Plinian eruptions of phenocryst-rich, viscous silicic magmas have been reported, and in several cases the climactic eruptions were preceded by a small eruption of the magmas formed by mixing with more mafic magmas. The eruptive sequence suggests that the less viscous mixed magmas formed a dike prior to the climactic eruption of the viscous silicic magmas. The viscosities of seven sets of precursory and climactic magmas were investigated. All the climactic silicic magmas had viscosities >∼106 Pa·s, in contrast to the precursory mixed magmas, which had viscosities one to five orders of magnitude smaller. These viscosity contrasts can be interpreted by adopting a hypothesis of “dike-propagation limit” for magma viscosity, above which the magmas are too viscous to form dikes without freezing. The dike-propagation limit estimated by the dike propagation model of Rubin and the maximum excess pressure retained by the tensile strength of a chamber's wall rock are consistent with the observation of the threshold viscosity of 105– 107 Pa·s between the precursory and climactic magmas. The consistency of the seven examples with the physical model suggests that precursory dike formation by less viscous mixed magma is one of the effective mechanisms for tapping silicic viscous magma chambers.