The 1991 eruption of Mount Pinatubo in the Philippines' Luzon Volcanic Arc was the second-largest volcanic eruption of the 20th century, after the 1912 eruption of Novarupta in Alaska. Eruptive activity began on April 2 with a series of steam explosions from a fissure that opened on the north side of Mount Pinatubo. Scientists placed seismographs to monitor the volcano for earthquakes. In late May, the number of seismic events under the volcano changed daily. Beginning on June 6, a group of earthquakes that became closer to the surface, along with ground movement on the upper east side of the mountain, led to the formation of a small lava dome.

On June 12, the volcano erupted for the first time in a dramatic way, sending an ash column 19 km (12 mi) into the atmosphere. More explosions happened overnight and on June 13. Seismic activity during this time became very strong. On June 15, highly gas-rich magma reached the surface, causing the volcano to erupt violently and sending an ash cloud 40 km (25 mi) into the atmosphere. Volcanic ash and pumice covered the surrounding area. Powerful pyroclastic flows moved down the sides of Pinatubo, filling valleys with volcanic deposits up to 200 m (660 ft) thick. The eruption removed so much magma and rock from beneath the volcano that the summit collapsed, forming a small caldera 2.5 km (1.6 mi) wide.

Fine ash from the eruption traveled as far as the Indian Ocean, and satellites tracked the ash cloud as it moved around the world multiple times. At least 16 commercial airplanes accidentally flew through the drifting ash cloud, causing about $100 million in damage. The falling ash caused darkness and the sounds of lahars (fast-moving mudflows) rushing down nearby river valleys. Several smaller lahars flooded Clark Air Base, flowing across the base in powerful sheets that hit buildings and scattered cars. Nearly every bridge within 30 km (19 mi) of Mount Pinatubo was destroyed. Several lowland towns were flooded or partially covered in mud. More than 840 people died when roofs collapsed under heavy wet ash, and many others were injured.

Rain continued to cause problems for several years, as volcanic deposits were remobilized into secondary mudflows. Damage to bridges, irrigation systems, roads, farmland, and urban areas occurred after each major rainfall. Many more people were affected for a long time by rain-induced lahars than by the eruption itself.

Build-up and evacuations

On July 16, 1990, a powerful earthquake measuring 7.8 on the Richter scale occurred in northern Central Luzon and the Cordilleras. This was the strongest earthquake recorded that year. The earthquake’s center was in the municipality of Rizal, Nueva Ecija, about 100 kilometers (62 miles) northeast of Mount Pinatubo. The earthquake’s movement extended northwest to southeast through three provinces. It also traveled west along the Philippine Fault System to Baguio, a city that suffered severe damage. Baguio is located about 80 kilometers (50 miles) north-northeast of Pinatubo. Scientists suggested this earthquake might have contributed to the 1991 volcanic eruption, though this cannot be proven for certain.

Two weeks after the earthquake, local people saw steam rising from Mount Pinatubo. Scientists who visited the area found only small rockslides, not signs of an eruption about to happen. On March 15, 1991, villagers near the northwest side of the volcano felt a series of earthquakes. Over the next two weeks, more earthquakes occurred, growing stronger. This showed that volcanic activity was likely happening.

On April 2, the volcano became active again. Explosions caused by water and steam occurred near the top of the volcano along a 1.5-kilometer (0.93-mile) long crack. For the next few weeks, small eruptions continued, spreading volcanic ash over nearby areas. Seismographs recorded hundreds of small earthquakes daily. Scientists quickly set up monitoring equipment and studied the volcano’s history. Carbon dating of charcoal found in old volcanic layers showed the last three major eruptions occurred about 5,500, 3,500, and 500 years ago. Geological maps showed that much of the surrounding land was formed by flood deposits from past eruptions.

Volcanic activity increased in May. Scientists measured sulfur dioxide emissions, a gas that comes from magma. These emissions rose from 500 tons (550 short tons) per day on May 13 to 5,000 tons (5,500 short tons) per day on May 28. This suggested fresh magma was rising beneath the volcano. After May 28, sulfur dioxide emissions dropped sharply, causing concern that magma might be blocked, leading to pressure buildup and a possible violent eruption.

