Developments in the study of volcanic and igneous plumbing systems: outstanding problems and new opportunities

Prior to and during eruptions, magma is stored and transported within volcanic and igneous plumbing systems (VIPS) that comprise a network of magma reservoirs and sheet intrusions. The study of these VIPS requires the combination of knowledge from the fields of igneous petrology, geochemistry, thermodynamic modelling, structural geology, volcano geodesy, and geophysics, which express the physical, chemical, and thermal complexity of the processes involved, and how these processes change spatially and temporally. In this contribution, we review the development of the discipline of plumbing system studies in the past two decades considering three angles: (1) the conceptual models of VIPS and paradigm changes, (2) methodological advances, and (3) the diversity of the scientific community involved in VIPS research. We also discuss future opportunities and challenges related to these three topics.


Introduction
Volcanic unrest recorded at the Earth's surface prior to and during eruptions is usually caused by sub-surface processes within the volcanic and igneous plumbing system (VIPS), such as magma transport within sheet intrusions and the establishment and evolution of magma reservoirs that feed those sheet intrusions (e.g. Biggs and Pritchard 2017;Sigmundsson et al. 2018;Sparks et al. 2019). Traditionally, VIPS have been studied in different sub-disciplines of the Earth sciences in parallel ( Fig. 1; Burchardt 2018). Examples include as follows: a. the study of the composition of igneous rocks and minerals to reconstruct the conditions in magma reservoirs (e.g. Sparks et al. 1977;Putirka 2008;Bindeman 2008), b. mapping of dyke swarms to shed light on the synemplacement stress conditions (e.g. Ernst et al. 1995;Hoek and Seitz 1995;Srivastava et al. 2019), c. the use of seismicity beneath active volcanoes to map zones of magma storage (e.g. Scarpa and Gasparini 1996;McNutt 2005;Lees 2007), and d. location of seismicity related to magma transport (e.g. Ebinger et al. 2008;Bell and Kilburn 2012;White et al. 2019).
Early research on plumbing systems was often restricted to single disciplines, such as the examples mentioned above. However, since the 1980s, volcanic eruptions, such as those at Mount St Helens, Pinatubo, and Montserrat, have shown the importance of multidisciplinary work to better understand the role of shallow reservoirs and conduits controlling or influencing eruption style (e.g. Eichelberger and Hayes 1982;Cashman 1988; Editorial responsibility: K.V. Cashman This paper constitutes part of a topical collection: Looking Backwards and Forwards in Volcanology: A Collection of Perspectives on the Trajectory of a Science 2014). It is through this multidisciplinary approach that the strongest advancements in VIPS studies have continued to be made through the second decade of the twentyfirst century (Figure 1).
Understanding magma transport and storage in VIPS is fundamental in estimating volcanic risk, as processes within the VIPS largely control whether an eruption will occur, what type of eruption it will be, and for how long it will last. Other areas where VIPS are of major significance are as follows: 1. crustal growth at plate boundaries and in intraplate settings (e.g. Christensen and Salisbury 1975;Brown 1994;Marinoni 2001), 2. the release of climate-active volatiles during the formation of Large Igneous Provinces (LIPs; e.g. (Aarnes et al. 2010;Svensen et al. 2018;Ernst et al. 2021), 3. heat flow, hydrocarbon maturation, and structural control on hydrocarbon reservoirs in sedimentary basins (Polteau et al. 2008;Senger et al. 2017;Spacapan et al. 2018), 4. the formation of economic deposits of ores and diamonds (e.g. Ganino et al. 2008;Sillitoe 2010;Elliott et al. 2018;Russell et al. 2019), and 5. the formation of geothermal resources (e.g. Sibbett 1988;Boyce et al. 2003;Stimac et al. 2015).
Hence, there are many reasons why VIPS research is both essential and relevant for research and society today.
VIPS studies are thus a dynamic and growing field that is, today, more integrated across the various sub-disciplines of volcanology. In the following assessment, we describe the developments that led to the integration of VIPS studies and the current state-of-the-art in VIPS research in terms of both conceptual understanding and methodological advances. We outline some general trends and explore how the methodological development is improving understanding of magma transport and storage, especially by exploiting opportunities and revealing new challenges. Moreover, we discuss how the scientific community involved in VIPS has evolved. We finally highlight open questions for future research and the need to include a diverse group of people if VIPS research is to continue to evolve in such a positive direction.

