Eruption of Mount Pinatubo
Pinatubo unleashed three major destructive agents, namely: ashfall, pyroclastic flow and lahar that caused destruction to Central Luzon's infrastructure and rendered its vast agricultural lands into virtual wastelands.
Hardest hit were the provinces of Zambales, Pampanga, and Tarlac where more than 86,000 hectares of agricultural lands and fishponds were affected by ashfalls and lahars.
Irrigation systems, water service facilities, power transmission and lateral lines, roads bridges and other infrastructures were damaged mainly by lahars while houses and public buildings collapsed from the weight of accumulated ash deposits. Commercial and industrial operations were suspended while more than 650,000 workers were forced out of work because of the destruction of their farms, shops, factories and work places. The Americans were forced to evacuate Clark Air Base after volcanic dust, ashfall and lahar rendered its runways useless and endangered the operations of its planes. In Metro Manila, volcanic dust also blanketed the metropolis forcing authorities to suspend classes for a few days. The Ninoy Aquino International Airport was also closed for four days and public buildings were converted into evacuation centers to accomodate the increasing number of refugees from the devastated areas.
From June 7 to 12, the first magma reached the surface of Mount Pinatubo. Because it had lost most of the gas contained in it on the way to the surface (like a bottle of soda pop gone flat), the magma oozed out to form a lava dome but did not cause an explosive eruption. However, on June 12 (Philippine Independence Day), millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption.
When even more highly gas charged magma reached Pinatubo's surface on June 15, the volcano exploded in a cataclysmic eruption that ejected more than 1 cubic mile (5 cubic kilometers) of material. The ash cloud from this climactic eruption rose 22 miles (35 kilometers) into the air. At lower altitudes, the ash was blown in all directions by the intense cyclonic winds of a coincidentally occurring typhoon, and winds at higher altitudes blew the ash southwestward. A blanket of volcanic ash (sand- and silt-size grains of volcanic minerals and glass) and larger pumice lapilli (frothy 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 searing hot ash, gas, and pumice fragments (pyroclastic flows) roared down the flanks of Mount Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as 660 feet (200 meters) thick. The eruption removed so much magma and rock from below the volcano that the summit collapsed to form a large volcanic depression (caldera) 1.6 miles (2.5 kilometers) across.
Much weaker but still spectacular eruptions of ash occurred occassionally through early September 1991. From July to October 1992, a lava dome was built in the new caldera as fresh magma rose from deep beneath Pinatubo.
In all, the eruption ejected about ten cubic kilometres (2.5 mile³) of material, making it the largest eruption since that of Novarupta in 1912 and some ten times larger than the 1980 eruption of Mount St. Helens. Ejected material such as tephra fallout and pyroclastic flow deposits are much less dense than magma, and the volume of ejected material was equivalent to about four cubic kilometres (1 mile³) of unerupted material. This colossal eruption had a Volcanic Explosivity Index of 6. The former summit of the volcano was replaced by a caldera 2.5 km wide. The highest point on the caldera rim now stood 1,485 m above sea level, some 260 m lower than the pre-eruption summit.
Over 800 people were killed by the eruption, mostly by roofs collapsing under the weight of accumulated wet ash, a hazard that was greatly exacerbated by the simultaneous arrival of Typhoon Yunya. The evacuation in the days preceding the eruption certainly saved tens of thousands of lives, and has been hailed as a great success for volcanology and eruption prediction.