The eruption of 1991 of Monte Pinatuboa stratovulcan located in the Luzon region in the Philippinesis considered the second largest in the twentieth centurysecond only to that of Mount Novarupta (Alaska) of 1912. The Plinian erupting event began on June 15, 1991 and generated a high eruptive column 35 km. The ashes covered an over surface 7,500 km²transported by the cyclonic winds produced by typhoon yunyawhich hit the island in conjunction with the volcanic eruption. THE gas volcanic placed in the atmosphere they caused a lowering global temperature about 0.5 ° Ca effect that lasted for almost three years. Constant monitoring of seismic and volcanic activity in the previous months allowed a timely evacuation almost 200,000 peoplesignificantly reducing the number of victims.
The 1991 eruption of Monte Pinatubo, one of the most violent of the last century
The Monte Pinatubo is one stratovulcano asset 1,460 meterslocated in the mountain range Zambalesein the western region of Luzonthe largest and most populous island of Philippines. The June 15, 1991the volcano gave rise to the second largest eruption of the twentieth centurywith an index of volcanic explosiveness (Vei) equal to 6 and beyond 5 km³ of volcanic material emitted.
The eruption, of the type Plinianhe generated a high erutive column 35 km and over larger 400 km. Ash And Pomiciwith diameter up to 4 cm, covered large areas. The arrival of typhoon Yunyacontemporary to the eruption, dispersed the materials on an area of 7,500 km²with accumulations up to 1 cm in different areas of the world and 50 cm around the volcano. Huge pyroclastic flows – mixtures of gas, ash and incandescent rock fragments – and huge plows of mud and volcanic debris (Lahar) filled the valleys with thick deposits up to 200 meters. Also, about 20 million tons of sulfur dioxide (I know₂) were placed in the stratosphere, causing a collapse of the temperature global Of 0.5 ° C who forgiven until 1993.
The precursors of the eruption in the Philippines
The eruption of Monte Pinatubo was preceded by several precursor signs who allowed to alert the population and evacuate the area in time. According to several volcanologists, the first signals traced at about a year earlier, at 16 July 1990when an earthquake of magnitude 7.8 He affected the region, with an epicenter about 100 km north-east of the volcano, causing landslides and temporary increases in smokyroly activity in some geothermal areas.
Among the months of March And April of 1991a series of small earthquakeswith an epicenter on the north-western side of the volcano, announced the beginning of the volcanic phase. The April 2nda sequence of explosions Freatics He opened three new flues along a crack about 3 km on the northern side. Among the months of April And Maythe Region was affected by an intense seismic phase during which the seismometric network recorded daily between 30 and 180 events seismic of small magnitude. In parallel, the air measurements of the gases emitted by the flues revealed a marked Increase in the emission of SO₂ between the 13 and the 28 Maypassing from about 500 tons al A beyond 5,000 tons.
The June 1sta new seismic swarm with hypocenter about 1 km of depth indicated that the magma He was approaching the surface. In the following days, the volcano produced small intermittent explosions, accompanied by ash emissions. The June 7thone column eruptive of steam and ash he raised up to 7 – 8 km in height. Between8 and 12 Junethe eruptions of ash and earthquakes increased both in frequency and in intensity, culminating in two large volcanic explosions that took place on June 12, with high eruptive columns up to 20 km. The character of the eruption changed the June 14thmarking the approach to the peak of the eruptive phase.
Monitoring, evacuation and victims
The monitoring of the volcanic and seismic activity of Monte Pinatubo intensified since March 1991. The local seismometric network managed by Phivolcs (Institute of volcanology and seismology of the Philippines), was extended thanks to the support of theUSGS (Geological service of the United States), improving the precision in the location of epicentrie hypocenter of earthquakes. Thanks to the efficient monitoring network, it was possible to issue alerts timely and evacuate the population.
The first evacuation took place in April and concerned the residents within a radius of 10 km from the top of the volcano. On June 7, with the raising of the alert level to “eruption in progress“, The evacuation radius was extended to 20 kminvolving about 40,000 people. Finally, the June 12ththe evacuation area was further extended to 30 kmleading to the safety of beyond 60,000 additional residents.
Timely evacuations limited the number of victims a 847 deaths. Most was not caused directly by volcanic activity, but by the effects of typhoon Yunya, who transported ash to Luzon. The accumulation of teas around wet on the roofs of the houses caused numerous structural collapsescontributing significantly to the budget of the victims.
Volcanic activity prior to the 1991 eruption
The eruption of 1991, however, was neither the first nor the most intense in the history of Mount Pinatubo. Experts divide the history of the volcano into two distinct phases: the Ancestral Pinaturbowhose activity is placed between approximately one million and 35,000 years ago, and the Modern Pinaturbowhich includes the eruptions that occurred in the Last 35,000 years.
The information on the ancestral phase is limited, but suggest that the main crater was about 2,3000 meters above sea leveltherefore at an altitude higher than the current one. The lavas of this phase were mainly rich in silica, with compositions variable between the lavas dacitic And Andesitic.
The history of the modern Pinaturbo, on the other hand, begins with theinhale eruptionabout about 35,000 years ago and considered its greatest eruption. The experts, in fact, have estimated a Vei greater than 7therefore a order of magnitude greater than the eruption of 1991. During the event, they were issued among the 25 – 35 km³ of volcanic material, including impressive pulician pyroclastic flows that deposited thicknesses above 100 meters along all the slopes of the volcano.
In the following millennia, other explosive eruptions have followed, generally of decreasing intensity. Among the main events we remember: the eruption Sacobia (17,000 years ago), the Pasbul (9,000 years ago), the Crow Valley (Between 6,000 and 5,000 years ago), the Maraunot (Between 3,900 and 2,300 years ago) and finally the Buagwhich took place in 1450 A.D..
