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How does magma move through sill-complexes?

Dr Craig Magee (SEE), Dr William McCarthy (University of St Andrews), Prof Douglas Paton (SEE)

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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 predominantly moves vertically through the crust. However, recent studies champion a new idea where the lateral extent of volcano plumbing systems is greater than their vertical extent, with magma primarily transported in a network of sub-horizontal sills (i.e. a sill-complex) as opposed to dykes. 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 prior to eruption. 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. However, we do not know how entire sill-complexes are built and 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 or along sill boundaries , 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. Fieldwork will primarily focus on the Loch Scridain Sill-complex on the Isle of Mull, Scotland but there is scope to study other examples. The 3D seismic reflection datasets to be used are from the Rockall Basin offshore western Ireland and from offshore Australia and New Zealand.

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: (i) how sill-complexes localise mineral and metal accumulations; (ii) magma volumes and storage conditions during continental break-up and Large Igneous Province formation; and (iii) the role of sill-complexes, which can form independent of plate tectonics, in shaping geological processes on other planetary bodies. Through training (e.g. scientific writing, statistics and data analysis, problem-solving, time management, and developing independent research planning skills), you will become a confident and independent researcher with transferable skills applicable to both academic and non-academic jobs. 

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Related undergraduate subjects:

  • Earth science
  • Geological science
  • Geology
  • Geophysical science
  • Geophysics
  • Geoscience
  • Natural sciences