1991 eruption of Mount Pinatubo
The 1991 eruption of Mount Pinatubo in the Philippines' Luzon Volcanic Arc was the second-largest volcanic eruption of the 20th century, behind only the 1912 eruption of Novarupta in Alaska. Eruptive activity began on April 2 as a series of phreatic explosions from a fissure that opened on the north side of Mount Pinatubo. Seismographs were set up and began monitoring the volcano for earthquakes. In late May, the number of seismic events under the volcano fluctuated from day-to-day. Beginning June 6, a swarm of progressively shallower earthquakes accompanied by inflationary tilt on the upper east flank of the mountain, culminated in the extrusion of a small lava dome.
On June 12, the volcano's first spectacular eruption sent an ash column into the atmosphere. Additional explosions occurred overnight and the morning of June 13. Seismic activity during this period became intense. When even more highly gas-charged magma reached Pinatubo's surface on June 15, the volcano exploded, sending an ash cloud into the atmosphere. Volcanic ash and pumice blanketed the countryside. Huge pyroclastic flows roared down the flanks of Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as thick. The eruption removed so much magma and rock from beneath the volcano that the summit collapsed to form a small caldera across.
Fine ash from the eruption fell as far away as the Indian Ocean and satellites tracked the ash cloud as it traveled several times around the globe. At least 16 commercial jets inadvertently flew through the drifting ash cloud, sustaining about $100 million in damage. With the ashfall came darkness and the sounds of lahars rumbling down nearby river valleys. Several smaller lahars washed through the Clark Air Base, flowing across the base in enormously powerful sheets, slamming into buildings and scattering cars. Nearly every bridge within of Mount Pinatubo was destroyed. Several lowland towns were flooded or partially buried in mud. More than 840 people were killed from the collapse of roofs under wet heavy ash and several more were injured.
Rain continued to create hazards over the next several years, as the volcanic deposits were remobilized into secondary mudflows. Damage to bridges, irrigation-canal systems, roads, cropland, and urban areas occurred in the wake of each significant rainfall. Many more people were affected for much longer by rain-induced lahars than by the eruption itself.
Build-up and evacuations
On July 16, 1990, a magnitude 7.8 earthquake struck northern Central Luzon and the Cordilleras. This was the largest earthquake recorded in 1990. Its epicenter was in the municipality of Rizal, Nueva Ecija, about northeast of Pinatubo, and faulted northwest–southeast through three provinces. It also followed the Philippine Fault System west as far as Baguio, which was devastated, and is located about north-northeast of Pinatubo, leading volcanologists to speculate that it might ultimately have triggered the 1991 eruption, although this is impossible to prove conclusively.Two weeks after the earthquake, local residents reported steam coming from the volcano, but scientists who visited there in response found only small rockslides rather than any pre-eruptive activity. On March 15, 1991, a succession of earthquakes were felt by villagers on the northwestern side of the volcano. Further earthquakes of increasing intensity were felt over the next two weeks, and it became clear some kind of volcanic activity was likely.
On April 2, the volcano woke up, with phreatic eruptions occurring near the summit along a long fissure. Over the next few weeks, small eruptions continued, dusting the surrounding areas with volcanic ash. Seismographs recorded hundreds of small earthquakes every day. Scientists immediately installed monitoring equipment and analyzed the volcano for clues as to its previous eruptive history. Radiocarbon dating of charcoal found in old volcanic deposits revealed the last three major explosive eruption periods in recent millennia, about 5500, 3500 and 500 years ago. Geological mapping showed that much of the surrounding plains were formed by lahar flood deposits from previous eruptions.
Volcanic activity increased throughout May. Measurements of sulfur dioxide emissions showed a rapid increase from per day by May 13 to per day by May 28. This implied that there was a rising column of fresh magma beneath the volcano. After May 28, the amount of being emitted decreased substantially, raising fears that the degassing of the magma had been blocked somehow, leading to a pressure build-up in the magma chamber and a high likelihood of violent explosive eruptions.
In early June, tiltmeter measurements had shown that the volcano was gradually inflating, evidently due to fast-growing amounts of magma filling the reservoir beneath the summit. At the same time, seismic activity, previously concentrated at a depth of a few kilometers below a point about northwest of the summit, shifted to shallow depths just below the summit. Such an event is a precursor of volcano tectonic earthquakes.
Given all the signs that a very large eruption was imminent, the Philippine Institute of Volcanology and Seismology – assisted by the United States Geological Survey – worked to convince local inhabitants of the high severity of the threat. A false warning might have led to cynicism about any later warnings, but delaying a warning until an eruption began might lead to thousands of deaths, so the volcanologists were under some pressure to deliver a timely and accurate assessment of the volcanic risk.
