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A prehistoric lahar-dammed lake and eruption of Mount Pinatubo
described in aPhilippine aborigine legend
Kelvin S. Rodolfo a,b,, Jesse V. Umbal a,1
a Department of Earth and Environmental Sciences, University of
Illinois at Chicago, 845 W. Taylor St., Chicago Il 60607 USAb
National Institute of Geological Sciences, University of the
PhilippinesDiliman, Quezon City, Philippines
A B S T R A C TA R T I C L E I N F O
Article history:Accepted 24 January 2008Available online 27 May
2008
Keywords:aboriginal volcano legendAytacaldera-forming
eruptionlahar-dammed lakePinatubo volcano
The prehistoric eruptions of Mount Pinatubo have followed a
cycle: centuries of repose terminated by acaldera-forming eruption
with large pyroclastic ows; a post-eruption aftermath of
rain-triggered lahars insurrounding drainages and dome-building
that lls the caldera; and then another long quiescent period.During
and after the eruptions lahars descending along volcano channels
may block tributaries fromwatersheds beyond Pinatubo, generating
natural lakes. Since the 1991 eruption, the Mapanuepe River
valleyin the southwestern sector of the volcano has been the site
of a large lahar-dammed lake. Geologic evidenceindicates that
similar lakes have occupied this site at least twice before. An
Ayta legend collected decadesbefore Mount Pinatubo was recognized
as a volcano describes what is probably the younger of these
lakes,and the caldera-forming eruption that destroyed it.
2008 Elsevier B.V. All rights reserved.
1. Introduction
MountPinatubovolcanoandMountsNatib andMariveles to the
southbelong to a chain of stratovolcanoes along western Luzon
Island in thePhilippines, resulting from eastward subduction of
South China Basinlithosphere along the Manila Trench (Fig. 1).
Isolated areas in centralLuzon are inhabited by Aytadark-skinned,
curly-haired aborigines ofshort stature also called Ita, Agta, and,
most commonly Aeta. At Pinatubo,these people prefer to be called
Ayta, as the word sounds and as itwould be correctly spelled in
Pilipino. The etymology of Pinatubo ispertinent to this report;
-tubo is the root in both the Pilipino (Tagalog) andAyta languages
that pertains to anything concerned with growth. Here,the prex
pina- creates made to grow. Pinatubo may as easily meanallowed to
grow, in the same sense that not felling or pruning a treeallows it
to grow. Volcanologists speculate that the namemay imply
thatAytaswitnessed the post-eruption growth of a newpeak
(Rodolfo,1995).Interestingly, the Ayta commonly refer to themselves
as the katutubo.Shimizu (1992, p. 7) translated the term tomean
theones fromthe land,but it transliterates to those who grew (or
sprouted) with.
Pinatubo's cataclysmic 1991 eruption, and what is known of
itsgeologic history, are described in encyclopedic detail in the 1
126-pagecompendium Fire and Mud (Newhall and Punongbayan, 1996).
Beforethe 1991 eruption the Pinatubo peak was the highest point in
the
Zambales Range. Standing 1745 m above sea level, it served as
thebenchmark for the point where Pampanga, Zambales and
Tarlacprovinces meet. Pinatubo was recognized as a volcano by only
a fewgeologists before 1991, being nestled inconspicuously in
surroundingophiolitic mountains that stood only 200 m or so lower.
Localinhabitants had little experience or folk memory of an
eruption beyondthe Ayta legend discussed in this report, due to its
centuries of repose.
During the paroxysmal eruption of 15 June 1991, the
volcanocollapsed to form a roughly circular, 2.5 kmwide caldera
with its oorat an altitude of 820 m (Fig. 2). Pinatubo's new
highest point on thesoutheastern 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
thevolcano anks. During the ensuing 5 years, torrential monsoonal
rainsenhanced by typhoons mobilized great volumes of the debris
intodestructive lahars that owed 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 laharicaftermath are estimated at a billion US dollars (Mercado
et al., 1996).Aytas were the worst affected among as many as two
million peoplearound the volcano. The frequency and sizes of lahars
have greatlydiminished, but voluminous lahar deposits along the
volcano front maystill be remobilized by exceptional future storms
(Umbal, 1997).
