ABSTRACT The Keanakāko‘i Tephra offers an exceptional window into the explosive portion of Kīlauea’s recent past. Once thought to be the products of a single eruption, the deposits instead formed through a wide range of pyroclastic activity during an ~300 yr period following the collapse of the modern caldera in ca. 1500 CE. No single shallow conduit or vent system prevailed during this period, and most of the deposits are confined to distinct sectors around the caldera. Vent position shifted abruptly and repeatedly throughout this time period. This combination of circumstances, influenced by prevailing wind direction, led to rapid lateral changes in the stratigraphy. We define and describe 12 units, several of which are subdivided into subunits or beds, and place them in a framework that reflects volcanologic processes as well as temporal succession. Eruption style and intensity are exceptionally diverse for a basaltic shield volcano. Bulk tephra volumes range from 10 6 to 10 7 m 3 , and the volcanic explosivity index (VEI) ranges from 1 to 3. The most intense activity included high Hawaiian fountaining eruptions, probably associated with caldera-confined lava flows, and subplinian and phreatoplinian explosions. There was also a wide range of less intense phreatomagmatic activity characterized by different magma/water ratios, with products ranging from ballistic block falls, to cross-bedded pyroclastic density current deposits, to fine-grained ash falls commonly bearing accretionary lapilli. Resumption of a Keanakāko‘i style and pattern of volcanism, which seems possible given events unfolding in May–July 2018, has serious implication in terms of future volcanic risk. The hazards associated with every style of explosive activity at Kīlauea summit are quite distinct from the dominantly effusive style of the past 200 yr and should be factored into any future evaluation of risk.

Hudson Volcano is one of the most active volcanoes in the southernmost Southern Andean volcanic zone, characterized by an ice-filled caldera 10 km in diameter. Tephrochronological studies indicate records of explosive activity from the late Pleistocene to historical times. In fact, the last large eruption occurred in August 1991 and is considered to be one of the largest eruptions of the twentieth century. The volcano is located in a remote and roadless region of the Patagonian Andes, which means that numerical models play an important role in assessing volcanic hazards at Hudson. In particular, these models are used to identify areas susceptible to be impacted by lahar flows and tephra fallout. In addition, a proximal-hazard zone was built using the energy cone model, which is useful when little or no prior geologic data are available. Lahar-inundation hazard zones were delineated using the LAHARZ model, based on empirical relationships. Several volumes were considered because of the range of potential lahar-initiating events, such as ice melting or mobilization of loose pyroclasts. Simulations indicate that valleys located west of the volcano are likely to be inundated by lahars, even small-volume lahars triggered by small eruptions, as have been recorded during historical episodes. In contrast, only large events would likely affect main populated settlements located farther west from the volcano. Tephra-fall deposits were simulated with an advection-diffusion model, Tephra2, employing wind data derived from atmospheric global data sets. Both spatial distribution of deposits and thickness derived from the August 1991 eruption were satisfactorily validated. Three eruptive scenarios were selected according to the geological record of the volcano. Results of simulations are outlined as probabilistic maps of mass accumulation on the surface and also as exceedance probability curves for selected localities. This analysis shows that regions east of the volcano are more vulnerable to tephra fallout throughout the year, and therefore no major interseasonal variability is recognized. However, the arrival of weather fronts, common during autumn and winter, could trigger tropospheric wind shifts, which may increase the chance of meridional (north-south) transport of pyroclasts. Finally, according to available tephrochronological data, the occurrence of a large eruption was estimated, indicating 10%–20% likelihood of an eruption ≥VEI 4 (where VEI is volcanic explosivity index) during the next 100 yr.

Misti volcano in southern Peru has a record of explosive eruptions and a nearby population of over 810,000, making it a hazardous volcano. The city center of Arequipa, Peru's second most populous city, is 15 km from the summit of Misti, and many neighborhoods are closer. As the population increases yearly, the urban boundary continues to move up the south side of the volcano. Many parts of the city are built upon the deposits from Misti's most recent Plinian eruption at ca. 2 ka. The 2 ka Plinian eruption (Volcanic Explosivity Index [VEI] 5) produced a 1.4 km 3 tephra-fall deposit and 0.01 km 3 of pyroclastic-flow deposits in ~2–5 h. Column height varied during the eruption but ascended up to 29 km. Pyroclastic flows descended only the south side of the volcano. The tephra fall spread southwest, resulting in ~20 cm of tephra accumulation in the area now occupied by the city center. The flowage deposits were previously identified as pyroclastic-flow deposits, but new sedimentologic and textural evidence suggests that ~80% (by volume) of the deposits were emplaced wet and relatively cold. As such, they are lahar deposits. A Neoglacial advance concurrent with the eruption supports evidence for voluminous snow and ice on the edifice. Pyroclastic flows melted between 0.01 km 3 and 0.04 km 3 of ice and snow on the volcano, triggering lahars that descended the volcano and inundated channels and some interfluves on the south flank. The lahars evolved downstream from proximal debris flows to distal hyperconcentrated flows, emplacing ~0.04 km 3 of deposits. Four facies of lahar deposits are present in the channels and another facies occurs on the interfluves. Such a comprehensive understanding of the 2 ka eruption will help to inform future volcanic hazards assessments. Pyroclastic-flow and tephra-fall deposits of the same magnitude could occur again and are useful in hazards assessment. The 2 ka lahars required voluminous water, which is no longer available on the volcano, and, within modern climate conditions, these deposits are not representative of possible future events. Estimations of water available from modern rain and snow suggest that lahars with volumes between 1 × 10 5 m 3 and 3 × 10 6 m 3 are possible. Lahars are more likely if an eruption occurs during a period of high snow accumulation or during subsequent heavy rainfall. Lahars up to 1 × 10 7 m 3 are possible if the Río Chili is dammed during an eruption. Lahar hazard zones generated using these volumes suggest the largest lahars could enter Arequipa.