Insights into eruption dynamics from the 2014 pyroclastic deposits of Kelut volcano, Java, Indonesia, and implications for future hazards

On 13 to 14 February 2014 a ~4 h long, VEI 4 eruption occurred at Kelut volcano (Java, Indonesia). Pyroclastic density currents (PDCs) and extensive ash fall led to 7 fatalities and disruption to flights across the Asia-Pacific region. New sedimentological descriptions of the pyroclastic deposits from the 2014 eruption were compared with eyewitness and satellite reports to elucidate temporal variations in eruptive dynamics. The stratigraphy of the deposits is presented in 3 stages, associated with two eruptions that occurred approximately ~15–30 min apart. Stage 1 PDC deposits originate from the smaller onset eruption. The PDC deposits from Stage 2, and tephra fall deposits from Stage 3 originate from the second, plinian eruption. During the onset eruption (Stage 1), low energy PDCs were produced that ran out to <2.6 km. Basal layers show characteristics of deposits similar to ash-cloud surges that carried dominantly fine ash and crystal fragments. These are capped by deposits typical of high-particle concentrated pyroclastic flows. All Stage 1 deposits have high contents of dense lithic fragments (up to 44% by vol.), sourced from the 2007–2008 lava dome and conduit walls, indicating that the eruption onset was driven by an explosive release of gas-overpressure below the vent-capping dome. Increases in the magma flux and transition to a more constant eruption led to a growing eruption column during the ~2-hour long Stage 2 plinian eruption. Pumice rich (>70% by vol.) PDC deposits ran out to 4.7 km from the vent. The deposits reflect an increased output of fresh fragmented magma, and some conduit widening evidenced by dense lithic fragments. Vent instabilities and incorporation of dense material into basal margins of the plume led to the marginal collapse and formation of these PDCs. Stage 3 occurred in the final hour at the peak of the plinian eruption, around 01:00 to 02:00, and produced reversely graded lapilli fall deposits with ≤90 vol% pumice from a 26 km-high plume. This indicates that there was a sustained flux of juvenile magma to the now open vent system, and expansion and fragmentation of the gas-rich magma was at its most efficient. Our study of the eruptive sequence of Kelut provides some constraints on predicted patterns for future explosive activity, critical for further hazard assessment of the volcano. Since 1901 Kelut has erupted on intervals of 1 to 23 years, and the 2014 event characterises a typical "explosive" style of eruption that alternates regularly with effusive dome-formation and collapse events. This pattern depends on the dynamics of magma renewal to the system, and degassing conditions in the shallow magma reservoir and upper conduit. If this pattern holds, and the next eruption occurs within the next two decades, a return of prolonged dome growth could be anticipated.