The Eruption Of Mount Pinatubo - Case Study - Geography

The Eruption of Mount Pinatubo

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:

Ð'* Zero to 10 km from the volcano,

Ð'* 10 to 20 km from the volcano and

Ð'* 20 to 40 km from the volcano, this included the Clark Air Base and the Angeles city with the population of 300,000 people.

On the 7th of April, the zero Ð'- 10km zone was evacuated. On the 7th of June, when the alert level was raised to 4, the 10 Ð'- 20 km zone was evacuated. And a level 5 alert triggered evacuation of the 20Ð'-40 km zone on the 14th of June. For the people who weren't willing to leave the area, a video showing the extreme hazards of the eruption was played to convince them of the dangers of the eruption. Also, on the 10th of June, the military finally issued evacuation orders and the 18,000 Clark Air Base personnel along with their families were transported to Subic Bay Naval Station (most were returned to the US). In total, the authorities were able to evacuate 200,000 people.

From June 7 to 12, SO2 gas emissions, which had increased tenfold from May 13 to May 28, suddenly decreased. The US government wanted definite answers so the USGS geologists went to the site to look for evidence. They monitored the SO2 emissions for two weeks and the results indicated that either the volcano had postponed its attack or that the chamber was blocked and the magma was pressurizing within the volcano.

The Eruption

On the 12th of June (Philippine Independence Day), millions of cubic yards of gas-charged magma reached the surface and resulted in the first major eruption which was then followed by a series of eruptions. It produced a 19 km long ash column and pyroclastic flows that extended up to four km from the summit to the river valleys. Later, a 15 minute eruption hurled ash straight up to the sky, reaching the heights of 24 km. The super heated air and the friction between the very fine ash in the ash column created bolts of lightening. At 08.41 on June 13, a third large eruption occurred which lasted about five minutes and generated a 24 km high ash column. And on the next day, a 21 km high ash colum was generated as a result of a three minute eruption. Thus it could concluded that the ash fall was massive in the area. After this, larger pyroclastic flows were seen, which had a temperature of up to 800˚C and burned everything in its way.

On the June 15, when even more highly gas charged magma reached the surface on, the volcano exploded in a disastrous eruption that ejected more than 5 cubic km of material. Its ash column reached a height of 35 km approximately, although direct visual observations were impossible on the day. The pyroclastic flows roared down the flanks of Mount Pinatubo and reached as far as 16 km away from the valley, filling it with fresh volcanic deposits as much as 200 meters thick. However,