Studies of the evolution of hotspot swells, mantle plumes, and Hawaiian basalts are presented in three parts in this dissertation. In part 1, the evolution of mid-plate hotspot swells are simulated numerically as an oceanic plate rides over a hot, upwelling mantle plume. The transient heat transfer equations, with time- and space-dependent boundary conditions, are solved in cylindrical coordinates. Geophysical data are used to constrain the models. Formation of the Hawaiian swell requires a mechanism of convective thinning of the lithosphere. The models constrain the Hawaiian heat source to have a maximum anomalous temperature of 250-300°C, and a perturbing heat flux 5-6 times the background value. On the other hand, the Bermuda swell is likely produced by heat conduction due to weakness of the heat source. In part 2, an analytic model of axisymmetric mantle plumes is presented. Plume parameters beneath the lithosphere, which are constrained from the swell models, are used to infer the plume source regions. The Hawaiian plume likely originates near the core-mantle boundary, but other hotspots may have shallower sources. Chemical plumes are much narrower than thermal plumes because of low chemical diffusivity in the mantle. For mantle plumes driven by combined thermal-chemical diffusion, the chemical signature of the source regions may only be observed near plume centers. Finally, melt generation and extraction along the Hawaiian volcanic chain are discussed in part 3. As a part of the plate moves over the heat source, melting largely takes place in the region where the lithospheric material is engulfed and swept away by the flow of the heat source. At least three mantle components must be involved in the melt generation: the plume material, the asthenosphere, and the engulfed lithospheric material. Significant amount of melts may also come from direct melting of the upwelling plume at depths below the initial plate-plume boundary. Melt extracts continuously from an active partial melting zone of 10-20 km thick, which moves outward as heating and compaction proceed. The models explain quantitatively the general characteristics of Hawaiian volcanism as the result of plume-plate interaction.

Cinder Cone is the most recent cinder cone eruption in the continental United States at ~350 years old. This study examines the physical characteristics of the explosive deposit of the volcano in order to infer eruption timing, style, and mechanisms. Building on previous work and using spatial extent, field relationships, and grain size, componentry, and textural data of ten samples from one column, this study demonstrates that Cinder Cone erupted in at least two distinct phases with at least two distinct eruption styles. This speaks to the changing magma supply and transport processes occurring beneath the volcano. Curiosities about the eruption include the extensive degree of contamination that contributed abundant quartz xenocrysts to all the deposits. Future work includes determining the extent and mechanism/s of contamination and tephra component creation. These data are important for informing hazard assessments of areas with abundant cinder cone volcanoes.

Includes one folded map in pocket in back cover. / Includes bibliographical references. / iv, 214, [46] leaves, [10] leaves of plates : ill. (some col.), col. maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics, 1994

Thesis (doctoral)--Universität Potsdam, 2002. / "April 2003"--P. [2] of cover. Lebenslauf. Includes bibliographical references (p. 93-101). Also available via the World Wide Web.

Risks posed by geohazards to urban centres are constantly increasing, due to the continuous increase in population and associated infrastructure. A major risk to North Island urban centres is impacts from volcanic ashfall. This study analyses the vulnerabilities of selected New Zealand lifelines infrastructure to volcanic ash, to better understand and mitigate these risks. Telecommunications and wastewater networks are assessed, as is the vulnerability of Auckland Airport and grounded aircraft. The ability of vehicles to drive on ash covered roads is also tested, to determine the extent to which emergency services, utility providers and the public will be able to travel during and immediately after ashfall. Finally, air-conditioners have been identified as a significant vulnerability during ashfall, due to the high dependence on cooling for infrastructure and lifelines providers. These are examined to quantify the effects of ashfall on their performance. Each of the selected infrastructure types is assessed through a review of past impacts of ashfall, and experimentation either in the field or under laboratory conditions. Where appropriate, mitigation options that reduce identified vulnerabilities are considered. In most cases these options are operational rather than physical engineering solutions, and indicate pre-planning and response requirements. Key recommended mitigation options include the acquirement or strategic relocation of resources prior to ashfall, regular cleaning and maintenance of essential air conditioners during ashfall to enable their continued use, access to appropriate vehicles for utility providers to reach infrastructure, and discharge of untreated wastewater into Waitemata harbour at Orakei during ashfall on Auckland, to preserve the ability to continue treating wastewater at the Mangere treatment plant

