This spectacular, near-vertical photograph of Mt. Pinatubo can be used to map out the distribution of ash, mud and debris flows around the mountain after the catastrophic 1991 eruption. Subsequent heavy rains have remobilized the thick, unstable ash and mud deposits on the mountain's slopes and caused massive floods and mudflows downstream. The mudflows (lahars) buried many towns and much cultivated land. Every bridge within 30 km of the mountain was destroyed, and most river valleys are now filled to capacity with mud. The mudflows continue after every heavy rain. Violent secondary explosions also occur as groundwater meets the still-cooling pyroclastic flows from the June 1991 eruption.
Approximately 1.5 million people live in the area encompassed by this photograph. This photograph, taken shortly after the July 1992 eruption, shows several important changes on and around the mountain:
1) The crater lake inside the 2 km wide caldera is clearly visable, as is the newly formed lava dome (the small islet in the middle of the lake). Philippine volcanologists estimated the dome (at this stage) to be about 300 m wide and 100 m high.
2) A newly-formed muddy lake appears near the headwaters of the Pasig River (upslope from Clark AFB). Drainage was completely disrupted after the June eruption. Pyroclastic flows buried the divide between the Pasig and Sacobia River and diverted water from the Sacobia River into the Abacan River (which flows just south of Clark AFB). The resulting floods and mudflows eroded through the city of Angeles, a large city on the outskirts of Clark, and buried part of the major highway which runs north from Manila.
3) The large, year-old lake at the junction of the Mapanuepe and Marella Rivers is easily visable (south of the mountain). This is the largest of the new lakes formed from clogged drainages. Should the dam (formed from loosely consolidated ash and debris) fail, a large amount of flooding will occur downstream.
4) Fresh lahar deposits can be identified in the Bucao, Balin Baquero and O'Donnell Rivers (late July deposits). Other new lahars were deposited in the Sacobia and Pasig drainages.
5) A large area appears flooded/buried near Poonbato (Bucao River).
6) One can identify and map the distal end of lahars along the mountains eastern slope where the rivers enter into the central valley.
Mt. Pinatubo on the island of Luzon (15.0N, 120.0E) erupted catastrophically in June 1991 after over 600 years of inactivity. In this vertical view, the full extent of the eruption is obvious. Thick layers of ash completely surrounds the crater and the effect of mudflows in this previously heavily forested and agricultural region can be traced as ribbons flowing downhill. Clark AFB, once the crossroads of the SW Pacific can only partially be seen.