Mount Pinatubo

Mount Pinatubo

Overview
Mount Pinatubo is an active stratovolcano
Stratovolcano
A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

 located on the island of Luzon
Luzon
Luzon is the largest island in the Philippines. It is located in the northernmost region of the archipelago, and is also the name for one of the three primary island groups in the country centered on the Island of Luzon...

, near the tripoint
Tripoint
A tripoint, or trijunction , is a geographical point at which the borders of three countries or subnational entities meet....

 of the Philippine provinces of Zambales
Zambales
Zambales is a province of the Philippines located in the Central Luzon region. Its capital is Iba. Zambales borders Pangasinan to the north, Tarlac and Pampanga to the east, and Bataan to the south. The province lies between the South China Sea and the Zambales Mountains. With a land area of...

, Tarlac
Tarlac
Tarlac is a landlocked province of the Philippines located in the Luzon Island. Its capital is Tarlac City. Tarlac borders Pampanga to the south, Nueva Ecija to the east, Pangasinan to the north, and Zambales to the west...

, and Pampanga
Pampanga
Pampanga is a province of the Philippines located in the Central Luzon region. Its capital is the City of San Fernando, Pampanga. Pampanga is bordered by the provinces of Bataan and Zambales to the west, Tarlac and Nueva Ecija to the north, and Bulacan to the southeast...

. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26.1 mi) west of the dormant and solitary Mount Arayat
Mount Arayat
Mount Arayat is an extinct stratovolcano on Luzon Island, Philippines, rising to a height of There is no recorded eruption of the volcano, and its last activity probably dates to the Holocene era.The volcano is located in a flat agricultural region at...

. Before 1991, the mountain has no recorded historical eruptions. It was heavily eroded
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

, inconspicuous and obscured from view.
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Mount Pinatubo is an active stratovolcano
Stratovolcano
A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

 located on the island of Luzon
Luzon
Luzon is the largest island in the Philippines. It is located in the northernmost region of the archipelago, and is also the name for one of the three primary island groups in the country centered on the Island of Luzon...

, near the tripoint
Tripoint
A tripoint, or trijunction , is a geographical point at which the borders of three countries or subnational entities meet....

 of the Philippine provinces of Zambales
Zambales
Zambales is a province of the Philippines located in the Central Luzon region. Its capital is Iba. Zambales borders Pangasinan to the north, Tarlac and Pampanga to the east, and Bataan to the south. The province lies between the South China Sea and the Zambales Mountains. With a land area of...

, Tarlac
Tarlac
Tarlac is a landlocked province of the Philippines located in the Luzon Island. Its capital is Tarlac City. Tarlac borders Pampanga to the south, Nueva Ecija to the east, Pangasinan to the north, and Zambales to the west...

, and Pampanga
Pampanga
Pampanga is a province of the Philippines located in the Central Luzon region. Its capital is the City of San Fernando, Pampanga. Pampanga is bordered by the provinces of Bataan and Zambales to the west, Tarlac and Nueva Ecija to the north, and Bulacan to the southeast...

. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26.1 mi) west of the dormant and solitary Mount Arayat
Mount Arayat
Mount Arayat is an extinct stratovolcano on Luzon Island, Philippines, rising to a height of There is no recorded eruption of the volcano, and its last activity probably dates to the Holocene era.The volcano is located in a flat agricultural region at...

. Before 1991, the mountain has no recorded historical eruptions. It was heavily eroded
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

, inconspicuous and obscured from view. It was covered with dense forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 which supported a population of several thousand indigenous people, the Aeta
Aeta
The Aeta , Agta or Ayta are an indigenous people who live in scattered, isolated mountainous parts of Luzon, Philippines. They are considered to be Negritos, who are dark to very dark brown-skinned and tend to have features such as a small stature, small frame, curly to kinky afro-like textured...

, who had fled to the mountains from the lowlands during the Spanish
Spanish Empire
The Spanish Empire comprised territories and colonies administered directly by Spain in Europe, in America, Africa, Asia and Oceania. It originated during the Age of Exploration and was therefore one of the first global empires. At the time of Habsburgs, Spain reached the peak of its world power....

 conquest of the Philippines.

The volcano's Plinian / Ultra-Plinian eruption (~VEI 6) on June 15, 1991 produced the second largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta
Novarupta
Novarupta, meaning "new eruption", is a volcano located on the Alaska Peninsula in Katmai National Park and Preserve, about southwest of Anchorage. Formed in 1912 during the largest volcanic eruption of the 20th century, Novarupta released 30 times the volume of magma as the 1980 eruption of...

 in the Alaska Peninsula
Alaska Peninsula
The Alaska Peninsula is a peninsula extending about to the southwest from the mainland of Alaska and ending in the Aleutian Islands. The peninsula separates the Pacific Ocean from Bristol Bay, an arm of the Bering Sea....

.
The colossal 1991 eruption had a Volcanic Explosivity Index
Volcanic Explosivity Index
The Volcanic Explosivity Index was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions....

 (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
1980 eruption of Mount St. Helens
The 1980 eruption of Mount St. Helens, a stratovolcano located in Washington state, in the United States, was a major volcanic eruption. The eruption was the only significant one to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California...

), and some 1000 years after previous VEI 6 eruptive activity. A VEI of 6 corresponds to 10 to 100 cubic km of released material (Pinatubo released an estimated 6 to 16 cubic km of ash). 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 flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

s, ash deposits, and later by lahar
Lahar
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...

s 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
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosol  into the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 – more than any eruption since that of Krakatoa
Krakatoa
Krakatoa is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island , and the volcano as a whole. The island exploded in 1883, killing approximately 40,000 people, although some estimates...

 in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 haze. Global temperatures dropped by about 0.5 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

 (0.9 °F
Fahrenheit
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...

