A prehistoric lahar-dammed lake and eruption of Mount Pinatubo described in a Philippine aborigine legend

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Abstract

The prehistoric eruptions of Mount Pinatubo have followed a cycle: centuries of repose terminated by a caldera-forming eruption with large pyroclastic flows; a post-eruption aftermath of rain-triggered lahars in surrounding drainages and dome-building that fills the caldera; and then another long quiescent period. During and after the eruptions lahars descending along volcano channels may block tributaries from watersheds beyond Pinatubo, generating natural lakes. Since the 1991 eruption, the Mapanuepe River valley in the southwestern sector of the volcano has been the site of a large lahar-dammed lake. Geologic evidence indicates that similar lakes have occupied this site at least twice before. An Ayta legend collected decades before Mount Pinatubo was recognized as a volcano describes what is probably the younger of these lakes, and the caldera-forming eruption that destroyed it.

Introduction

Mount Pinatubo volcano and Mounts Natib and Mariveles to the south belong to a chain of stratovolcanoes along western Luzon Island in the Philippines, resulting from eastward subduction of South China Basin lithosphere along the Manila Trench (Fig. 1). Isolated areas in central Luzon are inhabited by Ayta—dark-skinned, curly-haired aborigines of short stature also called Ita, Agta, and, most commonly Aeta. At Pinatubo, these people prefer to be called “Ayta”, as the word sounds and as it would be correctly spelled in Pilipino. The etymology of Pinatubo is pertinent to this report; -tubo is the root in both the Pilipino (Tagalog) and Ayta languages that pertains to anything concerned with growth. Here, the prefix pina- creates “made to grow”. Pinatubo may as easily mean “allowed to grow”, in the same sense that not felling or pruning a tree allows it to grow. Volcanologists speculate that the name may imply that Aytas witnessed the post-eruption growth of a new peak (Rodolfo, 1995). Interestingly, the Ayta commonly refer to themselves as the katutubo. Shimizu (1992, p. 7) translated the term to mean “the ones from the land”, but it transliterates to “those who grew (or sprouted) with”.

Pinatubo's cataclysmic 1991 eruption, and what is known of its geologic history, are described in encyclopedic detail in the 1 126-page compendium Fire and Mud (Newhall and Punongbayan, 1996). Before the 1991 eruption the Pinatubo peak was the highest point in the Zambales Range. Standing 1745 m above sea level, it served as the benchmark for the point where Pampanga, Zambales and Tarlac provinces meet. Pinatubo was recognized as a volcano by only a few geologists before 1991, being nestled inconspicuously in surrounding ophiolitic mountains that stood only 200 m or so lower. Local inhabitants had little experience or folk memory of an eruption beyond the Ayta legend discussed in this report, due to its centuries of repose.

During the paroxysmal eruption of 15 June 1991, the volcano collapsed to form a roughly circular, 2.5 km wide caldera with its floor at an altitude of 820 m (Fig. 2). Pinatubo's new highest point on the southeastern jagged caldera rim stands only 1485 m above the sea, 260 m lower than the old peak.

The eruption left more than 5 km3 of pumiceous debris on the volcano flanks. During the ensuing 5 years, torrential monsoonal rains enhanced by typhoons mobilized great volumes of the debris into destructive lahars that flowed down all major drainages of the volcano, damaging or obliterating many villages and towns (Bautista, 1996, Spence et al., 1996). Economic losses from the eruption and its laharic aftermath are estimated at a billion US dollars (Mercado et al., 1996). Aytas were the worst affected among as many as two million people around the volcano. The frequency and sizes of lahars have greatly diminished, but voluminous lahar deposits along the volcano front may still be remobilized by exceptional future storms (Umbal, 1997).

During and after the eruption, lahars descending along volcano drainages dammed the confluences of tributary streams that drain areas outside the Pinatubo watershed, impounding ponds and lakes of variable permanence. When the impounded water overtopped the debris dams, the released floods would generate “lake-breakout” lahars, including extremely devastating events in 1991, 1992, and 1994 (Newhall et al., 1996). Unlike rain-triggered lahars, those generated by lake breakouts were difficult to predict, happened after typhoons when people had dropped their guard, and often occurred at night, thus were especially dangerous. Mapanuepe Lake in the southwestern sector of the volcano (Fig. 3; Umbal, 1994, Umbal and Rodolfo, 1996) is the only such lahar-dammed lake large enough to survive until this writing.