During and after the eruption, lahars descending along
volcanodrainages dammed the conuences of tributary streams that
drainareas outside the Pinatubowatershed, impounding ponds and
lakes ofvariable permanence. When the impounded water overtopped
thedebris dams, the released oods would generate lake-breakout
Journal of Volcanology and Geothermal Research 176 (2008)
432437
Corresponding author. E8022 BakkomRd, ViroquaWI 54665, USA.
Tel.: +1608 6376159.E-mail address: [email protected] (K.S.
Rodolfo).
1 Present address: Avocet Bolaang Mongondow, Jl.Kolonel Sugiono
No. 24, Kotaban-gon, Kecamatan Kotamobagu, Sulawesi Utara,
Indonesia.
0377-0273/$ see front matter 2008 Elsevier B.V. All rights
reserved.doi:10.1016/j.jvolgeores.2008.01.030
Contents lists available at ScienceDirect
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lahars, including extremely devastating events in 1991,1992, and
1994(Newhall et al., 1996). Unlike rain-triggered lahars, those
generated bylake breakouts were difcult to predict, happened after
typhoonswhen people had dropped their guard, and often occurred at
night,thus were especially dangerous. Mapanuepe Lake in the
southwesternsector of the volcano (Fig. 3; Umbal, 1994; Umbal and
Rodolfo, 1996) isthe only such lahar-dammed lake large enough to
survive until thiswriting.
Pinatubo eruptivity, its timing and associated events are
poorlyunderstood, and clues regarding them are urgently needed.
Toward thatend, folklore has been scrutinized by Gaillard et al.
(2006a,b). Closerexamination of a legend transcribed in 1915
(Rodriguez, 1918), the focus
of this report, provides clear evidence that a lahar-dammed lake
wasformed during the penultimate prehistoric eruption and survived
until itwas destroyedby the last eruption before the1991 event. To
examine thelegendproperly requires familiaritywithPinatubo's
eruptive chronology,and with two of its geomorphic features: an
ancient caldera and, in thesouthwestern sector of the volcano,
Mapanuepe Lake and its ancestors.
2. Eruption chronology of Mount Pinatubo
Newhall et al. (1996) have cited evidence that Pinatubo's
pasteruptions have followed a cycle of centuries of repose
terminated by apowerful eruption with a caldera collapse, large
pyroclastic ows andlahars, followed by an aftermath period of
post-eruption lahars.Finally, a period of dome-building associated
with eruptions ofuncertain intensity and duration lls the caldera
before the volcanoenters another long quiescent period.
Fig. 1. Locations of the Philippines and Mount Pinatubo (shaded
gray).
Fig. 2. The new Pinatubo caldera, and arcuate scarps A, B and C
interpreted as remnantsof the Tayawan Caldera. Modied from Lagmay
et al., 2007.
Fig. 3. Mapanuepe Lake in relation to the new caldera. Base:
composite of Landsatphotographs 116_050_2611_2001 and
117_049_01_2001.
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Most of the data onwhich thismodel of cyclic eruption is
basedweregathered shortly after precursory seismic activity began
at Pinatubo on15 March 1991 and the rst small explosions occurred
north of thesummit on 2 April (Wolfe and Hoblitt, 1996). To develop
a hazard map,geologists of the Philippine Institute of Volcanology
and Seismologyimmediately began a heroic attempt to study the style
andmagnitude ofpast eruptions. Theywere joined in late April by
geologists of the UnitedStates Geological Survey. Given the short
time before the eruption, aremarkable amount of evidence was
gathered, but the data werenecessarily incomplete. Thus, drawing on
radiocarbon dates of 51samples, Newhall et al. (1996) hypothesized
that the modern series ofsix prehistoric eruptions began around 35
ka BP, and that eruptionsmaybe getting weaker over time after
successively shorter periods of repose(Fig. 4). These authors
acknowledged, however, that those trendsmaybeartifacts of the
erosion or burial of older deposits. Dates for the earliestthree
eruptions are each based on only one or two samples, and a gap
inthe coverage was recently demonstrated by a single sample from
anancient caldera-lake deposit with an uncalibrated C14 age of
14,70040yBP (Lagmayet al., 2004; Rodolfo, 2005; Lagmayet al.,
2007;Gaillardet al., 2006b). The last two of the prehistoric
eruptions, the Maraunotand Buag episodes that respectively occurred
ca. 3.93.3 and 0.80.5 kaBP are of immediate interest here.
