Date: June 12-16, 1991
Place: Luzon, Philippines
Result: About 350 dead (mostly from collapsed roofs); extensive damage to homes, bridges, irrigation-canal dikes, and cropland; 20 million tons of sulfur dioxide spewed into the stratosphere up to an elevation of 15.5 miles
Prior to the 1991 eruption, Pinatubo had the appearance of a steep, domelike spheroid that rose about 2,297 feet (700 meters) above a gently sloping apron made of pyroclastic and epiclastic materials. Such a volcano belongs to the class of stratocones, of which such 803 well-known exemplars as Fuji and Mayon are considerably larger than Pinatubo. The extensive pyroclastic apron of Pinatubo, however, indicated that the volcano was extremely active in prehistoric times. Until the collapse of the summit in the 1991 eruption, Pinatubo rose 5,725 feet (1,745 meters) above sea level, surrounded by older volcanic centers, including an ancestral Pinatubo due south, east, and northeast

Chatterjee, Monish Ranjan

English

University of Dayton

University of DaytoneCommonsElectrical and Computer Engineering FacultyPublicationsDepartment of Electrical and ComputerEngineering20071991: Pinatubo EruptionMonish Ranjan ChatterjeeFollow this and additional works at: https://ecommons.udayton.edu/ece_fac_pubPart of the Geology Commons, and the Volcanology CommonsThis Encyclopedia Entry is brought to you for free and open access by the Department of Electrical and Computer Engineering at eCommons. It hasbeen accepted for inclusion in Electrical and Computer Engineering Faculty Publications by an authorized administrator of eCommons. For moreinformation, please contact frice1@udayton.edu, mschlangen1@udayton.edu.eCommons CitationChatterjee, Monish Ranjan, "1991: Pinatubo Eruption" (2007). Electrical and Computer Engineering Faculty Publications. 403.https://ecommons.udayton.edu/ece_fac_pub/403■ 1991: Pinatubo eruptionVolcanoDate: June 12-16, 1991Place: Luzon, PhilippinesResult: About 350 dead (mostly from collapsed roofs); extensivedamage to homes, bridges, irrigation-canal dikes, and cropland;20 million tons of sulfur dioxide spewed into the stratosphere upto an elevation of 15.5 milesAs early as April 2, 1991, people from a small villagenamed Patal Pinto, on the Philippine island of Luzon,observed steam and gases smelling of rotten eggs (indicating hydro-gen sulfide) emanating from near the crest of Mount Pinatubo,along with intermittent minor explosions. Within ten weeks, theseearly ominous activities culminated in a volcanic eruption that hascome to be regarded among the largest that occurred in the twenti-eth century.Pinatubo, located about 62 miles (100 kilometers) northwest ofManila, belongs to a chain of composite volcanoes constituting a vol-canic arc in the Philippines. It is believed to have been the result of alava dome that formed about five hundred to six hundred years agoduring the last-known eruption. Its lower slopes and foothills werecomposed primarily of pyroclastic and lahar (volcanic mudflow) de-posits from voluminous eruptions that occurred in prehistoric times.More than 30,000 people inhabited the foothills of the volcano be-fore the 1991 eruption. Cities and villages surrounding the base ofthe volcano on gently sloping alluvial plains were populated by asmany as 500,000 inhabitants. Located about 15.5 miles (25 kilome-ters) to the east of the volcano was Clark Air Base, and 25 miles (40 ki-lometers) to the southwest was Subic Bay Naval Station, both belong-ing to the United States.Prior to the 1991 eruption, Pinatubo had the appearance of asteep, domelike spheroid that rose about 2,297 feet (700 meters)above a gently sloping apron made of pyroclastic and epiclastic mate-rials. Such a volcano belongs to the class of stratocones, of which such803well-known exemplars as Fuji and Mayon are considerably largerthan Pinatubo. The extensive pyroclastic apron of Pinatubo, how-ever, indicated that the volcano was extremely active in prehistorictimes. Until the collapse of the summit in the 1991 eruption, Pin-atubo rose 5,725 feet (1,745 meters) above sea level, surrounded byolder volcanic centers, including an ancestral Pinatubo due south,east, and northeast.The Onset of Eruption. Following the emission of hydrogensulfide gas and steam together with a few minor, phreatic (steam-charged) explosions along a 1-mile-long chain of vents on the northside of the volcano around April 2, 1991, the Philippine Institute ofVolcanology and Seismology (PHIVOLCS) installed seismometersnear the mountain, which immediately began recording several hun-8041991: Pinatubo eruptionS o u t hC h i n aS e aP h i l i p p i n eS e aPHILIPPINESMALAYSIABRUNEIINDONESIAManilaBaguioIlocosProvinceLuzonVisayasPalawanMindanaoSuluArchipelagoCagayande OroLaoagZamboangaBataanPeninsulaDavaoPinatuboAngelesdred earthquakes a day. By April 5, nontelemetered seismographs in-stalled on the northwest side of