Dissertations / Theses on the topic 'Pyroclastic flow'

A series of analogue flume experiments are used to investigate initiation, flow and deposition of static piles of polymict materials, the sorting during transport, and the three dimensional geometry of the resulting deposits. Sequential charges are used to investigate the effects and extent of reworking. The particle heterogeneity is designed to simulate typical PDC make-up, with analogues for juvenile pumice and lithic clasts, as well as the fine-grained pumiceous material which makes up the bulk of the flow. Analogue flume experiments are used to investigate the generation of complex facies variations typical of pyroclastic density current (PDC) deposits. Polymict charges are developed to behave as analogues for the particle size and density contrasts present in PDC (i.e. lithic and juvenile pumice clasts), and investigate the effect of granular sorting during flow on the geometry of deposit architectures. Multiple charges are used to simulate pulses or sequences of separate PDC in order to assess the extent and effects of reworking. 3D visualisation of the resulting deposits reveals stratigraphies analogous to those seen in PDC, including pumice ‘rafting' or over-passing and inverse grading of pumice, and normal grading of lithics by simple gravitational granular sorting. Reworking between differentially-coloured layers makes several complex shear-derived Kelvin-Helmholtz instability features apparent, from fully developed rotational eddies, to less developed recumbent flame structures. The implications for the formation of these in PDC are assessed, including the potential influences on temperature proxy data, radiogenic dating by included phenocrysts (40Ar/39Ar) or charcoals (14C), calculation of eruptive volumes, sedimentation rates and flow velocity.

Radar bright deposits on Venus that have diffuse margins suggest eruptions that distribute debris over large areas due to ground-hugging flows from plume collapse. We examine deposits in eastern Eistla, western Eistla, Phoebe, and Dione Regiones using Magellan data and Earth-based radar maps. The radar bright units have no marginal lobes or other features consistent with viscous flow. Their morphology, radar echo strength, polarization properties, and microwave emissivity are consistent with mantling deposits composed of few centimeters or larger clasts. This debris traveled downhill up to similar to 100km on modest slopes and blanketed lava flows and tectonic features to depths of tens of centimeters to a few meters over areas up to 40x10(3)km(2). There is evidence for ongoing removal and exhumation of previously buried terrain. A newly identified occurrence is associated with a ridge belt south of Ushas Mons. We also note radar bright streaks of coarse material west of Rona Chasma that reflect the last traces of a deposit mobilized by winds from the formation of Mirabeau crater. If the radar bright units originate by the collapse of eruption columns, with coarse fragmental material entrained and fluidized by hot gases, then their extent suggests large erupted volatile (CO2 or H2O) amounts. We propose that these deposits reflect the early stage of renewed magmatic activity, with volatile-rich, disrupted magma escaping through vents in fractured regions of the upper crust. Rapidly eroding under Venus surface conditions or buried by subsequent eruptions, these markers of recently renewed activity have disappeared from older regions.

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.

The thinning trend of a low-aspect ratio ignimbrite (LARI) in a direction of increasing topographic relief at the Pululagua Volcanic Complex, Ecuador, is established by correlating continuous ground penetrating radar (GPR) profiles and radar reflector behavior with stratigraphic measurements and unit behavior. Minimum density-current and vertical (cross-sectional) velocity analyses of the LARIs parent pyroclastic density-current are performed by analyzing the exchange of kinetic energy for potential energy in an upslope direction. Continuous GPR profiles were acquired in a direction of increasing topographic relief with the intent of identifying the LARI within the GPR record and examining the relationships between the LARI and the underlying paleo-topographical surface. Stratigraphic measurements recorded throughout the field area demonstrate that the LARI thins 7.5 m in an upslope direction (over 480 m distance and 95 m elevation). Stratigraphic measurements enable correlations with GPR profiles, resulting in LARI identification. By utilizing GPR derived paleo-topographical surface elevations, minimum flow velocities of the LARI-producing parent pyroclastic density-current at the base of upslope flow are shown to be at least 25 m/s. Vertical velocity analyses based on the identification of internal GPR reflectors, interpreted as flow streamlines, yield pyroclastic surge-like cross-sectional velocity profiles of the LARIs parent density-current. Maximum density-current velocities at the base of upslope flow reach 24 m/s and diminish toward the base of the current.

