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Bertin D.,Servicio Nacional de Geologia y Mineria
Journal of Geophysical Research: Solid Earth | Year: 2017

An innovative 3-D numerical model for the dynamics of volcanic ballistic projectiles is presented here. The model focuses on ellipsoidal particles and improves previous approaches by considering horizontal wind field, virtual mass forces, and drag forces subjected to variable shape-dependent drag coefficients. Modeling suggests that the projectile's launch velocity and ejection angle are first-order parameters influencing ballistic trajectories. The projectile's density and minor radius are second-order factors, whereas both intermediate and major radii of the projectile are of third order. Comparing output parameters, assuming different input data, highlights the importance of considering a horizontal wind field and variable shape-dependent drag coefficients in ballistic modeling, which suggests that they should be included in every ballistic model. On the other hand, virtual mass forces should be discarded since they almost do not contribute to ballistic trajectories. Simulation results were used to constrain some crucial input parameters (launch velocity, ejection angle, wind speed, and wind azimuth) of the block that formed the biggest and most distal ballistic impact crater during the 1984–1993 eruptive cycle of Lascar volcano, Northern Chile. Subsequently, up to 106 simulations were performed, whereas nine ejection parameters were defined by a Latin-hypercube sampling approach. Simulation results were summarized as a quantitative probabilistic hazard map for ballistic projectiles. Transects were also done in order to depict aerial hazard zones based on the same probabilistic procedure. Both maps combined can be used as a hazard prevention tool for ground and aerial transits nearby unresting volcanoes. ©2017. American Geophysical Union. All Rights Reserved.

Castruccio A.,University of Chile | Clavero J.,Servicio Nacional de Geologia y Mineria
Journal of Volcanology and Geothermal Research | Year: 2010

The Villarrica and Calbuco volcanoes, of the Andean Southern Volcanic Zone, are two of the most active volcanoes in Chile and have erupted several times in the XX century. The 1961 eruption at Calbuco volcano generated lahars on the North, East and Southern flanks, while the 1971 eruption at Villarrica volcano generated lahars in almost all the drainages towards the north, west and south of the volcano. The deposits from these eruptions in the Voipir and Chaillupén River (Villarrica) and the Tepú River (Calbuco) are studied. The 1971 lahar deposits on Villarrica volcano show a great number of internal structures such as lamination, lenses, grading of larger clasts and a great abundance of large floating blocks on top of the deposits. The granulometry can be unimodal or bimodal with less than 5% by weight of silt + clay material. SEM images reveal a great variety of forms and compositions of clasts. The 1961 lahar deposits on Calbuco volcano have a scarce number of internal structures, steeper margins and features of hot emplacement such as semi-carbonized vegetal rests, segregation pipes and a more consolidated matrix. The granulometry usually is bimodal with great quantities of silt + clay material (> 10% by weight). SEM images show a uniformity of composition and forms of clasts. Differences on deposits reveal different dynamics on both lahars. The Villarrica lahar was generated by sudden melt of ice and snow during the paroxysmal phase of the 1971 eruption, when a high fountain of lava was formed. The melted water flowed down on the flanks of the volcano and incorporated sediments to become transition flows, highly energetic and were emplaced incrementally. Dilution of the flows occurs when the lahars reached unconfined and flatter areas. In cases where the lahar flow found large water streams, dilution is enhanced. The Calbuco lahars were generated by the dilution of block and ash pyroclastic flows by flowing over the ice or snow or by entering active rivers, transforming to debris flows. The differences on dynamics of both flows show the importance to understand initiation processes of lahars in order to make better hazard assessment due to laharics flows. © 2009 Elsevier B.V. All rights reserved.