), and ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 depletion temporarily increased substantially.

Overview of the area


The volcano is located 87 km (54.1 mi) northwest of Manila
Manila
Manila is the capital of the Philippines. It is one of the sixteen cities forming Metro Manila.Manila is located on the eastern shores of Manila Bay and is bordered by Navotas and Caloocan to the north, Quezon City to the northeast, San Juan and Mandaluyong to the east, Makati on the southeast,...

, 14 km (8.7 mi) west of the former Clark Air Base
Clark Air Base
Clark Air Base is a former United States Air Force base on Luzon Island in the Philippines, located 3 miles west of Angeles City, about 40 miles northwest of Metro Manila. Clark Air Base was an American military facility from 1903 to 1991...

, and 37 km (23 mi) north of the former U.S. Naval Base Subic Bay
U.S. Naval Base Subic Bay
U.S. Naval Base Subic Bay was a major ship-repair, supply, and rest and recreation facility of the United States Navy located in Zambales, Philippines. It was the largest U.S...

. 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
Angeles City
The City of Angeles , located within the province of Pampanga in the Philippines, is locally classified as a first-class, highly-urbanized city. Its name is derived from El Pueblo de los Ángeles in honor of its patron saints, Los Santos Ángeles de los Custodios , and the name of its founder, Don...

.

The Aetas


An indigenous group of people, the Aetas (also spelled as Ayta/Ita), had lived on the slopes of the volcano and in surrounding areas for several centuries, having fled the lowlands to escape persecution by the Spanish during their conquest of the Philippines which began in 1565. They were a hunter-gatherer
Hunter-gatherer
A hunter-gatherer or forage society is one in which most or all food is obtained from wild plants and animals, in contrast to agricultural societies which rely mainly on domesticated species. Hunting and gathering was the ancestral subsistence mode of Homo, and all modern humans were...

 people who were extremely successful in surviving in the dense jungles of the area. These people also grew some staple crops such as wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

, barley
Barley
Barley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...

, and rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

.

In total, about 30,000 people lived on the flanks of the volcano in about 25 established barangay
Barangay
A barangay is the smallest administrative division in the Philippines and is the native Filipino term for a village, district or ward...

s (villages) and other small settlements like Tarukan village and Maruglu. The dense jungle covering most of the mountain and surrounding peaks supported the hunter-gathering Aeta, while on the surrounding flatter areas, the abundant rainfall (almost 4 m annually) provided by the monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...

 climate and the fertile volcanic soils provided excellent conditions for agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

, and many people grew rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

 and other staple foods.

Geological history


Mount Pinatubo's summit was 1745 m (5,725.1 ft) above sea level, but only about 600 m (1,968.5 ft) above nearby plain
Plain
In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or...

s, and about 200 m (656.2 ft) higher than surrounding peaks, which largely obscured it from view. It is a part of a chain of volcanoes which lie along the western side of the edge of the island of Luzon
Luzon
Luzon is the largest island in the Philippines. It is located in the northernmost region of the archipelago, and is also the name for one of the three primary island groups in the country centered on the Island of Luzon...

 (West Luzon volcanic arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...

) specifically referred to as the Tri-Cabusilan Mountain Range which consists of Mt. Cuadrado, Mt. Negron, Mt. Tayawan and Mt. Pinatubo. They are subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...

 volcanoes, formed by the Philippine Mobile Belt
Philippine Mobile Belt
The Philippine Mobile Belt is a complex portion of the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate, comprising most of the country of the Philippines. It includes two subduction zones, the Manila Trench to the west and the Philippine Trench to the east, as well as the...

 sliding over the Eurasian Plate
Eurasian Plate
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia , with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia...

 along the Manila Trench
Manila Trench
The Manila Trench is an ocean trench in the South China Sea, west of the Philippines. It reaches a depth of about 5,400 m, in contrast with the average depth of the South China Sea of about 1,500 m...

 to the west. Mount Pinatubo and the other volcanoes of the volcanic arc arise due to magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 occlusion from this subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...

 plate
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 boundary.

Pinatubo is flanked on the west by the Zambales Ophiolite Complex, which is an easterly-dipping section of Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 ocean crust which was uplifted during the late Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

. The Tarlac Formation consists of marine, nonmarine and volcaniclastic sediments in the north, east and southeast of Pinatubo which was formed in the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 and Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

. Pinatubo was an andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...

 and dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...

 stratovolcano whose center was in the same location of the current volcano. The old volcano is exposed in the walls of an old 3.5 x 4.5 km wide 3.5 by 4.5 km (2.2 by 2.8 mi) caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

. Ancestral satellite vents include the domes of Mount Negron, Mount Cuadrado, Mt. Mataba, Bituin plug, and Tapungho plug.

Ancestral Pinatubo


Much of the rugged land surrounding the present volcano consists of remnants of 'ancestral' Pinatubo (~1 Ma to an unknown time before 35 ka). 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
M
M is the thirteenth letter of the basic modern Latin alphabet.-History:The letter M is derived from the Phoenician Mem, via the Greek Mu . Semitic Mem probably originally pictured water...

 (7,550 ft) above sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

, based on profile fitting to the remaining lower slopes.

Several mountains near modern Pinatubo are old satellite vents of ancestral Pinatubo, formed from volcanic plug
Volcanic plug
A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an...

s and lava dome
Lava dome
|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

s. Some nearby peaks are also remnants of ancestral Pinatubo, formed from erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

-resistant parts of the old mountain slopes left behind when the less resistant parts were eroded by weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

.

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.

