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U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM HAZARDS The Cataclysmic 1991 Eruption of Mount Pinatubo,

he second-largest volcanic Teruption of this century, and by far the largest eruption to affect a densely populated area, occurred at Mount Pinatubo in the Philip- pines on June 15, 1991. The erup- tion produced high-speed ava- lanches of hot ash and gas, giant mudflows, and a cloud of hundreds of miles across. The impacts of the eruption continue to this day.

Precursors to the 1991 Eruptions On July 16, 1990, a magnitude 7.8 earth- quake (comparable in size to the great 1906 San Francisco, California, earthquake) struck about 60 miles (100 kilometers) northeast of Mount Pinatubo on the island A huge cloud of volcanic ash and gas rises above Mount Pinatubo, Philippines, on June 12, 1991. Three of in the Philippines, shaking and days later, the volcano exploded in the second-largest volcanic eruption on Earth in this century. Timely forecasts of this eruption by scientists from the Philippine Institute of and Seismology and squeezing the Earth’s crust beneath the vol- the U.S. Geological Survey enabled people living near the volcano to evacuate to safer distances, sav- cano. At Mount Pinatubo, this major earth- ing at least 5,000 lives. quake caused a landslide, some local earth- quakes, and a short-lived increase in steam disturbed. In March and April 1991, how- The Eruptions emissions from a preexisting geothermal ever, molten rock () rising toward the From June 7 to 12, the first magma area, but otherwise the volcano seemed to surface from more than 20 miles (32 kilo- reached the surface of Mount Pinatubo. be continuing its 500-year-old slumber un- meters) beneath Pinatubo triggered small Because it had lost most of the gas con- earthquakes and caused powerful tained in it on the way to the surface (like steam that blasted three a bottle of soda pop gone flat), the magma craters on the north flank of the vol- oozed out to form a but did not cano. Thousands of small earth- cause an explosive eruption. However, on quakes occurred beneath Pinatubo June 12 (Philippine Independence Day), through April, May, and early June, millions of cubic yards of gas-charged and many thousand tons of noxious magma reached the surface and exploded gas were also emitted in the reawakening volcano’s first spec- by the volcano. tacular eruption. When even more highly gas charged magma reached Pinatubo’s surface on June 15, the volcano exploded in a cata- clysmic eruption that ejected more than 1 Following Mount Pinatubo’s cataclysmic June 15, 1991, eruption, thousands of roofs collapsed cubic mile (5 cubic kilometers) of mate- under the weight of ash made wet by heavy rial. The ash cloud from this climactic rains (see example in photo above). Ash depos- eruption rose 22 miles (35 kilometers) its from the eruption have also been remobi- lized by monsoon and typhoon rains to form gi- into the air. At lower altitudes, the ash was ant mudflows of volcanic materials (), blown in all directions by the intense cy- which have caused more destruction than the clonic winds of a coincidentally occurring eruption itself (photo at right shows village bur- ied by lahars). (Photo above courtesy of Peter typhoon, and winds at higher altitudes Baxter, University of Cambridge.) blew the ash southwestward. A blanket of

U.S. Department of the Interior USGS Fact Sheet-113-97 U.S. Geological Survey Reprinted, 1998 PHILIPPINES