Les écoulements pyroclastiques sont des écoulements volcaniques complexes dont le comportement physique fait encore l'objet de débats. Ils sont composés de deux parties : l'écoulement dense basal, riche en particules et en blocs, surmonté par la déferlante, diluée et turbulente. Les interactions entre ces deux parties ne sont pas bien comprises, tout comme leurs échanges de masses et de quantités de mouvement. Partant de ce constat, cette thèse se concentre sur l’étude des mécanismes de formation de la déferlante à partir de l’écoulement dense.Les expériences mettent en évidence un mécanisme de formation d'un écoulement dilué par l’alternance d’incorporation d'air et d’élutriation des particules fines d’un lit granulaire dense soumis à des vibrations. L'air est aspiré dans le lit granulaire pendant les phases de dilatation puis expulsé pendant les phases de contraction. Une partie des particules est alors soutenue par l'air turbulent expulsé et forme un mélange de gaz et de particules qui, plus dense que l’air, se transforme en un écoulement de gravité. Extrapolé à l’échelle d’un volcan, ce mécanisme d’incorporation d’air et d’élutriation peut être reproduit par une topographie rugueuse, où chaque obstacle génère une compaction puis une dilation de l’écoulement dense. La quantification du mécanisme a été effectuée et l’approche expérimentale a permis d’aboutir à une loi reliant le flux de masse de la partie dense vers la déferlante à la vitesse de l’écoulement dense. Le modèle numérique est utilisé dans un premier temps pour étudier la rhéologie de l’écoulement dense qui, en contrôlant sa vitesse, contrôle le flux de masse précédemment évoqué. Un chapitre est consacré à l’effet de la fluidisation de l’écoulement dense sur sa rhéologie. Les résultats montrent que la fluidisation par les gaz est capable d’expliquer à la fois la grande mobilité de ces écoulements, ainsi que la formation des morphologies terminales en lobes et chenaux. L’ingestion d’air dans un écoulement au cours de sa mise en place semble pouvoir expliquer une partie de la dynamique des écoulements denses. Des rhéologies simples, de premier ordre, ont également été analysées : la rhéologie de Coulomb, la rhéologie plastique, et la rhéologie à coefficient de frottement variable. Les résultats montrent que la rhéologie plastique semble la mieux adaptée pour reproduire la vitesse et l’extension des écoulements denses.Ce modèle numérique a ensuite été utilisé pour tester la loi de flux de masse obtenue suite aux expériences de laboratoire. Appliqués à l’effondrement de dôme du 25 juin 1997 à la Soufriere Hills de Montserrat, les résultats montrent que les simulations reproduisent des dépôts de déferlantes dont l’épaisseur et l’extension sont tout à fait réalistes. Les simulations reproduisent même les écoulements denses secondaires issus de la sédimentation de la déferlante puis de la remobilisation des dépôts. Les cycles d’ingestion/expulsion d’air dans l’écoulement dense, par interaction avec la topographie, expliqueraient donc à la fois la grande fluidité des écoulements denses et la formation des déferlantes pyroclastiques. Les résultats de cette thèse mettent à jour un mécanisme nouveau qui pourrait être la clé de la mise en place des écoulements pyroclastiques et pourrait permettre d’améliorer la prévision future des risques et des menaces par modélisation numérique
Small volume pyroclastic density currents are complex volcanic flows, whose physical behaviour is still debated. They comprise two parts: the pyroclastic flow, rich in particles and blocks, overridden by the ash-cloud surge, a turbulent and dilute flow. The interactions between these two parts are not fully understood, as well as their exchanges of mass and momentum. Therefore, the thesis focuses on the investigation of ash-cloud surge formation mechanisms from the pyroclastic flow. The experiments reveal a mechanism of dilute flow formation by alternation of air incorporation into and elutriation of fine particles from a dense granular bed subjected to vibrations. The air is aspirated into the granular bed during dilatations, and expulsed during the contraction phases. A part of the particles are then sustained by the turbulent expulsed air and form a mixture of gas and particles that transforms into a gravity current. Extrapolated to a volcanic edifice, this mechanism of air incorporation and elutriation can be reproduced by a rough topography, where each obstacle generates a compaction followed by a dilatation of the pyroclastic flow. The quantification of the mechanism has been accomplished and the mass flux from the dense flow to the ash-cloud surge has been deduced.The numerical model is first used to study the pyroclastic flow rheology, which controls the velocity of the flow, and then the mass flux previously mentioned. One chapter is dedicated to the fluidization effect on the pyroclastic flow rheology. Results show that this mechanism can explain the long runout of these flows, and also the formation of levées and channel morphologies. The air ingestion in the flow during its movement could explain a part of the pyroclastic flows dynamic. Simple rheologies has also been analyzed: a Coulomb rheology, a plastic rheology, and a variable friction coefficient rheology. Results show that the plastic rheology seems to be the most adapted rheology to simulate the pyroclastic flow dynamic. Then, the numerical model has been used to test the mass flow law obtained through experiments. Applied to the 25 June 1997 dome collapse at Soufrière Hills Volcano at Montserrat, results show that the simulations reproduce accurately the extension and the thickness of the surge deposits. The simulations are also able to reproduce the surge derived pyroclastic flow, generated by remobilisation of surge deposits. The cycles of ingestion/expulsion of air in the pyroclastic flow by interactions with the topography could explain both the great fluidity of these flows and the formation of ash-cloud surge. These results highlight a new mechanism that could be a key process in pyroclastic flow dynamic, which could improve significantly the hazard and risk assessment using numerical model

North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre and represents one of at least eleven vents which have been active on Tongariro since the last glacial maximum. The most recent cone-forming activity at North Crater is thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped andesite cone. This project focused mainly on the cone-building eruptions at North Crater, including stratigraphic correlations with distal tephra, interpreting eruptive processes, and establishing the sequence of events during cone construction. Detailed field work identified key stratigraphic sections and facies in the proximal, medial and distal environments. These sections allowed stratigraphic correlations to be made between proximal cone-building facies and distal sheet-forming facies at North Crater, and established a complete North Crater eruption stratigraphy. In the proximal environment, welded and non to poorly welded facies formed from fallout of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the medial and distal environment, the lithofacies consist of fallout from a convecting plume and minor pyroclastic flow. Convective fall and non to poorly welded pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting explosive eruptions involving a gas-rich magma. A change to welded deposits produced from lava-fountaining occurs later in the cone-building sequence and suggest a change to lower explosively and eruption of gas-poor magma. Grain size, componentry data, density, petrography and SEM analysis were carried out on representative samples to characterise the different facies, and reveal information about eruption processes. The non to poorly welded deposits are typically made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and lapilli. The welded facies are relatively dense and clast outlines are often difficult to distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and indicate crustal assimilation. Mingled clasts of light and dark glass were found to have microlites present in the dark glass, but were absent in the light glass. Electron microprobe analyses found that the dark and light glass components in a single clast had similar compositions, showing that the contrasting physical appearance of the glass is not due to a different chemical composition. Forty three whole rock XRF analyses showed that the magmas ranged from basaltic andesite to andesite, and Harker variation plots display linear trends typical of magma mixing. Magma mixing as the most important magmatic process is supported by disequilibrium of phenocryst compositions and phenocryst textures. Magma viscosity, bulk density and temperature was determined using MAGMA (Kware), and indicate that they fall into the range of typical andesites. Eruptive activity involved vigorous lava-fountaining, minor convecting eruption plumes and dominant collapsing eruption plumes. This activity has produced welded and non-welded pyroclastic flow and fall deposits to form the large cone seen today. There are significant volcanic hazards associated with this style of activity at North Crater, characterised by lava-fountaining, eruption plume fallout, and widespread pyroclastic flows and lahars extending beyond the ring plain. These could all be potentially devastating to the central North Island of New Zealand.

