The basement of the Tonga intraoceanic forearc comprises Eocene arc volcanic crust formed during the earliest phases of subduction. Volcanic rocks recovered from the forearc include boninites and arc tholeiites, apparently erupted into and upon older mid-oceanic ridge tholeiites. Rock assemblages suggest that the forearc basement is a likely analog for large supra-subduction zone (SSZ) ophiolites not only in structure and Ethology, but also in the style of hydrothermal alteration. Dredged volcanic samples from the central Tonga forearc (20-24�� S) exhibit the effects of seafloor weathering, low (<200��C, principally <100��C) alteration, and high temperature (>200��C) alteration. Tholeiites and arc tholeiites are significantly more altered than boninites. Seafloor weathering is due to extensive interaction with cold oxidizing seawater, and is characterized by red-brown staining and the presence of Fe-oxyhydroxides. Low temperature alteration is due to circulation of evolving seawater-derived fluids through the volcanic section until fluid pathways were closed by secondary mineral precipitation. Low temperature alteration is characterized by smectites, celadonite, phillipsite, mixed-layer smectite/chlorite, carbonates, and silica. All phases fill veins and cavities; clay minerals and silica also replace the mesostasis and groundmass phases. Low temperature alteration enriches the bulk rock in K, Ba, and Na, and mobilizes other elements to varying extents. The few high temperature samples are characterized by mobilizes other elements to varying extents. The few high temperature samples are characterized by epidote, chlorite, quartz, oxides, and fibrous amphibole, which replace groundmass and phenocrysts, and fill cavities, and are presumed to have originated in zones of concentrated hydrothermal upflow.These three alteration types are similar to those seen in many ophiolites such as Troodos, where low temperatures prevailed in the volcanic section except in localized upflow zones. Alteration mineral chemistries are also broadly similar to those observed for the Troodos Ophiolite. Tonga forearc alteration differs from mid-oceanic ridge alteration in the presence of Al-rich dioctahedral smectites (not common in mid-oceanic ridge crust), the high Al content of saponite, and the predominance of K as an interlayer cation in clays. Hydrothermal alteration of the Tonga forearc is likely the product of extensive interaction with compositionally evolving seawater-derived fluids beginning at the time of emplacement. The distribution and intensity of alteration in these crustal sections depend principally on the porosity and permeability of the crust during alteration, which are influenced by the primary porosity, igneous morphology, and the presence of faults and fractures which could affect fluid flow. / Graduation date: 1999

This thesis describes laboratory measurements of the complex refractive index of volcanic ash particles. These measurements are needed to model the radiative impact of volcanic ash, vital for accurate satellite remote sensing. Three experimental methods have been developed, and the results for the complex refractive index and optical properties of a wide range of volcanic ash samples are presented. Measurements were made of the spectral transmission of radiation through suspended volcanic ash particles inside an aerosol cell, using a Fourier transform spectrometer at infrared wavelengths and two diffraction grating spectrometers covering ultraviolet, visible, and near-infrared wavelengths. In addition to the optical measurements, a suite of sampling and sizing instruments were connected downstream of the aerosol cell to measure the particle size distribution. The method was calibrated using two quartz samples. Mass extinction coefficients for nine volcanic ash samples, at 0.3-14 μm, are presented and show considerable variation. These variations are linked to the composition of the samples, measured using X-ray fluorescence (XRF) analysis. The complex refractive index, at 0.3-14 μm, of the two quartz samples and two samples of volcanic ash from the 2010 Eyjafjallajökull eruption were retrieved from the extinction measurements. The forward model used Mie theory and a classical damped harmonic oscillator (CDHO) model to represent the complex refractive index of the samples in terms of a finite set of band parameters, as well as the real refractive index of the sample in the small wavelength limit. Previous studies have shown that there is a redundancy in the retrievals between the band strength parameters and the real refractive index in the small wavelength limit, which can lead to spurious values for the retrieved complex refractive index. This problem was overcome by using an independent measurement of the real refractive index at a visible wavelength, to constrain the model parameter of the real refractive index in the short wavelength limit. Independent measurements of the complex refractive index at visible wavelengths are also important because the extinction produced at these wavelengths is highly sensitive to the particle size distribution, and any uncertainty in the measured size distribution will contribute to significant systematic error in the refractive index retrieved from extinction. The retrieved spectral complex refractive index of Eyjafjallajökull ash was applied using the ORAC retrieval scheme to measurements of the 2010 Eyjafjallajökull eruptionmade by theMODIS instrument