Abstract
The early evolution of basaltic fissure eruptions is often marked by vent localisation, shutdown, migration, and changes in eruption vigour and style, all of which add significant challenges to hazard assessment. These surface dynamics are in part the expression of thermal evolution within the feeder dyke, where heat loss to the surrounding country rock forces cooling and changes to the magma’s viscosity. The effect triggered by this thermal mechanism then controls where and for how long a dyke can sustain an eruption. However, our ability to monitor these processes is limited as it is currently not possible to study the surface expression of flow localisation and its shallow subsurface features jointly. This leaves a critical knowledge gap of how a basaltic fissure eruption evolves in real-time.
To compensate for this problem, here I use a small-scale volcanic fissure, ‘Artfish’, that reproduces the key processes of viscous flow and cooling in active dykes. Using PEG600 as the magma analogue, a fluid with temperature-dependent viscosity, it is possible to simulate in a controlled environment the thermal, viscous and velocity gradients that would develop in a natural magmatic system. In an experiment, temperature distribution and flow velocity is quantified, while flow patterns and their evolution are qualitatively described. The experiments determine the requirements under which sealing of the dyke or meltback can develop, and the sensitivity to dyke width, country rock temperature and magma flow rate. The experimental results show how long-lived conduits can remain active and how slow-flowing viscous domains seal and block segments of a dyke. The addition of non-planar features such as obstructions and channelised paths in the dykes further enhances feedback in the viscous flow, producing both stable and unstable domains of slow-flowing wax that eventually seal off regions along the length of the model dyke.
The features observed in the ‘Artfish’ were tested against those of eroded basanite dykes of the Jagged Rocks volcanic complex in the Hopi Buttes Volcanic Field. The ~1,500 m long segmented dyke records a complex, short-lived history of sub-horizontal magma transport and heterogenous thermal impact on the adjacent mudrock. Detailed mapping of over 10,000 vesicles and 2,500 phenocrysts reveals margin-parallel banding and shifting flow directions over metre-scale distances, indicating dynamic and complex magma flow while the dykes were active. While field evidence of contact metamorphism is sparse or ambiguous, paleomagnetic transects confirm that the dyke heated and remagnetised the mudrock up to 0.5 m from the contact. Together, these textural and magnetic signatures characterise the dyke as a transient but thermally significant conduit.
By combining experiments with field-based observations, this thesis provides a basis for understanding how flow localisation is driven by the sensitive feedback between magma rheology, dyke geometry and country rock temperature and thermophysical properties. These findings highlight the importance of subsurface flow dynamics in controlling the eruptive behaviour of basaltic fissures, offering a new perspective on the triggers of vent cessation and localisation in dyke-fed fissure eruptions.