   #copyright

Mount Pinatubo

2007 Schools Wikipedia Selection. Related subjects: Geography of Asia


   This is a featured article. Click here for more information.
                  Mount Pinatubo
   Ash plume of Pinatubo during 1991 eruption
   Elevation     1,486 metres (4,875 feet)
   Location      Luzon, Philippines
   Range         Zambales Mountains
   Coordinates   15°7.8′N 120°21.0′E
   Type          Stratovolcano
   Age of rock   1.1 million years
   Last eruption 1993

   Mount Pinatubo is an active volcano located on the island of Luzon in
   the Philippines, at the intersection of the borders of the provinces of
   Zambales, Tarlac, and Pampanga. 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 when the Spanish
   conquered the Philippines in 1565.

   The volcano's eruption in June 1991 came after 500 years of dormancy,
   and produced one of the largest and most violent eruptions of the 20th
   century. 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 as the surrounding areas were
   severely damaged by pyroclastic flows, ash deposits, and later, lahars
   caused by rainwater remobilising earlier volcanic deposits, thousands
   of houses were destroyed.

   The effects of the eruption were felt worldwide. 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 destruction increased
   substantially.

Overview of the Mount Pinatubo area

   Location of Mt. Pinatubo, showing area over which ash from the 1991
   eruption fell
   Enlarge
   Location of Mt. Pinatubo, showing area over which ash from the 1991
   eruption fell

   Pinatubo is part of a chain of volcanoes which lie along the western
   edge of the island of Luzon. They are subduction volcanoes, formed by
   the Eurasian Plate sliding under the Philippine Plate along the Manila
   Trench to the west. The word 'pinatubo' means 'to have made grow' in
   the Tagalog and Sambal languages, which may suggest a knowledge of its
   previous eruption in about AD 1500, although there is no oral tradition
   among local people of earlier large eruptions. Pinatubo might instead
   mean a fertile place where crops can be made to grow.

   Before the catastrophic eruption of 1991, Pinatubo was an inconspicuous
   volcano, unknown to most people in the surrounding areas. Its summit
   was 1,745 m (5,725 ft) above sea level, but only about 600 m above
   nearby plains, and about 200 m higher than surrounding peaks, which
   largely obscured it from view. An indigenous people, the Aeta (also
   spelt Ayta), 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. They were a hunter-gatherer people who were
   extremely successful in surviving in the dense jungles of the area.

   In total, about 30,000 people lived on the flanks of the volcano in
   barangays (villages) and other small settlements. 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 climate
   and the fertile volcanic soils provided excellent conditions for
   agriculture, and many people grew rice and other staple foods. About
   500,000 people continue to live within 40 km of the mountain, with
   population centres including the 150,000 in Angeles City, and 20,000 at
   Clark Air Base.

   Several important river systems have their sources on Pinatubo, with
   the major rivers being the Bucao, Santo Tomas, Maloma, Tanguay 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.

Geological history

   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.

Ancestral Pinatubo

   Pinatubo before the major eruption of 1991
   Enlarge
   Pinatubo before the major eruption of 1991

   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 away 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.

Modern Pinatubo

   The birth of modern Pinatubo occurred in the most explosive eruption in
   its history, which deposited pyroclastic flow material up to 100 metres
   thick on all sides of the mountain. The total volume of material
   erupted may have been up to 25 cubic kilometres (6 mile³), and the
   removal of this amount of material from the underlying magma chamber
   led to the formation of a large caldera.

   Later 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 500 years ago,
   and after that, the volcano lay dormant. Its slopes became completely
   covered in dense rainforest, and eroded into gullies and ravines.

1991 awakening

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

   On July 16, 1990, an earthquake of magnitude 7.8 (comparable in size to
   the 1906 San Francisco earthquake and the 2005 Kashmir Earthquake)
   struck central Luzon. Its epicentre was about 100 km northeast of
   Pinatubo, leading some 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 coming from the volcano, but scientists
   visiting the mountain found that small landslides rather than any
   eruptive activity were responsible.

   On March 15, 1991, a succession of earthquakes was felt by villagers on
   the north-western 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 imminent. On April 2, the
   volcano awoke, with phreatic eruptions occurring near the summit along
   a 1.5 km long fissure. Over the next few weeks, small eruptions
   continued, dusting the surrounding areas with ash. Seismographs
   detected hundreds of small earthquakes every day.

   Scientists immediately installed monitoring equipment and analysed the
   volcano for clues as to its previous eruptive history. Radiocarbon
   dating of charcoal found in old volcanic deposits revealed the 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 deposits from previous eruptions.

   Volcanic activity increased throughout April and May. Measurements of
   sulfur dioxide emission showed a rapid increase from 500  tonnes per
   day on May 13 to 5,000 tonnes/day by May 28. This implied that there
   was a rising column of fresh magma beneath the volcano. After May 28,
   the amount of SO[2] being emitted decreased substantially, raising
   fears that the degassing of the magma had been blocked somehow, leading
   to a pressure build-up in the magma chamber and a high likelihood of
   explosive eruptions.

