1 10 2 https://theses.otagogeology.org.nz/files/original/48572e4d61711ae8b65345e14aad2f84.pdf 7161d97d9bf6331b7bf4380fe1e78fe6 Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Geology theses OU Geology thesis Thesis or dissertation completed by University of Otago Geology students Location WKT (WGS84) The location stored in WKT (WGS84) format MULTIPOLYGON (((-18.4711923145008 63.5416956421709,-18.9703197424945 63.50349596139,-19.1271883627211 63.813963623143,-18.6280609347274 63.8517463583428,-18.4711923145008 63.5416956421709))) Author last name Last name of the Author Gorny Project type Is it an MSc, PhD, BSc(Hons) or PGDipSci? PhD Advisers Who supervised/advised this student White, J.D.L. Gudmundsson, M.T. Abstract The Abstract for this thesis Large scale subglacial eruptions are enigmatic. Their eruption and emplacement dynamics are not well understood due to the incompleteness of preserved units. The Snæbýlisheiði unit, south Iceland, forms a ca. 27 cubic km elongate, flat-topped ridge of volcaniclastic debris coupled with and intruded by coherent basalt and represents a large-scale subglacial basaltic fissure eruption, where both the eruption and ridge growth occurred subglacially. It is preserved in its entirety from the eruption site stretching over 34 km towards the modern day coastline. Snæbýlisheiði differs from typical Icelandic subglacial deposits in being elongate perpendicular to the controlling rift direction reflecting the high eruption rates and the overlying glacier slope at the time of eruption. Investigation of the source area reveals voluminous volcaniclastic debris encased by and complexly intruded into by coherent basalt; I infer early and continuing production of pyroclastic deposits with near-synchronous emplacement of peperitic intrusions. Volcaniclastic debris accumulated at the eruption site and prograded towards the present coastline by deposition in an enlarging drainage network. Deposition took place both in migrating and converging tunnels and during short intervals of sheet flow during outbreak floods. The main body of the deposit is characterized by complexly bedded volcaniclastic debris coupled with and intruded by a longitudinally extensive basalt sheet. The deposits result from an intricate depositional and intrusive history, with a longitudinally extensive internal basalt sheet comprising complex and irregular coherent bodies with dikes, apophyses, horns, tendrils, and lobate fingers that extend into the surrounding host debris. Peperitic margins, where dynamic mingling or quench fragmentation occurred, are common. The basalt sheet fed higher-level intrusions through irregular apophyses as it propagated down-flow to produce the multilevel intrusions exposed today. During propagation the coherent basalt sheet concentrated into gently meandering and locally bifurcating conduits having sheet-lobe extensions. This channelized or conduit-concentrated magma propagation was very thermally and mechanically efficient, insulating the molten basalt and hence reducing rheological changes from cooling. This efficiency is reflected by the intrusions’ propagation more than 34 km from the source area or eruption site with little change to overall intrusion morphology. The sheet forms apophyses and tendrils into the overlying sediment where it dynamically mingled to form extensive peperitic textures. In places, apophyses locally intersected the sediment-water interface and shed clasts into flowing water. These “contorticlasts” have “rolled-up” forms encasing matrix-forming debris. The main body of the intrusion propagated through the host in a channelized fashion, gently meandering within the edifice, with thin sheet-like extensions along the margins. Tendrils and apophyses extending from the main body extended into the overlying host, where they experienced enhanced cooling and frictional interactions resulting in efficient brittle fragmentation. The resulting fragments are typically angular, dense, and glassy medium lapilli to coarse ash sized and make up the bulk of the distal deposits. Where apophyses remain attached to the basal intrusion the margins fragment in the ductile regime forming fluidal-type fragments. Bed contacts are sharp to diffuse, locally erosive, indicating deposition from traction or/and high-concentration flows. Shedding of contorticlasts into bedload beneath flowing water requires that their source intrusions shoaled, hence implying the voluminous intrusions would have also been transferring heat into the water with only a thin, deforming clastic blanket to reduce the heat flux into water and hence overlying ice. As the eruption is influenced by the ice, the ice is influenced by the eruption. The vast amounts of heat release during fragmentation will melt ice above the fragmentation zone. Using simplified ice mechanics and thermodynamic principles the total volume of ice melted can be calculated for a subglacial volcano of any type. For the Snæbýlisheiði unit, the thermal model was constructed to consider the consequences of the formation of the unit based on field and other observational data. The thermal model for the extent of ice melted by the emplacement of the Snæbýlisheiði unit combined with field observations and dissolved volatile contents in matrix glass suggests that the eruption likely occurred during the PreBoreal or Younger Dryas stadials. This is the first description of a complete unit formed by a large-scale subglacial basaltic fissure eruption. OURArchive handle The handle from the Otago University Research Archive (OURArchive) <a href="http://hdl.handle.net/10523/7031">http://hdl.handle.net/10523/7031</a> OURArchvive access level Abstract Only Department The department where the student is studying primarily. Geology Named locality Named locality describing the field area location. South