FIRE and MUD: Eruptions and Lahars of Mount Pinatubo, Philippines


On the afternoon of June 15, 1991, Mount Pinatubo erupted between 3.7 and 5.3 km 3 of magma (8.4 to 10.4 km3 of porous, pumiceous deposit), devastated more than 400 km2, and blanketed most of Southeast Asia with ash (W.E. Scott and others; Paladio-Melosantos and others). For comparison, Mount St. Helens erupted only a tenth of this volume of magma but devastated and blanketed similar areas, more thinly. Within the 20th century, only the eruption of 13+-3 km3 of magma from Novarupta (Katmai) in 1912, in a remote part of Alaska, was larger.

Even more remarkable than the volume, area of impact, and eruption rate is that this was the first event of this size to be monitored in detail (Wolfe and Hoblitt). Many startling and fascinating phenomena were observed. Some, such as the occurrence of long-period earthquakes before explosive eruptions, were previously known but magnified at Pinatubo. Others, such as early separation of a volatile phase deep within dacite reservoirs, were noticed first at Pinatubo and are now being found elsewhere (Gerlach and others). In another example, a sudden, short-lived decrease in SO2 emission shortly before an explosion was seen first at Pinatubo (Daag, Tubianosa, and others) and is now being discovered elsewhere. Still other phenomena remain unique for the moment. For example, deep long-period earthquakes marked the intrusion of basalt into a body of crystal-rich dacitic magma (White), and mixing of the magmas triggered the eruption (Pallister and others). Both processes were previously known on geologic grounds; neither had been constrained in time and space. Similarly, interaction between the eruption column and the ground generated tremor with periods of 200 to 300 s that was recorded around the world (Zürn and Widmer), and caldera collapse did not generate earthquakes larger than magnitude 5 (Bautista and others). Geomorphic changes that a geologist might normally expect to occur within millennia have occurred within days and years (Punongbayan, Newhall, and Hoblitt).

The most remarkable fact of all is that, although Pinatubo threatened 1,000,000 people, only a few hundred perished. The eruption was accurately predicted and tens of thousands of people were evacuated to safe distances. The 1991 eruption was the first to occur at Pinatubo in about 500 years, and the lack of baseline information for an eruption of Pinatubo and for other large eruptions made forecasts highly uncertain. Local disbelief that Pinatubo was even a volcano, much less one that could erupt, posed a horrific challenge for scientists and civil defense leaders. Indeed, some of this skepticism was never overcome (Tayag and others).

Long-term accumulation of volatiles within a capped, viscous dacitic magma, and sudden intrusion of that dacite by basalt, may have contributed to a surprisingly orderly and rapid progression from first precursors to climactic eruption. Only slightly more than 2 months elapsed from the first confirmed precursors to the massive eruptions, and the most diagnostic signs occurred within just 10 days of systematic escalation of unrest in early June 1991. A hazard map and a simple 5-level alert scheme, the primary warnings to public officials, proved to be remarkably accurate. In addition, a graphic video summary of volcanic hazards, made by the late Maurice Krafft for the International Association of Volcanology and Chemistry of the Earth's Interior, grabbed even the skeptics' attention (Punongbayan, Newhall, Bautista, and others). Fast action by civil defense and local leaders led to massive evacuations and saved many lives, and, especially on U.S. military bases, much property as well. Not to be overlooked, luck also played a role: the 1991 eruption was not as large as some prehistoric eruptions, and a series of moderate-scale eruptions that preceded the climax convinced skeptics and those clinging to their land and homes that evacuation was the only prudent option.

The muddy aftermath of the eruption, in which heavy rains continue to remobilize large volumes of 1991 deposit as lahars, still batters central Luzon. More than 2x109 m3 of sediment have buried about 400 km2 of lowland alluvial fans. Steaming hot debris flows with peak discharges of more than 1,000 m3/s continue for hours after heavy rains and leave deposits of up to several tens of million cubic meters. Different watersheds yielded sediment at different rates, depending on factors such as slope, area versus thickness of deposit, and, later, vegetation recovery (Janda and others). Forecasts of sediment volume have been used for planning long-range mitigation projects; forecasts of imminent lahars have been used to trigger immediate evacuations. In principle, long- and short-range forecasts have been intended to help people to stay safely in their own homes and villages until they absolutely must move to higher or more distant ground; in practice, scientists, public officials, and citizens alike are still struggling to understand changing hazards and to neither underreact nor overreact. Hope runs eternal among those at risk from lahars, and people still delay their precautions until lahars are upon them (Cola). Technical accuracy in warnings is not enough to save lives; warnings must also convince those who are at risk to actually take the necessary precautions.

Dramatic changes to the face of the volcano are matched by dramatic cultural change. The minority Aeta population of about 20,000 was completely uprooted from their mountain life, wild fruits and animals, and supernatural spirits (C.B. Bautista). An even larger number of lowlanders were displaced, more physically than culturally, by the distal reaches of the eruption and by consequent lahars. Economic development of the region suffered severe short-term setbacks, and the large American military presence in the Philippines was brought to an early end.

In summary, the eruption of Mount Pinatubo in June 1991 and the subsequent lahars were the first of their magnitude to occur in a densely populated area among a people with the will and technological means to actually mitigate risks. With the benefit of good forecasts and generally constructive public responses, thousands of lives and untold amounts of property were saved. About 250 died during the eruption and a hundred more have died in subsequent lahars; without the forecasts and constructive public response, the toll would have been much greater.


Ash-covered newsstand at Clark Air Base; headline tells the story.
Photograph by Val Gempis, USAF.


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Last updated 09.16.98