Memoria (geógrafo) / El riesgo es una condición latente que se ve intensificada por la existencia de población humana, producción e infraestructura, la cual se conjuga con el nivel de vulnerabilidad que estos términos puedan poseer. Por ende, la construcción social del riesgo se produce de manera distinta en función del nivel socioeconómico, cultural y educacional de un determinado grupo social. Si se pretende tipificar la forma en cómo se crea el riesgo por parte de la acción humana, entre las causas más importantes y recurrentes se encuentran: población pobre expulsada de sectores rurales que se ven obligadas a asentarse en sectores inseguros y vulnerables de la ciudad (principalmente zonas periféricas); población de ingresos medios o medios bajos ocupando viviendas regulares situadas en sectores de peligrosidad, que no poseen los sistemas adecuados de mitigación y protección; población de altos recursos que se asientan en áreas de gran valor estético, que teniendo conocimiento del grado de peligrosidad que poseen, las ocupan debido al estatus social y los beneficios en cuanto a paisaje que éstas implican. El estudio de la construcción social el riesgo debe estar destinado a la solución del paradigma conocido como “Gestión de Desastres” el cual debe abordar los esquemas de prevención, respuesta y recuperación de manera integral y debe tener la capacidad de adaptarse a los cambios de la sociedad y a los nuevos desafíos que ésta le impone, como lo es la aparición de nuevas amenazas (LAVELL, 1996). A través de una gestión integral se logra abordar tanto las causas como consecuencias de los desastres socio-naturales, permitiendo con esto la disminución de la vulnerabilidad dado que éste es un factor posible de reducir y modificar. Si bien el Plan Nacional de Emergencia ya lleva más de nueve años de implementación, aún surgen inconvenientes en las etapas de prevención y mitigación, debido al cambio cultural que esto implica, ya que, el nuevo plan asume una labor más exhaustiva en el desarrollo de actividades destinadas a la prevención, preparación y educación de la población para así reducir y aminorar los costos sociales, económicos y de infraestructura que conllevan los escenarios de desastres; cosa muy distinta a la que ocurría con el antiguo programa de protección civil (Plan Nacional de Emergencias de 1977) el cual se centraba sólo en la etapa de post-desastre. El presente estudio busca ahondar y evaluar la construcción social del riesgo al interior de la ciudad de Caldera con el fin de visualizar la capacidad de acción tanto de las autoridades como de la población ordinaria ante un posible evento de tsunami. Además, lo anterior contribuirá a vislumbrar el escenario social y cultural del área de estudio que es, en definitiva, la base de la construcción social del riesgo. / Proyecto Fondecyt 1100223/2010

La résurgence, définie comme le soulèvement du plancher des calderas postérieurement à leur effondrement, est largement répandue mais encore très mal comprise. L'objectif de cette thèse est l'étude pluridisciplinaire d'un dôme résurgent: le Yenkahe, au sein de la caldera de Siwi au Vanuatu. L'intérêt de ce dôme est multiple. D'une part, la résurgence est active, d'autre part, elle est très rapide donc elle est associée à des structures relativement préservées. Enfin le Yenkahe présente l'originalité d'être associé à un cône volcanique en éruption permanente depuis au moins plusieurs centaines d'années: le Yasur. Une première étude tectonique basée sur des observations de terrain, complétées par l'exploitation d'images satellites et de modèles numériques de terrain à basse résolution existants, a permis de mettre en évidence une histoire en deux temps (au moins) de la croissance du dôme résurgent. Une première phase de croissance verticale engendrant un graben longitudinal aurait été suivie par une seconde phase de déplacement vers l'est de la source de la déformation, occasionnant un soulèvement de la partie est du dôme par rapport à la partie ouest. Ces grands traits structuraux ont pu être raffinés, par la suite, par le calcul d'un modèle numérique de surface à haute résolution issu de la photogrammétrie. Outre le haut niveau de détails permettant une cartographie plus fine des failles, ce modèle a apporté la mise en évidence de nombreuses traces de déstabilisations sur les flancs du Yenkahe. Dans l'avenir, de telles déstabilisations pourraient engendrer des tsunamis atteignant des zones habitées en quelques minutes. Les caractéristiques de la source de déformation à long terme du Yenkahe, et d'autres dômes résurgents présentant une