What lies beneath: feeding a basaltic volcanic eruption.

Basaltic volcanic eruptions span a wide spectrum of styles, from relatively gentle effusion of lava, to violent explosions that produce lofting ash plumes. The eruption style, and its evolution in time and space, depends to a large extent on multiphase fluid dynamic processes that occur within the magma as it travels through the sub-volcanic plumbing system. Most basaltic eruptions are fed by dykes, which are a ubiquitous feature of eroded basaltic volcanoes. When a dyke intersects the surface, it first forms a fissure eruption. Usually, the eruption localizes to a single vent within a few days, and often builds a scoria cone or shield, depending on its vigour.
Beneath the vent, the conduit retains a dyke geometry, and various lines of evidence indicate that complex magma flow patterns develop within the dyke. Understanding the magma flow pattern within a dyke, and how it changes with time, is crucial because it controls the nature of the eruption at the surface. In particular, localization of gas-rich magma into rapidly upwelling jets is expected to promote more explosive eruption styles.
The main goals of this project are:
1. to collect field evidence and use laboratory experiments and/or numerical models to reconstruct magma flow processes in volcanic conduits from textural evidence in dykes;
2. to interpret magma flow patterns in exposed volcanic conduits in the UK and Tenerife.
Bubbles and crystals are very common in dykes, and often show preferential alignment that is imposed by the magma flow. These textures provide a record of the flow history through an eruption, however, their interpretation has been hindered by a lack of understanding of the processes that create them. In this project we will investigate and quantify the process of marginal accretion of crystal- and bubble-bearing magma as it flows along a conduit, using laboratory experiments and/or numerical modelling, depending on the skills of the applicant. Results will be used to analyse and interpret field samples from dykes exposed at different depths within the subvolcanic plumbing system, allowing us, for the first time, to reconstruct dyke flow processes through time and space.
Experiments:
Suspensions of bubbles and particles in molten wax will be pumped through a model dyke (a high-aspect-ratio duct) with one cooled wall. The wax will solidify against the wall, producing a marginally-accreted facies that will be texturally analysed using x-ray computed tomography. Experimental parameters will be systematically varied, including: size, shape and concentration of particles; size and concentration of bubbles; wax flow rate; cooling rate; duct geometry. Parameters will be scaled to the natural system.
Fieldwork:
Textures exposed in dykes of the British Tertiary Igneous Province will be mapped at the crystal/vesicle scale in the field, and samples will be collected for XRCT and SEM analysis. The samples will inform laboratory experimental parameters. Later fieldwork in the Teno Massif in Tenerife will focus on mapping spatial and temporal changes in textural indicators of magma flow, in order to reconstruct 4D flow patterns.