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The science of volcanology advances disproportionately during exceptionally large or well-observed eruptions. The 2018 eruption of Kīlauea Volcano (Hawai'i) was its most impactful in centuries, involving an outpouring of more than one cubic kilometer of basalt, a magnitude 7 flank earthquake, and the volcano's largest summit collapse since at least the nineteenth century. Eruptive activity was documented in detail, yielding new insights into large caldera-rift eruptions; the geometry of a shallow magma storage-transport system and its interaction with rift zone tectonics; mechanisms of basaltic tephra-producing explosions; caldera collapse mechanics; and the dynamics of fissure eruptions and high-volume lava flows. Insights are broadly applicable to a range of volcanic systems and should reduce risk from future eruptions. Multidisciplinary collaboration will be required to fully leverage the diversity of monitoring data to address many of the most important outstanding questions. Unprecedented observations of a caldera collapse and coupled rift zone eruption yield new opportunities for advancing volcano science. Magma flow to a low-elevation rift zone vent triggered quasi-periodic step-like collapse of a summit caldera, which pressurized the magma system and sustained the eruption. Kīlauea's magmatic-tectonic system is tightly interconnected over tens of kilometers, with complex feedback mechanisms and interrelated hazards over widely varying timescales. The eruption revealed magma stored in diverse locations, volumes, and compositions, not only beneath the summit but also within the volcano's most active rift zone. [ABSTRACT FROM AUTHOR]

Forecasting heightened magmatic activity is key to assessing and mitigating global volcanic hazards, including eruptions from lateral rift zones at basaltic volcanoes. At Kīlauea volcano, Hawai'i (United States), planar dikes intrude its east rift zone (ERZ) and repeatedly affect the same segments. Here we show that Kīlauea's upper and middle ERZ dikes in the last four decades intruded at regular intervals of ~8 or ~14 yr. Segments with shorter recurrence intervals are adjacent to faster-moving parts of the flank, and ~1-5 MPa of tension accumulates from flank spreading in the time between dike events. Intrusion frequency was neither advanced nor delayed during magma supply variations, supporting the role of longterm flank motion on the timing of dike intrusions. Although fewer historical dikes have occurred near the 2018 CE eruption site in the lower ERZ and the adjacent slowly sliding lower eastern flank, similar tension accumulated between the 1955 and 2018 eruptions. Regular dike intrusion recurrence intervals indicate the importance of including both extrusive and (commonly neglected) intrusive activity in eruption hazard analyses. [ABSTRACT FROM AUTHOR]

Mammoth Mountain, California, has exhibited unrest over the past ~30 years, characterized by seismicity over a broad range of depths, elevated 3He/4He ratios in fumarolic gas, and large‐scale diffuse CO2 emissions. This activity has been attributed to magmatic intrusion, but minimal ground deformation and the presence of a shallow crustal gas reservoir beneath Mammoth Mountain pose a challenge for estimating magma supply rate. Here, we use the record of fumarolic 3He/4He ratios and CO2 emissions to estimate that of the ~5.2 Mt of CO2 released from Mammoth Mountain between 1989 and 2016, 1.6 Mt was associated with active intrusion and degassing of ~0.05–0.07 km3 of basaltic magma. Intrusion at an average rate of ~0.002–0.003 km3/year into a postulated zone of partial melt at ~15‐km depth could occur without detection by local Global Navigation Satellite System stations. Plain Language Summary: Magma supply rate exerts a fundamental control on a volcano's eruptive and intrusive activity. Seismicity and large‐scale diffuse CO2 emissions at Mammoth Mountain, California, over the past ~30 years have been attributed to magmatic intrusion. Estimating magma supply rate beneath Mammoth Mountain is challenging, however, because (1) ground deformation, a useful indicator of magmatic intrusion, has been minor and (2) a gas reservoir in the shallow crust traps rising CO2 for unknown periods. In this study, we use fumarole helium isotopes to estimate CO2 emissions associated with active magmatic intrusion beneath Mammoth Mountain from 1989 to 2016, volume of basaltic magma degassed, and average intrusion rate. Based on ground deformation source modeling, we find that this rate of intrusion could potentially occur into a postulated zone of partial melt at ~15‐km depth without detection by geodetic monitoring, although other (e.g., deeper) sources are possible. Key Points: Fumarole helium isotopes are used to quantify CO2 emissions associated with active magmatic intrusion beneath Mammoth MountainFrom 1989 to 2016, ~1.6 Mt of CO2 emissions was associated with active intrusion and degassing of ~0.05‐0.07 km3 of basaltic magmaAn average intrusion rate of ~0.002‐0.003 km3/year into a zone of partial melt at ~15‐km depth might occur undetected by geodetic monitoring [ABSTRACT FROM AUTHOR]

Long Valley Caldera (LVC, eastern California) and the surrounding region represent a geophysically complex area due to the presence of an active magmatic system and tectonic motion with associated seismicity. Furthermore, being located at the eastern edge of the Sierra Nevada block, this area is also influenced by hydrological forcing associated with the huge amount of precipitation that falls on the Sierra Nevada and its spatiotemporal variability. These superimposed tectonic and non-tectonic processes cause complex deformation including long-term tectonic motion, episodic inflation of the resurgent dome and variable seasonal and multi-year displacements from surface water loading and subsurface discharge/recharge.Snow accumulation and subsequent runoff play an important role in LVC, as demonstrated by the presence of a diffuse and active hydrothermal system and the occurrence of swarms of shallow earthquakes induced by the spring runoff. Combining displacement data from permanent GPS networks with various hydrological records (spring discharge, lake and groundwater level, rainfall, snow depth), we analyze the non-tectonic deformation affecting the LVC region and the adjacent Sierra Nevada range. We observe a clear vertical deformation pattern whose spatiotemporal variability is likely related to surface loading acting both at the large (regional or continental) and local scales. We highlight anomalous horizontal deformation in the GPS sites on the Sierra Nevada range front, showing its clear correlation with spring discharge. We investigate possible causes of this significant horizontal deformation, including response to snowpack loading and the influence of groundwater recharge. [ABSTRACT FROM AUTHOR]