Pinatubo is known for itsVEI-6eruption on June 15, 1991, the second-largest terrestrial eruption of the 20th century after the 1912 eruption ofNovarupta inAlaska.[7] The eruption coincided withTyphoon 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 withpyroclastic surges,pyroclastic falls, and later, floodinglahars caused by rainwater re-mobilizing volcanic deposits. This destruction affected infrastructure and altered river systems for years.[7][8] Minordome-forming eruptions inside the caldera continued from 1992 to 1993.
The 1991 eruption had worldwide effects. It released roughly 10 billiontonnes (1.1×1010short tons) or 10 km3 (2.4 cu mi) of magma, bringing large amounts of minerals and toxic metals to the surface. It also released 20 million tonnes (22 million short tons) ofSO 2. It ejected moreparticulate into thestratosphere than any eruption sinceKrakatoa in 1883. In the following months, aerosols formed a global layer ofsulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F) in the years 1991–1993,[9] andozone depletion temporarily increased significantly.[10]
The volcano is about 87 kilometers (54 mi) northwest ofManila, the capital of the Philippines. Near Mount Pinatubo are former military bases that were maintained by the United States. TheU.S. Naval Base Subic Bay was 37 kilometers (23 mi) south of Pinatubo, and the extent ofClark Air Base was just 14 kilometers (8.7 mi) east of the volcano's summit.[11] The volcano is near to about 6 million people.[12]
Even before the 1991 eruption, Mount Pinatubo had littletopographic prominence: it was 1,745 m (5,725 ft) above sea level, only about 600 m (2,000 ft) above nearbyplains, and only about 200 m (660 ft) higher than surrounding peaks, which largely obscured it from view.[13] It is part of a chain ofvolcanoes which lie along the western side of the island ofLuzon called theZambales Mountains.[14]
Pinatubo is flanked on the west by theZambalesOphiolite Complex, which is an easterly-dipping section ofEocene oceanic crust uplifted during the lateOligocene. The Tarlac Formation north, east and southeast of Pinatubo consists of marine, nonmarine and volcanoclastic sediments formed in the lateMiocene andPliocene.[17]
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 thePhilippine National Oil Company as part of the surface investigations of the area before exploratory drilling and well testing forgeothermal energy sources in 1988 to 1990. He recognized two life histories of the mountain, which he classified as "ancestral" and "modern" Pinatubo.[17][18]
Aerial view of Mount Pinatubo before the 1991 eruption, March 1933Pinatubo in April 1991, approximately two months before the eruption.[19][20]
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 anandesite anddacitestratovolcano 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 2,300 m (7,500 ft), or 1.43 miles abovesea 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.[17]
The old volcano is exposed in the walls of an old 3.5 km × 4.5 km (2.2 mi × 2.8 mi) widecaldera, referred to asTayawan 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 byweathering. Ancestral Pinatubo is asomma 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, formingvolcanic 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.[17]
c. 79,000 BC:[21] After a long period ofdormancy, Modern Pinatubo was born in Ancestral Pinatubo's cataclysmic and most explosive eruptions, estimated to be five times larger than the June 1991 eruption. It deposited all around the volcano up to 25 km3 (6.0 cu mi) ofpyroclastic surge material up to 100 meters (330 ft) thick. The total volume of volcanic material ejected during the eruptions is unknown. The removal of so much material from the underlyingmagma chamber resulted in the Tayawancaldera. The violent eruptive period started by the eruption is referred to by Delfin as theInararo Eruptive Period, named after a village that was destroyed in the 1991 eruption.[17]
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:
c. 15,000 BC (Sacobia Eruptive Period)
c. 7000 BC (Pasbul Eruptive Period). Its eruptions were as energetic, if not as voluminous as the Inararo eruptions.
c. 4000–3000 BC (Crow Valley Eruptive Period). This and the Mara-unot period's eruptions were smaller than the Inararo eruptions but about two to three times as big as that of 1991 based on the pyroclastic flow runout distances and depths of valley filling.
c. 1900–300 BC (Maraunot Eruptive Period)
c. AD 1500 (Buag Eruptive Period). Its eruptions were roughly the same size as those of 1991.