morphologie similaire, ont fait l'objet d'une étude en modélisation analogique (intrusion de silicone dans un mélange de sable-plâtre). Les résultats de cette étude montrent que la taille du graben engendré en surface par l'intrusion d'une source allongée dépend surtout de la profondeur de cette source. On tire de cette étude que l'intrusion associée au Yenkahe, supposée magmatique, serait située aux alentours d'un kilomètre sous la surface.Une seconde étude en modélisation numérique, basée sur un processus de poinçonnement dans un milieu élastoplastique, met en avant une géométrie interne de dôme différente mais confirme l'ordre de grandeur obtenu pour la profondeur de source (1-2 km), rendant le résultat plus robuste. Cette source magmatique est peut-être connectée au Yasur, qui montre une activité de dégazage en conduit ouvert depuis plusieurs centaines d'années. Ceci qui impliquerait un mode de croissance incrémental gouverné par des événements de surpression transitoires (tels que des injections magmatiques). Enfin, un ensemble de méthodes géophysiques appliquées à l'étude de la caldera de Siwi (gravimétrie, mesures magnétiques et électriques, etc.) révèle que l'histoire post-effondrement de cette caldera comporte, outre la résurgence tectonique, des événements effusifs et la construction de plusieurs édifices volcaniques successifs. Ces méthodes mettent également en évidence la présence d'un système hydrothermal très étendu, particulièrement profond (plus de 300 m) et actif à l'est du dôme. L'altération hydrothermale associée pourrait favoriser des déstabilisations de l'édifice résurgent. / Resurgence, defined as the post-collapse uplift of the caldera floor, is widespread phenomenon worldwide but it is still poorly understood. This work is a multidisciplinary study of a resurgent dome: the Yenkahe dome, located inside the Siwi caldera, in Vanuatu. The relevance of this dome is multiple: firstly, resurgence is currently active, secondly it is fast, so the associated structures are well-preserved, and lastly, the Yenkahe dome presents the originality to be associated with a volcanic cone which has been permanently active for at least several hundred years: the Yasur volcano. A primary tectonic study based on field observations, satellite images and available low-resolution digital elevation models brought the evidence of a two-stage (at least) dome growth history. The first stage is a vertical growth that produced the longitudinal graben on top of the dome. It was followed, in the second stage, by an eastward displacement of the deformation source, generating an uplift of the eastern dome relative to the western dome. The Yenkahe structural map was then refined through the computation of a high-resolution photogrammetric digital surface model (DSM). Besides the great number of details allowing precise fault mapping, this DSM highlights the presence of numerous destabilization scars associated with the evolution of the dome. In the future, similar destabilization events may produce tsunamis reaching inhabited areas in a couple of minutes. The characteristics of the long-term deformation source of the Yenkahe dome, and of other resurgent domes with a similar morphology, were investigated by analogue modeling (intrusion of silicone putty in a sand-plaster mixture). The results of the study show the width of the graben created by an elongated intrusion source mostly depends on the depth of this source. The source of the Yenkahe dome, presumably a magmatic intrusion, would be approximately one-kilometer deep. A second study, based on a punching process in a numerically modeled elastoplastic medium, shows a different internal structure for the dome. However, it confirms the order of magnitude obtained for the source depth (1-2 km), making this result more robust. The presumed magmatic source may be connected to the Yasurvolcano, exhibiting an open conduit activity for several hundred years, which would imply an incremental growth governed by transient over-pressurizing events at depth (such as magma injections). Lastly, a panel of geophysical methods were carried out within the caldera (magnetics, gravimetry, electrical methods, etc.). They revealed that the post-collapse history of Siwi involved, not only resurgence tectonics, but also the emplacement of lava fields and of several volcanic edifices. They also demonstrate the presence of an extended hydrothermal system, particularly deep (more than 300 m) and active on the eastern side of the dome. The associated extended alteration may favor the destabilization of the resurgent edifice.

