Mafic magma is commonly associated with effusive eruptions, however several mafic volcanoes throughout the globe have produced explosive eruptions. Here we present one such volcano – Colli Albani. Colli Albani is 20 km SE of Rome and produced seven large volume ignimbrites. Field observations, mineral chemistry, and Sr and Nd isotopes in clinopyroxene show that the high potassic, silica undersaturated and CO₂ -rich magma typical of Colli Albani is produced by partial melting of a metasomatized mantle. Clinopyroxene based thermobarometry combined with thermal modelling, indicates rapid accumulation of magma into the shallow crust preceding the last caldera forming event (355 ka). The crystallization of high Mg# and high Cr₂O₃ clinopyroxenes at low pressures and high temperatures indicates rapid magma ascent from the mantle. We suggest that a final rapid input of this deeply sourced magma destabilised the shallow and fast assembled magma reservoir and lead to the caldera forming event. Our findings have significant implications for the evaluation of the timescales of reactivation of Colli Albani and other similar long-quiescent calderas erupting low viscosity magmas, as rapid migration of magma to shallow reservoirs may result in short unrest periods prior to a large eruption.