Each of these eruptions seems to have been very large, ejecting more than 10 km3 (2.4 cu mi) 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.[17]
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.[17]
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 denserainforest 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.[17]
Theeruption column of Mount Pinatubo on June 12, 1991, three days before the climactic eruptionView to the west from Clark Air Base of the major eruption of Pinatubo on June 15, 1991. The June 15–16 climatic phase lasted more than fifteen hours, sent tephra about 35 km (22 mi) into the atmosphere, generated voluminouspyroclastic flows, and left a caldera in the former summit region. Later dubbed Black Saturday, the day of darkness stretched for 36 hours.
In March and April 1991, magma rising toward the surface from more than 32 km (20 mi) beneath Pinatubo triggered smallvolcano tectonic earthquakes and caused powerfulsteam explosions that blasted threecraters 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 noxioussulfur dioxide gas were also emitted by the volcano.[7] 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.
A small blast at 3:41 PST on June 12 marked the beginning of a new, more violent phase of the eruption as millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption.[7] A few hours later the same day, massive blasts lasting about half an hour generated big eruption columns, which quickly reached heights of over 19 kilometres (62,000 ft) and which generated largepyroclastic surges extending up to four kilometres (2.5 mi) from the summit in some rivervalleys. Fourteen hours later, a 15-minute blast hurled volcanic matter to heights of 24 km (15 mi). Friction in the up-rushing ash column generated abundantvolcanic lightning.
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 5 km3 (1.2 mi3) of material. Theash cloud from this climactic eruption rose 35 km (22 mi) into the atmosphere. At lower altitudes, thevolcanic 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 largerpumicelapilli blanketed the countryside. Fine ash fell as far away as theIndian Ocean and satellites tracked the ash cloud several times around the globe.[7]
Huge pyroclastic flows roared down the flanks of Mount Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as 200 m (660 ft) thick. The eruption removed so much magma and rock from below the volcano that the summit collapsed to form a 2.5 km (1.6 mi) wide caldera.[7]
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.
The location of Mount Pinatubo and the regional ash fallout from the 1991 eruption
Lake Pinatubo, the crater lake resulting from the 1991 eruption, pictured here in 2008
Activity at the volcano remained low until July 1992 when a new lava dome started growing in thecaldera. 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.[1]
An aerial view of Mt. Pinatubo andLake Pinatubo in 2006
The 1991 caldera afterwards filled with water from annual monsoon rains and acrater 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 highlyacidic, with a minimumpH of 2 and a temperature of about 40 °C (104 °F). Subsequent rainfall cooled and diluted the lake, lowering the temperature to 26 °C (79 °F) and raising the pH to 5.5 by 2003.
The lake deepened by about 1 meter (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 largeflood was accidentally triggered. Workers cut a 5-meter (16 ft) notch in the crater rim and drained about a quarter of the lake's volume.[22]
On July 10, 2002, the west wall of the crater collapsed, slowly releasing approximately 160 million cubic meters (5.7×10^9 cu ft) of water and sediment into the Maraunot River inBotolan, Zambales.[23]
On July 26, 2011, a 5.9 magnitude earthquake struck close to Pinatubo; however, no major damages or casualties were reported.[24]
On March 4, 2021, the PHIVOLCS raised Alert Level 1 over Mount Pinatubo after reporting an increase on its seismic activity. Since January 2021, 1,722 volcanic earthquakes have been recorded within the vicinity of the volcano.[25]
On August 12, 2021, 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 to August 1, 2021. The agency also downgraded Mt. Pinatubo's Alert Level 1 to Level 0, due to "continued decrease in earthquake activity and a return to baseline seismic parameters."[26]
On November 30, 2021, PHIVOLCS reported that a weak explosion occurred at Mt. Pinatubo between 12:09 p.m. and 12:13 p.m. (PST), which produced a plume.[27] The agency later confirmed it was a phreatic explosion produced byhydrothermal fluids near the surface, rather than a magmatic eruption.[28]