Precaldera volcanism between 3.2-2.6 M.a. produced a basalt -trachybasalt -trachyandesite -quartz latite suite peripheral to the present Long Valley caldera from a heterogeneous, interactive, deep crustal magmatic -system. The suite consists of ( 1) widespread, predominately porphyritic olivine-augite basalt / trachybasalt / trachyandesite flow sequences (> 24 km³), (2) local orthopyroxene -phyric silicic trachyandesite flows (> I km³), and (3) sparsely -phyric orthopyroxene -hornblende -plagioclase quartz latite dome-flows and coarsely -phyric biotite -hornblende -plagioclase quartz latite dome-flows ( > 4 km³) erupted in that general sequence. Field, petrographic, and major-, minor-, and trace-element, as well as Sr isotopic studies of representative precaldera lavas on the northwest periphery of the caldera suggest that: (I) the basaltic magmas were generated from a lherzolite partial melt modified by minor crystal fractionation (limited fractionation due to their high incompatible element content) and contamination by older sialic rocks or their derivatives (represented by granitic inclusions, quartz xenocrysts, and progressively higher ⁸⁷Sr/⁸⁶Sr, 0.7062 to 0.7067), (2) the silicic trachyandesite was probably the result of intimate mixing of basaltic and quartz latite magmas (reflected in compositional gaps in progressively more silicic bulk compositional trends and the similarity of the quartz latite and silicic trachyandesite initial ⁸⁷Sr/⁸⁶Sr ratios, 0.7070-0.7074), and (3) the quartz latite was derived by crustal melting at different depths (as reflected in the variable ⁸⁷Sr/⁸⁶Sr, 0.7072-0.7095) and underwent radically changing crystallization conditions and contamination by trachyandesite (represented by heterogeneous mineral assemblages, chemistry, and textures indicating changing equilibrium conditions most evident in the trachyandesite enclave-rich quartz latite). The basaltic magmas provided the heat and mass to the crust promoting partial melting and generation of quartz latitic magmas. Synchronous basaltic intrusion and generation of crustal melts interacted and hybridized to yield trachyandesite. The isolated occurrence of trachyandesite enclaves in the youngest quartz latite dome-flows, suggests the disruption of a quartz latite-trachyandesite interface during late stages of the eruptive drawdown of a small volume magmatic system. Heat from continued basaltic input and coalesence of initially separate quartz latite bodies could possibly have resulted in development of the larger silicic magma chamber from which the younger rhyolitic (Glass Mountain-Bishop Tuft) magmas erupted. / M.S.

Rhyolites of the Picabo volcanic field (10.4-6.6 Ma) of the Yellowstone hotspot in eastern Idaho are preserved as thick ignimbrites and lavas along the margins of the Snake River Plain. This study presents new O and Hf isotope data and U-Pb geochronology from individual zircons, O isotope data from major phenocrysts, whole rock Sr and Nd isotope data, whole rock geochemistry, and trace element and volatile analyses of quartz-hosted melt inclusions, which were used to characterize the evolution of rhyolite generation through the eruptive sequence. The chemical composition of the first eruption of the caldera complex, the Tuff of Arbon Valley, suggests magma generation through repeated magma injection into the crust, remelting, crystallization, mixing, and crustal assimilation. Subsequent eruptions have diverse and low δ18O signatures indicating rhyolite generation through the remelting of variably hydrothermally altered volcanics, followed by rapid batch assembly. This thesis includes co-authored material previously published.

The Toba caldera located in north Sumatra, Indonesia, is the source of the largest volcanic eruption in the Quaternary (Rose & Chesner 1987). Its enormous volume of 2800 km3 has been a matter of debate for decades and it is still unclear where and how the Toba magma was assembled. This study documents oxygen isotope data for a suite of whole rocks and minerals erupted as part of the Young Toba Tuff (YTT), some 74 ka ago (cf. Chesner et al. 1991). Oxygen isotope data has been obtained in-situ from quartz crystals (SIMS), whole rocks (conventional), as well as quartz, feldspar, amphibole and biotite (laser fluorination). In combination with cathodoluminescence (CL) imaging on the quartzes, the data are used to test the relative roles of shallow magmatic processes such as crystal fractionation, magma-crust interaction and crystal recycling within the Toba magmatic system. In addition, thermobarometric calculations have been performed on plagioclase and amphibole phenocrysts from the YTT to help unravel the magma storage and plumbing system that gave rise to the YTT. The combined evidence will be used to derive a model for shallow magma evolution and storage at Toba some 74 ka ago. The CL images of quartz crystals exhibit defined patterns of magmatic zoning, which broadly coincide with fluctuations in δ18O values in the quartz crystals, allowing correlation of textural and compositional data. Measured δ18Oquartz values range from 6.7 ‰ to 9.4 ‰, independent of position on crystal core or rim. Values for δ18Omagma have been calculated from quartz phenocrysts (assuming Δquartz-magma is 0.7 ‰ at magmatic temperatures). The lowest magma value is 6.0 ‰, apparently reflecting a primitive isotopic signal (Taylor & Sheppard 1986). The maximum calculated magma value is 8.7 ‰, indicating a significant crustal component and thus multiple sources to the Toba magmatic system. Several crystals, however, show internal zoning with gradually lower values towards the rims, pointing to a late-stage low-δ18O input, most probably from the shallow volcanic edifice. The crystals therefore record a complex and heterogeneous origin of the YTT magma, comprising a primitive and an evolved magmatic component topped up with several substantial crustal contributions to finally assemble the massive volume of the 74 ka Toba eruption.

