Magma Plumbing Systems: A Geophysical Perspective

- Guest blog! Craig Magee

Volcanic systems are complicated. Nearly as complicated, it turns out, as figuring out how to introduce your first blogpost. I’ve finally decided on...

This is me trying to be artistic with a 
jaunty selfie in front of Lake Taupo

Hello! I’m Craig.

Usually I introduce myself as a volcanologist. It’s much easier and sounds more exciting than saying ‘I’m a structural igneous geologist’, which then requires deciphering.Unfortunately, given the expertise of the usual reporters for this most excellent blog, I cannot really pass myself off as a volcanologist. The only active volcanoes I’ve been to are on Lanzarote, which I can’t even remember the name of, and Taupo in New Zealand. I’ve never even seen a volcano erupt! Instead, I specialise in mapping ancient magma plumbing systems in 3D and reconstructing their formation.

To circle back to my opening remark, we have learned a lot about volcanoes but it seems the more we learn the more complicated they become. One problem that I’m sure we’re all aware of and that becomes immediately apparent whenever you stand on an active volcano is how do we actually know what is going on inside and below active volcanoes?

There are two main ways we explore what is going on within volcanoes: (1) we can examine the structure and distribution of ancient intrusions now exposed at the surface that may be analogous to the plumbing systems beneath active volcanoes; or (2) we can use geophysical (and geodetic) techniques to image and/or piece together what is actually happening beneath the surface. However, a new problem arises in how to integrate observations, whether they are from ancient and active systems and/or obtained by geophysical or more classical (e.g. petrological and geochemical) techniques.

Top: Complex network of exposed dyke, sill, and inclined sheet intrusions cross-cutting volcaniclastic deposits on Madeira (photo by Craig Magee). Bottom: 3D view of the -600 kg m3 density contrast isosurface beneath the Laguna del Maule, derived from gravity data, which is interpreted to define a magma reservoir (>50% melt) within a larger region of a crystal mush system hosted by volcanic and sedimentary rocks; the 2D planes show slices through the dataset (redrawn from Miller et al., 2017).

I personally get a bit annoyed when I read or see work on a volcano or ‘magma chamber’, which may incorporate extensive quantitative textural analyses or detailed isotope work, and it is all summarised by a cartoon showing a volcano underlain by ‘blobs’ of magma. I say this not to disparage any such diagrams as I completely understand the need to simplify and summarise information. But out of all the sub-volcanic systems I’ve looked at from around the world, I would confidently say such simple systems rarely exist.

What has taken me awhile to learn is that the opposite of my view is just as true. I produce diagrams detailing the structure of plumbing systems, incorporating information on host rock deformation, and then hurriedly throw in some geochemical or petrological data to briefly support my interpretation. I can now imagine that my flippant use of such data, which is borne more out of my ignorance for the power and subtleties of the techniques involved, must annoy volcanologists.

We are all aware that we need to integrate data and techniques, whether it’s to get work published or a grant funded, but I would argue that such integration is usually limited. Of course, there are many cases where this is not true, particularly where a multitude of scientists with very different expertise have got together to solve a problem. Limited integration is a failing I have recognised in my own research and one that I have come to realise is not necessarily easy to rectify. It’s quite obvious that integration is easier when you find and work with a colleague who is an expert in something you are not, but that integration will always be limited if you yourself cannot get a grasp of the technique you want to synthesise your work with.

Hopefully having explained my viewpoint, I can (finally!) get to the main reason for this blog. In 2016 I was offered an opportunity to help satisfy my relatively new desire to promote integration and collaboration. Through Carl Stevenson, my former PhD supervisor, I was asked by Marian Holness to put together a review article on how geophysical techniques could be used to study magma plumbing systems for the Journal of Petrology’s (JPet) exciting new ‘Perspectives in Petrology’ initiative. The end result was our recently published review ‘Magma Plumbing Systems: A Geophysical Perspective’. Here’ I want to provide a brief explanation of what we did and why.

Marian’s vision for the article was to provide fundamental information for readers of JPet on how geophysical techniques could be integrated with petrological and chemical approaches to study igneous systems, all in the hope of promoting collaboration between the two overarching disciplines. Whilst I jumped at the opportunity, I soon remembered I’m not a geophysicist. I’m a field geologist at heart and whilst I use some geophysical techniques, I don’t necessarily understand all about their underlying physical principals. There’s also many geophysical techniques with which I am totally unfamiliar. It was therefore necessary to assemble a team of experts to write what I could not; I am entirely grateful to all 16 co-authors who volunteered their time to help out and would like to think they had as much fun as I in building the review.

