Tens or even hundreds of kilometers beneath Earth’s surface, magmas are born when water, carbon, heat, and pressure are present in the right proportions and minerals in the mantle begin to melt. This melting marks the beginning of a long journey, in which microscopic amounts of buoyant magma gradually make their way upward, migrating along crystal edges and forcing their way through fractures in the surrounding rock. As these tiny amounts of melt find each other, they merge into larger magma bodies that pry open jagged conduits through which they ascend further.
Mantle supply refers to the flux of these magmas (and to the volatile chemical species like water and carbon dioxide that they carry) from the mantle to magmatic systems that span the thickness of the crust. This supply is the essential driver of volcanism and volcanic hazards at the surface, from explosive eruptions and towering ash plumes to lava and debris flows and volcanic gas emissions.
Mantle supply is poorly understood because it is difficult to “see”—using geochemical and geophysical techniques—through shallower parts of magmatic systems to their roots in the mantle.
Yet despite its fundamental importance, mantle supply is poorly understood because it is difficult to “see”—using geochemical and geophysical techniques—through shallower parts of magmatic systems to their roots in the mantle. Compounding this issue, different geoscientific disciplines determine mantle supply constraints (i.e., flow rates and volumes) by looking at processes over a wide range of temporal and spatial scales.
To better understand magma supplies deep beneath volcanoes in subduction zones and other tectonic settings, scientists must catalyze progress in research to address several grand challenges. These challenges, illuminated by 70 participating scientists during a June 2024 workshop, include improving modeling and geophysical imaging of mantle magma systems and reconciling knowledge of these systems across disciplines.
Making the needed progress will depend upon leveraging recent scientific advances in geophysics, gas geochemistry, petrology, and geodynamic modeling; developing interdisciplinary approaches to study mantle supply; and synthesizing new strategies to geophysically image magmatic systems from the mantle to the surface.
Mantle Magmatic Models
Over the past few decades, our understanding of volcanic systems has advanced in transformative ways. Scientists have adopted a new conceptual model of transcrustal magmatic systems underpinned by petrologic studies of erupted volcanic rocks. In this model, magma bodies, encased to different extents within crystal-rich mush zones, are interconnected across crustal depths [e.g., Cashman et al., 2017].
Extending this conceptual model to integrate the mantle is an important step toward addressing profound questions about the origins of and controls on magmatic systems (Figure 1). Such questions include how and why volcanoes emerge where they do and how mantle supply influences magma storage and transport in the crust and eruptions and hazards at the surface.
The lower crust is a key interface between the mantle and upper crustal portions of magmatic systems. Geochronologic studies of the ages of exhumed lower crustal magmatic rocks and petrologic studies of the compositions of crystals from primitive magmas from volcanic arc settings (e.g., the Andes, Cascades, and Central American arcs) offer the potential to elucidate how the lower crust controls magma supply to the upper crust and the initiation of eruptions. For example, diffusion timescales gleaned from primitive olivines from the Central American arc suggest rapid, subdecadal resupply of magmas from the mantle to transcrustal magmatic systems, with magmas then transiting the lower and middle crust at rates of tens of meters per day [Ruprecht and Plank, 2013].
Advances in geodynamics are crucial for developing robust mantle-to-surface magmatic models.
Advances in geodynamics, which focuses on processes and properties controlling the movement of materials in Earth, are also crucial for developing robust mantle-to-surface magmatic models. Geodynamic models can break down at key interfaces between different regimes, such as the lithosphere-asthenosphere boundary and the Moho (the Mohorovičić discontinuity, which marks the boundary between Earth’s crust and mantle). Consequently, modeling the pathways followed by magma en route to the surface can be extremely challenging.
Likewise, treating aqueous fluids and magmas as generic fluids without any distinction between them, as is often done in models of fluid migration, also complicates geodynamic interpretations. In reality, aqueous fluids and magmas have distinct or hybrid properties. More accurate modeling of the complex interplay among melting, freezing (solidifying), and dissolution reactions that control fluid compositions and properties is likely critically important for tracking these fluids through mantle and crustal magmatic systems.
