Stratigraphy, chronology, and magma evolution of Holocene volcanic products from Mt. Slamet deposited in the Guci Valley, Central Java, Indonesia

Abstract

The active cone of Mt. Slamet is situated above the Guci Valley, making it vulnerable to volcanic eruptions. This is a fast-growing resort area with a permanent population of thousands and annual visitation in the hundreds of thousands. In this study, we combined stratigraphic observations with petrologic and geochemical analysis of volcanic rocks deposited in the valley to better understand the potential volcanic hazards. Within 7–9 km from the summit, the valley contains four massive lavas, seven pyroclastic density current deposits, one scoria fall, and one lahar deposit. Two of the three massive pyroclastic density current deposits (3–15 m thick) were dated to 8040 ± 30 BP and 3910 ± 30 BP; the third was overlain by the 8040 ± 30 BP deposit. Therefore, during the Holocene at least 7 explosive eruptions have occurred at Mt. Slamet. The valley's volcanic deposits were characterized by lower CaO numbers, higher FeO*, and higher FeO/MgO ratios than older products from Mt. Slamet, representing a distinct character of modern magma. Thus, our stratigraphic record suggests that the Guci Valley may be exposed to future eruptive activity with corresponding hazards to the population.