Going round the twist: The effect of stress on magma intrusion in Scotland and the Canary Islands

Introduction: Magma is transported through the crust in fractures called dykes that cut across rock layers. Dykes are important in all stages in the life of a volcano; they transport magma from depth but may stall during ascent or reach the surface to feed eruptions (see Kavanagh 2018 for a review). They are capable of transporting magma 100’s of kilometres both vertically from their source at depth, but also laterally as shown during the recent dyke intrusion in Kilauea’s lower east rift zone in Hawaii (Figure 1). The majority of dykes do not erupt, and so dyke arrest is an important process that helps build long-lived volcanic centres from within. Understanding the state of stress within and around a volcanic edifice, the geometry and extent of the volcanic plumbing system itself, and what controls magma flow will improve our ability to forecast when and where the next volcanic eruption will be.

Cone-sheets are thin, arcuate dykes that confocally dip into the crust to creating an overall conical intrusion geometry. They have been used to infer the depth of the magma chamber in sub-volcanic systems. The Ardnamurchan central complex, NW Scotland, exposes the deeply eroded base of an ancient volcanic edifice that formed ~58 Ma as part of the British Palaeogene Igneous Province. Structural studies of this world-famous field area have largely shaped our understanding of cone sheet emplacement; and yet contrasting models exist to explain the origin of the Ardnamurchan cone sheets, as either from a single, elongate magma reservoir (e.g. Burchardt et al. 2013) or due to lateral emplacement by regional dykes from the adjacent Mull volcanic system (e.g. Magee et al. 2012).

Recent volcanic eruptions fed by cone sheets have been linked to volcanoes that have a pre-existing caldera, where previous explosive events have resulted in a topographic depression due to subsidence into a partially drained magma chamber at depth. Laboratory models have shown that the unloading stress field caused by caldera-forming events can affect the subsequent pathways of dyke propagation and the overall stress budget of a volcano (Corbi et al. 2016), and in doing so may favour cone-sheet emplacement. Cone sheets are often observed in ancient volcanic systems and this suggests they are a common feature in long-lived volcanic centres that are not only associated with calderas. Understanding the dynamics of cone sheet development, dyke arrest or dyke eruption will aid volcanic hazard assessment at caldera volcanoes.

Project Summary: Using field sites in the Canary Islands and Scotland as case studies, you will use a multidisciplinary approach combining analogue laboratory experiments, field observations and rock magnetism to study the effects of stress on the dynamics of dyke emplacement.
In the field, you will collect large-scale images of well-exposed dykes using UAV (drone) technology and use photogrammetry software to reconstruct their geometry (e.g. Thiele et al. 2017). You will ground truth your measurements using GPS mapping, and collect closely-spaced rock samples for micro-scale crystal shape and magnetic fabric analysis. The samples you collect will be analysed for their rock fabrics using petrographic imaging techniques, such as scanning electron microscopy (SEM), and Anisotropy of Magnetic Susceptibility (AMS), which will aid in the reconstruction of the magma flow pathways in the intrusions. The AMS datasets will be complemented by other rock magnetic experiments, including Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Curie Point determination, to constrain magnetic mineralogy, in the University of Liverpool’s Geomagnetism Laboratory.

Experimental dykes will be created in the laboratory using scaled analogue materials (e.g. Galland et al. 2014; Corbi et al. 2016). Working in the University of Liverpool’s MAGMA Laboratory, you will use imaging and analytical techniques such as digital image correlation (DIC), particle image velocimetry (PIV) and laser scanning to document the coupled host-deformation and fluid flow trajectories (Kavanagh et al. 2018; Figure 2) as the dyke develops. The analogue model results will be compared with the AMS and field observations to reconstruct the emplacement history of arrested dykes, feeder dykes and cone sheets in Scotland and the Canary Islands.
This project is suitable for applicants with a degree in Geology, Geophysics or a related discipline. Field experience and/or knowledge of working in a research laboratory is desirable.