Mount Pinatubo 1991 An Eruption with Global Impact
Pinatubo as it slept Pinatubo Volcano, one of the 22 active volcanoes dotting the Philippines, is part of the chain of volcanoes which borders the western side of Luzon and lies in the central portion of the Zambales Range, a mountain belt that extends 220 kms. from Lingayen Gulf in the north and Bataan in the South. The Philippine Institute of Volcanology and Seismology (PHILVOLCS) says that as early as 7 million years ago, the eruptive activity of the Zambales range had already begun. This lasted for millions of years, subsiding only 450 years ago.
Pinatubo prior to eruption Steam On April 2, 1991, a hydrothermal explosion at Pinatubo’s crater interrupted its 450 years of slumber. Rumbling sounds were heard as steam clouds and a small amount of ash were ejected from several active vents, shooting them to heights ranging from 500 to 800 meters.
Pinatubo prior to eruption Quakes Three days later, in a hastily installed temporary seismic station at Sitio Yamut, scientists recorded 223 high frequency volcanic quakes (HFVQ) during its first 24 hours of operation. Then, for two months from April 6 to June 6, the seismograph’s daily count varied from a minimum of 26 to a maximum of 178 incidents of HFVQs. Then on June 7, daily incidents leaped to a daily average of 1,500 to 2,000. On June 9, seismographs record harmonic tremors indicating intensified seismic activities. On June 12, more harmonic tremors were recorded which continued during the volcano’s eruption.
Pinatubo prior to eruption Ash It was 7:30 in the morning of June 3, 1991, when Pinatubo Volcano first started ejecting ash which lasted for about 30 minutes. The following day, more incidents of ash ejections, although at short intervals, were observed. After a few days of quiescence, Pinatubo’s seismic activities intensified as scientists recorded an explosion type earthquake at around 3:25 pm on June 8.
Pinatubo earth-shaking eruption Sunday, 9 June 1991 Eight hours of ash-laden steam clouds ejection ushered one of the world’s most violent and destructive eruptions of the century, beginning at around 6 AM. This was followed by pyroclastic flows which flowed down Pinatubo’s gullies into the Maraunot and Moraza rivers. These pyroclastic flows reached some 4-5 kilometers from the center of activity.
Pinatubo earth-shaking eruption Wednesday, 12 June 1991 At around 8:51 on Independence Day morning, intense seismic activity was followed by three major explosions, the most powerful of which ejected a "huge, grey, mushroom-shaped cloud" that reached 20 kilometers above the vent. Ash, pumice, and other larger volcanic fragments were also ejected by Pinatubo. Major rivers and tributaries radiating from the volcano overflowed with cascading pyroclastic flows. Shortly before midnight, a second series of strong explosions hurled clouds of ash and pyroclastic materials 25 kilometers into the air.
Pinatubo earth-shaking eruption Thursday, 13 June 1991 Only 24 hours after the first major explosions, another violent eruption occurred triggering heavy ashfalls that blanketed most of Zambales, Tarlac and Pampanga. Prevailing winds also blew the volcanic ash hundreds of kilometers away in all directions.
Pinatubo earth-shaking eruption Friday, 14 June 1991 The fourth major eruption occurred at 1:09 in the afternoon of June 14, with Pinatubo ejecting another vertical ash column that reached a maximum height of 25 kilometers above the vent. This was followed by smaller, intermittent eruptions. But at 3:30 in the afternoon, a much bigger eruption produced a cauliflower-shaped column that ejected volcanic debris to a maximum height of 30 kilometers. Pyroclastic flows ran down the Maraunot River towards Sitio Ogik reaching 15 kilometers from the source.
Pinatubo earth-shaking eruption Saturday, 15 June 1991 Two explosions occurred at dawn accompanied by incandescent pyroclastic flows that moved at speeds between 70 kph to 80 kph. These were followed by seven more eruptions lasting up to midmorning, producing ash columns from 15-18 kilometers wide at the base and heights of up to 25 kilometers above sea level. This time, pyroclastic flows advanced to areas as far as 16 kilometers away from the center of activity. At 10:27 am, a violent eruption ejected a 40 kilometer high eruption column.
