A long time ago in a galaxy far away, I had the opportunity to do a couple weeks of flying over and around the Island of Luzon in the Philippines. I was temporarily assigned to Clark Air Base, PI taking part in an exercise called Cope Thunder, basically a Pacific Air Forces version of the stateside training called Red Flag. http://en.wikipedia.org/wiki/Clark_Air_Base
Clark Air Base was laid out in the flat part of Luzon, some 90 km NW of Manila. It is surrounded by two volcanoes, one obvious, and the other not so much. Some 16 km to the east of Clark and Angeles City is Mount Arayat, thought to be an extinct stratovolcano that is just over 1,000 m high. It is topped with a crater that is over a kilometer in diameter and a breached crater and debris flow from its WNW summit. There is no modern record of eruptions of Arayat, though there is occasional fumarole activity. From a distance, it is a truly beautiful mountain. When the news of the Pinatubo eruption hit, this was the mountain that I thought had exploded, as it was the only visibly obvious volcano in the area. http://volcano.si.edu/volcano.cfm?vn=273084
Some 26 km to the WSW of Clark and Angeles City is the Pinatubo complex. From the base, it does not look like a volcano, rather more like a small mountain range. Aviation and topographic maps do depict a roughly circular complex some 24 km in diameter. Its flanks are heavily eroded, with jagged ravines, canyons, and greenery everywhere. Flying around the mountain in early 1979, there were also a couple active fumaroles near the highest point.
At the time, there was an active gunnery range on its northern flank called the Crow Valley Range that American aircraft from Clark and Subic Bay used to drop practice ordinance on. The range sat in a river valley between two fairly steep cliffs. It was possible to hide on the other side of the ravine, poke your nose above the top and deliver a rocket or bomb onto the range and then quickly leave line of sight from the target. http://en.wikipedia.org/wiki/Crow_Valley_Range_Complex
Flying up and down the ravines was great fun as long as you remembered the rate of climb in an aircraft was not necessarily greater than that of the rising terrain you were flying up, especially on a hot Philippine day. Desert terrain in the Western US claims no small number of pilots every year who poke their noses up box canyons they can neither turn or climb out of and figure it out too late to do anything about it. Pinatubo was the second place I tried to kill myself in an aircraft for precisely that reason. Fortunately God had other plans for me.
The other interesting thing about Crow Valley Range was the local natives who lived on the flanks of Pinatubo. The official name of natives was Aeta, which we did not know at the time. However, there was a location named Negrito depicted on the aviation maps very near the range at the time so that is what the military referred to them as.
These guys were entrepreneurial and watched the range complex closely. Following each weapons delivery pass, they would run onto the range and recover brass, steel, shrapnel, and other metals that they would make things out of and sell in Angeles and other cities. This was a source of no small amount of concern for range personnel and fliers, for nobody wanted to put anyone at risk while training. They were also among the first on the crash scene when an aircraft went down, helping the survivor(s) until rescue arrived.
The eruption of Pinatubo forced the evacuation of Clark AB in 1991. The base which had been in operation since 1903 was eventually returned to the Philippine government. Crow Valley was completely buried under pyroclastic flows and lahars following the eruption, though the area was retained by the government of the Philippines for defense force training after they turned over Pinatubo to the Aeta who claimed it as ancestral ground.
Pinatubo is the northern most member of a string of volcanoes on the western part of Luzon called the Luzon Arc. They are essentially subduction volcanoes. The Philippines is comprised by a number of microplates at the margin of the Philippine Plate as it subducts under the Eurasian Plate. There are subduction trenches to the west of the Philippines and transverse faults on Luzon itself.
The volcano is underlain by a magma chamber some 6 – 11 km deep. The chamber is thought to contain some 40 – 90 km3 of relatively cool, highly crystalized (up to 50%) mush. The crystal mush was destabilized by an injection of basalt into the chamber and erupted andesitic pumice and ash that shifted into dacitic as the 1991 eruption progressed. Analysis of eruptions over the last 35,000 years indicates that the process observed in 1991 has repeated many times over the period.
There are two historic periods for Pinatubo. The ancestral Pinatubo dates back as far as some 1.1 million years (MY) ago. It was an andesite and dacite stratovolcano that eventually destroyed itself, leaving a 3.5 – 4.5 km caldera. There are remains of domes and satellite vents around the complex. It occupies about the same location as the modern Pinatubo.
