Postglacial activity on the Askja volcanic system, north Iceland, has been dominated by basaltic volcanism. Over 80% of Askja's postglacial basalts fall within a relatively narrow compositional range containing between 4 and 8 wt.% MgO. The 'main series' is further divided into two groups separated by a distinct compositional gap evident in major and trace element concentrations. The most evolved basalts formed by fractional crystallisation within shallow magma reservoirs, followed by the extraction of residual liquid from a semi-rigid, interconnected crystal network. This process is analogous to the formation of melt segregations within single lava flows, and was responsible for generating several small-volume, aphyric basaltic lavas erupted along caldera ring fractures surrounding the Oskjuvatn (Askja lake) caldera in the early 20th century. Further examples of evolved basalt are found throughout Askja's postglacial volcanic record. However, Askja's early postglacial output is dominated by more primitive compositions. Some of the most primitive basalts erupted within the Askja caldera are found in phreatomagmatic tuff cone sequences which crop out in the walls of Oskjuvatn caldera. one such tuff sequence has been dated at between 2.9 and 3.6 ka. This tuff cone shares geochemical source characteristics, such as Nb/La and Nb/Zr, with basaltic tephras erupted during precursory activity to the Plinian-phreatoplinian eruption of 28th-29th March 1875. It may therefore be considered to be compositionally representative of the primitive basaltic magmas supplied to Askja during the postglacial period. The predominance of relatively primitive basalt (6.8 wt.% MgO) within Askia's postglacial lava succession suggests that it did not have a permanent shallow magma chamber during the postglacial period. It is envisaged that the postglacial Askja magmas evolved by a process of polybaric factionation in transient, sill-like magma storage zones located at various levels in the crust. The most primitive magmas erupted directly from deeper reservoirs, while the more evolved magmas experienced longer crustal residence times. The buoyant rise of volatile-enriched melt from these sill-like bodies, without mobilising phenocryst phases, explains the observation that almost all lavas on Askja's eastern and southern lava aprons are essentially aphyric. The 28th-29th March 1975 eruption marked the climax of a volcanotectonic episode on the Askja volanic system lasting from late 1874 to early 1876. Fissure eruptions also occurred at the Sveinagja graben, 45-65 km north of Askja, between February and October 1875, producing the Nyjahraun lava. A strong similarity exists between whole-rock major element concentrations from Myjahraun and the Askja 20th century basalts. This has led to the suggestion that these basalts originated from a common shallow magma reservoir beneath Askja central volcano, with the Nyjahraun eruptions being fed by a lateral dyke extending northwards from Askja. This theory also offers an explanation for the observation that the volume of phyolitic ejecta from 28th-29th March 1875 is significantly less than the volume of Oskjuvatn caldera, which was formed as a result of this eruption. New major and trace element data from whole-rock and glass samples indicated that Nyjahraun and the Askja 20th century basalts did not share a common parental magma. A detailed investigation of historical accounts from explorers and scientists who visited Askja between 1875 and 1932 reveals that Oskjuvatn caldera took over 40 years to reach its current form, and that its size in 1876 was equal to the volume erupted on 28th-29th March 1875. Small injections of magma into an igneous intrusion complex beneath Askja, coupled with background deflation, are sufficient to provide the required accommodation space for continued caldera collapse after 1876. Lateral flow is therefore not required to explain the volume of Oskjuvatn caldera, nor the eruption of evolved basaltic magma on the Askja volcanic system in 1875. It has been conjectured that the Holuhraun lava, located at the southern tip of the Askja volcanic system, was also connected with the 1874-76 Askja volcanotectonic episode. However, major and trace element data from whole-rock samples, glass and melt inclusions receal the Holuhraun is geochemically more similar to basalts erupted on the Bardarbunga-Veidivotn volcanic system than to postglacial basalts from Askja. The division between the 'Askja' and 'Veidivotn' geochemical signatures appears to be linked to east-west-striking lineations in the region south of Askja. This indicates that a particular geochemical signature is not necessarily confined to the tectonic expression of a single volcanic system, and has important implications for the identification and delineation of individual volcanic systems beneath the northwest sector of Vatnajokull.

