THE JUNE 1991 ERUPTION OF MOUNT PINATUBO, PHILIPPINES

Mount Pinatubo is an andesitic island arc volcano, located on southern Luzon Island, Philippines. The hornblende andesite dacit dome complex is located 100 km northwest of Manila. When the dormant volcano last erupted more than 635 years ago, it produced pyroclastic flows that filled the Marella River Valley.

On April 2, 1991, the volcano, which had been rumbling for months, stirred to life with an explosion at the east end of a geothermal area. After the explosion, a line of new fumaroles roughly one kilometer long developed. Over the next six weeks several very small earth tremors and minor explosions occurred. By June 3, seismic activity had increased dramatically. Four days later a dome appeared on the mountain, oozing molten rock. On June 10, 15,000 Americans were evacuated from Clark Air Base (located approximately 12 km southeast of Pinatubo); essential personnel stayed behind. On June 12, a tephra column rose to about 20 km. By June 14, 55,000 people had been evacuated from villages within a 12-mile radius of Pinatubo. Then at dawn, just before 6:00 A.M. on June 15, 1991, a cataclysmic eruption began with a tremendous explosion. The eruption destroyed ten deserted villages and earthquakes shook surrounding towns.

The towering plume of Mt. Pinatubo soared an astonishing 40 km into the atmosphere. Snow-like ash fell in Manila, 100 km to the south. Locally, the ash blocked out the sun. Searing gas, ash, and molten rocks (pyroclastics) raced down the lush northern and western slopes of the volcano. The high velocity and low density of the flows allowed them to drain off the steep slopes immediately around the crater. These pyroclastic flows, together with heavy rains, later produced mudflows and landslides on all flanks of the volcano.

After a typhoon on July 18, lahars (hot mudflows) as deep as three meters along rivers east of the volcano buried sections of some towns. The filling of river channels and ruptures of dikes caused lahars as wide as 4 km. Mudflows five meters high rushed through the streets of Concepcion, sweeping away several people. By July 26th, 100,000 homes had been crushed or buried. When the rain fell on the ash-covered roofs, many collapsed, resulting in more deaths. On August 20, more than two months following the major eruption, the largest of the mudflows resulted in thirty-one casualties. One pyroclastic flow made a dam behind which a lake formed. On September 7, the dam ruptured causing muddy flash floods that destroyed 800 homes and killed seven. Flooding and mudflows caused twelve more deaths on September 15.

The eruptions at Mount Pinatubo and the aftermath events killed 722 people. Of these, 358 deaths were attributed to disease that broke out in the evacuation camps, 281 to the initial eruption, and 83 to mudflows. An additional 184 people were injured and 23 were reported missing. More than 108,000 houses were partially or totally damaged. The event left more than 200,000 people homeless, destroyed businesses, and ruined Clark Air Base. About 20,000 Americans were evacuated from Subic Bay Naval Station to the United States after Subic Bay was extensively damaged.

Besides local damage, Pinatubo ejected huge quantities of particles into the global wind system in the stratosphere to heights exceeding 30 km. These particles affect the weather in two ways. Tiny aerosol droplets reflect sunlight away from Earth's surface. A maximum global cooling of about 1.5 degrees C has been observed and is gradually diminishing.

In addition, the aerosols from the eruption help reactivate inert chlorine molecules drifting in cold clouds in the stratosphere. In sunlight, these chlorine molecules combine with oxygen molecules in the ozone layer reducing its density. Until the ozone regenerates, it cannot effectively shield that portion of Earth from the sun. The aerosol increase may cause a 15% reduction in mid-latitude ozone. Preliminary measurements suggest that Pinatubo was one of the largest volcanic eruptions of the 20th Century.

The threat of more destruction continues. The combination of rains and loose pyroclastic materials, sediment-filled channels, area seismicity, and small ash and steam explosions will continue to make the area dangerous for years to come. On the positive side, volcanic deposits have enriched the soil on the plains around Pinatubo.