Peaks like Mt. Negron, Mt. Cuadrado, Mt. Dorst, Mt. Tayawan and Mt Donald are believed to have been part of the volcano's original peak. Despite becoming heavily silted with mud deposits, rivers like Tarlac River
Tarlac River
The Tarlac River is a river in the Philippines, located in the province of Tarlac.It is visible from the dike near the Tarlac City Wet Market. It was formerly the site of traditional balsa or bamboo raft riding, until the river was heavily silted by sticky lahar or mud flow brought by Mt. Pinatubo...

 and Abacan River still flow through the mountain's summit.

Modern Pinatubo


The birth of Modern Pinatubo (>35 ka to the present) occurred in the most explosive eruption in its history, which deposited pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

 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
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...

 led to the formation of a large caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

 which was filled with water by monsoon rains several months after the climatic eruption, Lake Pinatubo has since become a good tourist attraction; the preferred route is through Barangay Santa Juliana in Capas, Tarlac
Capas, Tarlac
Capas is a first class municipality in the province of Tarlac, Philippines. According to the latest census, it has a population of 122,084 people in 18,333 households. It is a part of the Third Municipal district of Tarlac with Mayor Antonio "TJ" Capitulo Rodriguez, Jr. as its incumbent Mayor and...

.

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
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...

 and eroded into gullies and ravines.

Possible precursor in 1990


On July 16, 1990, the major 1990 Luzon earthquake
1990 Luzon earthquake
The Luzon earthquake occurred on Monday, July 16, 1990, at 4:26 PM local time in the Philippines. The densely populated island of Luzon was struck by an earthquake with a 7.8 Ms...

 of magnitude 7.7 struck central Luzon. This was the largest earthquake recorded in 1990,
comparable in size to the 1906 San Francisco earthquake
1906 San Francisco earthquake
The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, California, and the coast of Northern California at 5:12 a.m. on Wednesday, April 18, 1906. The most widely accepted estimate for the magnitude of the earthquake is a moment magnitude of 7.9; however, other...

 and the 2008 Sichuan earthquake
2008 Sichuan earthquake
The 2008 Sichuan earthquake or the Great Sichuan Earthquake was a deadly earthquake that measured at 8.0 Msand 7.9 Mw occurred at 14:28:01 CST...

. Its epicenter was in Rizal, Nueva Ecija
Rizal, Nueva Ecija
Rizal is a 3rd class municipality in the province of Nueva Ecija, Philippines. According to the latest census, it has a population of 52,465 people in 10,001 households....

 municipality,
about 100 km northeast of Pinatubo, and faulted northwest-southeast through three provinces. It also followed the Philippine Fault System
Philippine Fault System
The Philippine Fault System is an inter-related system of faults throughout the whole of the Philippine Archipelago, primarily caused by tectonic forces compressing the Philippines into what tectonic geophysicists call the Philippine Mobile Belt....

 west as far as Baguio City
Baguio City
The City of Baguio is a highly urbanized city in northern Luzon in the Philippines. Baguio City was established by Americans in 1900 at the site of an Ibaloi village known as Kafagway...

, which was devastated, and is located about 80 km north-northeast of Pinatubo, leading volcanologists to speculate that it might ultimately have triggered the 1991 eruption, although this is impossible to prove conclusively. Two weeks after the earthquake, local residents reported steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

 coming from the volcano, but scientists who visited the mountain in response found only small landslide
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

s rather than any eruptive activity.

1991 activities leading to the eruption


On March 15, 1991, a succession of earthquakes was felt by villagers on the northwestern side of the volcano. Further earthquakes of increasing intensity were felt over the next two weeks, and it became clear some kind of volcanic activity was likely. On April 2, the volcano awoke, with phreatic eruption
Phreatic eruption
A phreatic eruption, also called a phreatic explosion or ultravulcanian eruption, occurs when rising magma makes contact with ground or surface water. The extreme temperature of the magma causes near-instantaneous evaporation to steam, resulting in an explosion of steam, water, ash, rock, and...

s occurring near the summit along a 1.5 km long fissure. Over the next few weeks, small eruptions continued, dusting the surrounding areas with Volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

. Seismographs recorded hundreds of small earthquakes every day.

Scientists immediately installed monitoring equipment and analyzed the volcano for clues as to its previous eruptive history. Radiocarbon dating
Radiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...

 of charcoal found in old volcanic deposits revealed three major explosive eruptions in recent millennia, about 5500, 3500 and 500 years ago. Geological mapping showed that much of the surrounding plains were formed by lahar
Lahar
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...

 deposits from previous eruptions.

Volcanic activity increased throughout May. Measurements of sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 emissions showed a rapid increase from 500 ton
Ton
The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...

s per day by May 13 to 5,000 tons/day by May 28. This implied that there was a rising column of fresh magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 beneath the volcano. After May 28, the amount of SO2 being emitted decreased substantially, raising fears that the degassing of the magma had been blocked somehow, leading to a pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 build-up in the magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...

 and a high likelihood of explosive eruptions.

The first magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

tic eruptions occurred on June 3, and the first large explosion on June 7 generated an ash column 7 km (4.3 mi) high. The Philippine Institute of Volcanology and Seismology
Philippine Institute of Volcanology and Seismology
The Philippine Institute of Volcanology and Seismology is a Philippine national institution dedicated to provide information on the activities of volcanoes, earthquakes and tsunamis, as well as other specialized information and services primarily for the protection of life and property and in...

 (PHIVOLCS) issued a warning indicating the possibility of a major eruption within two weeks.

Evacuations


Given all the signs that a very large eruption was imminent, PHIVOLCS – assisted by the U.S. Geological Survey – worked to convince local inhabitants of the severity of the threat. A false warning might have led to cynicism about any later warnings, but delaying a warning until an eruption began might lead to thousands of deaths, so the volcanologists were under some pressure to deliver a timely and accurate assessment of the volcanic risk.

