Understanding and forecasting sudden explosive eruptions.

Explosive eruptions of VEI ≤ 3 commonly occur with few warning signs. Such eruptions can be magmatic, phreatomagmatic, or phreatic in nature, and they are driven by the catastrophic release of pressurized gas. Our challenge is how to better forecast these eruptions and better understand them with existing and new tools. Here we examine a number of such eruptions, some lethal to humans, which have occurred during the last decade. We first describe key precursory signals that preceded these events, examine whether they developed in a bottom-up or top-down fashion, and compare the different timescales of precursory activity. In an attempt to understand how, when, and where these systems become pressurized, we then outline the different processes and crustal locations leading to the overpressure. We further identify a number of precursory signals that may be generally applicable and exportable to such systems, and we discuss effective means of using thresholds of these precursory signals and eruptive transitions to improve our forecasting abilities. We conclude by outlining three grand challenges for the next decade: (1) complete forecasts of explosive eruptions including when, where, how big, and what type, (2) a full view of subsurface volcano plumbing, and (3) monitoring networks that are comprehensive, similar, and systematic in nature.