The latest eruptive episode of Merapi volcano, Java, Indonesia, produced block-and-ash flows (BAFs) that affected the densely populated areas on the volcano's southern flank. The flows were not confined to the existing valleys but spilled over the valley sides to create overbank flows that resulted in fatalities and strong damages in the village of Kaliadem, -5 km away from the summit. This testifies the unpredictable behaviour of BAFs, not only at this volcano but also at similar locations around the world, and the need for an improved understanding of the flow transport and depositional mechanisms. In this respect, the new approaches, descriptive schemes and numerical simulations presented in this thesis address some of the key concepts of the dynamics and hazards of BAFs. The 2006 deposits were examined both immediately after flow emplacement and after the first rainy season following the eruption, allowing detailed correlations between their surface characteristics and internal architecture. Two main types of BAFs (short- to mediumrunout and long-runout BAFs) are recognized based on parameters including generation mechanism, flow volume, travel distance, deposit morphology, distribution, lithology and grain size distribution. The effects of topography on flow dynamics have been examined through the development of conceptual models for the two types of BAFs. Integration of high-resolution field-based data into numerical simulations using the Titan2D and VolcFlow models allows the validity of these models to be tested and rapid quantification of best-fit input parameters. Sensitivity analyses and inundation maps based on the probability of impact were used to produce a suite of potentially inundated areas from future collapse events affecting the Gendol valley and adjacent areas on the southern flank. Results provide the basis for defining hazard zonations of key areas at risk from BAFs at Merapi

Die Sedimente des Halleschen Permokarbonkomplexes gaben schon immer Raum für Spekulationen. Aufgrund ihrer Dominanz an rhyolithischen Geröllen wurden sie über einen langen Zeitraum einheitlich als Postporphyrschutt ausgehalten. Vielfältig wechselnde Faziesbedingungen machten es jedoch notwendig, die Sedimente aufzugliedern. Neuere Erkenntnisse in der Erforschung des Halleschen Permokarbonkomplexes erfordern eine Überprüfung v. a. der nach KUNERT (1995) aufgestellten allgemeinen stratigraphischen Gliederung der Unterrotliegendsedimente in Halle,- Hornburg,- Sennewitz- und Brachwitz-Formation anhand einiger ausgewählter Beispiele. Der ursprüngliche Gedanke der Diplomarbeit bestand darin, eine Fazies- und eine Geröllanalyse der unterpermischen Abtragungsprodukte des Halle-Vulkanitkomplexes anzufertigen. Zur Verfügung standen zwei Kernbohrungen und zwei Aufschlüsse, sowie diverse Unterlagen zu angrenzenden Bohrungen in der Saale-Senke. Die beiden Oberflächenaufschlüsse Riveufer und Teichgrund sollten stratigraphisch aufgenommen werden, so dass eine Fazieszuordnung möglich ist. Die Bohrung Brachwitz 2/62 wurde mit dem Ziel aufgenommen, neuere Theorien über den Ablagerungszeitraum der Rotliegend-Sedimente in Bezug auf den permokarbonen Vulkanismus zu widerlegen oder zu bekräftigen. Die zweite Bohrung (Kb Lochau 7/65) wurde am Rande mit in die Diplomarbeit einbezogen, da sie das immense Spektrum der spätvulkanischen Aktivitäten im Halle Permokarbonkomplex erweitert. Ergebnis ist eine Neugliederung des Rotliegend im Halleschen Permokarbonkomplex, in der nur noch die Halle-Formation mit ihrem ausgeprägten Vulkanismus und die Hornburg-Formation, stellvertretend für alle jüngeren Abtragungsprodukte des Halle Vulkanitkomplexes, unterschieden werden. Mit einem großen Hiatus folgt anschließend die Eisleben-Formation.

