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Pre-eruptive processes and their timescales are critical information for risk management at explosive volcanoes, and Santorini caldera (Greece) provides an excellent context in which to approach this subject. We ask two questions. First, are pre-eruptive processes the same for small and big eruptions? To investigate, we performed a multi-mineral diffusion timescale study of a small explosive eruption of Kameni Volcano and compared the results with those published for larger caldera-forming eruptions at Santorini. The Kameni dacite resembles products of larger eruptions in being crystal-poor, containing plagioclase with antecrystic cores and autocrystic rims, bearing orthopyroxene with sector zoning and phantom skeletal morphologies, and showing evidence for mixing of different silicic magmas prior to eruption. Diffusion timescales from Mg-Fe profiles in orthopyroxene and clinopyroxene phenocrysts are

Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpreting the signals of potential reawakening at active volcanoes. In this paper, we provide the first volcanological and petrochronological characterization of the Singkut caldera, a young volcanic system located in northern Sumatra (Indonesia), in close proximity to Medan, one of the country's most popoluous cities. Singkut formed at ∼44 ka during a VEI 6 explosive eruption that deposited at least ∼26 km3 of tephra (dense rock equivalent, DRE). The cataclysmic eruption was preceded by >200 ky of mostly effusive pre-caldera activity and followed by effusive to mildly explosive post-caldera activity. The lavas and pumices have high crystallinity (up to 62% crystals) with andesitic to dacitic bulk-rock composition and rhyolitic glass. Mineral textures and matrix glass compositions indicate resorption of quartz, plagioclase and zircon. Zircon crystallization ages show a complete overlap with the eruption ages in pre-caldera lavas, while a time gap in zircon crystallization (>50 ky) is identified in the caldera-forming tuff and post-caldera lavas. Ti-in-zircon thermometry shows that the Singkut magma body experienced a temperature increase starting approximately upon eruption of the pre-caldera lavas (∼254 ka). Such thermal perturbation determined progressive melting of mineral phases in the cumulate crystal mush, caused the resorption of the youngest zircon domains before the caldera-forming eruption, and hampered zircon crystallization between the caldera-forming eruption and the effusion of the post-caldera lavas (∼16 ka). Our data demonstrate how cumulate melting processes played a key role in leading the volcanic system towards a caldera-forming eruption and controlled the transitions in eruptive style between the effusive phases and the explosive climactic eruption. • Singkut caldera formed at 44 ka during a large explosive eruption (∼26 km3 DRE). • Caldera-forming eruption was preceded by >200 ky of effusive volcanic activity. • Magmas are crystal-rich with textural and geochemical evidence of cumulate melting. • Temperature increase in magma reservoir started upon eruption of pre-caldera lavas. • Cumulate mush rejuvenation was triggered by increasing mafic recharge flux. [ABSTRACT FROM AUTHOR]

Crustal magma chambers can grow to be hundreds to thousands of cubic kilometers, potentially feeding catastrophic caldera‐forming eruptions. Smaller volume chambers are expected to erupt frequently and freeze quickly; a major outstanding question is how magma chambers ever grow to the sizes required to sustain the largest eruptions on Earth. We use a thermo‐mechanical model to investigate the primary factors that govern the extrusive:intrusive ratio in a chamber, and how this relates to eruption frequency, eruption size, and long‐term chamber growth. The model consists of three fundamental timescales: the magma injection timescale τin, the cooling timescale τcool, and the timescale for viscous relaxation of the crust τrelax. We estimate these timescales using geologic and geophysical data from four volcanoes (Laguna del Maule, Campi Flegrei, Santorini, and Aso) to compare them with the model. In each of these systems, τin is much shorter than τcool and slightly shorter than τrelax, conditions that in the model are associated with efficient chamber growth and simultaneous eruption. In addition, the model suggests that the magma chambers underlying these volcanoes are growing at rates between ~10−4 and 10−2 km3/year, speeding up over time as the chamber volume increases. We find scaling relationships for eruption frequency and size that suggest that as chambers grow and volatiles exsolve, eruption frequency decreases but eruption size increases. These scaling relationships provide a good match to the eruptive history from the natural systems, suggesting that the relationships can be used to constrain chamber growth rates and volatile saturation state from the eruptive history alone. Plain Language Summary: Magma chambers in the Earth's crust grow by incremental addition of new magma from deeper reservoirs, and over time can reach volumes that would fill the entire Grand Canyon. However, small magma chambers in the earliest stages of formation are prone to frequent eruptions and will lose heat quickly to the surrounding crust, both of which supposedly impede growth. Therefore, an important question is how magma chambers can possibly grow to such large sizes. Here we present results of physics‐based modeling aimed at understanding what conditions allow magma chambers to grow. We test effects of chamber size, rate of magma supply, water content in the magma, and plasticity of the crust hosting the chamber. Results suggest that growth is promoted when chambers cool slowly and are hosted within pliable crust that can easily relax pressures that build within the chamber. Surprisingly, we found that for a particular range of crustal pliability, growth is accompanied by frequent volcanic eruptions. We compared these results to four large volcanoes in Chile, Italy, Greece, and Japan. Model predictions for eruption frequency and chamber growth rates are a good match to what we observe at these volcanoes from the rock record and active monitoring systems. Key Points: Coupled thermo‐mechanical modeling suggests that magma chambers grow when recharge and viscous relaxation of the crust are fast compared to chamber coolingMagma chambers containing exsolved volatiles may shrink over time despite constant recharge because high magma compressibility leads to large eruption volumesEruptive phases at Laguna del Maule, Campi Flegrei, Santorini, and Aso reflect growth of chambers; amount of growth can be estimated from eruption frequency and size [ABSTRACT FROM AUTHOR]