The day the Philippines’ Mount Pinutabo exploded back into life — the second biggest volcanic eruption in the 20th century

FOR five or six centuries Mount Pinatubo had remained dormant, but its re-awakening on June 15, 1991, shook the Philippines.

Filipino tribesman walk their bullocks as volcanic ash spews from Mount Pinatubo on June 22, 1991.

FOR five or six centuries the volcano had slept. Although Mount Pinatubo on the island of Luzon in the Philippines was still considered an active volcano, occasionally emitting gas and hissing steam, it appeared it had ceased to be a threat to the people living nearby.

Apart from the occasional rock slide during rains the mountain, viewed as a god by some locals, had not shown its full wrath in hundreds of years. Villagers had built their homes on the mountain’s slopes, the last eruption lost to human memory.

But in March 1991 there was an increase in volcanic activity. During the next few months earthquakes and emissions of gas, steam, ash and magma indicated that something might be about to happen. Warnings were issued and evacuations began.

On June 15, 1991, 25 years ago today, an eruption measuring six on the Volcanic Explosivity Index (VEI) rocked the mountain. It was the second biggest volcanic eruption in the 20th century. Despite the evacuations and precautions, more than 800 people were killed by Pinatubo’s fury.

A soldier walks towards an abandoned house as Mount Pinatubo continues to spew ash as high as 19km, four days after it erupted on June 15, 1991.

For thousands of years the forests surrounding Mount Pinatubo had been home to the Aeta people, thought to have been the original inhabitants of the Philippines.

Ancestors of the modern Aeta would have experienced the power of Pinatubo, which was preserved in legends relating to their gods.

There are differing theories on the meaning of the name of the mountain.

The word Pinatubo means “made to grow” which may refer to the mountain increasing in size due to volcanic activity or a reference to how well crops grow in the rich volcanic soil.

Over time memories of the eruption in the 15th or 16th century dimmed and people began to make their homes on the slopes. In July 1990 a magnitude 7.8 earthquake struck about 100km northeast of Mount Pinatubo. There were rockslides and an increase in steam seeping through fissures. At the time geologists were not concerned that it was a sign of an impending eruption.

Buildings collapse under the weight of volcanic ash mixed with rain from Typhoon Yunya, which hit the same day as Mount Pintuba erupted on June 15, 1991.

The rumbling went away but in March 1991 came more local quakes and more steam. Hundreds of people began leaving the area. In June magma made its way to the surface and the Philippine Institute of Volcanology and Seismology, assisted by the US Geological Survey, began monitoring the situation. Wary of issuing a warning without being sure when the volcano might erupt would cause people to become sceptical, authorities waited.

On June 7 a level 4 alert was issued, a warning that an eruption was imminent, and major evacuations began. Tens of thousands had been moved to safety when, on June 15, a major eruption occurred. The explosion shifted five cubic kilometres of material, sending an ash cloud 35km into the air.

The eruption coincided with Typhoon Yunya, which hit Luzon the same day. The rain-soaked volcanic ash caused many houses to collapse and was responsible for many of the 847 reported deaths, along with mudflows and rockslides.

Despite the terrible death toll, the evacuations prompted by warnings from vulcanologists had saved thousands of lives. Several weaker eruptions occurred throughout the rest of the year and in subsequent years, but nothing like catastrophic event of June 15. The sulphur dioxide and particles emitted by the volcano caused global temperatures to drop by an average of 1C.

VOLCANIC ERUPTIONS THAT SHOOK THE WORLD

● Krakatoa 1883 VEI 6: Destroyed the island of Krakatoa, in the Sunda Strait between Java and Sumatra, and set off a tsunami killing more than 36,000 people.

● Santa Maria 1902 VEI 6: A volcano in Guatemala which killed 5000.

● Novarupta 1912 VEI 6: The only 20th century volcanic eruption bigger than Pinatubo took place in Alaska but is little known because it is so isolated and only two people are believed to have died.

● Mt Tambora 1815 VEI 7: On the island of Sumbawa, Indonesia, death toll is believed to be more than 80,000. The eruption, which was the largest ever recorded, is also thought to have caused global cooling and crop failures.