Paradigm shifts and state of the art in VIPS studies
Prior to the twenty-first century, several different conceptual models of igneous plumbing system were developed (Figure 2a), some of which have now been abandoned. Prior to 2000, the most prominent concept was that the large amount of granitoid rocks exposed at the Earth's surface originated from crystallisation of huge, fully molten, and long-lived magma chambers. How these magma chambers were emplaced in the crust had been a long-standing dilemma throughout the nineteenth and twentieth centuries (termed 'the space problem'; e.g. Bowen 1948;Read 1957;Hutton 1996). However, methodological advances in the fields of geochronology and geochemistry in the early 2000s demonstrated that granitoid plutons were in fact assembled by the amalgamation of multiple small magma batches over millions of years (Coleman et al. 2004;de Saint Blanquat et al. 2011). This insight agreed with seismic tomography studies of the crust beneath active volcanoes that found the presence of only a few percent of melt (Lees 2007). Diffusion chronometry in crystals from volcanic products also revealed magma storage at rather 'cold', i.e. below magma liquidus temperature, conditions for most of the residence time of erupted crystals (Cooper and Kent 2014).
A new paradigm thus began to emerge that encompassed magma storage in a plumbing system mainly composed of an uneruptible, crystal-dominated 'mush', where melt exists between crystals or in small pockets (Figure 2b). The concept is not completely new (Marsh 2004;Bachmann and Bergantz 2004), but has gained recent momentum as it elegantly explains the diversity of crystal ages within erupted materials and facilitates the fast assembly of mostly molten, but ephemeral, magma reservoirs (Cashman et al. 2017). Thermodynamic modelling of the 'mush plumbing system' (e.g. Petford et al. 2000;Annen et al. 2006;Bachmann and Huber 2016;Jackson et al. 2018;Huber et al. 2019;Annen and Burgisser 2020) has also shown how the system evolves thermally through the addition of small magma batches, how the magma composition can evolve, and how shallow magma storage and magma degassing can contribute to the eruptive potential of volcanoes. However, thermodynamic modelling also  (Burchardt 2018) and b) how aspects of these now are reconciled within the new 'mush' plumbing system paradigm shows that mush extent and longevity are limited by the amount of heat available in the crust (Glazner 2021).
Current research addresses the properties of crystal mush, its generation, and how magma is remobilised within the mush before large eruptions (e.g. Spera and Bohrson 2018;Wieser et al. 2020;Humphreys et al. 2021). Moreover, the mush paradigm will need to reconcile evidence which supports the existence of long-lived and largely molten magma bodies in the geological record (e.g. Barboni et al. 2016;Kruger and Latypov 2020;Rout et al. 2021), to explain the formation of the thermal anomaly required for the generation and persistence of large volumes of mush in the upper crust, and to acknowledge that the lack of high melt fraction reservoirs recovered by seismic tomography may be due in part to a problem of spatial resolution (Paulatto et al. 2019).
Another major conceptual advance in the field of VIPS studies focuses on the transport of magma through sheetlike (dyke and sill) and cylindrical (conduit) pathways. In the last two decades, the insight that magma transport is most efficient through magmatic sheet intrusions has been confirmed by field-based monitoring of seismicity related to propagating intrusions and remote observation of surface deformation patterns (e.g. Ebinger et al. 2008;Bell and Kilburn 2012;Sigmundsson et al. 2014). Even within the lower crust, magma transport is believed to be possible through dyking resulting from self-organisation and convergence of thin magma-filled veins (Cruden and Weinberg 2018).
A broad array of methods, such as field studies in areas with exposed, solidified sheet intrusions, high-resolution 3D seismic surveys of sedimentary basins, and active plumbing systems at mid-ocean ridges, and numerical and laboratory models have all highlighted: 1. the variety of sheet intrusion geometries, including subvertical dykes, inclined cone-sheets, and concordant sills (e.g. Kavanagh 2018;Burchardt et al. 2018), 2. their internal complexity (e.g. Magee et al. 2016;Schmiedel et al. 2021;Köpping et al. 2022), 3. the abundance of these sheets in the plumbing systems (Walker 1992;Walker and Eyre 1995;Tibaldi and Pasquarè 2008), and 4. their interaction with host rock structures and lithologies (e.g. Krumbholz et al. 2014;Spacapan et al. 2017;Norcliffe et al. 2021).
Within this framework, ongoing research on sheet intrusions thus addresses the relationship between the dynamic emplacement and associated volcanic unrest signals, as well as the interplay between tectonics and the magma/hostrock properties, and how understanding of the macro-and micro-scale temporal and spatial processes can be reconciled in such meso-scale structures (cf., Kavanagh 2018;Burchardt et al. 2018).
