Mount Pinatubo

The eruption of Mount Pinatubo on June 15, 1991, was one of the most violent eruptions of the twentieth century. The use of modern techniques of eruption prediction and danger assessment allowed the safe evacuation of nearly 150,000 people from the area adversely affected by the eruption.

Dormant Since 1541

Mount Pinatubo, on the island of Luzon in the Philippines, is one of the nearly three hundred active volcanoes that rim the Pacific Ocean Basin in a narrow belt called the Ring of Fire. This stratovolcano had been dormant since the Spanish first settled the Philippines in 1541. The awakening of Mount Pinatubo was of considerable concern to the United States because Clark Air Base was located only 20 kilometers (12 miles) to the east of the summit, and the Subic Bay Naval Station was 40 kilometers (25 kilometers) to the southwest.

Pinatubo rumbled into activity on April 2, 1991, when a series of small explosions formed a row of craters 1.5 kilometers (about 1 mile) long just northwest of the volcano's summit. The explosions lasted several hours and deforested an area of several square kilometers. Villagers living on the volcano's flank reported hearing the explosions and awoke the morning of April 3 to find a row of fumaroles extending along the western side of Pinatubo. Civil officials evacuated a circular area in a 10-kilometer (six-mile) radius from the volcano's summit.

Seismic Wake-up

Throughout April and the early part of May, Mount Pinatubo experienced dozens of small earthquakes each day as magma began to move beneath the volcano. Magma fractured the brittle rocks as it cleared a pathway upward to form a shallow magma body. The center of the earthquake activity was about five kilometers (three miles) northwest of the summit and four kilometers (2 1/2 miles) below the surface.

During May, earthquake activity increased, with more than 1,800 low-intensity quakes being recorded between May 7 and June 1. During the last two weeks of May, the fumaroles showed a tenfold increase in the emission of sulfur dioxide.

At the beginning of June, a second area began to experience a series of shallow earthquakes several kilometers closer to the summit. The magma was rising beneath the volcano, and it was forcing a conduit to form above the magma chamber. The rate of emission of gas reversed its May trend, decreasing from a production of 5,000 tons of sulfur dioxide on May 28 to a production of 1,800 tons on May 30. By June 5, the total had dropped to only 260 tons. Apparently, the rising magma had sealed the fractures that allowed the escaping gases to reach the fumaroles.

Harmonic Tremors

On June 3, 1991, a series of small explosions blew ash from the summit area, and the earthquakes moved into a type of activity called “harmonic tremor”: a prolonged rhythmic shaking of the earth, in contrast to the single sharp jolts of earlier earthquakes. Harmonic tremor occurs when underground magma flows in a more continuous manner through established subsurface channels. On June 6, the summit region of the volcano began to bulge outward, with the upper flank showing a measurable increase in the slope of the land surface. As the tilting increased, there was an increase in earthquake activity, leading to a strong shock and explosion on June 7. This explosion sent a column of ash and steam to a height of between seven and eight kilometers (four and five miles). The explosion allowed the summit area to stop its inflation temporarily, and there was a brief period of reduced earthquake activity. Based upon these warning signs, the area of evacuation was extended to a radius of 15 kilometers (nine miles).

Magma first appeared on the surface of the volcano the next morning, June 8. Observers reported that a small lava dome 150 meters (492 feet) in diameter was growing near one of the fumaroles just northwest of the summit. The vents associated with this dome emitted a series of weak ash clouds that rose only to the level of Pinatubo's summit over a three-day period, ending on June 12. During this interval, harmonic tremors occurred almost continuously. An examination of the composition of ejected ash revealed that a new, very fluid magma had invaded an old magma chamber that contained residual magma from the last eruption nearly five hundred years ago. This intruding magma signaled the potential of a major eruption, and authorities extended the evacuation area to a radius of 20 kilometers (66 feet) from the volcano. On June 10, 14,000 U.S. military personnel left Clark Air Base along with their aircraft, never to return. They left behind three helicopters and a contingent of fifteen hundred security and maintenance personnel.

