京都大学 / 新制・課程博士 / 博士(情報学) / 甲第25434号 / 情博第872号 / 新制||情||146(附属図書館) / 京都大学大学院情報学研究科社会情報学専攻 / (主査)教授 多々納 裕一, 教授 矢守 克也, 教授 井口 正人 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DGAM

En comparaison aux éruptions basaltiques effusives, les éruptions basaltiques explosives sont relativement rares et leurs propriétés physiques sont mal comprises. L’objectif de cette thèse est d’étudier les causes à l’origine de l’explosivité des magmas basaltiques en caractérisant les produits volcaniques de l’éruption subplinienne trachybasaltique des Puys de La Vache-Lassolas (Chaîne des Puys, France) et des éruptions phréatomagmatiques et subpliniennes de magma basaltique tholéiitique du Grímsvötn (Islande). Les minéraux, verres matriciels et inclusions magmatiques (MI) ont été analysés à la microsonde électronique afin d’estimer les concentrations en éléments volatils (initiales et finales) et les pressions (P) et températures (T) de cristallisation, en utilisant la thermobarométrie basée sur l’équilibre clinopyroxène-liquide. Une étude expérimentale a été réalisée afin de mieux comprendre les limites de la thermobarométrie clinopyroxène-liquide à des pressions crustales et pendant l’ascension magmatique. Quatre éruptions de Grímsvötn ont été étudiées : deux phréatomagmatiques (celles de 1823 et 2004) et deux (sub-)pliniennes (1873 et 2011). Les éruptions subpliniennes ont des concentrations en éléments volatils plus élevées que celles des phréatomagmatiques. De plus, les premières sont également sujettes à une exsolution plus poussée du gaz dissous, ce qui se traduit par des émissions atmosphériques plus importantes. Quelle que soit l’explosivité d’une éruption du Grímsvötn, les équilibres clinopyroxène-liquide (verre matriciel ou MI) ont enregistré la même profondeur de cristallisation : ~15 km. Ceci suggère que la chambre magmatique superficielle du système magmatique du Grímsvötn puisse agir comme un réservoir dans lequel le gaz exsolvé d’un magma plus profond est stocké et accumulé entre les éruptions. La gamme de variabilité thermique des équilibres clinopyroxène-MI est plus restreinte pour les éruptions du 19ème siècle, ce qui suggère que le système magmatique du Grímsvötn était plus perméable en réponse à l’évènement volcano-tectonique du Laki (1783-84). La température moyenne des équilibres clinopyroxène-verre matriciel des quatre éruptions décroît avec le temps, suggérant que le système magmatique du Grímsvötn, dans son ensemble, est en cours de refroidissement. Les produits des Puys de La Vache-Lassolas ont enregistré une contribution magmatique plus profonde : vers 30 km. Les concentrations initiales en éléments volatils et leurs rapports sont plus proches de ceux des magmas d’arc que de tout autre contexte tectonique. Cette observation remet en question l’existence d’un point chaud sous la Chaîne des Puys. La contribution significative de magma d’origine profonde est en accord avec la cristallisation simultanée du clinopyroxène et de l’olivine. L’éruption des Puys de La Vache-Lassolas a émis 0,5 Tg de HF, 0,7 Tg de HCl et 7,3 Tg de SO2, illustrant le risque volcanique auquel est confrontée la population auvergnate. Afin de mieux contraindre les estimations pétrologiques de pression et température, une étude expérimentale au piston-cylindre a été réalisée. Elle porte sur la stabilité du clinopyroxène dans le magma tholéiitique de l’éruption 2014-15 d’Holuhraun (Islande). L’absence d’olivine dans la gamme de pression entre 0,5 et 1,0 GPa, associée à la présence de clinopyroxène indique que le clinopyroxène est la première phase au liquidus. Ceci confirme l’importance de la thermobarométrie clinopyroxène-liquide pour l’estimation des P et T de cristallisation des magmas tholéiitiques islandais. Les expériences au cours desquelles la pression a été diminuée de 1,0 à 0,5 GPa, puis maintenue constante, montrent que l’équilibre entre clinopyroxène et liquide n’est pas encore atteint après 24 heures à 0,5 GPa. Par conséquent, un temps de résidence de plusieurs jours à P et T constantes est nécessaire pour obtenir des estimations thermobarométriques fiables. / Explosive basaltic eruptions are rare compared to effusive ones and the underlying physical mechanisms are poorly understood. The goal of this thesis is to study the causes of basaltic magma explosivity by characterizing the volcanic products of the subplinian eruption of trachy-basalt at the Puys de La Vache-Lassolas complex (Chaîne des Puys, France) and of phreatomagmatic and sub-plinian eruptions of tholeiitic magma at Grímsvötn volcano (Iceland). Minerals, matrix glasses and melt inclusions (Mis) were analysed with an electron microprobe to estimate volatile concentrations (initial and final), and pressure (P) and temperature (T) of crystallization using clinopyroxene-liquid thermobarometry. In addition, an experimental