In 2018, Kīlauea Volcano experienced its largest lower East Rift Zone (LERZ) eruption and caldera collapse in at least 200 years. After collapse of the Pu‘u ‘Ō‘ō vent on 30 April, magma propagated downrift. Eruptive fissures opened in the LERZ on 3 May, eventually extending ~6.8 kilometers. A 4 May earthquake [moment magnitude (Mw) 6.9] produced ~5 meters of fault slip. Lava erupted at rates exceeding 100 cubic meters per second, eventually covering 35.5 square kilometers. The summit magma system partially drained, producing minor explosions and near-daily collapses releasing energy equivalent toMw4.7 to 5.4 earthquakes. Activity declined rapidly on 4 August. Summit collapse and lava flow volume estimates are roughly equivalent—about 0.8 cubic kilometers. Careful historical observation and monitoring of Kīlauea enabled successful forecasting of hazardous events.

Caldera-forming eruptions are among Earth’s most hazardous natural phenomena, yet the architecture of subcaldera magma reservoirs and the conditions that trigger collapse are poorly understood. Observations from the formation of a 0.8–cubic kilometer basaltic caldera at Kīlauea Volcano in 2018 included the draining of an active lava lake, which provided a window into pressure decrease in the reservoir. We show that failure began after <4% of magma was withdrawn from a shallow reservoir beneath the volcano’s summit, reducing its internal pressure by ~17 megapascals. Several cubic kilometers of magma were stored in the reservoir, and only a fraction was withdrawn before the end of the eruption. Thus, caldera formation may begin after withdrawal of only small amounts of magma and may end before source reservoirs are completely evacuated.

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