Imaging Beneath the Upper Crust
Geophysical techniques using gravity, seismic, and magnetotelluric data have captured increasingly complex snapshots of magmatic systems in the upper crust, for example, revealing the locations of upper crustal magma chambers (Figure 1). These methods are poised to play key roles in shedding light on the architecture and properties of deeper magmatic systems as well.
However, determining magmatic properties such as melt fraction, temperature, and porosity from geophysical images is not trivial because a given set of seismic or gravity data could result from multiple distinct combinations of values for these variables. Moreover, imaging deep portions of magmatic systems in the first place remains a pressing challenge, one that will require coordinated application of multiple methods, buttressed by modeling, field, and experimental data that will aid in the interpretation of imaging results.
A key first step is developing a geophysical model intercomparison project. In this project, conceptual models of transcrustal magmatic systems and geodynamic models of the mantle will be compared and combined to produce a more thermodynamically realistic understanding and physical models of mantle supply. Among the key properties considered in these models will be temperature, melt composition, melt fraction, and melt volatile content.
The results of this intercomparison project will guide the design of future geophysical deployments in the field.
These synthetic mantle magmatic models will be used to investigate applications of different combinations of geophysical techniques—and different arrays of deployed instrumentation—to assess questions that can and cannot be addressed. The results of this intercomparison project will then guide the design of future geophysical deployments in the field, including those of the Subduction Zones in Four Dimensions (SZ4D) effort, a large-scale research community initiative to explore processes underlying subduction zone hazards.
Even within the relatively well studied upper crust, major questions about magma systems remain. For example, how quickly do magma reservoirs from the Moho to the upper crust change, and how quickly does magma move through them? The emerging use of time-lapse geophysical imaging that can capture changes in magmatic systems on human timescales offers especially exciting prospects for answering these questions.
Reconciling Mantle Supply Constraints
Accurately quantifying mantle supply from a single method is difficult, but comparing and reconciling magma supply constraints obtained from different methods are perhaps even more so. This difficulty arises because different methods suggest estimates of mantle supply over a wide range of temporal and spatial scales (Figure 2). Further, different disciplines constrain supply rates at different depths in different magmatic systems using different measurement units. For example, cubic kilometers per year is used for hot spot settings, individual plutons, and global estimates, whereas cubic kilometers per kilometer (of trench or ridge) per million years is commonly used to express supply per kilometer of trench in subduction zones or per kilometer of ridge at divergent boundaries.
Understanding the ratio of intrusive to extrusive rock in magmatic systems, which reflects the proportion of igneous rocks that have solidified in the crust (intrusive) to the proportion that erupted at the surface (extrusive), is especially important for reconciling different magma supply constraints because the eruptive flux likely represents only a fraction of the total magma supply.
Coordinating efforts to estimate mantle supply promises to bridge the gap between our understanding of deep magmatic processes and volcanic eruptions.
Estimates of this ratio in volcanic arc settings vary widely, from <1:1 to 35:1 [White et al., 2006], but it is not clear whether this range is due to true natural variability or to differences in the approaches and data used to quantify the ratio. Eruption histories are sometimes used, for instance, but they record only magmas that reach the surface, whereas records of volcanic sulfur emissions also track shallow intrusive magmas that do not reach the surface (but still emit gas to the atmosphere) and records of noble gas emissions (e.g., helium) help constrain mantle-to-crust magma fluxes. At larger scales, geophysical estimates of net crustal growth relate to magma supply and inform intrusive-to-extrusive ratios, although processes that remove crust, such as delamination at the base of the crust and erosion, must also be accounted for.
Agreeing on consistent and robust magma supply constraints across scales will require interdisciplinary collaboration between gas geochemists, field geologists, geodynamicists, geochemists, geochronologists, and others. A sensible first step is to establish common units for these constraints. Ultimately, coordinating efforts to estimate mantle supply—and to develop novel indirect approaches that inform these estimates, such as tracking noble gas emissions or surface erosion rates—promises to bridge the gap between our understanding of deep magmatic processes and volcanic eruptions.