Les écoulements pyroclastiques sont des mélanges à haute température de gaz et de particules volcaniques qui peuvent se propager sur de très grandes distances. Cette forte « mobilité » est souvent attribuée à leur capacité à se fluidiser, c’est-à-dire à générer et conserver une forte pression interstitielle de gaz qui réduit les forces de friction interne. L’objectif principal de cette thèse est de comprendre comment les irrégularités des terrains sur lesquelles se propagent les écoulements pyroclastiques peuvent favoriser leur fluidisation. Une première série d’expériences de laboratoire a consisté à générer des écoulements de particules fines (diamètre de 45-90 μm) sur des substrats de différentes rugosités. Les résultats montrent que la distance de parcours des écoulements augmente avec la rugosité, allant jusqu’à doubler par rapport à la distance de parcours sur fond lisse. Des analyses de vidéos haute vitesse et des mesures de pression interstitielle d’air à la base des écoulements montrent que la tête (partie antérieure) des écoulements qui se propagent sur un substrat rugueux s’auto-fluidisent en conséquence de la sédimentation des particules dans les interstices du substrat, chassant l’air qui remonte et percole dans l’écoulement. Ce mécanisme d’auto-fluidisation est efficace pour toutes les inclinaisons étudiées (0-30°), suggérant qu’il est susceptible de se produire tout au long de la mise en place d’un écoulement pyroclastique. Une seconde étude a consisté à faire chuter des lits de particules dans une colonne statique. Les résultats montrent que même pour une hauteur de relâchement relativement faible (20 cm), le mélange peut entièrement s’auto-fluidiser durant sa chute. Quand les particules sont suffisamment fines (<100 μm) la pression interstitielle dans le dépôt diffuse pendant plusieurs secondes, la durée de cette diffusion augmentant avec l’augmentation de l’épaisseur du lit et la diminution de taille des particules. Les temps de diffusions les plus longs sont observés avec un matériau provenant d’un dépôt d’écoulement pyroclastique (~30 s pour des lits de 28.5 cm d’épaisseur). Ces résultats suggèrent que les écoulements pyroclastiques qui se propagent sur des terrains accidentés peuvent s’auto-fluidiser et conserver une faible friction au cours de leur mise en place
Pyroclastic flows are hot mixtures of gas and particles that can propagate over large distances. This high “mobility” is often attributed to their ability to be fluidized, that is, to generate and retain high gas pore pressure that reduces internal friction forces. The main objective of this thesis is to understand how irregularities of substrates on which pyroclastic flows propagate can enhance their fluidization. A first set of laboratory experiments consisted of the generation of fine-grained flows (diameter of 45-90 μm) on substrate of various roughness. Results show that the flow runout distance increases with the substrate roughness, and is up to twice the runout on a smooth substrate. High speed video analyses and air pore pressure measurements at the flow base show that the flow head propagating over a rough substrate can auto-fluidize because of particles sedimentation into the substrate interstices, which forces the air to escape upward and percolate through the flow. This auto-fluidization mechanism is efficient at all inclinations investigated (0-30°), suggesting that it could occur during the whole emplacement of a pyroclastic flow. A second study consisted of the vertical release of beds of particles in a static column. Results show that the granular mixture can be fully fluidized, even when collapsing from a relatively low height (20 cm). When particles are fine enough (<100 μm), pore pressure in the deposit diffuses for several seconds, the diffusion duration increasing with increasing bed thickness and decreasing particle size. The longest diffusion durations are observed for pyroclastic flow deposit materials (~30 s for 28.5 cm thick beds). These results suggest that pyroclastic flows propagating on irregular terrains can auto-fluidize and preserve low internal friction during their emplacement

>Magister Scientiae - MSc
The Saldania Belt in southern Africa, a product of the Pan-African Saldanian Orogeny, forms part of a system of Neoproterozoic mobile belts that border and weld older cratons on the African continent. It is a low-grade orogenic belt situated along the southwestern margin of the Kalahari Craton and is composed of several inliers of greenschist facies metasedimentary and metavolcanic rocks (Malmesbury Group), unroofed in megaanticlinal hinges of the Permo-Triassic Cape Fold Belt. The Malmesbury Group rocks were syn- and post-tectonically intruded in a pervasive transpressive regime between 555 Ma and 515 Ma by Neoproterozoic to early Cambrian S-, I- and A-type granites, monzodiorites, gabbros and quartz syenites, which collectively constitute the rocks of the Cape Granite Suite (CGS). Along the south-western coastline of South Africa, the Saldanha Bay Volcanic Complex (which forms part of the CGS) is divided into two eruption centres both of which have been identified as “intra-caldera pyroclastic ignimbrites”. The Postberg eruption centre is situated to the south of the Saldanha Bay entrance and the Saldanha eruption centre is situated to the north of the entrance. Both eruption centres display distinct geochemical signatures, the most apparent being the greater TiO2 concentrations (> 0.25 wt. %) of the Saldanha centre ignimbrites when compared to its Postberg centre counterparts. The Postberg eruption centre consists of S-type rhyolitic ignimbrites which are subdivided into the two geochemically distinct Plankiesbaai and Tsaarsbank Ignimbrites. Small amounts of the Jacobs Bay and Saldanha Ignimbrites (less felsic tephra from the Saldanha eruption centre) are also present in the Postberg eruption centre. A robust geochemical analysis of both the Plankiesbaai and Tsaarsbank magma groups display high SiO2 content (>76 wt. %), a lack of variation in TiO2 and Zr, high Al2O3 and ASI (aluminium saturation index) values (> 1.0 and generally >1.1 which, on average, is higher than the Saldanha eruption centre ignimbrites), low CaO and Na2O, and a highly ferroan character. The Plankiesbaai ignimbrite also display lower #Mg concentration compared to the Tsaarsbank ignimbrite. Typical geochemical trends in the Postberg eruption centre include the lack of variation in Zr content, higher Rb content and lower Sr, Ba, V and Zn concentrations when compared to the tephra of the Saldanha eruption centre found in the Postberg area.