Flow variability plays an important role in structuring lotic communities, yet comparatively little is known about processes governing assemblage dynamics in stream ecosystems with stable environmental conditions, such as spring-fed rivers. Volcanic spring-fed rivers (hereafter spring-fed rivers) occur in geologically active landscapes of the western USA and around the globe. We sampled invertebrate assemblages and quantified primary productivity and habitat characteristics of spring-fed and runoff rivers in northern California over 4 seasons. We predicted that abiotic factors would be more stable and nutrient availability greater and that invertebrate density would be greater and diversity lower in spring-fed than in runoff rivers. Runoff rivers exhibited high variability in discharge and temperature, whereas spring-fed rivers were relatively stable with high naturally occurring nutrient levels. On average, NO3- and PO43- concentrations were 40x greater in spring-fed than in runoff rivers. Spring-fed rivers supported nearly 7 to 16x greater densities of invertebrates than runoff systems, depending on season. However, invertebrate species richness was greater in runoff rivers in all seasons. Spring-fed river invertebrate assemblages were strongly correlated with elevated nutrient concentrations and basal C sources, whereas runoff assemblages were associated with discharge variability and median substrate size. We suggest that strong differences in abiotic variability between spring-fed and runoff rivers play an important role in determining invertebrate assemblage structure. Because spring-fed rivers exhibit more stable temperatures throughout the year and lower temperatures during the summer than runoff rivers, they may provide essential refugia for coldwater taxa in a warming climate.

Zuni Salt Lake (ZSL) is a large maar in the Red Hill-Quemado volcanic field located in west-central New Mexico in the southwestern USA. Stratigraphic analysis of sections in and around the maar, coupled with optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) C-14 dating, indicate that ZSL volcanic activity occurred between similar to 13.4 and 9.9 ka and was most likely confined to a <= 500-year interval sometime between similar to 12.3 and 11.0 ka. The basal volcanic unit consists of locally widespread basaltic ash fallout interpreted to represent a violent or wind-aided strombolian eruption tentatively attributed to Cerro Pomo, a scoria cone similar to 10 km south of ZSL. Subsequent eruptions emanated from vents near or within the present-day ZSL maar crater. Strombolian eruptions of multiple spatter and scoria cones produced basaltic lava and scoria lapilli fallout. Next, a phreatomagmatic eruption created the maar crater and surrounding tephra rim and apron. ZSL eruptions ended with strombolian eruptions that formed three scoria cones on the crater floor. The revised age range of ZSL is younger and more precise than the 190-24 ka 2-sigma age range derived from previous argon dating. This implies that other morphologically youthful, argon-dated volcanoes on the southern margin of the Colorado Plateau might be substantially younger than previously reported.

This thesis presents the development of various methods for measuring rainfall rates using horizontally-pointing millimetre-wave radars. This work builds from the combination of a T-matrix scattering model that allows the scattering from almost arbitrarily pro led rotationally symmetric particles to be calculated, and drop shape models that allow the effects of temperature and pressure on the shape to be taken into account. Many hours of rain data have been collected with 38 and 94 GHz FMCW radars, as well as with a disdrometer and weather station. These have been used to develop single- and dual-frequency techniques for measuring rainfall rate. A temperature, polarisation and attenuation corrected application of simple power-law relationships between reflectivity and rainfall rate has been successfully demonstrated at 38 GHz. However, at 94 GHz it has been found that more detailed functions relating reflectivity, attenuation and rainfall rate are beneficial. A reflectivity-based determination of attenuation has been adapted from the literature and successfully applied to the 94 GHz data, improving the estimate of rainfall rate at longer ranges. The same method for estimating attenuation has also been used in a dualfrequency technique based on the ratio of the extinction coefficients at 38 and 94 GHz, but with less success. However, a dual-frequency reflectivity ratio based approach has been successfully developed and applied, producing good estimates of rainfall rate, as well as reasonable estimates of two drop-size distribution parameters. Simulations of radar measurements of airborne volcanic ash have also been carried out, demonstrating that for most reasonable measurement configurations the optimal frequencies would typically be 35 GHz or 94 GHz, not the more commonly used 3-10 GHz. It has also been shown that various existing millimetre-wave radars could be used to detect ash. Finally, there is a discussion of the optimal frequencies for dual-frequency measurement of volcanic ash.

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.

