Eruptional and post-eruptional processes in rhyolite domes.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Four field areas, representing different tectonic settings, have been chosen to study eruption and post-eruption processes in rhyolite domes. These are Ngongotaha Dome, on the west side of Lake Rotorua; Mt Tarawera Volcanic Complex, in the Okataina Caldera Complex; Gebbies Pass, Banks Peninsula; and Mt Somers, South Canterbury. These field areas provide different vertical and horizontal sections through rhyolite dome structures and allow the investigation of internal structures at different levels within a dome. A holistic approach aims to integrate the various physical and chemical processes that have taken place during and after the emplacement of rhyolite domes. An extensive review of these processes forms a substantial part of the thesis and the basis for later investigations in the field and laboratory. A main aspect of the review is the structure of rhyolite melts and glasses and its influence on diffusion rates which in turn determine diffusion-controlled crystallisation processes. Physico-chemical changes at the transition melt-glass are also discussed. Rheological properties of rhyolitic melts have a pronounced effect on flow behaviour and diffusion processes. The influence of temperature, pressure, melt composition, water content (especially), solid particles and vesicles, on the viscosity are outlined, and the various methods for estimating the viscosity of rhyolitic rocks are compared. Crystallisation processes under conditions of a high degree of undercooling are reviewed and parameters which allow one to distinguish between crystallisation from the melt and crystallisation from the glassy state (devitrification) are identified. Both crystallisation processes frequently result in very similar products. Spherulitic growth occurs generally above the glass-transition temperature where growth rates are fastest. Overall growth times are estimated to range from hours to months depending on the spherulite morphology and size. Compositionally, spherulites connect the fields of plagioclase phenocrysts and glass matrix in the An-Ab-Or ternary system. Opening structures, such as lithophysae, 'lip'-structures and spherulites with central voids, form above the glass transition and ideas about their formation are given. In the context of crystallisation, the textural terminology of rhyolitic rocks is critically reviewed and the use of simple textural terms proposed. A wide variety of small-scale flow features in rhyolitic rocks such as foliation, lineation and deformation structures are investigated and their use in inferring local flow directions is evaluated. A strong similarity to analogous features in metamorphic rocks is pointed out. Large-scale extensional and compressional structures in close proximity in dome lobes point to a complex flow behaviour. The occurrence of prominent columnar joints in two field areas, Gebbies Pass and Mt Somers, required a detailed study of this type of jointing process. Existing theories focus on rocks of basaltic nature, but it is shown that same ideas can be applied to silica-rich rocks. Column features, such as their thickness, regularity of cross sections, surface morphology and their overall pattern are used to outline the cooling history and morphology of single lava domes and numerous domes with different spatial and temporal relationships to each other. A review of rhyolite domes outlines common and specific surface and internal structures. The transition from an explosive to an effusive eruption style is investigated and characteristics of endogenous and exogenous dome growths are compared. Ngongotaha and Wahanga Domes are predominantly exogenous in character and comprise between 6-11 and 4-5 dome lobes, respectively. Dome lobe lengths vary between 350-450 m and thicknesses between 200-300 m. This length of dome lobes represents the maximum lobe extent of highly viscous lavas after which dome lobes start to pile onto each other. Large rhyolite domes (diameter ≥900 m) with slow effusion rates and/or rapid solidification are therefore predominantly exogenous in character. Domes with a small diameter (<500 m), such as the small dome at Ruawahia Dome, are predominantly endogenous. The results of recent dome growth simulations are applied to domes in the field areas and these allow the estimation of eruption rates and overall extrusion times. The eruption rate of Ngongotaha Dome was in the order of 20 m3 8-1 and its overall emplacement time close to one year (336 days). The extension of a 1D cooling model to two dimensions allows the study of the overall cooling time of a lava dome. Depending on the dome thickness, the transition of the entire dome to the glassy (solid) state is considerable and is estimated as 22 years for Ngongotaha Dome and 41 years for Wahanga Dome. A combination of the 2D cooling model with the distribution of certain lithologies allows the estimation of the onset of crystallisation such as defined by the 'nucleation lag' time of spherulite crystallisation. 'Nucleation lag' times of spherulitic growth ranges from 3-12 years for the upper obsidian layer (U.OBS) to 15-23 years for the central felsitic rhyolite (CRHY) at Ngongotaha Dome. In rhyolite domes, main lithologies are distributed concentrically following the overall outline of the domes rather than individual dome lobes. Their distribution is therefore largely independent of the emplacement mode of the dome but controlled by the cooling history of the dome. In domes with complexly developed lithologies the following main types can be recognised: a carapace breccias (BB), a finely-vesicular pumice (FVP), an upper and lower obsidian layer (U.OBS and L.OBS), a felsitic/poikilitic rhyolite (RHY) and a central rhyolite with abundant opening structures (CRHY). The dome base is characterised by a basal breccia (BB) which frequently interfingers with obsidian bands of the L.OBS layer. Absence of obsidian layers is attributed to insulating effects of thick FVP and BB layers in relatively thick domes. Finally, alteration processes of rhyolites and resulting alteration products are discussed with a major emphasis on the hydration of rhyolitic glasses, formation of perlite and zeolites as well as lunitisation. Detailed studies in the field, involving sampling and measurements of structural elements in combination with the above outlined background, allow the extrusion and emplacement history of the domes to be investigated. They also show that rhyolite domes are highly individual in character and differences between them arise mainly from differences in setting, extrusion rate, dome morphology and cooling history.