Three successive evacuation zones were defined, the innermost containing everything within 10 km of the volcano's summit, the second extending from 10 to 20 km from the summit, and the third extending from 20 to 40 km from the summit (Clark Air Base
Clark Air Base
Clark Air Base is a former United States Air Force base on Luzon Island in the Philippines, located 3 miles west of Angeles City, about 40 miles northwest of Metro Manila. Clark Air Base was an American military facility from 1903 to 1991...

 and Angeles City
Angeles City
The City of Angeles , located within the province of Pampanga in the Philippines, is locally classified as a first-class, highly-urbanized city. Its name is derived from El Pueblo de los Ángeles in honor of its patron saints, Los Santos Ángeles de los Custodios , and the name of its founder, Don...

 were in this zone). The 10 km and 10–20 km zones had a total population of about 40,000, while some 331,000 people lived in the 20–40 km zone. Five stages of volcanic alert were defined, from level 1 (low level seismic disturbances) up to level 5 (major eruption in progress). Daily alerts were issued stating the alert level and associated danger area, and the information was announced in major national and local newspaper
Newspaper
A newspaper is a scheduled publication containing news of current events, informative articles, diverse features and advertising. It usually is printed on relatively inexpensive, low-grade paper such as newsprint. By 2007, there were 6580 daily newspapers in the world selling 395 million copies a...

s, on radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 and television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 stations, by nongovernmental organizations (NGOs), and directly to the endangered inhabitants.

Many of the Aeta
Aeta
The Aeta , Agta or Ayta are an indigenous people who live in scattered, isolated mountainous parts of Luzon, Philippines. They are considered to be Negritos, who are dark to very dark brown-skinned and tend to have features such as a small stature, small frame, curly to kinky afro-like textured...

 who lived on the slopes of the volcano left their villages of their own volition when the first explosions began in April, gathering in a village about 12 kilometres (7.5 mi) from the summit. They moved to increasingly distant villages as the eruptions escalated, some Aeta moving up to nine times in the two months preceding the cataclysmic eruption.

The first formal evacuations were ordered from the 10 kilometres (6.2 mi) zone on April 7. Evacuation of the 10 – zone was ordered when a level 4 alert was issued on June 7. A level 5 alert triggered evacuation of the 20 – zone on June 13, and in all some 60,000 people had left the area within 30 kilometres (18.6 mi) of the volcano before June 15. Most people temporarily relocated to Manila
Manila
Manila is the capital of the Philippines. It is one of the sixteen cities forming Metro Manila.Manila is located on the eastern shores of Manila Bay and is bordered by Navotas and Caloocan to the north, Quezon City to the northeast, San Juan and Mandaluyong to the east, Makati on the southeast,...

 and Quezon City
Quezon City
Quezon City is the former capital and the most populous city in the Philippines. Located on the island of Luzon, Quezon City is one of the cities and municipalities that make up Metro Manila, the National Capital Region. The city was named after Manuel L...

, with some 30,000 using the Amoranto Velodrome
Amoranto Velodrome
The Amoranto Velodrome is located inside the Amoranto Sports Complex in Quezon City, Philippines. In 2005, it was the venue for the cycling track events of the 23rd Southeast Asian Games. The stadium holds 15,000 people.-Source:*...

 in Quezon City as an evacuee camp
Emergency evacuation
Emergency evacuation is the immediate and rapid movement of people away from the threat or actual occurrence of a hazard. Examples range from the small scale evacuation of a building due to a bomb threat or fire to the large scale evacuation of a district because of a flood, bombardment or...

.

Climactic eruption buildup



In early June, tiltmeter
Tiltmeter
A tiltmeter is an instrument designed to measure very small changes from the horizontal level, either on the ground or in structures. A similar term, in less common usage, is the inclinometer...

 measurements had shown that the volcano was inflating, evidently due to growing amounts of magma filling the reservoir beneath the summit. At the same time, seismic activity, previously concentrated at a depth of a few kilometers below a point about 5 kilometres (3.1 mi) northwest of the summit, shifted to shallow depths just below the summit. On June 7, the first magmatic eruptions took place with the formation of a lava dome
Lava dome
|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

 at the summit of the volcano. The dome grew substantially over the next five days, reaching a maximum diameter of about 200 metres (656.2 ft) and a height of 40 metres (131.2 ft).

A small explosion at 03:41 on June 12 marked the beginning of a new, more violent phase of the eruption. A few hours later, large explosions lasting about half an hour generated an eruption column
Eruption column
An eruption column consists of hot volcanic ash emitted during an explosive volcanic eruption. The ash forms a column rising many kilometres into the air above the peak of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km, penetrating the stratosphere...

 which quickly reached heights of over 19 kilometres (11.8 mi), and which generated pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

s extending up to 4 kilometres (2.5 mi) from the summit in some river valley
Valley
In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...

s. Fourteen hours later, a 15-minute eruption hurled ash to heights of 24 km. Friction in the uprushing ash column generated abundant lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

.

A third large eruption began at 08:41 on June 13, after an intense swarm of small earthquakes over the previous two hours. It lasted about five minutes, and the eruption column once again reached 24 km. After three hours of quiet, seismic activity began, growing more and more intense over the next 24 hours, until a three-minute eruption generated a 21 kilometres (13 mi) high eruption column at 13:09 on June 14.

Tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

 fall from these four large eruptions was extensive to the southwest of the volcano. Two hours after the last of these four explosions, a series of eruptions began which lasted for the next 24 hours, and which saw the production of much larger pyroclastic flows and surges which traveled several kilometres down river valleys on the flanks of the volcano.

Dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...

 was the igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

 making up the tephra in these eruptions and in the following climactic event. The most abundant phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...

 minerals were hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....

 and plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

, but an unusual phenocryst mineral was also present—the calcium sulfate
Calcium sulfate
Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

, anhydrite
Anhydrite
Anhydrite is a mineral – anhydrous calcium sulfate, CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium and strontium sulfates, as might be expected from the...