Master of Science / Department of Geology / Matthew E. Brueseke / Low-[delta][superscript]1[superscript]8O silicic magmas are found in many volcanic provinces throughout the world, including the Snake River Plain-Yellowstone volcanic province (SRPY). The origin of SRPY low-[delta][superscript]1[superscript]8O silicic magmas is controversial, and centers on two disputed models: [1] a caldera collapse model that proposes reworking of the hydrothermally altered intra-caldera fill into the underlying silicic magma body, where each successive eruption lowers the [delta][superscript]1[superscript]8O of the magma eventually producing a low-[delta][superscript]1[superscript]8O magma and [2] melting previously hydrothermally altered mid-upper crust to form low-[delta][superscript]1[superscript]8O magmas. The mid-Miocene Santa Rosa-Calico volcanic field (SC) lies in northern Nevada. Brueseke and Hart (2008) described the geology and petrology of the SC, but did not deal with the [superscript]1[superscript]8O compositions of any locally sourced silicic magma. In the existing geological framework of the SC, this project aims to evaluate the two disputed models for low-[delta][superscript]1[superscript]8O silicic magma generation by analyzing the [delta][superscript]1[superscript]8O values of SC silicic eruptive products. Fifteen representative samples of locally erupted silicic units (e.g. ash-flow tuffs and lavas) were chosen for [superscript]1[superscript]8O analyses based on Sr-Nd-Pb isotope compositions, whole rock geochemistry, and field/temporal relationships. Each sample was crushed, sieved, and quartz and feldspar crystals were handpicked, described, and analyzed for their [superscript]1[superscript]8O compositions. Our results show that low-[delta][superscript]1[superscript]8O values exist in the SC and are limited to the youngest erupted silicic unit, the 15.8 to 15.4 Ma Cold Springs tuff, which was also the only unit erupted from a caldera. Cold Springs tuff [delta][superscript]1[superscript]8O feldspar values range from 2.36 to 4.05[per mil]; the unit is not quartz-bearing. Older silicic lavas that are not petrogenetically related to the Cold Springs tuff are characterized by normal [delta][superscript]1[superscript]8O feldspar values that range from 7.19 to 10.04[per mil]. Magma mixing models indicate that the source of the Cold Springs is a mixture of hydrothermally altered Granite Peak-Sawtooth granitoid and local mid-Miocene basalt, with an approximate range of [delta][superscript]1[superscript]8O values of 2-4[per mil], by fluids (with [delta][superscript]1[superscript]8O values ranging from -12[per mil] to + 7[per mil]) from the nearby hydrothermal system at Buckskin Mountain. This result follows the model by Boroughs et al. (2005) of prior alteration and melting, forming low-[delta][superscript]1[superscript]8O silicic magmas.

Lors de cette étude, 47 nouveaux âges ont été obtenus par la technique Cassignol-Gillot, complétant à 128 âges le nombre de données disponibles sur l'île de Basse-Terre. La très bonne reproductibilité des âges obtenus dans cette étude, et la cohérence de ces derniers sur l'ensemble des massifs, appuie l'utilisation de la méthode K-Ar pour la datation des laves des Petites Antilles. Les données géochronologiques ont été associés à des analyses géochimiques et géomorphologiques dans le but de contraindre l'évolution spatio-temporelle du volcanisme de Basse-Terre, mais également d'apporter de nouvelles contraintes sur les taux de construction et d'érosion en contexte tropical.Le volcanisme récent de Basse-Terre, i.e. inférieur à 1 Ma, se concentre dans la moitié sud de l'île. Composée de trois massifs volcaniques (Piton de Bouillante, Sud Chaîne Axiale et le Complexe Volcanique de Grande-Découverte), son activité a débuté au nord-ouest par la mise en place du Piton de Bouillante entre 906 ± 13 et 712 ± 12 ka, avec un taux de construction de 0.7 ± 0.2 km3/kyr. Les nouvelles données obtenues lors de cette étude montrent qu'aucun effondrement de flanc majeur n'a affecté cet édifice. L'activité volcanique a ensuite rapidement migrée vers le sud-est pour former entre 681 ± 12 et 509 ± 10 ka les volcans de Moustique, Matéliane, Capesterre et Icaque, qui constitue le massif Sud Chaine Axiale. La contemporanéité des âges obtenus sur l'ensemble des édifices, et le taux de construction calculé à 0.5 km 