2.1. The prehistoric Tayawan Caldera
While Pinatubo was still regarded as extinct, Deln and his
co-workers (1984, 1996) conducted the rst systematic studies of
itsgeology to assess its geothermal resources. They mapped three
arcuatescarps, labeledA, B andC in Fig. 2. Newhall et al. (1996)
interpreted thosefeatures as remnants of an old, 3.54.5 km collapse
depression, whichthey named the Tayawan Caldera and interpreted as
having beenproduced by the earliest, and presumably strongest
episode, the Inararoepisode ca. 35 ka BP (Fig. 4). A problem is
posed by the date of 14,70040 y BP recently obtained from
lacustrine deposits nowwidely exposedin the walls of the new
caldera. Those sediments may have beendeposited in yet another
caldera, formed after the Inararo eruption.Either that event was a
previously unrecognized eruption, or ithappened late during the
Sacobia eruptive period, which would havehad to last more than 2000
years longer than indicated in Fig. 4.Alternatively, if those
sediments really were deposited in the TayawanCaldera, it is much
younger than previously thought.
2.2. Mapanuepe Lake and its prehistoric predecessors
Before the 1991 eruption, the Mapanuepe River drained 88 km2
ofmountainous watershed south of Mount Pinatubo. It joined
theMarella River descending the southwest Pinatubo ank, to form
theSanto Tomas River, which follows the northern margin of its
broad,235-km2 alluvial plain en route to the South China Sea.
During the1991 eruption, large lahars descending along the Marella
River
blocked the Mapanuepe River, transforming it into the lake
thatnow bears the same name (Fig. 3). The evolution of Mapanuepe
Lakewas observed and documented from its inception (Umbal,
1994;Umbal and Rodolfo,1996). At its maximum extent the lake had an
areaof 6.7 km2 and a volume of about 7.5107 m3.
Sedimentary ll (Fig. 5) and geomorphology (Fig. 6) indicate
thatsimilar lahar-dammed lakes occupied the site during at least
twoprevious eruptions. Several terraces rim theMapanuepe catchment
areaand the adjacent valley of the Santo Tomas River. One terrace,
15 mhigher than the pre-eruption oor of the Santo Tomas River at
Sitio(hamlet) Dalanaoan is composed of pyroclastic-ow deposits
overlainby lahar deposits that yielded charcoal samples dated at
299070 and293080 14C years, which place them in theMaraunot
eruptive episodeof Newhall et al. (1996). Similar, undated
pyroclastic-owdeposits occurat a site near the now-buried Sitio
Aglao. There, another terrace 5 mlower than the Dalanaoan section
was composed of old lahar depositscontainingyoungercharcoal,
about7606014C years old, correspondingto the earlier part of the
Buag episode.
3. Anthropological evidence
3.1. The Aytas
When the Spaniards conquered the Pampanga region north ofManila
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 havebeen
alive. This suggests that the main Buag eruption occurred
earlierrather than later (Gaillard et al., 2006b), or that the
later phases of itsactivity occurred away from the southern and
eastern sectors.
At the time of the 1991 eruption, as many as 10,000 Aytas
inhabitedPinatubo and its mountainous environs (Seitz, 2004). These
aboriginessubsist by hunting, shing, gathering and swidden farming
(Shimizu,1989).
The 16th century Spanish conquistadores collectively referred to
theAytas and similar Philippine tribes as Negritosowing to their
dark skin,curly hair, and short stature. Modern scholars include as
Negritos notonly the twelve ethnically related mountain tribes
isolated onmountainousareasof fourPhilippine islands, but
theAndaman islandersin the eastern Bay of Bengal and tribes
inwesternMalaysia and Thailandaswell. In the early 20th century the
Negritoswere commonly assumedto be related to the pygmies of Africa
(Waterman,1924). In the Forewordtohis 1963book, however,
Garvan,whoclaimed tobe probably theonlyanthropologist who knows
both the Asiatic Negritos and the AfricanPygmies by personal
experience, said, Their comparison showed thatthey formby nomeans a
racial or cultural unit.Omoto (1981) arrived atthe same conclusion
from genetic information from blood analyses. TheNegritos are now
generally regarded as the only survivors of
aboriginal,pre-mongoloid Asians (Headland and Reid, 1989).
