Mt Pinatubo Volcano | John Seach


Luzon, Philippines

15.13 N, 120.35 E
summit elevation 1486 m

The Mt Pinatubo eruption of 1991 was the second largest eruption of the 20th century. Mount Pinatubo Volcano had been dormant for 400 years.

Before 1991 Pinatubo was an inconspicuous volcano which was known to be active in the past 1000 years and was the site of a failed geothermal development. The area was the home of the Aetas people, who considered the mountain the home of Apo Namalyari, the Great Protector and Provider.

Before 1991 Pinatubo had a height of 1745 m, and was only 200 m higher than nearby mountains which obscured it from view. The surrounding mountains are remains of an older Mt Pinatubo.

Geology of Mt Pinatubo
Pinatubo is flanked on the west by the Zambales Ophiolite Complex, which is an easterly-dipping section of Eocene ocean crust which was uplifted during the late Oligocene. The Tarlac Formation consists of marine, nonmarine and volcaniclastic sediments in the north, east and southeast of Pinatubo which was formed in the late Miocene and Pliocene. Ancestral Pinatubo was an andesite and dacite stratovolcano whose centre was in the same location of the current volcano. The old volcano is exposed in the walls of an old 3.5 x 4.5 km wide caldera. Ancestral satellite vents include the domes of Mount Negron, Mount Cuadrado, Mataba, Bituin plug, and Tapungho plug.

Eruptions of Mount Pinatubo
The largest eruption at Mt Pinatubo volcano occurred 35,000 years ago, and produced pyroclastic flow deposits up to 100 m thick on all sides of the volcano. The Sacobia eruption period 17,000 years ago produced two debris flow deposits which are visible on the north bank of the Sacobia River.

Pasbul eruptive period occurred 9,000 years ago and is visible as pyroclastic flow and tephra deposits exposed along the road between Sitio Pasbul, Camias, Porac, and Gumain River. Crow Valley eruptions 6,000-5,000 years ago produced pyroclastic flow deposits on both sides of upper Crow valley. Maraunot eruptions occurred from about 3,900 to 2,300 years ago.

The previous eruptions before 1991 (Baug Eruptions) occurred in 1450 ± 50 years and produced pyroclastic flow deposits in all valleys of Pinatubo except Gumain and Porac rivers.

2021 Seismic events
Seismic unrest was recorded at Pinatubo commencing in early 2021 with 826 small earthquakes between 20-27 January. Magnitudes range from 1.0-2.5 and had a focus at depths of 15-28 km. The volcano remained at level 0 alert (quiescent).

2009 Earthquake
On 31st October 2009 a magnitude 5.0 earthquake hit 37 km WSW of the summit of Mt Pinatubo volcano. The earthquake focus was at a depth of 79 km.

2009 Mudflows
Heavy monsoon rains caused by typhoon Kiko triggered mudflows which 12 people and flooded the town of Botolan in August 2009. There were some volcanic materials mixed with water, but the flows were not a lahar.

During 2001 the crater lake on Mount Pinatubo reached dangerous levels, and the crater lake changed colour in January 2004.

1991 eruption
On 16th July 1990 a magnitude 7.8 earthquake hit 100 km northeast of Mt Pinatubo.
3rd August 1990
Loud rumbling heard, a landslide near the summit, and steaming ground. First volcanic activity in 400 years.
August 1990
Five earthquakes near Pinatubo.
15th March 1991
Rumbling and earth tremors.
2nd April 1991
Steam and ash exploded from a 1.5 km long fissure high on the northern slopes of Pinatubo. Smell of sulphur and ash fell 10 km away.
3rd April 1991
200 small earthquakes registered at the volcano.
23rd April 1991
US Geological Survey arrived with monitoring equipment.
26th April 1991
Monitoring station set up at Clark Air Base 25 km to the east of the volcano.
13th May 1991
30-180 earthquakes per day were being recorded. Volcano releasing sulphur dioxide indicating molten magma was rising beneath the volcano. Alert level 2 issued.
23rd May 1991
First hazard map completed.
28th May 1991
A tenfold increase in sulphur dioxide emissions in previous 2 weeks.
1st June 1991
Earthquakes became centered 5 km below the volcano.
5th June 1991
Sulphur dioxide emissions almost stopped as the vents became blocked. Earthquakes continued and the mountain started to bulge as magma moved towards the surface.
7th June 1991
1500 earthquakes under Pinatubo. An ash eruption sent a cloud to 8 km high. Alert level 4 issued stating an eruption could happen within 24 hours. Danger zone extended to a 20 km radius.
Lava reached the surface and formed a small dome 1 km northwest of the main crater.
9th June 1991
Sulphur dioxide began escaping again from the volcano. The first nuee ardentes rolled down the volcano. A large eruption of ash. People evacuated from a 20 km radius.
10th June 1991
Evacuations begun at Clark Air Base. (14,400 people)
12th June 1991
Ash eruptions to 20 km above the volcano on Philippine Independence Day.
13th June 1991
Ash eruption to 25 km high followed by 28 hours without an explosion. Earthquakes continued.
14th June 1991
Violent explosion at 1:09 pm. Eruption column to 21 km in 15 minutes. Nuee ardentes devastated the main valleys radiating from Mt Pinatubo.
15th June 1991. The Main Eruption
Typhoon Yunya passes 100 km northeast of Pinatubo. Mudflows raged down the mountain at 30 km/hr. At 3:39 pm the main eruption began. In 9 hours 90 percent of the total material for the eruption was expelled from the volcano. Cauliflower columns of ash, gas and steam rose 34 km high and traveled 400 km. At 4:30 pm the summit of Pinatubo began to sink. The evacuation area was increased to 40 km radius. The eruption stopped at 10:340 pm.
Effects of the Eruption
740 people killed.
A huge caldera was formed 2.5 km across.
260 m was lost off the summit of the volcano.
The ash entered the stratosphere and covered the whole earth within 12 months.
Global temperatures were reduced by 0.5 degree C the year after the eruption.
Forests buried under 50-200 m deep ash and pumice.
During the last five months of 1991 200 mudflows raced down the valleys of Pinatubo.
Damage amounted to $450 million dollars.
8,000 houses were destroyed and 75,000 houses damaged.
2 million people were affected by the eruption.
The biggest volcanic disaster of the 20th century was avoided due to good planning and monitoring.

Further reading
Soden, B.J., Wetherald, R.T., Stenchikov, G.L. and Robock, A., 2002. Global cooling after the eruption of Mount Pinatubo: A test of climate feedback by water vapor. science296(5568), pp.727-730.

Book, Christopher G. Newhall, Raymundo Punongbayan, Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines, 1996.

McCormick, M.P., Thomason, L.W. and Trepte, C.R., 1995. Atmospheric effects of the Mt Pinatubo eruption. Nature373(6513), pp.399-404.

Bluth, G.J., Doiron, S.D., Schnetzler, C.C., Krueger, A.J. and Walter, L.S., 1992. Global tracking of the SO2 clouds from the June, 1991 Mount Pinatubo eruptions. Geophysical Research Letters19(2), pp.151-154.

Hansen, J., Lacis, A., Ruedy, R. and Sato, M., 1992. Potential climate impact of Mount Pinatubo eruption. Geophysical Research Letters19(2), pp.215-218.

Mt Pinatubo Volcano Eruptions

1992, 1991, 1450 ± 50 years, 1050 BC ± 500, 3550 BC?, 7030 BC ± 300, 7460 BC ± 150, 15,000 BC, 33,000 BC.