When did Pinatubo form?
pinatubo was formed in 2 stages. The ancestral Pinatubo started to form about 1 million years ago, and built an andesite - dacite stratovolcano whose center was at the same location as today. Remnants of this precessor are seen in the ancient 3.5x4.5 wide caldera. Ancient Pinatubo had a number of flank vents, that formed the domes of Mount Negron, Mount Cuadrado, Mataba, Bituin plug, and the volcanic plug of Tapungho.
Deep erosion in the Sacobia, Porac, Marimla, and Porac River valleys, and weathering of the lavas suggests that activity of the ancestral volcano ended several tens of thousands of years (or more) before the caldera-forming eruption and initial growth of the modern Pinatubo (ca. 35,000 years ago).
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