3/kyr, appuient la mise en place du sud de la Chaine Axiale par un unique massif volcanique, contredisant les hypothèses d'effondrement de flanc précédemment proposées. L'homogénéité géochimique observée sur l'ensemble du massif supporte l'hypothèse d'un seul édifice. Entre 500 et 450 ka, le flanc ouest du massif Sud Chaîne Axial a été affecté par un slump actuellement matérialisé par le volcan d'Icaque. La dépression formée a permis la mise en place du volcan du Sans-Toucher entre 451 ± 13 et 412 ± 8 ka. Entre 400 et 200 ka, très peu d'activité effusive a pu être mise en évidence. Depuis 200 ka, l'activité volcanique se concentre dans le sud de l'île, avec la mise en place du Complexe Volcanique de la Grande-Découverte, par une succession de phases de construction et de destruction. La dernière activité volcanique a permis la construction du dôme actuel de La Soufrière. Les investigations géomorphologiques nous ont également permis de contraindre les taux d'érosion ayant affecté l'île de Basse-Terre. Ainsi, le Piton de Bouillant subit une érosion de 1 250 ± 700 t/km'/an depuis 700 ka. Pour les volcans du Sans-Toucher, et des Monts-Caraïbes, nous avons obtenus un taux d'érosion similaire, respectivement de 940 ± 380 et 610± 550 t/km2/an. Malgré une localisation et une morphologie initiale différentes, la similarité des taux d'érosion obtenus pour les volcans de Basse-Terre met en évidence l'absence d'un effet barrière sur l'érosion à long terme, pourtant majeur à plus courte échelle de temps. / In this study, forty-seven new ages have been obtained by the Cassignol-Gillot technique, increasing to 128, the geochronological database available for the Basse-Terre Island. The very good reproducibility of the ages obtained in this study, added to a strickly consistency observed between the volcanic edifices, support the use of the K-Ar method in the dating of the Lesser Antilles lavas. This new geochronological dataset has been combined with geochemical and geomorphological analyses in order to constrain the volcanic history of Southern Basse-Terre Island as well as to compute construction and erosion rates.Southern part of Basse-Terre hosts the recent volcanic activity since the last 1 Myr. Composed by three volcanic massifs (Piton de Bouillante, Southern Axial Chain and the Grande-Découverte Volcanic Complex), its activity has begun in the northwest part by the construction of the Piton de Bouillante between 906 ± 13 and 712 ± 12 ka, with a construction rate of 0.7 ± 0.2 km3/kyr. Our new data show that no major flank collapse have affected this volcano. Then, volcanic activity has migrated to the southeast, constructing between 681 ± 12 and 509 ± 10 ka the Southern Axial Chain massif, composed by Moustique, Matéliane, Capesterre and Icaque volcanoes. The contemporaneity of the ages for the whole massif together with the construction rate computed at 0.5 km3/kyr suggest the formation of the southern Axial Chain by a unique volcanic edifice, which did not experienced major flank collapses as previously proposed. The geochemical homogeneity observed throughout the massif supports this single volcano hypothesis. Between 500 and 450 ka, a slump has affected the western part of the Southern Axial Chain and forming the Icaque volcano. The resulting depression has allowed the construction of the Sans-Toucher volcano from 451 ± 13 to 412 ± 8 ka. After the construction of the Sans-Toucher volcano, only few evidences for an effusive activity occurring between 400 and 200 ka can be found. Since 200 ka, volcanic activity is present in the southern part with the construction of the Grande-Découverte Volcanic complex (GDVC), alternating constructive and destructive phase. The last volcanic activity formed the 1530 AD La Soufrière dome. Geomorphological investigations have also allowed us to constrain the erosion rates having affected Basse-Terre Island. Thus, Piton de Bouillante volcano have suffered of an erosion rate of 1,250 ± 700 t/km²/yr since 700 ka. We have obtained for the Sans-Toucher and Monts-Caraïbes volcanoes similar rates of 940 ± 380 and 610 ± 550 t/km²/yr, respectively. Despite a different location and different initial morphology, the similarity erosion rates observed for each massif suggest that the barrier effect does not significantly affect the long-term erosion budget while it plays a major role at much shorter time-scale.