This thesis investigates normal faulting and its influence on fluid flow over a wide range of spatial and temporal scales using tunnel engineering geological logs, outcrop, surface fault traces, earthquakes, gravity, and volcanic ages. These data have been used to investigate the impact of faults on fluid flow (chapter 2), the geometry and kinematics of the Taupo Rift (chapter 3), the hydration and dehydration of the subducting Pacific plate and its influence on the Taupo Volcanic Zone (chapter 4), the migration of arc volcanism across the North Island over the 16 Myr and the associated changes in slab geometry (chapter 5) and the Pacific-Australia relative plate motion vectors since 38 Ma and their implications for arc volcanism and deformation along the Hikurangi margin (chapter 6). The results for each of these five chapters are presented in the five paragraphs below. Tunnels excavated along the margins of the southern Taupo Rift at depths < 500 m provide data on the spatial relationships between faulting and ground water flow. The geometry and hydraulic properties of fault-zones for Mesozoic basement and Miocene strata vary by several orders of magnitude approximating power-law distributions with the dimensions of these zones dependent on many factors including displacement, hostrock type and fault geometries. Despite fault-zones accounting for a small proportion of the total sample length (≤ 15%), localised flow of ground water into the tunnels occurs almost exclusively (≥ 91%) within, and immediately adjacent to, these zones. The spatial distribution and rate of flow from fault-zones are highly variable with typically ≤ 50% of fault-zones in any given orientation flowing. The entire basement dataset shows that 81% of the flow-rate occurs from fault-zones ≥ 10 m wide, with a third of the total flow-rate originating from a single fault-zone (i.e. the golden fracture). The higher flow rates for the largest faults are interpreted to arise because these structures are the most connected to other faults and to the ground surface. The structural geometry and kinematics of rifting is constrained by earthquake focal mechanisms and by geological slip and fault mapping. Comparison of present day geometry and kinematics of normal faulting in the Taupo Rift (α=76-84°) with intra-arc rifting in the Taranaki Basin and southern Havre Trough show, that for at least the last 4 Myr, the slab and the associated changes in its geometry have exerted a first-order control on the location, geometry, and extension direction of intra-arc rifting in the North Island. Second-order features of rifting in the central North Island include a clockwise ~20° northwards change in the strike of normal faults and trend of the extension direction. In the southern rift normal faults are parallel to, and potentially reactivate, Mesozoic basement fabric (e.g., faults and bedding). By contrast, in the northern rift faults diverge from basement fabric by up to 55° where focal mechanisms indicate that extension is achieved by oblique to right-lateral strike-slip along basement fabric and dip-slip on rift faults. Hydration and dehydration of the subducting Pacific plate is elucidated by earthquake densities and focal mechanisms within the slab. The hydration of the subducting plate varies spatially and is an important determinant for the location of arc volcanism in the overriding plate. The location and high volcanic productivity of the TVZ can be linked to the subduction water cycle, where hydration and subsequent dehydration of the subducting oceanic lithosphere is primarily accomplished by normal-faulting earthquakes. The anomalously high heat flow and volcanic productivity of the TVZ is spatially associated with high rates of seismicity in the underlying slab mantle at depths of 130-210 km which can be tracked back to high rates of deeply penetrating shallow intraplate seismicity at the trench in proximity to oceanic fluids. Dehydration of the slab mantle correlates with the location and productivity of active North Island volcanic centres, indicating this volcanism is controlled by fluids fluxing from the subducting plate. The ages and locations of arc volcanoes provide constraints on the migration of volcanism across the North Island over the last 20 Myr. Arc-front volcanoes have migrated southeast by 150 km in the last 8 Ma (185 km since 16 Ma) sub-parallel to the present active arc. Migration of the arc is interpreted to mainly reflect slab steepening and rollback. The strike of the Pacific plate beneath the North Island, imaged by Benioff zone seismicity (50-200 km) and positive mantle velocity anomalies (200-600 km) is parallel to the northeast trend of arc-front volcanism. Arc parallelism since 16 Ma is consistent with the view that the subducting plate beneath the North Island has not rotated clockwise about vertical axes which is in contrast to overriding plate vertical-axis rotations of ≥ 30º. Acceleration of arc-front migration rates (~4 mm/yr to ~18 mm/yr), eruption of high Mg# andesites, increasing eruption frequency and size, and uplift of the over-riding plate indicate an increase in the hydration, temperature, and size of the mantle wedge beneath the central North Island from ~7 Ma. Seafloor spreading data in conjunction with GPlates have been used to generate relative plate motion vectors across the Hikurangi margin since 38 Ma. Tracking the southern and down-dip limits of the seismically imaged Pacific slab beneath the New Zealand indicates arc volcanism in Northland from ~23 Ma and the Taranaki Basin between ~20 and 11 Ma requires Pacific plate subduction from at (or beyond) the northern North Island continental margin from at least 38 Ma to the present. Pacific plate motion in a west dipping subduction model shows a minimum horizontal transport distance of 285 km preceding the initiation of arc volcanism along the Northland-arc normal to the motion vector, a distance more than sufficient for self-sustaining subduction to occur. Arc-normal convergence rates along the Hikurangi margin doubled from 11 to 23 mm/yr between 20 and 16 Ma, increasing again by approximately a third between 8 and 6 Ma. This latest increase in arc-normal rates coincided with changes in relative plate motions along the entire SW Pacific plate boundary and steepening/rollback of the Pacific plate.

