Mount Pinatubo

Pinatubo eruption, June 12, 1991

The Pinatubo is an active volcano in the Philippines in the center of the island of Luzon. The 1,486 m high mountain today ( before the outbreak of 1991, he was 1745 m high) is located 93 km north-west of Manila and 26 kilometers west of Angeles City. It is located in the border triangle of the provinces of Zambales, Bataan and Pampanga, in the Zambales Mountains. By the year 1991, the volcano was considered extinct. When the Spaniards conquered the Philippines in 1565, served the mountain of the fleeing indigenous people, the Aeta, as a hiding place, as he was then covered by dense forest.

The last eruption of Pinatubo occurred on 15 June 1991 after a 550 -year rest period and was one of the most powerful of the 20th century. The prediction of the impending eruption led to the evacuation of the surrounding areas, of which tens of thousands of people were affected. Nevertheless, at least 875 people died from the outbreak and its consequences. In addition, the surrounding land was destroyed by pyroclastic flows, ash deposits, and later by lahars.

The effects of this explosive eruption were felt worldwide. They caused a greater release of aerosols into the stratosphere than any other volcanic eruption since the eruption of Krakatoa in 1883. Has led in the following months to a global layer of mist containing sulfuric acid. Furthermore, a temperature drop of 0.5 ° C and an increased ozone reduction was recorded.

• 6.1 sign of an outbreak
• 6.2 evacuation
• 6.3 Structure of the eruption until the climax
• 6.4 Main eruption
• 6.5 effects
• 6.6 Social and economic impact
• 6.7 Global impacts

The Pinatubo and the surrounding area

The Pinatubo is part of a volcanic chain extending along the western tip of the island of Luzon. It involves a subduction zone volcanoes, which are fed by the lowering of the Philippine plate under the Eurasian plate along the Manila trench and thereby rising magma.

Population

The word " Pinatubo " means in the language of Tagalog and Sambal " make it grow ", which could be related to the knowledge of an outbreak in 1500, although reports have not been handed down from earlier eruptions. Instead, you could " Pinatubo " to be understood also as a place thrives particularly well to the grain. Before the disaster of 1991, the mountain was an inconspicuous volcano, whose existence was the most people in the surrounding areas unknown. Although Its highest point was 1745 m above sea level, but only 600 m higher than the surrounding plains and 200 meters higher than the surrounding peaks, the more likely it concealed. The population, especially indigenous people ( Aeta or Negrito ), had lived for several centuries on the slopes of the volcano, since she had fled from the lowlands before the Spanish conquerors. There were hunters and gatherers who had survived in this area. After the outbreak in 1991 in many areas have been resettled by the government, which were not or less affected by the outbreak. Overall, about 30,000 people lived on the slopes, either in barangays ( villages ) or in small settlements.

Vegetation

The thick jungle that covered most of the many surrounding mountains, the hunters brought an advantage. In contrast, the shallower areas due to the monsoon caused heavy rainfall of a total of around 4000 mm per year and the fertile volcanic soil were more suitable for agriculture. So today live in a radius of 40 km around the volcano again half a million people, with the major population centers Angeles (150,000 ) and Clark Air Base (20,000 ) were included in all calculations.

Waters

Many major rivers originate at Pinatubo. Among other things, the Bucao, Santo Tomas, Maloma, Tanguay and Kilengfluss. Before the eruption of 1991, these rivers were an important ecosystem, but the eruption showered many valleys with pyroclastic layers. So the rivers are now blocked with sediment and in the valleys occur frequently lahars. Studies have shown that the river system will take many years and about 15 years after the eruption, in order to recover.

The last remnants of the once up to 200 m high Laharablagerungen be gone by the heavy rains of the rainy season and with the overgrowth through the lush vegetation soon or to the human eye.

Geological History

Although there was no knowledge about previous major eruptions in the area around Mount Pinatubo, some Aeta report that their elders can remember some small explosions in the past. Pinatubo was already known before the 1991 eruption as geothermal area and smaller explosions in such areas nothing special. It was only after 1991 began the geologists to examine the eruption history in detail. They can be in the two eras, ancient and modern Pinatubo divided.