. The dacite magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 was more oxidized than most magmas, and the sulfur-rich nature of the eruption was probably causally related to the redox state
Mineral redox buffer
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions at which a rock forms and evolves can be important for interpreting the rock history...

.

The climactic eruption


All the seismographs at Clark Air Base
Clark Air Base
Clark Air Base is a former United States Air Force base on Luzon Island in the Philippines, located 3 miles west of Angeles City, about 40 miles northwest of Metro Manila. Clark Air Base was an American military facility from 1903 to 1991...

 had been rendered inoperative by 14:30 on June 15, mostly by pyroclastic density currents. Intense atmospheric
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 pressure variation was also recorded.
On the same day, Typhoon Yunya
1991 Pacific typhoon season
Tropical Storm Enrique formed in the eastern Pacific basin, where it reached it's peak intensity as a category 1 Hurricane, becoming Hurricane Enrique. Enrique lasted for 6 days before becoming a remnant low, shortly after entering in the central Pacific....

 struck the island, passing about 75 km (46.6 mi) north of the volcano. The typhoon rains made direct visual observations of the eruption impossible, but measurements showed that ash was ejected to heights of 34 km (21.1 mi) by the most violent phase of the eruption, which lasted about three hours. Pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

s poured from the summit, reaching as far as 16 km away from it. Typhoon rains mixed with the ash deposits caused massive lahars.

The ash cloud from the volcano covered an area of some 125,000 km² (50,000 mi²), bringing total darkness to much of central Luzon. Almost all of the island received some ashfall, which formed a heavy, rain-saturated snow-like blanket. Tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

 fell over most of the South China Sea
South China Sea
The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...

 and ashfall was recorded as far away as Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...

, Cambodia
Cambodia
Cambodia , officially known as the Kingdom of Cambodia, is a country located in the southern portion of the Indochina Peninsula in Southeast Asia...

 and Malaysia.

Twelve days after the first magmatic eruptions of June 3, on June 15, 1991, by about 22:30, and about nine hours after the onset of the most recent climactic phase, atmospheric pressure waves had decreased to the pre-eruption levels. No seismic records were available at this time, but volcanologists believe 22:30 marked the end of the climactic eruption.

Vast quantities of minerals and metals were brought to the surface. Overall, introduced to the surface environment, was an estimated 800,000 tons of zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

, 600,000 tons of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, 550,000 tons of chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

, 300,000 tons of nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

, and massive amounts of toxic heavy metals such as 100,000 tons of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

, 10,000 tons of arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

, 1000 tons of cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...

, and 800 tons of mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

.

The eruption effects on aircraft


At least sixteen commercial aircraft made damaging encounters with the ash cloud ejected by the June 15 eruption, as well as others on the ground. The encounters caused loss of power to one engine on each of two different aircraft. A total of 10 engines were damaged and replaced, including all four engines of a single jumbo jet. Longer term damage to aircraft and engines was reported, including accumulation of sulfate deposits on engines.

Aftermath of the 1991 eruption



Explosivity of the eruption


In all, the eruption ejected about ten cubic kilometres (2.4 mile³) of material 10 cubic kilometres (2.4 cu mi), making it the largest eruption since that of Novarupta
Novarupta
Novarupta, meaning "new eruption", is a volcano located on the Alaska Peninsula in Katmai National Park and Preserve, about southwest of Anchorage. Formed in 1912 during the largest volcanic eruption of the 20th century, Novarupta released 30 times the volume of magma as the 1980 eruption of...

 in 1912 and some ten times larger than the 1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
The 1980 eruption of Mount St. Helens, a stratovolcano located in Washington state, in the United States, was a major volcanic eruption. The eruption was the only significant one to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California...

. Ejected material such as tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

 fallout and pyroclastic flow deposits are much less dense than magma, and the volume of ejected material was equivalent to about four cubic kilometres (1 mile³) of unerupted material. This colossal eruption had a Volcanic Explosivity Index
Volcanic Explosivity Index
The Volcanic Explosivity Index was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions....

 of 6.
The former summit of the volcano was replaced by a caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

 2.5 km wide. The highest point on the caldera rim now stood 1,485 m above sea level, some 260 m lower than the pre-eruption summit.

Death toll


Over 800 people were killed by the eruption, mostly by roofs collapsing under the weight of accumulated wet ash, a hazard that was greatly exacerbated by the simultaneous arrival of Typhoon Yunya. The evacuation in the days preceding the eruption certainly saved tens of thousands of lives, and has been hailed as a great success for volcanology
Volcanology
Volcanology is the study of volcanoes, lava, magma, and related geological, geophysical and geochemical phenomena. The term volcanology is derived from the Latin word vulcan. Vulcan was the ancient Roman god of fire....

 and eruption prediction. However, damage to healthcare facilities, and the spread of illnesses in relocation facilities due to poor sanitation, led to soaring death rates in the months following the eruption.

After the eruption, about 500,000 people continue to live within 40 km (24.9 mi) of the mountain, with population centers including the 150,000 in Angeles City
Angeles City
The City of Angeles , located within the province of Pampanga in the Philippines, is locally classified as a first-class, highly-urbanized city. Its name is derived from El Pueblo de los Ángeles in honor of its patron saints, Los Santos Ángeles de los Custodios , and the name of its founder, Don...

, and 30,000 at Clark Freeport Zone
Clark Freeport Zone
Clark Freeport Zone is a redevelopment of the former Clark Air Base, a former United States Air Force base in the Philippines. It is located on the northwest side of Angeles City and borders the municipality of Mabalacat in the province of Pampanga. It is located about 40 miles northwest of...

.

