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Mount Pinatubo

scene of one of the largest volcanic eruptions in modern history

Pinatubo prior to 1991general location of Mount PinatuboMount Pinatubo is located on the island of Luzon in the Philippines, about 55 miles northwest of Manila. Prior to June 1991, the mountain's peak was 5,725 feet above sea level and its slopes were covered with forests.

One of a chain of volcanoes that parallels the Manilla Trench, where part of the Eurasian plate is subducting under the Philippines, the "original" Mount Pinatubo began forming about a million years ago. Geological evidence shows that the "ancestral peak" grew to a height of about 7,500 feet before going dormant. "Modern" Pinatubo first emerged from the original volcano with a massive explosive eruption about 35,000 years ago. While "ancestral Pinatubo" was marked by relatively frequent but "mild" outpourings of lava, modern Pinatubo has been marked by long periods of inactivity being broken by very explosive eruptions.

Pinatubo had been dormant for about 500 years before July 16, 1990, when a magnitude 7.8 earthquake (comparable to the San Francisco earthquake of 1906) struck about 60 miles to the northeast. The area immediately surrounding Pinatubo experienced several smaller earthquakes and a landslide, and residents noted a short-lived increase in emissions from a geothermal vent. There was no evidence at the time, however, that the earthquake was in any way connected with activity under Mount Pinatubo.

On March 15, 1991, a succession of earthquakes were felt by villagers on the northwestern side of the volcano. Further earthquakes of increasing intensity were felt over the next two weeks, and by April geologists had determined that magma was rising towards the surface from 20+ miles beneath the mountain. By June geologists were warning that an eruption was imminent, and by June 5th an area extending out 12.4 miles from Pinatubo had been avacuated. The first magma reached the surface on June 7th, but since most of its gas had been lost during its "journey" it simply oozed out into the crater to form a lava dome. Authorities chose not to take chances however, and the evacuation zone had been extended out to 18.6 miles by June 12th.

explosion of Mount Pinatubo on June 12eruption of June 15thThe most violent volcanic eruption since Krakatoa in 1883 began with an explosion marking the emergence of gas-charged magma at 3:41 am (local time) on June 12th. Larger explosions began a few hours later, generating an ash column that at its peak reached a height of about 15 miles, as well as pyroclastic flows extending up to 2.5 miles from the summit. Explosive eruptions continued for three days, with the climax coming on June 15th. The final eruption, which lasted about nine hours, generated an ash cloud that rose 22 miles into the air and was 250 miles wide, and filled valleys with pyroclastic flows to depths of 660 feet. The last seismic activity was recorded about 10:30 pm on the 15th, and the mountain went silent again. On August 1st a science team was able to report that the summit of Mount Pinatubo had collapsed into a caldera 1.6 miles across and that the mountain had lost 853 feet of height.

valley filled by pyroclastic flowsMount Pinatubo on August 1, 1991

Clark Air Force Base after the eruptionSubic Bay after the eruptionWhile authorities were able to prevent thousands of deaths by evacuating the area around Mount Pinatubo prior to the eruption, Tropical Storm Yunya happened to be striking northeast of the volcano at the same time as the eruption. The heavy rains accompanying that storm mixed with the ash fall from the volcano, causing many buildings to collapse under the weight. Those collapses, along with heavy mudslides generated by a combination of volcanic eruption and tropical storm caused the deaths of approximately 800 people. The eruption also forced the evacuation of Clark U.S. Air Force Base, which was devastated by the ashfall and never reopened. The ash cloud also damaged the U.S. Naval Base at Subic Bay, to which Clark AFB personnel had been evacuated, and it was subsequently abandoned by the U.S. Navy. The ash cloud generated by Pinatubo ultimately circled the earth at least once and directly affected weather patterns worldwide for at least two years, with the 1993 floods along the Mississippi River being one of the most visible results.

map showing area affected by the eruption

crater of Mount Pinatubo today

Volcano Discovery


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  The Robinson Library > Science > Geology > Dynamic and Structural Geology

This page was last updated on 01/04/2015.

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