Pinatubo aftermath Pinatubo unleashed three major destructive agents, namely: ashfall, pyroclastic flow and lahar that caused destruction to Central Luzon's infrastructure and rendered its vast agricultural lands into virtual wastelands. Hardest hit were the provinces of Zambales, Pampanga, and Tarlac where more than 86,000 hectares of agricultural lands and fishponds were affected by ashfalls and lahars.
Pinatubo aftermath Irrigation systems, water service facilities, power transmission and lateral lines, roads bridges and other infrastructures were damaged mainly by lahars while houses and public buildings collapsed from the weight of accumulated ash deposits. Commercial and industrial operations were suspended while more than 650,000 workers were forced out of work because of the destruction of their farms, shops, factories and work places. In Metro Manila, volcanic dust also blanketed the metropolis forcing authorities to suspend classes for a few days. Public buildings were converted into evacuation centers to accommodate the increasing number of refugees from the devastated areas.
Pinatubo aftermath By December 1991, 61 municipalities and two cities had been declared as calamity areas. But more than the destruction of buildings and infrastructure, Pinatubo's eruption affected more than 249,000 families or about 1.18 million people, including 847 deaths, 184 injuries and 23 missing.
<ul><li>Millions of tons of sulfur dioxide were discharged into the atmosphere, resulting in a decrease in the temperature worldwide over the next few years. </li></ul>Pinatubo aftermath
Pinatubo aftermath – Typhoon Yunya Typhoon Yunya made landfall midday on June 15, 1991, in Central Luzon.
Pinatubo aftermath – Typhoon Yunya It produced heavy rainfall which in return, caused flooding that wiped away bridges. The direct impact of the typhoon was fairly minimal due to its subsequent influence on the cloud of ash and dust produced by the eruption of Mt. Pinatubo the same day. As Yunya was crossing central Luzon, its deep cyclonic circulation re-disturbed the ash that would have been carried out over the South China Sea, over land. This highly aggravated the impact of the water-laden ash fall-out on Clark Air Base, resulting in downed power lines and collapsing of flat-roofed buildings.
Pinatubo aftermath – Global effects AVHRR satellite data depicting the thickness of aerosol optical products measured on May 28 to June 5, 1991, and again after the Pinatubo eruption on July 4 to July 10, 1991 (about 20 days following the major eruption). Ten days after the June 15 eruption the aerosol cloud formed a nearly continuous band that stretched 11,000 km from Indonesia to Central Africa.
Pinatubo – Powerful force of nature Pyroclastic flows are fluidized masses of rock fragments and gases that move rapidly in response to gravity. Pyroclastic flows can form in several different ways. They can form when an eruption column collapses, or as the result of gravitational collapse or explosion on a lava dome or lava flow. These flows are more dense than pyroclastic surges and can contain as much as 80 % unconsolidated material. The flow is fluidized because it contains water and gas from the eruption, water vapor from melted snow and ice, and air from the flow overriding air as it moves downslope.
Pinatubo – Powerful force of nature Pyroclastic flows and lahars are the greatest volcanic hazards. More people have died due to these hazards than any other volcanic hazard. Pyroclastic flows can incinerate, burn, and asphyxiate people. Gases within a pyroclastic flow can explode and cause ash to rain down on nearby areas. Pyroclastic flows travel long distances so their threat is far reaching. What is worse is they also can transform into lahars which travel even farther distances from the volcano and can produce even greater hazards.
Pinatubo – Powerful force of nature Scientists recognize the hazards of pyroclastic flows, and so there is currently a lot of research going on in this area. Important research with regard to hazards prevention is the study of past pyroclastic flow deposits. Areas that have old pyroclastic flow deposits are likely to receive new pyroclastic flow deposits if the volcano erupts again. People living near the summit of an active volcano, especially those in valley areas, are most likely to be in danger from a pyroclastic flow.
Pinatubo – Powerful force of nature The best course of action for these people to take when a volcano erupts is to evacuate valley areas and head for higher ground away from the volcano. Of course, if the volcano gives ample warning that it is going to erupt, then the best thing to do is evacuate the area and get as far away from the volcano as possible.
Pinatubo – One of the world’s greatest volcanoes
Produced by: Aisha Alhazza References and Images from: park.org smate.wwu.edu geo.mtu.edu February 2009