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Laharic debris-flows from Villarrica Volcano
José A. Naranjo
Servicio Nacional de Geologla y Minerla. Av. Santa María 0104. Santiago, Chile
[email protected]
HugoMoreno
Servicio Nacional de Geologia y Minerla, P.O. Box 23-D, Temuco. hmoreno@ sernageomin.cl
ABSTRACT
The Villarrica volcano (2.847 m a.s.l.) is one of the most active Andean volcanoes being a perfect cone of basalts to basaltic-andesites. The 200 m across open crater has an active fumarolic lava-lake. An extensive glacier fills an older caldera and partially covers the Holocene cone. Snow covers most of the volcano from May to October, up to 20 m thick around the summit. Lava eruptions have taken place, sporadically, through the main crater at low effusion rates (-20 m/s as in 1984}, or frequently through upper flan k fissures at higher effusion rates (-500 m/s as in 1971). One of the most hazardous places around the volcano is Pucón town, 1O km north, built on top of historie (1908) and recent laharic deposits. Within the XX century, six big lahars have occurred. Also. debris remobilisation by heavy rains is common and their effects are significant. Fissure orientation in the crater rim define the most probable lahar flow directions, with high effusion rates as a triggering condition. Finally. a source of coarse debris anda suitable channel slope are the driving factors for the resulting damage of tl1e neighbouring areas.
INTRODUCTION
Modern and ancíent lahar flows have ínvaded and ínundated the m a in river systems that drain the Víllarrica volcano. Above the 1 ,900 m, both the summitflanks and the 6.5 x4.2 km caldera are capped by perennial ice. Total glaciers cover an area of -40 km 2 with a volume of -8 km 3 of ice, equivalent to -6,5 km 3 of water (Moreno, 1993, 2000). As a consequence, almost all effusive eruptíons have produced lahars, whích have caused most of the dísasters around the volcano. Among the main documented eruptions, only those of 1908 (October-Oecember, with lavas and laharic flows to the north), 1948-1949 (October to February, the largest historícal event that emitted lavas and laharíc flows in dífferent directíons), 1963 (March-May), 1964 and 1971, added up more than 100 fatalíties, together wíth an unknown number of missing peop le and severe damages to the infrastructure. Thus, laharic flows have been the most risky and recurrent hazard of the Villarrica volcano. The m a in goal of this article is to summaríse what is known about the characteristics of the lahar deposíts in the Villarrica volcano on the basis of field observations, especially in the deposíts and riverbeds to the north of the vol cano, a long the Turbio, Pedregoso and Zanjón Seco ríver systems. Al so the authors pursued the descriptíon of lahar occurrences and the determination oftheir nature, possible origin and behaviour.
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MODERN ERUPTION-TRIGGERED DEBRIS FLOWS
Reliable chronicles (e.g., Petit-Breuilh and Lobato, 1994) about eruption-triggered lahars in the Villarrica volcano are mainly available since the XX century (Table 1). They include lahars and floods that occurred not only in the northeast drains of the volcano, but also in other radial river systems as Voipir-Molco, Chaillupén, Coñaripe and Palguín (Fig. 1).
FIG. 1. Location map of Villarrica volcano and the river network over its flanks .
RIVERBEDS ANO LAHAR DEPOSIT CHARACTERISTICS
THE TURBIO-PEDREGOSO RIVER SYSTEM
Laharic flows generated during the 1971 event along the Turbio-Pedregoso rivers obliterated ancient deposits. However, larger lahar events were preserved as terraces placed higherthan those formed in 1971. These deposits crops out 1 km downstream the junction ofTurbio and Pucón rivers (Fig. 1), and were driven through the Turbio riverheads underthe eastern and more voluminous glacier tongue of the Villarrica volcano. That glacier is situated between the northeastern caldera wall and the frontal glacial till that is al so present above the 1,300m contour. Coarse sediment moraine ridges, up to 20m high, are the main source for debris flows (Fig. 2). The glacial till has been partially eroded by young lahars on slopes that exceed 0.27 m/m. Debris deposits were left as lahar tails between 1 and 3 m thick over the Holocene lavas that fill the glacial val ley at Palguín, Turbio and Pedregoso headwaters, where the stream has gradients of 0.18-0.24 m/m . These tail facies exhibit an increase in the matrix/boulder
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TABLE 1.ERUPTION-TRIGGERED LAHARS AT VILLARRICAVOLCANO DURING THE XX CENTURY.
Date 1904
Riverbed Turbio-Pedregoso Zanjón Seco-Carmelito Molco Chaillupén
1908
Turbio-Pedregoso Zanjón Seco (to Pucón)
Mean
Mai n features
Velocity (km/h)
Severa! debris flows. A dam was formed upstream the junction of Turbio and P ucón rivers. which collapsed alter lew hours. Lütgens (1909) indicated that along the Turbio river, the debris flow completely razed the native lorest covering an approximated area of 15 x 3.8 km. describing a 3m higt1 wave 111at left in the mentioned junction a 1 m th ick deposit. Ice blocks were shed into the Villarrica lake.
?
Tidal marks up to 1O m high were identified in trees along the Pucón river.
?
No records.
?
Large trunks and boulders up to 20m:" were carried. Witnesses told that the water level of the Villarrica lake rose 1 m, making clear that the total discharge was really high.
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The Oiuco drain was able to contain the debris flow for 12 to 15 km, but 2 km befare the mouth it split up and one third destroyed the western half of Coñaripe (22 fatalities). A maximum flow section of 3m high and 250 m wide witll a discha rge of 4,000 to 8,000 m '/s was documentad.
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Correntoso Molco-Huichatio Chaillupén 1920
Turbio-Pedregoso
Early
Molco-Huichatio
December
Voipir
1948-1949
Turbio-Pedregoso
(October-
Zanjón Seco-Carmelito
February)
Correntoso
PaiQuín?
Molco-Huichatio Chaillupén Seco-Ca~melito
1964
Zanjón
March 2.,,
Correntoso Molco-Huichatio Diuco
1971
Turbio-Pedregoso
December
Chaillupén
29'"
Zanjón Seco-Cannelito Correntoso Voipir
3
Large trunks arel boulders up to 30-40 m w ere carried. Estimated discharges of 10,000 to 20.000 mis. The total volume of sediments and
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water mobilised along the Pu eón river in different pulses during eight hours. exceeded 40 x 1Cfm". A dam similar to that of the 1908 event was formed. Particularly voluminous '.'/ere the flows in Turbio-Pedregoso and Chaillupén rivers with waves up to 10m high that devastated all bridges. The fan d ella ol the Pucón river in the Villarrica lake was completely transformad. The Carmelita creek channeled reduced volumes of debris flows comparad to those of Turbio-Pedregoso Chaillupén and V oipir rivers. dueto the efficient sediment deposition occurred upstream at El Playón (Zanjón Seco Valley). The 1971 eruption gave key evidences about lahar velocities as the eruption clímax was observed. In fact, the night of December 29 of 1971. a 2 km long N1 O E fissure produced a deep cleft across t11e Villarrica crater that generated a 400 m high glowing lava fountain. The 50-60 rn thick glacier was partially rnelted after 50 minutes of activity. triggering a water outburst at 0040 h of !he Oecernber 30. Laharic flov.s reached mean velocities ol 36 kmih (from 30 to 40 krn/h). and were channel1zed by the Turbio, Zanjón Sew, Correntoso, Voipir and Chaillupén rivers.
ratio and occupies the entire 1 km wide val ley (Fig. 3). However, where narrow 30m deep cannyons were present, lahars were more erosive than common sediment-rich debris-flows and no-deposition occurred, moreover, substrate lavas and previous deposits were eroded. In the western branch of the Pedregoso river (drain width less than 120m), below the 450 m contour, exists evidence of wave marks (3 to 6 m abo ve the riverbed) with peak-flow levels indicating flow depths 3 to 4 times greater than the deposit thickness, as boulders of lava (up to 2m diameter) in addition to angular clasts (up to 5 m diameter) derived from piecemeal eros ion of the steep bedrock walls may have been transported by different lahar pulses during different eruptive events.
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FIG. 2. Coarse sediment moraine ridges are the main source for the lahar flow at the heads of Pedregoso and Turbio rivers .
Gradient decreases to 0.018 m/m 1 km upstream of the junction of Turbio and Pedregoso rivers causing an increase in the sediment accumulation. Large clasts migrated towards the debris flow margins as the velocity was greater in the central channel, forming 4 m high laterallevées (Fig. 4 ). As a consequence, the accretion of large clasts and reverse-normal gradation is observed in the levées. The central channel depth was estimated in 8 m, and considering 130m width for the entire flow, its total section could have been estimated between 800 and 1,000 m2 . Blocks of less than 1 m diameter formed more than 20% in volume of the levées , while the deposits left in the channel typically form thin lags with a concentration of the coarser and denser particles in detriment of the matrix. In contrast, at the same altitude the wider Turbio river branch has facies notably richer in fine sandy matrix . Thus, the larger and denser FIG . 3. Oblique aerial view of the lahar facies salid charge of the lahars moved through the Pucón along the highest gradient segments of river was mainly transported through the Pedregoso the Pedregoso river . rather than the Turbio river (Fig. 1). The lower segment of the Turbio river corresponds toan alluvial fan and has a gradient of 0.026 m/m. lt is characterised by laharflow deposits of 300m wide and 2m thick. However, the 19711ahar spilled over the river terrace to the west and runned straightforward to the Pucón's aerodrome (Figs. 1, 5). The Prehistoric facies that dominates here corresponds to hyperconcentrated sediments with scarce floating blocks of 1 to 2.5 m in diameter with very flat tops. A summary of the main characteristics of the riverbeds and lahar debris flows occurred in the Villlarrica volcano during the 1964 and 1971 eruptions is presented in table 2, as it is inferred from witnesses and direct observations at the field . lt is evidenced that most of the modern Villarrica lahars ha ve been lowerthan those of historie eruptions such as Mt. St. Helens in 1980 (Washington , USA) and Nevados del Ruiz, in 1985 (Colombia, Voight, 1990). Data correspond to sites nearwhere substantial deposition of debris flows first occurred (after Pierson, 1995).