Multiple Eruptions

The first major explosion of Pinatubo occurred at 8:51 a.m. on June 12, signaling the final and most powerful phase of the 1991 eruption. This very violent phase lasted about ten days, with the most intense activity occurring on June 15 and 16. The June 12 eruption lasted thirty-five minutes and spewed a column of steam and ash to a height of 19 kilometers (12 miles). A small pyroclastic flow (a turbulent, dense mixture of volcanic gases, magma, ash, and rock that is ejected from a vent and flows rapidly down the flanks of a volcano) traveled a short distance from the vent down the northern flank of the volcano into already evacuated villages along the Maraunot River. Clark Air Base evacuated six hundred maintenance personnel, and Filipino authorities extended the evacuation zone to a 30-kilometer radius. Ash from the eruption was so dense and spread so far that the airport in Manila, more than 50 kilometers (31 miles) away, was forced to close.

Similar explosions continued over the next few days, producing ash clouds that prevented visibility of the volcano. The arrival of Typhoon Yunya on June 13 meant that most of these explosions were completely unseen from the ground. Seismic stations recorded the associated earthquake activity, and military weather radar verified the presence of an eruption cloud.

Beginning on June 14, the main, violent phase peaked in a three-day period with more than fifty short, violent explosions in which numerous vents erupted simultaneously. The intensity of the earthquakes generated by these eruptions was more than a hundred times greater than those recorded during May. These eruptions grew progressively more violent during this time, sending ash to a height of more than 40 kilometers (25 miles). At 5:55 a.m. on June 15, the ejected ash switched from a vertical orientation to a more horizontal one. The onset of laterally directed explosions indicated that the summit region was beginning to collapse. Several pyroclastic flows erupted and traveled up to 13 kilometers (eight miles) from the summit. The remaining personnel at Clark Air Base were evacuated that morning.

At about 3:30 p.m. on June 15, the climax occurred. A series of strong earthquakes started, lasting all afternoon and throughout the night. The seismographic equipment on the volcano was destroyed during the night by a series of large pyroclastic flows. Pyroclastic deposits that exceeded 200 meters (656 feet) in thickness formed in many valleys around the volcano. A 2-kilometer (1-mile) caldera formed when the summit area collapsed, causing the volcano to drop by more than 300 meters (984 feet) in elevation.

Ashfall, Mudflow, and Poisonous Gas

About 0.8 cubic kilometers (1/2 cubic mile) of ejected material covered the west-central portion of the island of Luzon. The falling ash blanketed villages and buried crops around the volcano. Many rooftops collapsed under the added weight of the ash, causing the majority of deaths attributed to the volcanic eruption. The total volume of ash ejected was close to five cubic kilometers (three cubic miles), most of which fell into the South China Sea.

The water runoff from the heavy rains associated with the typhoon mixed with the recently fallen ash and caused disastrous volcanic mudflows known as “lahars.” The lahars swept down twelve river valleys, including the Abacan and Sacobia River Valleys, where the town of Angeles City was completely destroyed. These mudflows damaged seven other towns. The July monsoons caused secondary lahars, one of which inundated the city of Pabanlog along the Gumain River. All these lahars destroyed bridges and farmland along the floodplains and caused widespread economic and social disruption. Special early-alert systems detected the lahars and sharply reduced the loss of life. However, the region continued to experience secondary lahars caused by heavy rains, and this threat was expected to last well into the twenty-first century.

The cloud associated with the June 15 eruption released four billion pounds of chlorine gas and forty billion pounds of sulfur dioxide into the stratosphere. By June 25, satellite images showed that a 7,750-kilometer-long (4,815 mile-long) cloud of sulfur dioxide had spread across the tropical Northern Hemisphere.

Damages and Death Toll

After June 16, the eruption slowly grew less intense. Occasional ash explosions into 1992 sent ash columns upward of 10 kilometers in height. During July and August of 1991, a dome grew in the fuming caldera that was 300 meters (984 feet) across and nearly 100 meters (328 feet) high. With an average of more than twenty centimeters of ash covering the region around Clark Air Base, base personnel were unable to stop ash infiltration into the jet engines, and the base had to be abandoned.

More than 108,000 homes were partially or totally destroyed. The final death toll for the region was 722; of these, 281 died as a result of ejected material either as ashfalls or pyroclastic flows, 83 died from primary and secondary lahars, and 358 died from disease related to the social turmoil associated with the interruption of the country's infrastructure. The loss of life was undoubtedly compounded by the typhoon that hit the island during the climax of the eruption.

Global Impact

The world's largest volcanic eruptions have all produced global climate and atmospheric changes. Fine volcanic ash and gases are blasted into the high atmosphere and dispersed around the world. Upper-level winds can keep volcanic ash suspended in the atmosphere for many years. The suspended ash from Mount Pinatubo's eruption produced colorful sunsets worldwide in 1991 and 1992.