Age Pinatubo

Much of the rocky area around the volcano today comes from the remains of the old Pinatubo. This volcano was approximately where the current Pinatubo is now. The activity seems to have started before 1.1 million years. The old Pinatubo may have reached a size of about 2300 m above sea level, which can be seen from the profile of the remaining lower slope.

Many mountains near modern Pinatubo are old satellite vents of the old Pinatubo. They were formed by lava domes and Schlotpfropfen. Some mountains are also remains of the old Pinatubo. They survived because they were erosion- resistant parts of the old mountain slope, while the less resistant parts are eroded by the weather influence.

Today Pinatubo

The formation of the present-day Pinatubo began 35,000 years ago by the most powerful eruption in its history. Up to 100 m thick layers of pyroclastic material were thrown in all directions of the mountain. The total amount of ejected material was 25 km3. The disappearance of this enormous amount of material from the underlying magma chamber led to the formation of a large caldera.

1991 eruption

Sign of an outbreak

On July 16, 1990, there was an earthquake of magnitude 7.8 on the Richter scale in the center of Luzon. The epicenter was 100 km northeast of Pinatubo. If one believes leading volcanologists, this could be the cause of the outbreak in 1991, although this is not clearly demonstrated today. Two weeks later, the volcano began to steam. However, scientists then studied the volcano, were more likely to believe that smaller landslides and not eruption activities were responsible.

Early 1991 were villagers perceive a small earthquake on the northwest side of the volcano. Over the next two weeks followed by further earthquakes of increasing intensity, which became clear that volcanic activity was imminent. On April 2, erupted. First came phreatic eruptions in the vicinity of the summit along a 1.5 km long column. Over the next two weeks stopped at the smaller eruptions that covered the surrounding areas with ash. Seismographs recorded hundreds of earthquakes every day.

Scientists immediately began to operate analyzes to obtain evidence of earlier eruptions. The radiocarbon method of charcoal that has been found in ancient volcanic layers, brought three major eruptions in earlier millennia to light. They took place 5500, 3500 and 500 years ago. Geological maps show that many of the surrounding plains are formed by layers of lahars during previous eruptions. The volcanic activity increased during the months of April and May. Measurements of sulfur dioxide emissions showed an increase in the values ​​of 500 tons on 13 May 5000 tonnes per day on 28 May. The amount of ejected SO2 was then abruptly less. It was feared that the outgassing of the magma had been blocked, which would lead to an increase in pressure in the magma chamber and make an explosive eruption likely.

On June 3, there was the first magma eruption and on June 7, the first big explosion with a 7 km- high ash column. The Philippine Institute of Volcanology and Seismology ( PHIVOLCS ) put up a warning before a major eruption out in the next two weeks.

Evacuation

Since all the circumstances indicated that a large explosion was imminent, PHIVOLCS worked with the U.S. Geological Survey, to convince the locals of the seriousness. A false alarm would have led to the ineffectiveness of subsequent warnings during the wait would have brought death to thousands. Thus, the volcanologists were under pressure to develop a timely and accurate assessment of the outbreak risk.

Three evacuation zones were defined. The innermost had a radius of 10 kilometers around the volcano, the second a distance of 10 to 20 km to the center and the last from 20 to 40 km. In the first and second zone there were about 40,000 people, while 331,000 people were in the third zone. Five warning levels of volcanic activity were defined, from level 1 (low seismic disturbances ) up to level 5 ( major eruption is currently underway ). Daily alerts were issued. They each contained the stage for the zones that were in danger. The information was passed on both national newspapers, radio, television, and non-governmental organizations spread and directly to the person concerned.

Many of the Aeta who lived on the slopes of the volcano left their villages voluntarily, as in April began the first explosions. They gathered in a village 12 km from the summit. As the eruptions escalated, they moved further and further away from the volcano. Some Aeta moved into the two months up to nine times.

The first official evacuation was carried out in the ten- kilometer zone on April 7. The evacuation of the second zone was arranged as a warning level 4 was issued on June 7. When they wanted on 14 June went on alert level 5, and the third zone was evacuated. Before June 15, 60,000 people had to leave an area 30 km around the volcano. Most people were briefly housed in Manila or Quezon City. 30,000 of them were using the Amoranto Stadium in Quezon City as a refugee camp.