Pinatubo about 6 to 9 miles (10 to 15kilometers) from the summit recorded between 40 and 140 seismicevents (of magnitude less than 1.0 on the Richter scale) each day.On April 23, a team of volcanologists from the United States Geo-logical Survey (USGS) arrived at the scene following a request byPHIVOLCS to assist in the monitoring of the seismic activities nearthe mountain. Together, the Philippine and American experts in-stalled a radio-telemetered seismic network and tiltmeters. These de-vices could locate the earthquakes and detect any new ground move-ment, respectively. They also measured fractures that opened duringthe early steam and vapor emissions from the chain of vents near thesummit of the mountain.Between May 13 and May 28, the geologists, with the help of theU.S. Air Force, measured a tenfold rise in the sulfur dioxide gas con-tent of the steam plumes emanating from the summit. These andother measurements indicated that magma was rising within the vol-cano, and immediate preventive measures were necessary for thesafety of people living in surrounding communities. The geologistsestablished a set of alert levels ranging from 1 (implying low-level un-rest) to 5 (indicating that eruption had started). On May 13, the alertlevel was set at 2, which meant that the seismic unrest probably in-volved magma.Before April 2, 1991, the available geologic information on Pina-tubo was quite limited. It was known to be a dacite dome complexabout 2 miles (3 kilometers) in diameter, with voluminous fans ofash-flow deposits that were geologically young (less than ten thou-sand years old). The volcano was known to be thermally active, how-ever, and had previously been explored as a potential geothermal en-ergy source by the Philippine National Oil Company.Anticipating that an eruption might be imminent, the geologistswent to work designing a hazard map, in preparation for the worst-case scenario. This was an urgent matter, especially since a large num-ber of small villages lay scattered on the northwest slope of the vol-cano and part of Clark Air Base and several urban communities (suchas the city of Angeles, with a population of 300,000) lay within the po-tential range of pyroclastic and debris flows extending well beyondthe volcano. Based on knowledge of the best-known distribution of8051991: Pinatubo eruptioneach type of volcanic deposit from past eruptions, a joint USGS-PHIVOLCS team rapidly compiled a worst-case hazard map showingareas most susceptible to ash flows, mudflows, and ashfall.Around May 23, the hazard map was distributed to officials of thePhilippine civil defense organization, the local governments in neigh-boring communities, and the U.S. military. Based on data obtainedfollowing the actual eruption on June 15, the predictions by the haz-ard map vis-à-vis areas where the impact would be most severe wereproven to be fairly accurate.Near the end of May, the number of seismic events per day wasfluctuating in a random fashion, and measurements of key seismicparameters such as earthquake hypocenter locations proved quite in-conclusive. The likelihood of an actual eruption, though highly plau-sible, could not be precisely forecast. From late May until early June,indicators such as relatively long earthquake periods interspersedwith periods of tremor, as well as the location of hypocenters beneaththe steam vents, were clear precursors of imminent eruption. It wasalso observed that the emission rate of sulfur dioxide, which had dra-matically increased during the preceding two weeks, had suddenlydecreased. This finding was consistent with the escape vents of thegas being sealed off by magma rising within the volcano.The Eruption and Its Aftermath. During the second week ofJune, the east flank of the mountain became tilted by inflation, and asmall lava dome extruded near the most vigorous steam vent. Thetectonic earthquakes became progressively shallower and weaker,while the emission of low-level ash became continuous.PHIVOLCS raised the alert level to 3 on June 5, indicating thateruption was likely within two weeks. On June 7, the extrusion of asmall dome on the north flank, accompanied by numerous smallearthquakes, triggered a level 4 alert, signifying eruption withintwenty-four hours. Residents of Zambales, Tarlac, and PampangaProvinces, within 12.4 miles (20 kilometers) of the volcano, wereevacuated. As the dome continued to grow and ash emissions in-creased to alarming levels, alert level 5 (signifying eruption had be-gun) was declared on June 9. On June 10, a total of 14,500 nonessen-tial personnel and dependents were moved by road from Clark AirBase to Subic Bay Naval Station. Most of the aircraft had already beenremoved from Clark Air Base at this time.8061991: Pinatubo eruptionOn June 12, the first of several major explosions occurred at 8:51a.m., spewing airborne ash to the west of the mountain and sendingpyroclastic