Mount Pinatubo's 1991 Eruption: Cataclysmic Forces Unleashed

Where: Mount Pinatubo, Philippines, Asia

When: June 12th-15th, 1991

Kind of volcano: Strato or composite volcano

Type of eruption: Explosive - the second greatest eruption this century

Deaths: 300 people passed away, 1000's were evacuate

Mount Pinatuba had been dormant for 500 years. The first indication that this circumstance may be altering happened on July 16, 1990 when a magnitude 7.8 earthquake (roughly the size of the 1906 San Francisco earthquake) struck about 60 miles (100 kms.) northeast of Mount Pinatubo on the island of Luzon in the Philippines.

This caused the shaking and squeezing of the Earth's crust underneath the volcano. At Mount Pinatubo, scientists tape-recorded a landslide, some local earthquakes, and a short-lived boost in steam emissions from a pre-existing geothermal area, but otherwise the volcano appeared to be undisturbed. In March and April 1991, however, lava started rising towards the surface area from more than 20 miles (32 kms.) beneath Pinatubo. This triggered more small earthquakes and caused effective steam surges that blasted 3 craters on the north side of the volcano.

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Countless small earthquakes took place beneath Pinatubo throughout April, May, and early June 1991, and many thousand heaps of toxic sulphur dioxide gas were likewise released by the volcano.

Scientists had been able to forecast Pinatubo's 1991 eruption and this resulted in the saving of many lives and much property. Commercial aircraft were warned about the hazard of the ash cloud from the June 15 eruption, and most avoided it. Although much equipment was successfully protected, buildings on two U.S. military bases in the Philippines--Clark Air Base and Subic Bay Naval Station--were heavily damaged by ash.

Nearly 20 million tons of sulphur dioxide were injected into the stratosphere and the spread of this gas cloud around the world caused global temperatures to drop temporarily (1991-1993) by about 0.5°C. About 20,000 Aeta highlanders, who had lived on the slopes of the volcano, were completely displaced, and most still wait in resettlement camps for the day when they can return home. About 200,000 other people who evacuated from the lowlands surrounding Pinatubo before and during the eruptions have returned home but face continuing threats from lahars (mudflows) that have already buried numerous towns, villages and field

On June 7th 1991, the first magma reached the surface of Mount Pinatubo but because it had lost most of the gas contained in it on the way to the surface, the magma merely oozed out to form a lava dome. However, on June 12th, large amounts of gas-charged magma reached the surface and exploded in the volcano's first spectacular eruption. When even more highly gas charged magma reached Pinatubo's surface on June 15th, the volcano exploded in a massive eruption that ejected more than 5 cu. kms. of volcanic material. The ash cloud from this huge eruption rose 22 miles (35 kms.) into the air. A blanket of volcanic ash and larger pumice pebbles blanketed the countryside. Fine ash fell as far away as the Indian Ocean, and satellites tracked the ash cloud several times around the globe. Huge avalanches of red hot ash, gas, and pumice fragments called pyroclastic flows roared down the sides of Mount Pinatubo, filling the deep valleys with fresh volcanic deposits as much as 660 ft. (200 m.) thick. The eruption removed so much magma and rock from below the volcano that the summit collapsed to form a large volcanic depression or caldera 1.6 miles (2.5 kms.) across.