As brechas vulcanoclásticas massivas correspondem a maciços rochosos de fragmentos vulcânicos geralmente imersos numa matriz mais fina que estão caraterizadas por apresentar baixo fraturamento ou ausência de descontinuidades. Embora as brechas vulcânicas massivas sejam de baixa resistência, possuem condições mecânicas que permitem escavar taludes altos ou túneis moderadamente profundos mostrando comportamentos estáveis. Neste trabalho foi estudado um afloramento de brechas vulcânicas da zona Pacífica Central da Costa Rica, com o objetivo de caracterizar mecanicamente a rocha. Avaliou-se a influência dos clastos nas propriedades globais de resistência e deformação. Estudou-se também o fraturamento progressivo e o efeito escala sobre as propriedades mecânicas da rocha sob carregamentos de compressão uniaxial e diametral. Caracterizaram-se os constituintes da brecha vulcanoclástica e a relação da proporção e a distribuição dos tamanhos de blocos para as escalas de afloramento e de laboratório. Mostram-se as propriedades mecânicas dos blocos e da matriz, avaliando-se o contraste mecânico existente e a influência na resistência e deformabilidade da brecha. A resistência à compressão e o módulo de deformação da brecha obtidos nos ensaios de laboratório não apresentaram influência significativa com a variação da porcentagem de blocos entre 10% até 65%. A coesão mostra um decréscimo entre o 7%-19%, e o ângulo de atrito aumentou 1.1° e diminuiu 2.6° com o acréscimo da porcentagem volumétrico de clastos de 25% para 50%. Em termos comparativos com outras rochas similares, a brecha analisada apresenta pouca influência mecânica significativa dos blocos. Quanto ao processo de fraturamento no ensaio de compressão axial e diametral, mostram-se os tipos curva completas dos ensaios e sua relação com a emissão acústica (EA) e inspeções visuais. Realizaram-se observações do microfissuramento preexistente e do obtido ao longo do ensaio de compressão diametral, mediante ensaios de microtomografia de rádio X (&#956;CT). De acordo com as observações nos ensaios de microtomografia, a propagação das fissuras é tortuosa e preferencial nos contornos dos clastos. Ocasionalmente, as fissuras podem cortar alguns blocos de densidade superior à matriz sem se perceberem mudanças na curva de força deslocamento ou acréscimos de resistência. Na inspeção macro dos testemunhos carregados, as trajetórias do fraturamento percorrem a matriz da rocha, a interface blocos-matriz, ou o interior dos blocos. O efeito escala sobre o comportamento mecânico foi estudado para amostras de diâmetro de 31 até 145 mm. A brecha vulcânica estudada se considera como uma rocha pouco fissurada e não apresenta efeito escala na resistência à compressão e tração, no comportamento da curva completa de compressão uniaxial, no módulo de deformação, no coeficiente de Poisson ou no fator de intensidade de tensão crítico. As curvas de força versus deslocamento horizontal no ensaio de compressão diametral apresentam comportamentos mais dúcteis para as amostras pequenas e tendências mais frágeis em tamanho maior. Finalmente o trabalho apresenta o caso de estudo de brechas vulcânicas massivas escavadas no túnel de condução da usina hidroelétrica Cachí na Costa Rica. Mostram-se a caracterização do maciço, a comparação comportamento teórico esperado da escavação comparado com o real percebido e as experiências da produtividade de escavação com métodos com explosivos e fresadora. / Massive volcanoclastic breccias are rock masses of volcanic fragments generally immerse in a fine matrix, and they are characterized by little or no presence of jointing. Despite massive volcanoclastic breccias being soft rock, they have mechanic conditions that allow them to be excavated in high slopes and moderately deep tunnels that present stable behavior. Volcanoclastic breccias of the Central Pacific of Costa Rica were studied for this dissertation aiming at mechanically characterizing this type of rock. The influence of the proportion of clasts was evaluated on the global properties of resistance and deformation. In addition, the progressive fracture process and the scale effect were studied focusing on the mechanical properties under uniaxial and diametrical compression loading. The constituents, the proportion and size distribution of clasts were characterized for outcrop and laboratory scales. The mechanical properties of blocks and the matrix are presented evaluating the mechanical contrast and the influence on the strength and