Building on Lessons Learned
The SZ4D initiative aims to compare subduction zone processes across the Cascades, Aleutian, and Chilean volcanic arcs. Understanding mantle supply, which links subducting tectonic plates with surface volcanism, is central to answering the scientific questions driving SZ4D’s investigation of subduction zone volcanic hazards (see the SZ4D Implementation Plan).
The well-studied Cascades arc has provided an excellent natural laboratory for examining connections between crustal structure and mantle-derived magma supply, and at the 2024 workshop, participants evaluated past and ongoing research across the Cascades arc as a template for coordinated research efforts elsewhere. For example, heat flow observations help constrain magmatic fluxes in the Cascades [e.g., Till et al., 2019], but this type of data is not available for all arcs.
Systematically tracking heat flow elsewhere (e.g., the Andes) could enable comparisons of magma supply among arcs globally, especially when this information is combined with other data, such as chemical measurements of gas emissions. Furthermore, new geophysical techniques pioneered in the Cascades, such as scattering of seismic waves by magma bodies, demonstrate the potential to image upper crustal magma reservoirs using only sparse instrumental arrays. These advances raise the exciting possibility of improved capabilities for comparing observations from many magmatic systems across different arc segments.
Work on the Cascades arc also highlights the challenges of studying mantle supply. For example, sulfur emissions from Cascades volcanoes are undetectable, precluding comparison to other arcs for which robust sulfur measurements are available. Consequently, using multiple overlapping methodologies at different arcs will be essential for scaling our understanding of magma supply from local to regional and global scales.
The key message that emerged from the workshop is that progress in understanding mantle supply across scales is needed to yield new insights into the life cycles and behaviors of magmatic systems. On long timescales of millions of years, mantle supply builds arcs and continental crust. On shorter timescales, mantle supply governs the tempo and style of volcanic eruptions and the hazards they cause at the surface. Yet many facets of how these processes play out remain mysterious.
Recent technological advances and new opportunities for interdisciplinary collaboration among geophysicists, geochemists, and geodynamicists promise to improve the accuracy of magma supply estimates and elucidate the deep-seated controls on Earth’s prolific volcanism.
Acknowledgments
The 3-day Workshop on Mantle Magma Supply and Imaging Magmatic Systems, held in June 2024 at the Lamont-Doherty Earth Observatory of Columbia University in New York, was sponsored by the National Science Foundation (NSF) and cosponsored by the SZ4D initiative. We thank Terry Plank for invaluable insights and logistical support, and we appreciate conversations with all the workshop participants. Support was provided by NSF grant EAR 2404913.
References
Cashman, K. V., R. S. J. Sparks, and J. D. Blundy (2017), Vertically extensive and unstable magmatic systems: A unified view of igneous processes, Science, 355, eaag3055, https://doi.org/10.1126/science.aag3055.
Ruprecht, P., and T. Plank (2013), Feeding andesitic eruptions with a high-speed connection from the mantle, Nature, 500, 68–72, https://doi.org/10.1038/nature12342.
Till, C. B., et al. (2019), The causes of spatiotemporal variations in erupted fluxes and compositions along a volcanic arc, Nat. Commun., 10(1), 1350, https://doi.org/10.1038/s41467-019-09113-0.
Ulberg, C. W., et al. (2020), Local source Vp and Vs tomography in the Mount St. Helens region with the iMUSH broadband array, Geochem. Geophys. Geosyst., 21(3), e2019GC008888, https://doi.org/10.1029/2019GC008888.
White, S. M., J. A. Crisp, and F. J. Spera (2006), Long‐term volumetric eruption rates and magma budgets, Geochem. Geophys. Geosyst., 7(3), Q03010, https://doi.org/10.1029/2005GC001002.
Author Information
Ben Black (bblack@eps.rutgers.edu), Department of Earth and Planetary Sciences, Rutgers University, Piscataway, N.J.; Samer Naif, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta; Forrest Horton, Woods Hole Oceanographic Institution, Woods Hole, Mass.; and Andrea Goltz and Cian Wilson, Carnegie Institution for Science, Washington, D.C.
