Kawerau is located in the Bay of Plenty on the north-east coast of the North Island, New Zealand. Kawerau is an active geothermal field where fluids have been extracted for energy use since the 1950’s when a pulp and paper mill was constructed due to the close proximity to forestry areas and the geothermal energy source. Kawerau has seen significant development in the last 10 years with the commissioning of a 100 MW geothermal power station by Mighty River Power in 2005. Kawerau is located on the south-western edge of the Rangitaiki Plains; these plains have been modified considerably over the last 125 years since the 1886 Tarawera eruption by both natural and anthropogenic mechanisms. Processes at work in the Kawerau area include active volcanism, rifting, fluvial processes, shallow and deep water extraction, anthropogenic river modification and diversion, and construction of buildings and factories. Subsidence is an issue in geothermal and oil fields worldwide and Kawerau is no different. This research aims to determine the origin of localised subsidence features identified by levelling surveys within the Kawerau Geothermal Field. Ground subsidence surrounding the pulp and paper mill, geothermal power station and residential properties in Kawerau has been monitored with levelling surveys since the 1970’s. The potential effects of continued subsidence and tilt within this area could negatively affect the operation of the industry in the area, particularly the pulp and paper mill due to the sensitivity of the paper rollers to tilt. Subsidence in Kawerau occurs on two scales: the first is a large, field-wide subsidence feature, the second is a series of smaller, localised subsidence features which this thesis focuses on. First, identifying the location and characterising the properties of historic river channels, as well as their response to human demand, such as land and water use has been the primary approach in determining the origin of subsidence features. This helped build a picture of how the area appeared 125 years ago and add to our understanding of the history and landscape of the Rangitaiki Plains. Second, to determine the cause(s) and mechanism(s) of the subsidence in Kawerau, field and laboratory investigations were undertaken. Site investigations included geomorphological mapping, ground penetrating radar (GPR), electrical imaging, hand augering and face logging. Laboratory investigations included permeability testing, determination of Atterberg Limits, dispersion testing, grain size distributions, microscopy and allophane detection testing. Aerial photograph and LiDAR interpretation as well as a literature review has shown the approximate location of where the Tarawera River used to flow before it was diverted to aid in the draining of the Rangitaiki Plains. In the approximate location of the old Tarawera River, the geophysical survey identified an extension of the Onepu fault. This fault may have influenced the original location of the Tarawera River by creating low points in the topography as the result of seismic events. The Tarawera River path was diverted to its current path in the early 1900s following the large outbreak flood from Lake Tarawera. Basin wide subsidence at Kawerau has been attributed to geothermal fluid extraction and the resulting contraction and/or cooling of the reservoir. This has caused low rates of subsidence across the whole field. This subsidence is unlikely to cause any damage to surface features due to its low rate and low angles of tilt. Basin wide subsidence is not the focus of this thesis so is not covered in detail. This thesis focuses on two main sites of subsidence. Site 1 lies between the mill site and the Mighty River Power geothermal power station. Site 2 lies in farm land to the north of the mill and the old air strip. The mechanisms controlling subsidence at these sites is believed to be acting independently of each other. Primary mechanisms of subsidence at Site 1 include indirect seismic activity, direct disturbance by construction, vibration, apparent subsidence, the influence of drainage through the site, and wetting and drying sequences associated with rainfall and the soak ponds immediately adjacent to Site 1. Subsidence at Site 2 is likely to be caused by direct seismic activity, indirect seismic activity, consolidation of sediment due to changes in the groundwater table, and the influence of perched water tables.