flows down its northwest slope. The ash column reached aheight of 62,335 feet (19,000 meters) above sea level, according tomeasurements by the weather radar at Clark Air Base. Explosionscontinued through the night of June 12 and the morning of June 13.Part of the dome was destroyed, and a small crater was formed adja-cent to it. There was intense seismic activity, with buildup periods last-ing as long as several hours prior to the explosions during June 12through 14. The long buildup periods permitted short-term notifica-tion to Philippine civil authorities and U.S. military authorities re-garding impending eruptions. The city of Angeles was placed onevacuation alert.The climactic eruptive phase began around 1:09 p.m. on June 14,following an eight-hour episode of vigorous seismic activity. Explo-sive eruptions continued through the night and into the morning ofJune 15. Around 5:55 a.m., a massive lateral blast spread north, west,southwest, and northwest from the volcano, sending a broad columnof ash 39,370 feet (12,000 meters) above sea level. This climactic blastwas followed by six more eruptive pulses, after which the eruption be-came essentially continuous, lasting between the afternoon of June15 through the early hours of June 16.Coincidentally, Typhoon Yunya approached Pinatubo around thesame time. The extreme combination of hazards, including the ex-plosive eruption, a complete loss of telemetry between the summitand the observatories, and uncertainty regarding the effect of Yunyaon the flow of volcanic debris, made it necessary to rapidly evacuateall remaining USGS, Air Force, and PHIVOLCS personnel fromClark Air Base. This task was accomplished by around 2:30 p.m. onJune 15.The volcano continued to erupt a column of ash rising 32,808 feet(10,000 meters) above sea level for several weeks, even though theoverall seismic activity started to decline by late June 15. When theweather cleared on June 16, it was observed that the top of the vol-cano had been replaced by a 1-mile-wide caldera, and vast areas sur-rounding the volcano were covered by around 6,540 or 7,847 cubicyards (5 or 6 cubic kilometers) of pyroclastic deposits.The presence of Yunya exacerbated the volcanic mudflows and8071991: Pinatubo eruptionthe dispersal of water-saturated ash across a large number of citiesand villages. Cyclonic winds spread tephra over at least 7,722 squaremiles (20,000 square kilometers) surrounding the volcano. Theweight of the wet, heavy ash led to the collapse of many buildings,which turned out to be the leading cause of the loss of 350 or so livesfrom the eruption. Mudflows triggered by the typhoon and heavyrainfall destroyed homes, bridges, and irrigation-canal dikes and bur-ied vast areas of cropland.The Pinatubo eruption was one of the largest in the twentieth cen-tury (being about ten times larger than the eruption of Mount St.Helens in the United States in 1980) and potentially threatened1 million lives. Overall, it must be concluded that the evacuation andsafety procedures followed jointly by the USGS, PHIVOLCS, and thevarious military and civil defense organizations via effective commu-nication and timely, responsible action, helped avert disaster of a farhigher magnitude. In fact, it is estimated that timely and effective in-tervention saved many thousands of lives (the actual casualty figurewould be much lower had it not been for the presence of the ty-phoon), and at least $1 billion in property which might otherwisehave been lost. In terms of accurate eruption prediction and highlyeffective response, the 1991 Pinatubo eruption provides an impor-tant model for future volcanic eruptions and other geological cata-clysms.Monish R. ChatterjeeFor Further Information:Fiocco, Giorgio, Daniele Fuá, and Guido Visconti, eds. The MountPinatubo Eruption: Effects on the Atmosphere and Climate. New York:Springer, 1996.Newhall, Christopher G., James W. Hendley II, and Peter H. Stauffer.The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines. Van-couver, Wash.: U.S. Geological Survey, 1997.Newhall, Christopher G., and Raymundo S. Punongbayan, eds. Fireand Mud: Eruptions and Lahars of Mount Pinatubo, Philippines. Seat-tle: University of Washington Press, 1996.Pinatubo Volcano Observatory Team. “Lessons from a Major Erup-tion: Mount Pinatubo, Philippines.” EOS/Transactions of the Ameri-can Geophysical Union 72 (1991): 554-555.8081991: Pinatubo eruptionScarth, Alwyn. Vulcan’s Fury: Man Against the Volcano. New ed. NewHaven, Conn.: Yale University Press, 2001.Shimizu, Hiromu. The Orphans of Pinatubo. Manila, Philippines: Soli-daridad, 2001.Wolfe, Edward. “The 1991 Eruptions of Mount Pinatubo, Philip-pines.” Earthquakes and Volcanoes 23, no. 1 (1992): 5-37.8091991: Pinatubo eruption

1991: Pinatubo Eruption

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1991: Pinatubo Eruption

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