The Breitenbush Hot Springs area lies on the boundary of folded middle to late Tertiary Western Cascade rocks and younger High Cascade rocks. Within the mapped area the Western Cascade rocks are represented by four formations. The Detroit Beds, a sequence of interstratified tuffaceous sandstone, mudflow breccia, and tuff, is overlain unconformably by the Breitenbush Tuff. The Breitenbush Tuff consists of three units of welded pumice-rich crystal-vitric ash-flow tuffs interbedded with tuffaceous sedimentary rocks. The Outerson Formation unconformably overlies the Breitenbush Tuff and consists primarily of basaltic lava and breccia. The Outerson Formation includes three localized members: a basal, glassy, aphanitic basalt, the Lake Leone Sediments, and the Outerson Tuff. The Outerson Formation is cut by a number of feeder dikes and plugs and is unconformably overlain by the Cheat Creek Sediments, composed of volcanic sedimentary rocks and a distinctive basaltic tuff. The Western Cascade formations total more than 1660 m {5500 ft) of strata and range from Oligocene to Pliocene in age. The High Cascade rocks are represented by two formations: the Triangulation Peak Volcanics of basalt and andesite lava and breccia, lying unconformably atop the Cheat Creek Sediments; and unconformably beneath the Collowash Volcanics, a series of thin basaltic lava flows and breccias. The Western and High Cascade rocks are covered extensively by surficial deposits, primarily glacial drift. The High Cascade formations are at least 840 m (2800 ft) thick, ranging in age from Pliocene to Pliestocene. The Western Cascade rocks have been folded and faulted in the Breitenbush Hot Springs area, and form the eastern limb of the north-trending Breitenbush Anticline. The folded rocks and the erosional unconformities between the rock units probably represent two local episodes of orogeny: one in early to middle Miocene and another in late Pliocene to Pleistocene time. The Outerson Formation represents a depositional sequence between the periods of uplift and deformation. Faulting accompanied the orogenic sequences. The primary volcanic landforms in the area have been destroyed by erosion but skeletal remains of High Cascade volcanoes are recognized. Stream erosion and glaciation are responsible for the present landforms. Breitenbush Hot Springs occurs, in part, along basaltic dikes which channel the water through impermeable Breitenbush Tuff. The dikes are believed to be associated with the Outerson basalts. The Hot Springs discharge upwards at 3400 l/min. (900 gpm) of water at temperatures up to 92°C (198°F).

The Tillamook Highlands is a largely unmapped volcanic pile located in the north end of the Coast Range of Oregon. The 36 square miles of T. 1 N., R. 8 W., on the southcentral margin of the Highlands, was chosen for detailed study. The study area is composed of Eocene age sedimentary and volcanic units which were deposited in a filling basin. The lowest units were deposited in moderate to deep marine waters; the uppermost were deposited subaerially.

<p> The Rotoiti Breccia Formation is a pyroclastic flow deposit located at the northern end of the Taupo volcanic zone, North Island New Zealand. It was formed by a fairly low energy subaerial eruption and subsequent flowage, which restricted it to the topographic low between two older ignimbrites.</p> <p> Samples from proximal, medial and distal areas of the flow were examined to determine changes in grain size and particle type distribution as the flow progressed. It was found that on the whole, the flow was very homogenous. However, some initial turbulence at the very beginning of the flow has produced a bimodal grain size distribution presumably due to increased mechanical breakage. A much more prominant crystal population was found in the distal areas of the flow due to rounding of pumice grains and the elutriation of the resultant fines.</p> / Thesis / Bachelor of Science (BSc)

No description available.

