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.

In the late morning of 12 July 2008, the Alaska Volcano Observatory (AVO) received an unexpected call from the U.S. Coast Guard, reporting an explosive volcanic eruption in the central Aleutians in the vicinity of Okmok volcano, a relatively young (∼2000‐year‐old) caldera. The Coast Guard had received an emergency call requesting assistance from a family living at a cattle ranch on the flanks of the volcano, who reported loud “thunder,” lightning, and noontime darkness due to ashfall. AVO staff immediately confirmed the report by observing a strong eruption signal recorded on the Okmok seismic network and the presence of a large dark ash cloud above Okmok in satellite imagery. Within 5 minutes of the call, AVO declared the volcano at aviation code red, signifying that a highly explosive, ash‐rich eruption was under way.

This page left intentionally blank. flows or surges probably would be limited to within the caldera, but ash fall could affect extensive regions downwind. Another voluminous eruption, such as Aniakchak II, is considered unlikely in the near future. Although the InSAR study indicates subsidence, the possibility that evolved magma may be accumulating beneath the central part of the caldera is worthy of consideration when interpreting future monitoring data. Pleistocene composite volcano that consists of basaltic andesite to dacite lava flows and fragmental rocks. Early in postglacial (Holocene) time, a voluminous eruption (Aniakchak I; Miller and Smith, 1987) left geochemically distinctive andesite pyroclastic-flow deposits (ignimbrite) in valleys and on the flanks of the edifice. This event was followed by dacite eruptions, at least one of which produced a widespread pumice fall deposit ca. 7,000 yr B.P. (VanderHoek, 2009). Unless otherwise noted, ages reported here are given in radiocarbon years before present (yr B.P., "present" being 1950 C.E.). This chronicle begins with a brief overview of Pleistocene ancestral Aniakchak and an account of the state of knowledge of postglacial, precaldera volcanism.

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