Mount Pinatubo is one of a chain of composite volcanoes (see Figure 2) that constitute the Luzon volcanic arc (Wolfe, E. & Hoblitt, R., 1999). Composite volcanoes, also called Strato-Volcanoes, are formed by alternating layers of lava and rock fragments (Oracle Thinkquest, 2011). 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 m [meter] 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 (Wolfe, E. & Hoblitt, R., 1999).
The first recognized signs that Pinatubo was reawakening after a 500-year slumber were a series of small steam-blast explosions in early April 1991 (Hendley II, J., Newhall, C. & Stauffer, P., 2005). Scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) intensified their monitoring of the volcano; they were soon joined by scientists from the U.S. Geological