study was performed to better understand the limitations of clinopyroxeneliquid thermobarometry at moderate pressures and during magma ascent. Four eruptions were studied at Grímsvötn: two phreatomagmatic (AD1823 and AD2004) and two subplinian (AD1873 and AD2011). The subplinian eruptions have volatile concentrations higher than the phreatomagmatic ones, together with more efficient gas exsolution, leading to significantly higher atmospheric mass loading. Regardless of the eruptive regime, clinopyroxene-liquid (matrix glass or MI) equilibria in Grímsvötn magmas record the same depth of crystallization, namely ~15 km. This suggests that the shallow magma chamber of Grímsvötn plumbing system may only act as a reservoir in which gas exsolved from deeper origin is stored and accumulates between eruptions. The lower variability of clinopyroxene-MI equilibrium T in the 19th century eruptions, suggests important Grímsvötn magma system permeability following the Laki volcano-tectonic event end of 18th century. Average matrix glass-clinopyroxene equilibrium T decreases with time suggesting that Grímsvötn magma plumbing system as a whole may be slowly cooling over the last two centuries. Puys de La Vache-Lassolas products record a deeper magma contribution, from approximately 30 km depth. Initial volatile concentrations and ratios of the ~8.6 ka eruption are closer to those of arc magmas than to magmas from any other tectonic settings. This observation calls into a question the Chaîne des Puys hotspot hypothesis. The significant contribution of deep-derived magma is coherent with the crystallization of clinopyroxene contemporaneously with olivine. The Puys de La Vache-Lassolas eruption carried 0.5 Tg of HF, 0.7 Tg of HCl and 7.3 Tg of SO2 into the atmosphere illustrating the volcanic hazard to the Auvergne population. In order to better constrain petrological P-T estimations, a piston-cylinder experimental study of clinopyroxene stability in a tholeiitic magma (Holuhraun 2014-15, Iceland) was undertaken. The absence of olivine in the pressure range 0.5-1 GPa and the presence of clinopyroxene indicate that clinopyroxene is the first phase at the liquidus, confirming the importance of clinopyroxene-liquid thermobarometers when estimating P and T of crystallization of Icelandic tholeiitic magmas. Experiments in which P was decreased from 1 to 0.5 GPa and then kept constant show that equilibrium between clinopyroxene and liquid is still not reached after 24 hours at 0.5 GPa. Thus a residence time of several days at constant P and T is required to obtain reliable thermobarometric estimates.

The transport and emplacement mechanisms of the highly energetic pyroclastic density current (PDC) generated in the blast style eruption of Soufriere Hills Volcano, Montserrat, on 26 December 1997 are examined through detailed lithological mapping and sedimentological analysis of the deposits. The PDC formed deposits which range in grain size from coarse breccias to fine ash, with distinctive bipartite layering and well-developed grading and stratification. On a large scale the PDC was highly erosive, sculpting large bedforms and depositing relatively thin deposits. However, locally, centimetre scale topographic protuberances were responsible for significant variations in deposit thickness, grain size, and the development of dune bedforms. The strong lateral and vertical lithofacies variations are attributed to well-developed density stratification, which formed during explosive expansion of the dome prior to PDC formation. Experimental modelling of stratified inertial gravity currents was carried out to investigate the effects of density stratification prior to release of the current. The degree of stratification governs the rate of mixing in the current, which in turn influences the velocity. Well·stratified currents initially move faster than homogenous currents but are slower in the latter stages of current propagation. The results have important implications for deposition from particle-laden flows, which may become stratified with coarser material concentrated at the base of the current. The role of PDCs jn the formation of unit US2-B, emplaced during the Upper Scoriae 2 eruption (79± 8 ka) on Santorini, Greece, was investigated through sedimentological analysis and mapping. Proximally, the unit exhibits features characteristic of emplacement from a flow, such as thickening into palaeochannels and erosive basal contacts. Distally, the unit is of uniform thickness and grain size parameters suggest the deposit is more characteristic of exnplacement from a fallout mechanism. Discrete lenses of fine-grained material within US2-B, and a gradational upper contact with PDC deposits suggest that there may have been contemporaneous deposition resulting the development of a hybrid deposit.