Effects on agriculture


Many reforestation
Reforestation
Reforestation is the natural or intentional restocking of existing forests and woodlands that have been depleted, usually through deforestation....

 projects were destroyed in the eruption, with a total area of 150 square kilometres (37,000 acre
Acre
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...

s) valued at 125 million pesos destroyed. Agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 was heavily disrupted, with 800 square kilometres (200,000 acres) of rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

-growing farmland destroyed, and almost 800,000 head of livestock and poultry
Poultry
Poultry are domesticated birds kept by humans for the purpose of producing eggs, meat, and/or feathers. These most typically are members of the superorder Galloanserae , especially the order Galliformes and the family Anatidae , commonly known as "waterfowl"...

 killed , destroying the livelihoods of thousands of farmers. The cost to agriculture of eruption effects was estimated to be 1.5 billion pesos.

Many farmers near Pinatubo began growing crops such as peanuts, cassava and sweet potatoes, which are quick ripening and can be harvested before the threat of lahar flows during the late summer rainy season.

Local economic and social effects


In total, 364 communities and 2.1 million people were affected by the eruption, with livelihoods and houses being damaged or destroyed. More than 8,000 houses were completely destroyed, and a further 73,000 were damaged. In addition to the severe damage sustained by these communities, roads and communications were damaged or destroyed by pyroclastic flows and lahar throughout the areas surrounding the volcanoes. Total losses in 1991 and 1992 alone were estimated at 10.6 and 1.2 billion pesos respectively, including damage to public infrastructure estimated at 3.8 billion pesos. Education
Education
Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...

 for thousands of children was seriously disrupted by the destruction of school
School
A school is an institution designed for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is commonly compulsory. In these systems, students progress through a series of schools...

s in the eruption.

The eruption of Pinatubo severely hampered the economic development
Economic development
Economic development generally refers to the sustained, concerted actions of policymakers and communities that promote the standard of living and economic health of a specific area...

 of the surrounding areas. The gross regional domestic product of the Pinatubo area accounted for about 10% of the total Philippine gross domestic product
Gross domestic product
Gross domestic product refers to the market value of all final goods and services produced within a country in a given period. GDP per capita is often considered an indicator of a country's standard of living....

. The GRDP had been growing at 5% annually before the eruption, but fell by more than 3% from 1990 to 1991. In 1991, damage to crops and property was estimated at $374 million, to which continuing lahar flows added a further $69 million in 1992. It was estimated that 8,000 houses had been destroyed and 75,000 more were severely damaged. 42 percent of the cropland around the volcano was affected by mudflows, dealing a severe blow to the agricultural economy in the region.

Lahars



Since the eruption, each rainy season has brought destructive lahars, which have caused the displacement of thousands of people. Extensive damage to buildings and infrastructure
Infrastructure
Infrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...

 cost billions of peso
Philippine peso
The peso is the currency of the Philippines. It is subdivided into 100 centavos . Before 1967, the language used on the banknotes and coins was English and so "peso" was the name used...

s to repair, and further costs were incurred in constructing dikes and dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

s to control the post-eruption lahars.

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 ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s, but the eruption filled many valleys with deep pyroclastic deposits. Since 1991, the rivers have been clogged with sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

, and the valleys have seen frequent lahar
Lahar
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...

s which continued for years after the eruption. Studies show that the river systems will take many years as well to recover from the 1991 eruption.

Clark Air Base and Subic Naval Base


The United States maintained two large military bases in the region; U.S. Naval Base Subic Bay
U.S. Naval Base Subic Bay
U.S. Naval Base Subic Bay was a major ship-repair, supply, and rest and recreation facility of the United States Navy located in Zambales, Philippines. It was the largest U.S...

 was 75 km (46.6 mi) to the southwest, while Clark Air Base
Clark Air Base
Clark Air Base is a former United States Air Force base on Luzon Island in the Philippines, located 3 miles west of Angeles City, about 40 miles northwest of Metro Manila. Clark Air Base was an American military facility from 1903 to 1991...

 was less than 25 km (15.5 mi) to the east of the volcano's summit. The United States Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

 initiated a massive airlift
Airlift
Airlift is the act of transporting people or cargo from point to point using aircraft.Airlift may also refer to:*Airlift , a suction device for moving sand and silt underwater-See also:...

 effort to evacuate American service members and their families during and immediately following the eruption, named Operation Fiery Vigil
Operation Fiery Vigil
Operation Fiery Vigil was the emergency evacuation of all non-essential military and United States Department of Defense civilian personnel and their dependents from Clark Air Base and U.S. Naval Base Subic Bay during the June 1991 volcanic eruption of Mount Pinatubo in the Philippines...

. Most personnel were initially relocated to Guam
Guam
Guam is an organized, unincorporated territory of the United States located in the western Pacific Ocean. It is one of five U.S. territories with an established civilian government. Guam is listed as one of 16 Non-Self-Governing Territories by the Special Committee on Decolonization of the United...

, Okinawa, and Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, although some returned to the continental United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. Clark Air Base was ultimately abandoned by the United States military, and Subic Bay reverted to Philippines control the next year following the breakdown of lease negotiations.

Global environmental effects


The powerful eruption of such an enormous volume of lava and ash injected significant quantities of aerosols and dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

 into the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

. Sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 oxidised in the atmosphere to produce a haze of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 droplets, which gradually spread throughout the stratosphere over the year following the eruption. The injection of aerosols into the stratosphere is thought to have been the largest since the eruption of Krakatoa
Krakatoa
Krakatoa is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island , and the volcano as a whole. The island exploded in 1883, killing approximately 40,000 people, although some estimates...

 in 1883, with a total mass of SO2 of about 17 million tons being injected—the largest volume ever recorded by modern instruments (see chart and figure).