In early June, instruments showed the volcano was slowly expanding, likely because magma was filling the space beneath the summit. At the same time, earthquake activity shifted from deep underground near the northwest side of the volcano to shallow areas just below the summit. This change is often a sign that a volcanic eruption is about to happen.

Because all signs pointed to a major eruption, scientists from the Philippine Institute of Volcanology and Seismology, with help from the United States Geological Survey, worked to warn people of the danger. A false warning could have made people ignore future alerts, but waiting until the eruption began might have caused many deaths. Scientists had to balance timing and accuracy.

Three evacuation zones were created. The innermost zone included areas within 10 kilometers (6.2 miles) of the volcano’s top. The second zone covered areas 10–20 kilometers (6.2–12.4 miles) from the summit. The third zone extended 20–40 kilometers (12–25 miles) from the summit, including Clark Air Base and Angeles City. About 40,000 people lived in the first two zones, and more than 331,000 people lived in the third zone.

Five levels of volcanic alert were established, from level 1 (low-level shaking) to level 5 (a major eruption). Daily alerts were shared through newspapers, radio, television, and NGOs, and directly to people in danger.

Many Aetas, a group of people who lived on the volcano’s slopes, left their homes when eruptions began in April. They gathered in a village about 12 kilometers (7.5 miles) from the summit and moved farther away as eruptions worsened. Some Aetas moved up to nine times in two months before the big eruption. The first official evacuation of the 10-kilometer (6.2-mile) zone began on April 7. Evacuation of the 10–20-kilometer (6.2–12.4-mile) zone started on June 7, when a level 4 alert was issued. A level 5 alert on June 13 triggered evacuation of the 20–40-kilometer (12–25-mile) zone. By June 15, about 60,000 people had left the area within 30 kilometers (19 miles) of the volcano. Most relocated to Metro Manila, with about 30,000 staying at an evacuee camp in Quezon City.

On June 7, the first eruptions of molten rock occurred, forming a lava dome at the volcano’s summit. The dome grew rapidly over five days, reaching a maximum width of about 200 meters (660 feet) and a height of 40 meters (130 feet).

Escalation of eruption

On June 12 at 03:41, a small explosion marked the start of a more violent phase of the eruption. A few hours later, large explosions lasting about 30 minutes created eruption columns that rose over 19 kilometers (12 miles) high. These explosions also caused fast-moving hot flows that traveled up to 4 kilometers (2.5 miles) from the volcano’s summit in some river valleys. A witness at Subic Bay described the explosion as feeling like a sudden rush of pressure. Fourteen hours later, a 15-minute blast sent volcanic material to heights of 24 kilometers (15 miles). Friction in the rising ash column caused frequent volcanic lightning.

A third large eruption began at 08:41 on June 13, following a series of small earthquakes over the previous two hours. This eruption lasted about five minutes, with an eruption column reaching 24 kilometers (15 miles) high. After three hours of calm, seismic activity increased over the next 24 hours, leading to a three-minute eruption at 13:09 on June 14 that created an eruption column 21 kilometers (13 miles) high.

Ash and rock from these four major eruptions spread widely to the southwest of the volcano. Two hours after the last of these eruptions, a series of eruptions lasting 24 hours produced large fast-moving hot flows and surges that traveled several kilometers down river valleys on the volcano’s sides. In total, nearly 400 square kilometers (150 square miles) of land were covered by fast-moving hot flows that spread in all directions, reaching as far as 16 kilometers (9.9 miles) from the volcano. The land closest to the eruption was worn away by these flows, leaving little material behind.

Dacite, a type of volcanic rock, was the main material in the tephra from these eruptions and the final major event. The most common minerals in the rock were hornblende and plagioclase, but a rare mineral called anhydrite (a type of calcium sulfate) was also present. The magma was more oxidized than usual, and the high sulfur content may have been related to the level of oxidation.

The final major eruption began at 13:42 on June 15. It caused many strong earthquakes due to the collapse of the volcano’s summit and the formation of a crater 2.5 kilometers (1.6 miles) wide. This event reduced the volcano’s height from 1,745 meters (5,725 feet) to 1,486 meters (4,875 feet).

By 14:30, all seismographs near Clark Air Base were no longer working, likely due to powerful fast-moving hot flows. Changes in atmospheric pressure were also recorded.