The gases and ash combined to produce an aerosol that blocked both incoming sunlight and infrared radiation emitted from the earth. The loss of solar radiation causes cooling, whereas the absorption of the earth's transmitted infrared radiation leads to global warming. The two effects did not balance equally, and by 1993, measurements from the National Aeronautics and Space Administration's Earth Radiation Budget Satellite provided the first conclusive evidence of a significant change in global energy as the result of a volcanic eruption. The net effect of a loss of solar radiation and a greater retention of infrared radiation resulted in a period of global cooling in which the average global temperature dropped by one-half degree Celsius (about one degree Fahrenheit). Approximately 2 to 3 percent of the sun's energy was blocked out, counteracting prevailing global warming trends and temporarily setting the earth's climatological clock back to the 1950s.

The gases also altered the chemistry of the upper atmosphere. Three months after the eruption, there was 50 percent less ozone in the tropical stratosphere over an area roughly coincident with Mount Pinatubo's volcanic plume. The ozone layer over the United States was 10 percent thinner than normal, translating to a 20 to 30 percent increase in the amount of cancer-causing ultraviolet radiation reaching the earth's surface. There was fear that an ozone hole might open over the populated areas of the Northern Hemisphere. By 1996, however, measurements of the levels of ozone-depleting chemicals had dropped to lower-than-average values.

Assessing the Response

The successful evacuation of nearly 150,000 people during the Mount Pinatubo eruption required both successful short-term forecasting of eruption events and a well-organized program of danger assessment. The activities that transpired during the ten weeks from the first steam explosion on April 2 to the culminating eruption of June 15 were a model of cooperation between scientific personnel, Filipino authorities, and the local citizenry. Immediately following the first gas explosion, volcanologists from the U.S. Geological Survey joined forces with other scientists from the Philippine Institute of Volcanology and Seismology to coordinate the scientific work of eruption prediction. They jointly established the Pinatubo Volcano Observatory (PVO).

Modern eruption prediction involves determining the nature of the volcano's past eruptions and monitoring the active volcano for any changes in its physical and chemical behavior. An array of scientific equipment must be deployed around the volcano and the active vents to measure earthquake activity, volumes and composition of emitted gases, changes in the slope of the land surface, changes in the horizontal distance across vents and fissures, and fluctuations in the temperature of the volcano.

Active volcanoes have sporadic eruptions that are separated by time intervals called “periods of repose.” Most active volcanoes display a repetitive history in terms of their repose interval, their eruptive violence, and the kinds of materials that they expel. The nature of a future eruption can often be established by an examination of the geological record of previous eruptions.

The PVO team realized the value of knowing the previous eruptive history at Mount Pinatubo and did a rapid geological reconnaissance of the volcano during the month following the initial steam explosion. They found that three previous eruptions had occurred about 500 years ago, 2,500 years ago, and 4,800 years ago. Each eruption had been dominated by highly explosive activity. Pyroclastic flows had swept down the volcano's flanks, leaving deposits more than 20 kilometers (12 miles) from the summit. Debris from lahars from these prehistoric eruptions were found in six river valleys leading away from the volcano for more than 40 kilometers (25 miles).

On the basis of their geological study of past eruptions, the PVO team compiled a hazard map showing the regions that were likely to be affected by pyroclastic flows, ashfall, and lahars. The map was used by both civil defense officials and military commanders during the various stages of the eruption. A map of the areas affected by the actual 1991 eruption shows a remarkable correlation with the pre-eruption hazard map. A videotape depicting the various volcanic hazards was produced by filmmaker and geologist Maurice Krafft, and the film aired repeatedly on local television.

Danger assessment is a complex task that goes beyond the accurate forecasting of eruptions. For example, in 1985, the Nevado del Ruiz volcano erupted in a region of Colombia that was much less populated than the area around Mount Pinatubo. The eruption generated a mudflow that killed 25,000 people. Although volcanologists were successful in predicting the eruption and recommended evacuation of the region, the civil authorities did not view the impending eruption to be particularly dangerous, and no evacuation occurred.

