The Eruption
The first major explosion of Mt. Pinatubo’s 1991 eruption began on the morning of June 12. The explosive eruption shot a column of ash and steam that rose 19 km into the air, and generated pyroclastic flows that poured down the mountain. This was a Plinian type eruption, characterized by the explosive eruption of tephra, ash and pyroclastic flows.
From June 12 to June 15 many briefer, explosive eruptions occurred. Tephra and ash were blown by wind to cover most of Luzon with a blanket several centimeters thick.
Area of Ash Fall From 1991 Eruption
http://www.answers.com/topic/mount-pinatubo
http://vulcan.wr.usgs.gov/Volcanoes/Philippines/Pinatubo/images.html
At the same time as the eruption, Typhoon Yunya passed through the area. When the falling ash became wet it became even heavier, causing many buildings to collapse. The water combined with pyroclastic flows to create devastating lahars. These lahars filled in many river valleys and forced runoff to new areas. With the arrival of monsoon season shortly after the eruption, the debris from the volcano continued to create lahars after it was no longer erupting.
A River Valley Before and After the Eruption
http://www.answers.com/topic/mount-pinatubo
Map of Pyroclastic Flows
http://pubs.usgs.gov/fs/1997/fs115-97/
Before the 1991 eruption Mt. Pinatubo was 1,745 meters high, after the eruption a collapse caldera had formed that was 2.5 kilometers across, lowering the height of the volcano to 1,485 meters.
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