Herve F.,University of Chile | Herve F.,Andrés Bello University | Calderon M.,Servicio Nacional de Geologia y Mineria | Fanning C.M.,Australian National University | And 2 more authors.
Gondwana Research | Year: 2013

We present detrital zircon U. Pb SHRIMP age patterns for the central segment (34-42°S) of an extensive accretionary complex along coastal Chile together with ages for some relevant igneous rocks. The complex consists of a basally accreted high pressure/low temperature Western Series outboard of a frontally accreted Eastern Series that was overprinted by high temperature/low pressure metamorphism. Eleven new SHRIMP detrital zircon age patterns have been obtained for meta-turbidites from the central (34-42°S) segment of the accretionary complex, four from previously undated metamorphic complexes and associated intrusive rocks from the main Andean cordillera, and three from igneous rocks in Argentina that were considered as possible sediment source areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early Paleozoic zircons are an essential component of the provenance, and Grenville-age zircons and isolated grains as old as 3. Ga occur in most rocks, although much less commonly in the Western Series of the southern sector. In the northernmost sector (34-38°30'S) Proterozoic zircon grains constitute more than 50% of the detrital spectra, in contrast with less than 10% in the southern sector (39-42°S). The youngest igneous detrital zircons in both the northern Western (307. Ma) and Eastern Series (345. Ma) are considered to closely date sedimentation of the protoliths. Both oxygen and Lu. Hf isotopic analyses of a selection of Permian to Neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions. The results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian) containing material recycled from cratonic areas of South America supplied detritus to this part of the paleo-Pacific coast. In contrast, in the southern exposures of the Western Series studied here, Permian detrital zircons (253-295. Ma) dominate, indicating much younger deposition. The northern sector has scarce Early to Middle Devonian detrital zircons, prominent south of 39°S. The sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of Devonian (Achalian) detritus by a topographic barrier formed by the Precordillera, and possibly Chilenia, terranes. Devonian subduction-related metamorphic and plutonic rocks developed south of 39°S, beyond the possible southern limit of Chilenia, where sedimentation of accretionary rocks continued until Permian times. © 2012.

Jordan T.E.,Cornell University | Nester P.L.,Cornell University | Blanco N.,Servicio Nacional de Geologia y Mineria | Hoke G.D.,Syracuse University | And 2 more authors.
Tectonics | Year: 2010

The western flank of the Central Andean Plateau is a crustal-scale monoclinal fold, expressed in the geomorphology and in the westward tilt of fore-arc basin strata. Data from three fore-arc basins quantify the magnitude and time of displacement of the plateau system relative to the fore arc. From 18°30'S to 22°S there is a single monocline strand. There, other authors documented ∼2000 m (±500 m) of early and middle Miocene structural relief growth across small-scale monoclines, and our data reveal 810 m (±640 m) of ∼11-5 Ma relief growth and 400 m (±170 m) relief growth since ∼5 Ma across a long-wavelength monoclinal fold limb. This structural relief growth since ∼11 Ma approximates the topographic relief growth between the fore arc and the Altiplano plateau. From 22°S to 24°S there are two subparallel long-wavelength monoclines. Structural relief on the east side of the fore arc increased by 2840 m (±2510 m) during ∼17-10 Ma and by 2320 m (±1050 m) since ∼10 Ma. Some part of this monoclinal limb rotation led to topographic uplift of the adjacent Puna plateau. Rotation across the western monocline could have increased the altitude of the fore-arc basins by <900 m since ∼6 Ma and may result from intense heating of the eastern margin of the cold and strong Atacama lithosphere block. Although the wavelengths of the monoclines vary along strike and through time, the monoclinal style of deformation dominated the uplift history of the eastern fore arc of the central Andes throughout the Neogene. © 2010 by the American Geophysical Union.

Pritchard M.E.,Cornell University | Jay J.A.,Cornell University | Aron F.,Cornell University | Henderson S.T.,Cornell University | Lara L.E.,Servicio Nacional de Geologia y Mineria
Nature Geoscience | Year: 2013

Large earthquakes provoke unrest in volcanic areas hundreds of kilometres away from their epicentre. For example, earthquakes can induce ground deformation, thermal anomalies, additional earthquakes, hydrological changes or eruptions in volcanic regions. Two earthquakes in the Chilean subduction zone, in 1906 and 1960, triggered eruptions in the Andean southern volcanic zone within one year, yet no significant eruptions in the past three years are clearly associated with the 2010 M w 8.8 Maule, Chile earthquake. We use satellite Interferometric Synthetic Aperture Radar (InSAR) and night-time thermal infrared data to analyse subtle changes in ground deformation and thermal activity at volcanoes in the southern volcanic zone since 2010. We document unprecedented subsidence of up to 15 cm in five volcanic areas within weeks of the earthquake, but no detectable thermal changes. We suggest that the deformation is related to coseismic release of fluids from hydrothermal systems documented at three of the five subsiding regions. The depth and shape of these hydrothermal reservoirs can also be constrained by our deformation data, implying that coseismic volcano subsidence could be used to prospect for geothermal resources. Similar subsidence observed at Japanese volcanoes following the 2011 Tohoku earthquake suggests this phenomenon is widespread.

