Lahar Grand Canyon and Crater

Lahar Grand Canyon and Crater
Mt. Pinatubo, Zambales

An aerial view of the crater and lahar canyons of Mt. Pinatubo.

Mount Pinatubo is a stratovolcano on the island of Luzon. Its eruptive history is divided into two distinct parts. The first part of this history involves an ancestral volcano to Pinatubo. Ancestral Pinatubo was a stratovolcano made of andesite and dacite. There is no evidence of large explosive eruptions from this volcano. Ancestral Pinatubo was centered where the modern Pinatubo currently stands. It may have risen as high as 7,550 ft (2300 m) above sea level at one time. Remains of a 2.2 by 2.8 mile (3.5 by 4.5 km) wide caldera from the old Pinatubo are still standing in the area today.

The modern Pinatubo is a dome complex and stratovolcano made of dacite and andesite. This complex is surrounded by pyroclastic flow and lahar deposits from large explosive eruptions. These explosive eruptions have been clustered into 6-12 eruptive periods. The explosive eruption of June 15, 1991 is one of the smallest of these eruptive periods. The 460 (+/- 30) year period of dormancy that preceded this eruption was actually relatively short compared to other such periods in the history of the volcano.

Pinatubo rose about 5725 ft ( 1745 m) above sea level before the June 1991 eruption. This means almost 500 ft (150 m) of the volcano was blasted away by this eruption. This seems to be the history of the volcano. After a relatively short explosive eruption, it will undergo a period of slow growth through extrusions. Then the volcano becomes dormant for a longer period, and much of the material deposited during the last eruption is eroded. Finally, another explosive eruption will occur and destroy whats left of the growth from the last eruptive period. This process seems to have repeated itself over time, creating a somewhat stable size for the volcano.

Pinatubo typically erupts large quantities of relatively cool ash and dacite magma. It follows that a large, shallow magma chamber exists beneath the volcano. When gas and crystal rich magma evolves in this chamber, explosive eruptions occur. There is great evidence of mafic magma mixing with the dacitic magma in these chambers causing these eruptions.

Pinatubo has been relatively quiet since the 1991-1992 eruption, but it is still active. It remains to be determined whether or not more explosions at the volcano are likely during the current eruptive period.

This photo shows a view of Mt. Pinatubo looking northeast at sunrise.