deformability of the breccia. The compressive strength and the deformation modulus obtained in the laboratory testing do not present significance influence when the block proportion measured in the breccia rock cores changed varying from 10% to 60%. The cohesion decreased between a 7% and a 19% and the friction angle increased with the increment of the volumetric proportion blocks from 25% to 50%. The volcanoclastic breccias analyzed exhibit little influence of the mechanical properties of the blocks when compared to other similar rocks. When discussing the fracturing process in uniaxial and diametral compression test, the dissertation presents the types of complete curves and their relationship to acoustic emissions (AE) and visual observations. There are remarks of preexistent micro-fissuring and the fracturing status obtained in the diametral compression test by the ray-X micro tomography technique (&#956;CT). In line with the observations in the micro tomography tests, the crack propagation path is tortuous preferably going through the surface of the clasts. Occasionally, the fracture can split some blocks denser than the matrix without changes in the force-horizontal displacement curve nor in the resistance. The fracture path goes through the matrix of the rock, the interface between the rock and the matrix and even the blocks. The scale effect in the mechanical behavior was studied in cores ranging from 31 mm to 145 mm of diameter. The volcanoclastic breccias are considered as a lightly fissured rock, and they do not present scale effect in the compressive and tensile strength, the deformation modulus, the complete curve behavior over the uniaxial compressional test, the Poisson´s ratio, and the critical fracture toughness. The complete force-horizontal displacement curves in the diametral compression test display ductile behaviors in the smaller diameters, and a trend of fragile behavior in bigger samples. Finally, this dissertation presents the study case of massive volcanic breccias excavated in the pressure tunnel of the Cachí hydroelectric project in Costa Rica. The characterization of the rock mass, a comparison of the excavation expected theoretical behavior with the real behavior and the experience of the excavation performance with the conventional methods and the roadheader mechanical methods are presented.

Foram realizadas medidas de espectroscopia gama simples, de espectroscopia gama-gama em anti-coincidência e coincidência em dezessete amostras de rochas vulcânicas irradiadas com nêutrons térmicos. Foram determinadas, utilizando cálculo covariante, 33 elementos químicos (Al, As, Ba, Ca, Ce, Cl, Co, Cr, Cs, Dy, Eu, Fe, Hf, K, La, Lu, Mg, Mn, Na, Nd, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, Ti, U, Yb, Zn e Zr), dos quais 21 elementos traço. Foi determinado o limite máximo de detecção para o irídio em 11 amostras. Foi obtido um valor máximo de 0,7 ppb. Os resultados foram comparados com o processo astrofísico de formação da Terra. A concentração de urânio e tório variou entre 3 ppm e 9 ppm, com exceção das amostras de Kilimanjaro, que variou entre 40 ppm e 45 ppm. Abaixo da concentração de 12 ppm presentes na crosta terrestre. Espera-se concentrações menores devido a estimativas de tório e urânio através do calor emitido pela Terra. Foram determinados 10/17 lantanídeos, com destaque para a abundância de Ce em Kilimanjaro, cujos valores obtidos foram de 272 (6) ppm e 319 (11) ppm. / Single gamma ray spectroscopy, gama-gama coincidence and anticoincidence spectroscopy have been performed to study 17 volcanic rocks irradiated with neutrons. We have determined, using covariant calculations, 33 chemical elements including 21 trace elements: Al, As, Ba, Ca, Ce, Cl, Co, Cr, Cs, Dy, Eu, Fe, Hf, K, La, Lu, Mg, Mn, Na, Nd, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, Ti, U, Yb, Zn e Zr. We set a maximum limit of detection for iridium in 11 samples. We have determined a maximum value of 0,7 ppb. The results were associated to the astrophysical process that led to the Earth formation. Thorium and uranium abundance ranged between 3 ppm e 9 ppm, except for the samples of Kilimanjaro, which ranged from 40 ppm to 45 ppm. Less than 12 ppm, the Earth\'s Crust abundance of thorium and uranium. Using the heat emitted by Earth to estimate the abundance of them led to significant lower values. We have also determined 10/17 lanthanides, like Ce that has an abundance of 272 (6) ppm and 319 (11) ppm, in the case of Kilimanjaro\'s samples.