Introduction - Mount Pinatubo Revisited: A Study of Lahar Erosion

The Pinatubo deposits are subdivided into two general groups based on the lithology and age of emplacement: the “ancestral” and the “modern.” The ancestral Pinatubo (+ 1 million to +35,000 years B.P.) is an andesite-dacite stratovolcano of mostly laval flows and breccia deposits of laharic origin. On its slopes are numerous elongated to sub-rounded hills made up of breccia, created mostly by the ancestral lahars.

The modern Pinatubo (+ 35,000 years to present) shows signs of repeated, very explosive eruptions which have produced large volumes of pumiceous pyroclastic flows. Pyroclastic flows, also known as nuée ardentes, or glowing avalanches, are extremely hot (+1,000° Celsius), often incandescent, highly fluid, gravity-driven density currents of gas and volcanic fragments that sweep down slope and travel at hurricane speed (+100 km per hour). Pyroclastic flows are generated when the density of the rising column of volcanic fragments and gas exceeds that of the surrounding atmosphere. Gravity causes a portion or all of the column to collapse and flow down the flanks of the volcano.

Most of the 1991 pyroclastic flow deposits were emplaced during the June 15 eruption. The pre-eruption magma temperature of Pinatubo was about 800° Celsius and the temperature of the emplaced pyroclastic flow was on the order of 600° Celsius. The deposits are non-welded, dry, and very loose.

The accumulated thickness of the pyroclastic flows varies, depending on the proximity to the crater and the pre-eruption morphology. It reaches more than 200 meters along deep pre-eruption valleys. The pyroclastic flow deposits of 1991 affected eight major watersheds around the slopes of the volcano and radically altered the hydrological regimes, leading to unprecedented amounts of erosion and sediment delivery in the form of destructive lahars.

Lahars predominantly occur during the rainy season in the monsoon period, which lasts from June until November. Long-duration and high-intensity rainfall, associated with the occurrence of strong typhoons, are responsible for the production of large-magnitude lahars. Other factors contributing to the rapid erosion of lahars are: failure of lahar dams, secondary explosions produced by rapid vertical and lateral erosion of the pyroclastic flow deposits, and stream and river capture bringing deposits into new drainage systems.

The rapid erosion or removal of the pyroclastic flow deposits has been one of the major social and scientific concerns after the 1991 eruption of the Pinatubo Volcano. The lahars from Pinatubo have resulted in loss of lives and damage to properties in areas surrounding the volcano. About 50,000 persons were left homeless and indirect impacts such as flooding and isolation have affected more than 1.3 million people in 39 different towns and four large cities. About 1,000 km2 of prime agricultural lands are at risk.

Lahars will continue to be an important hazard on the lower slopes of Pinatubo for a number of years.

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