Pierson T.C.,U.S. Geological Survey | Major J.J.,U.S. Geological Survey | Amigo A.,Servicio Nacional de Geologia y Mineria | Moreno H.,Servicio Nacional de Geologia y Mineria
Bulletin of Volcanology | Year: 2013

The 10-day explosive phase at the start of the 2008-2009 eruption of Chaitén volcano in southern Chile (42.83°S, 72.65°W) blanketed the steep, rain-forest-cloaked, 77-km2 Chaitén River drainage basin with 3 to >100 cm of tephra; predominantly fine to extremely fine rhyolitic ash fell during the latter half of the explosive phase. Rain falling on this ash blanket within days of cessation of major explosive activity generated a hyperconcentrated-flow lahar, followed closely by a complex, multi-day, muddy flood (streamflow bordering on dilute hyperconcentrated flow). Sediment mobilized in this lahar-flood event filled the Chaitén River channel with up to 7 m of sediment, buried the town of Chaitén (10 km downstream of the volcano) in up to 3 m of sediment, and caused the lower 3 km of the channel to avulse through the town. Although neither the nature nor rate of the sedimentation response is unprecedented, they are unusual in several ways: (1) Nearly 70 percent of the aggradation (almost 5 m) in the 50-70-m-wide Chaitén River channel was caused by a lahar, triggered by an estimated 20 mm of rainfall over a span of about 24 h. An additional 2 m of aggradation occurred in the next 24-36 h. (2) Direct damage to the town was accomplished by the sediment-laden water-flood phase of the lahar-flood event, not the lahar phase. (3) The volume of sediment eroded from hillslopes and delivered to the Chaitén River channel was at least 3-8 × 106 m3-roughly 15-40 % of the minimum tephra volume that mantled the Chaitén River drainage basin. (4) The acute sedimentation response to rainfall appears to have been due to the thickness and fineness of the ash blanket (inhibiting infiltration of rain) and the steepness of the basin's hillslopes. Other possible factors such as the prior formation of an ash crust, development of a hydrophobic surface layer, or large-scale destruction of rain-intercepting vegetation did not play a role. © 2013 The Author(s).

Major J.J.,Cascades Volcano Observatory | Lara L.E.,Servicio Nacional de Geologia y Mineria
Andean Geology | Year: 2013

Chaitén Volcano erupted unexpectedly in May 2008 in one of the largest eruptions globally since the 1990s. It was the largest rhyolite eruption since the great eruption of Katmai Volcano in 1912, and the first rhyolite eruption to have at least some of its aspects monitored. The eruption consisted of an approximately 2-week-long explosive phase that generated as much as 1 km3 bulk volume tephra (~0.3 km3 dense rock equivalent) followed by an approximately 20-month-long effusive phase that erupted about 0.8 km3 of high-silica rhyolite lava that formed a new dome within the volcano's caldera. Prior to its eruption, little was known about the eruptive history of the volcano or the hazards it posed to society. This edition of Andean Geology contains a selection of papers that discuss new insights on the eruptive history of Chaitén Volcano, and the broad impacts of and new insights obtained from analyses of the 2008-2009 eruption. Here, we summarize the geographic, tectonic, and climatic setting of Chaitén Volcano and the pre-2008 state of knowledge of its eruptive history to provide context for the papers in this edition, and we provide a revised chronology of the 2008-2009 eruption.