Three successive evacuation zones were defined, the innermost containing everything within of the volcano's summit, the second extending from the summit, and the third extending from from the summit. The and zones had a total population of about 40,000 inhabitants, while some more 331,000 inhabitants lived in the zone.
Five stages of volcanic alert were defined, from level 1 up to level 5. Daily alerts were issued stating the alert level and associated danger area, and the information was announced in major regional and national newspapers, on radio and television stations, by nongovernmental organizations and directly to the endangered inhabitants.
Many of the Aetas who lived on the slopes of the volcano left their villages of their own volition when the first blasts began in April, gathering in a village about from the summit. They moved to increasingly distant settlements as the eruptions escalated, some Aetas moving up to nine times in the two months before the colossal eruption.
The first formal evacuations were ordered for the zone on April 7. Evacuation of the zone was ordered when a level 4 alert was issued on June 7. A level 5 alert triggered evacuation of the zone on June 13, and in all some 60,000 people had left the area within of the volcano before June 15. Most people temporarily relocated to Metro Manila, with some 30,000 using the Amoranto Velodrome in Quezon City as an evacuee camp.
On June 7, the first magmatic eruptions took place with the formation of a lava dome at the summit of the volcano. The dome grew substantially over the next five days, reaching a maximum diameter of about and a height of.
Escalation of eruption
A small blast at 03:41 on June 12 marked the beginning of a new, more violent phase of the eruption. A few hours later the same day, massive blasts lasting about half an hour generated big eruption columns, which quickly reached heights of over and which generated large pyroclastic surges extending up to from the summit in some river valleys. One witness at Subic Bay described the explosion as 'hitting him as a whoosh of pressure'. Fourteen hours later, a 15-minute blast hurled volcanic matter to heights of. Friction in the up-rushing ash column generated abundant volcanic lightning.A third large eruption began at 08:41 on June 13, after an intense swarm of small earthquakes over the previous two hours. It lasted about five minutes, and the eruption column once again reached. After three hours of quiet, seismic activity began, growing more and more intense over the next twenty-four hours, until a three-minute eruptive blast generated a high eruption column at 13:09 on June 14.
Tephra fall from these four large eruptions was extensive to the southwest of the volcano. Two hours after the last of these four explosions, a series of eruptions began which lasted for the next twenty-four hours, and which saw the production of much larger pyroclastic flows and surges which travelled several kilometres down river valleys on the flanks of the volcano. In total, almost of land was buried by pyroclastic density currents which travelled in all directions, and reached at least, and as far as, from the volcano. The land closest to the eruption was eroded by the pyroclastic currents, which did not leave much deposit there.
Dacite was the dominant igneous rock making up the tephra in these eruptions and in the following climactic event. The most abundant phenocryst minerals were hornblende and plagioclase, but an unusual phenocryst mineral was also present – the calcium sulfate called anhydrite. The dacite magma was more oxidized than most magmas, and the sulfur-rich nature of the eruption was probably causally related to the redox state.
The final, climactic eruption of Mount Pinatubo began at 13:42 on June 15. It caused numerous major earthquakes due to the collapse of the summit and the creation of a caldera in diameter, reducing the peak from to.
All the seismographs close to Clark Air Base had been rendered completely inoperative by 14:30, mostly by super-massive pyroclastic surges. Intense atmospheric pressure variation was also recorded.
On the same day, Typhoon Yunya, locally named Diding, struck the island, with its center passing about north of the volcano. The typhoon rains mostly obscured the eruption, but measurements showed that ash was ejected to a height of by the most violent phase of the eruption, which lasted about three hours. Pyroclastic surges poured from the summit, reaching as far as away from their origin point. Typhoon rains and flooding, mixed with the ash deposits, caused a messy rain of mud and massive lahars.
The volcanic column from the crater covered an area of some, bringing total darkness to much of Central Luzon for 36 hours. Almost all of the island received some wet ash fall, which formed a heavy, rain-saturated snow-like blanket. Tephra fell over most of the South China Sea and ash falls were recorded as far away as Vietnam, Cambodia, Singapore, Malaysia and Indonesia.
Twelve days after the first magmatic eruptions of June 3, on June 15, 1991, by about 22:30, and about nine hours after the onset of the most recent climactic phase, atmospheric pressure waves had decreased to the pre-eruption levels. No seismic records were available at this time, but volcanologists believe 22:30 marked the end of the climactic eruption.
Vast quantities of light and heavy metal minerals were brought to the surface. Overall an estimated of zinc-, of copper-, of chromium-, of nickel-, and massive amounts of potentially toxic heavy metal mineral such as of lead-, of arsenic-, of cadmium-, and of mercury-minerals comingled with the other magmatic rock, came forth.