Les écoulements (ou coulées) pyroclastiques (PFs) sont des mélanges concentrés de gaz et de particules à haute température qui représentent l’aléa volcanique le plus meurtrier qui soit. La protection des populations nécessite la mise au point de cartes des menaces précises, qui requièrent une connaissance fine de ces phénomènes. Cependant, les causes et les conséquences de l’érosion/incorporation associée aux PFs au cours de leur mise en place restent encore largement méconnues. Cette thèse se propose de caractériser la capacité érosive des PFs, de définir des mécanismes d’érosion, et de quantifier leurs impacts sur les budgets éruptifs et sur l’aléa associé. Pour cela, cette étude se concentre sur les PFs de volumes modestes mis en place pendant l’éruption d’août 2006 du volcan Tungurahua (Equateur) et adopte une démarche double, basée sur des investigations sédimentologiques et texturales des dépôts, couplées à la modélisation numérique. Une méthode originale, basée sur l’analyse d’images haute résolution corrigées par stéréologie, sur des études texturales détaillées des dépôts, et sur des bilans massiques de matière, permet de déterminer la granulométrie, la composition lithologique et la morphologie des produits sur l’ensemble de leur gamme de taille. Le calcul des bilans de matière montre que près de 50 wt. % des dépôts de PFs sont composés de matériaux non-juvéniles incorporés lors de la mise en place. Ces derniers proviennent principalement de la partie supérieure du volcan. La pente est ainsi le paramètre contrôlant au premier ordre l’intensité de l’érosion. Les budgets éruptifs complets indiquent un VEI de 3 (0,09 km 3 ) pour l’éruption, et soulignent l’importance de considérer séparément les matériaux juvéniles et non-juvéniles pour estimer la taille d’une éruption. L’étude détaillée des constituants met en évidence une ségrégation dynamique des clastes par densité au cours du transport, avec un taux de sédimentation de ≈10 cm.s -1 . Les données lithologiques, granulométriques et morphologiques démontrent la présence de phénomènes de fragmentation-abrasion des clastes pendant leur transport. Les clastes massifs (ex : laves anciennes) sont le principal agent de fragmentation des clastes scoriacés (ex : bombes). Des populations granulométriques fines, capables d’être transférées depuis l’écoulement dense principal vers les déferlantes et/ou le panache co-pyroclastique sont produites pendant toute la durée de la mise en place. Les modèles numériques basés sur une nouvelle loi d’érosion développée ici (et intégrée au code VolcFlow), démontrent la capacité de la rhéologie plastique à reproduire des PFs érosifs. L’érosion est associée à des variations dynamiques du rapport des contraintes normales/cisaillantes pendant la mise en place des écoulements, provoquées par des fluctuations d’épaisseur lors de phases de décélération. Le front fin des PFs, fortement frictionnel et érosif, est poussé par une tête et un corps plus épais, tous deux non érosifs. L’incorporation s’accompagne d’une augmentation de distance de parcours de l’ordre de 10-30% en fonction du taux d’incorporation, qui dépend de la quantité de matière affouillable disponible sur le volcan avant l’éruption. Ces résultats montrent que l’érosion peut avoir un rôle majeur sur les zones impactées par les PFs, et soulignent l’importance de prendre en compte cette capacité lors de la définition de l’aléa, ainsi que pour les études futures
Pyroclastic flows (PFs) are hot mixtures of gas and particles that represent the most deadly volcanic hazard. To protect the populations, it is necessary to work on precise risk maps, which require having a deep knowledge of these phenomena. However, the causes and consequences of erosion and incorporation of non-juvenile material during PFs emplacement remain poorly known. This thesis aims at characterizing the erosive capacity of pyroclastic flows, defining erosion mechanisms and quantifying their impact on eruptive budgets and associated hazards. Here, we focus on small-volume PFs and use an approach based on field and textural investigations coupled with numerical modeling of PFs emplacement. The August 2006 PF-forming eruption of Tungurahua volcano (Ecuador) is used as a case-study for this work.An original method, based on high-resolution, stereologically-corrected image analyses, detailed textural analyses of PFs deposits and mass budget, enables determining the grain size distribution and the componentry of PFs products along their entire clast size range. Volume calculation and mass budgets show that about 50 wt. % of the whole deposit consists of non-juvenile materials incorporated during PFs emplacement, and mostly coming from the upper part of the volcano. The slope is a prevailing parameter that controls PFs erosive power. Eruptive budgets support a VEI 3 event (0.09 km 3 ) for the 2006 eruption of Tungurahua and highlight the importance of separating juvenile from non-juvenile material. Detailed analyses of deposits’ componentry suggest a strong dynamic density-driven segregation of the clasts during PFs emplacement, associated with sedimentation rates of ≈10 cm.s -1 . Lateral variations of lithological, grain size, and morphological data demonstrate the occurrence of componentry-driven clast fragmentation and abrasion processes. Massive components (e.g. old lavas) are the main grinding agents of scoriaceous components (e.g. bombs). During emplacement, these processes continuously create fine grained populations, which are transferred from the main dense flow to pyroclastic surge or Co-PF cloud. Numerical models of erosive PFs based on a new erosion law integrated into VolcFlow code show the ability of plastic rheology to reproduce natural erosion patterns of PFs. The erosion is produced by dynamic variations of normal stress / shear stress ratio during emplacement, due to thickness unsteadiness during flow deceleration. The thin, highly frictional and erosive front of PFs pulses is pushed by the thicker and non-erosive head and flow body. Incorporation implies longer PFs runouts of about 10-30%, depending on the amount of incorporated material, which is related to the quantity of erodible material available on the volcano’s flanks before the eruption. These results show that erosion has a significant role on PFs runouts, and thus in hazard assessment, which should be closely taken into account in future works

