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

Abstract

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 's to 10 -6 C kg-•). 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.

Multiparametric observation of volcanic lightning: Sakurajima Volcano, Japan

“…That this process is very efficient is demonstrated by the very small grain size (few tens of microns) of the ash ejected, as shown by the fine-ash fallout depositing in the proximity of the crater. Thus, it is difficult to distinguish the relative contributions of tribocharging and fractocharging which are considered the main mechanisms of ash electrification in volcanic plumes [James et al, 2000;Houghton et al, 2013]. During the prolonged activity of ash venting at Showa crater, no visible flashes were recorded.…”

“…Thus, volcanic lightning may provide a valuable monitoring tool for active volcanoes, allowing detection of ash emissions from safe distance and in inclement weather conditions [Behnke and McNutt, 2014]. However, the use of volcanic lightning to probe the properties of volcanic plumes (ash concentration, mass eruption rate, turbulence, etc) has been hampered so far largely by (i) the lack of systematic instrumental observation of electric activity in volcanic plumes and (ii) the limited number of experimental investigations on the electrification processes of volcanic materials [James et al, 2000;Houghton et al, 2013;Méndez-Harper et al, 2015] and the mechanism of plume electrification [Cimarelli et al, 2014].…”

Impact compressive and bending behaviour of rocks accompanied by electromagnetic phenomena

“…However, there are many mechanisms that may result in the electromagnetic phenomena generated from the dynamic or impact fracture of rocks. Examples of such fracture occurrences include earthquakes, volcanic eruptions [17] and the collision of a meteorite with the Earth. Therefore, it is quite important to investigate the electromagnetic emission induced by the dynamic or impact deformation of rocks.…”

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

“…Laboratory-based experiments are also essential to studying volcanic charge generation mechanisms in a controlled environment and can allow different charge mechanisms to be examined individually. Laboratory experiments by Büttner et al (2000) and James et al (2000) have studied the fractoemission mechanism, whereby James et al generated silicate particles by fracture during collisions between pumice samples. During the experiments, there was evidence of ion release during the fracture process.…”

“…On Earth, volcanic lightning is often present during eruptions (see Harrison and Mather 2006; McNutt and Williams 2010 for reviews), providing strong evidence for the electrical charging of volcanic ash as well as demonstrating that charge separation sufficiently large to initiate breakdown within the volcanic plume environment. Numerous mechanisms have been suggested by which volcanic ash in Earth-based volcanoes can become electrified including fractoemission (James et al 2000), contact or triboelectrification (Houghton et al 2013) and thunderstorm-style ice-contact charging ('dirty thunderstorm' mechanism; Williams and McNutt 2005), each of which may occur at different altitudes throughout the plume (Fig. 9).…”