The small volcanic islands of the central Greater Kuril Chain (GKC) produce strong explosive eruptions, two of which occurred in the recent years. The latest eruption took place at Raikoke volcano, which occurred on June 21-25 (2019) and was one of the largest on the Kuril Islands in the 21st century. This study presents mineralogical, petrographic and geochemical data on the air fall ash and pyroclastic density current deposits to elucidate the causes of high explosivity of this eruption. The magma involved in the Raikoke volcanic eruption is likely to have had a mantle origin and basaltic composition. Prior to forming a near-surface reservoir, the magma fractionated (i.e. due to crystallization of olivine and pyroxene) at depths of about 26 km, which is close to the Moho discontinuity in the middle part of GKC. The presence of amphibole among the 2019 pyroclastic minerals indicates that the magma contained significant (i.e. no less than 4 wt%) amounts of water at depth. This magma was degassed prior to intrusion into a shallower reservoir. The magma in the shallow reservoir had high crystallinity and contained viscous felsic residual melts, which prevented the development of effusive eruptions or Strombolian explosions. It is likely that magmatic fluid bubbles were still present, but their abundance was insufficient for lava fragmentation in the conduit. This led to low degree of vesiculation of the erupting melts. The results of this study demonstrate that the explosivity of the eruption is most likely related to the interaction in the volcanic edifice between meteoric waters and the magma, which was largely degassed and contained significant amounts of crystals and felsic melt (i.e. had high viscosity and low mobility).