On 10 July the western wall of Pinatubo's crater collapsed, gradually releasing about 160 million m3 of water and sediments. The collapse occurred at the crater wall's lowest point leading to the Maraunot River in Botolan, Zambales. About a year earlier workers had removed a section of the crater wall in an effort to gradually release the rapidly rising water in the crater and prevent a larger crater-wall collapse. PHIVOLCS reported to Philstar news that if the notch had not been created at least 320 million m3 of water and sediment could have been released into inhabited areas.

Geological Summary. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Source: Philstar News