Durant les trois années de la thèse, j’ai eu le plaisir de travailler en collaboration avec à la fois des volcanologues, des physiciens de laboratoire et des mathématiciens. Ce mémoire est l’occasion de présenter la démarche et les résultats de mes recherches dans le domaine de la modélisation d’écoulements granulaires denses fluidisés. Ces derniers consistent à développer un nouveau modèle mathématique et son étude théorique et numérique. Sur la base d’observations faites lors d’expériences de laboratoire, nous proposons une façon de modéliser le changement comportemental d’un écoulement granulaire initialement fluidisé au travers de la définition de sa rhéologie viscoplastique à seuil variable. Plus précisément, le seuil de plasticité est défini par la différence entre la pression lithostatique et la pression du fluide interstitiel. La nouveauté apportée par ce modèle ouvre de nouvelles perspectives à la fois pour le champ de recherche en mathématiques et pour la compréhension des lits granulaires fluidisés et leur application à la volcanologie. Du point de vue mathématique, une étude théorique du modèle a été menée. En proposant une preuve de l’existence de solutions faibles à un problème lié à la version homogène du modèle, nous apportons une extension au champ de connaissances autour des écoulements des fluides non-newtoniens. D’autre part, dans le but de reproduire numériquement des expériences de laboratoire de chute de colonne granulaire fluidisée, nous avons développé un code de simulation numérique incluant une nouvelle méthode de résolution des équations d’écoulement de fluides à seuil. Dans ce manuscrit, je décris et justifie les différents choix stratégiques pour le développement de ce code. Par ailleurs, je présente quelques tests académiques permettant de valider le code. Enfin, je donne les résultats de simulation de chute de colonne granulaire, qu’elle soit fluidisée ou non. Une comparaison avec les données de laboratoire est effectuée afin d’évaluer les points forts et les défauts du modèle par rapport à la réalité des expériences. En conclusion, dans la continuité du travail mené dans ce projet, des perspectives d’amélioration sont proposées
During these three years, I enjoyed to work with collaborators from volcanology, laboratory physics and mathematics. This document presents the steps and results of my research in the field of modelling of fluidised granular flows. The last consists in the development of a new mathematical model and its theoretical and numerical study. Based on observations made on experimental studies, the model focuses on the change in the behaviour of an initially fluidised granular flow through the definition of its viscoplastic rheology with variable threshold. More precisely, the threshold (aslo called yield stress) is defined via the difference between the lithostatic pressure and the pressure of the interstitial fluid. The innovation of this model opens perspectives for the mathematical research as well as for the study of fluidised granular flows and their application to volcanology. From a mathematical point of view, a theoretical study has been conducted. Proving the existence of weak solution for the homogeneous version of the model, we offer an extension in the field of knowledges of non-newtonian fluid flows. Also, we have developped a numerical code to simulate dambreak experiments with fluidised granular media. This one includes a new method to solve the flow equations of viscoplastic fluids. In this thesis, I describe and justify the numerical strategy chosen. Moreover, I present some academic tests to validate the code. At the end, I give the numerical results in the case of the dambreak simulation for dry and fluidised fluids. By comparing with experimental data, we evaluate the validity of the model and its resolution, and highlight the advantages and inconvenients. To conclude the project, I propose some perspectives of improvement for later work

Our interest is the risk assessment of rare natural hazards, such as

large volcanic pyroclastic flows. Since catastrophic consequences of

volcanic flows are rare events, our analysis benefits from the use of

a computer model to provide information about these events under

natural conditions that may not have been observed in reality.

A common problem in the analysis of computer experiments, however, is the high computational cost associated with each simulation of a complex physical process. We tackle this problem by using a statistical approximation (emulator) to predict the output of this computer model at untried values of inputs. Gaussian process response surface is a technique commonly used in these applications, because it is fast and easy to use in the analysis.

We explore several aspects of the implementation of Gaussian process emulators in a Bayesian context. First, we propose an improvement for the implementation of the plug-in approach to Gaussian processes. Next, we also evaluate the performance of a spatial model for large data sets in the context of computer experiments.

Computer model data can also be combined to field observations in order to calibrate the emulator and obtain statistical approximations to the computer model that are closer to reality. We present an application where we learn the joint distribution of inputs from field data and then bind this auxiliary information to the emulator in a calibration process.

One of the outputs of our computer model is a surface of maximum volcanic flow height over some geographical area. We show how the topography of the volcano area plays an important role in determining the shape of this surface, and we propose methods

to incorporate geophysical information in the multivariate analysis of computer model output.