For the first part of the review, the assembled team of experts included Susi Ebmeier, Derek Keir, James Hammond, Jo Gottsmann, and Kathy Whaler, who helped collate sections on how geophysical techniques are used to examine active volcanoes. For example, in the review we discuss: (1) Interferometric Synthetic Aperture Radar (InSAR) and other geodetic techniques can relate ground deformation to the geometry and location of moving magma in the subsurface; (2) seismicity (earthquakes) can track magma intrusion and link to petrological indicators of changes in magma dynamics; (3) various seismological techniques (e.g. P-wave travel-time tomography) allow us to map locations and volumes of melt and/or crystallised intrusions beneath active volcanoes; (4) gravimetry can be used, particularly in conjunction with geodetic techniques, to identify regions of magma accumulation; and (5) electromagnetic techniques (e.g. magnetotellurics) provides unique insights into the distribution of fluids, including magma, in the subsurface. These techniques mentioned above are primarily used to study active volcanic systems and, although they all have limitations, their integration is critical to understanding active magma plumbing systems. As we highlight in the review, there is incredible potential, as evidenced by a few studies, in further integrating these techniques with petrological, geochemical, and geochronological information.

Top: Schematic showing how InSAR is collected and how ground deformation patterns may correspond to different intrusion types (Magee et al., 2018). Bottom: Correlation between seismicity, SO2 flux and number of calculated crystal ages over the 1980-1986 Mt St Helens eruption period (from Saunders et al., 2012).
Putting the sections mentioned above together emphasised to me the further importance of integrating such studies with geophysical analyses of ancient intrusions. To shed some light on how this could be achieved and the benefits, I worked together with Chris Jackson, Nick Schofield, Carl Stevenson, Brian O’Driscoll, and Mike Petronis, to discuss how: (1) seismic reflection data, which provides ultrasound-like images of Earth’s subsurface, reveals lateral magma transport in networks of sills and inclined sheets (i.e. sill-complexes) can transport magma over significant lateral (>4000 km!!!) and vertical (>12 km) distances; and (2) rock magnetic techniques, principally anisotropy of magnetic susceptibility (AMS), allows fabrics related to magma flow, layering, textural modification, and/or deformation to be identified and interpreted. These techniques reveal how magma plumbing systems are structured and constructed, providing an important framework in which geophysical data from active plumbing systems can be interpreted.

Uninterpreted and interpreted seismic reflection images from the Rockall Basin, offshore Ireland showing an interconnected network of sills and inclined sheets extending >20 km laterally (modified from Magee et al., 2014).

Beyond a review of what geophysical techniques are already used to study igneous systems, we also highlight the future role that Full Wave-form Inversion (FWI) (Jo Morgan), Unmanned Aerial Vehicles (UAVs) (Sandy Cruden, Steve Micklethwaite, Stefan Vollgger, Greg Dering), and numerical modelling (Matt Jackson) may play in developing our understanding of magma plumbing systems. FWI offers amazing potential in imaging the 3D structure of plumbing systems beneath active volcanoes, allowing insights from the worlds of seismology and seismic reflection data to be combined. UAVs provide an unprecedented resource for mapping and photographing outcrops in detail, but with the potential addition of geophysical sensors to drones, they can help integrate field and geophysical studies. Numerical modelling similarly allows data from different techniques to be integrated and hypotheses tested.

To wrap up, it is important to emphasise that this review is by no means exhaustive and there are so many other techniques and examples that we would have liked to have included and explored: apologies to all those works we were just unable to incorporate and acknowledge.

Hopefully the review will form a foundation for anyone wanting to develop their understanding of how different geophysical techniques can be used to investigate different aspects of active and ancient magma plumbing systems and, in particular, how those techniques could be integrated with each other as well as more classical methodologies (e.g. petrology, geochemistry, geochronology, etc...).

If anything, I had a great time putting this review together and with all that I learnt from the experience I can see an exciting and integrative future within igneous geology. I would thus like to finish by once again thanking all my co-authors for their time and effort in helping put together the review article. I also extend my gratitude to Juliet Biggs, Martyn Unsworth, John Bartley, and Magnús Gudmundsson who dedicated the time and effort to provide four excellent reviews. Lastly, I would thank Marian Holness for the invitation to put together what has certainly been a very rewarding piece of work.

If you are interested in learning more about the techniques discussed in the review or wish to collaborate and integrate one or several of those discussed, please feel free to contact me and I can direct you to the relevant co-authors. I would also particularly love to hear any stories where this review has sparked off an idea and/or collaboration!

Kind regards,

Craig Magee

(Twitter: @DrCraigMagee)


Magee C., Jackson CL., Schofield N. (2014). Diachronous sub‐volcanic intrusion along deep‐water margins: insights from the Irish Rockall Basin. Basin Research, 26, 85–105.

Magee C., Stevenson CTE., Ebmeier SK., Keir D., Hammond JOS., Gottsmann JH., Whaler KA., Schofield N., Jackson CAL., Petronis MS., O’Driscoll B., Morgan J., Cruden A., Vollgger SA., Dering G., Micklethwaite S., Jackson MD. (2018). Magma Plumbing Systems: A Geophysical Perspective. Journal of Petrology, egy064,

Miller, C.A., Williams-Jones, G., Fournier, D. and Witter, J. (2017). 3D gravity inversion and thermodynamic modelling reveal properties of shallow silicic magma reservoir beneath Laguna del Maule, Chile. Earth and Planetary Science Letters, 459, 14–27.

Saunders K., Blundy J. D., Dohmen R., Cashman K. (2012). Linking Petrology and Seismology at an active volcano. Science, 336, 1023–1027.

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