Mount Pinatubo

Mount Pinatubo is an active stratovolcano located on the island of Luzon, at the intersection of the borders of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26 mi) west of the dormant and more prominent Mount Arayat, occasionally mistaken for Pinatubo. Ancestral Pinatubo was a stratovolcano made of andesite and dacite. Before 1991, the mountain was inconspicuous and heavily eroded. It was covered in dense forest which supported a population of several thousand indigenous people, the Aeta, who had fled to the mountains from the lowlands during the protracted Spanish conquest of the Philippines which began in 1565.

Mount Pinatubo, boundaries of Pampanga,
Tarlac and Zambales, Philippines

Elevation:  1.445 km (height before eruption was 1.745 km)       

Base Diameter:  40 km                                          

Type of Volcano:  Compound

Caldera Lake:  Pinatubo Crater Lake 

 (2 km in diameter and depth of 600 to 800 meters)   

Adjacent Volcanic Edifice:  

 Quadrado, Malasimbo, Balakibok, Negron

Rock Type:  Dacite with andesitic xenoliths, inclusions       

Tectonic Setting:  Western Bataan Lineament      

Age of Deposits:  1.10+0.09 Ma; 635+80 B.P; 1991;1992

 

The volcano's ultra-Plinian eruption in June 1991 produced the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta) and the largest eruption in living memory. The colossal 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, and came some 450–500 years after the volcano's last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 1000 years after previous VEI 6 eruptive activity. Successful predictions of the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but surrounding areas were severely damaged by pyroclastic flows, ash deposits, and later by lahars caused by rainwater remobilizing earlier volcanic deposits: thousands of houses and other buildings were destroyed.
The effects of the eruption were felt worldwide. It ejected roughly 10 billion metric tonnes (10 cubic kilometres) of magma, and 20 million tons of SO₂, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosols into the stratosphere – more than any eruption since that of 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), and ozone depletion temporarily increased substantially.

The eruption column of Mount Pinatubo
during the 1991 eruption.

Mount Pinatubo is part of a chain of volcanoes which lie along the western side of the edge of the island of Luzon. They are subduction volcanoes, formed by the Philippine Mobile Belt sliding over the Eurasian Plate along the Manila Trench to the west. Mount Pinatubo and the other volcanoes of the West Luzon volcanic arc arise due to magma occlusion from this subduction plate boundary.
The volcano is located 87 km (55 miles) northwest of Manila, 14 km (9 miles) west of the former Clark Air Base, and 37 km (23 mi) north of the former U.S. Naval Base Subic Bay. Clark Air Base's residential areas and petroleum storage facilities were in much closer proximity to the volcano than the airfield complex and neighboring Angeles City.
Several important river systems have their sources on Pinatubo, with the major rivers being the Bucao, Santo Tomas, Maloma, Tanguay, Ashley, and Kileng rivers. Before the eruption, these river systems were important ecosystems, but the eruption filled many valleys with deep pyroclastic deposits. Since 1991, the rivers have been clogged with sediment, and the valleys have seen frequent lahars. Studies show that the river systems will take many years yet to recover from the 1991 eruption.
About 500,000 people continue to live within 40 km of the mountain, with population centres including the 150,000 in Angeles City, and 30,000 at Clark Freeport Zone.

One of the early explosive eruptions at
Pinatubo after the April 1991 onset of ash eruptions

Although there seems to be no local knowledge of the previous large eruptions in the Pinatubo area, several Aeta residents reported in 1991 that their elders recalled small explosions in the past. Pinatubo was a known geothermal area before the 1991 eruption, and small steam explosions are quite common in such areas. It was only after volcanic activity began in 1991 that geologists studied the eruptive history of the region in any detail. Eruptions at the site can be divided into two major eras.

Much of the rugged land surrounding the present volcano consists of remnants of 'ancestral' Pinatubo. This volcano was located roughly in the same place as the present mountain, and activity seems to have begun about 1.1 million years ago. Ancestral Pinatubo may have reached a height of up to 2,300 m (7,550 ft) above sea level, based on profile fitting to the remaining lower slopes.
Several mountains near modern Pinatubo are old satellite vents of ancestral Pinatubo, formed from volcanic plugs and lava domes. Some nearby peaks are also remnants of ancestral Pinatubo, formed from erosion-resistant parts of the old mountain slopes left behind when the less resistant parts were eroded by weathering.
The eruptive activity of ancestral Pinatubo was much less explosive than modern Pinatubo, and probably ended about 45,000 years ago. After a long period of dormancy, modern Pinatubo was born in eruptions beginning about 35,000 years ago.

Mount Pinatubo's caldera contains a
lake of the same name, pictured here in 2008

The birth of modern Pinatubo occurred in the most explosive eruption in its history, which deposited pyroclastic flow material up to 100 meters thick on all sides of the mountain. The total volume of material erupted may have been up to 25 cubic kilometers (6 mile³), and the removal of this amount of material from the underlying magma chamber led to the formation of a large caldera.
Earlier large eruptions occurred 17,000, 9000, 6000 – 5000, and 3900 – 2300± years ago. Each of these eruptions seems to have been very large, ejecting more than 10 km³ of material and covering large parts of the surrounding areas with pyroclastic flow deposits. Scientists estimate that the most recent eruption before 1991 happened about 450 years ago, and after that, the volcano lay dormant. Its slopes became completely covered in dense rainforest and eroded into gullies and ravines.


Coordinates: 15°08′30″N 120°21′00″E