The relative importance of infrequent, episodic geomorphic events (e.g. floods, landslides, debris flows, earthquakes, tsunamis, etc.) in the evolution of the landscape has been a long-discussed question in the geomorphology community. These events are large in magnitude, but low in frequency, posing the complex question of how effective these events are at shaping the landscape. Unfortunately, the frequencies of these events are so low that it is extremely difficult to observe these events over human time scales. Also, the dangerous nature of these events makes them extremely difficult to observe and measure. However, the last few decades have brought new technology and techniques that provide a way to measure and calculate the magnitudes of these events more accurately and completely. In the present study, we use Next-Generation-Radar (NEXRAD) precipitation products, LiDAR tools, and multiple denudation-rate techniques to approach the magnitude and frequency of episodic events in different ways. Using NEXRAD precipitation products in conjunction with flow-routing algorithms, we were able to improve upon the traditional flood-envelope curves used to estimate the largest possible flood for a given basin area within a region. Improvements included adding frequency and uncertainty information to curves for the Upper and Lower Colorado River Basin, which in turn makes these curves more informative for flood hazard and policy applications. This study allowed us to improve upon a known flood-analysis method for identifying the distribution of the maximum floods with basin area. Both airborne and terrestrial LiDAR methods were used to measure the magnitude and time scale of the post-wildfire erosional response in two watersheds after the Las Conchas fire of 2011 in the Valles Caldera, NM. We found that sediment yield (measured by differencing LiDAR-derived DEMs) decreased exponentially with time in one watershed, while sediment yield in the other watershed decreased in a more complex way with time. Both watersheds had a recovery time (i.e. time interval over which sediment yields recovered to pre-wildfire levels) of one year. LiDAR was also used to understand the complex response of, and the processes on, the piedmonts adjacent to the watersheds. Overall, LiDAR proved to be extremely useful in measuring the magnitude and time scale of post-wildfire geomorphic response and observing the piedmont dynamics associated with elevated sediment yield. To understand the effects of wildfire on the long-term evolution of the landscape, techniques ranging from the relatively simple, traditional techniques (i.e. suspended-sediment-load sampling and paleosurface and modern surface differencing) to more complex and new techniques (i.e. ¹⁰Be and LiDAR) were used to measure the volumes and rates of denudation over multiple time scales in the Valles Caldera, NM. Long-term denudation rates were higher than short-term, non-wildfire-affected denudation rates, but lower than short-term, wildfire-affected denudation rates. Wildfire-affected denudation rates occurring at previously predicted frequencies (occurring<3% of the time interval) were found to account for the majority of long-term denudation, attesting to the importance of these episodic and extreme events in the evolution of the landscape.