Two mid-Miocene (16.5-15 Ma) rhyolite volcanic centers in eastern Oregon, the Buchanan rhyolite complex and Dooley Mountain rhyolite complex, were investigated to characterize eruptive units through field and laboratory analysis. Results of petrographic and geochemical analysis add to field observations to differentiate and discriminate the eruptive units. Additionally, new geochemical data are used to correlate stratigraphically younger and older basalt and ash-flow tuff units with regional eruptive units to constrain the eruptive periods with modern Ar-Ar age dates. Previous work at the Buchanan rhyolite complex was limited to regional mapping (Piper et al., 1939; Greene et al., 1972) and brief mention of the possibility of multiple eruptive units (Walker, 1979). Observed stratigraphic relationships and geochemical analysis were used to identify eight distinct eruptive units and create a geologic map of their distribution. Slight differences in trace element enrichment are seen in mantle normalized values of Ba, Sr, P, Ti and Nd-Zr-Hf and are used to differentiate eruptive units. New geochemical analyses are used to correlate the overlying Buchanan ash-flow tuff (Brown and McLean, 1980) and two underlying mafic units to the Wildcat Creek ash-flow tuff (~15.9 Ma, Hooper et al., 2002) and flows of the Upper Steens Basalt (~16.57 Ma, Brueseke et al., 2007), respectively, bracketing the eruptive age of the Buchanan rhyolite complex to between ~16.5 and ~15.9 Ma (Brueseke et al., 2007; Hooper et al., 2002). The Dooley Mountain rhyolite complex was thoroughly mapped by the U.S. Geological Survey (Evans, 1992) and geochemically differentiated in a previous Portland State University M.S. thesis (Whitson, 1988); however, discrepancies between published interpretations and field observations necessitated modern geochemical data and revisions to geologic interpretations. Field and laboratory studies indicate that the Dooley Mountain rhyolite complex consists of multiple eruptive units that were effusive domes and flows with associated explosive eruptions subordinate in volume. At least four geochemically distinct eruptive units are described with variations in Ba, Sr, Zr and Nb. Picture Gorge Basalt flows and Dinner Creek Tuff units found within the study area both overlay and underlie the Dooley Mountain rhyolite complex. These stratigraphic relationships are consistent with the one existing Ar-Ar age date 15.59±0.04 Ma (Hess, 2014) for the Dooley rhyolite complex, bracketing the eruptive period between ~16.0 and ~15.2 Ma (Streck et al., 2015; Barry et al., 2013). The findings of this study indicate that the Buchanan rhyolite complex and the Dooley Mountain rhyolite complex are the westernmost and northernmost rhyolite complexes among the earliest (16-16.5 Ma) mid-Miocene rhyolites associated with initiation of Yellowstone hot spot related volcanism.

Two adjacent caldera systems, the Mahogany Mountain and the Three Fingers caldera constitute voluminous rhyolitic volcanic deposits on the eastern margin of the Oregon-Idaho graben during the middle-Miocene. Both calderas are part of the Lake Owyhee volcanic field that in turn is part of widespread rhyolite deposits associated with the Columbia River Basalt province. We focus on establishing relationships between intracaldera units of Three Fingers caldera and caldera-forming tuff of Spring Creek and surveying the distribution of entrained mafic clasts which often display anomalous concentrations of rare earth elements. Previous mapping identified two intra-caldera facies and one outflow facies of the tuff of Spring Creek, in addition to a younger rhyolite within the caldera (Trp). New 40Ar/39Ar dates show these units are nearly time equivalent at 15.64 ± 0.08 Ma for Trp and 15.64 ± 0.09 Ma for tuff of Spring Creek. Field evidence shows extensive coverage of Trp and associated facies emplaced after a period of sedimentation within the caldera. The main reinterpretations are: i) the mostly devitrified units of Trp are time equivalent to flows and domes of glassy, vesicular, or brecciated rhyolite previously mapped as intra-caldera tuff of Spring Creek; and ii) mafic clasts present in dense glass and porous rhyolite are fragments of mafic lava flows entrained by the subsequent eruptions. New geochemical and mineralogical evidence clearly distinguish the outflow tuff of Spring Creek and intracaldera rhyolites. Compared to the outflow tuff, intracaldera rhyolite flows are less Fe-rich, (2 vs. 3 wt.% FeO), and higher silica (77 vs. 74 wt.% SiO2) rhyolites that lack vitrophyric texture. I interpret the investigated area as a rhyolite dome field, erupted subsequent to caldera collapse. The proximity of vents resulted in a complex stratigraphic overlap of rhyolite flows and clastic debris issued from coalescing domes. The predominance of high-standing dome interiors reflects the more resistant nature of dense devitrified rhyolite as compared to pumiceous, glassy, or brecciated facies of intra-caldera rhyolite. Enrichment of REE in mafic clasts is highly variable, and does not correlate with their entrainment in a specific facies of intra-caldera rhyolite. Individual clasts contain up to 2400 ppm Nd, 1800 ppm Ce, and 1400 ppm La in the most enriched samples. Linear regression shows these highly anomalous concentrations are not correlated with variations in major element chemistry between enriched and un-enriched clasts. The geographic extent of mafic clast-bearing units is limited to less than 5 percent of the area mapped, and their distribution within these units is typically volumetrically insignificant, limiting their economic potential. Mechanisms for enrichment of REE within these rocks is however significant to our understanding of a yet unexplained phenomenon and may lead to further discoveries with greater economic potential.

