Relationships between crater diameter (D in meters) and ejecta volume (V in cubic meters) of recent magmatic and phreato-magmatic eruptions are expressed as D=0.11 V0.42, and D=0.97 V0.36, respectively. Crater diameters of phreato-magmatic eruptions are ca. 2--5 times larger than those of magmatic eruptions of similar ejecta volume, suggesting that magma-water interaction in phreato-magmatic eruptions generated mechanical energies of crater formation 1--2 orders of magnitude larger than equivalent-volume magmatic eruptions. The conversion ratio of thermal energy to kinetic energy in phreato-magmatic eruptions is estimated to be 0.7--10 percent, which is in accord with the presently available data of laboratory experiments on melt-water interaction [Wohletz and McQueen, 1984>.¿ 1997 American Geophysical Union