Recognition of the volcanic unrest at Mt. Pinatubo in the Philippines began when steam explosions occurred on April 2nd, 1991. The unrest culminated ten weeks later in the world's largest eruption in more than half a century. Volcanologists from the Philippine Institute of Volcanology and Seismology joined in late APril by colleagues from the US Geological Survey successfully forecasted the eruptive events and their effects, enabling Philippine civil leaders to organize massive evacuations that saved thousands of lives.
Mount Pinatubo is one of a chain of composite volcanoes known as the Luzon volcanic arc. The arc parallels the west coast of the island of Luzon and reflects the eastward dipping subduction along the Manila Trench to the west. Its former summit lay at the 1745 meter crest of a 3 kilometer diameter dacite dome formed during an earlier eruption.
Before the eruption, about 15000 people lived in small villages on the volcano's
flanks. A much larger population, about 500000, lived in cities and villages
on broad, gently sloping alluvial fans surrounding the volcano. During the
afternoon of April 2nd 1991,villagers were surprised by a series of small
explosions from a 1.5 kilometer long line of vents high on the volcano and
near the north flank of the summit dome. The explosions, which occurred over
a period of several hours, formed a line of new craters, deposited a meter
of more of poorly sorted rock debris near the vents, and dusted villages
10 kilometers to the west southwest with ash. By the next morning, a line
of new fumaroles on the northwest flank of the summit dome extended southwest.
At the request of civil officials, about 5000 people evacuated a zone of
10 kilometer radius around the summit of Mt. Pinatubo.
Numerous small earthquakes were recorded through May, some too small to be felt, while 1800 earthquakes were recorded at magnitudes less than 2.5. About June 1st, a second cluster of earthquakes began to develop between the surface and a depth of 5 kilometers. A small explosion early on June 3rd initiated an episode of increasing volcanic unrest characterized by intermittent minor emission of ash, and increasing seismicity beneath vents led to the issue of an announcement that a major pyroclastic eruption would occur within two weeks. On June 9th, the evacuation was up to 25000 people and an ash cloud rose to the crest of the volcano, and daylight showed a plume of ash and steam rising 3 kilometers above Mt. Pinatubo. The first major explosive eruption began on June 12th, generating a column of ash and steam that rose 19 kilometers. It lasted 35 minutes, as ash was transported southwestward past communities north of Subic Bay, and small pyroclastic flows traveled northwest and north from the vent. Pinatubo continued eruptions through June til the end of the year.
By June 15th, climatic eruption clouds rose into the stratosphere and spread like an umbrella, more than 200 kilometers in all directions. Pinatubo's original summit was completely destroyed, replaced by a 2 kilometer caldera, formed in response to the withdrawal of a large volume of magma. The estimated total volume of volcanic debris was 2 to 4 cubic kilometers. More than 300 people died, mostly from roof collapses. The combination of typhoon Yunya, the death toll might have been smaller. The typhoon brought heavy rain, which saturated the accumulating tephra, and strong winds contributed to the widespread dispersal of the stratospheric eruption cloud.
The 1991 eruption of Mount Pinatubo was the largest in half a century. Calculations based on the volume of erupted debris indicate that about 3 - 5 cubic kilometers of dense magma, ten times the volume of magma from the eruption of Mt. St. Helens, had fallen. Although an unfortunate number of lives were lost, thousands of casualties were averted by timely warnings and evacuations. The eruption of Mount Pinatubo exemplifies the way volcanoes can wreak sudden widespread havoc on the lives of people who live near them. However, with sufficient volcanic monitoring and emergency preparation, the lost of life and property can be minimized.