Large explosive volcanic eruptions inject SO2 into the stratosphere where it is oxidised to sulphate aerosols which reflect sunlight. This causes a reduction in global temperature and precipitation lasting a few years. Here the robust features of this precipitation response are investigated, using superposed epoch analysis that combines results from multiple eruptions. The precipitation response is first analysed using the climate model HadCM3 compared to two gauge based land precipitation datasets. The analysis is then extended to a large suite of state-of-the art climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). This is the first multi-model study focusing on the precipitation response to volcanoes. The large ensemble allows analysis of a short satellite based dataset which includes ocean coverage. Finally the response of major world rivers to eruptions is examined using historical records. Whilst previous studies focus on the response of just a few rivers or global discharge to single eruptions, here the response of 50 major world rivers is averaged across multiple eruptions. Results are applicable in predicting the precipitation response to future eruptions and to geoengineering schemes that seek to counteract global warming through reducing incoming solar radiation. The main model-simulated features of the precipitation response include a significant global drying over both land and ocean, which is dominated by the wet tropical regions, whilst the dry tropical ocean regions get significantly wetter following eruptions. Monsoon rainfall decreases, whilst in response to individual eruptions the Intertropical Convergence Zone shifts away from the hemisphere with the greater concentration of volcanic aerosols. The ocean precipitation response is longer lived than that over land and correlates with near surface air temperature, whilst the land response correlates with aerosol optical depth and a reduction in land-ocean temperature gradient Many of these modelled features are also seen in observational data, including the decrease in global mean and wet tropical regions precipitation over land and the increase of precipitation over dry tropical ocean regions, all of which are significant in the boreal cold season. The land precipitation response features were robust to choice of dataset. Removing the influence of the El Nino Southern Oscillation (ENSO) reduces the magnitude of the volcanic response, as several recent eruptions coincided with El Nino events. However, results generally remain significant after subtraction of ENSO, at least in the cold season. Over ocean, observed results only match model expectations in the cold season, whilst data are noisy in the warm season. Results are too noisy in both seasons to confirm whether a long ocean precipitation response occurs. Spatial patterns of precipitation response agree well between observational datasets, including a decrease in precipitation over most monsoon regions. A positive North Atlantic Oscillation-like precipitation response can be seen in all datasets in boreal winter, but this is not captured by the models. A detection analysis is performed that builds on previous detection studies by focusing specifically on the influence of volcanoes. The influence of volcanism on precipitation is detectable using all three observational datasets in boreal winter, including for the first time in a dataset with ocean coverage, and marginally detectable in summer. However, the models underestimate the size of the winter response, with the discrepancy originating in the wet tropics. Finally, the number of major rivers that undergo a significant change in discharge following eruptions is slightly higher than expected by chance, including decreased flow in the Amazon, Congo, Nile, Orange, Ob and Yenisey. This proportion increases when only large or less humanly influenced basins are considered. Results are clearer when neighbouring basins are combined that undergo the same sign of CMIP5 simulated precipitation response. In this way a significant reduction in flow is detected for northern South American, central African and less robustly for high-latitude Asian rivers, along with a significant increase for southern South American and SW North American rivers, as expected from the model simulated precipitation response.