aboard NASA's Terra satellite. Significant difference were found in the retrieved plume parameters of optical path, effective radius, and plume altitude, compared to assuming a literature measurement for the refractive index of pumice. For three discrete visible wavelengths (450, 546.7, and 650 nm) an optical microscope was used to make measurements of the complex refractive index of the volcanic ash samples. The long-established Becke line method was used to measure the real refractive index of the samples. For the imaginary refractive index, a new and novelmethod was developed involving measurements of the attenuation of light in individual particles. A strong linear correlation was found between the SiO₂ content of the samples and both their real and imaginary refractive indices at the visible wavelengths investigated. Furthermore, from the XRF compositional analysis of the samples values were calculated for the ratio of non-bridging oxygen atoms per tetrahedral cation (NBO/T), and it was found that NBO/T was an even stronger predictor of real refractive index at visible wavelengths. The optical microscope measurements could only be applied to particles with a radius larger than 10 μm. A new refractometer method was investigated for retrieving the real refractive index of submicron particles from colloidal reflectance measurements close to the critical angle in an internal reflection configuration. A coherent scattering model (CSM) was used to model the coherent reflection from a half-space of monodisperse or polydisperse particles, and a simple extension of the model is presented to properly account for the modified size distribution at the interface in an internal reflection set-up. A rigorous sensitivity analysis was performed to determine how experimental uncertainties propagate into uncertainty associated with the retrieved real refractive index, and the uncertainty due to non-spherical effects was estimated using T-matrix methods. Experimental reflectance data at a wavelength of 635 nm were obtained for spherical monodisperse polystyrene calibration particles, a polydisperse sand sample, and a polydisperse volcanic ash sample. The retrieved values for the real refractive index agreed, within propagated uncertainties, with values measured using other techniques. The method is shown to be a viable technique for measuring the real refractive index of small quantities of submicron particles, and can also retrieve the concentration and size of particles.

The aims of this thesis are to refine the tephrochronology of the Öræfi district, SE Iceland and assess the Holocene glacier fluctuations of the Öræfajökull ice cap. The pattern and timing of glacier fluctuations are determined using glacial geomorphology and tephrochronology, and the implications for palaeoclimate are assessed. Iceland is important to the study of global and regional climatic change because it is located close to both the marine and atmospheric Polar Fronts widely regarded as the key factors in the climate of the North Atlantic region. Six outlet glaciers were studied: Svinafellsjökull, Virkisjökull, Kotárjökull and Kviárjökull originating from the Öræfajökull ice cap and Skaftafellsjökull and Morsárjökull originating from the Vatnajökull ice cap. A long history of glacier fluctuations were found with a similar temporal pattern of glacier oscillation between the outlets of Vatnajökull and Öræfajökull. A maximum of eight advances have been identified. The oldest advance is inferred to date from the maximum of the last Glaciation ca. 18000 yrs BP. The first advance in the Holocene occurred ca. 9700 BP during a still-stand of the last Termination. The onset of the Neoglaciation occurred between 6000 BP and 4600 BP with an expansion of all of the studied glaciers. Subsequent smaller advances have been dated to ca. 3200 BP, ca. 1800 BP, ca. 700 BP, ca. 200 BP and ca. 80 BP. The most significant movement of the Polar front during the Holocene is likely to have occurred around 5000 BP, and, as a consequence, an estimated temperature cooling of ca. 2.5°C took place in Iceland, perhaps the greatest cooling since the last Termination. Within the broad pattern of change, glaciers in the study area show variability which represents local precipitation patterns, contrasting topography and change in glacier process. In this thesis a total of 22 silicic tephra layers are identified from over 90 profiles in the study area. The majority of these layers are dated to the latter part of the Holocene. Three silicic tephras were deposited during historical time (post 900 AD) namely, Vö ca. 900AD,HI104 and Ö1362. The Vö ca. 900 AD and the H1104 tephras are located for the first time. Specific prehistoric (pre 900 AD) tephras identified include Hekla-Ö, Hekla-4 and Hekla-S. The tephrochronology of the Öræfi district is also used to assess the eruption history of the Öræfajökull stratovolcano during the Holocene. Prehistoric eruptions are dated to ca. 9200 BP, ca. 6500 BP(?), ca. 4700 BP, ca. 2800 BP and ca. 1500 BP. Jökulhlaups accompanied the eruptions of 1727 AD, 1362 AD and ca. 1500 BP and are likely to have followed older eruptions of the volcano. A strong relationship occurs between volcanic activity of the Öræfajökull stratovolcano and the pattern of glacier fluctuations. This is explained as a response to isostatic crustal adjustment during ice cap growth and decay, and indicates a general relationship between volcanic activity and climate change.

