The volcanic eruptions of different types are followed by volcanic tremor that is supposed to be initiated by the moving of the gases along the vent. Under some conditions the gas flow velocities can reach sonic values. The volcanic tremor is produced by the self-oscillations, which in turn are generated by the transition of subsonic to supersonic gas flow. The field evidences and ballistic calculations show that the volcanic explosions and voluminous gas emissions display supersonic volcanic gas flow.