The PVO team worked closely with civil and military personnel to assess the level of danger posed by the various stages of the Mount Pinatubo eruption. Using the hazard map and the video, the Filipino scientists developed a five-level alert system. Alert level 1 was to be declared when low-level seismic activity was coupled with fumarolic emission. The level 1 alert occurred on April 3, when fumaroles developed in a row along the northwest flank of Mount Pinatubo. Alert level 2 was to be invoked when moderate levels of seismic activity occurred and there was positive evidence for the existence of subsurface magma. Level 2 was reached in late May, when 1,800 earthquakes were recorded within a three-week interval.

Authorities were instructed to interpret alert level 3 to mean that the magma was intruding into the volcano and could eventually lead to a major eruption. Level 3 was to be issued when high levels of gas emission occurred with simultaneous ground deformation. This level of danger was attained on June 5. Level 3 meant that a major pyroclastic eruption could occur within two weeks; it actually occurred ten days later. Alert level 4 was to be issued when extensive harmonic tremors indicated the magma was moving more freely beneath the volcano. Level 4 was announced immediately following the June 7 explosion at Pinatubo that generated the seven-kilometer-high (four-mile-high) ash column. Civil authorities were told that level 4 meant that a major pyroclastic eruption was possible within twenty-four hours. The highest level of danger assessment was alert level 5, which meant that an eruption was in progress. This level was issued on June 9, after a pyroclastic flow engulfed several evacuated villages on the northern flank of Pinatubo. The day after the level 5 alert was issued, the U.S. military evacuated fourteen thousand personnel from Clark Air Base.

Lessons Learned

The violent eruption of Mount Pinatubo was classified as a magnitude 6. The largest recorded eruption has been classified as magnitude 7, and the largest geologically known eruption has been ranked as a magnitude 8. Such violent eruptions as these have historically caused high death tolls. However, as shown by Mount Pinatubo experience, the most dangerous volcanoes can cause far fewer deaths when proper surveillance is employed.

There are nearly 300 active volcanoes that should be monitored for the eruption of pyroclastic flows and lahars, and many of these volcanoes are located in underdeveloped countries. Surveillance and risk-reduction programs are very expensive, requiring equipment, trained scientists, education programs for local populations, and development of cooperative military and civil planning. The expenses associated with such programs are beyond the capabilities of many less-developed countries. Consequently, the United States, Russia, Iceland, Italy, and Japan lead the world in this field of volcanology.

The United States has two facilities dedicated to monitoring active volcanoes, both staffed by geologists from the U.S. Geological Survey. The Hawaiian Volcano Observatory has been successfully predicting eruptions of Mount Kilauea for more than forty years. After the eruption of Mount St. Helens in 1980, the Cascades Volcano Observatory was established to monitor magma amounts and movements beneath the active volcanoes in Washington, Oregon, and northern California. Japan has twenty-one volcano observatories.

It is very likely that Mount Pinatubo will erupt again, although the volcano has entered a period of repose that may last as long as a few hundred years. The volcano is located on the margin of two converging plates that have produced a very active chain of volcanoes. For example, Mount Mayon lies 300 kilometers (186 miles) to the south of Pinatubo and has had more than forty historical eruptions, with the last as recent as 1993. A key factor indicating continued life for Mount Pinatubo is that a 1996 study found that between 40 and 100 cubic kilometers (25 and 62 cubic miles) of magma remained in a reservoir below the volcano's summit. This volume is the largest quantity of magma (by almost a factor of three) ever detected beneath any volcano. The volcano experienced a weak explosion in 2021, which caused concern. It was discovered to be caused by surface level hydrothermal fluids rather than true volcanic activity.

Principal Terms

ash: small fragments of volcanic material less than 2 millimeters in diameter formed by explosive ejection from a vent

caldera: a large circular depression around a summit vent that typically forms by collapse when large volumes of magma are rapidly ejected

dome: a small, steep-sided mass of volcanic rock formed from nonexplosive, viscous lava that solidifies in or above a vent

fumarole: a volcanic vent from which only gases are emitted

harmonic tremor: a type of earthquake activity in which the ground undergoes continuous shaking in response to subsurface movement of magma

lahar: a volcanic mudflow resulting from the mixing of erupted lava and ash with surface water, rain, or melted snow

pyroclastic flow: a turbulent, dense mixture of volcanic gases, magma, ash, and rock that is ejected from a vent and flows rapidly down the flanks of a volcano

stratovolcano: a large, steep-sided volcano consisting of alternating layers of coherent lava and explosively ejected fragmental material; also called a “composite volcano”

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