Volcanic plume electrification: Experimental investigation of a fracture‐charging mechanism

Abstract

Although ashfall from particulate volcanic plumes is known to be highly electrically charged, little is known about the charging mechanism. We describe experiments designed to investigate the particle charges generated from the fracture of pumice. Small silicate particles were produced in the laboratory during collisions between two samples cut from pumice clasts. The net charge magnitudes detected on these particles are similar to those previously measured on ashfall from volcanic plumes (∼10−5 to 10−6 C kg−1). This net charge is also shown to be the result of a small imbalance between the sums of individual particle charges of both polarities, which are up to several orders of magnitude larger than the net charge. The magnitude of both the net and single polarity specific charges were only weakly affected by changes of relative humidity, but single polarity charges increased steadily with increasing sample impact velocities. The dominant charging process during the experiments was that of material fracture. The charging mechanism is thus interpreted to be fractoemission (the release of nuclear particles from fresh crack surfaces) occurring during the production of the silicate particles. This implies that the electrification of volcanic plumes could be the result of brittle fragmentation of magma or pumice clasts within the upper regions of the conduit and in the jet region of the plume.