Dissertation

abstract: Silicic volcanoes produce many styles of activity over a range of timescales. Eruptions vary from slow effusion of viscous lava over many years to violent explosions lasting several hours. Hazards from these eruptions can be far-reaching and persistent, and are compounded by the dense populations often surrounding active volcanoes. I apply and develop satellite and ground-based remote sensing techniques to document eruptions at Merapi and Sinabung Volcanoes in Indonesia. I use numerical models of volcanic activity in combination with my observational data to describe the processes driving different eruption styles, including lava dome growth and collapse, lava flow emplacement, and transitions between effusive and explosive activity. Both effusive and explosive eruptions have occurred recently at Merapi volcano. I use satellite thermal images to identify variations during the 2006 effusive eruption and a numerical model of magma ascent to explain the mechanisms that controlled those variations. I show that a nearby tectonic earthquake may have triggered the peak phase of the eruption by increasing the overpressure and bubble content of the magma and that the frequency of pyroclastic flows is correlated with eruption rate. In 2010, Merapi erupted explosively but also shifted between rapid dome-building and explosive phases. I explain these variations by the heterogeneous addition of CO2 to the melt from bedrock under conditions favorable to transitions between effusive and explosive styles. At Sinabung, I use photogrammetry and satellite images to describe the emplacement of a viscous lava flow. I calculate the flow volume (0.1 km3) and average effusion rate (4.4 m3 s-1) and identify active regions of collapse and advance. Advance rate was controlled by the effusion rate and the flow’s yield strength. Pyroclastic flow activity was initially correlated to the decreasing flow advance rate, but was later affected by the underlying topography as the flow inflated and collapsed near the vent, leading to renewed pyroclastic flow activity. This work describes previously poorly understood mechanisms of silicic lava emplacement, including multiple causes of pyroclastic flows, and improves the understanding, monitoring capability, and hazard assessment of silicic volcanic eruptions.
Dissertation/Thesis
Doctoral Dissertation Geological Sciences 2016

Die Sedimente des Halleschen Permokarbonkomplexes gaben schon immer Raum für Spekulationen. Aufgrund ihrer Dominanz an rhyolithischen Geröllen wurden sie über einen langen Zeitraum einheitlich als Postporphyrschutt ausgehalten. Vielfältig wechselnde Faziesbedingungen machten es jedoch notwendig, die Sedimente aufzugliedern. Neuere Erkenntnisse in der Erforschung des Halleschen Permokarbonkomplexes erfordern eine Überprüfung v. a. der nach KUNERT (1995) aufgestellten allgemeinen stratigraphischen Gliederung der Unterrotliegendsedimente in Halle,- Hornburg,- Sennewitz- und Brachwitz-Formation anhand einiger ausgewählter Beispiele. Der ursprüngliche Gedanke der Diplomarbeit bestand darin, eine Fazies- und eine Geröllanalyse der unterpermischen Abtragungsprodukte des Halle-Vulkanitkomplexes anzufertigen. Zur Verfügung standen zwei Kernbohrungen und zwei Aufschlüsse, sowie diverse Unterlagen zu angrenzenden Bohrungen in der Saale-Senke. Die beiden Oberflächenaufschlüsse Riveufer und Teichgrund sollten stratigraphisch aufgenommen werden, so dass eine Fazieszuordnung möglich ist. Die Bohrung Brachwitz 2/62 wurde mit dem Ziel aufgenommen, neuere Theorien über den Ablagerungszeitraum der Rotliegend-Sedimente in Bezug auf den permokarbonen Vulkanismus zu widerlegen oder zu bekräftigen. Die zweite Bohrung (Kb Lochau 7/65) wurde am Rande mit in die Diplomarbeit einbezogen, da sie das immense Spektrum der spätvulkanischen Aktivitäten im Halle Permokarbonkomplex erweitert. Ergebnis ist eine Neugliederung des Rotliegend im Halleschen Permokarbonkomplex, in der nur noch die Halle-Formation mit ihrem ausgeprägten Vulkanismus und die Hornburg-Formation, stellvertretend für alle jüngeren Abtragungsprodukte des Halle Vulkanitkomplexes, unterschieden werden. Mit einem großen Hiatus folgt anschließend die Eisleben-Formation.:Inhalt Abbildungsverzeichnis Tabellenverzeichnis Abkürzungsverzeichnis 1. Einleitender Teil 1 1.1 Einleitung 1 1.2 Aufgabenstellung und Problematik 1 1.3 Geographischer Überblick über die Bohrungen und Aufschlüsse 2 2. Regionalgeologischer Teil 4 2.1 Aufbau des Halle Vulkanitkomplexes 4 2.2 Beckenentwicklung des Permokarbons im Bereich des Halle- Vulkanitkomplexes 5 2.3 Historischer Rückblick über die Einstufung der Rotliegend-Formationen im Halle Vulkanitkomplex 10 2.4 Neueste Entwicklungen in der Erforschung des Saale-Beckens 15 2.4.1 Die Ablagerungen der Halle-Formation 15 2.4.2 Die Ablagerungen der Sennewitz-Formation 16 2.4.3 Die Ablagerungen der Hornburg-Formation 17 2.4.4 Die Ablagerungen der Brachwitz-Formation 19 2.4.5 Die Ablagerungen der Eisleben-Formation 20 2.4.6 Aktuelle Stratigraphische Gliederung 22 2.5 Die späte Phase des Halle Vulkanitkomplexes und ihr Bezug zur Diplomarbeit 23 3 Arbeitsmethodik 24 3.1 Aufnahme der Bohrungen Brachwitz 2/62 und Lochau 7/65 24 3.2 Aufnahme des Aufschlusses am Teichgrund bei Döblitz 26 3.3 Aufnahme des Aufschlusses am Riveufer im Stadtgebiet von Halle 26 4. Vulkanische und sedimentäre grobklastische Transport- und Ablagerungssysteme 27 4.1 Vulkanische Massentransporte 27 4.1.1 Pyroklastische Ablagerungen 27 4.1.1.1 Pyroklastische Fallablagerungen 28 (1) Aschefallablagerungen 28 (2) Bimsführende Fallablagerungen 29 (3) Scoriaführende Fallablagerungen 29 4.1.1.2 Pyroklastische Stromablagerungen 29 (1) Bimsführende pyroklastische Stromablagerungen oder Ignimbrite 29 (2) Block- und Aschestromablagerungen 31 (3) Scoriaführende pyroklastische Stromablagerungen 32 4.1.1.3 Pyroklastische Surge-Ablagerungen 32 (1) Surgeablagerungen durch Aschewolken 32 (2) Ablagerungen am Boden der pyroklastischen Surge 33 (3) Ablagerungen an der Basis der pyroklastischen Surge 33 4.1.2 Explosive vulkanische Eruptionen 33 (1) Hawaiianische Eruptionen 34 (2) Plinianische Eruptionen 34 (3) Strombolianische Eruptionen 35 (4) Vulkanianische und Surtseyanische Eruptionen 35 4.1.3 Produkte phreatomagmatischer Eruptionen 36 (1) Maare 37 (2) Tuffkegel und Tuffringe 37 4.1.4 Tephraablagerungen 38 4.2 Sedimentäre Massentransporte 39 4.2.1 Alluviale Fächer 40 4.2.2 Schichtfluten 42 4.2.3 Flußsyteme 42 4.2.4 Überflutungsebenen 43 4.2.5 Deltas und Ästuare 44 5. Lithologien und Faziestypen 45 6. Aufschlüsse und Bohrungen 45 6.1 Aufschlußkomplex am Riveufer im Stadtteil Giebichenstein in Halle 48 6.1.1 Allgemeine Aussagen 48 6.1.2 Das Faziesmodell eines verflochtenen Flußsystems 48 (1) Ausbildung von Rinnen 48 (2) Einfallen der Rinnen 50 (3) Prallhänge 50 (4) Seitenanschnitte an beiden Enden des Aufschlusses 51 6.1.3 Ein tuffgefülltes Spaltensystem als syn- bis postsedimentäres Ereignis 52 6.1.4 Interpretation 53 6.2 Aufschluß am Teichgrund bei Döblitz 55 6.2.1 Allgemeine Aussagen 55 6.2.2 Sedimentäre Lithofaziestypen und -assoziationen 56 6.2.3 Dokumentation der einzelnen Aufschlüsse 56 6.2.3.1 Aufschluß T1 56 (1) Detaildarstellung Aufschluß am Teichgrund T1-1 56 6.2.3.2 Aufschluß T2 59 6.2.3.3 Aufschluß T3 59 6.2.4 Fazielle Diskussion 59 6.3 Kernbohrung Brachwitz BrwSk 2/62 südöstlich der Ortschaft Friedrichsschwerz 61 6.3.1 Allgemeine Informationen 61 6.3.2 Erläuterungen zu den Lithofaziestypen 61 (1) SFT-B1 Konglomerat der Eislebenformation 61 (2) SFT-T1 Sedimentäre Brekzie 61 (3) SFT-T4 Mittel- bis Grobsandstein 62 (4) SFT-B2 Schluffstein 62 (5) VFT-T0 Rhyolith, brekziös/ VFT-T1 Porphyrbrekzie, monomikt 63 (6) VFT-B12 Porphyrbrekzie mit Obsidianmatrix 64 (7) VFT-B2 Porphyrbrekzien, oligomikt und polymikt 64 (8) VFT-B3 Mittelsand, vulkanogen 65 (9) VFT-B5 Schluffstein, brekziiert 66 6.3.3 Auswertung 66 6.4 Kernbohrung Lochau 7/65 südöstlich Halle 68 6.4.1 Allgemeines 68 6.4.2 Erläuterungen zu den Vulkanischen Faziestypen 68 (1) VFT-L1 Aschentuff 68 (2) VFT-L2 Surges 69 (3) VFT-L3 Surge oder Explosionsbrekzie 70 (4) VFT-L4 Explosionsbrekzie mit Tuffzwickelfüllung 71 (5) Tuff mit einzelnen Ballistischen Bomben 72 6.4.4 Beispiel Ha-Lo7/17 73 6.4.5 Diskussion 74 7. Zusammenfassung und Ausblick 76 8. Literatur- und Quellenverzeichnis 78 9. Anhang Anlage 1: Allgemeines Anlage 2: Teichgrund bei Döblitz Anlage 3: Riveufer im Stadtzentrum von Halle (Saale) Anlage 4: Kb Brachwitz 2/62 Anlage 5: Kb Lochau 7/65

