Thematic Articles

Do supervolcanoes form metallic ore deposits? If so, what types of deposits do they form and how large are they?

Rare but extremely large explosive supereruptions lead to the catastrophic formation of huge calderas, devastation of substantial regions by pyroclastic flow deposits, and ash falls that cover continent-sized areas. The effects of future supereruptions will be felt globally or at least by a whole hemisphere. The most widespread effects are likely to derive from the volcanic gases released, particularly sulfur gases that are converted into sulfuric acid aerosols in the stratosphere. These will remain for several years, promoting changes in atmospheric circulation and causing surface temperatures to fall dramatically in many regions, bringing about temporary reductions in light levels and producing severe and unseasonable weather (‘volcanic winter’). Major disruptions to global societal infrastructure can be expected for periods of months to years, and the cost to global financial markets will be high and sustained.

Although giant calderas (“supervolcanoes”) may slumber for tens of thousands of years between eruptions, their abundant earthquakes and crustal deformation reveal the potential for future upheaval. Any eventual supereruption could devastate global human populations, so these systems must be carefully scrutinized. Insight into dormant but restless calderas can be gained by monitoring their output of heat and gas. At Yellowstone, the large thermal and CO2 fluxes require massive input of basaltic magma, which continues to invade the lower to mid-crust, sustains the overlying high-silica magma reservoir, and may result in volcanic hazard for millennia to come. The high flux of CO2 may contribute to the measured deformation of the caldera floor and can also modify the pressure, thermal, and chemical signals emitted from the magma. In order to recognize precursors to eruption, we must scrutinize the varied signals emerging from restless calderas with the goal of discriminating magmatic, hydrothermal, and hybrid phenomena.

Pyroclastic deposits and lava flows generated by supereruptions are similar to, but tens of times larger than, those observed in historic eruptions. Physical processes control eruption styles, which then dictate what products are available for sampling and how well the eruption sequence can be determined. These erupted products and their ordering in time permit reconstruction of the parental magma chamber. Supervolcanoes also have smaller eruptions that provide snapshots of magma chamber development in the lead-in to and aftermath of supereruptions. Many aspects of supereruption dynamics, although on a vast scale, can be understood from observations or inferences from smaller historic and prehistoric events. However, the great diversity in the timings of supereruptions and in the eruptive behaviour of supervolcanoes present continuing challenges for research.

Along-recognized correlation between the volume of major eruptions and the time interval between them suggests that magma may accumulate for about a million years before a supereruption. However, radiometric ages and time-dependent phenomena like crystal growth and compositional homogenization show that the duration of supervolcano magma accumulation could be significantly shorter than this. Crystals in supervolcano magmas may have protracted growth histories and may grow from chemically different hosts as crystallization progresses. Semisolid crystal mushes rather than liquid-rich magma chambers may be the prevalent state of supervolcano feeder systems and should be the focus of geophysical studies aimed at predicting future supereruptions.

The vigor and size of volcanic eruptions depend on what happens in magma reservoirs in the Earth’s crust. When magmatic activity occurs within continental areas, large reservoirs of viscous, gas-rich magma can be generated and cataclysmically discharged into the atmosphere during explosive supereruptions. As currently understood, large pools of explosive magma are produced by extracting interstitial liquid from long-lived “crystal mushes” (magmatic sponges containing >50 vol% of crystals) and collecting it in unstable liquid-dominated lenses.

Earth’s largest volcanic eruptions were an order of magnitude larger than any witnessed by humans since the advent of civilization. These “supereruptions” have played an important role in our species’ past and they pose a serious future threat. In this issue of Elements, we consider key issues that reflect both the scientific and social importance of these aweinspiring phenomena: the products and processes of the eruptions themselves, the nature and evolution of the shallow magma chambers that feed them, the monitoring of active supervolcano systems, and the potential consequences to humans of future supereruptions.