Investigating the magmatic drivers behind temporal variations in eruption frequency and style at Kelut volcano, Indonesia

Stratigraphic, petrological, mineralogical and geochemical records of volcanism are vital to understanding and forecasting the magmatic processes governing the cyclic effusive-explosive eruptive styles and patterns. Understanding the full range of eruptive behaviour at a volcano is fundamental in helping to reduce the disastrous impacts of eruptions to nearby vulnerable populations. Java (Indonesia) contains a large concentration of historically active and deadly volcanoes. However, few of these have well-constrained stratigraphic records that allow the variety of eruption styles and the eruption frequency to be estimated. Kelut volcano is one of the more deadly volcanoes in Indonesia, yet its geological record is poorly known. This thesis aims to improve our knowledge of Kelut, by examining detailed records of Kelut’s pyroclastic deposits and geochemistry, including both the most recent eruption (2014) and a series of units emplaced over the last 1500 years. The dynamics and chronology of individual eruptions of Kelut are complex. From AD ~560 to1800’s, glass shard compositions reveal that magmatic processes are the first-order control on eruptive behaviour. Mafic recharge events triggered the most explosive episodes. The system was maintained by variable rates and volumes of mafic magma influxes. In the early record, recharges were small or infrequent, and enough time between eruptions allowed homogenised high-Si magmas to erupt. More frequent eruptions took place from AD 1000 and variability of magmas was greater. From AD 1920 to 1990, eruptions tapped a continuously evolving magma system.The homogeneity of compositions from recent effusive-explosive rocks suggested that regular, small injections of gas-rich magma allowed for efficient mixing, buffering the system for some time. During this phase, other processes were of greater influence on eruptive styles. In some cases the presence of a conduit-capping lava dome inhibited gas release, eventually leading to overpressure and triggering a more explosive eruption, e.g., 2014. In the 2014 eruption, there is also potential evidence of mafic intrusion before eruption. This sudden intrusion left little trace,except in feldspar-rim compositions. Since 1920, explosive eruptions occured every ~21 years at Kelut. If the alternate explosive/effusive cycle holds, the next eruption of Kelut is likely to be a period of lava dome effusion.