This very large stratospheric injection resulted in a reduction in the normal amount of sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 reaching the Earth's surface by roughly 10% (see figure). This led to a decrease in northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

 average temperatures of 0.5–0.6 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

 (0.9–1.1 °F), and a global fall of about 0.4 °C (0.7 °F). At the same time, the temperature in the stratosphere rose to several degrees higher than normal, due to absorption of radiation by the aerosol. The stratospheric cloud from the eruption persisted in the atmosphere for three years after the eruption.


The eruption had a significant effect on ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 levels in the atmosphere, causing a large increase in the destruction rate of ozone. Ozone levels at mid-latitudes reached their lowest recorded levels, while in the southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

 winter of 1992, the ozone hole over Antarctica reached its largest ever size until then, with the fastest recorded ozone depletion rates. The eruption of Mount Hudson
Mount Hudson
Mount Hudson is a stratovolcano in southern Chile, and the site of one of the largest eruptions in the twentieth century. The mountain itself is covered by a glacier. There is a caldera at the summit from an ancient eruption; modern volcanic activity comes from inside the caldera...

 in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

 in August 1991 also contributed to southern hemisphere ozone destruction, with measurements showing a sharp decrease in ozone levels at the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...

 when the aerosol clouds from Pinatubo and Hudson arrived.

Another noticeable effect of the dust in the atmosphere was the appearance of lunar eclipse
Lunar eclipse
A lunar eclipse occurs when the Moon passes behind the Earth so that the Earth blocks the Sun's rays from striking the Moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a...

s. Normally even at mid-eclipse, the moon is still visible although much dimmed, but in the year following the Pinatubo eruption, the moon was hardly visible at all during eclipses, due to much greater absorption of sunlight by dust in the atmosphere.

Impact on the indigenous people of Pinatubo


The Aeta people were the hardest hit by the eruption. After the areas surrounding the volcano were declared safe, many Aetas returned to their old villages only to find them destroyed by pyroclastic and lahar deposits. Some were able to return to their former way of life, but most moved instead to government-organized resettlement areas. Conditions on these were poor, with each family receiving only small plots of land not ideal for growing crops. Many Aeta found casual labor working for lowland farmer
Farmer
A farmer is a person engaged in agriculture, who raises living organisms for food or raw materials, generally including livestock husbandry and growing crops, such as produce and grain...

s, and overall Aeta society became much more fragmented, and reliant on and integrated with lowland culture.

Activity since 1991


Following the climactic eruption of June 15, 1991, activity at the volcano continued at a much lower level, with continuous ash eruptions lasting until August 1991 and episodic eruptions continuing for another month. Activity then remained low until July 1992 when a new lava dome
Lava dome
|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

 started growing in the caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

. Volcanologists suspected that further violent eruptions could be possible, and some areas were evacuated. However, the eruption was only minor and since that episode, the volcano has been quiet.

Lake Pinatubo


A crater lake
Crater lake
A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not...

, Lake Pinatubo
Lake Pinatubo
Lake Pinatubo is the summit crater lake of Mount Pinatubo formed after its climactic eruption on June 15, 1991. The lake is located near the boundaries of Pampanga, Tarlac and Zambales provinces in the Philippines and is the deepest lake in the country at...

, eventually formed in the 1991 caldera, with the 1992 lava dome forming an island
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...

. Initially, the lake was hot and highly acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic, with a minimum pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of 2 and a temperature of about 40 °C. Subsequent rainfall cooled and diluted the lake, lowering the temperature to 26 °C and raising the pH to 5.5 by 2003.

The lake increased in depth by about 1 metres (3.3 ft) per month on average eventually submerging the lava dome, until September 2001, when fears that the walls of the crater might be unstable prompted the Philippine government to order a controlled draining of the lake. An estimated 9,000 people were once again evacuated from surrounding areas in case a large flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

 was accidentally triggered. Workers cut a 5 m notch in the crater rim, and successfully drained about a quarter of the lake's volume.

Recent activity


On the 26th of July 2011, a 5.9 magnitude earthquake struck close to Pinatubo, however no major damages or casualties were reported.

Cultural history


The word pinatubo could mean a fertile place where one can make crops grow or could mean "made to grow" in Tagalog
Tagalog language
Tagalog is an Austronesian language spoken as a first language by a third of the population of the Philippines and as a second language by most of the rest. It is the first language of the Philippine region IV and of Metro Manila...

 and Sambal, which may suggest a knowledge of its previous eruption in about AD 1500. There is an oral tradition amongst local people suggestive of a folk memory of earlier large eruptions. Ancient legend tells of Bacobaco, the terrible spirit of the sea, who could metamorphose into a huge turtle and who threw fire from his mouth. When being chased by the spirit hunters, Bacobaco fled to the mountain and dug a great hole in its summit showering the surrounding land with rock, mud, dust and fire for three days; howling so loudly that the earth shook.

History among Aetas


Aeta elders tell many stories about the history of the mountain, the best known being that it was once a Batung Mabye (Kapampangan for living stone). It was said to have been planted on a kingdom by a displeased sorcerer but relocated by a hero. The mountain was soon turned into the abode of Apo Namalyari ("The lord of happenings/ events"), the pagan deity of the Sambal, Aeta, and Kapampangans living on the Zambales range. It was said to comprise the whole mountain range until Sinukuan of Mount Arayat
Mount Arayat
Mount Arayat is an extinct stratovolcano on Luzon Island, Philippines, rising to a height of There is no recorded eruption of the volcano, and its last activity probably dates to the Holocene era.The volcano is located in a flat agricultural region at...

 (the god of the Kapampangans) became a strong rival of Namalyari. Their fight, which took place over the center plains, involved the shattering of the mountain into smaller bodies while Mount Arayat lost its center peak. Other versions have it that Pinatubo's peak shattered because of Namalyari's immense fury in an attempt to teach humans the meaning of fear and show how misdeeds will be punished.