   The first magmatic eruptions occurred on June 3, and the first large
   explosion on June 7 generated an ash column 7 km (4.5 miles) high. The
   Philippine Institute of Volcanology and Seismology (PHIVOLCS) led by
   Raymundo Punongbayan issued a warning indicating the possibility of a
   major eruption within two weeks.

Evacuation

   Pinatubo evacuation zones
   Enlarge
   Pinatubo evacuation zones

   Given all the signs that a very large eruption was imminent, PHIVOLCS
   worked in cooperation with the US Geological Survey to convince people
   in the local area 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 and Angeles City 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
   newspapers, radio and television stations, nongovernmental
   organizations (NGOs), and directly to the endangered inhabitants.

   Many of the Aeta 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 km from the summit. They moved
   to increasingly distant villages as the eruptions escalated, with some
   Aeta moving up to nine times in the two months preceding the
   cataclysmic eruption.

   The first formal evacuations were ordered from the 10 km zone on 7
   April. Evacuation of the 10–20 km zone was ordered when a level 4 alert
   was issued on 7 June. A level 5 alert triggered evacuation of the 20–40
   km zone on 14 June, and in all some 60,000 people had left the area
   within 30 km of the volcano before 15 June. Most people temporarily
   relocated to Manila and Quezon City, with some 30,000 using the
   Amoranto Stadium in Quezon City as a refugee camp.

Eruptions build to a climax

   Explosive eruption, early June 1991
   Enlarge
   Explosive eruption, early June 1991

   In early June, tiltmeter 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 km 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 at the summit of the volcano. The
   dome grew substantially over the next five days, reaching a maximum
   diameter of about 200 m and a height of 40 m.

   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 which quickly
   reached heights of over 19 km, and which generated pyroclastic flows
   extending up to 4 km from the summit in some river valleys. Fourteen
   hours later, a 15 minute eruption hurled ash to heights of 24 km.
   Friction in the uprushing ash column generated abundant lightning.

   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 km-high eruption column at 13:09 on June 14.

   Tephra 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 travelled several kilometres down river valleys on the
   flanks of the volcano.

The climactic eruption

   The eruption cloud shortly before the climactic eruption
   Enlarge
   The eruption cloud shortly before the climactic eruption

   June 15 saw the onset of the climactic eruption. Large tremors starting
   at 13:42 saturated all the seismographs at Clark Air Base, and by 14:30
   all had been rendered inoperative, mostly by pyroclastic density
   currents. Intense atmospheric pressure variation was also recorded.

   On the same day, Typhoon Yunya struck the island, passing about 75 km
   (50 miles) 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 by the most violent phase of the
   eruption, which lasted about three hours. Pyroclastic flows poured from
   the summit, reaching as far as 16 km away from it. Typhoon rains mixed
   with the ash deposits causing 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 fell over most of the South
   China Sea and ashfall was recorded as far away as Vietnam, Cambodia and
   Malaysia.

   By about 22:30, nine hours after the onset of the 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.

The aftermath

   Snow-like ashfall caused by heavy rain mixing with ash columns
   Enlarge
   Snow-like ashfall caused by heavy rain mixing with ash columns

   In all, the eruption ejected about ten cubic kilometres (2.5 mile³) of
   material, making it the largest eruption since that of Novarupta in
   1912 and some ten times larger than the 1980 eruption of Mount St.
   Helens. Ejected material such as tephra 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 of 6 . The former summit of the volcano was replaced by a caldera
   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.

   About 300 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 and eruption prediction.
   Before and after the eruption: a river valley filled in by pyroclastic
   flow deposits
   Enlarge
   Before and after the eruption: a river valley filled in by pyroclastic
   flow deposits

   However, since the eruption, each rainy season has brought further
   lahars, which have caused the displacement of thousands of people.
   Hundreds have died from poor sanitation in relocation camps.
   Agriculture in the region also suffered badly from the effects of the
   eruption, with hundreds of square kilometres of formerly arable land
   being rendered infertile, destroying the livelihoods of thousands of
   farmers.

   The United States maintained two large military bases in the region;
   U.S. Naval Base Subic Bay was 75 km (50 mi.) to the southwest, while
   Clark Air Base was a mere 40 km (25 mi.) to the east of the volcano's
   summit. Both were abandoned after being severely damaged by the
   eruption.

   Although the 1991 eruption was one of the largest and most violent of
   the 20th century, it was weaker than any of the historical eruptions
   uncovered by geologists. There is some evidence that eruptions at
   Pinatubo are getting weaker over time, but this is by no means
   conclusively established.

