Plinian-type eruptions are extremely hazardous, producing pyroclastic fallout and flows extending many kilometres from the vent. The most commonly invoked eruption trigger for Plinian-type eruptions is the intrusion of fresh magma, generally associated with precursory ground deformation and seismicity days/weeks before eruption. Closed-system internal triggering has also been proposed, such as protracted crystallisation of magma, which can produce a build-up of exsolved volatiles and thus pressurise the system prior to eruption. On 22–23 April 2015 Calbuco volcano, Chile, produced a sub-Plinian eruption with <3 h seismic precursory activity and no clear deformation signals in the preceding months. Here, we show that petrological and geochemical evidence do not support a hypothesis of eruption triggering due to pre-eruptive intrusion of fresh magma, but instead are consistent with an internal trigger. We found that basaltic andesitic magma was stored at depths between 8 and 12 km (i.e. 230–320 MPa) beneath Calbuco volcano before the 2015 eruption. The stored magma had an initial temperature of 900–950 °C, was water-saturated (5.5–6.5 wt% H2O) and formed phenocrysts of titanomagnetite, orthopyroxene, clinopyroxene and plagioclase cores (An78–93). Gradual cooling of the magma chamber produced thermal gradients and magma convection, evidenced by plagioclase overgrowth rims (An58–77) and blocky microlites (25–250 μm). Our interpretation is that this continuing crystallisation induced second boiling and an over-pressurisation of the system, leading to the rapid onset of the 2015 eruption. Petrological and geochemical evidence therefore shows that a closed-system magma chamber can evolve into a highly explosive eruption with very little precursory warning, posing a challenge for current volcano monitoring paradigms. We propose that internal triggering should be carefully considered as a mechanism for unexpected sub-Plinian eruptions, prompting a potential revision of existing hazard management strategies.