The central segment of the Taupo Volcanic Zone (TVZ) is one of the world’s most productive areas of silicic volcanism and geothermal activity. Rhyolites largely predominate the eruptive output in the central TVZ, with only minor basalts, andesites and dacites. The rhyolites show diversity in composition, and form a compositional continuum between two end-member types (R1 and R2), as suggested in previous studies. In this thesis I present results from a quartz- (and rare plagioclase-) hosted melt inclusions study, focussing on the volatile concentration (i.e. H2O, Cl, F, CO2) and their relative distribution between R1 and R2 rhyolites. The main objective is to add further constraints on the magmatic systems with regard to their contribution to the hydrothermal systems in the central TVZ. A comparative study between R1 and R2 melt inclusions show distinct volatile, fluid-mobile, and highly incompatible element compositions. Differences in the bulk volatile concentration of the parental magmas (i.e. basalts intruding the lower crust) are suggested to be at the origin of these volatile disparities. Further analysis on the volatile exsolution of R1 and R2 melts lead to the observation that the two rhyolite types exsolve a volatile phase at different stages in their magmatic history. From Cl and H2O concentrations, it is suggested that R1 magmas exsolve a vapour phase first, whereas R2 rhyolites more likely exsolve a hydrosaline fluid phase. These results have considerable implications for the magmatic contribution into the hydrothermal systems in the central TVZ, as differences in the composition of the resulting volatile phase may be expected. The hydrothermal systems in the central TVZ are subdivided into two groups based on their gas and fluid chemistry; and the current model suggests that there are two distinct contributions: a typical ‘arc’ system, with geochemical affinity with andesitic fluids, located along the eastern margin of the TVZ, and a typical ‘rift’ system, with geochemical affinity with rhyolitic/basaltic fluids, located along the central and/or western region of the TVZ. The addition of the new data on the rhyolitic melt inclusions, leads to a re-evaluation of the magmatic contribution into the hydrothermal systems, with a particular focus on B and Cl. The results indicate a more diverse variety of contributions to the meteoric water in the hydrothermal systems, and also show that the east-west distribution of ‘arc’ and ‘rift’ fluids is not a viable model for the central TVZ. This work emphasises that melt inclusion data and their volatile degassing history cannot be underestimated when characterising and quantifying the magmatic component in hydrothermal fluids. The melt inclusion data also provide further insight into the pre-eruptive magmatic plumbing systems and are particularly important from a hazard perspective. Included in the thesis is a detailed petrological analysis of rhyolite melt inclusions across the central TVZ and an interpretation that large silicic magma systems (in the TVZ) are typically comprised of multiple batches of magma emplaced at some of the shallowest depths on Earth. Tectonic activity is suggested to play an important role in triggering large caldera-forming eruptions as the evacuation of one magma batch could cause a regional-scale readjustment that is sufficient enough to trigger and allow simultaneous eruption of an adjacent melt batch.

Lichen have been used as indicators of atmospheric pollutants since Grindon (1859) observed lichen populations declining in a polluted Southern Lancashire in the mid-1800s. Since then lichen have been used in a number of atmospheric studies. A study by Tozer et al. (2005) attempted to use nitrogen isotopes of lichen and free-living algae as indicators of geothermal ‘pollution’ near Rotorua and the Te Kopia Geothermal Area, but was unable to show a correlation with distance to geothermal features. This thesis aims to build from Tozer et al. (2005) and use both carbon and nitrogen isotopes in lichen as an exploration tool in geothermal areas. Three transects were completed: one across the South Island from Christchurch to Greymouth (non-geothermally influenced area), and two along (north-south) and across (east-west) the Taupo Volcanic Zone (TVZ) in the North Island (geothermally influenced area). In addition to these three transects, sampling at higher spatial resolution was conducted in the immediate vicinity of the Orakonui Stream geothermal springs at the Ngatamariki Geothermal Area. The three transects showed large variation, largely due to the type of land use from which the sample was collected. The highest nitrogen contents (1.62 ± 0.39%) and less negative nitrogen isotopic compositions (-9.44 ± 0.39‰) were found over farmland, while both exotic and native forests had low nitrogen (1.08 ± 0.35% and 1.03 ± 0.44‰, respectively) and highly negative isotopic compositions (-12.94 ± 0.26‰ and -12.09 ± 0.45‰, respectively). The statistical difference between land use classes is hypothetically explained by variations in nitrogen sources, with intensive farmland volatilizing NH3 with δ15N values of -6 to -10‰ (Tozer et al., 2005), while forest areas are expected to produce biogenic nitrogen from decomposition with more negative δ15N. At Ngatamariki, δ13C and δ15N isoscapes were produced, with both showing a large isotopic anomaly (>-23.5 and >-8‰, respectively) to the north and north-west of the study area, correlating with areas of farmland, although in some places the δ15N values exceed 0‰, which is unexplained. A study by Hanson (2014) identified diffuse soil flux using δ13C in the vicinity of the Orakonui South Main Crater to have a geothermal signature, the same location in which a small relatively less-negative δ13C anomaly (>-23.5‰) is seen in lichen isotopes. While this could be attributed to a geothermal influence, it could also be due to the effect of substrate the lichen lives on and a reduction in carbon sourced from biogenic respiration. Ultimately, there is the potential for isotopes in lichen to be used as a geothermal exploration tool, although this method needs to be investigated in a higher flux geothermal area, such as Rotokawa, 7km to the south of Ngatamariki.