Bonali F.L.,University of Milan Bicocca | Tibaldi A.,University of Milan Bicocca | Corazzato C.,University of Milan Bicocca | Tormey D.R.,ENTRIX Inc. | Lara L.E.,Servicio Nacional de Geologia y Mineria
Tectonophysics | Year: 2013

Four earthquakes with Mw≥8 occurred in proximity to 60 Holocene volcanoes, in the Southern Volcanic Zone of the Andes (SVZ) since 1906. We analyzed these events by numerical modeling and field data to understand the key attributes of each volcano that may lead to a seismically-triggered eruption and the general mechanisms by which earthquakes could trigger volcanic new activity. We developed a new approach by resolving the earthquake-induced normal static stress change on the magma pathway of each volcano instead of considering the general crustal volume. We also considered other parameters that may lead to eruption, such as magma chamber depth, magma composition and viscosity, local tectonic settings and volcano dimension. The dataset includes a total of 18 new volcanic activities following large earthquakes (15 eruptions and 3 minor increases in volcanic activity) at 10 volcanoes. Of these, 9 out of 18 events representing unrest occurred at volcanoes that had no activity in the five years before the earthquake. Results indicate that the static stress changes were capable of triggering the observed volcanic phenomena up to a distance of 353km from the epicenter. Eleven out of 18 new volcanic events occurred at volcanoes with shallow magma chambers (2-3km) under conditions of unclamping or very weak clamping. New activity at volcanoes with deeper magma chambers (>7km) occurred only by magma pathway unclamping. Considering the regional tectonics of the SVZ, 5 volcanoes lying along strike-slip faults, 3 on thrust faults and 2 along a transition zone, experienced unrest. Our results show that magma pathway unclamping plays a fundamental role in dictating unrest at volcanoes that are already in a critical state. These studies contribute to possible individuation of those volcanoes that are more prone to seismically-triggered eruptive events. © 2012 Elsevier B.V.

Watt S.F.L.,UK National Oceanography Center | Pyle D.M.,University of Oxford | Mather T.A.,University of Oxford | Naranjo J.A.,Servicio Nacional de Geologia y Mineria
Geophysical Research Letters | Year: 2013

Global arc magmatism is sustained by a continuous fluid flux that is returned to the mantle in subduction zones. Despite considerable advances in simulations of melting processes, models of arc magmatism remain incompletely tested against erupted products. Here, we show that a suite of primitive volcanic rocks from across the southern Chilean arc preserves the signature of a systematic down-slab gradient in fluid chemistry. The chemical gradient is consistent with predictions from modeling, geothermometry and experiments. We infer that increasing slab-surface temperatures cause the sub-arc slab flux to become less water-rich and increasingly dominated by hydrous melts over a distance of a few kilometers behind the arc front. This change exerts a first-order control on magma chemistry, and implies discrete melt-transport pathways through subduction zones. Our results replicate patterns in other arcs, implying common sub-arc slab-surface temperature ranges in thermally-diverse subduction zones. © 2013 American Geophysical Union. All Rights Reserved.

Amigo A.,Servicio Nacional de Geologia y Mineria | Lara L.E.,Servicio Nacional de Geologia y Mineria | Smith V.C.,University of Oxford
Andean Geology | Year: 2013

Tephra fall deposits and one large ignimbrite close to Chaitén and Michinmahuida Volcanoes were analyzed for chemistry and radiocarbon dated to correlate the eruptive units and establish the timing of eruptions. These data suggest that both volcanoes were the source of large (VEI ≥5) and small to moderate (VEI <5) explosive eruptions throughout the Holocene. Four deposits are associated with volcanic activity from Chaitén Volcano, with two from Plinian eruptions at 9.9-9.5 and 5.3-4.9 (cal) ka BP that also generated pyroclastic density currents. The last event recognized from Chaitén (prior 2008) occurred a few hundred years ago, producing deposits that are similar to those of the 2008 eruption. All products from Chaitén are high-silica rhyolites; whole-rock compositions are indistinguishable but glass compositions are subtly different for some of the units. Seven deposits are related to eruptions of Michinmahuida Volcano, including a Plinian fall deposit at 7.6-7.3 ka BP and a large ignimbrite deposit at 10.5-10.2 ka BP. The chemical compositions of these products range from andesite to dacite. The last substantial explosive eruption event from Michinmahuida Volcano appears to have been a 0.5-0.3 ka BP sub-Plinian eruption, although younger scoria fall deposits likely derived from local pyroclastic cones are also found. Both volcanoes pose a wide variety of potential hazards to the region ranging from those derived from ignimbrite-forming eruptions to pyroclastic-cone formation. Valleys adjacent to the volcanoes were the areas most heavily affected by volcanic activity, because they were inundated by pyroclastic density currents and lahars. However, even regions located tens of kilometers east and north of the volcanoes experienced accumulations of tephra, which could harm both agriculture and infrastructure if similar events occurred today.

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