Mt. Pinatubo - About

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Mount Pinatubo is an active stratovolcanic caldera 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.


The volcano's Ultra Plinian eruption on June 15, 1991 produced the second-largest terrestrial eruption of the 20th century.  Complicating the eruption was the arrival of Typhoon Yunya (Diding), bringing a lethal messy 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, ash falls, and subsequently, by the flooding lahars caused by rainwater re-mobilizing earlier volcanic deposits. This caused extensive destruction to infrastructure and changed river systems for years after the eruption.


The effects of the eruption were felt worldwide. It ejected roughly 10,000,000,000 tonnes (1.1×1010 short tons) or 10 km3 (2.4 cu mi) of magma, and 20,000,000 tones (22,000,000 short tons) of SO2, bringing vast quantities of minerals and toxic metals to the surface environment. It injected more particulate into the stratosphere than any eruption since Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F) in the years 1991–93,[7] and ozone depletion temporarily increased substantially.


Overview of the area

The location of Mount Pinatubo and the regional ash fallout from the 1991 eruption.
The volcano is 87 km (54 mi) northwest of Manila, the capital of the Philippines. Near Mount Pinatubo, the United States maintained two large military bases in the region. The U.S. Naval Base Subic Bay was 37 km (23 mi) south of Pinatubo, and the extent of Clark Air Base was just 14 km (8.7 mi) east of the volcano's summit.[9] The volcano is near to about 6 million people.[10]


History
Mount Pinatubo's summit before the 1991 eruption 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 Mt. Cuadrado, Mt. Negron, Mt. Mataba and Mt. 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.