Merging geophysical, petrochronologic, and modeling perspectives to understand large silicic magma systems

Abstract:

How does one quantify the hazard from a large but dormant silicic caldera system with no volcanic eruptions for 70,000 years? Unlike more recently erupting calderas such as Campi Flegrei, Taupo, or Laguna del Maule, Yellowstone enjoys a long slumber characterized by considerable unrest and geophysical activity, yet without any magma extrusion. Compared with many other large silicic magma systems, though, Yellowstone displays more abundant evidence for continuing magma intrusion. Up to several thousand earthquakes shake the region annually, most within discrete swarm episodes. Decimeter-scale uplift and subsidence occurs in tandem with the ongoing earthquakes, and extends north of the caldera as well as from at least two sources within. Gas discharge reflects continual input of mantle-derived basalts on a scale similar to that of Kilauea, Etna, and Earth’s other most prolific natural CO₂ sources. Heat loss is also commensurate with basaltic input on the order of 0.1 to 0.6 km³ per year. Most workers agree that such considerable heat supply and degassing requires the assembly of a deep gabbro body beneath the upper crust silicic magma reservoir.

Geophysical imaging clearly confirms the presence of an upper crustal magma body, though with apparent melt fractions inconsistent with the eruptible volume required for a supereruption. Petrologic data implies the slow cooling and crystallization of the magma prior to cessation of eruptions 70,000 years ago. Has the magma solidified such that it cannot re-awaken? How much new basaltic input is required to renew silicic volcanism, or has the system been rendered too refractory to generate new eruptible magmas? Attempts to answer such questions require sophisticated numerical models based on a set of variables with large uncertainties. Indeed, even our subsurface images of magmatic systems will remain blurry at best without additional subsurface exploration, sampling, observations, and testing. Our future knowledge of deep magmatic and hydrothermal systems will require considerable effort to design scientific drilling programs at Yellowstone or elsewhere that provide direct information on the temperatures, pressures, permeabilities, material properties and phase relations in the near- and suprasolidus environments that hold so many secrets.