The Philippines is host to more than two dozen active volcanoes. These volcanoes occur in a
variety of shapes and sizes. However, none of these volcanoes in the country are as famous
as Mount Pinatubo. After more than 500 years of dormancy, this volcano produced a cataclysmic
eruption in 1991. This event was the second largest volcanic eruption of the entire
20th century. Although not much activity has happened since its major eruption, the volcano is
still active. This video will discuss this active volcano, and mention what its future hazards are.
Mount Pinatubo can be found on the island of Luzon in the central western Philippines. Specifically,
it is 90 kilometers northwest of Manila. Like the Taal Volcano to the southeast,
Mount Pinatubo exists due to the same tectonic features. It exists due to geologic activity
related to the subduction of the Eurasian plate underneath the Philippine Mobile Belt.
The subducted Eurasian plate crust from this collision then melts and migrates upwards until
it erupts on the surface, creating a volcano. When you look at this volcano from satellite,
you can truly see the full extent of its 1991 eruption. This eruption created a 2.5 kilometer
wide crater which is referred to as a caldera. However, the volcano didn't always look like this.
Additionally, this is not the first caldera to form at the mountain. (pause 2 seconds)
Mount Pinatubo originated after an explosive eruption of its Ancestral volcano 35,000
years ago. This eruption formed a 4.5 kilometer wide caldera, from which the
modern volcano grew (Tayawan caldera). Over tens of thousands of years, different periods
of volcanic activity built the modern volcano. These eruptive periods primarily consisted of lava
dome building eruptions, but each also featured a very large explosive eruption similar in size to
its 1991 eruption. These large explosive eruptions are one reason why Mount Pinatubo never grew to a
significant height, as highly explosive eruptions periodically destroy a portion of the volcano.
Within the last 10,000 years, five highly explosive eruptions have been documented
in rock layers observed at the volcano. The oldest of these occurred 9,500 years
ago and created deposits of rock which at completely filled the Gumain River.
The next large eruption occurred in 3550 BC during the Crow Valley Eruptive Period. Pyroclastic
flow related deposits from this period can be found along the Sacobia river where they are
20 meters thick. The two subsequent eruptive periods had a similar 2500 year gap of volcanic
activity between them. They occurred in 1050 BC and more recently in 1450.
For the next several hundred years, there were no definitive records of eruptions from the volcano.
However, there is some contemporary evidence which suggests that several steam explosions
known as phreatic eruptions may have occurred. After a 540 year dormancy, volcanic activity began
again in April of 1991. Initially, it started as a series of phreatic eruptions which took place
along a kilometer and a half long fissure. Over the next several weeks, these eruptions scattered
ash over nearby areas. Throughout the month of May, the volcano produced an ever increasing
amount of sulfur dioxide, indicating a rising column of magma. However, at the end of May
the rate of sulfur dioxide suddenly dropped. This did not indicate a decrease in activity,
but rather that something was blocking the gas. In early June, scientific instruments showed that
the volcano was slowly rising over a large area. Scientists recommended the extreme danger this
posed and suggested a much larger evacuation zone. The mandatory evacuation area was subsequently
expanded to anyone living within 40 kilometers. The eruption escalated until it reached a climatic
phase on June 15th. A column of ash rose from Pinatubo to 40 kilometers into the air. Super
heated pyroclastic flows raced down the mountain, and reached up to 16 kilometers away. Dangerous
lahars swept down the volcano and traveled more than 40 kilometers away. During this eruption,
the volcano lost about 300 meters in elevation and erupted 13 cubic kilometers of tephra.
So, what can we expect in the next few decades from this volcano? In my opinion, the next several
hundred years will not contain any new large eruptions similar to what occurred in 1991.
Rather, a series of phreatic eruptions or lava domes could erupt at this volcano, as hinted by
past periods of activity. The main hazard posed in the near future from this volcano is lahars.
Every time a large monsoon hits, some debris from the 1991 eruption washes down into riverbeds,
which can effect a number of low lying areas around the volcano. Additionally, potential
ashfall from future eruptions is also a concern. I hope that you enjoyed this video.
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