Ngauruhoe is an active basaltic andesite to andesite composite cone volcano at the southern end of the Tongariro volcanic complex, and most recently erupted in 1954-55 and 1974-75. These eruptions constructed the inner crater of Ngauruhoe, largely composed of 1954-55 deposits, which are the basis of this study. The inner crater stratigraphy, exposed on the southern wall, is divided into seven lithostratigraphic units (A to G), while the northern stratigraphy is obscured by the inward collapse of the crater rim. The units are, from oldest to youngest: Unit A, (17.5 m thick), a densely agglutinated spatter deposit with sharp clast outlines; Unit B, (11.2 m) a thick scoria lapilli deposit with local agglutination and scattered spatter bombs up to 1 m in length; Unit C, (6.4 m thick) a clastogenic lava deposit with lateral variations in agglutination; and Unit D, (10 m thick) a scoria lapilli with varying local agglutination. The overlying Unit E (15 cm thick) is a fine ash fallout bed that represents the final vulcanian phase of the 1954-55 eruption. Unit F is a series of six lapilli and ash beds that represent the early vulcanian episode of the 1974-75 eruption. The uppermost Unit G (averaging 10 m thick) is a densely agglutinated spatter deposit that represents the later strombolian phase of the 1974-75 eruption. Units A-D juvenile clasts are porphyritic, with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, minor olivine, within a microlitic glassy groundmass. Quartzose and greywacke xenoliths are common in most units, and are derived from the underlying basement. The 1954-55 and 1974-75 eruptions are a product of a short-lived, continental arc medium-K calc-alkaline magma. The magma originated from the mantle, then filtered through the crust, undergoing assimilation and fractionation, and evolving to basaltic andesite and andesite compositions. The magma body stagnated in shallow reservoirs where it underwent further crustal assimilation and fractionation of plagioclase and olivine, and homogenisation through magma mixing. Prior to the 1954-55 eruption a more primitive magma body was incorporated into the melt. The melt homogenised and fed both the 1954-55 and 1974-75 eruptions, with a residence time of at least 20 years. The 1954-55 eruption produced alternating basaltic andesite and andesite strombolian activity and more intense fire fountaining, erupting scoria and spatter that built up the bulk of the inner crater. A period of relative quiescence allowed the formation of a cooled, solid cap rock that resulted in the accumulation of pressure due to volatile exsolution and bubble coalescence. The fracturing of the cap rock then resulted in a vulcanian eruption, depositing a thin layer of fine ash and ballistic blocks. The 1974-75 eruption commenced with the rupturing of the near-solid cap rock from the 1954-55 eruption in an explosive vulcanian blast, the result of decompressional volatile exsolution and bubble coalescence, and possible magma-water interaction. The eruption later changed to strombolian style, producing a clastogenic lava that partially flowed back into the crater.