According to the elders, Apo Namalyari, caused the 1991 eruption because of displeasure toward illegal loggers and Philippine National Oil Company
Philippine National Oil Company
Philippines National Oil Company, owners of 40% stake in Petron, is a Philippines state owned company.On April 25 2008, Gloria Macapagal-Arroyo appointed former Negros Oriental 1st district congressman Jacinto "Jing" Paras as chairman of its subsidiary, the Philippine National Oil Company -...

 executives who performed deep exploratory drilling and well testing on the mountain looking for geothermal heat from 1988 to 1990. Discouraging results from the wells forced the abandonment of the prospect 13 months before the April 2, 1991 explosions.

Aetas granted ownership of Pinatubo


After being driven away by the 1991 eruption of Mount Pinatubo, in May 2009 some 454 Aeta families in Pampanga were given the first clean ancestral land ownership on Mount Pinatubo with the Certificate of Ancestral Domain Title (CADT) by the National Commission on Indigenous Peoples (NCIP), the government agency that deals with issues concerning indigenous people of the Philippines. The approved and declared net land area of 7440.1 hectares (18,384.9 acre) covers the barangay
Barangay
A barangay is the smallest administrative division in the Philippines and is the native Filipino term for a village, district or ward...

s of Mawakat and Nabuklod in Floridablanca, Pampanga
Floridablanca, Pampanga
Floridablanca is a 1st class municipality in the province of Pampanga, Philippines. According to the 2007 census, Floridablanca has a population of 103,388, with an area of 17,548 hectares or 175.48 square kilometers....

, plus a portion of San Marcelino, Zambales
San Marcelino, Zambales
San Marcelino is a 1st class municipality in the province of Zambales, Philippines. According to the latest census, it has a population of 29,052 people in 5,866 households.-Etymology:...

, and a portion of Barangay Batiawan in Subic, Zambales
Subic, Zambales
Subic is a 1st class municipality in the province of Zambales, Philippines. According to the latest census, it has a population of 77,118 people in 13,882 households....

.

On January 14, 2010, some 7,000 Aeta families from Zambales were officially granted the Certificate of Ancestral Domain Title (CADT) covering the Zambales side of Pinatubo which includes the summit and Lake Pinatubo, officially becoming their lutan tua (ancestral land). The ancestral domain title covers 15984 hectares (39,497.3 acre) and includes the villages of Burgos, Villar, Moraza and Belbel in Botolan and portions of the towns of Cabangan
Cabangan, Zambales
Cabangan is a 4th class municipality in the province of Zambales, Philippines. According to the latest census, it has a population of 21,519 people in 4,032 households. Don Cabangan is believed to be the largest filipino in recorded history.-Barangays:...

, San Felipe
San Felipe, Zambales
San Felipe is a 4th class municipality in the province of Zambales, Philippines. According to the 2007 Philippine census, it has a population of 21,322 people in 4,094 households.-Barangays:San Felipe is politically subdivided into 11 barangays....

 and San Marcelino
San Marcelino, Zambales
San Marcelino is a 1st class municipality in the province of Zambales, Philippines. According to the latest census, it has a population of 29,052 people in 5,866 households.-Etymology:...

.

Having the land title will protect them from others — including foreigners — from exploiting their land without compensation to the indigenous tribes. In the past, the Aetas had to contend with mining companies, loggers, and recently, with tourists who earn from Mount Pinatubo but do not compensate the local tribes.

Ancestral domain titles are awarded to a certain community or indigenous group who have occupied or possessed the land continuously in accordance with their customs and traditions since time immemorial. They have the legal right to collectively possess and to enjoy the land and its natural resources to the exclusion of others.

Pinatubo in popular culture


Long before Mt. Pinatubo became famous for its cataclysmic eruption, Philippine President
President of the Philippines
The President of the Philippines is the head of state and head of government of the Philippines. The president leads the executive branch of the Philippine government and is the commander-in-chief of the Armed Forces of the Philippines...

 Ramon Magsaysay
Ramon Magsaysay
Ramón del Fierro Magsaysay was the third President of the Republic of the Philippines from December 30, 1953 until his death in a plane crash in 1957. He was elected President under the banner of the Nacionalista Party.-Early life:Ramon F...

, a native of Zambales, named his C-47
C-47 Skytrain
The Douglas C-47 Skytrain or Dakota is a military transport aircraft that was developed from the Douglas DC-3 airliner. It was used extensively by the Allies during World War II and remained in front line operations through the 1950s with a few remaining in operation to this day.-Design and...

 presidential plane "Mt. Pinatubo". The plane crashed in 1957
1957 Cebu Douglas C-47 crash
The 1957 crash of a Douglas C-47 plane named "Mt. Pinatubo" on the slopes of Mount Manunggal, Cebu, Philippines, killed the 7th President of the Philippines, Ramon Magsaysay, and 24 other passengers. The crash is estimated to have occurred at 1:40:00 AM, March 17, 1957, Philippine Standard Time...

, killing the President and 24 others onboard.

See also

  • Active volcanoes in the Philippines
    Active volcanoes in the Philippines
    Active is commonly termed for volcanoes with historical eruptions. Moreover, the Philippine Institute of Volcanology and Seismology classify active volcanoes in the country as having erupted within historical times , with accounts of these eruptions documented by man; or having erupted within the...

  • Inactive volcanoes in the Philippines
  • Potentially active volcanos in the Philippines
  • List of volcanic eruptions by death toll
  • Timetable of major worldwide volcanic eruptions
    Timetable of major worldwide volcanic eruptions
    This article is a list of volcanic eruptions of approximately at least magnitude 6 on the Volcanic Explosivity Index or equivalent sulfur dioxide emission around the Quaternary period. Some cooled the global climate; the extent of this effect depends on the amount of sulfur dioxide emitted...


External links