History of Geophysics Opinion

We Need a New Definition for “Magma”

Confusion over the meaning of "magma" can generate popular misperceptions, including a nonexistent molten sea underneath Yellowstone National Park. We propose a different definition.

By , John M. Bartley, and Drew S. Coleman

“Magma,” definition 5: A confused or disordered body or mass of something.

Oxford English Dictionary Online (20 September 2016)

Magma is a fundamental constituent of the Earth. Issues as diverse as volcanic hazard assessment and planetary evolution studies rely on knowledge of magma’s properties, origin, evolution, and significance. Thus, the definition of “magma” should be simple and universally agreed upon, but the term means fundamentally different things to different people.

This inconsistency has led to miscommunication between petrologists, geophysicists, the press, and the public, making the “confused or disordered” definition of the word unintentionally appropriate. It is time to agree on a clearer geologic definition of “magma.”

To the public, magma is the stuff of lava—hot, glowing red liquid that flows out of volcanoes—and such lava is unquestionably magma that has reached the surface. The point of contention is whether partially molten rock that resides below the surface and is too crystalline to flow should also be called magma.

Because this distinction is critical when geologists communicate about magma, especially to the press and public, we contend that highly crystalline immobile material should not be called “magma.” Rather, magma should mean material that is capable of moving within the Earth and onto its surface.

Can Magma Be Mostly Solid?

“Magma” is commonly used to mean any rock that is at least a little bit molten. For example, a recent seismic study of areas around Yellowstone National Park published in Science [Huang et al., 2015] reported a “lower-crustal magma body [that] has a volume of 46,000 cubic kilometers.” That is a lot of magma!

Basaltic lava erupts from an active cone in Hawaii. This lava came from magma, but what is the definition of “magma?”
Basaltic lava erupting from an active parasitic cone (about 5 meters tall) on the side of Puʻu ʻŌʻō, Hawaii, 1997. The flowing material is unquestionably erupted magma, but whether its partially molten source region should be called magma is debatable. Credit: Allen Glazner

Several national news outlets picked up on this figure, including National Public Radio, NBC News, the Washington Post, and Fox News. All of them reported that there is enough magma “to fill the Grand Canyon 11 times.”

This may have created a public perception of a vast pool of liquid perched under Yellowstone, waiting to explode. However, the authors of the scientific study were careful to note that nothing has changed—the amount of molten material under Yellowstone is the same, and the risk has not changed—but they had produced an improved image of the partially molten rock.

The study also noted that this large volume in their image contains only a small percentage of molten material. That is still a lot of melt—enough to fill 20% of the Grand Canyon—but it is dispersed through a large volume of rock, not gathered for an eruption. It is unlikely that the features that Huang’s group imaged could produce an eruption unless the molten material were collected into a much smaller volume with a much higher melt fraction.

What’s in a Name?

Whether rock that contains a small percentage of partial melt should be called magma is debatable. Indeed, use of the word “magma” to refer both to material that can flow across the Earth’s surface and to a largely solid volume that contains a small fraction of melt is akin to using the same word to refer to a river and to an aquifer. To do so ignores and obscures fundamental differences, and broad usage of “magma” is clearly causing such conceptual problems.

The term “magma” was used in pharmacy as early as the 17th century for suspensions such as magnesia magma (now called milk of magnesia). George Scrope was among the first to use the term in a geological context [Scrope, 1862, p. 121]. He wrote that lava “is not a homogeneous molecular liquid, such as any melted or completely fused substance, but … a ‘magma’ or compound of crystalline or granular particles to which a certain mobility is given by an interstitial fluid.” Scrope clearly viewed the ability to flow or intrude as a defining quality.

Should I Stay or Should I Flow?

A critical control on mobility in a crystal-liquid mixture is the volume ratio of crystals to liquid; the apparent viscosity of a mixture (the ratio of how much shear stress is applied to the rate at which the material deforms) depends upon the proportion of particles suspended in it. For a low percentage of solid particles, less than 20% by volume, for example, the particles are sufficiently dispersed that they scarcely interact during flow.

