REDUCING THE RISK FROM VOLCANO HAZARDS Lahars of Mount Pinatubo, Philippines n June 15, 1991, Mount O Pinatubo in the Philip- pines exploded in the second largest volcanic eruption on Earth this century. This eruption deposited more than 1 cubic mile (5 cubic kilome- ters) of volcanic ash and rock fragments on the volcano’s slopes. Within hours, heavy rains began to wash this material down into the sur- rounding lowlands in giant, fast-moving mudflows called lahars. In the next four rainy seasons, lahars carried about half of the deposits off the volcano, causing even more destruction in the lowlands In August 1991, 2 months after Mount Pinatubo, Philippines, exploded in a cataclysmic volcanic eruption, this house (see roof) along the Sacobia-Bamban River 15 miles from the volcano was buried by a fast-moving than the eruption itself. mudflow of volcanic ash and debris (lahar) caused by heavy rains. Inset photo shows the house before it was buried. (Photos by Raymundo S. Punongbayan, Philippine Institute of Volcanology and Seismology.) Background nology and Seismology (PHIVOLCS) inten- fragments—making them dense and viscous Before April 1991, when steam explo- sified their monitoring of the volcano; they like wet concrete—they actually flow faster sions blasted out three small craters on the were soon joined by scientists from the U.S. than clear-water streams. These mudflows north flank of Mount Pinatubo on the island Geological Survey (USGS) and other orga- can rush down the flanks of a volcano at of Luzon in the Philippines, the volcano had nizations. While evaluating the threat from speeds as great as 40 miles (65 kilometers) been dormant for 500 years, allowing lush the volcano, these scientists discovered that per hour and can travel more than 50 miles tropical vegetation to cover its slopes. Even- the gentle slopes surrounding Pinatubo had (80 kilometers). Lahars that contain the most tually, the population of the region grew to largely been created by countless giant mud- debris (as much as 90%) move the fastest and nearly 1,000,000 people, as towns, cities, and flows of volcanic debris (lahars) from the are the most destructive. hundreds of villages, as well as the largest powerful ancient eruptions that built the vol- U.S. military base in the Philippines (Clark cano. They also concluded that lahars would Conditions that Create Lahars at Pinatubo Air Base), were built on the broad gentle again rush down Pinatubo’s slopes in its next Annual rainfall at Mount Pinatubo ranges slopes surrounding the volcano. major eruption, threatening the communities from 80 inches (2,000 millimeters) on the When Pinatubo reawakened in April 1991, that were built while it slumbered. volcano’s northeast flank to more than 160 scientists from the Philippine Institute of Volca- inches (4,000 millimeters) on its summit and Nature and Causes of Lahars southwest flank. Nearly all of this rain falls Explosive eruptions can deposit huge during the time of the southwest monsoon amounts ash and other volcanic debris on a (June to September) and the overlapping ty- volcano’s slopes. Lahars form when water phoon season (June to October). Rainfall dur- from intense rainfall, melting snow and ice, ing typhoons can be as intense as 2 inches or the sudden failure of a natural dam, mixes (50 millimeters) per hour. Since the volcano's with this loose volcanic material, creating cataclysmic June 1991 eruption, the maxi- mudflows that can be particularly dangerous mum rainfall at Pinatubo in one 24-hour pe- and destructive. Although lahars contain at riod was more than 30 inches (780 millime- least 40% (by weight) volcanic ash and rock ters)—as much rain as many parts of the The village of Lourdes, built on lowlands 15 miles United States receive in a whole year! northeast of Mount Pinatubo, was inundated by Ash and other deposits from Mount giant, fast-moving mudflows of volcanic debris Pinatubo’s 1991 eruption destroyed or bur- (lahars) in the months following the volcano’s ied much of the lush vegetation that had cov- cataclysmic June 1991 eruption. Since that eruption, lahars have destroyed the homes of more than ered the volcano. When rainfall on Pinatubo 100,000 people in the area surrounding Pinatubo. exceeded about 0.5 inches (12 millimeters) U.S. Department of the Interior U.S. Geological Survey PHILIPPINES Extent of ash cloud from the MOUNT PINATUBO Cabanatuan June 15, Manila Tarlac 1991 in 30 minutes, rapid runoff down the still bare eruption Within hours of Mount r Pinatubo’s explosive slopes quickly grew into torrents that became ll Rive ne r n e June 15, 1991, o iv lahars by eroding these deposits. Fortunately, 