Fig. 4. Chronologies of Pinatubo eruptions (from Newhall et al.,
1996; the new date from Lagmay et al., 2004, 2007; Rodolfo,
2005),the hypothesized arrival in the Philippines of theAytas and
their presence at Pinatubo indicated by the legend.
434 K.S. Rodolfo, J.V. Umbal / Journal of Volcanology and
Geothermal Research 176 (2008) 432437
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The Aytas and other Philippine Negritos do not build boats
(Kreiger,1942), and it is generally believed that they are
descendants of peoplewho crossed land bridges from the southeast
Asian mainland to thePhilippines during the last Pleistocene drop
in sea level, which attainedits nadir of about 130mbelowpresent sea
level some18 kaBP (Pirazzoli,1998). The scientic consensus is that
the inux of Austronesian-speaking people from mainland Southeast
Asia occurred about vemillennia ago, which would have been sometime
between Pinatubo'sCrow Valley and Maraunot eruptive episodes (Fig.
4). It has long beenclaimed that the technologically more advanced
newcomers killed theNegritos or drove them into the hills (Brinton,
1898). That, however, iscontradicted by the fact that the Aytas of
western Pinatubo have losttheir original language and adopted that
of the lowland Zambalnewcomers (Peralta, 1990), from whom they also
borrowed manyother cultural traits (Headland and Reid, 1989). If
the Aytas wereeventually forced to retreat to Pinatubo, it would
have been at someconsiderable timeafter the Zambal arrival. TheAyta
legend suggests thatthey were already at Pinatubo no later than
during the repose periodbetween the Maraunot and Buag
eruptions.
3.2. The legend
Gaillard et al. (2006a,b) discussed several legends of the
lowlandKapampangan people that may suggest knowledge of
Pinatubovolcanism, but these are very equivocal. Hardly less vague
is theAyta tradition, before the 1991 eruption, of warning their
children thatApo Pinatubo Namalyari [the god of the mountain they
worship] willwake up and start throwing stones if you don't behave
(Rodolfo, 1995p. 88). But an Ayta legend contains much detail that
strongly indicatesfolk memory of an eruption. Collected in 1915
(Rodriguez, 1918), thelegend survives only as a somewhat garbled
typewritten transcriptionon microlm in the Filipiniana collection
of the main library of theUniversity of the Philippines in Diliman,
Quezon City. It carries specialweight because it was collected long
before geologists recognized thatPinatubo was a volcano.
Like most legends, it has several confusing elements. Its two
principalprotagonists are a mighty, supernatural hero named Aglao,
king of theSpirit Hunters, and a powerful, villainous spirit named
Bacobaco. Thename Aglao is signicant only because it was the name
of a hamlet near
Fig. 5. Stratigraphic sections near the Mapanuepe basin. Modied
from Umbal (1994).
435K.S. Rodolfo, J.V. Umbal / Journal of Volcanology and
Geothermal Research 176 (2008) 432437
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theMarellaMapanuepeconuence thatwasburiedby the
lake-damminglahars in 1991 following the paroxysmal eruption (Fig.
3); otherwise, it isthe name of an herb that Aytas use as a
medicine for gastrointestinal ills(Fox, 1952). Bacobaco is
the terrible spirit of the sea who makes the storms and
thewaves. Bacobaco was also extremely fond of deer's meat
andsometimes he would transform himself into an enormous turtle;and
suddenly appearing on the shore of the sea, he would sallyforth
into the hunting grounds of the Spirit Hunters and gorgehimself
with deer Aglao would greatly resent this, but howcould he face
Bacobaco who carried his thick shield on his back,and who threw re
from his mouth?
However, one day, he consultedWasi, the spirit of thewind.
AndWasiwhispered into his ear, Why don't you ask Blit, my brother,
to helpyou? He is the only one capable of killing Bacobaco, for if
he hits [himwith] the tipofhis tail [?] ora toeof oneofhis feet,
itwill kill Bacobaco.
It is unclear who Blit isan earthquake god, perhaps, judging
fromhis strengthbut he joins Aglao and the other Spirit hunters,
andattacks Bacobacowith arrows. Bacobaco manages to avoid being
hit,but ees towardthe lake at the foot of Mount Pinatubu [sic].