On the same day, Typhoon Yunya (locally called Diding) hit the island, passing about 75 kilometers (47 miles) north of the volcano. The typhoon’s rain made it hard to see the eruption, but measurements showed that ash was thrown 34 kilometers (21 miles) high during the most intense phase, which lasted about three hours. Fast-moving hot flows reached as far as 16 kilometers (9.9 miles) from the volcano. Rain from the typhoon mixed with ash and caused heavy mudflows.

The volcanic ash cloud covered an area of about 125,000 square kilometers (48,000 square miles), causing darkness over much of Central Luzon for 36 hours. Most of the island received wet ash, which formed a thick, rain-soaked layer resembling snow. Ash also fell over much of the South China Sea and was recorded as far away as Vietnam, Cambodia, Singapore, Malaysia, and Indonesia.

Twelve days after the first eruptions on June 3, on June 15, 1991, by about 22:30, atmospheric pressure waves had returned to normal levels. No seismic records were available at this time, but scientists believe this marked the end of the final major eruption.

Large amounts of light and heavy metal minerals were brought to the surface. It is estimated that 800,000 tons of zinc, 600,000 tons of copper, 550,000 tons of chromium, 300,000 tons of nickel, and large amounts of potentially harmful minerals such as 100,000 tons of lead, 10,000 tons of arsenic, 1,000 tons of cadmium, and 800 tons of mercury were released along with other volcanic rock.

Effects on aircraft

At least 16 commercial aircraft experienced damaging encounters with ash from the June 15 eruption. Many aircraft that were not flying also suffered significant damage. During these encounters, one engine on two separate aircraft lost power. A total of ten engines were damaged and had to be replaced, including all three engines of one DC-10 aircraft. Long-term damage to aircraft and engines was reported, such as the buildup of sulfate deposits on engines. The eruption also caused permanent damage to the Philippine Air Force’s recently retired fleet of Vought F-8 aircraft, as these planes were stored in open areas at Basa Air Base at the time.

Aftermath

The 1991 eruption had a rating of 6 on the Volcanic Explosivity Index. It happened about 450 to 500 years after the volcano's last known activity. The eruption sent more than 10 cubic kilometers of material into the air, making it the largest eruption of the 20th century since the 1912 eruption of Novarupta. It was also ten times larger than the 1980 eruption of Mount St. Helens. Material like tephra and pyroclastic flow deposits are less dense than magma. The total volume of material sent into the air was equal to about 5 cubic kilometers of material that had not yet erupted. The energy released during the eruption was as strong as 70 megatons of TNT.

The top of the volcano was destroyed completely, and a caldera 2.5 kilometers wide formed in its place. The highest point on the caldera’s edge is now 1,485 meters above sea level, which is 260 meters lower than the volcano’s peak before the eruption.

About 847 people died during the eruption, mostly because roofs collapsed under the weight of volcanic material. This danger was made worse by the arrival of Typhoon Yunya at the same time.

Evacuations before the eruption saved the lives of tens of thousands of people. This effort is considered a major success for volcanology and eruption prediction.

After the eruption, about 500,000 people lived within 40 kilometers of the volcano. Cities like Angeles City, with 150,000 people, and Clark Freeport Zone, with 30,000 people, were among the population centers nearby.

Many reforestation projects were destroyed, with 150 square kilometers of land valued at 125 million pesos lost. Agriculture was heavily damaged, with 800 square kilometers of rice fields destroyed and nearly 800,000 animals, including chickens and other birds, killed. This hurt the livelihoods of thousands of farmers. The cost to agriculture was estimated at 1.5 billion pesos.

Farmers near Pinatubo began growing crops that can be harvested quickly, such as peanuts, cassava, and sweet potatoes, to avoid damage from lahar floods during the rainy season.

In total, 364 communities and 2.1 million people were affected by the eruption. More than 8,000 homes were destroyed, and 73,000 were damaged. Roads and communication systems were also damaged or destroyed by pyroclastic surges and lahar floods. Total losses in 1991 and 1992 were estimated at 10.6 and 1.2 billion pesos, respectively. Damage to public infrastructure alone was 3.8 billion pesos. Schools for thousands of children were temporarily closed because of the destruction caused by the eruption.

