The Crystal Cargo Reveals the Build-up Phase to Caldera-Forming Mafic Eruptions

The magma accumulation phase preceding caldera-forming eruptions is crucial to identify the signs of an impending catastrophic event. While extensively studied in silicic systems, mafic volcanoes present unique challenges. The high eruptive temperatures of these magmas imply short storage in the cold upper crust and, thus, short periods of unrest preceding eruption, which could challenge our capacity to mitigate the impact of an imminent event. We analyse crystals in erupted magma to reconstruct the lead-up phase to the last caldera-forming eruption of Colli Albani, an active volcano near Rome (Italy). Here, we show that the caldera-forming eruption was preceded by effusive to mildly explosive eruptions fed by multiple crustal magma reservoirs. Following a pause in volcanic activity, magma rose directly from the mantle and accumulated to form a reservoir of a few tens of cubic kilometres. The ascent of one of these pulses ultimately triggered the last caldera-forming eruption of Colli Albani. Our results provide a new framework to identify the processes leading to caldera-forming eruptions in mafic volcanic systems.