Again, Bacobaco escaped injury byhiding himself under his
shield. Heimmediately jumped into the lake but the water was so
clear, that Blitcould see him at the bottom. Finding the lake a
useless place of refuge,he climbed the Mount Pinatubu in exactly
twenty-one tremendousleaps.When he had reached the top, he at once
began to dig a big holeinto the mountain. Bit [sic] pieces of rock,
mud, dust, and other thingsbegan to fall in the showers around
themountain. During all thewhile,he howled and howled so loudly
that the earth shook under the foot[sic] of Blit [and] Aglao There
that escaped fromhismouth becameso thick and so hot that the
pursuing party had to turn away.
For three days the turtle continued to burrow itself, throwing
rocks,mud, ashes, and thunderingawayall the time in [a] deafening
roar. Atthe end of the three days he stopped, and all was quiet
again in themountain. But the lake, with its clear water was now
lled withrocks, and mud covered everything. On the summit of the
Pinatubuwas the great hole, through which Bacobaco had passed, and
from
which smoke could be seen constantly coming out. This showed
thatalthough he was already quiet he was still full of anger, since
recontinued to come from his mouth
The legend's elements of clear volcanological interest are a
pre-eruption lake of clear water at the base of Pinatubo, and a
thunderouseruption with ery pyroclastic ows lasting three days that
threw outgreat quantities of debris that lled the lake below and
left a summitcaldera.
4. Discussion and conclusion
The chronology depicted in Fig. 4 and the salient elements of
thelegend suggest that Aytas were already occupying Pinatubo after
theMaraunot eruption had formed a proto-Mapanuepe Lake,
andwitnessed its destruction by tephra-falls, pyroclastic ows and
laharsduring the calderagenic Buag eruption. Pyroclastic ows are
alsoindicated by the basal pyroclastic-ow deposits of
Maraunot-episodeage at Dalanaoan, which likely correlate with
similar basal deposits inthe upper Aglao stratigraphic section
(Fig. 5).
Perhaps, as Newhall et al. (1996) have suggested, the Buag
eruptionwas not as strong as its predecessor and produced less
debris tomobilize into lahars that blocked the drainage of the
Mapanuepevalley. The resulting smaller proto-Mapanuepe Lake would
have beenlled in more easily and over less time by deposits that
weresubsequently eroded, leaving the lower terrace (Fig. 6).
Similarly, the1991 eruption and its aftermath lled the Mapanuepe
valley even lessthan did the Buag event, lending weight to the
conjecture thatPinatubo eruptions have become successively weaker,
at least for thelast three events.
Legends commonly distort the time dimension, but a
calderacollapse lasting three days equates reasonably well with the
1991counterpart event. The legend tells us nothing about whether
theAytas experienced any of the pre-Buag eruptions. If these indeed
werelarger than the Buag event, it would not be surprising if no
witnessesever returned. Quiescent intervals lasting as long or
longer than theone that preceded the 1991 eruption would have
diminished thepossibility that folk memories of them would have
survived.
No legend relates how dome-building eruptions lled the
Buagcaldera and erected the pre-1991 summit peak. This is
unfortunate,because large, lahar-generating oods expelled by
dome-buildingeruptions under the present caldera lakemay pose
Pinatubo's greatestfuture hazard (Lagmay et al., 2007). As
described elsewhere (Rodolfo,1995), the Aytas are the most
environmentally aware people we haveever had the good fortune to
work among and learn from; perhaps
Fig. 6. Cartoon recreating the history of successive lacustrine
inllings and incisions of the Mapanuepe area since the Maraunot
eruptive episode. Modied from Umbal (1994).
436 K.S. Rodolfo, J.V. Umbal / Journal of Volcanology and
Geothermal Research 176 (2008) 432437
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loitering around Pinatubo to watch its dome grow would simply
havebeen too foolhardy.
The Ayta narrator does, however, conclude the legend by
fore-telling the 1991 eruption: But now, you do not see smoke
coming outof the Pinatubo mountain and many believe that the
terriblemonster is already dead; but I think that he is just
resting after hisexertions, and that someday hewill surely come out
of his hiding placeagain
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