	Aerial View of Area Devastated by April 2 Explosions ERUPTION PHASES Aerial view taken in late April looking south at the north flank of Pinatubo and showing the area devastated by the April 2 explosions. The explosions occurred about 1.5 km northwest of the summit, at the east end of Pinatubo's geothermal area. They denuded about one km2 of forested land, and ejected small steam/ash clouds, depositing ash 10 km away. The road in the foreground provided access to geothermal drilling stations. Photo credit: C.G. Newhall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-01
	Aerial View of April 2 Explosion Craters ERUPTION PHASES  Aerial view of the April 2 explosion craters, taken in late April. Following the explosion, a NE-WSW-trending line of new fumaroles with six main vents developed. Two of these are visible in the background. Vegetation and leaves were stripped over several square kilometers. Downed trees were usually aligned north-south. Photo credit: C.G. Newhall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-02
	Close-Up of April 2 Explosion Craters ERUPTION PHASES  View of April 2 explosion craters;the slide was taken in late April. The April 2 explosion, which was about two-thirds of the way down the volcano's flank, ejected clouds of steam and minor quantities of ash to 500-800 m height. Ash (shown here around one of the craters) was carried to the west and dropped at distances of 2 to 10 km, over an area of about 10,000 km2. It forced about 5,000 residents on the west flank to leave the area. Photo credit: R. Batalon, U.S. Air Force.  File: vo-Mt-Pinatubo-03
	Geologists Examine Ash Deposits from April 2 Eruption ERUPTION PHASES  Geologists examine ash deposits on the rim of the April 2 explosion crater. Gray vent emissions, such as those visible in the background, consisted mainly of steam with the odor of hydrogen sulfide. The slide was taken in late April. Photo credit: U.S. Geological Survey.  File: vo-Mt-Pinatubo-04
	June 12, 1991, Eruption Cloud from Clark Air Base ERUPTION PHASES  View to the west from Clark Air Base of the first major eruption of Pinatubo on June 12, 1991. After more than two months of increasing seismicity, localized deformation, and emission of small plumes, a series of strong explosions and a tephra column on this date heralded the start of the major pyroclastic phase. The tephra column rose to about 20 km. Photo credit: R.S. Culbreth, U.S. Air Force.  File: vo-Mt-Pinatubo-05
	View of June 12, 1991, Eruption Cloud ERUPTION PHASES View to the west from Clark Air Base of the first major eruption of Pinatubo on June 12, 1991. Pyroclastic flows (clouds of ash and steam) advanced 5-15 km down the north, northwest, and southwest flanks of the volcano. Most of the remaining personnel at Clark Air Base were evacuated along with thousands of people at nearby Angeles City. However, personnel returned to Clark Air Base within a few hours of the start of this first eruption, as winds had blown the volcanic ash south of the base. Photo credit: K. Jackson, U.S. Air Force. File: vo-Mt-Pinatubo-06
	View of Major Eruption Cloud on June 15, 1991 ERUPTION PHASES  View to the west from Clark Air Base of the major eruption of Pinatubo on June 15, 1991. The June 15-16 climatic phase lasted more than fifteen hours, sent tephra 30-40 km into the atmosphere, generated voluminous pyroclastic flows, and left a caldera in the former summit region. Later dubbed "Black Saturday," the day of darkness stretched for 36 hours. The day included a black blizzard of coarse sand, more than 50 earthquakes, volcanic thunder, brilliant lightning, and orange fireballs. Photo credit: R. Lapointe, U.S. Air Force.  File: vo-Mt-Pinatubo-07
	Aerial View of Pinatubo Crater after Major Eruption ERUPTION PHASES  Aerial view of the north side of Pinatubo crater with a small explosion in progress on June 22, 1991. The crater is approximately two km in diameter. The caldera was offset to the north of the former summit. The major eruption lowered the 1,755 m (5,758 ft) summit by 145 m (476 ft). The caldera wall, 200 m high at one point, dropped to the level of the crater floor on the east side. The floor of the caldera was smooth and covered with ash. Photo credit: R. Batalon, U.S. Air Force.  File: vo-Mt-Pinatubo-08
	Aerial View Pinatubo Pyroclastic Deposits ERUPTION PHASES  Aerial view on June 22 of the northeast side of Pinatubo showing valley-filling pyroclastic deposits from the June 15 eruption. Thick pyroclastic-flow deposits (locally more than 200 m) occurred in main valleys at distances of 12-18 km from the caldera, and caused surface-drainage diversions. These flows were accompanied by ash clouds whose deposits ranged in thickness from a few to tens of centimeters. Photo credit: R.P. Hoblitt, U.S. Geological Survey.  File: vo-Mt-Pinatubo-09