A look at viscosity versus crystal volume percent may help constrain what material can be called “magma.”
Fig. 1. Schematic illustration (background, two-dimensional representation of a three-dimensional crystal framework) of the transition from liquid with suspended crystals to a rigid crystal framework with interstitial liquid, with a plot (foreground) of the increase in viscosity that accompanies a greater percentage of crystalline material. Lockup occurs when the crystal framework gains enough connectivity that shear deformation can only occur through processes governed by deformation of the solid fraction. Credit: Lejeune and Richet [1995]
However, when the percentage of particles reaches approximately 40%–60% by volume, the apparent viscosity of the suspension increases by several orders of magnitude as crystals lock up and jam together. Crystals within these logjams hold each other in place in a phenomenon called force chains [Cates et al., 1998], and the system transitions from a melt with suspended crystals to a crystal network with interstitial melt (Figure 1) [Lejeune and Richet, 1995].

During cooling, crystals grow onto one another and interlock to produce a welded framework that is even stronger than one produced by nonreactive particles such as pebbles in water. Such a material can only flow by processes such as crystal plasticity and solution-reprecipitation, at rates dramatically slower than those at which even highly viscous silica-rich melts can flow. Collecting melt from such a material is a slow process.

Two Names for Two Materials

This fundamental difference in deformation and flow (rheology) between partially molten rock that is melt rich (more than about 50% melt by volume) and its melt-poor counterpart (less than about 50%) is reason to give the two materials different names. Here we suggest that the term “magma” be reserved for melt-rich materials that can flow as fluids on timescales consonant with volcanic eruptions. We suggest that more crystal-rich and largely immobile partially molten rock be referred to by another name such as “crystal mush” or “rigid sponge” [Hildreth, 2004].

By this definition, highly viscous materials such as water-poor rhyolite lavas (Figure 2), with viscosities that can reach 1010 pascal seconds or greater [Pinkerton and Stevenson, 1992], are magma, whereas highly crystal rich materials are not. The former can ascend to the Earth’s surface sufficiently rapidly to be erupted, whereas the latter cannot. This is consistent with the general observation that volcanic rocks with more than about 50% crystals by volume are rare [Marsh, 1981].

Aerial view to the south of a rhyolite dome in the Mono Craters chain of eastern California.
Fig. 2. Aerial view to the south of a rhyolite dome in the Mono Craters chain of eastern California. This dome erupted about 600 years ago. Although lavas such as these have viscosities that can range up to 1010 pascal seconds or greater [Pinkerton and Stevenson, 1992], they are clearly capable of movement on eruption timescales and thus would be considered erupted magma by the definition proposed here. Credit: Allen Glazner
This distinction is not, and cannot be, precise, yet it is useful for reasons both internal and external to the magma community. For example, many interpretations of plutonic rocks depend on the inferred existence of large bodies of melt-rich magma in which processes such as crystal settling play out—the “big tank” model, illustrated in myriad textbooks and papers. Such processes are largely impossible in a system such as the body imaged under Yellowstone, which consists of more than 90% solid rock with melt in its pore spaces. The magma research community intuitively understands this distinction, but using “magma” as a one-size-fits-all term for rock with any proportion of melt obscures it.

For the press, the public, and even Earth scientists who do not specialize in magmatic systems, “magma” conjures up dramatic images of lava flowing down hillsides. Using the same term to describe large rock volumes that contain small melt fractions as well as large bodies of mobile magma can engender such mistaken perceptions as a sea of potentially eruptible magma underneath Yellowstone.

A Better Definition

We want to start a conversation about a more precise definition of “magma.” We suggest this as a starting point:

Magma: naturally occurring, fully or partially molten rock material generated within a planetary body, consisting of melt with or without crystals and gas bubbles and containing a high enough proportion of melt to be capable of intrusion and extrusion.

This proposed definition naturally reflects our particular scientific perspective and concerns; we hope that it will stimulate a broad-based discussion that will yield a consensus definition.

Acknowledgments

Conversations with numerous geologists and geophysicists over the years have helped to clarify various viewpoints about the meaning of “magma,” and Christoph Breitkreuz and Bernard Bonin offered interesting German and French perspectives. Comments by anonymous reviewers are greatly appreciated. Supported by NSF grant EAR-1250505 to A.F.G.

References

Cates, M. E., J. P. Wittmer, J.-P. Buchaud, and P. Claudin (1998), Jamming, force chains and fragile matter, Phys. Rev. Lett., 81, 1841–1844.