Currently, knowledge on VIPS processes based on field studies (e.g. Holness and Humphreys 2003;Stephens et al. 2017;Spera and Bohrson 2018;Mattsson et al. 2018;Martin et al. 2019) and modelling (e.g. Gudmundsson et al. 2016;Guldstrand et al. 2018;Drymoni et al. 2020) is integrated into the interpretation of volcanic unrest and eruptive activity. In the past, volcano deformation modelling has successfully reproduced the signals recorded by monitoring networks (Neal et al. 2019). However, the modelled solutions were generally non-unique and not always geologically plausible ). In the next decade, the integration of VIPS research on, e.g., the mechanisms of dyke propagation and deformation associated with the establishment of magma bodies into unrest modelling will contribute to more realistic interpretations of volcano monitoring data, which will lead to better risk management and hazard mitigation.

Methodological developments in VIPS studies
Since the 1950s, a major trend in all fields of the Earth sciences has been the transition from a qualitative, descriptive science towards quantification and accuracy. This began when analogue and numerical models appropriate for magmatic and volcanic systems began to emerge (e.g. Morton et al. 1956;King et al. 1957;Kavanagh et al. 2018). Studies of industrial plumes were applied to model volcanic plumes (Morton et al. 1956), and hydraulic fractures in boreholes were developed as analogues for dyke and sill emplacement (King et al. 1957). Magma chamber processes were then explored in several pioneering analogue experiment studies in the 1980s and combined with insights from numerical models and igneous petrology ); his was a key moment of driving forwards the discipline (e.g. Huppert and Turner 1981;Huppert and Sparks 1981;Turner et al. 1983;Huppert et al. 1983). These models were very effective, and are still held as formative studies in VIPS because they involved multidisciplinary collaborations between mathematicians, fluid dynamicists, engineers, volcanologists, petrologists, and geochemists. The current developments in analogue modelling in volcanology are discussed by Poppe et al. (2022, in press).
The development of multidisciplinary quantitative studies is aided today by rapidly increasing computational power and increasing precision of analytical techniques. Access to commercial software, often developed for industrial purposes, such as the petroleum industry, engineering, or material science, allows VIPS research to benefit from the improvements in these, often better-funded, fields. At the same time, open access to numerical codes designed by researchers allows everyone to apply codes developed for specific Earth science problems. In addition, broad access to satellite images has opened up new opportunities ranging from fieldwork planning and detailed remote sensing to near real-time deformation monitoring at active volcanic systems (Burchardt and Galland 2016). Even in the field, the use of low-cost unmanned aerial vehicles (drones) allows access to previously inaccessible localities, as well as the quantitative study of structural features on virtual outcrops created through photogrammetry. This advance allows for studying aspects of plumbing systems, such as the growth of (crypto)domes, the quantification of magmainduced deformation, and the detailed study of intrusion geometries (e.g. Belousov et al. 2005;Thiele et al. 2021;Rhodes et al. 2021).
Although these and other methodological developments have provided data and models of previously unachievable precision, new challenges have emerged in how to handle, store, and share these data (cf. Science Europe, 2021). Another challenge is connected to the ease of access to tools such as numerical modelling software and bench-top analogue experiments. It is now perhaps easy to create a seemingly correct model output even if one lacks understanding of the physics behind the modelling process and how appropriate the model output is to the natural phenomena. This dilemma stresses the need for ground truthing of modelling output by means of empirical and experimental studies. In addition, it leads us to look at how the research community involved in VIPS studies has developed in the last two decades.

Past, present, and future of the VIPS research community
At the beginning of the twenty-first century, VIPS were still studied as part of separate and different disciplines within the Earth Sciences, most not necessarily linked to volcanology at all (Burchardt 2018). For instance, igneous intrusions were studied by both structural geologists and igneous petrologists, and the results were published in separate specialised journals for structural geology and igneous petrology without linking to each other. On the other hand, books were published in volcanology that barely mentioned the plumbing systems which feed the eruptive products.
However, a transition towards a more integrated and multidisciplinary approach to studying VIPS has become more common in the last decade (e.g. Voight and Sparks 2010;Sigmundsson et al. , 2014Pritchard et al. 2018;Neal et al. 2019). In our opinion, this transition has been fuelled by three factors: 1. volcanic eruptions that have inspired the formation of multi-disciplinary consortia, such as the eruption of Eyjafjallajökull in 2010 that spurred a number of largescale projects, such as FutureVolc, and the 2021 eruption of La Palma, Canary Islands. 2. scientific conference sessions, symposia, and workshops with a clear, multidisciplinary scope, such as at the EGU, AGU, and IAVCEI general assemblies, and 3. the formation of the IAVCEI Commission on Volcanic and Igneous Plumbing Systems (https:// vipsc ommis sion. org/).