'Natural' disasters have traditionally been viewed as the result of an extreme physical environment. A radical backlash against this dominant view, in the nineteen seventies and eighties, moved the debate to the opposite extreme and in doing so replaced physical with social determinism. Vulnerability analysis is proposed as a methodology that bridges these extremes. It takes into account individual decision making, social milieu and physical hazard when describing human habitation in areas of volcanic activity. It is argued that vulnerability should be defined in terms of universal human needs in order to avoid it simply being a measure of the chance of death and injury or losing its meaning in the uncertainty of cultural relativism. Once vulnerability is identified it is important to explore why it has come to exist. A contextual theory of vulnerability change is presented. Vulnerability to volcanic activity was explored in the area around Mt. Etna in Sicily (Italy) and Furnas volcano San Miguel in the Azores (Portugal) using a case study methodology. This included: collecting data through interviews (semistructured and structured) and field surveying, utilising census and other secondary data sources, and examining historical documents and texts. The volcanic hazard on Mt. Etna is related to regular (4-7 years) effusive lava flows which threaten property and land rather than people. Living in a European state, it is likely that a victim of Mt. Etna will have their basic needs provided for in the long-term and therefore they are not vulnerable. In contrast the irregular explosive eruptions of Furnas, last eruption 1630, not only damage property and land but also endanger lives. The limited ability of individuals to protect themselves in the event of an eruption and organisations to aid them in this means that, in spite of state insurance, many around Furnas are vulnerable. The production of vulnerability around Etna and Furnas is strongly related to the socio-economic nature of the region and wider European and global contexts. Opportunities and constraints that exist across socio-physical space encourage behaviour and forms of life which, in tum, produce various levels of vulnerability. Individuals seem to cognitively diminish their perceptions of this threat within a context of social representations of low risk. They, and society as a whole, rarely seem to engage directly with the risk itself.

No description available.

The composition of the atmosphere has changed substantially over Earth's history, with important implications for past climate. A number of case studies will be presented which employ coupled climate model simulations to assess the strength of these chemical feedbacks on the climate. The eruption of Mount Samalas in 1257 led to the largest stratospheric volcanic injection of aerosol precursor gases in the Common Era, however climate model simulations of the last millennium typically overestimate the resulting climatic cooling when compared with tree-ring proxy records. A novel configuration of the Met Office UM-UKCA climate model is presented which couples an atmosphere-ocean general circulation model to a rigorous treatment of the relevant atmospheric chemistry and microphysical aerosol processes. This permits the climate response to a particular stratospheric injection of reactive volatile gases to be quantified and for the first time to date applied to a historical volcanic eruption. This model configuration compares favourably to observational data for simulations of the 1991Mount Pinatubo eruption. Results from an ensemble of model simulations are presented, with different assumptions about the sulfur dioxide and halogen loadings based on a recent geochemical reconstruction. These show a muted climate response, in reasonable agreement with tree ring records. Emissions of halogenated compounds lead to an increase in the sulfur dioxide lifetime, widespread ozone depletion and a prolonged climatic cooling. Strong increases in incident ultraviolet radiation at Earth's surface also occur. Oxygen levels may have varied fromas little as 10% to as high as 35% in the Phanerozoic (541Ma - Present). An increase in atmospheric oxygen increases atmospheric mass which leads to a reduction in incident shortwave radiation at Earth's surface due to Rayleigh scattering. However, this is offset by an increase in the pressure broadening of greenhouse gas absorption lines. Dynamical feedbacks also lead to increased meridional heat transport, warming polar regions and cooling tropical regions. An increase in oxygen content using the HadCM3-BL and HadGEM3-AO climate models leads to a global mean surface air temperature increase for a pre-industrial Holocene base case, in agreement with idealised 1D and 2D modeling studies. Case studies from past climates are investigated using HadCM3-BL which show that in the warmest climates, increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Maastrichtian (72.1 - 66.0Ma), increasing oxygen content leads to a better agreement with proxy reconstructions of surface temperature at that time irrespective of the carbon dioxide content. There is considerable uncertainty in the timing of the rise in atmospheric oxygen content from values around 1% in the Neoproterozoic (1000 Ma - 541 Ma) to the 10- 35% values inferred in the Phanerozoic with respect to two global glaciation episodes (717-635Ma). Results of simulations with HadCM3-BL which investigate the impact of oxygen content on the Neoproterozoic Snowball Earth glaciations are presented. These demonstrate that a smaller reduction in carbon dioxide content is required to initiate a Snowball Earth at low oxygen content. Geological evidence suggests the presence of a basaltic large igneous province before the Sturtian Snowball Earth episode. This could have caused episodes of paced explosive volcanism, injecting sulfate aerosol precursors into the stratosphere. Results of simulations to investigate the impact of different volcanic aerosol emission scenarios are presented. 500 Tg SO2 is investigated with a range of aerosol sizes. For aerosol size distributions consistent with the aerosol evolution in the aftermath of the Mount Pinatubo eruption, the Earth enters a Snowball Earth in between 30 and 80 years. Using a larger size of aerosols, consistent with a larger eruption, does not lead to a Snowball Earth. These simulations show that changes to the chemical composition of the atmosphere, whether reactive gases or bulk chemical composition may have played an important role in the past climate of Earth.