The evolution of rhyolitic lava from effusion to cessation of activity is poorly understood. Recent lava dome eruptions at Unzen, Colima, Chaiten and Soufrière Hills have vastly increased our knowledge on the changes in behaviour of active domes. However, in ancient domes, little knowledge of the evolution of individual extrusion events exists. Instead, internal structures and facies variations can be used to assess the mechanisms of eruption. Rhyolitic magma rising in a conduit vesiculates and undergoes shear, such that lava erupting at the surface will be a mix of glass and sheared vesicles that form a permeable network, and with or without phenocryst or microlites. This foam will undergo compression from overburden in the shallow conduit and lava dome, forcing the vesicles to close and affecting the permeable network. High temperature, uniaxial compression experiments on crystal-rich and crystal-poor lavas have quantified the evolution of porosity and permeability in such environments. The deformation mechanisms involved in uniaxial deformation are viscous deformation and cracking. Crack production is controlled by strain rate and crystallinity, as strain is localised in crystals in crystal rich lavas. In crystal poor lavas, high strain rates result in long cracks that drastically increase permeability at low strain. Numerous and small cracks in crystal rich lavas allow the permeable network to remain open (although at a lower permeability than undeformed samples) while the porosity decreases. Flow bands result from shear movement within the conduit. Upon extrusion, these bands will become modified from movement of lava, and can therefore be used to reconstruct styles of eruption. Both Ngongotaha and Ruawahia domes, from Rotorua caldera and Okataina caldera complex (OCC) respectively, show complex flow banding that can be traced to elongated or aligned vents. The northernmost lobe at Ngongotaha exhibits a fan-like distribution of flow bands that are interpreted as resulting from an initial lava flow from a N – S trending fissure. This flow then transitioned into intrusion of obsidian sheets directly above the conduit, bound by wide breccia zones which show vertical movement of the sheets. Progressive intrusions then forced the sheets laterally, forming a sequence of sheets and breccia zones. At Ruawahia, the flow bands show two types of eruption; long flow lobes with ramp structures, and smaller spiny lobes which show vertical movement and possible spine extrusion. The difference is likely due to palaeotopography, as a large pyroclastic cone would have confined the small domes, while the flow lobes were unconfined and able to flow down slope. The vents at Ruawahia are aligned in a NE – SW orientation. Both domes are suggested to have formed from the intrusion of a dyke. The orientations of the alignment or elongation of vents at Ngongotaha and Ruawahia can be attributed to the overall regional structure of the Taupo Volcanic Zone (TVZ). At Ngongotaha, the N – S trending elongated vent is suggested to be controlled by a N – S trending caldera collapse structure at Rotorua caldera. The rest of the lobes at Ngongotaha, as well as other domes at Rotorua caldera, are controlled by the NE – SW trending extensional regional structure or a NW – SE trending basement structure. The collapse of Rotorua caldera, and geometry of the deformation margin, are related to the interplay of these structures. At Ruawahia, the NE – SW trending vent zone is parallel to the regional extension across the OCC, as shown by the orientation of intrusion of the 1886AD dyke through the Tarawera dome complex. The NE – SW trending regional structures observed at both Rotorua caldera and Okataina caldera complex are very similar to each other, but differ from extension within the Taupo rift to the south. Lava domes, such as Ngongotaha, that are controlled by this structure show that the ‘kink’ in the extension across Okataina caldera complex was active across Rotorua caldera during the collapse at 240 ka, and possibly earlier. This study shows the evolution of dyke-fed lava domes during eruption, and the control of regional structures in the location and timing of eruption. These findings improve our knowledge of the evolution of porosity and permeability in a compacting lava dome, as well as of the structures of Rotorua caldera, the longevity of volcanic activity at dormant calderas and the hazard potential of dyke-fed lava domes.

Carbonatites are carbonate-rich magmatic rocks that are rare and of great relevance for our understanding of crustal and mantle processes. Although found on all continents and in settings ranging from Archaean to present-day, their deeper plumbing system is still poorly understood. Therefore, the main goal of this thesis is to broaden the existing knowledge of carbonatite systems, often limited to surface geological observations, by providing depth constraints using a number of geophysical methods and petrophysical measurements. The Alnö alkaline and carbonatite complex in central Sweden was chosen for this purpose. Data from three reflection seismic lines, ground gravity and magnetic measurements are presented. These data are complemented by a series of petrophysical measurements, including ultrasonic velocities, density, magnetic bulk susceptibility, anisotropy of magnetic susceptibility (AMS), and magnetic remanence, to aid in the interpretation of the geophysical data. The reflection seismic data indicate a solidified saucer-shaped fossil magma chamber at about 3 km depth. Caldera-style volcanism, constrained by surface geological observations, provides a plausible scenario to explain the emplacement of the complex, suggesting that carbonatite magmas have been stored, transported and erupted in a similar manner to known emplacement mechanisms for silicic calderas, although these are compositionally different. The AMS data from most of the carbonatite sheets in Alnö show a strong degree of anisotropy and oblate-shaped susceptibility ellipsoids. A set of syn- and post-emplacement processes that may control the AMS signature is evaluated based on the dataset. Overprinting of the primary flow patterns by processes related to sheet closure at the terminal stage of magma transport may explain the AMS observations. A complementary study using 3D inversion of ground gravity and aeromagnetic data was then carried out to better delineate the 3D internal architecture of the complex. Resulting models indicate a depth extent of the complex to about 3-4 km, consistent with the interpretation of the reflection seismic data. The modelling results of a ring-shaped magnetic anomaly observed in the Klingefjärden bay adjacent to Alnö Island further suggest that the complex may extend laterally about 3 km towards the north.