Hildreth, W. (2004), Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: Several contiguous but discrete systems, J. Volcanol. Geotherm. Res., 136(3–4), 169–198.

Huang, H.-H., F.-C. Lin, B. Schmandt, J. Farrell, R. B. Smith, and V. C. Tsai (2015), The Yellowstone magmatic system from the mantle plume to the upper crust, Science, 348(6236), 773–776.

Lejeune, A., and P. Richet (1995), Rheology of crystal-bearing silicate melts: An experimental study at high viscosities, J. Geophys. Res., 100, 4215–4229.

Marsh, B. D. (1981), On the crystallinity, probability of occurrence, and rheology of lava and magma, Contrib. Mineral. Petrol., 78, 85–98.

Pinkerton, H., and R. J. Stevenson (1992), Methods of determining the rheological properties of magma at sub-liquidus temperatures, J. Volcanol. Geotherm. Res., 53, 47–66.

Scrope, G. P. (1862), Volcanos: The Character of Their Phenomena, Their Share in the Structure and Composition of the Surface of the Globe, and Their Relation to Its Internal Forces; with a Descriptive Catalogue of All Known Volcanos and Volcanic Formations; with a Map of the Volcanic Areas of the Globe, 490 pp., Longman, Green, Longmans, and Roberts, London.

—Allen F. Glazner, Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill; email: [email protected]; John M. Bartley, Department of Geology and Geophysics, University of Utah, Salt Lake City; and Drew S. Coleman, Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill

Citation: Glazner, A. F., J. M. Bartley, and D. S. Coleman (2016), We need a new definition for magma, Eos, 97, doi:10.1029/2016EO059741. Published on 22 September 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • Howard R Naslund

    The real problem is not the definition of magma, but the description of the body beneath Yellowstone by the press. The press should have reported this as a 46,000 km3 igneous body containing a small per cent of magma. If we redefine our terminology every time the press gets it wrong, we will be constantly redefining everything in science.

    • Donna Goss

      I agree that the press should have described the seismically determined igneous body beneath Yellowstone properly. Of concern is determining locations and changes involving magmatic activity of molten or partially molten portions, volumes of magma, and what volume has potential for eruption.

  • Donna Goss

    I agree with Henry Dick, that magma is molten rock in the subsurface. Defining magma is similar to defining ice cream. The more different types of additives, nuts, fruit, and swirls in it, the more it becomes something other than ice cream. When it gets so jam packed with all sorts of stuff, it doesn’t resemble or act like ice cream anymore, and people are examining the stuff in it and some expound on what the ice cream had become. Of course, similarly magma is subsurface molten rock, and people can start thinking of crystals and all sorts of other ingredients and how it acts. I liked to spend time looking at outcrops, examine shapes of igneous rock and imagine that it was once molten and what it did then as magma until it crystallized into solid rock.

  • Henry Dick

    Good grief. Either Hilary or Donald are going to be elected president, Syria is in flames, and my last grant was turned down – and we are supposed to worry about this? Magma is molten rock in the subsurface – end of story. Crystals not included, unless you wish to open up a big can of worms as you go from liquid to mush to grain supported aggregate. Leave it be.

  • Henry Dick

    Good grief,

  • Donna Goss

    I had learned that the definition of magma is for below surface, and eruption can be flowing lava, extruding domes, lapilli, and explosive ash. The erupted material indicates what the magma consisted of. I find it interesting to question the definition of magma. I previously thought of magma as molten or partially molten rock, such as crystals with interstitial fluid and gas, and potentially capable of melting, intruding, and erupting. This brings up the question about using the word “potentially” It could have been cooling so much as to become incapable of doing anything but crystallize entirely, and be at a temperature and pressure maintaining a crystalline state. However, some of it may become pockets with interstitial fluid and gas, with a potential for eruption determined by increase in temperature, increasing stress with resultant structural changes, or heating with additional mantle plume activity. Defining a hot shape as magma, while explaining its variations such as composition, percentage crystals locked into place, interstitial fluid and gas, temperature, and potential for change, seems to be a sufficient verbalization. The hot shape could be porous and increasingly permeable when very hot fluid and gas accumulates with an increasing percentage, facilitating its mobility, and liquefy for eruption.