Extent of ash cloud MOUNT PINATUBO from the Cabanatuan June 15, Tar lac 1991 eruption r ll Rive ne r n e o iv 'D R O a g n a River p n m ba a am P volcanic ash (sand- and silt-size grains of B The June 15, 1991, explo- ia- sive eruption of Mount cob volcanic minerals and glass) and larger Sa Pinatubo, Philippines, (frothy pebbles) blanketed (U.S.) was the second largest Pa s volcanic eruption of this the countryside. Fine ash fell as far away ig Angeles MOUNT - P PINATUBO ot City century and by far the as the , and satellites tracked re a ro ll largest eruption to affect e R r r i a e v a densely populated the ash cloud several times around the iv e San Fernando M R r area. The eruption pro- globe. BASA AIR BASE duced high-speed ava- Huge avalanches of searing hot ash, lanches of hot ash and er iv gas, and pumice fragments (pyroclastic R gas (pyroclastic flows), a g giant mudflows (lahars), flows) roared down the flanks of Mount n a p and a cloud of volcanic m Pinatubo, filling once-deep valleys with Y a A P ash hundreds of miles S B NAVAL STATION (U.S.) fresh volcanic deposits as much as 660 O C CUBI POINT across. (U.S. bases have I U B NAVAL AIR STATION (U.S.) U reverted to Philippine feet (200 meters) thick. The eruption re- T S H MOUNT control since 1991.) moved so much magma and rock from be- NATIB C low the volcano that the summit collapsed H Manila I N to form a large volcanic depression A MANILA BAY S () 1.6 miles (2.5 kilometers) E A MARIVELES across. 0 10 MILES Pyroclastic flows Much weaker but still spectacular Lahars eruptions of ash occurred occassionally 0 10 KILOMETERS through early September 1991. From July to October 1992, a lava dome was built in 5,000 lives and at least $250 million in the eruptions have returned home but face the new caldera as fresh magma rose from property. Commercial aircraft were continuing threats from lahars that have deep beneath Pinatubo. warned about the hazard of the ash cloud already buried numerous towns and vil- from the June 15 eruption, and most lages. Rice paddies and sugar-cane fields Continuing Hazards avoided it, but a number of jets flying far that have not been buried by lahars have Even after more than 5 years, hazard- to the west of the Philippines encountered recovered; those buried by lahars will be ous effects from the June 15,1991, cli- ash and sustained about $100 million in out of use for years to come. mactic eruption of Mount Pinatubo con- damage. Although much equipment was tinue. The thick, valley-filling successfully protected, structures on the Chris Newhall, James W. Hendley II, and pyroclastic-flow deposits from the erup- two largest U.S. military bases in the Phil- Peter H. Stauffer tion insulated themselves and have kept ippines—Clark Air Base and Subic Bay Graphic design by Sara Boore and Susan Mayfield much of their heat. These deposits still Naval Station—were heavily damaged by Banner design by Bobbie Myers had temperatures as high as 900°F ash from the volcano’s climactic eruption. COOPERATING ORGANIZATIONS (500°C) in 1996 and may retain heat for Nearly 20 million tons of sulfur diox- decades. When water from streams or un- ide were injected into the stratosphere in Armed Forces of the Philippines derground seepage comes in contact with Pinatubo’s 1991 eruptions, and dispersal National Disaster Coordinating Council, Philippines these hot deposits, they explode and of this gas cloud around the world caused Philippine Institute of Volcanology and Seismology spread fine ash downwind. Since the cli- global temperatures to drop temporarily U.S. Agency for International Development mactic 1991 eruption, ash deposits have (1991 through 1993) by about 1°F United States Air Force also been remobilized by monsoon and ty- (0.5°C). The eruptions have dramatically United States Navy phoon rains to form giant mudflows of changed the face of , home volcanic materials (lahars). to about 3 million people. About 20,000 indigenous Aeta highlanders, who had For more information contact: U.S. Geological Survey Impacts of the Eruptions lived on the slopes of the volcano, were Cascades Volcano Observatory Fortunately, scientists from the Philip- completely displaced, and most still wait 5400 MacArthur Blvd., Vancouver, WA 98661 pine Institute of Volcanology and Seis- in resettlement camps for the day when Tel: (360) 696-7693, Fax: (360) 696-7866 http://volcanoes.usgs.gov/ mology and the U.S. Geological Survey they can return home. About 200,000 See also Benefits of Volcano Monitoring Far Outweigh had forecast Pinatubo’s 1991 climactic people who evacuated from the lowlands Costs—the Case of Mount Pinatubo (USGS Fact Sheet eruption, resulting in the saving of at least surrounding Pinatubo before and during 115-97)

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