'D R eruption, heavy rains O a g n began to wash the ash vegetation is beginning to grow on the 1991 a River p an m and debris deposited by deposits, and new stream channels are be- mb a Ba P this explosion down into coming established, reducing the runoff and ia- the surrounding cob Sa CLARK erosion that create lahars. AIR BASE lowlands in giant, fast- (U.S.) moving mudflows called Pa s lahars. By 1993, lahars MOUNT ig Angeles Lahars at Pinatubo Since 1991 - P had already caused PINATUBO ot City re more devastation in the The volume of Pinatubo’s lahars stag- a ro ll e R r r i lowlands than the a e v gers the mind. In the first few years fol- iv e San Fernando M R r eruption itself. lowing the cataclysmic 1991 eruption, they BASA Fortunately, a system have deposited more than 0.7 cubic miles AIR BASE to monitor and warn of er lahars was rapidly (3 cubic kilometers; equivalent to 300 mil- iv R established in the days a g lion dump-truck loads) of debris on the n a following the eruption Olongapo p m by the Philippine lowlands surrounding the volcano, bury- Y a A SUBIC BAY P S B NAVAL STATION (U.S.) Institute of Volcanol- ing hundreds of square miles of land. Dur- O C CUBI POINT I ogy and Seismology, U B NAVAL AIR STATION (U.S.) ing heavy rains, lahars at Pinatubo can U U.S. Geological Survey, T S H MOUNT and other organizations. transport and deposit tens of millions of NATIB C This system has saved cubic yards of mud in a single day. H Manila hundreds of lives by I yy When the largest and fastest lahars reach N enabling warnings to A MANILA BAY the lowlands surrounding Pinatubo, they have S be sounded for most E yy but not all major lahars speeds of more than 20 miles (32 kilome- A MARIVELES MOUNTAIN at Pinatubo. (U.S. ters) per hour, are as much as 30 feet (10 0 10 MILES bases have reverted to meters) thick and 300 feet (100 meters) wide, Lahars Philippine control since 0 10 KILOMETERS and can transport more than 35,000 cubic feet 1991.) (1,000 cubic meters) of debris and mud per raced to trap the sediment behind small rainfall in lahar source regions, acoustic second. Within minutes, such powerful la- dams and to confine it between levees, but flow monitors on stream banks detect hars can erode many feet into the banks or most of these measures were too small to ground vibration as lahars pass, and floor of a channel. They can carry large boul- contain the lahars. Near the base of the manned watchpoints further confirm that ders, cars, or even bridges and can destroy or volcano, lahars eroded whole city blocks, lahars are rushing down Pinatubo’s slopes. bury almost anything in their paths. When and in the lowlands they inundated and bur- This system has enabled warnings to be lahars reach low-lying areas, they spread out, ied entire towns. sounded for most but not all major lahars at slow down, and deposit their vast loads of Since the 1991 eruption, lahars from Pinatubo, saving hundreds of lives. debris over many square miles. Pinatubo have destroyed the homes of more One particularly dangerous type of lahar than 100,000 people. With most old stream Chris Newhall, Peter H. Stauffer, and has occurred at Mount Pinatubo as a result channels filled, lahars can now spread James W. Hendley II of the damming of stream channels and tribu- widely on the gentle slopes and lowlands Graphic design by tary valleys by the 1991 ash deposits or by surrounding Mount Pinatubo, threatening Sara Boore and Susan Mayfield subsequent lahars, creating temporary lakes. successors to the early dams and levees. After a few weeks or months, the lakes over- Fortunately, by 1995, the volume of mate- top and quickly erode through their dams, rial carried by Pinatubo’s lahars had al- COOPERATING ORGANIZATIONS Armed Forces of the Philippines releasing large volumes of water that gener- ready decreased to less than a quarter of National Disaster Coordinating Council, ate major lahars. Because these large and that carried in 1991, and new towns had Philippines Philippine Institute of Volcanology and Seismology powerful mudflows can occur without warn- been built on high ground for many of those U.S. Agency for International Development ing, even when there is no rain, they are displaced or threatened by lahars. None- United States Air Force among the most dangerous of Pinatubo’s la- theless, 100,000 people remain at risk from United States Navy hars. One lake on the Pasig-Potrero River, the volcano’s lahars. on the volcano’s east flank, has formed and broken out three times (1991, 1992, and Lahar Warnings For more information contact: 1994), creating giant lahars that have killed Once the potential for lahars at Mount U.S.