Yucca Mountain, Nevada is the proposed site for the underground storage of high-level civilian nuclear waste in the United States. The repository must be isolated from the general environment for at least 10,000 years. Ground water and gases are potential carriers of radioactive materials. Fractures and connected pores in the host rock are the major pathways for ground water and gases. Therefore, the mechanical and flow properties of the host rock should be understood and utilized in the design of the underground repository. Samples of Topopah Spring tuff from Yucca Mountain were used in this study. Cylindrical specimens were prepared to perform uniaxial and triaxial "damage" tests where specimens are loaded to a particular stress level to induce damage and fracturing and then unloaded. Mechanisms of microcracking at different fracturing levels have been studied by using both an optical microscope and a Scanning Electron Microscope (SEM). The original rock sample without loading was also observed under the microscopes. Many kinds of defects including pores, preexisting fractures, and soft inclusions were found in the undamaged Apache Leap tuff samples. Pores were determined to be the main microstructures in Topopah Spring tuff that could influence the mechanical and hydrologic properties. Under compressive stresses, microcracking initiates from some of the pores. These microcracks will interact and coalescence to form large microcracks or macroscopic cracks as the load is increased. Crack propagation phenomena, such as pore cracking, pore linking, pore collapse and the formation of en echelon arrays were all found in specimens unloaded prior to complete failure. The failure of tuff specimens is often through a process of shear localization. In summary, the deformation and failure of both tuffs occurred by progressive fracturing, starting from microcracking on the small scale, and ending as fractures coalesced to form macroscopic fractures and shear localization. On the basis of the experimental studies, micromechanical models such as the pore collapse model and the pore linking model have been developed based on fracture mechanics theory. These models are used to predict the constitutive behavior for tuff and the predicted stress-strain curves match well with the experimental curves.

Porous plates delivered calcium chloride at a negative potential to the top of blocks of partially welded (20.1 x 20.1 x 66.6 cm) and densely welded (30.1 x 20.1 x 48.1 cm) tuff with discrete fractures. During infiltration, flux increased through the partially welded block's fracture as the applied suction was lowered to 2.3 cm. The wetting front advanced 66.6 cm in 239 days. Chloride concentration and temporal moments from five tracer tests with 0 to 5 cm of applied suction indicated that preferential fracture flow occurred. Displacement transducer data reflect a decrease in fracture aperture at several months prior to but not during tracer tests. Fracture transmissivities decreased an order of magnitude (6.4 x 10⁻⁹ to 4.2 x 10⁻¹⁰ M²/s) as the applied suction increased from 0 to 5 cm while the tensiometer data indicated a suction of about 20 cm of water within the fracture and matrix. Highest during infiltration to an initially dry block, inflow losses of 3 to 44 percent due to evaporation are the greatest source of error for the constant potential method used.

