Mount Pinatubo

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 . 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

left|thumb|Mount Pinatubo relief map

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.

Ancestral Pinatubo

thumb|left|Aerial view of Mount Pinatubo before the 1991 eruption, March 1933

thumb|Pinatubo in April 1991, approximately two months before the eruption.|left

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. After a long period of dormancy, 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 of pyroclastic surge material up to thick. The total volume of volcanic material ejected during the eruptions is unknown. The removal of so much material from the underlying magma chamber resulted in the Tayawan caldera. The violent eruptive period started by the eruption is referred to by Delfin as the Inararo Eruptive Period, named after a village that was destroyed in the 1991 eruption.

1991 eruption

thumb|right|The [[eruption column of Mount Pinatubo on June 12, 1991, three days before the climactic eruption]]

thumb|right|View 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 into the atmosphere, generated voluminous [[pyroclastic 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 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.

Recent activity

thumb|The Mount Pinatubo (Drone footage)

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. Since January 2021, 1,722 volcanic earthquakes have been recorded within the vicinity of the volcano.

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."

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. The agency later confirmed it was a phreatic explosion produced by hydrothermal fluids near the surface, rather than a magmatic eruption.

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. The stolen instruments were valued at approximately ₱1.2 million, according to PHIVOLCS.

Activity monitoring

Alert levels

{| class="wikitable" style="text-align:left; font-size:95%;"

|+ Pinatubo Volcano Alert Level Scheme

! scope="col" style="width:5%;" | Alert Level

! scope="col" style="width:50%;" | Monitoring Criteria

! scope="col" style="width:30%;" | Interpretation

! scope="col" style="width:15%;" | Recommendations

! scope="col" style="width:35%;" | Step-down Criteria

! scope="col" style="width:5%;" | Alert Level (Step-down)

! scope="row" | 0

(Normal)

| Background parameters: Volcanic earthquakes typically <5/day; Caldera lake CO2 flux <1000 tonnes/day.

| Quiescence; no major eruption in foreseeable future

| Communities at risk must continue preparedness efforts.

! scope="row" | 1

(Low-level unrest)

| Abnormal parameters: Volcanic earthquake clusters along regional faults or lineaments or within the hydrothermal system; Changes in fumarolic activity in the crater; increased crater lake CO2 flux and CO2/H2S ratio changes.

| Hydrothermal or local tectonic activity beneath/near the volcano may be occurring; no eruption imminent.

| Extreme caution when venturing into the Pinatubo Caldera recommended.

| Alert Level 1 monitoring criteria have been in decline for at least one (1) month before lowering to Alert Level 0.

! scope="row" | 2

(Increasing unrest)

| Increasing changes in parameters: Increased and shallowing high-frequency (HF) or volcano-tectonic (VT) earthquake clusters, shallow sporadic low-level tremor with long-period (LP) or low-frequency (LF) events, DLPs (deep long period) events at sub-crustal depths; Continued increases in crater lake CO2 flux and CO2/H2S ratios, significant SO2 emission may be detected; Increased fumarolic activity with discrete weak phreatic eruptions; Ground deformation detected by satellite systems.

| Deep-seated magmatic intrusion and/ or increased hydrothermal activity may be occurring, with increased chances of phreatic, gas or hydrothermal explosions; could eventually lead to an eruption.

| No entry into Pinatubo Caldera; Preparation of communities in case of escalation of unrest.

| Alert Level 2 monitoring criteria have been in decline for one (1) month before lowering to Alert Level 1.

! scope="row" | 3

(Intensified unrest/Magmatic unrest)

| Intensifying changes in parameters: VT/ HF earthquake clusters or swarms within shallow depths of the edifice, increase in the intensity and duration of LP/LF events, DLP clusters at lower crustal depths; Sustained or increasing SO2 emission, increased phreatic activity with explosion-type earthquakes or tremor episodes; Increasing ground deformation or inflation of the edifice.

| Certain magmatic intrusion into the shallow magma system or edifice, with higher chances of a major eruption; precursory eruptive activity due to disruption of the hydrothermal system; lava dome growth may occur.

| Evacuation of upland communities up to 10-kilometers from the summit caldera.

| Alert Level 3 monitoring criteria have been in decline for at least two (2) weeks before lowering to Alert Level 2.

