We report on the experimental study of electrical phenomena during explosive volcanic eruptions, which provides qualitative and quantitative insight into different fragmentation and eruption mechanisms of magmatic melt. The experiments show that air friction and surface enlargement by hydro- and/or aerodynamic magma fragmentation are only minor contributors to electrical charging of erupted particle clouds in comparison to thermo-hydraulic fracturing of magma by explosive magma/water interaction. This process has the potency to explain the frequently observed occurrence of lightning in eruption clouds of explosive volcanic events. ©1997 American Institute of Physics.