Magma reservoir failure and the onset of caldera collapse at Kīlauea Volcano in 2018

INTRODUCTION The 2018 rift zone eruption of Kīlauea Volcano, Hawai‘i, drained large volumes of magma from the volcano’s summit reservoir system, causing high-rate subsidence of the ground surface and withdrawal of an active lava lake. Over the span of 1 week, the surface of the lava lake fell more than 300 m. Continued withdrawal of magma caused the rock above the reservoir to fail, triggering the onset of episodic caldera collapse. Surface collapse began near the evacuated lava lake vent, but as the eruption continued over 3 months, the area of the new caldera expanded to ~5 km2 and its volume grew to 0.8 km3. The precursory activity and subsequent growth of the caldera were recorded in far greater detail than was possible at the handful of other caldera collapses observed in the past century. These comprehensive observations permit new insights into the conditions that lead to magma reservoir host rock failure and caldera collapse. RATIONALE Volcanic caldera collapses can be highly destructive and create prominent topographic features, but little is known about the architecture of subcaldera magma storage zones or the critical decrease in pressure that triggers collapse. Withdrawal of Kīlauea’s lava lake in 2018 can be used to gauge pressure change in the underlying magma reservoir. We developed a model of time-evolving reservoir depressurization to jointly explain lava lake withdrawal rate and the rate and spatial pattern of ground subsidence obtained from radar satellites and a dense local monitoring network. RESULTS We tracked the evolution of the magmatic system from steady elastic decompression to inelastic failure. We were able to estimate the location, geometry, volume, and time-evolving pressure within the reservoir as well as conditions required to trigger failure of the overlying crust. Before the onset of collapse, the ground at Kīlauea’s summit was subsiding at nearly 10 cm/day, and the lava lake surface was retreating at ~50 m/day. We found that these phenomena were caused by drainage of magma at a high rate from a storage reservoir centered ~2 km below the surface, with a volume of several cubic kilometers. Drainage rapidly reduced reservoir pressure, stressing the surrounding crust. Two weeks after the rift zone intrusion and eruption began to drain magma from the summit, withdrawal of CONCLUSION Our results tightly constrain the pressure decrease in the magma reservoir before the onset of collapse. Together with geodetic data, this bounds the magma storage volume and the stress changes needed to cause failure of the host rock above the reservoir. Our results demonstrate that a magma reservoir’s roof may begin to fail after withdrawal of only a small fraction of the stored magma. At Kīlauea, this process was likely influenced by a relatively thin and wide reservoir roof and preexisting crustal weaknesses, including an established caldera ring-fault system and the lava lake vent. Roof collapses maintained magma pressure, sustaining the eruption, but they did not (as is sometimes assumed) completely repressurize the reservoir. This indicates residual frictional strength on the collapse-bounding faults. The eruption was not terminated by complete evacuation of stored magma, contrary to assumptions sometimes made when interpreting data from past caldera collapses, and indicates that a different process was responsible for the cessation of the eruption. Joint monitoring of ground deformation and lava lake elevation at other volcanoes, when possible, may yield rich insights into magmatic processes and conditions.