! scope="row" | 4

(Hazardous eruption imminent)

| Accelerating changes or abrupt decline in parameters: Strong VT/ HF earthquakes within a few kilometers depth of the edifice with felt intensities, episodic swarms of hybrid or LP earthquakes, episodic tremor with continuous ash emission, sustained and intensifying volcanic tremor, episodic explosion earthquakes with explosive activity; Increasing incidence and magnitudes of volcanic earthquakes; Successive explosions with pronounced eruption columns and small-volume pyroclastic density currents (PDCs); Intense ground deformation or bulging of the summit region; Abrupt increase or drop in SO2/volcanic gas flux.

| Phreatomagmatic or pre-climactic magmatic eruption; If magma ascent rates increase, highly explosive eruption probable within hours to a few days; if magma ascent rates decrease, prolonged lava dome growth may occur.

| Evacuation of communities within pre-determined hazard zones for PDCs, heavy ashfall and syn-eruption lahars.

| Alert Level 4 monitoring criteria have been in decline for two (2) weeks before lowering to Alert Level 3.

! scope="row" | 5

(Highly hazardous eruption in progress)

| Highly explosive magmatic eruption ongoing: Successive explosion-type earthquakes or large-amplitude volcanic tremor and VT earthquakes; Successive or sustained explosions with large eruption column and pronounced umbrella cloud that could exceed 20-kilometer heights above the crater; extensive PDCs emplaced around the edifice; widespread ashfall; syn-eruption lahars

| Climactic Subplinian to Plinian eruption; Volcanic hazards expected within 30-kilometer radius of the crater and downwind of the eruption plume.

| Evacuation of additional communities, downwind of the eruption plume, along major river systems and in buffer extensions of hazard zones.

| Alert Level 5 conditions have ceased for at least 24 hours before lowering to Alert Level 4.

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. Discouraging results from the wells forced the abandonment of the prospect 13 months before the April 2, 1991, explosions.

Aetas granted ownership of Pinatubo

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 the National 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 covers the barangays of Mawakat and Nabuklod in Floridablanca, Pampanga, plus a portion of San Marcelino, Zambales, and a portion of Barangay Batiawan in Subic, Zambales.

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 their lutan tua (ancestral land). The ancestral domain title covers and includes the villages of Burgos, Villar, Moraza and Belbel in Botolan and portions of the towns of Cabangan, San Felipe and San Marcelino.

In popular culture

thumb|Hiking to Mount Pinatubo's Crater

Long before Mount Pinatubo became famous for its cataclysmic eruption, Philippine president Ramon Magsaysay, a native of Zambales, named his C-47 presidential plane Mt. Pinatubo. The plane crashed into Mount Manunggal in Cebu province in 1957, killing the president and twenty-four others on board.

The shape of Mount Pinatubo's caldera inspired New Clark City Athletics Stadium in Capas, Tarlac.

Hiking activity

Since June 15, 1991, the recent caldera and Lake Pinatubo have become a tourist attraction. The preferred route is through Barangay Santa Juliana in Capas, Tarlac.

In April 2025, Aeta communities held a protest, calling attention to their exclusion from revenues from tourism on Mount Pinatubo. The protesters said that their calls for the NCIP to address the issue had been ignored. Indigenous rights activist Beverly 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".

The Zambales forest mouse (Apomys zambalensis) is a species of rodent endemic to Zambales Mountain Range, Philippines.

Notes

References

Decker, R. and Decker, B. (1997) Volcanoes, 3rd edition, WH Freeman, New York.
Dhot, S. Mt Pinatubo Safety.

External links

Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines, United States Geological Survey site
"Weather effects of the 1991 eruption" EOS Volcanology.
"The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines". United States Geological Survey site
Pinatubo, Philippines (volcanic images)
Entry for Pinatubo in the Smithsonian Institution's Global Volcanism Program (GVP)