Mount Pinatubo, Philippines: June 1991 Eruptions
Mount Pinatubo is an andesitic island arc volcano, located on the southern Luzon Island,
Philippines. Prior to 1991 it had been dormant for more than 635 years. On April 2,
1991, the volcano, which had been rumbling for months, stirred to life with an explosion.
The next two and a half months were marked by generally increasing volcanic and seismic
activity at the site. On June 15, a cataclysmic eruption began with a tremendous explosion.
This slide set shows the early stages of the eruption phase, the cataclysmic eruption
of June 15, and the effects and aftermath of the eruption. Subsequent calamities caused
by the ash, pyroclastic flows, mud flows, and flooding are also depicted. The hornblende
andesite dacit dome complex is located 100 km northwest of Manila. When the dormant
volcano last erupted more than 635 years ago, it produced pyroclastic flows that filled
the Marella River Valley. On April 2, 1991, the volcano, which had been rumbling for
months, stirred to life with an explosion at the east end of a geothermal area. After
the explosion, a line of new fumaroles roughly one kilometer long developed. Over
the next six weeks several very small earth tremors and minor explosions occurred.
By June 3, seismic activity had increased dramatically. Four days later a dome appeared
on the mountain, oozing molten rock. On June 10, 15,000 Americans were evacuated from
Clark Air Base (located approximately 12 km southeast of Pinatubo); essential personnel
stayed behind. On June 12, a tephra column rose to about 20 km. By June 14, 55,000
people had been evacuated from villages within a 12-mile radius of Pinatubo. Then
at dawn, just before 6:00 A.M. on June 15, 1991, acataclysmic eruption began with
a tremendous explosion. The eruption destroyed ten deserted villages and earthquakes
shook surrounding towns. The towering plume of Mt. Pinatubo soared an astonishing
40 km into the atmosphere. Snow-like ash fell in Manila, 100 km to the South. Locally,
the ash blocked out the sun. Searing gas, ash, and molten rocks (pyroclastics) raced
down the lush northern and western slopes of the volcano. The high velocity and low
density of the flows allowed them to drain off the steep slopes immediately around
the crater. These pyroclastic flows, together with heavy rains, later produced mudflows
and landslides on all flanks of the volcano. After a typhoon on July 18, lahars (hot
mudflows) as deep as three meters along rivers east of the volcano buried sections
of some towns. The filling of river channels and ruptures of dikes caused lahars as
wide as 4 km. Mudflows five meters high rushed through the streets of Concepcion,
sweeping away several people. By July 26, 100,000 homes had been crushed or buried.
When the rain fell on the ash-covered roofs, many collapsed, resulting in more deaths.
On August 20, more than two months following the major eruption, the largest of the
mudflows resulted in thirty-one casualties. One pyroclastic flow made a dam behind
which a lake formed. On September 7, the dam ruptured causing muddy flash floods that
destroyed 800 homes and killed seven. Flooding and mudflows caused twelve more deaths
on September 15. The eruptions at Mount Pinatubo and the aftermath events killed 722
people. Of these, 358 deaths were attributed to disease that broke out in the evacuation
camps, 281 to the initial eruption, and 83 to mudflows. Additionally, 184 people were
injured and 23 were reported missing. More than 108,000 houses were partially or totally
damaged. The event left more than 200,000 people homeless, destroyed businesses, and
ruined Clark Air Base. About 20,000 Americans were evacuated from Subic Bay Naval
Station to the United States after Subic Bay was extensively damaged. Besides local
damage, Pinatubo ejected huge quantities of particles into the global wind system
in the stratosphere to heights exceeding 30 km. These particles affect the weather
in two ways. Tiny aerosol droplets reflect sunlight away from Earth's surface. A maximum
global cooling of about 1.5 degrees C has been observed and is gradually diminishing.
In addition, the aerosols from the eruption help reactivate inert chlorine molecules
drifting in cold clouds in the stratosphere. In sunlight, these chlorine molecules
combine with oxygen molecules in the ozone layer reducing its density. Until the ozone
regenerates, it cannot effectively shield that portion of Earth from the sun. The
aerosol increase may cause a 15% reduction in mid-latitude ozone. Preliminary measurements
suggest that Pinatubo was one of the largest volcanic eruptions of the 20th Century.
The threat of more destruction continues. The combination of rains and loose pyroclastic
materials, sediment-filled channels, area seismicity, and small ash and steam explosions
will continue to make the area dangerous for years to come. On the positive side,
volcanic deposits have enriched the soil on the plains around Pinatubo.