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-volume silicic eruptions are often evacuated from magma reservoirs which display gradients in composition, temperature, crystallinity, and volatile content. The 9.7 Ma Devine Canyon Tuff (DCT) of eastern Oregon represents such an eruption, with >300 km³ of compositionally zoned pyroclastic material deposited as a variably-welded ignimbrite. The ignimbrite displays homogenous bulk tuff major element compositions with a wide range of trace element compositions, allowing for the investigation of how these magmas were generated, stored, and modified in the magma reservoir by studying pumices which represent the primary magmas composing the DCT. Five pumices ranging from dacite to rhyolite bulk compositions were selected across the range of trace element compositions and were crushed and sieved to measure how crystallinity and mineral abundances change within each pumice at different particle size fractions. Single alkali feldspar and clinopyroxene crystals were analyzed using EMP and LA ICP-MS from each pumice. Physical results yielded a systematic decrease in crystallinity from 22% to 3% going from the dacite to the most evolved rhyolite composition, with the highest crystallinity occurring between <991-425 microns for all pumices analyzed. The dacite pumices displayed a glomerocrystic texture not observed in rhyolite pumices. Two populations of crystals were distinguished using single crystal chemical data, one belonging to the rhyolitic magmas and another belonging to the dacitic magma. Acquired mineral data have relevance for how strongly zoned with regard to trace elements the rhyolitic magmas of the DCT were, how these magmas were generated, and how they were stored within the magma reservoir. Applying melt extraction models to explain observed patterns in trace element compositions between pumices is problematic. In this model, the observed range of trace elements in rhyolite pumices would be attributed to two separate melt extraction events from an intermediate crystal mush where the first expulsion of melt from the mush produced the most evolved rhyolite composition and a second expulsion coupled with partial melting produced a second rhyolite with an indistinguishable major element composition but less evolved trace element composition. Mixing of these two rhyolite end members would then be needed for generating the range of intermediate rhyolite compositions. Magma mixing modeled using a mixing equation produced a poor fit for trace elements, suggesting the range of observed trace element compositions cannot be solely generated through the mixing of the extracted rhyolite melts but require processes that subsequently modify the mixed rhyolite compositions. The occurrence of crystal aggregates in the dacite may represent fragments of the crystal mush. However, the dacite was unlikely produced by partial remelting of the crystal mush, generating a less evolved, more intermediate bulk composition. In summary, mush extraction combined with partial melting of the crystal mush and mixing of compositional end members cannot fully explain the trace element patterns observed in the DCT pumices thus warranting further study.

The Dinner Creek Tuff erupted during a period of rhyolitic volcanism coeval to the flood volcanism associated with the Columbia River Basalt Group. The High Rock Caldera Complex, Lake Owyhee and McDermitt volcanic fields account for ~90% of the rhyolites erupted between 16.7-15.0 Ma. Situated at the northern end of the Lake Owyhee volcanic field, the Dinner Creek Tuff was originally mapped as a ~2,000 km2 single ignimbrite confined to the Malheur Gorge. Streck et al. (2015) correlated tuff outcrops previously mapped as generic Miocene welded tuff as well as local units such as the "Mascall" or "Pleasant Valley" tuff of eastern Oregon to individual cooling units that comprise the newly redefined Dinner Creek Tuff, enclosing an area of ~25,000 km2. Areal extents defined in this study show that all outcrops now determined to be Dinner Creek Tuff enclose an area of ~31,800 km2 not including any fallout deposits that likely extended beyond the defined area. Although Dinner Creek Tuff rhyolites have nearly identical compositions, different ages and subtle geochemical and mineralogical differences exist and were used to divide the Dinner Creek Tuff into four discrete cooling units. Except for unit 4, the units are lithologically very similar. Unit 1 is the Dinner Creek Tuff unit associated with the Malheur Gorge type section. The four cooling units have ages of 16.15-16 Ma (unit 1), 15.6-15.5 Ma (unit 2), 15.46 Ma (unit 3) and 15.0 Ma (unit 4). Areal extents were established for all four cooling units based on feldspar compositions along with lithological and bulk rock geochemical data. Minimal extents of individual units are as follows: ~22,590 km2 (unit 1), ~17,920 km2 (unit 2), ~14,170 km2 (unit 3) and ~8,370 km2 (unit 4). Using conservative thicknesses, determined erupted tuff volumes are ~170 km3 (unit 1), ~125 km3 (unit 2), ~99 km3 (unit 3) and ~46 km3 (unit 4), totaling ~440 km3 and dense rock equivalents are ~152 km3 (unit 1), ~96 km3 (unit 2), ~76 km3 (unit 3) and ~31 km3 (unit 4), totaling ~356 km3. These extents and volumes are the absolute minimum based solely on the locations of exposed tuff sections and the inclusion of the source. Centering eruptive units on source areas where they are known, expands the tuff extents into a more radial pattern as would be expected for low-aspect ratio, high energy ash-flow tuff eruptions. These probable extents increase the areal extents of the individual units to: ~36,900 km2 (unit 1), ~31,660 km2 (unit 2), ~17,290 km2 (unit 3) and ~10,150 km2 (unit 4) distributed over a ~43,490 km2 area. Likewise, erupted tuff volume and dense rock equivalents also increase: volume-- ~277 km3 (unit 1), ~222 km3 (unit 2), ~121 km3 (unit 3) and ~56 km3 (unit 4); DRE-- ~248 km3 (unit 1), ~170 km3 (unit 2), ~93 km3 (unit 3) and ~38 km3 (unit 4). New mapping confirms previous hypotheses that the Castle Rock caldera erupted unit 1 and identified the new Ironside Mountain caldera as the source for unit 2 while precise source areas for unit 3 and 4 are not yet known but are thought to lie within the Dinner Creek Eruptive Center. Minimal calculated caldera volumes for units 1 and 2 are ~98.5 km3 (unit 1) and ~31.1 km3 (unit 2). Adding the thick ponded intra caldera tuff volume to the determined and probable erupted tuff volumes determined in this study, increases the erupted volumes to ~268 km3 (determined) and ~375 km3 (probable) for unit 1 along with ~157 km3 (determined) and ~253 km3 (probable) for unit 2. DREs increase to ~251 km3 (determined) and ~347 km3 (probable) for unit 1 along with ~128 km3 (determined) and ~202 km3 (probable) for unit 2.