Social and economic effects

   Aircraft hangars at Clark Air Base destroyed by ashfall
   Enlarge
   Aircraft hangars at Clark Air Base destroyed by ashfall

   The eruption of Pinatubo severely hampered the economic development of
   the surrounding areas. Extensive damage to buildings and infrastructure
   cost billions of pesos to repair, and further costs were incurred in
   constructing dikes and dams to control the post-eruption lahars.

   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 lahars throughout the areas surrounding the
   volcanoes. The estimated cost of repairing the damage to infrastructure
   was 3.8 billion pesos.

   Many reforestation projects were destroyed in the eruption, with a
   total area of 150 square kilometres (37,000  acres) valued at
   125 million pesos destroyed. Agriculture was heavily disrupted, with
   800 square kilometres (200,000 acres) of rice-growing farmland
   destroyed, and almost 800,000 head of livestock and poultry killed. The
   cost to agriculture of eruption effects was estimated to be 1.5 billion
   pesos.

   Damage to healthcare facilities, and the spread of illnesses in
   relocation facilities, led to soaring death rates in the months
   following the eruption. Education for thousands of children was
   seriously disrupted by the destruction of schools in the eruption. The
   gross regional domestic product of the Pinatubo area accounted for
   about 10% of the total Philippine gross domestic product. The GRDP had
   been growing at 5% annually before the eruption, but fell by more than
   3% from 1990 to 1991.

Global effects

   Space Shuttle (Mission STS-43) photograph of the Earth over South
   America taken on August 8, 1991, showing double layer of Pinatubo
   aerosol cloud (dark streaks) above high cumulonimbus tops
   Enlarge
   Space Shuttle (Mission STS-43) photograph of the Earth over South
   America taken on August 8, 1991, showing double layer of Pinatubo
   aerosol cloud (dark streaks) above high cumulonimbus tops

   The powerful eruption of such an enormous volume of lava and ash
   injected significant quantities of aerosols and dust into the
   stratosphere. Sulfur dioxide oxidised in the atmosphere to produce a
   haze of sulfuric acid 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 in 1883, with a total mass of SO[2] 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 reaching the earth's surface by up to 5% (see
   figure). This led to a decrease in northern hemisphere average
   temperatures of 0.5–0.6 ° C (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 aerosols. The stratospheric cloud from the eruption
   persisted in the atmosphere for three years after the eruption.
   Satellite measurements of ash and aerosol emissions from Mount
   Pinatubo.
   Enlarge
   Satellite measurements of ash and aerosol emissions from Mount
   Pinatubo.

   The eruption had a significant effect on ozone 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 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 in Chile
   in August 1991 also contributed to southern hemisphere ozone
   destruction, with measurements showing a sharp decrease in ozone levels
   at the tropopause when the aerosol clouds from Pinatubo and Hudson
   arrived.

   Another noticeable effect of the dust in the atmosphere was the
   appearance of lunar eclipses. 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.

The area since 1991

   Thick ash deposits from Pinatubo are evident in this 1992 photo from
   the Space Shuttle Atlantis
   Enlarge
   Thick ash deposits from Pinatubo are evident in this 1992 photo from
   the Space Shuttle Atlantis

   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
   began to grow in the caldera.

   This dome appeared to be composed of fresh lava from the deep magma
   reservoir beneath the volcano, rather than material 'left over' in a
   shallow reservoir from the 1991 eruption. Thus, volcanologists
   suspected that further violent eruptions could be possible, and some
   areas were once again evacuated. However, the eruption did not become
   violent, perhaps due to outgassing from the deep reservoir reducing the
   explosivity of the lava reaching the surface. Since 1992, the volcano
   has been dormant.

   The Aeta people were the hardest hit by the eruption. The total
   destruction of many villages by pyroclasts and lahar deposits meant
   that many Aeta were unable to return to their former way of life. After
   the areas surrounding the volcano were declared safe to return to,
   those whose villages had not been destroyed moved back, but most people
   moved instead to government-organised resettlement areas. Conditions on
   these were poor, with each family receiving only small plots of land,
   which were not ideal for growing crops. Many Aeta found casual labour
   working for lowland farmers, and overall Aeta society became much more
   fragmented, and reliant on and integrated with lowland culture.

   After eruptions ended, a crater lake formed in the 1991 caldera, with
   the 1992 lava dome forming an island. At first, the lake was small, hot
   and highly acidic, with a minimum pH of 2 and a temperature of about
   40°C. Abundant 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 metre per month on average,
   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. 9,000 people were once again evacuated from
   surrounding areas in case a large flood was accidentally triggered.
   Workers cut a 5 m notch in the crater rim, and successfully drained
   about a quarter of the lake's volume.

Related images

   Mount Pinatubo and crater lake, May 2005

   Sulfur dioxide emissions.

   Solar radiation reduction due to volcanic eruptions.

   Sulfur dioxide emissions by volcanoes.

   The Pinatubo crater lake in January 2006

   Map showing major volcanoes of the Philippines

   Retrieved from " http://en.wikipedia.org/wiki/Mount_Pinatubo"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
