On the possible role of potassium enrichment for controlling the morphological evolution of stratovolcanoes into compound or caldera (Java Island, Indonesia)

This study focuses on integrating the morphometry analysis (acquired from image processing) with the published geochemical and eruptive parameters data on the Late Quaternary Java Volcanoes (LQJV). First, we qualitatively discriminate the volcano type into stratovolcano, compound, and caldera. Next, we performed quantitative morphometrical analyses and compiled geochemistry data from previous works. Stratovolcano marks the youngest stage of a volcano, can be either small or large, but exhibits a typically steep and sharp morphology, with basaltic and andesitic magmas as the primary magma compositions. As time goes by and the magma differentiates into more viscous compositions (up to dacite and rhyolite), a stratovolcano may develop into a compound type, forming multiple cones and a larger edifice size. Finally, a caldera marks the most developed volcano type due to the typically large and nearly flat morphology as a result of the highly catastrophic eruptive event. Both compound and caldera volcanoes show a significantly overlapping silica content, suggesting that silica is not the main factor that controls the formation of a caldera on Java Island. Interestingly, all caldera volcanoes on Java Island always exhibit high-K magma affinity, while compound volcanoes are dominated by medium- and low-K affinity. This means that potassium occurs as the most important factor that controls the fate of a volcano during the growth process, either just becoming a compound or evolving into a caldera. This idea is further supported by the recent findings from other works, where the solubility of volatiles (e.g., CO₂ and H₂O) in magmas increases with increasing potassium content. The fact that most calderas in the Sunda Arc (e.g., Toba, Maninjau, Ranau, Rinjani, and Tambora) also exhibit high-K affinity (or even shoshonitic) does not deny this idea. Finally, we also pointed out that irregularity and the ellipticity index are more controlled by the exogenous factors (e.g., the preexisting morphology) instead of magma composition and eruption style.