  • davidlaing

    This is good, but it could be expanded to include the rheology of the material in question.

  • George Bergantz

    While the authors cite the excellent paper by Cates et al., they didn’t fully appreciate that one of the essential points made there, and in subsequent related literature (see papers by Ness and Sun for example), is that the notion and progress of “strength” is through a sequence of (micro-and-macroscopic) fragile states (ultimately producing discontinuous shear thickening) with an apparent yield strength, prior to hard jamming. Invoking the notion of some magic 50% crystallinity as being a robust criteria, and the continued application of suspension rheology, is “adding bricks to a crumbling foundation” as far as the description of viscous crystal-rich mixtures. There are a number of dynamic and kinematic (deformation) regimes that emerge under crystal-rich hydrogranular conditions involving lubrication, sustained intermittent friction, granular shear, viscous forces, strain localization and small changes in pore pressure that are not captured in the simplified approach here. Small excursions in pore pressure can quickly fluidize a crystal-rich mixture, producing commonly observed mineral fabrics, sorting and even mixing. True jamming (see Peters, Nature, 2016)- which can happen in even frictionless materials, is not a function of shear rate or volume fraction, but the absolute value of shear stress, it occurs (macroscopically) when shear stress overcomes lubrication forces causing melt films to break down inducing stress chain propagation to a rheological boundary. But the point here is that crystal-rich mixtures are not “rheologically dead” at some fixed value of the volume fraction, (short of the total loss of permeability such that pore pressure is no longer defined) and are able to be mechanically “unzipped” and activated on short timescales. And there is ample evidence for that in crystal zoning and chronology even in very viscous systems.

    In my view advocating for a new definition of magma without embracing the still emerging science of the dynamics of crystal-rich mixtures does not open new doors going forward, and it is doubtful, in my experience, that there has been any persistent confusion on the part of the community regarding the current usage that would warrant that change.

  • George Bergantz

    While the authors cite the excellent paper by Cates et al., they didn’t fully appreciate that one of the essential points made there, and in subsequent related literature (see papers by Ness and Sun for example), is that the notion and progress of “strength” is through a sequence of (micro-and-macroscopic) fragile states (ultimately producing discontinuous shear thickening) with an apparent yield strength, prior to hard jamming. Invoking the notion of some magic 50% crystallinity as being a robust criteria, and the continued application of suspension rheology, is “adding bricks to a crumbling foundation” as far as the description of viscous crystal-rich mixtures. There are a number of dynamic and kinematic (deformation) regimes that occur under crystal-rich hydrogranular conditions conditioned by lubrication, sustained friction, granular shear, viscous forces, and small changes in pore pressure that are not captured in the simplified approach here. Small excursions in pore pressure can quickly fluidize a crystal-rich mixture, producing commonly observed mineral fabrics, sorting and even mixing. True jamming (see Peters, Nature, 2016)- which can happen in even frictionless materials, is not a function of shear rate or volume fraction, but the absolute value of shear stress, it occurs (macroscopically) when shear stress overcomes lubrication forces causing melt films to break down inducing stress chain propagation to a rheological boundary. But the point here is that crystal-rich mixtures are not “rheologically dead” at some fixed value of the volume fraction, (short of the total loss of permeability such that pore pressure is no longer defined) and are able to be mechanically “unzipped” and activated on short timescales. And there is ample evidence for that in crystal zoning and chronology.

    In my view advocating for a new definition of magma based on the still emerging science of the dynamics of crystal-rich mixtures does not open new doors going forward, and it is doubtful, in my experience, that there has been any persistent confusion on the part of the community regarding the current usage that would warrant that change.

  • Duff

    I fail to see any difference between the definition of magma given in my copy of the Glossary of Geology, Second Edition (1980) and the definition suggested by the authors of this essay. It’s also a bit “confusing” that none of the photos that the authors use in their essay are of magma. They are photos of lava … the subsurface- versus surface-environment distinction.

  • Mario Aigner-Torres

    Magma is just molten rock; Lava is just subaerial magma. All the rest being equal…really.

  • Mark Elowitz

    If one takes the first-derivative of the function in Plot 1, there should result a derivative peak at the location of the inflection point in the steep slope (at ~50% volume % crystals). Could the location of the derivative peak be used as a quantitative criteria at which lockup occurs?