Iceland Thesis description Number of pages, maps, CDs, etc. 791 pages A4, 2 A1 maps in back pocket folded Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context 2017Gorny Creator An entity primarily responsible for making the resource Gorny, Carolyn Fine (Carolyn) Date A point or period of time associated with an event in the lifecycle of the resource 2017 Title A name given to the resource The Origin of Lava-Hyaloclastite Sheets, South Iceland Subject The topic of the resource Volcanology Dalsheidi Eruption dynamics Fissure eruption Glaciofluvial Glaciovolcanism Glass microtextures Ice melting Iceland Intrusions Juvenile clast microtextures lava-hyaloclastite sheets Lithofacies analysis Magma fragmentation Magma-water interactions Particle analysis Peperite Pipe-flow Sida Group Slurry Subglacial eruption Subglacial sediment transport/deposition Surtseyan eruption Thermal model Tholeiitic basalt Vesicle number density volatiles Volcano-ice interactions https://theses.otagogeology.org.nz/files/original/d63c34745f52b6d20eb179916555e303.pdf 52a198a7321c7e4bc81a25c93ee14a08 Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Geology theses OU Geology thesis Thesis or dissertation completed by University of Otago Geology students Author last name Last name of the Author Murtagh Project type Is it an MSc, PhD, BSc(Hons) or PGDipSci? PhD Advisers Who supervised/advised this student White, J.D.L. Abstract The Abstract for this thesis Explosive basaltic eruptions present a challenge to modelling volcanic behaviour. The main focus of this thesis is the examination of pyroclastic micro-textures and morphology of the ash fraction in order to constrain the conduit conditions during ascent and at the time of fragmentation, respectively in the lead up to such explosive volcanic eruptions. Three sites, Black Point, California, Pahvant Butte, Utah and Ilchulbong, Korea, were considered. I propose that, in certain cases, conduit processes can collaborate to modify basaltic magma rheology, which can be complemented by powerful water-magma interactions to result in an intense and violent eruption. Results of micro-textural data show: (1) vesicularity of shallow subaqueous basaltic (“Surtseyan") tephra can reach high values (up to 92% at Pahvant Butte) and vesicularity ranges are broad (e.g. from 6% to 92%, for Pahvant Butte pre-emergent mound). Quantitative values combined with textural observations (e.g. vesicle shape and pattern) indicate a strong heterogeneity of vesiculating magma. (2) vesicle number densities are high (the minimum is at Black Point, 5.4 x 102 vesicles per mm3 and the maximum is seen at both Black Point and Pahvant Butte, 4.8 x 104 vesicles per mm3). Finally, (3) extensive microlite formation is common within Surtseyan pyroclasts examined here. Results of ash morphology analyses show 75% of Black Point ash and 80% of Pahvant Butte ash fragments fall within the hydromagmatic fragmentation field. Furthermore, strong evidence for water-magma interaction (fracture-bound, stepped, cracked and etched surfaces) is seen, as is a dominance of thermohydraulic mechanism of fragmentation. Due to the thermodynamic properties of water and magma this fragmentation regime is considered the most efficient and explosive. A model, common to the three study sites, of shallow conduit conditions and fragmentation leading to eruption is proposed here. Early nucleation at depth is followed by ascent and temporary arrest of magma resulting in a mature (relatively large equivalent diameters) vesicle population and a variable amount of microphenocrysts, both juvenile and xenocrystic. A decompression event, possibly induced by injection of a fresh batch of magma and/or, later in the eruption, flank instability and failure, induces a late stage of bubble nucleation and consequently the final ascent of the magma is rapid and thought to follow a parabolic ascent profile. Simultaneous to rapid ascent microlite nucleation occurs. This burst of nucleation is prompted by high volatile contents (Black Point 0.64-1.92 wt% and Pahvant Butte 0.80-1.29 wt% H2Ot) and decompression. Magma rheology is progressively altered, particularly the viscosity. These processes in combination are sufficient drivers of an explosive eruption. In the Surtseyan cases presented here, conduit processes act to prime the basaltic magma for explosive interaction with an external body of water, however, explosivity cannot be directly linked to the presence of external water alone. In summary, the Surtseyan eruptions presented in this study are interpreted to have been as a result of exceptional conduit conditions which aided explosive eruptive behaviour that is uncharacteristic for basaltic magmas. It is speculated that had these eruptions occurred in a subaerial setting the outcome could have been as dramatic as well documented sub-Plinian and Plinian eruptions. OURArchive handle The handle from the Otago University Research Archive (OURArchive) <a href="http://hdl.handle.net/10523/1982">http://hdl.handle.net/10523/1982</a> OURArchvive access level Open Department The department where the student is studying primarily. Geology Named locality Named locality describing the field area location. USA Thesis description Number of pages, maps, CDs, etc. xi, 242 p. : ill. (some col.) ; 30 cm. + 1 CD-ROM (4 3/4 in.) Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context 2011Murtagh Creator An entity primarily responsible for making the resource Murtagh, Rachel M. (Rachel Maria) Date A point or period of time associated with an event in the lifecycle of the resource 2011 Title A name given to the resource An investigation into the explosivity of shallow subaqueous basaltic eruptions Subject The topic of the resource Igneous volcanology basalt MFCI Surtseyan eruption Vesicle micro-texture Vesicle number density volcanic ash