A decade of geodetic change at Kīlauea’s summit—Observations, interpretations, and unanswered questions from studies of the 2008–2018 Halemaʻumaʻu eruption

On March 19, 2008, a small explosion heralded the onset of an extraordinary eruption at the summit of Kīlauea Volcano. The following 10 years provided unprecedented access to an actively circulating lava lake located within a region monitored by numerous geodetic tools, including Global Navigation Satellite System (GNSS), interferometric synthetic aperture radar (InSAR), tilt, and gravity. These datasets revealed a range of processes occurring on widely different timescales. Over years, pressure change within the summit magmatic system, determined from ground deformation and lava-lake surface height, seems to have been governed by broad variations in the rate of magma supply from the mantle to the volcano’s shallow magmatic system, as well as changes in the efficiency of East Rift Zone (ERZ) magma transport and eruption. Over weeks to months, intrusions at the summit and along the ERZ, where new eruptive vents commonly formed and intrusions were primed by extension from south-flank motion, were a result of short-term increases in magma supply or waning lava effusion from the ERZ. Waning lava effusion caused magma to back up behind the ERZ eruptive vent all the way to the summit. ERZ intrusions and eruptions caused rapid depressurization of the summit magmatic system, whereas summit intrusions resulted in complex deformation patterns as magma moved to and from two main sub-caldera storage areas. Over hours to days, pressure changes were caused by episodic deflationinflation (DI) events and possibly small summit intrusions, and deformation of the rim of the summit eruptive vent revealed instabilities that indicated an increased potential for collapse and minor explosive activity. Finally, over timescales of minutes to hours, gas pistoning, summit explosions, very-longperiod seismic events, and even the airborne eruptive plume had clear manifestations in geodetic datasets, providing insights into the causes and consequences of those processes. The diversity and quantity of geodetic observations shed important light on this exceptional and well-documented decade-long summit eruption and its accompanying phenomena, yet numerous questions remain about the causal mechanisms, physical processes, and magmatic conditions associated with eruptive and intrusive activity. Introduction The 2008–2018 summit eruption of Kīlauea Volcano, Hawaiʻi, provided an exceptional opportunity for research and discovery (Patrick and others, 2021). The eruptive vent, inside which an actively circulating lava lake was usually present, was easily accessible and located in the middle of a dense and comprehensive monitoring network within which new equipment could quickly be added. Moreover, this eruption was at a well-studied volcano where a century of research had established a strong framework for understanding the shallow magmatic plumbing system and associated volcanic activity. As a result, an outstanding record of observations was obtained (for example, Patrick and others, 2021) during the eruption, leading to a vast array of detailed insights into magmatic processes ranging from bubble nucleation (for example, Carey and others, 2012, 2013) to the hydraulic connectivity of Kīlauea’s extensive magmatic system (for example, Patrick and others, 2019a). The correlation between surface deformation and lava-lake level proved particularly important, providing information on such parameters as the density of the lava lake (Carbone and others, 2013; Poland and Carbone, 2016, 2018), the potential for intrusions and eruptions at the summit and along the volcano’s East Rift Zone (ERZ; fig. 1) (Patrick and others, 2015), and the volume of the shallow magmatic system (Anderson and others, 2019). In this study, we utilize records from numerous types of geodetic sensors to examine a variety of signals related to changes within the summit magmatic system, as well as within and around the summit vent. We begin by describing geodetic activity at Kīlauea’s summit prior to 2008, changes to the magmatic system that this activity implied, and the network of instrumentation that monitored the 2008–2018 summit eruption, from its explosive onset on March 19, 2008, through 10 years of lava lake activity, to its conclusion coincident with the summit collapse and lower ERZ eruption of 2018 (Neal and others, 2019). We then examine the changes that occurred over the course of the eruption, first from a broad spatiotemporal perspective and then zeroing in on ever smaller (in magnitude) and shorter (in duration) signals that reflect a diversity of processes occurring in and around 2 A Decade of Geodetic Change at Kīlauea’s Summit AA7512_fig 01 SDH SMC IKI