Today's Punto
Today's Punto
Feature Article
Mount Pinatubo erupts

Jun 15, 2017

THE SECOND-LARGEST volcanic eruption of the 20th century, and by far the largest eruption to affect a densely populated area, occurred at Mount Pinatubo on June 15, 1991. The eruption produced high-speed avalanches of hot ash and gas, massive lahar floods and huge clouds of superheated volcanic material hundreds of kilometers across.

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 5 kilometres northwest of the summit, shifted to shallow depths just below the summit.

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 200 m and a height of 40 m.

Timely forecasts of an eruption by scientists from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey enabled people living near the volcano to quickly evacuate to safer distances, saving at least 5,000 lives.

A small blast at 03:41 PST 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 19 kilometres and which generated large pyroclastic surges extending up to 4 km from the summit in some river valleys. Fourteen hours later, a 15-minute blast hurled volcanic matter to heights of 24 km. Friction in the up-rushing ash column generated abundant 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 24 kilometers. After three hours of quiet, seismic activity began, growing more and more intense over the next 24 hours, until a three-minute eruptive blast generated a 21 km high eruption column at 13:09 on June 14.

On June 15, 1991, the eruption plume minutes after the climactic eruption.

The final massive eruption of Mount Pinatubo began at 13:42 PST on June 15. It caused numerous major earthquakes due to the collapse of the summit of Mount Pinatubo and the creation of a caldera 2.5 km in diameter, reducing the peak from 1,745 m to 1,485 m.

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 75 km north of the volcano. The typhoon rains mostly obscured the eruption, but measurements showed that ash was ejected to 34 km high by the most violent phase of the eruption, which lasted about three hours. Pyroclastic surges poured from the summit, reaching as far as 16 km away from their origin point. Typhoon rains mixed with the ash deposits caused a messy rain of mud and massive lahars.

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