here, and also small channels of a broad area just south of the preeruption Maraunot. Barangay Villar was destroyed (right of center, on the interfluve between the voluminous deposits and the unfilled channel of the Maraunot). (R.P. Hoblitt, June 24, 1991)
Figure 25D. Similar view as in A and B, but from slightly south of A, 3.5 years after the eruption. New vegetation on what were thin 1991 tephra-fall and ash-cloud surge deposits contrasts with gray, largely unvegetated 1991 valley-filling pyroclastic-flow deposits. Runoff from the west side of Pinatubo, together with some that was diverted from upper Maraunot River (lower right, at the foot of the elongate green hills), has cut a new channel through the pyroclastic-flow field (straight down the field of view). Former site of Villar is between the old course of the lower Maraunot River (right center) and its temporary(?) new course (straight center). (R.S. Punongbayan, January 11, 1995)
Figure 26A. Balin Baquero River, Zambales, May 28, 1991. The Balin Baquero, a major tributary of the Bucao River, drains the western slopes of Mount Pinatubo. It is joined by the Maraunot River at the left edge of the photograph, about 15 km west-northwest of Mount Pinatubo's summit. View is to the south, upstream. (R.S. Punongbayan)
Figure 26B. Lahar deposits along the Balin Baquero River. The flood plains of the Balin Baquero and Maraunot Rivers were covered with lahar deposits up to 30 m thick; here, the deposit is about 10 m thick. A small lake at the right side of the picture formed when lahar deposits blocked drainage from the adjoining hillslope watershed. The road, its bend, and the village (Barangay Burgos) are the same in both A and B. (R.S. Punongbayan, November 22, 1992)
Figure 27A. Bridge to Poonbato, Botolan, Zambales, across the Bucao River, May 28, 1991. Access to the northwestern and western slopes of Mount Pinatubo was provided by this bridge. Barangay Poonbato was mostly right of the photograph, south of the bridge. View is to the east, upstream. Bucao River heads to the right, on Mount Pinatubo; Balintawak River heads on non-Pinatubo terrain in the background (R.S. Punongbayan)
Figure 27B. Poonbato bridge was buried (but not swept away) by lahars of 1991 and 1992. Deposits are approximately 25 m thick. Barangay Poonbato (immediately to the right of the field of view) was buried. View is to the east, upstream. (R.S. Punongbayan, May 16, 1994)
Figure 28A. Bucao River, Zambales (view upstream), May 28, 1991. The Bucao River drains the northwestern slopes of Mount Pinatubo. This segment of the Bucao River is about 1 km downstream from sitio Maguiguis, Botolan, Zambales (location shown by arrow 28 on figure 1), and about 12 km northwest of Mount Pinatubo's summit. Rice and corn were grown on the flood plain. (R.S. Punongbayan)
Figure 28B. Lahar deposits along the Bucao River, as of March 11, 1993. Lahar deposits completely covered the flood plain to depths as much as 25 m. (R.S. Punongbayan)
Figure 29A. Mouth of the Bucao River, April 16, 1991. White-sand beaches of Zambales Province, consisting of pumice and coralline debris, attracted many tourists. The ocean was clear blue; little sediment was carried by the Bucao River. (R.S. Punongbayan)
Figure 29B. Muddy water at the mouth of the Bucao River, October 1, 1991. Massive amounts of sediment were carried from Mount Pinatubo into the Bucao River valley; an unknown but relatively small percentage of that sediment is carried into the South China Sea. (R.S. Punongbayan)
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