The modern Pinatubo started its construction some 35,000 years ago. Its eruptive products are typically loose, with pyroclastic flows, tephras and pumices. Other than dome-building, the destabilized crystal mush produces little lava flows. This leads to significant erosion along the flanks due to the wet climate of that part of the world. The lack of lava was also one of the reasons it took a long time to identify Pinatubo as a volcano, which started taking place in the mid to late 1970s.
The mountain was verified as a volcano following investigations into possibilities for geothermal power in the early 1980s.
Eruptive periods are marked by an intense, short series of eruptions followed by occasional dome building, caldera formation, and years of lahar action as water moves the loose pyroclastic and tephra debris from the remaining mountain. After the eruption(s), there is a period of repose as the destabilized crystal mush stabilizes.
Eruptive periods from 35,000, 17,000, 9,000, 6 – 5,000, 3,900 – 2,300, and 500 years ago have been identified. Volume of magma erupted during these eruptive periods appear to have decreased over time while the period of repose between eruptive periods appears to some to have decreased. There is no small amount of disagreement over the notion that Pinatubo is erupting more often.
Additionally, while the major deposits of an eruptive period are able to be determined, it is not possible to tell how many single eruptions emplaced those deposits. With a destabilized magma chamber and new basalt entering it from below, the possibility of additional eruptions from Pinatubo exists.
Finally, one of the great ongoing threats from Pinatubo is lahars, as rain removes loosely deposited (and welded) tuff, drags it downstream and deposits the material on the lower parts of the surrounding valley in Luzon. This action has gone on for decades after the eruption and is expected to continue. It was worst immediately following the climatic eruption of 1991 which coincided with the passage of a major typhoon. A few months later the monsoon season and its rains started, moving yet more debris downhill.
Eruption of 1991
The best compendium of Pinatubo information was put together by a cooperative between PHIVOLCS, University of Washington and the USGS. It is called Fire and Mud and was last updated in 1999. http://pubs.usgs.gov/pinatubo/
The following summary is paraphrased from the Fire and Mud Overview by Wolfe and Hoblitt.
Local earthquakes started in March, 1991. The first phreatic eruption took place on April 2. Continuing earthquakes from June 1 – 7 localized the activity at the throat of the volcano. There was dome-building from June 7 – 12. Four brief vertical eruptions accompanied continuing buildup of tremor and earthquakes. June 14 – 15 saw 13 brief surge producing eruptions that became progressively closer together until the 9-hour long climatic eruption on June 15. Continuous ash emissions continued until mid-late July, tailing off along with earthquake intensity over the period. Small ash eruptions continued until early September. It was followed by dome-building through October 1992.
Some 500,000 lived close enough to the volcano to be at risk from lahars following the eruption, with some 35,000 living on the flanks on the actual volcano. Ash coverage stretched from the Philippines to Southeast Asia.
PHIVOLCS progressively raised the level of alert during the increase in activity and started evacuating people off the mountain and from close to the mountain. This kept the death toll from the eruption and following lahars to around 800. There were some 100,000 left homeless following the eruption.
The eruption was the second-largest measured in the 20th Century, ejecting some 8.4 – 10.4 km3 of tephra and pyroclastic deposits. It was also Sulphur-rich injecting some 17 megatons (Mt) of SO2 into the atmosphere. The eruption was also water-rich, ejecting up to 921 Mt of water. The high volatile content of the crystal mush is one of the reasons that the eruption was so explosive in nature. The plume was measured by radar and has been estimated as high as 34 km during the climatic eruption.
The SO2 and ash injected into the atmosphere has been blamed for global temperature impact (overall cooling for a year or two), though like all things climate these days, there are those who disagree with these conclusions.
Pinatubo is the first example of a large volcanic eruption in a relative modern, densely inhabited area. This time around, the governments involved gave timely warnings and managed to get the people out of the danger zone as it grew. While the property damage and loss was very large, the actual cost in human life for such a large eruption was relatively small.
This was the first huge eruption that was closely monitored, that is until the eruption destroyed emplaced seismographs and other devices. It will not be the last one.