The mechanisms and timescales associated with the triggering of caldera-forming eruptions remain ambiguous and poorly constrained. Do such eruptions start vigorously, then escalate, or can there be episodicity? Are they triggered through internal processes (e.g. recharge, buoyancy), or can external modulations play an important role? Key to answering these questions is the ability to reconstruct the state of the magma body immediately prior to eruption. My dissertation research seeks to answer these questions through detailed investigation of four voluminous caldera-forming eruptions: (1) 650 km3, 0.767 Ma Bishop Tuff, Long Valley, (2) 530 km3, 25.4 ka Oruanui eruption, Taupo, (3) 2,500 km3, 2.08 Ma Huckleberry Ridge Tuff, Yellowstone and (4) 250 km3, 26.91 Ma Cebolla Creek Tuff, Colorado. The main techniques I applied integrated glass geochemistry (major, trace and volatile), diffusion modeling, and detailed field sampling. In chapters two, three, and four these methods are applied to the initial fall deposits of three supereruptions (Bishop, Oruanui and Huckleberry Ridge) that preserve field-evidence for different opening behaviors. These behaviors range from continuous deposition of fall deposits and ignimbrite (Bishop), to repetitive start/stop behavior, with time breaks between eruptive episodes on the order of weeks to months (Oruanui, Huckleberry Ridge). To reconstruct the timescales of opening activity and relate this to conduit processes, I used two methods that exploit diffusion of volatiles through minerals and melt, providing estimates for the rate at which magmas ascended to the surface. This knowledge is then integrated with the pre-eruptive configuration of the magma body, based on melt inclusion chemistry, to interpret what triggered these systems into unrest. Finally, in chapter five I take a different approach by integrating geochemical data for melt inclusions and phenocryst minerals to test whether the mechanism of heat and volatile recharge often called upon to trigger crystal-rich dacitic magmas (the so-called monotonous intermediates), is applicable to the Cebolla Creek Tuff. This dissertation includes both previously published and unpublished co-authored material, and three online supplementary excel files.

xii, 76 p. ： ill. (some col.) / Large-volume calderas are responsible for producing large deposits of rhyolite and high-silica rhyolite, but the mechanisms by which these deposits are produced are still poorly understood. The Kamchatka Peninsula of Russia contains several large calderas and is one of the most volcanically active areas on Earth. Karymshina Caldera, the largest (25 km x 15 km) caldera in Kamchatka, produced an estimated 800 km 3 of high-silica rhyolitic ignimbrites and post-caldera extrusions, which erupted 1.78 and 0.5-0.8 Ma, respectively. SiO2 content ranges from 66.27-71.89 wt% in the ignimbrites and 70.16-77.70 wt% in the post-caldera extrusions studied. Crystal content is primarily quartz and plagioclase, 0.5-2 mm in size, with other minerals. Values of δ18 O, δD, 87 Sr/86 Sr, and 144 Nd/143 Nd indicate little assimilation of crustal material, in contrast to modeling results. XRF analysis indicates a homogeneous magma. The rhyolite-MELTS program was used to model crystallization of a basaltic source with addition of amphibolite partial melt and hydrothermally-altered silicic rock to reproduce the observed compositions. This thesis contains both previously published and co-authored material. / Committee in charge: Dr. Ilya Bindeman， Chairperson； Dr. Paul Wallace， Member； Dr. Mark Reed， Member