碩士
國立成功大學
自然災害減災及管理國際碩士學位學程
105
In this thesis, a depth-averaged model in a terrain-fitted coordinate system is applied to simulate the pyroclastic flows during the eruption of Mt. Merapi in 2010. Through the numerical simulation we are able to obtain more precise flow information, such as the local flow fluxes, the distributions of flow depth and velocity as well as the deposits, for the goal of hazard mitigation or hazard management. In the numerical computation, the terrain-fitted coordinate system is based on the digital elevation model (DEM). Because of the fact that the real topography might consists of highly varying elevation, e.g. obstacles or cliffs, which is not applicable in the terrain-fitted coordinate, a compromised treatment is to smooth (filter) the topographic surface. However, the employment of the smooth basal surface might introduce deviation because of the lack of the sudden change of topography. In this study, we introduce the so-call “sub-topography” over the smoothed topographic surface, to mimic the real topography. In the simulations, we also introduce the “Upwelling” to mimic the exploded material, especially by the multiple eruptions, so that we may have a more reasonable spreading/supply of flow material. In addition, we also investigate the impacts of the key parameters on the flow behaviors, such as the friction coefficient, momentum correction factor and the amount of upwelling. Besides, the Voellmy rheology has also been taken into account. The results of these tests were compared to theoretical findings of rheological analysis presented in previous studies as well as historical records, and satellite images to figure out the representative values of the relevant parameters.