Active stratovolcano in Luzon, Philippines
Mount Pinatubo is an active stratovolcano in the Zambales Mountains in Luzon in the Philippines. Located on the tripoint of Zambales, Tarlac and Pampanga provinces, most people were unaware of its eruptive history before the pre-eruption volcanic activity in early 1991. Dense forests, which supported a population of several thousand indigenous Aetas, heavily eroded and obscured Pinatubo.
Pinatubo is known for its VEI-6 eruption on June 15, 1991, the second-largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in Alaska. The eruption coincided with Typhoon Yunya making landfall in the Philippines, which brought a dangerous mix of ash and rain to nearby towns and cities. Early predictions led to the evacuation of tens of thousands of people, saving many lives. The eruption severely damaged surrounding areas with pyroclastic surges, pyroclastic falls, and later, flooding lahars caused by rainwater re-mobilizing volcanic deposits. This destruction affected infrastructure and altered river systems for years. Minor dome-forming eruptions inside the caldera continued from 1992 to 1993.
The 1991 eruption had worldwide effects. It released roughly or of magma, bringing large amounts of minerals and toxic metals to the surface. It also released of SO2. It ejected more particulate into the stratosphere than any eruption since Krakatoa in 1883. In the following months, aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about in the years 1991–1993, and ozone depletion temporarily increased significantly.
Geography.
The volcano is about northwest of Manila, the capital of the Philippines. Near Mount Pinatubo are former military bases that were maintained by the United States. The U.S. Naval Base Subic Bay was south of Pinatubo, and the extent of Clark Air Base was just east of the volcano's summit. The volcano is near to about 6 million people.
History.
Even before the 1991 eruption, Mount Pinatubo had little topographic prominence: it was above sea level, only about above nearby plains, and only about higher than surrounding peaks, which largely obscured it from view. It is part of a chain of volcanoes which lie along the western side of the island of Luzon called the Zambales Mountains.
Pinatubo belongs to the Cabusilan sub-range of the Zambales Mountains, which consists of Mount Cuadrado, Mount Negron, Mount Mataba and Mount Pinatubo. They are subduction volcanoes, formed by the Eurasian plate sliding under the Philippine Mobile Belt along the Manila Trench to the west. Mount Pinatubo and the other volcanoes on this volcanic belt arise due to magmatic occlusion from this subduction plate boundary.
Pinatubo is flanked on the west by the Zambales Ophiolite Complex, which is an easterly-dipping section of Eocene oceanic crust uplifted during the late Oligocene. The Tarlac Formation north, east and southeast of Pinatubo consists of marine, nonmarine and volcanoclastic sediments formed in the late Miocene and Pliocene.
The most recent study of Mount Pinatubo before the activities of 1991 was the overall geological study in 1983 and 1984 made by F. G. Delfin for the Philippine National Oil Company as part of the surface investigations of the area before exploratory drilling and well testing for geothermal energy sources in 1988 to 1990. He recognized two life histories of the mountain, which he classified as "ancestral" and "modern" Pinatubo.
Ancestral Pinatubo.
Activity of Ancestral Pinatubo seems to have begun about 1.1 million years ago and probably ended tens of thousands of years or more before the birth of "modern" Pinatubo. Much of the rugged land around the present volcano consists of remnants of "ancestral" Pinatubo. It was an andesite and dacite stratovolcano whose eruptive activity was much less explosive than modern Pinatubo. Its center was roughly where the current volcano is. The projected height of the mountain is up to , or 1.43 miles above sea level if it were a lone peak, based on a profile fitting to the remaining lower slopes, or lower if it had more than one peak.
The old volcano is exposed in the walls of an old wide caldera, referred to as "Tayawan Caldera" by Delfin. Some of the nearby peaks are the remnants of ancestral Pinatubo, left behind when the softer parts of the old mountain slopes were eroded by weathering. Ancestral Pinatubo is a somma volcano with modern Pinatubo as the new cone.
Mount Dorst, to the east, is part of the dip slope of the ancestral Pinatubo. Several mountains near modern Pinatubo are old satellite vents of ancestral Pinatubo, forming volcanic plugs and lava domes. These satellite vents were probably active around the same time as the ancestral volcano and include the domes of Mount Negron, Mount Cuadrado, Mount Mataba and the Bituin and Tapungho plugs.
Modern Pinatubo.
Later eruptions of modern Pinatubo occurred episodically and lasted for periods much shorter than the repose intervals between them. Subsequent eruptions and eruptive period occurred about:
Each of these eruptions seems to have been very large, ejecting more than of material and covering large parts of the surrounding areas with pyroclastic flow deposits. Some eruptive periods have lasted decades and perhaps as much as several centuries and might appear to include multiple large explosive eruptions.
The maximum size of eruptions in each eruptive period though has been getting smaller through the more than 35,000-year history of modern Pinatubo, but this might be an artifact of erosion and burial of older deposits. The oldest eruption of modern Pinatubo, Inararo, was also its largest.
The 1991 eruption was among the smallest documented in its geologic record.