How people pereceive the risks associated with natural hazards contributes to their willingness to take protective action. Such action may be constrained by prevailing socio-economic and place-specific conditions that restrict or inform the choice of protective measures available to the individual. Vulnerability to the impacts of extreme geophysical events increases when the range of alternatives is limited or misinformed. Many evacuees from a potentially violent volcanic eruption in Ecuador returned to their home town of Banos while it was still under an evacuation order in 2000 and considered to be a high risk area by officials. The research examined four main questions: (1) What economic conditions confronted Baños evacuees? (2) What political or other social events occurred while they were evacuated that limited their perceived range of options? (3) What information was available about prior eruptions of the volcano and other local natural hazards? and (4) What were the characteristics of the economic base of Baños? These questions were investigated using data from interviews with evacuees, government and non-governmental officials, census and other statistical information, scholarly texts and newspaper reports. The research suggests that economic conditions made it extremely difficult for people to relocate to other communities. When a violent eruption did not occur immediately, and few direct impacts of the eruptions were experienced in Baños, many people chose to return home in an effort to reestablish themselves economically. These people perceived the volcano hazard in Baños to be far less threatening than the economic destitution associated with evacuation. This perception may have been influenced by factors other than the socio-economic context, including efforts of political leaders and tourist business owners to effect the town's economic recovery. These efforts included an aggressive publicity campaign that minimized the risk posed by the volcano. Besides encouraging tourists to return, the campaign also encouraged evacuees to do so. Finally, among some residents, religious beliefs may have contributed to perceptions that they would not be harmed in the event of an explosive eruption.

Andesitic volcanoes are notorious for their rapid and unpredictable changes in eruptive style between and during volcanic events, a feature normally attributed to shallow crustal and intra-edifice magmatic processes. Using the example of eruptions during the last 1000 yrs at Mt. Taranaki (the Maero Eruptive Period), deposit sequences were studied to (1) understand lava dome formation and destruction, (2) interpret the causes of rapid shifts from extrusive to explosive eruption styles, and (3) to build a model of crustal magmatic processes that impact on eruption style. A new detailed reconstruction of this period identifies at least 10 eruptive episodes characterised by extrusive, lava dome- and lava flow-producing events and one sub- Plinian eruption. To achieve this, a new evaluation procedure was developed to purge glass datasets of contaminated mineral-glass analyses by using compositional diagrams of mineral incompatible-compatible elements. Along with careful examination of particle textures, this procedure can be broadly applied to build a higher degree of resolution in any tephrostratigraphic record. Geochemical contrasts show that the products of the latest Mt. Taranaki eruption, the remnant summit dome (Pyramid Dome) was not formed during the Tahurangi eruptive episode but extruded post-AD1755. Its inferred original maximum volume of 4.9×106 m3 (DRE) was formed by simultaneous endogenous and exogenous dome growth within days. Magma ascent and extrusion rates are estimated at =0.012 ms-1 and =6 m3s-1, respectively, based on hornblende textures. Some of the Maero-Period dome effusions were preceded by a vent-clearing phase producing layers of scattered lithic lapilli around the edifice [Newall Ash (a), Mangahume Lapilli, Pyramid Lapilli]. The type of dome failure controlled successive eruptive phases in most instances. The destruction of a pressurised dome either caused instantaneous but short-lived magmatic fragmentation (Newall and Puniho episodes), or triggered a directed blast-explosion (Newall episode), or initiated sustained magmatic fragmentation (Burrell Episode). The transition from dome effusion to a sustained, sub- Plinian eruption during the Burrell Lapilli (AD1655) episode was caused by unroofing a conduit of stalled magma, vertically segregated into three layers with different degrees of vesiculation and crystallisation. The resultant ejecta range from brown, grey and black coloured vesicular clasts to dense grey lithics. Bulk compositional variation of erupted clasts can be modelled by fractionation of hornblende, plagioclase, clinopyroxene, and Fe-Ti oxides. Pre-eruption magma ascent for the Maero Period events is assumed to begin at depths of c.9.5 km.