On August 7, 2025, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that earthquake-monitoring equipment at the San Jose observation station in Tarlac was stolen.[29] The stolen instruments were valued at approximately ₱1.2 million, according to PHIVOLCS.[30]
The wordpinatubo could mean "fertile place where one can make crops grow", or could mean "made to grow", inSambal andTagalog, 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.[31]
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 theSambal, Aetas andKapampangans living on the Zambales range.[32]
It was said to comprise the whole mountain range until Sinukuan ofMount 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.[32]
According to the native elders, Apo Namalyari induced the June 1991 eruption because of displeasure toward illegal loggers andPhilippine National Oil Company executives who performed deep exploratory drilling and well testing on the volcano looking for geothermal heat from 1988 to 1990.[33] Discouraging results from the wells forced the abandonment of the prospect 13 months before the April 2, 1991, explosions.[34]
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 theNational 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 7,440.1 ha (18,385 acres) covers thebarangays of Mawakat and Nabuklod inFloridablanca, Pampanga, plus a portion ofSan Marcelino, Zambales, and a portion of Barangay Batiawan inSubic, Zambales.[35]
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 theirlutan tua (ancestral land). The ancestral domain title covers 15,984 ha (39,500 acres) and includes the villages of Burgos, Villar, Moraza and Belbel in Botolan and portions of the towns ofCabangan,San Felipe andSan Marcelino.[36][37]
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.[37] Having the land title will protect them from others – including foreigners – exploiting their land without compensation to and consent from the indigenous tribes. In the past, the Aetas had to contend with mining companies, loggers, and recently, tourist companies who earn from Mount Pinatubo but do not compensate the local tribes.[37]
The caldera formed andLake Pinatubo has, since June 15, 1991, become a tourist attraction with the preferred route through Barangay Santa Juliana inCapas, Tarlac.[40]
In April 2025, Aeta communities held a protest action calling attention to their exclusion from revenues from tourism on Mount Pinatubo. The protesters said they their calls for the NCIP to address the issue had been ignored.[41][42] Indigenous rights activistBeverly Longid of KATRIBU said that the NCIP has a history of neglecting the Aeta indigenous peoples. Longid stated that there is a need to recognize "the Aeta and their rights over their ancestral land and territory".[42]
^Rodolfo, K.S. & Umbal, J.V. (2008) "A prehistoric lahar-dammed lake and eruption of Mount Pinatubo described in a Philippine aborigine legend", J. Volcanol. Geotherm. Res.,176, 432–437
^abHeaney, Lawrence R.; Balete, Danilo S.; Veluz, Maria Josefa; Steppan, Scott J.; Esselstyn, Jacob A.; Pfeiffer, Andrew W.; Rickart, Eric A. (January 2014). "Two new species of Philippine forest mice (Apomys, Muridae, Rodentia) from Lubang and Luzon Islands, with a redescription ofApomys sacobianus Johnson, 1962".Proceedings of the Biological Society of Washington.126 (4):395–413.doi:10.2988/0006-324X-126.4.395.S2CID49347286.
Self S.; Zhao, Jing-Xia; Holasek, R.E.; Torres, R.C.; King, A.J. (1998). Newhall, C.G.; Punongbayan, R.S. (eds.).Fire and Mud, Eruptions and Lahars of Mount Pinatubo, Philippines. Washington: Smithsonian Institution – Global Volcanism Program. p. 1126.Archived from the original on November 17, 2017. RetrievedApril 21, 2010.The atmospheric impact of the 1991 Mount Pinatubo eruption
Stimac, James A.; Goff, Fraser; Counce, Dale; Larocque, Adrienne C. L.; Hilton, David R.; Morgenstern, Uwe (2004). "The crater lake and hydrothermal system of Mount Pinatubo, Philippines: Evolution in the decade after eruption".Bulletin of Volcanology.66 (2):149–167.Bibcode:2004BVol...66..149S.doi:10.1007/s00445-003-0300-3.S2CID128408824.
Wiesner, Martin G.; Wetzel, Andreas; Catane, Sandra G.; Listanco, Eddie L.; Mirabueno, Hannah T. (2004). "Grain size, areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea".Bulletin of Volcanology.66 (3):226–242.Bibcode:2004BVol...66..226W.doi:10.1007/s00445-003-0306-x.S2CID128818475.
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