On the 15th June of 1991, the second largest volcanic eruption of the twentieth century took place on the island of Luzon in the Philippines, 90 km northwest of the capital city Manila. It was also, by far, the largest eruption to affect a densely populated area. Mount Pinatubo, a stratovolcano, is part of a chain of volcanoes along the Luzon arc on the west coast (refer map). The arc of volcanoes is due to the subduction of the Manila trench to the west. The mountain has a very huge eruptive history. It was known to be thermally active and had been explored as possible geothermal energy resource by the Philippine National Oil Company. Mount Pinatubo is among the highest peaks in west-central Luzon. Its lower flanks were made, mostly, of pyroclastic deposits from voluminous, explosive prehistoric eruptions. Although the 1991 eruption was one of the largest and most violent of the 20th century, it was weaker than the previous Pinatubo eruptions.
During the eruption, approximately 350 people were killed by the 35 km ash column, the hot blast and, most importantly, from the collapsing roofs. But the casualties and death toll could have been greater. The Philippines Institute of Volcanology and Seismology (PHIVOLCS) and the USGS were able to inform the locals of the disaster ahead and were able to carry out the most successful volcanic hazards mitigation in the history, thus able to save lives of thousands of people and an estimated billion dollars worth of properties.
The Events Leading to the Eruption
The events of the 1991 eruption began back in July 1990, when a magnitude 7.8 earthquake struck a region 100 km northeast of Pinatubo. This shook the earth's crust beneath the volcano and caused a landslide, some local earthquakes and some small steam emissions too. The following year, sometime in March, villagers of Patal Pinto felt some earthquakes around the volcano. On the 2nd of April the villagers witnessed small explosions followed by steaming and the smell of rotten eggs (SO2). The Filipino geologists immediately installed portable seismographs 10 to 15 km away from the summit, recording over hundreds of earthquakes a day which meant that there was geothermal movement on steam and water beneath the surface. On the request of the Filipino government, personnel from the USGS arrived on the 23rd of May. The US showed particular interest in the situation because it had two military bases in Luzon, of which the Clark Air Base was the biggest US military base, on foreign soil. Within the next 2 weeks a radio-telemeter was installed that was capable of locating the increasing number of earthquakes. They later installed tiltmeters to detect new ground movement. The content of SO2 in the continuously visible steam plumes were measured by the help of the U.S Air Force. All these signals indicated that magma was rising towards the surface from more than 32 km's beneath the volcano. Meanwhile, geologists made a geological investigation of the volcano and established a set of alert (with 1 being a low level unrest and 5 being a high level unrest)to provide the public with Mount Pinatubo's status. As soon as the chart was completed on the 13th of May, the alert level was set at two. Throughout the end of May vigorous steam emissions continued from vents on the north side. By May 23, a hazard map was prepared and distributed to local officials, showing areas most vulnerable to pyroclastic flow, lahars and ashfall. The map shows the worst case scenario. The geologists did not want to take any risk in calling out the evacuation, if they called it too early then the people would return to their areas but if they called it out too late then thousands of lives could be lost. They then defined three successive evacuation zones: Zero to 10 km from the volcano,
10 to 20 km from the volcano and
20 to 40 km from the volcano, this included the Clark Air Base and the Angeles city with the...