Mount Pinatubo
Ash plume of Pinatubo during 1991 eruption
Elevation:	4,875 ft (1,486 m)
Latitude:	15° 8′ 0″ N
Longitude:	120° 21′ 0″ E
Location:	Luzon, Philippines
Range:
Type:	Stratovolcano

Mount Pinatubo is an active volcano located on the island of Luzon in the Philippines. It most recently erupted on June 15, 1991. Mount Pinatubo lies at the intersection of the borders of the provinces of Zambales, Bataan, and Pampanga. Before the 1991 eruption, Pinatubo had been dormant for more than 600 years.

Contents

1 Awakening 2 Eruptions build to a climax 3 The aftermath 4 Global effects 5 External links 6 References

Awakening

Most locals were not aware that 1700m-tall Pinatubo was even a volcano - its slopes were heavily eroded and covered in jungle. However, in March 1991 a succession of earthquakes were felt, and soon after, phreatic eruptions occurred near the summit.

Scientists immediately installed monitoring equipment and analysed the volcano for clues as to its previous eruptive history. Radiocarbon dating of charcoal found in old volcanic deposits revealed that there had been three major explosive eruptions in recent millenia, about 5500, 3500 and 600 years ago. Geological mapping showed that much of the surrounding plains were formed by lahar deposits from previous eruptions.

Volcanic activity increased throughout April and May, before the first magmatic eruptions occurred on June 3. The first large explosion on June 7 generated an ash column 7 kilometres high.

Eruptions build to a climax

The signs were that a very large eruption was imminent, and the Phillipines Institute of Volcanology worked in cooperation with the US Geological Survey to convince people in the local area of the severity of the threat. A false warning may have led to cynicism should a later warning need to be issued, while 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. After the first large eruption on June 7, evacuation of the local areas was ordered, and by June 14, some 60,000 people had left the area within 30km of the volcano. June 15 saw the onset of the climactic eruption.

On the same day, Typhoon Yunya struck the island, adding to the volcanic devastation. The typhoon rains made direct visual observations of the eruption impossible, but measurements showed that ash was ejected to heights of 34km by the most violent phase of the eruption, which lasted about three hours. Pyroclastic flows poured from the summit, reaching as far as 16km away from it. Typhoon rains mixed with the ash deposits causing massive lahars.

The aftermath

In all, the eruption ejected about four cubic kilometres of material, making it the largest eruption since that of Katmai in 1912. About 300 people were killed by the eruption, mostly by roofs collapsing under the weight of accumulated wet ash. The evacuation had saved tens of thousands of lives.

However, since the eruption, each rainy season has brought further lahars, which have caused the displacement of thousands of people. Hundreds have died from poor sanitation in relocation camps. Agriculture in the region also suffered badly from the effects of the eruption, with hundreds of square kilometres of formerly arable land being rendered infertile and thousands of farmers' livelihoods destroyed.

The United States maintained two large air bases in the region; Subic Bay Naval Base was 75 km (50 mi.) to the southwest, while Clark Air Base was a mere 40 km (25 mi.) to the east of the volcano's summit. Both were abandoned after being severely damaged by the eruption. These events were dramatized in the book One of the Guys by U.S. author Robert Clark Young.

Global effects

The powerful eruption of such an enormous volume of lava and ash injected significant quantities of aerosols and dust into the stratosphere. Sulphur dioxide oxidised in the atmosphere to produce a haze of sulphuric acid droplets, which gradually spread throughout the stratosphere over the year following the eruption. This resulted in a reduction in the normal amount of sunlight reaching the earth's surface, and global temperatures fell by about 0.4 °C.

Another noticeable effect of the dust in the atmosphere was the appearance of lunar eclipses; normally even at mid-eclipse, the moon is still visible although much dimmed, but in the year following the Pinatubo eruption, the moon was hardly visible at all during eclipses, due to much greater absorption of sunlight by dust in the atmosphere.

External links

• Mt. Pinatubo (http://volcano.und.nodak.edu/vwdocs/volc_images/southeast_asia/philippines/pinatubo.html)
• Weather effects of the 1991 eruption (http://eos.higp.hawaii.edu/education/slide_set1/)

References

• Decker R, Decker B, 1997, Volcanoes, 3rd edition, WH Freeman (New York)

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