The November 2004 explosive eruption of Grímsvötn volcano, Iceland (G2004) commenced as a subglacial event within the interior of the Vatnajökull ice cap before breaking through the ice cover to generate a 6-10 km high eruption column. This produced a tephra apron extending >50 km across the ice surface northwards from the eruption site, incorporating 0.047 km3 (DRE) of plagioclase-bearing, sparsely porphyritic, basaltic tephra. This study focuses on quantifying the key eruption parameters and evaluating the stratigraphy, grain size and geochemical characteristics of the proximal facies of the subaerial deposit with a level of detail and precision that has never previously been possible for a modern glacio-volcanic event. The G2004 deposit consists of a finely layered sequence which is subdivided into seven units (A-G) on the basis of differences in texture, grain size and componentry and the presence of sharp contacts between the layers. It is poorly sorted and finegrained with a median clast diameter of 1.5 Φ. The grain size characteristics and clast morphologies are indicative of intense phreatomagmatic fragmentation, despite a significant component of highly vesicular juvenile glass. A wide range in thicknesses and geometries of depositional units reflects variations in intensity and style of activity. Units C and E account for 80% of the total deposit volume, including the entire distal portion, and are interpreted to represent a mixture of (i) a widely dispersed component that fell from the upper margins of a strongly inclined (~45°) 6-10 km high plume and (ii) a locally dispersed (<3 km from source) component originating from (1) pyroclastic density currents generated by shallow explosions and tephra jets and (2) sedimentation from the jet region and lower convective column margins. The other units are only locally dispersed around the vent. A significant proportion of fine material was deposited in the near-vent region due to particle aggregation processes. The bulk of the G2004 deposit is therefore identified as the product of continuous incremental deposition during the passage of a single quasi-steady current supplied by a sustained explosive phreatomagmatic eruption, with a variable contribution of material from concurrent tephra fallout. Major oxide, trace element and volatile composition of the G2004 tephra were analysed and compared with that of the G1998 and Laki events. Results indicate that the G2004 magma originated within a shallow-level, compositionally stratified chamber and was discharged through an independent plumbing system. The parent magmas for each of these three Grímsvötn events were produced by different degrees of partial melting within a similar lower-crust or mantle source, but are not related by fractional crystallization or magma mixing. Despite episodic intense vesiculation, the G2004 magma was fragmented at very shallow levels by almost exclusively phreatomagmatic mechanisms – the effect of which was to arrest the degassing process such that only 75% of the potential magmatic sulphur budget escaped to the atmosphere.

Iceland is the land-based expression of the Mid-Atlantic Ridge and is one of the most volcanically active regions of the world. Volcanic eruptions on Iceland are a source of geological hazard to humans and the environment due to the release of ash, gases and lava. The composition of the material released is determined by the chemical composition of the surrounding bedrock and the magma upwelling from the Earth’s crust. The effects of historical eruptions on Iceland have been locally devastating and of global impact. The eruption of Lakagígar in 1783-1784 is known to have been the largest eruption in historical time, and is responsible for the death of ~22 % of theIcelandic population. Skeletal fluorosis is a disease that is sometimes observed following large volcanic eruptions. Volcanic ash can travel great distances in the upper atmosphere and spread over vast areas far away from the erupting volcano. Volcanic ash can change incomposition in the atmosphere, and bring about climate-changing effects. Most notably in recent times, violent ash eruptions can also cause problems to the aviation industry, when ash enters and damages airplane engines. Iceland has many active volcanoes and needs to ensure plans for future eruptions are in place. One such measure is an evacuation plan that protects people living close to an active volcano, such as the most lively on Iceland: Hekla, Katla and Eyjafjallajökull.

No description available.

No description available.