The Eruption of Mount Pinatubo

The image of the gigantic volcanic cloud slowly rising higher and higher is forever etched in Filipino memory. The immense pressure built up within Mt. Pinatubo was released with such energy that the force of its eruption was directed upwards. On the ground, there was an unusual eerie silence.

The eruption of Mt. Pinatubo in the Philippines on June 15, 1991 is the second largest eruption of the 20th century and is considered as the largest stratospheric disturbance since Krakatoa in 1883. Spewing ash and toxic gasses, it formed a huge volcanic plume (10 times that of Mt. St. Helens in 1980) that reached an altitude of 34 km. in just 2 hours and a width of 400 km. The cloud produced by the eruption covered the planet within a year reducing global temperature as much as 0.73 degrees Celsius. The change in global climate created unusual weather conditions that even affected the size of the hole in the Ozone layer over Antartica.

Located just 90 km. from Manila, Mt. Pinatubo is a part of a group of active volcanoes lining the west coast of the island known as the Luzon arc. It lies within the Zambales Range whose eruptive activities began as early as 7 million years ago and lasted for more than 4 million years. Inactive for more than 500 years, volcanologists contend that Pinatubo was likely awakened by an earthquake that hit the island about a year earlier. The June 1990 earthquake had a 7.8 magnitude with an epicenter directly northeast of the region and only 100 km. away.

Minor earthquakes were felt as early as March 1991, prompting the Philippine Institute Volcanology and Seismology (PHILVOLCS) and its then director the late Reynaldo Punongbayan to put the mountain to closer scrutiny. PHILVOLCS had to determine whether Pinatubo’s activity was normal or a sign of an impending eruption. The responsibility of if and when to issue an evacuation order lay squarely on Punongbayan’s shoulders.