The estimate is being made that Mount Pinatubo , which began to erupt on Saturday , will explode and send its deadly hot ash toward Clark Air Force Base on Luzon . Also at risk are 350 thousand residents of Angeles City . On clear days they fly above the crater to measure sulfur emissions. Even the threat of eruption is likely to keep the base closed indefinitely . Volcanic explosions are caused when one of the earth 's plates collides or moves apart and pressure from the earth 's interior melts the ash and forces it to the surface . Pinatubo 's last eruption 200 years ago produced as much molten rock as Mount St. Helens in 1980 . The ash produced by a catastrophic eruption would be especially dangerous to in - flight aircraft , whose engines would stall . Sensitive sensors placed by U.S. and Philippine forces continue to monitor the mountain .
The estimate is being made that Mount Pinatubo , which began to erupt on Saturday , will explode and send its deadly hot ash toward Clark Air Force Base on Luzon . Also at risk are 350 thousand residents of Angeles City . Even the threat of eruption is likely to keep the base closed indefinitely . Volcanic explosions are caused when one of the earth 's plates collides or moves apart and pressure from the earth 's interior melts the ash and forces it to the surface . Pinatubo 's last eruption 200 years ago produced as much molten rock as Mount St. Helens in 1980 . The ash produced by a catastrophic eruption would be especially dangerous to in - flight aircraft , whose engines would stall . Sensitive sensors placed by U.S. and Philippine forces continue to monitor the mountain .