The eruption of Pinatubo hurt the economic development of the surrounding areas. The region’s economy made up about 10% of the total Philippine economy before the eruption. It had been growing by 5% each year but dropped by more than 3% from 1990 to 1991. In 1991, damage to crops and property was estimated at $374 million. Lahar floods added another $69 million in damage in 1992. About 42% of cropland near the volcano was affected by lahars, which hurt the region’s agriculture.

Since the eruption, heavy rains have caused large lahars from the volcano, displacing thousands of people and damaging buildings and infrastructure. Billions of pesos were spent building dikes and dams to control these lahars.

Several

Humanitarian aid

Humanitarian aid received after the eruption included the following efforts:

The government started programs to help people recover and rebuild. These programs aimed to address problems caused by lahar, such as building large dikes to protect areas from flooding. To speed up the delivery of basic services for those affected, private companies and non-governmental organizations (NGOs) helped provide relief. They worked with the government to fill gaps in services that were not covered by official efforts.

Key areas of aid included:
1. Livelihood programs focused on agriculture and industry to quickly help affected families earn income.
2. Basic social services such as health care, education, and welfare.
3. Rebuilding and repairing infrastructure like roads, bridges, and public buildings.

The Asian Disaster Reduction Center was created in Kobe, Hyogo prefecture, Japan, in 1998. Its goal is to help its 50 member countries become more disaster-resilient, build safe communities, and support sustainable development. The center connects countries through programs like sharing knowledge and training. It works with groups such as the United Nations, the International Strategy for Disaster Reduction, and the World Health Organization. The center’s main focus includes:

After the eruption, many homes were destroyed, and areas affected by lahar were considered unsafe to live in. People, including the Aetas and lowlanders, needed to be resettled. This process had to consider their cultural and economic differences.

The destruction of farmland and the displacement of farmers and workers forced the government to find long-term solutions. Agricultural industries were also harmed. The closure of Clark Air Base created a need for short-term jobs and the use of the base’s land to help displaced workers.

The disaster required social service sectors to continue helping with health care, welfare, and education. These services were not limited to people in evacuation centers but also reached others affected. School classes were delayed because school buildings were destroyed. Providing safe places for evacuees to live was also a major challenge. Social services were set up in future resettlement areas to help victims prepare for new homes.

Damage to infrastructure was mainly caused by wet ash after the eruption. Roads, bridges, public buildings, communication systems, and flood control structures were major concerns for the government. Measures to prevent flash floods and lahar threats became urgent.

The eruption harmed not only buildings but also farmland, forests, and watersheds. River systems and the environment in the affected area were severely damaged by lahar flows. Careful planning of land use was needed to address these issues.

The event highlighted the need for scientific studies to review current policies and knowledge about areas at risk of eruptions. Research should also explore how ash fall could be used for industrial or commercial purposes. This concern affects both the government and private sectors.

Before the Philippine government officially asked for help, the U.S. Foreign Disaster Assistance (USAID/OFDA) sent shelter supplies to flood and lahar victims in July 1992. In August, they gave $375,000 for relief and rebuilding projects. The Department of Social Welfare and Development said during a meeting with international agencies that the government had enough resources to help victims. The UN-Disaster Management Team and the United Nations’ Department of Humanitarian Affairs/Disaster Relief Organization worked with the government to monitor the situation and plan further aid.

The national government only officially asked for international help after President Fidel V. Ramos declared the affected areas in a state of emergency. In response, the United Nations reached out to the international community for aid and continued working with the government.

Countries that provided humanitarian help included Australia, Belgium, Canada, China, Denmark, Finland, France, Germany, India, Indonesia, Italy, Japan, Malaysia, Malta, Myanmar, the Netherlands, New Zealand, Norway, Saudi Arabia, Singapore, South Korea, Spain, Sweden, Thailand, the United Kingdom, and the United States. International groups like the United Nations Development Programme, the United Nations Children’s Emergency Fund, the World Food Programme, and the World Health Organization also offered aid. This help came in the form of cash donations or supplies like food, medicine, and shelter materials.

Contributions from United Nations systems and participating countries included:

Specific projects led by the Department of Public Works and Highways, supported by foreign aid, included: