villarrica guia.cdr
Transcription
villarrica guia.cdr
4. Glaciology Villarrica 3 is characterised by the formation of the small 450 m-high summit cone within the youngest caldera at 2,400 m a.s.l., constituted by lava flows that have almost covered its rim. The recent cone (Holocene, <3,700 B.P.) reaches an altitude of 2,847 m asl and consists of a sequence of lava flows with interbedded pyroclastic flow, fallout and surge deposits (mainly basaltic to basaltic andesite in composition (50.554.5% SiO2), as well as lahar deposits. The sequence includes historic lava flows and lahars (since 1558 A.D. up to the penultimate eruption in 1984). The last eruption in 2000 did not generate lahars nor lava flows. 1. Introduction Volcán Villarrica (Figure 1) is located in the modern Southern Volcanic Zone (SVZ) of the Chilean Andes at 39°30’S, being one of the most active in Chile in historical times (Petit-Breuilh and Lobato, 1994). It forms a NW-SE volcanic chain together with the Pleistocene-Holocene Quetrupillán and Lanín stratovolcanoes which is oblique to the recent volcanic arc and main “Liquiñe-Ofqui Fault Zone” (LOFZ; Figure 2; Hickey et al., 1989; Cembrano, 1990; Cembrano et al., 1992; Cembrano and Moreno, 1994; López Escobar et al., 1995; Cembrano et al., 2000). The Middle?-Late Pleistocene to Historic compound Villarrica stratovolcano and its products cover an area of more than 700 2 km (Moreno, 1993 and 2000), being characterised by a conical shape with a 200 m diameter open crater and small lava lake showing weak strombolian activity (Witteretal, 2004). Its altitude reaches 2,847 m a.s.l. According to morphostructural and stratigraphic criteria, Volcán Villarrica edifice has been divided into three evolution stages which are described below. During the Late Glacial Maximum (LGM), lobule type glaciers reached 210 m asl at the lower end of Lago Villarrica and Lago Calafquén (Laugenie, 1971), where formed several coalescent moraine arcs still visible (also visible are sandurs, terraces, ablation moraines located down stream from the main arcs and the ice margin contact at the lake shore, among other geomorphological features), which were probably abandoned between 13,840 and 14,200 years B.P. (Clayton et al., 1997; Moreno & Clavero, 2006). Very few Little Ice Age (LIA) deposits have been detected near the present lower end of the glaciers in the surroundings of Volcán Villarrica, however, there are a couple of valleys (Pichillancahue, and Palguin), where arrow style LIA moraines are still visible. At present, the ice cap at Volcán Villarrica is reaching a minimum altitude of 1,750 m asl at Glaciar Pichillancahue, Palguin valley. Parasitic cones More than 30 pyroclastic cones, and their associated lava flows, basaltic andesite to andesite in composition (52 – 56% SiO2), have been erupted through the flanks of Volcán Villarrica (mainly through Villarrica 1 edifice). These parasite centres form two clusters: Los Nevados and Chaillupén volcanic groups, located on the northeastern and southern flanks of Volcán Villarrica, respectively. Glaciers and volcano interactions There is a close interaction between volcanism and glaciers. For Volcán Villarrica, it is recognised a highly explosive event soon after the ice started to retreat at the end of the Last Glaciation (Naranjo and Moreno, 1991; Clavero, 1996) presumably due to an “unloading” effect (Clavero, 1996). The explanation of this event, could be related to the effects of deglaciation, that can produce crustal rebound because of isostatic equilibrium restoration (i.e. Ivins and James, 1999; Ivins and others, 2000; Clague and James, 2001; Crucifix and others, 2001). The unloading effect produces a stress relief on the volcano magma chamber, helping magmas to reach the surface and eventually erupt (i.e. Finn and others, 1995; Maclennan and others, 2002). At the southern shore of Volcán Villarrica, there are several indications of an uplift of at least 60 m (Clavero, 1996) which is likely to be a response to the unloading effect of the disappearance of a large mass of ice in the area. Historical eruptive activity Volcán Villarrica is Chile’s most active eruptive centre, with 59 documented eruptions since 1558, when the first Spanish conquerors arrived to southern Chile. However, from 31 well-documented eruptions, 8 occurred in December and 23 between spring and summer. This suggests Villarrica volcano has a strong seasonal modulation present in the eruptions. Historical eruptive activity has been essentially effusive with few explosive eruptions (i.e. 1948-49 eruption). This effusive activity has produced several lava flows, both pahoehoe and ‘aa’ type, with some associated scoria fallout deposits, which have been directed towards the East and Southeast. The effusion of high temperature (ca.1,100°-1,250°C) lava flows over an ice-covered volcano has generated numerous lahars, which have traveled down the main valleys surrounding the volcano. Figure 1. Volcán Villarrica and debris covered glacier (Photo, Camilo Rada). 2. Volcanic geology 3. Volcanic hazards associated to Volcán Villarrica Villarrica Units Although most of the historic eruptions have been mainly effusive, the permanent glacier that covers the volcano, together with the seasonal snow-cap, generates a very important volcanic hazard to its surroundings. Historical eruptions have produced lava flows, lahars and the ejection of pyroclastic material. Hence, the main hazards expected from future eruptions of this volcano are those that derive directly from lava flows and tephra fallout, together with those induced by them, such as lahars and river flooding. There is an extensive description of these hazards, in the Volcanic Hazards Map of Volcán Villarrica published by Chilean Geological Survey (Moreno, 2000). The older unit (Villarrica 1) consists of a 500 m thick sequence of basaltic to andesitic lava flows, volcanic breccias, ignimbritic tuffs and agglomerates (Moreno and Clavero, 2006), being characterised by the formation of the main stratovolcano 3 km southeast of modern cone (Gaytán et al., 2005). Moreno and Clavero (2006) suggested two subunits, one pre-last glaciation (>90,000 B.P., with Ar-Ar dates from 600 to 90 ka) and another intraglacial (between 90,000 and 14,000 B.P.). The caldera collapse (ca. 100 ka; Clavero and Moreno, 2004) would represent the break in the early evolution of Volcán Villarrica at the beginning of the Last Glaciation (Llanquihue). 3 Villarrica 2 is considered the unit after the large Licán ignimbrite (~ 10 km ), which is responsible for the formation of a nested caldera stratovolcano, mainly with lava flow 14 eruptions, in the northwestern side of the wide caldera. According to C ages, its formation begun probably between 13,500 and 11,000 (Late Pleistocene) and ended with another large eruption ~3,700 (Holocene) that originated the Pucón ignimbrite (~ 3 1 5 km ) which is associated to the smaller summit caldera formation. This stratocone consists mostly of a lava flow sequence interbedded with pyroclastic flows, fallout tephra, laharic and surge deposits, mainly of basaltic to andesitic in composition (502 57% SiO2), covering an area over 2,000 km around the volcano. Figure 2. Geological map of Volcán Villarrica (Moreno and Clavero, 2006). 1 2 For instance, lava flows may virtually affect almost all around the volcano. According to their transport, they would travel down the main valleys and spread over wider areas at lowlands and lake shores. Lahars hazard varies according to the time of year and the thickness of the seasonal snow cover. In the December 1971 eruption, the lahars that descended along the river beds of Ríos Turbio and Correntoso, reached an estimated 6 volume of 20 x 10 m3. Tephra fallouts should affect areas located mainly on the eastern-southeastern side of the volcano. Although there are no important human settlements towards the East, if a highly explosive eruption occurs, the tephra fallout deposits could reach more populated areas further to the East, in Argentina. Pyroclastic flows are uncommon, but if they occur, they can be extremely destructive due to its widespread distribution. Moreover, it is important to say that there are no evidences that the explosive phase of Volcán Villarrica has finished yet. Glaciar Pichillancahue-Turbio Volcán Villarrica is at present covered by a glacier of 30.3 km2 (Rivera et al., 2006). The ice mass is mainly distributed towards the south and east side of the volcano, where the main glacier basin (Glaciar Pichillancahue-Turbio) of a size of 17.3 km2 is formed. The ELA is approximately at 2,000 m (in 2005) (Rivera et al., 2006). The glacier shows an accelerated retreat, mainly due to an increase of air temperature and decreases of precipitation (Rivera et al., 2008) like most other glaciers in the region (Brock et al., 2007). The glacier area loss accounts 25% from 1961 to 2003, as was detected by the analysis of satellite imagery and photogrammetric techniques (Figure 3). The frontal variations and present extent of Villarrica’s glaciers is shown in Figure 4. 3 The ice-elevation changes determined by subtracting topographic datasets (IGM DEM from 1961 and AirSAR C DEM from 2004) yielded at the lower parts of the glacier a rate of -0.81 ± 0.45 ma –1 , confirming the imbalance of the glacier with present climate conditions (Rivera et al., 2006). Finn C., Bell R., Blankenship D. and Behrendt J. 1995. The relation of crustal structure, warm mantle, and ice sheets to Cenozoic volcanism in West Antarctica. Abstracts of the VII International Symposium on Antarctic Earth Sciences. González O. 1972. Distribución del volcanismo activo de Chile y la reciente erupción del volcán Villarrica. Instituto Geográfico Militar, Primer Symposium Cartográfico Nacional, Santiago, Chile. Hickey-Vargas R., Moreno H., López L. and Frey F. 1989. Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanín volcanic chain (39.5°S): an evaluation of source heterogeneity, fractional crystallization and crustal assimilation. Contributions to Mineralogy and Petrology 103, 3: 361-386. Hickey-Vargas R., Sun M., López-Escobar L., Moreno H., Reagan M.K., Morris J.D. and Ryan JG. 2002. Multiple subduction components in the mantle wedge: Evidence from eruptive centres in the Central Southern volcanic zone, Chile. Geology,Vol. 30, no. 3, p.199-202. Several RES profiles measured since 2003 have allowed to obtain the ice thicknesses of the glacier (Casassa et al., 2004; Rivera et al., 2006). These profiles surveyed both in the ash/debris-covered area and the snow-covered surfaces, from the margins of the glacier up to 2,436 m a.s.l., yielded a mean thickness of 75 ± 4 m, the error being the mean difference between 663 crossing points (Rivera et al., 2006). Internal layers detected from RES profiles showed the boundary between snow and ash/debris covered ice, consisting of pyroclastic deposits originating from the volcano, being advected by ice flow and emerging on the ablation area of the glacier (Figure 5). These deposits are probably related to the large Pucón Ignimbrite eruption that occurred at 3,700 BP (Clavero and Moreno, 2004), as evidenced by its characteristic juvenile material, formed by phenocrysts-rich basaltic-andesite cauliform and breadcrusted bombs. Ivins E., Raymond C. and James T. 2000. The influence of 5000 year-old and younger glacial mass variability on present-day crustal rebound in the Antarctic Peninsula. Earth, Planets, and Space 52: 1023-1029. Figure 6. AWS at Volcán Villarrica used for the energy balance programme (Photo, Camilo Rada). References Figure 5. Topographic profile showing surface and subglacial topography of Glaciar Pichillancahue of Volcán Villarrica. In the middle is the radar non-migrated corresponding profile with subglacial returns in white. At the bottom are Bed Power Reflection (BRP) values obtained along this profile. The arrow indicates appearance of ash/debris covered layer on top of the glacier. (Rivera et al., 2006). Location of A-A´ profile in Figure 4. Credits Jorge Clavero, PhD., Geologist Andrés Rivera, PhD., Glaciologist Claudio Bravo, Geographer Martina Barandun, Bachelor in Geography FONDECYT 1090387 5 Tephra-covered glaciers are thought to be less sensitive to atmospheric temperature changes than ‘clean’ glaciers. In a possible future warmed climate, the melt of the winter snow cover in the ablation season would be accelerated and melt of any bare snow or ice surface would be increased (Brock et al., 2007). A climate warming would affect the winter mass balance since melt would most likely be extended to the top of the accumulation zone. The magnitude and frequency of winter melt events would be more likely, higher (Brock et al., 2007). Laugenie C. 1971. Elementos de la cronología glaciar de los Andes chilenos meridionales. Cuadernos Geográficos del Sur 1: 7 – 20. Casassa, G., Acuña C., Zamora R., Schliermann E. and Rivera, A. 2004. Ice thickness and glaciar retreat at Villarrica Volcano. In: LARA L. & CLAVERO, J. (Eds.). Villarrica Volcano (39.5ºS), Southern Andes, Chile. SERNAGEOMIN, Boletín 61, 53-60. Maclennan J., Jull M., McKenzie D., Slater, L. and K. Grönvold. 2002. The link between volcanism and deglaciation in Iceland. Geochemistry, Geophysics, Geosystems 3(11), 1062, DOI: 10.1029/2001GC00282. Cembrano J. 1990. Geología del Batolito Norpatagónico y de las rocas metamórficas de su margen occidental: 41º50’S-42º10’S. Unpublished thesis, University of Chile. Moreno H. 1993. Volcán Villarrica: Geología y Evaluación del Riesgo Volcánico, regiones IXª y Xª, 39°25’S. Unpublished Fondecyt report, 112p.* Cembrano J., Beck M., Burmester R., Rojas C., García A. and Hervé F. 1992. Paleomagnetism of Lower Cretaceous rocks from east of the Liquiñe-Ofqui fault zone, southern Chile: evidence of small in-situ clockwise rotations. Earth and Planetary Science Letters 113: 539-551. Moreno H. 2000. Mapa de Peligros Volcánicos del Volcán Villarrica. Documentos de Trabajo No. 17, Servicio Nacional de Geología y Minería, escala 1:75.000. Moreno H. and Clavero J. 2006. Mapa geológico del volcán Villarrica. Serie Geología Básica, No., p. Servicio Nacional de Geología y Minería. Cembrano J. and Moreno H. 1994. Geometría y naturaleza contrastante del volcanismo entre los 38ºS y 46ºS: ¿Dominios compresionales y tensionales en un régimen transcurrente? Abstracts 7th Chilean Geological Congress, Concepción, vol I, p.240-244. Overall, the glacier is experiencing both thinning and area reduction, in spite of presenting a thick layer of ash and debris (thicker than 1m in places) covering most of the ablation area. In some places where the glacier is more crevassed, backwasting seems to be an important process on steep ice walls. Apart from the possible volcanic component, which is affecting Glaciar Pichillancahue, the main glacier variations are driven by climate change or decadal atmosphere/ocean oscillations (i.e. the 1976 shift). However, not all changes are a direct response to warmer/drier conditions, as an important role is played by feedbacks triggered by climatic changes. Among these feedbacks, the ice surface elevation and glacier length responses are the most important (Rivera et al., 2006). Francisca Bown, MSc, Geographer Ivins E. and James T. 1999. Simple models for Late Holocene and present-day Patagonian glacier fluctuation and predictions of a geodetically detectable isostatic response. Geophysical Journal International 138: 601-624. Brock B., Rivera A., Casassa G., Bown F. and Acuña C. 2007. The surface energy balance of an active ice – covered volcano: Villarrica volcano, southern Chile. Annals of Glaciology 45: 104 – 114. In many surveyed areas, the subglacial topography was not visible or was confused by internal layers, requiring new and denser data to detect deeper ice. One of these areas is illustrated in Figure 5 where the subglacial topography is interrupted. These features were visible in all the records obtained from this sector of the glacier, suggesting that large crevasses obscure the bedrock returns. These crevasses could be related to a break or a large crater structure in the subglacial topography. The mean ice thickness obtained in Volcán Villarrica represents areas where signals were sufficiently clear to be distinguished from internal layers. 10 4 Crucifix M., Loutre M., Lambeck K. and Berger, A. 2001. Effect of isostatic rebound on ice volume variations during the last 200 yr. Earth and Planetary Science Letters 184:623-633. The influence of the volcanic activity on the overlaying glaciers shows two contrasting effects. On one hand a geothermal flux at the glacier’s base reinforces melt. On the other hand, where thick enough isolating debris and ash layers are present, the ice is protected from melting, reducing ablation and having a positive effect on the mass balance (Rivera et al., 2006; Brock et al., 2007). These findings were observed at Automatic Weather Stations (AWS) installed on the glacier together with mass balance measurements carried out on the glacier (Figure 6). Energy balance monitoring (Brock et al., 2007) has been combined to Global Positioning System (GPS) and Radio Echo Sounding (RES) measurements since 2004 (Rivera et al., 2006). Figure 4. Glacier variations, 1961-2009, at Volcán Villarrica. The yellow dot shows the summer AWS location. The star shows the location of the camera and the GPS station. (Updated from Rivera et al, 2008). A-A´ RES profile showed in Figure 5. Figure 3. Photographic camera (red arrow) setup for mapping glacier dynamics, albedo, and snow deposition (Rivera et al., 2008) (Photo, Camilo Rada). Naranjo J. and Moreno H. 1991. Actividad explosiva postglacial en el volcán Llaima, Andes del Sur. Revista Geológica de Chile, 18 (1): 69-80. Cembrano J., Schermer E., Lavenu A. and Sanhueza A. 2000. Contrasting nature of deformation along an intra-arc shear zone, the Liquiñe-Ofqui fault zone, southern Chilean Andes. Tectonophysics,Vol. 319, p. 129-149. Rivera A., Bown F., Mella R., Wendt J., Casassa G., Acuña C., Rignot E., Clavero J. and Brock B. 2006. Ice volumetric changes on active volcanoes in Southern Chile. Annals of Glaciology 43: 111 - 122. Clague J. and James T. 2001.History and isostatic effects of the last ice sheets in southern British Columbia. Quaternary Science Reviews. Clapperton C (1993) Quaternary Geology and Geomorphology of South America, Elsevier, Amsterdam, 466p. Rivera, A., Corripio J., Brock B., Clavero J. and Wendt J.. 2008. Monitoring icecapped active Volcán Villarrica, southern Chile, using terrestrial photography with automatic weather stations and global positioning system. Journal of Glaciology 54: 920 – 930. Clavero J. 1996. Ignimbritas andesítico-basálticas del Volcán Villarrica, Andes del Sur (39°30’S). Unpublished MSc thesis, University of Chile, 112p.* Witter J., Kress V., Delmelle P. and Stix J. 2004. Volatile degassing, petrology, and magma dynamics of the Villarrica lava lake,Southern Chile. Journal of Volcanology and Geothermal Research 134: 303-337. Clavero J. and Moreno H. 2004. Evolution of Villarrica volcano. In Lara and Clavero (eds.) Villarrica volcano, Southern Andes. Boletín No. 61 Servicio Nacional de Geología y Minería, Chile. 6 Clayton J., Clapperton C. and Antinao-Rojas J. 1997. Las glaciaciones pleistocénicas en la cuenca del Lago Villarrica, Andes del Sur. Actas del VIII Congreso Geológico Chileno, Antofagasta, Vol 1: 307 – 311. (*) Unpublished Document available at the Library of the Servicio Nacional de Geología Minería, Avenida Santa María 0104, Providencia,Santiago. Chile. 7 8 Stops Stop 1.- Outskirts of Coñaripe Coñaripe is a small touristic town located on the eastern shore of Lago Calafquén. In March 1964, the northern part of the town was completely destroyed by a lahar originated at Volcán Villarrica, killing more than 20 people. We will observe the few remaining trees of the old town and the marks left by the ca. 2 m high wave which reached the town, leaving half of the town into the lake. After this disaster, the area is once again urbanized and several touristic facilities have been built. Stop 2.- Chaillupén River We will look at one of the lavas which were generated during the 1971 eruption. This is an Aa-type andesitic lava, which also generated a lahar that reached Lago Calafquén triggering a small tsunami. The lava took several days for reaching this area, after moving for ca. 12 km from its source. Stop 3.- Outskirts of Licán Ray We will look at the distal deposits of the largest Postglacial explosive eruption of Volcán Villarrica: the Licán Ignimbrite. In this area (at ca.20 km from the western caldera rim) the deposit sits on top of morainic deposits of the Last Glaciation (Llanquihue Drift). The deposit is more than 8 m thick and shows, at least, 3 flow units. Its base shows local surge deposits, produced by the irregular topography on which it traveled and deposited. The upper flow units are generally massive, although some internal structures can be observed (bomb trends, parallel lamination, etc.). The contact with the morainic deposits shows sometimes a reddish color as well as carbonized wood and gas segregation pipes, suggesting a hot emplacement on top of still humid glacial deposits. Stop 4.- Willylafquen area-Ski center road Pucón Ignimbrite (3.7 ka) is the second large mafic pyroclastic flow at the top of Villarrica 2 unit, which produced a summit collapse of the stratovolcano generating caldera at 2,400 m a.s.l. Compared with the Licán ignimbrite, this deposit has much more lithics, thus it was much slower and in the Zanjón Seco valley, the flow was extremely canalized. In this place, the deposit consists roughly in four flow units, which individually show little structures, although some parallel bedding and lenses containing big scoriaceous bombs and/or lithics, together with carbonized trees usually imbricated along the flow direction. From base to top,the different flow units contain different amounts of juvenile and lithic material. At lower levels, it consists mainly of scoriaceous bombs (up to 60 cm in diameter) and lapilli, with plenty charred wood. A second horizon has scoria, lithics and an indurate ash matrix with carbonized trees. Then a third thinner hard layer consists of lithic fragments and sand,followed by a 4.5 m thick upper horizon with three lithic rich facies, the middle one with big angular fragments up to 35 cm in diameter. Stop 5.- Pucón viewpoint In this place there is a magnificent view of Villarrica volcano (Units 2 and 3), if not cloudy. The roadcut shows a lateral moraine of the Estero Zanjón Seco, which belongs to the last Glaciation (Llanquihue Drift), covered by almost the whole Postglacial pyroclastic sequence of Volcán Villarrica explosive record. The large Licán ignimbrite is missing here but outcrops are visible some hundredth of meters toward south along the road. The first ignimbrite layers over the moraine are quite weathered, and the higher one that underlies the noticeable pumice fall deposit from Mocho-Choshuenco volcano, has a 14C age of 9.7 ka. On top of it we can observe several weathered pyroclastic flow deposits that underlie a conspicuous but thin grey surge deposit equivalent to the Pucón event upper facies (3.7 ka). The surge layer is followed by two significant mafic air-fall tephra, covered by younger pyroclastic flow deposits on the top, all of them belong to Villarrica unit 3. View from the south to the remnants of Coñaripe village, after the March 1964 lahar which killed tens of people. 1971 lava flow at Chaillupén valley. From this place looking north and northeast, toward the Estero Zanjón Seco, it can be observed the Postglacial fast filling of ignimbrites, lava flows and laharic deposits that came from the volcano. Thus, when the Licán ignimbrite eruption took place at the Postglacial beginning, the Estero Zanjón Seco valley was much deeper than today. Therefore, most of it was confined to it, although some surge facies overflowed as seen upwards in the road cut. Far down the Pucón town can be seen, located on an evident hazardous area. Willylafquén. Channel juvenile-rich facies of the Pucón Ignimbrite deposit, showing several flow units. 1949 Volcán Villarrica eruption.