The Wildcat Creek Tuff is a thin (~3-12 m), rhyolite to andesitic ash-flow tuff with a minimal extent of 1500 km2 in Malheur county, eastern Oregon. The previously undated tuff yielded a single crystal, anorthoclase 40Ar/39Ar age of 15.49±0.02 Ma and thus is closely related to mafic and silicic volcanism of the Columbia River Province. The tuff texturally stands out by its high proportion of co-mingled mafic inclusions appearing as dark, scoriaceous, and phenocryst-poor fragments, and their proportion dictate bulk tuff compositions ranging from rhyolite (74% SiO2) to andesite (59% SiO2). Glass analyses confirm rhyolite end member at 74-75 wt.% SiO2 and two mafic members, one at 59-60 wt.% SiO2 and the other at 56-57 wt.% SiO2. Rare plagioclase and even rarer pyroxene phenocrysts with compositions clustering at An60-74 and An35-45, and Mg17-19 and Mg80-84, respectively, similarly suggest two andesitic magmas with the 60% member being the dominant mafic composition. It has distinctly lower TiO2 and CaO, slightly lower FeO, and comparable Al2O3, MgO, and alkalis. Eruption of crystal-poor dacitic to basaltic-andesitic cognate components is also observed in other Miocene ash-flow tuffs from eastern Oregon, like the Rattlesnake, Dinner Creek, and the Devine Canyon Tuffs, as well as other less voluminous tuffs. However, the high proportion of mafic components in the Wildcat Creek tuff seems currently unrivaled. The co-eruption of intermediate magmas with rhyolite implies that mafic magmas were tapped from a common reservoir, and these magmas increased in proportion during the course of the eruption(s). This continued up to the point where nearly all deposited tuff material consisted of andesite. This is consistent with progressively deeper magma withdrawal, in turn implying that mafic magmas resided below the rhyolites as a discrete magma batch. Dacitic components of voluminous rhyolitic tuffs have been recently interpreted as remelted samples of a crystal mush after crystal-poor rhyolites where extracted. Dacitic Wildcat Creek Tuff samples do not bear any evidence of this. To the contrary, small negative Eu anomalies, normal Ba and Sr concentrations, and nearly aphyric nature are consistent with a large portion of mixing between Wildcat Creek Tuff rhyolites and regional mid Miocene, Fe-rich, and crystal poor basaltic andesite magmas that occur ubiquitously as lava flows.

Detailed mapping and granulometric analyses of the 1992 Cerro Negro tephra blanket reveal remarkable departures from the expected distribution of tephra. Isomass maps show that the major axis of dispersion for the eruption was to the SW of the cone and that the coarser-grained particles, ranging from -4.0 -- 1.0 f, were deposited primarily along the major axis of dispersion with deposits thinning off of the axis. Comparable isomass maps for finer-grained particles, 1.5 - 3.5 f, show that these particles were primarily deposited along the edges of the deposit, off of the major axis of dispersion. Advection-diffusion models for tephra fallout currently widely used in volcanology do not account for this deposition pattern. Rather, it appears that interaction between the wind field, which developed a strong cross flow during the eruption, and the ascending tephra plume resulted in the formation of turbulent structure in the plume. Particles with a settling velocity greater than ~1-2m/s (diameter >0.5 mm) were able to overcome the turbulent structure and settled in a manner predicted by the advection-diffusion equation. Those with lower settling velocities were caught up in turbulent structure and deposited off of the major axis of dispersion, near the edges of the overall tephra blanket. Thus, this data set provides the first estimate of the strength of such turbulent structures in advecting plumes, and illustrates the limitations of the typical advection-diffusion models in describing some transport processes.