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How does magma move through sill-complexes?

  • Full or part time
  • Application Deadline
    Monday, January 06, 2020
  • Competition Funded PhD Project (European/UK Students Only)
    Competition Funded PhD Project (European/UK Students Only)

Project Description

Supervisors: Dr Craig Magee (Leeds), Dr William McCarthy (St Andrews), Dr Douglas Paton (Leeds)


Understanding how magma intrusion controls the location of volcanoes and pre-eruption warning signals is critical to hazard assessment. Textbooks suggest volcanoes are underlain by dykes, where magma moves vertically through the crust (Fig. 1A). However, recent studies champion a new idea where the lateral extent of volcano plumbing systems may be greater than their vertical extent, with magma transported through a network of sub-horizontal sills (i.e. a sill-complex) as opposed to dykes (Fig. 1A) (Magee et al. 2016). The Karoo Sill-complex (South Africa), spanning an area the size of Spain, and the Ferrar Sill-complex (Antarctica), which extended horizontally for >4000 km, provide excellent examples of volcano plumbing systems that channelled magma laterally through the crust (Leat et al. 2008; Svensen et al. 2012; Magee et al. 2016). Recognising that sill-complexes can play a major role in magma transport questions our current understanding of volcanology, which centres on vertical, dyke-dominated systems.
Field observations and seismic reflection data, which provide 3D ultrasound-like images of Earth’s subsurface, have allowed the broad structure of sill-complexes to be constrained and shown how individual intrusions are emplaced (e.g. Magee et al. 2016). However, we do not know how entire sill-complexes are built or how they transport magma over large areas without freezing. It has been suggested that hot but solidified sections of sill-complexes allow later magma injections, focused into channels (Fig. 1B) (Holness & Humphreys, 2013) or along sill boundaries (Fig. 1C) (Annen et al. 2015), to flow further and gradually extend the sill-complex. The aim of this project is to test these ideas by investigating how magma moves through sill-complexes, with a view to understanding how magma flow pathways influence the distribution, construction, and eruption of volcanoes.


To study how magma moves through sill-complexes, you will combine observations and data from fieldwork, rock magnetic analyses, petrological studies, and interpretation of seismic reflection data. You will work with the Institute of Geophysics and Tectonics (IGT) and Institute of Applied Geophysics (IAG) at the University of Leeds, and with Dr William McCarthy at the M3ORE Laboratory in the University of St Andrews. Four broad objectives have been identified for the project, but these are flexible and dependent on your research interests:

Year 1 – Seismic reflection interpretation

Map sill-complexes in 3D seismic reflection dataset from around the world, particularly from a 3D seismic dataset in the Rockall Basin offshore Ireland (see Magee et al. 2014).

Years 1 and 2 – Fieldwork and rock fabric analysis

Map the Loch Scridain Sill-complex on the Isle of Mull, focusing on constraining the geometry of sills and host rock deformation structures generated by magma emplacement. The Loch Scridain Sill-complex is an ideal laboratory to test how sill-complexes are intruded because it is well-exposed, easily accessible, and contains magma channels (Holness and Humphreys, 2003).
Analyse the Anisotropy of Magnetic Susceptibility (AMS) of oriented samples at the M3ORE Laboratory at the University of St Andrews. This technique allows alignments of minerals within rocks to be measured, which can be linked to the rotation of crystals within a flowing magma (e.g. sticks in a stream align with flow); AMS and complementary rock magnetic experiments will thus be used to map magma flow pathways.

Year 3 – Quantitative petrological analysis

Apply quantitative textural petrology techniques, such as Crystal Size Distribution (CSD), to evaluate how crystals grew, interacted, and how long they resided in the system. These techniques will help distinguish the construction history of the sill-complex.

Impact of Research and Publications

This work will identify how magma moves through and builds sill-complexes, addressing a pressing need to understand their impact on volcano distribution, interaction, and eruption warning signals. The research will also shed light on: (1) how sill-complexes localise mineral and metal accumulations; (2) magma volumes and storage conditions during continental break-up and Large Igneous Province formation; and (3) the role of sill-complexes, which can form independent of plate tectonics, in shaping geological processes on other planetary bodies.

Funding Notes

Funding available through the NERC Panorama DTP - View Website


Annen, C., Blundy, J.D., Leuthold, J. and Sparks, R.S.J., 2015. Construction and evolution of igneous bodies: Towards an integrated perspective of crustal magmatism. Lithos, 230, pp.206-221.

Holness, M.B. and Humphreys, M.C.S., 2003. The Traigh Bhan na Sgurra Sill, Isle of Mull: flow localization in a major magma conduit. Journal of Petrology, 44(11), pp.1961-1976.

Leat, P.T., 2008. On the long-distance transport of Ferrar magmas. Geological Society, London, Special Publications, 302(1), pp.45-61.

Magee, C., Jackson, C.L. and Schofield, N., 2014. Diachronous sub‐volcanic intrusion along deep‐water margins: Insights from the Irish Rockall Basin. Basin Research, 26(1), pp.85-105.

Magee, C., Muirhead, J.D., Karvelas, A., Holford, S.P., Jackson, C.A., Bastow, I.D., Schofield, N., Stevenson, C.T., McLean, C., McCarthy, W. and Shtukert, O., 2016. Lateral magma flow in mafic sill complexes. Geosphere, 12(3), pp.809-841.

Svensen, H., Corfu, F., Polteau, S., Hammer, Ø. and Planke, S., 2012. Rapid magma emplacement in the Karoo large igneous province. Earth and Planetary Science Letters, 325, pp.1-9.

Related Subjects

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FTE Category A staff submitted: 79.20

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