Interpreting dyke propagation and emplacement dynamics using crystal and vesicle textures

Abstract

The textures of crystals and vesicles within volcanic rocks are a product of cooling and flow conditions within the volcanic system. This thesis concerns the textures within basaltic dykes, which are the planar conduits that feed fissure eruptions, which produce more magma than any other type of eruption on Earth. Fissure eruptions pose a hazard to life by emitting toxic gases, and pose a hazard to infrastructure by releasing voluminous lava flows. Understanding dyke emplacement is integral to forecasting the likelihood, location and longevity of potential eruptions. However, our means of monitoring dyke emplacement are limited. We can interpret the earthquakes and surface deformation generated by magma fracturing through the crust, but these cannot provide a complete image of dyke structure or flow dynamics. As such, the textures captured within exposed, solidified dykes provide a valuable insight into the flow and cooling conditions that existed while the dyke was active.

In this thesis, dyke emplacement processes are inferred from crystal and vesicle textures. Analogue experiments are used to demonstrate the potential for crystal alignment and vesicle shapes to be read as a record of flow history. A key concept underpinning much of the work in this thesis is that dyke textures should be read as a time-series. Dykes solidify progressively inwards from their margins, so marginal textures record early-stage processes in the dyke tip, whereas central textures record late stage processes as the dyke is sealing shut. A conceptual model of dyke emplacement is developed by interpreting textures across the widths of dykes at numerous locations within the exposed dyke system, combining field study and petrographic analysis. Textures suggest that cooling and solidification play a significant role in mediating magma flow, and that dykes develop preferential flow pathways. The conceptual model challenges the common conception of dykes as simple, planar fractures, and highlights the importance of thermal processes, which should be included in future models that aim to capture the dynamics of dyke propagation and emplacement.