Magmatic activity in general, and volcanic activity in particular, involves complex physical and chemical processes. These unfold over time and length scales that span many orders of magnitude from seconds to millions of years and from nm to hundreds of km. Grasping even part of this complexity thus demands the combination of multiple methods capable of addressing this huge range in temporal and spatial scale. The different strengths of each approach partly overcome their weaknesses such that the whole is greater than the sum of its parts (Burchardt and Galland 2016;Edmonds et al. 2019). This can best be achieved in multidisciplinary consortia where experts from different fields work together with a common goal.
An immediate challenge for such consortia, as well as in multidisciplinary conference sessions and workshops, is the establishment of a common language. The history of Earth sciences has in the past been characterised by increasing specialisation, which fostered the development of specialised terminology and scientific jargon in different fields. An example is the diverse use of the term 'magma chamber' (cf. Lees 2007;Glazner et al. 2016;Burchardt 2018): while for instance petrologists may picture shapeless reservoirs hosting the chemically evolving melt, crystals, and volatiles they call magma, the structural geologists traditionally envision intrusions of characteristic shapes reflecting specific emplacement mechanisms of the passively emplaced fluid they call magma. Geophysicists on the other hand detect magma chambers as sub-surface anomalies of seismic wave speeds corresponding to hot rocks with some percentage of melt. Shaping an effective multidisciplinary research community that studies the complexities of VIPS thus needs to also focus on sharing knowledge through effective communication.
Scientific jargon is exclusive, and so is a lack of diversity among scientists regarding both disciplines and personal attributes. An inclusive and diverse scientific community will combine the strengths of different methods, build on knowledge of the past, and include new ideas to improve the understanding of magma transport and storage (Figure 3). This is why the IAVCEI Commission on VIPS, founded in 2016, is actively working to provide a platform with a global reach for multidisciplinary dialogue, collaboration, and outreach, including an ambitious plan for improving equality, diversity, and inclusivity (see www. VIPSc ommis sion. org; Kavanagh et al. 2022, this volume).
As in other branches of Earth Science, there is a tendency to extend the conclusions drawn from one case study into general models and ignore the diversity and complexity of the processes involved. Collaborative efforts and multidisciplinarity are the best ways to avoid this pitfall. The next decade of research on VIPS thus needs: 1. researchers that are experts in using their methods, but who are able to communicate beyond their own field, 2. generalists with an overview of different methods, thereby allowing identification of new connections or opportunities, 3. working environments that allow full, open, and respectful collaboration between Earth scientists and experts from other fields, 4. constructive communication between researchers working in the field, in laboratories, as well as modellers, and observatory personnel, and 5. above all, our discipline needs to support and nurture a diverse group of students with a solid education in the Earth sciences and a broad outlook so that they dare to question the state-of-the-art assumptions of our field and create new knowledge.

Outlook
The study of magma transport and storage in VIPS has, in the last two decades, emerged as its own sub-discipline within volcanology, from a history of evolution that was dispersed across separate fields. Our vision is of a science that embraces the diversity and complexity of volcanic and igneous systems, while focusing on defining the general laws that underpin the functioning of VIPS. One of the big challenges that researchers will have to address in the upcoming decade is the link between VIPS dynamics and eruption dynamics in order to improve our ability to forecast eruptions and to use VIPS knowledge to reduce the hazards and negative societal impacts of volcanism. We hope that the next decade thus will see an even closer integration of different methods Fig. 3 Our vision of a diverse and inclusive research community studying VIPS using many different methods: a expert field geologists, b expert analogue (experimental) modellers, c expert petrologists and geochemists, and d expert numerical modellers.
with the aim of better understanding the processes of magma transport and storage in the Earth's crust. This integration will happen in a more inclusive and diverse scientific community that will unravel some of the remaining big questions on VIPS and come up with many new questions to address.
Funding Open access funding provided by Uppsala University. JK acknowledges a UKRI Future Leaders Fellowship (MR/ S035141/1). CA was supported by a grant IXXI from the Institut rhônalpin des systèmes complexes. SB acknowledges a Wallenberg Academy Fellowship by the Knut and Alice Wallenberg Foundation (KAW 2017.0153).
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