page_title: Mount Pinatubo
section_title: Ancestral Pinatubo
hierarchical_section_title: Mount Pinatubo / History / Ancestral Pinatubo
caption_attribution_description: Mount Pinatubo in April 1991, before the onset of magmatic eruptions. Preeruption Mount Pinatubo, April 16, 1991. View from the northwest, up the Maraunot River valley. The river had become acidic and silty, owing to reactivation of the hydrothermal system and phreatic explosions of April 2, 1991 (the vents of which were just out of view at left edge of photograph). Steam was from 2-week-old fumaroles on the upper north slope of the volcano. The fumarole farthest to the right (behind a jagged ridge, right of the one visible on the valley floor) would later become the site of the preclimactic lava dome extrusion of June 7-12 (Hoblitt, Wolfe, and others, this volume). Mount Negron is behind and to the right of Pinatubo. (R.S. Punongbayan) USGS
context_page_description: Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history was unknown to most before the pre-eruption volcanic activities of 1991, just before June. Pinatubo was heavily eroded, inconspicuous and obscured from view. It was covered with dense forests which supported a population of several thousand indigenous Aetas.
  Pinatubo is most notorious for its VEI-6 eruption on June 15, 1991, the second-largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in Alaska.
  Complicating the eruption was the arrival of Typhoon Yunya, bringing a lethal mix of ash and rain to towns and cities surrounding the volcano. Predictions at the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives. Surrounding areas were severely damaged by pyroclastic surges, pyroclastic falls, and subsequently, by the flooding lahars caused by rainwater re-mobilizing earlier volcanic deposits.
context_section_description: Activity of Ancestral Pinatubo seems to have begun about 1.1 million years ago and probably ended tens of thousands of years or more before the birth of "modern" Pinatubo. Much of the rugged land around the present volcano consists of remnants of "ancestral" Pinatubo. It was an andesite and dacite stratovolcano whose eruptive activity was much less explosive than modern Pinatubo. Its center was roughly where the current volcano is. The projected height of the mountain is up to 2,300 m (7,500 ft), or 1.43 miles above sea level if it were a lone peak, based on a profile fitting to the remaining lower slopes, or lower if it had more than one peak.
  The old volcano is exposed in the walls of an old 3.5 km × 4.5 km (2.2 mi × 2.8 mi) wide caldera, referred to as Tayawan Caldera by Delfin. Some of the nearby peaks are the remnants of ancestral Pinatubo, left behind when the softer parts of the old mountain slopes were eroded by weathering. Ancestral Pinatubo is a somma volcano with modern Pinatubo as the new cone.
  Mount Dorst, to the east, is part of the dip slope of the ancestral Pinatubo. Several mountains near modern Pinatubo are old satellite vents of ancestral Pinatubo, forming volcanic plugs and lava domes. These satellite vents were probably active around the same time as the ancestral volcano and include the domes of Mount Negron, Mount Cuadrado, Mount Mataba and the Bituin and Tapungho plugs.
caption: Pinatubo in April 1991, a few months before the eruption.[17][18]
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