The Strawberry Volcanics of Northeast Oregon are a group of geochemically related lavas with a diverse chemical range (basalt to rhyolite) that erupted between 16.2 and 12.5 Ma and co-erupted with the large, (~200,000 km3) Middle Miocene tholeiitic lavas of the Columbia River Basalt Group (CRBG), which erupted near and geographically surround the Strawberry Volcanics. The rhyolitic lavas of the Strawberry Volcanics produced the oldest 40Ar/39Ar ages measured in this study with ages ranging from 16.2 Ma to 14.6 Ma, and have an estimated total erupted volume of 100 km3. The mafic and intermediate lavas of the Strawberry Volcanics include both tholeiitic and calc-alkaline compositions; calc-alkaline andesite is the dominant type by volume. 40Ar/39Ar ages of the mafic and intermediate lava flows range from 15.6 Ma to 12.5 Ma, and volume estimates of the intermediate lavas are approximately 1,100 km3. The magmas that gave rise to the Strawberry Volcanics traveled to the surface through numerous dikes, some of which have been exposed at the surface and supplied lava to fissure – style eruptions and/or shield volcanoes. Herein, we show that the Strawberry Volcanics are related to the CRBG in both time and space and share a chemical affinity, specifically to the Steens Basalt. Chemical similarities are observed in normalized trace element patterns, selected trace element ratios, and radiogenic isotopes. Comparison of the Strawberry Volcanic rhyolites to the other Middle Miocene rhyolites of eastern Oregon associated with the initiation of the Yellowstone – Snake River mantle plume reveals similar eruption ages, trace element compositions, including the rare earth elements (REEs), and "A-type" rhyolite characteristics. These data suggest that the Strawberry Volcanics are part of the regional volcanism (basalt to rhyolite) of the Columbia River Basalt Province. The petrogenesis of the Strawberry Volcanics can be explained as follows: 1) The tholeiitic, intermediate magmas were produced by fractional crystallization of mafic magmas, which have a commonality with the surrounding Columbia River Basalt Group; 2) The calc-alkaline magmas are a result of mixing between tholeiitic basalt, rhyolite, and crust. The arc-like signature of the calc-alkaline lavas (elevated large ion lithophiles) is a result of both the melting source region and the end-members with which the mafic magmas mixed/contaminated. Other authors have produced similar findings from within the Basin and Range/Oregon-Idaho graben and CRB province. The difference at the Strawberry Volcanics is that there is no need for a primitive calc-alkaline magma or extensive fractional crystallization to generate the calc-alkaline andesites. Alternatively, the calc-alkaline magmas of the Strawberry Volcanics were generated by a more primitive tholeiitic magma than erupted at the surface, which interacted with crustal melts and assimilated crustal lithologies from the complex zone of assimilated terranes that make up the basement of eastern Oregon.

The West Elk Breccia has been studied since the late 1800’s with many interpretations regarding its origin. One unrecognized possibility is that parts of it are debris-avalanche deposits. This study has recognized evidence for this interpretation at three scales: volcano scale, outcrop scale, and intra-outcrop scale. At the volcano scale, a scarp in the old volcano reveals underlying Mesozoic bedrock, suggesting sector collapse. At the outcrop scale, megablocks of the original edifice, up to hundreds of meters in length, have atypical orientations and are surrounded by a gravel matrix. At the intra-outcrop scale, jigsaw-fit fracturing and rip-up clasts are common in distal deposits, which are documented in analogous debris-avalanche deposits. Similar to the debris-avalanche deposit at Mt. Shasta, medial-to-distal-matrix volcaniclast content decreases by 23%; Paleozoic and Mesozoic clasts increase by 5%; and the size of megablocks decreases. The geochemical and petrographic signatures reveal breccia blocks composed of pyroxene-andesite, a more silicic matrix facies, and the andesitic-to-dacitic East Elk Creek Tuff, all compositions that corroborate previous work on this northern extension of the San Juan volcanic field. Measured sections in the 100-km² study area allow for an estimation of total formation volume of approximately 8.5 km3.

The Dabbahu-Manda Hararo rift segment is a ~25 x 60 km rift zone in Afar, Ethiopia, where a series of axial dyke intrusions has recently occurred. Basaltic eruptions associated with individual dyking events between 2007-2010 have been fed from fissures along the rift axis and been relatively short-lived events lasting less than 60 hours. The volume of melt delivered to the rift surface by these eruptions has been a minor component of the total melt volume supplied to the shallow crust since the onset of the active rifting phase in 2005 and the current intruded to erupted melt ratio for the 2005-2010 period is ~260:1. This is below typical values for magmatic rift zones and may suggest that further volcanism is likely to occur before this activity ceases. ⁴⁰Ar/³⁹Ar geochronology of basaltic lavas from the flank of the rift and from a region of off-axis volcanism to the west of the rift zone gives ages of 25 – 450 ka. These constrain the development of a prominent axial graben in the northern part of the rift to < 30 ka and based on the age-distribution of lavas across the rift flank suggests that volcanism has been focused to the present neo-volcanic zone for at least 200 ka. Geochemical and isotopic constraints on melt generation suggest ~4-6 % partial melting of fertile mantle beneath rift at depth of ~100-75 km. Lavas erupted at the rift axis and from off-axis volcanoes are derived from a common mantle source, however, axial lavas are shown to represent slightly greater extents of partial melting suggesting a focused mantle melting anomaly, such as those seen at ocean ridges, is forming beneath the rift zone.

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.