The volcano has never grown very large between eruptions, because it produces mostly unwelded, easily erodible deposits and periodically destroys the viscous domes that fill its vents. After the Buag eruption (c. 1500 AD), the volcano lay dormant, its slopes becoming completely covered in dense rainforest and eroded into gullies and ravines. The c. 500-year repose though between the Buag and present eruptive periods is among the shorter repose periods recognized in its geologic history.
1991 eruption.
A small blast at 03:41 PST on June 12 marked the beginning of a new, more violent phase of the eruption. A few hours later the same day, massive blasts lasting about half an hour generated big eruption columns, which quickly reached heights of over and which generated large pyroclastic surges extending up to from the summit in some river valleys. Fourteen hours later, a 15-minute blast hurled volcanic matter to heights of . Friction in the up-rushing ash column generated abundant volcanic lightning.
In March and April 1991, magma rising toward the surface from more than beneath Pinatubo triggered small volcano tectonic earthquakes and caused powerful steam explosions that blasted three craters on the north flank of the volcano. Thousands of small earthquakes occurred beneath Pinatubo through April, May and early June and many thousand of tons of noxious sulfur dioxide gas were also emitted by the volcano.
From June 7 to 12, the first magma reached the surface of Mount Pinatubo. Because it had lost most of the gas contained in it on the way to the surface, the magma oozed out to form a lava dome but did not cause an explosive eruption. However, on June 12, millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption.
When even more highly gas-charged magma reached Pinatubo's surface on June 15, the volcano exploded in a cataclysmic eruption that ejected more than of material. The ash cloud from this climactic eruption rose into the atmosphere. At lower altitudes, the volcanic ash was blown in all directions by the intense cyclonic winds of a coincidentally occurring typhoon, and winds at higher altitudes blew the ash southwestward. A blanket of ash and larger pumice lapilli blanketed the countryside. Fine ash fell as far away as the Indian Ocean and satellites tracked the ash cloud several times around the globe.
Huge pyroclastic flows roared down the flanks of Mount Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as thick. The eruption removed so much magma and rock from below the volcano that the summit collapsed to form a wide caldera.
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.
Later eruptions.
Activity at the volcano remained low until July 1992 when a new lava dome started growing in the caldera. Volcanologists suspected that further violent eruptions could be possible, and some areas were evacuated. However, the eruption was only minor. The last eruption of Mount Pinatubo took place in 1993.
Lake Pinatubo.
The 1991 caldera afterwards filled with water from annual monsoon rains and a crater lake, Lake Pinatubo, was formed. In 1992, a growing lava dome formed an island, which was eventually submerged by the lake. Initially, the lake was hot and highly acidic, with a minimum pH of 2 and a temperature of about . Subsequent rainfall cooled and diluted the lake, lowering the temperature to and raising the pH to 5.5 by 2003.
The lake deepened by about 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 was accidentally triggered. Workers cut a notch in the crater rim and drained about a quarter of the lake's volume.
Recent activity.
On July 10, 2002, the west wall of the crater collapsed, slowly releasing approximately of water and sediment into the Maraunot River in Botolan, Zambales.
On July 26, 2011, a 5.9 magnitude earthquake struck close to Pinatubo; however, no major damages or casualties were reported.
On March 4, 2021, the PHIVOLCS raised alert level 1 over Mount Pinatubo after reporting an increase on its seismic activity. 1,722 volcanic earthquakes were also recorded within the vicinity of the volcano since January 2021.
On August 11, 2021, PHIVOLCS downgraded Mt. Pinatubo's Alert Level 1 to Level 0, due to "continued decrease in earthquake activity and a return to baseline seismic parameters".
PHIVOLCS said it noted a "significant decrease" in volcanic earthquakes, with a total of 104 quakes or an average 2–3 events per day recorded from July 1 – August 1, 2021.
On November 30, 2021, PHIVOLCS reported a weak explosion occurred on Mt. Pinatubo between 12:09 p.m. and 12:13 p.m., which produced a plume. The agency later confirmed it was a phreatic explosion produced by hydrothermal fluids near the surface, rather than a magmatic eruption.
Cultural history.
The word "pinatubo" could mean "fertile place where one can make crops grow", or could mean "made to grow", in Sambal and Tagalog, which may suggest a knowledge of its previous eruption in about 1500 AD. There is a local oral tradition suggestive of a folk memory of earlier large eruptions. An ancient legend tells of Bacobaco, a terrible spirit of the sea, who could metamorphose into a huge turtle and throw fire from his mouth. In the legend, when being chased by the spirit hunters, Bacobaco flees to the mountain and digs 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 shakes.
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 language 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, Aetas and Kapampangans living on the Zambales range.
It was said to comprise the whole mountain range until Sinukuan of Mount Arayat (the god of the Kapampangans) became a strong rival of Namalyari. Their fight, which took place over the center plains, shattered the mountain into smaller bodies and 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 native elders, Apo Namalyari induced the June 1991 eruption because of displeasure toward illegal loggers and Philippine National Oil Company executives who performed deep exploratory drilling and well testing on the volcano looking for geothermal heat from 1988 to 1990. Discouraging results from the wells forced the