DESCRIPTION:
Mount Pinatubo, Philippines

Pinatubo91_eruption_plume_06-12-91.jpg
The June 12, 1991 eruption column from Mount Pinatubo taken from the east side of Clark Air Base. U.S. Geological Survey Photograph taken on June 12, 1991, by Dave Harlow
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Mount Pinatubo
Location: Philippines
Latitude: 15.13 North¹
Longitude: 120.35 East¹
Height: 1,745 meters before the June 15, 1991 eruption and 1,485 meters (high point caldera rim) after²
(Height: 1,600 Meters¹)
Type: Stratovolcano

Mount Pinatubo is one of a chain of composite volcanoes that constitute the Luzon volcanic arc. The arc parallels the west coast of Luzon and reflects eastward-dipping subduction along the Manila trench to the west. Mount Pinatubo is among the highest peaks in west-central Luzon. Its former summit, at 1,745 meters elevation, may have been the crest of a lava dome that formed about 500 years ago during the most recent previous major eruptive episode. The volcano's lower flanks, intricately dissected and densely sheathed in tropical vegetation prior to the 1991 eruptions, were composed largely of pyroclastic deposits from voluminous, explosive prehistoric eruptions.

Before the eruption, more than 30,000 people lived in small villages on the volcano's flanks. A much larger population -- about 500,000 -- continues to live in cities and villages on broad, gently sloping alluvial fans surrounding the volcano. Clark Air Base lies to the east of the volcano, within 25 kilometers of the summit, and Subic Bay Naval Station is about 40 kilometers to the southwest.

Until June 1991, Mount Pinatubo rose 1,745 meters above sea level but only about 600 meters above a gently-sloping apron and only 200 meters above nearby mountains that largely obscured Mount Pinatubo from view. The surrounding mountains are older volcanic centers and relics of an ancestral Mount Pinatubo ... The gently-sloping apron consists of thick pyroclastic-flow and lahar (volcanic mudflow) deposits. ...

... The June 1991 eruption (of Mount Pinatubo) created a 2.5-kilometer-wide collapse caldera and filled valleys around Pinatubo with about 5.5 +/- 0.5 cubic kilometers of pyroclastic-flow deposits. ... The new summit elevation of Mount Pinatubo is approximately 1,485 meters above sea level, reduced from a preeruption elevation of 1,745 meters; the elevation of the caldera lake is between 820 and 840 meters above sea level, or about 650 meters below the highest point on the new caldera rim.

Before its June 15, 1991, eruption, Mount Pinatubo consisted of a rounded, steep-sided, domelike mass that rose about 700 meters above a broad, gently sloping, deeply dissected apron of pyroclastic and epiclastic materials. Some relics of older volcanic edifices, including an ancestral Mount Pinatubo, lay south, east, and northeast of Mount Pinatubo. In comparison to well-known stratocones such as Mayon or Fuji, Mount Pinatubo was small and inconspicuous, but its extensive pyroclastic apron told of large prehistoric explosive eruptions.

Eruption of about 5 cubic kilometers of magma on June 15, 1991 created a new, 2.5-kilometer-diameter collapse caldera centered slightly northwest of the preeruption summit. The preeruption summit was included in the area of collapse, so the posteruption height of Mount Pinatubo was substantially reduced. Valleys that had existed in the pyroclastic apron were largely filled by eruptive products; valleys that had been carved into older volcanic terrain and partly filled by prehistoric eruptions of Mount Pinatubo were partly filled once again.

In the twentieth century, this (1991) eruption was second in size only to an eruption in Katmai, Alaska, in 1912. Ten times larger than the eruption of Mount St. Helens in 1980, Pinatubo's eruption threatened the lives of a million people. A giant ash cloud rose 35 kilometers into the sky and hot blasts seared the countryside, but a more serious disaster was averted by timely, accurate warnings. Philippine authorities were able to evacuate 60,000 people from the slopes and valleys, and the American military evacuated 18,000 personnel and their dependents from Clark Air Base below the mountain -- thus saving many thousands of lives and an estimated billion dollars in property and making this the most successful case of volcanic hazards mitigation in history.

The climactic (1991) eruption destroyed the volcano's original summit. In its place is a 2-kilometer-diameter caldera, the center of which is offset 1 kilometer northward from the pre-eruption summit. The caldera probably formed from collapse of the volcano's summit on June 15, during the period of abundant large earthquakes in response to withdrawal of a large volume of magma from the reservoir beneath the volcano. ...

Tephra-fall deposits 5 centimeters or more thick covered a land area of about 4,000 square kilometers surrounding Pinatubo. These deposits buried crops, and the weight of the rain-saturated tephra, no doubt with assistance from repeated intense seismic shaking and buffeting by wind, caused numerous roofs to collapse in the Philippine communities around the volcano and on the two large U.S. military bases. More than 300 people died during the eruption, most of them from collapsing roofs. Without typhoon Yunya, the death toll might have been far smaller. The typhoon brought heavy rain, which saturated the accumulating tephra, and strong winds that contributed to the widespread dispersal of tephra falling from the broad stratospheric eruption cloud.

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