Mount Pinatubo

Mount Pinatubo is an active stratovolcano in the Zambales Mountains in Luzon in the Philippines. Located on the tripoint of Zambales, Tarlac and Pampanga provinces, most people were unaware of its eruptive history before the pre-eruption volcanic activity in early 1991. Dense forests, which supported a population of several thousand indigenous Aetas, heavily eroded and obscured Pinatubo. Pinatubo is known 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. The eruption coincided with Typhoon Yunya making landfall in the Philippines, which brought a dangerous mix of ash and rain to nearby towns and cities. Early predictions led to the evacuation of tens of thousands of people, saving many lives. The eruption severely damaged surrounding areas with pyroclastic surges, pyroclastic falls, and later, flooding lahars caused by rainwater re-mobilizing volcanic deposits. This destruction affected infrastructure and altered river systems for years. Minor dome-forming eruptions inside the caldera continued from 1992 to 1993. The 1991 eruption had worldwide effects. It released roughly 10 billion tonnes (1.1×1010 short tons) or 10 km3 (2.4 cu mi) of magma, bringing large amounts of minerals and toxic metals to the surface. It also released 20 million tonnes (22 million short tons) of SO2. It ejected more particulate into the stratosphere than any eruption since Krakatoa in 1883. In the following months, aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F) in the years 1991–1993, and ozone depletion temporarily increased significantly.

Geography

The volcano is about 87 kilometers (54 mi) northwest of Manila, the capital of the Philippines. Near Mount Pinatubo are former military bases that were maintained by the United States. The U.S. Naval Base Subic Bay was 37 kilometers (23 mi) south of Pinatubo, and the extent of Clark Air Base was just 14 kilometers (8.7 mi) east of the volcano's summit. The volcano is near to about 6 million people.

History Even before the 1991 eruption, Mount Pinatubo had little topographic prominence: it was 1,745 m (5,725 ft) above sea level, only about 600 m (2,000 ft) above nearby plains, and only about 200 m (660 ft) higher than surrounding peaks, which largely obscured it from view. It is part of a chain of volcanoes which lie along the western side of the island of Luzon called the Zambales Mountains. Pinatubo belongs to the Cabusilan sub-range of the Zambales Mountains, which consists of Mount Cuadrado, Mount Negron, Mount Mataba and Mount Pinatubo. They are subduction volcanoes, formed by the Eurasian plate sliding under the Philippine Mobile Belt along the Manila Trench to the west. Mount Pinatubo and the other volcanoes on this volcanic belt arise due to magmatic occlusion from this subduction plate boundary. Pinatubo is flanked on the west by the Zambales Ophiolite Complex, which is an easterly-dipping section of Eocene oceanic crust uplifted during the late Oligocene. The Tarlac Formation north, east and southeast of Pinatubo consists of marine, nonmarine and volcanoclastic sediments formed in the late Miocene and Pliocene. The most recent study of Mount Pinatubo before the activities of 1991 was the overall geological study in 1983 and 1984 made by F. G. Delfin for the Philippine National Oil Company as part of the surface investigations of the area before exploratory drilling and well testing for geothermal energy sources in 1988 to 1990. He recognized two life histories of the mountain, which he classified as "ancestral" and "modern" Pinatubo.

Ancestral Pinatubo

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.

Modern Pinatubo c. 79,000 BC: After a long period of dormancy, Modern Pinatubo was born in Ancestral Pinatubo's cataclysmic and most explosive eruptions, estimated to be five times larger than the June 1991 eruption. It deposited all around the volcano up to 25 km3 (6.0 cu mi) of pyroclastic surge material up to 100 meters (330 ft) thick. The total volume of volcanic material ejected during the eruptions is unknown. The removal of so much material from the underlying magma chamber resulted in the Tayawan caldera. The violent eruptive period started by the eruption is referred to by Delfin as the Inararo Eruptive Period, named after a village that was destroyed in the 1991 eruption. Later eruptions of modern Pinatubo occurred episodically and lasted for periods much shorter than the repose intervals between them. Subsequent eruptions and eruptive period occurred about:

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