A phreatic eruption is a steam eruption without lava ejection. Phreatic eruptions are a common precursor of volcanic activity. The eruptions are caused by groundwater flashing to steam as it is heated by magma.
Dangers from phreatic eruptions include:
1) Toxic gas release. Fatalities at Larderello, 1282: uncertain number of deaths; Dieng, 1979:149 deaths; Nyos, 1986: over 1700 killed).
2) Base surges, lahars and directional blasts, avalanches (Bandai, 1888:461 deaths; Karkar, 2 deaths.
3) Falling blocks.