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An ephemeral dam was formed in the junction of Turbio and Pucón rivers (Fig. 1), causing a momentary discharge decrease. The dam invaded over 800 m upstream rising 1Om the water level for 3 to 4 hours. During the da m breakage in the Pucón river, the undercutting of steep slopes from the formerfluvial terraces (the northern terrace retreated for 1Om) probably was the most important and efficient way in which the lahar eroded and incorporated sediments in this segment. Thus, the entrance of these materials together with the huge amounts of wood incorporated as the flow inundated large areasofforestedterrain, produced an increase ofthefinallaharvolumetowardsthe urban area of Pucón. This rise in the volume occured in spite of the gradient drop (from -0.037 m/ m to 0.026 m/m) along the lower segment of the Turbio river (>235m a.s.l.), and the enlargement. almost three times, the width of the riverbeds.
T ABLE 2. SUMMARY OFTHE DEBRIS-FLOWS ANO CHANNELSCHARACTERISTICS OFTHE VI LLAR RICA VOLCANO, COMPARED TO SAINT HELENS (1980) ANO NEVADOS DEL RUIZ {1985) VOLCANOES (PIERSON, 1995). Distan ce Debris flows
ot site from volcano (km)
Peak
Channel
Peak mean
gradient
velocity (m/s)
(m/m)
hydraulic
Discharge
depth (m) (*)
(m~/s)
Site or source and reference
Víllarrica Coñaripe 1964
17.5
0.083
>10
-5
2,000-2.400
Turbio bridge
16
0.053
>10
-4
5,000-10,000
1971 Correntoso
18
0.046
>10
-6.7
-10.000
12.5
0.100
>10
-2.5
200
VillarricaPucón road
1971 Chaillupén
Pucón-Curarrehue road
1971 Chaillupén
Coíiaripe
Pino Guacho
17.5
0.050
10-14
-5
10,000-14,000
Calafquén lake
9
0.088
-14
-6
>7,000
Altitude 700 m
12
0.088
>14
-5.8
7.000-10,000
Altitud e 440 m
18
0.053
5-10
-5-6
1971 Pedregoso 700 m 1971 Pedregoso 440 m 1971 Turbio
-12,000-20,000 Turbio and
1971
Pucón rivers
Pucón river
Altitude 235 m
Pucón river
25
0.003
«10 (?)
?
?
14
0.020
14.4
4.7
20,000
20
0.019
11.0
9.0
10,000
Defta
Delta St. Helens Muddy Pine
M1 (Píerson, 1985)
river
P10 (Pierson, 1985)
creek Nevado del Ruiz Molinos river
47
0.018
6.4
7.4
8.000
MN7
72
0.011
-12
-5
28,000
Armero
74
0.020
6.2
5.8
15.000
(Pierson el al., 1990)
-Nereidas
(Pierson etal., 1990)
Azufrado river Guali river
G4 (Pierson et al., 1990)
(')Peak hydraulic depth: section area/upper maxímum width.
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FIG . 4. 'Levées' 1 ,500 m asl) , its maximum width reaches 2 km and no deposition is observed due to the high gradient of the heads of both Calabozos creek (eastern branch, 0.38 m/m) and Carmelita cree k (western double branch , 0.46 m! m). A long the transitional segment, between 1 ,500 and 1,000 m asl, both gradientsdecreaseto0.13 m/m and0.17-0.21 m/m, respectively. Thiscauses that from 1,000 m downwards two branches derived from Calabozos cree k to Carmelita cree k, turning the latter into the most active riverbed. Both gradients decrease (Calabozos=0.038 m/ m; Carmelita= 0 .035) below 1,000 m a.s.l. The m a in glacial valley ends at 600 m asl and splits into different riverbeds following ancient lava lobes (Fig . 6). Figure 6 also shows both the distribution of remnants of debris flow deposits that occurred during the XX century and lahar deposits accumulated during the 1971 event. Thin (< 1 m) easy removable lahar deposits are found between 1,100 and 1,500 m high,
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characterised by randomly distributed (< 2 m diameter) blocks in a matrix of sand and gravels. Thicker (