About 7,000 years ago two major tephra-falls blanketed the Pacific Northwest in volcanic ash. These two tephra-falls, identified as the Llao and climatic tephra-falls, were a part of the eruptive events that led up to the collapse of Mount Mazama to form Crater Lake in the southern Oregon Cascades. The tephra-falls occurred about 200 years apart at around 7,000 years B.P. and 6,800 years B.P. for the Llao and climatic eruptions respectively. The effects of the tephra-falls on the flora, fauna, and people of the period have been characterized by different researchers as ranging from minimal to catastrophic. In an attempt to better understand the affects of these two events on the flora, fauna, and people, a model is presented to help organize the various lines of research into a coherent whole and to suggest profitable areas of research which have not yet been completed. The model is based on ecological and anthropological theory with a strong reliance on analogy with modern ecosystems and volcanic hazards research. The model makes use of the ecosystem concept as the framework for the interaction of the abiotic, or nonliving habitat, with the biotic, or living system. The biotic organisms are adapted to the characteristics of the abiotic habitat and in many respects the composition, frequencies, and distributions of biotic organisms are determined by their tolorance levels to these characteristics. Tephra-falls act as environmental disturbances which change the abiotic habitat of the ecosystem. Therefore, any changes caused by such a disturbance in the abiotic characteristics that are not optimal or are outside of the tolorance limits of the biotic (flora and fauna) components should cause changes in the composition, distribution, and frequency of organisms within the ecosystem. The changes brought about by the tephra-falls may be described by successional and evolutionary processes through analysis of pollen and faunal remains, population demography as described by mortality profiles, and research into the reaction of specific flora and fauna within adaptational types to the properties of tephra-falls and the tephra as a soil body. The state factors used to describe the abiotic component of the ecosystem are: time, distribution, material properties, climate, and geomorphology. The state factor of time involves the determination of the occurrence in time of the event(s), the duration of the event(s), the season of occurrence of the event(s), and the residence time of tephra in the ecosystem. This state factor is used to define the specific point in time and duration of the effects of the tephra-fall(s) for individual ecosystems. The state factor of distribution describes the aerial extent and thickness of the air-fall deposits. This state factor determines the extent of the initial disturbance. The state factor of climate describes the specific components of rainfall, wind, and temperature which control ecosystem composition and development, and the changes to the climate which may have occurred due to volcanic aerosols associated with the eruption. The state factor of geomorphology describes the location of tephra and nontephra bodies across the landscape and through time as the tephra is reworked by wind, water, and gravity from the initial air-fall positions. The determination of the long term distribution of the tephra is important in determining post-event influences on ecosystems as described by the material properties of the tephra. It is argued that most people were not greatly harmed by the Mazama tephra-fall events themselves, but instead may have been greatly affected by a loss of food resources during and after the events. Changes in food resource availability and exploitation locations due to the tephra-falls may have resulted in changes in both settlement and subsistence activities. Changes in settlement and subsistence activities may be seen in a corresponding change in differential frequencies of functional tool types across space and time. The kind and amount of expected changes in settlement and subsistence systems are linked to distance from the source of the tephra, the stability and compostion of pre-disturbance ecosystems, the types and intensity of resource exploitation, and the amount of variability in subsistence and settlement traits which were available to the sociocultural system. / Graduation date: 1988