Structure of the eruption until the climax

Beginning in June 1991 showed inclination measurements, that the volcano inflated itself. It was clear that the cause was the increasing amount of magma, which gathered in memory under the volcano. At the same time, seismic activities were measured 5km north- west of the summit not far below the rock surface. On June 7, there was the first magma eruption in the form of a lava dome at the summit of the volcano. The lava dome grew substantially over the next five days. He reached a diameter of 200 m and a height of 40 m.

A small explosion on June 12, at 3:41 clock marked the beginning of a new phase of more powerful eruptions. A few hours later held large explosions for half an hour at. They produced an eruption column which quickly reached heights of over 19 km, resulting in pyroclastic flows in some river valleys up to 4 km from the summit stretched. 14 hours later threw an eruption of ash up to 24 km into the air. The outbreak originated many flashes.

The third large eruption began on June 13 at 8:41 clock after several minor earthquakes in the previous two hours. These were stronger in the next five minutes, and the eruption column reached again 24 km altitude. After three hours of rest, the seismic activity began again. It was within the next 24 hours and more intense, a three-minute eruption generated until June 14 at 13:09 clock a 21 km high eruption cloud. From the ash cloud fell through the four major eruptions, the volcanic tephra material particularly in the southwest of the volcano. Two hours after the last of the four explosions, there was a series of eruptions that lasted for the next 24 hours and resulted in much larger pyroclastic flows and many miles came down to the sides of the volcano, the river runs.

Main eruption

On June 15, you could see the beginning of the great eruption. Large earthquakes began at 13:42 clock. They disrupted the seismographs at Clark Air Base, so they had to be shut down at 14:30 clock. Also, a large variation of the atmospheric pressure was perceived.

On the same day the typhoon Yunya moved about 75 km north of the volcano on the island. This provides a direct observation of the eruption was impossible. Measurements, however, it was shown that ashes were thrown into the air during the most violent, three-hour period up to 34 km. Pyroclastic flows made up to a distance of 16 km from the summit.

The ash cloud covered an area of 125,000 km2 and resulted in total darkness over Luzon center. The rain of ash reached almost all the islands and formed a massive snow- like layer that increasingly vollsog with rainwater and became unstable. The large amounts of water mixed with the Taifunregen ash layers, so that massive lahars were the result. Tephra fell to the South China Sea and even in Vietnam, Cambodia and Malaysia, there was ash rain. Around 22:30 clock, nine hours after the peak of the main eruption, the pressure waves fell back to the strength that had prevailed before the main eruption. Although there were no seismographs in the vicinity of the Pinatubo longer in operation, but assume volcanologists that around 22:30 clock the main phase of the eruption was over.

Effects

A total of about 10 km3 tephra was ejected by the eruption. It is the largest eruption since Novarupta in 1912 and it is about 10 times larger than the eruption of Mount St. Helens in 1980. These massive explosion had the Vulkanexplosivitätsindex 6 The former summit of the volcano was now a 2.5 -km-wide crater. The highest point of the crater is now 1,486 m above sea level. These are 259 m less than the peak before the eruption had.

At least 875 people died during the eruption, most of them, because the ceilings of their houses collapsed due to the heavy ash - water mixture. This was a threat that was reinforced again by the typhoon. The evacuation of large areas before the eruption here probably saved tens of thousands of people's lives and can be seen for the volcanologists and the eruption forecast as a great success.

Since the eruption occurs during each rainy season to further lahars, which is why thousands of people have had to move. Hundreds have died because of poor sanitary conditions in the refugee camps. Agriculture also had to suffer the effects of the eruption. Hundreds of square kilometers of arable land was barren and the existence of thousands of farmers was destroyed.

The U.S. talked two large military bases in the region. The Subic Bay Naval Base was located 40 km south and the Clark Air Base, 20 kilometers east of the volcano summit. After having been seriously damaged by the eruption, both of which were returned to the Philippines as scheduled.

Although the eruption of 1991 was one of the most powerful of the 20th century, it was weaker than others in the history Pinatubos according to opinion of geologists.

Social and economic impact

The eruption hampered the economic development of the surrounding areas. Strong damage was caused to buildings and infrastructure, their repair cost billions of pesos. Other costs incurred by the construction of canals and dams to control the eruptive lahars.