Pinatubo eruptivity, its timing and associated events are poorly understood, and clues regarding them are urgently needed. Toward that end, folklore has been scrutinized by Gaillard et al., 2006a, Gaillard et al., 2006b. Closer examination of a legend transcribed in 1915 (Rodriguez, 1918), the focus of this report, provides clear evidence that a lahar-dammed lake was formed during the penultimate prehistoric eruption and survived until it was destroyed by the last eruption before the 1991 event. To examine the legend properly requires familiarity with Pinatubo's eruptive chronology, and with two of its geomorphic features: an ancient caldera and, in the southwestern sector of the volcano, Mapanuepe Lake and its ancestors.

Section snippets

Eruption chronology of Mount Pinatubo

Newhall et al. (1996) have cited evidence that Pinatubo's past eruptions have followed a cycle of centuries of repose terminated by a powerful eruption with a caldera collapse, large pyroclastic flows and lahars, followed by an aftermath period of post-eruption lahars. Finally, a period of dome-building associated with eruptions of uncertain intensity and duration fills the caldera before the volcano enters another long quiescent period.

Most of the data on which this model of cyclic eruption is

The Aytas

When the Spaniards conquered the Pampanga region north of Manila Bay and southeast of Mount Pinatubo in 1571, Austronesian-speaking, Muslim, Kapampangan agriculturists were already well-established there, but apparently had no tradition of a large eruption. Had the last one occurred 500 y BP, a few witnesses would still have been alive. This suggests that the main Buag eruption occurred earlier rather than later (Gaillard et al., 2006b), or that the later phases of its activity occurred away

Discussion and conclusion

The chronology depicted in Fig. 4 and the salient elements of the legend suggest that Aytas were already occupying Pinatubo after the Maraunot eruption had formed a proto-Mapanuepe Lake, and witnessed its destruction by tephra-falls, pyroclastic flows and lahars during the calderagenic Buag eruption. Pyroclastic flows are also indicated by the basal pyroclastic-flow deposits of Maraunot-episode age at Dalanaoan, which likely correlate with similar basal deposits in the upper Aglao stratigraphic

References (30)

  • BautistaC.B.

    The Mount Pinatubo disaster and the people of Central Luzon

  • BrintonD.G.

    The peoples of the Philippines

    American Anthropologist

    (1898)
  • DelfinF.

    Geology of the Mt. Pinatubo area

  • DelfinF. et al.

    Geothermal exploration of the pre-1991 Mount Pinatubo hydrothermal system

  • FoxR.B.

    The Pinatubo Negritos: their useful plants and material culture

    Philippine Journal of Science

    (1952)
  • GaillardJ.-C. et al.

    A reconstruction of the ca. 800–500 BP. Buag eruption of Mt. Pinatubo, Philippines

  • GaillardJ.-C. et al.

    The 800–500 y BP eruption of Mt. Pinatubo and its aftermath: hypotheses from the archaeological and geographical records

  • GarvanJ.M.

    The Negrito of the Philippines

  • HeadlandT.N. et al.

    Hunter-gatherers and their neighbors from prehistory to the present

    Current Anthropology

    (1989)
  • KreigerH.W.

    Peoples of the Philippines

    (1942)
  • LagmayA.M.F. et al.

    Geology and hazard implications of the Maraunot Notch in the Pinatubo caldera, Philippines

    Bulletin of Volcanology

    (2007)
  • LagmayA.M.F. et al.

    Geology and structures of the Maraunot caldera notch, Pinatubo, Philippines

  • MercadoR.A. et al.

    Socioeconomic impacts of the Mount Pinatubo eruption

  • NewhallC. et al.

    Eruptive history of Mount Pinatubo

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    Present address: Avocet Bolaang Mongondow, Jl.Kolonel Sugiono No. 24, Kotabangon, Kecamatan Kotamobagu, Sulawesi Utara, Indonesia.

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