	Aerial View of Small Explosion in Pinatubo Crater ERUPTION PHASES Aerial view to the south of Pinatubo crater (3 km wide) showing the start of a small explosion on August 1. Declining volcanic activity continued through October, 1991. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-10
	Aerial View of Crater Lake ERUPTION PHASES  Aerial view to the south of Pinatubo crater showing the crater lake on September 10. The turquoise-blue caldera lake is emitting steam along its south wall. Fresh landslides have occurred on the steep-sided caldera walls. Photo credit: T.C. Pierson, U.S. Geological Survey.  File: vo-Mt-Pinatubo-11
	Effects of Ash Fall on Plane STRUCTURAL DAMAGE DUE TO VOLCANIC ASH  Heavy ash fall caused this World Airways CD-10 airplane at Cubi Point Naval Air Station to set on its tail. The ash cloud caused eleven commercial aircraft emergencies. Engines were destroyed in three of the eleven aircraft. Most of the incidents occurred at night or during other periods of low visibility about 600 miles southwest of the volcano. The photograph was taken on June 17. Photo credit: R.L. Rieger, U.S. Navy.  File: vo-Mt-Pinatubo-12
	Effects of Ash Fall at Clark Air Base STRUCTURAL DAMAGE DUE TO VOLCANIC ASH Patio area at the Officer's Club at Clark Air Base as photographed on July 31. Pumice particles as large as 7cm in diameter fell at this location, 25 km east of the crater. Particles roughly 1.5 cm in diameter fell 33 km south- southwest (at Olongapo). Over 150 buildings were totally destroyed. Damage to Clark Air Base was estimated at $600 million. The eruption turned Clark Air Base and Subic Bay Naval Station (twice as far from the volcano) into wastelands buried in 15-30 cm of white ash. Vegetation was stripped and many buildings were merely twisted pieces of metal. Despite the widespread damage, all mission-essential personnel returned to Clark Air Base within 48 hours of the June 15 eruption. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-13
	Effect of Ash Fall on Native Hut STRUCTURAL DAMAGE DUE TO VOLCANIC ASH  A native house on the flanks of Pinatubo on June 28 after the major eruption of June 15. Within 3 km of the caldera rim, ash thicknesses averaged one to two meters. Photo credit: R. P. Hoblitt, U.S. Geological Survey.  File: vo-Mt-Pinatubo-14
	Resettlement Camp EVACUATION AND RESETTLEMENT  Aerial view taken on August 12 of the main resettlement camp for victims of the Pinatubo eruption. By this date more than 46 people in evacuation camps had died of illnesses. Prior to the eruption, almost 55,000 people had evacuated their homes to places outside of the danger area. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-15
	Aerial View of Valley Filled with Mudflows MUDFLOWS AND LAHARS Aerial view of the Sacobia River drainage on August 15. The valley was filled by June 15 deposits and was, at the time of this photo, being partially reworked by mudflows. Steam rises from the still hot interiors of the June 15 deposits. Five thousand people were evacuated from Tabon in the Pampanga region; 96 houses were washed away on August 14. Debris reached a depth of three meters. Mudflows on August 18 prompted another large evacuation with 3,000 people fleeing six towns in the Pampanga and Tarlac regions on the volcano's east flank. Up to 55,000 people evacuated their homes because of the August 20 mud flows. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-16
	Scientists Observe a Hot Mudflow MUDFLOWS AND LAHARS  Scientists observe a hot mudflow (lahar) in the Sacobia Valley below the Mactan Gate of Clark Air Base on August 14. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-17
	Mudflow Collapses Bridges in Angeles City MUDFLOWS AND LAHARS  Aerial view of the Abacan River channel as it passes through Angeles City near Clark Air Base on August 12. A mudflow has caused the collapse of main bridges across the Abacan River. Note makeshift bridges for pedestrians. On August 20, mudflows five meters high traveled down ten rivers damaging more than 9,000 houses and three bridges, and killing 31 people. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-18
	Aerial View Flooding in Village FLOODING Aerial view of flooding in the village of Santa Rita de Concepcion on July 23. The main flow of the Bamban River has migrated from its channel to cause extensive flooding in villages downstream from the volcano. Photo credit: T.J. Casadevall, U.S. Geological Survey.  File: vo-Mt-Pinatubo-19
	Satellite Photos Showing Movement of Aerosol Cloud AEROSOLS AND PARTICULATES IN THE STRATOSPHERE  AVHRR satellite data depicting the thickness of aerosol optical products measured on May 28 to June 5, 1991, and again after the Pinatubo eruption on July 4 to July 10, 1991 (about 20 days following the major eruption). Ten days after the June 15 eruption the aerosol cloud formed a nearly continuous band that stretched 11,000 km from Indonesia to Central Africa. Image credit: Dr. Larry Stowe, NOAA/NESDIS Office of Research and Applications.  File: vo-Mt-Pinatubo-20