A total of 364 communities and 2.1 million people were affected by the eruption. More than 8,000 houses were completely destroyed. More 73,000 houses were damaged. In addition to these serious damage to roads and communication facilities were everywhere around the volcano damaged or destroyed by pyroclastic flows or lahars. The total cost of repair amounted to 3.8 billion pesos. Many reforestation projects were zunichtegemacht by the eruption. A total area of ​​150 km2 worth 125 million pesos was destroyed. Agriculture was heavily disrupted. 8000 km2 area of ​​rice were destroyed and 800,000 head of livestock and poultry animals died. The agricultural damage amounted to 1.5 billion pesos.

The damage to medical facilities and the rapid spread of diseases in the refugee camps resulted in the months following the eruption of a rapidly rising death rate. The education could not be continued because of the destroyed schools. The area around Pinatubo contributed ten percent of the gross national product. It grew in this region previously annually by five percent, but fell after the eruption from 1990 to 1991 by three percent.

Global impacts

The massive eruption brought a tremendous amount of aerosols and dust into the stratosphere. Oxidized sulfur dioxide in the atmosphere and generated mist of sulfuric acid droplets, which gradually spread out throughout the year in the stratosphere. A total of 17 million tons of SO2 were injected into the stratosphere, the largest volume since the eruption of Krakatoa in 1883.

The result of this massive injection was a sun light reduction by 5 percent. This in turn resulted in an average drop in temperature by 0.5 to 0.6 ° C in most of the northern hemisphere and 0.4 ° ​​C. At the same time, the temperature rose in the stratospheric several degrees. The clouds in the stratosphere, which were caused by the eruption, remained three years.

The eruption had a significant effect on the ozone layer of the Earth's atmosphere. The ozone layer in the middle latitudes had the lowest level that has ever been measured and took over the Antarctic ozone hole set a new record size. The eruption of the Hudson in Chile in August 1991 also contributed to the destruction of the ozone layer. In the tropopause, a steep drop in ozone levels was found upon the arrival of the aerosol clouds from Pinatubo and Hudson.

Another effect of the dust in the atmosphere was the appearance of the lunar eclipse. Normally, the moon and in the core shadow is visible. However, in the year after the Pinatubo eruption, he was almost not visible during the eclipse, as was absorbed by the dust in the atmosphere to sunlight.

The area since 1991

After the main eruption on 15 June 1991, the activity of the volcano took on more and more. In August 1991, the ash eruptions ceased. More sporadic eruptions still held on for another month. The activity decreased thereafter continue until formation of a new lava dome in the crater in July 1992.

This lava dome was mainly composed of zurückgelassenem material of the eruption of 1991 and less from the magma reservoir deep beneath the volcano. Volcanologists expected that more massive eruptions were imminent, so some areas were evacuated again. However, the eruption was not huge. This could be due to the pressure exerted by a gas leak, could reduce before the lava reached the surface. Since 1993, the volcano is no activity.

After the eruption in 1991 formed a crater lake.

By -1992 lava dome formed in himself an island. At the beginning of the lake was small, hot ( 40 ° C) and strongly acidic ( pH value was 2). The temperature was lowered to 2003 at 26 ° C by the rain and increases the pH to 5.5.

The water level rose by an average of 1 m per month. In September 2001, the government feared that the crater walls could the increasing pressure soon no longer withstand. The surrounding areas were evacuated again, because an outflow of the lake should be triggered artificially. A 5 m wide opening was cut into the crater rim, causing the lake volume could be reduced by a quarter. About 9000 people were affected by this evacuation.

The people of the Aeta had the most to suffer from the disaster. Due to the almost complete destruction of their villages by pyroclastic flows or lahars, most of them could not return to their way of life. The few natives whose villages were not destroyed, returned. The other moved to the bodies set up by the government resettlement areas. The living conditions there were, however, poor. Each family was only a small piece of land that was unsuitable for cultivation. Many Aeta were casual employees of lowland farmers. Overall, the Aetavolk became more and more fragmented and dependent and had to be integrated into the lowland culture.

From Laharablagerungen of the volcano can produce high quality bricks for house construction. Around the Pinatubo some large quarries for building materials have emerged.

The crater lake of Mount Pinatubo has now opened up to tourists on a small scale. Local guides offer hikes there. Usually, visitors are driven with four wheel drive vehicles along the river bed of the Tarlac River until the terrain is impassable and you have to continue his journey on foot. The Philippine Air Force monitored from a small base of the mountain access.