Experimental investigation of a fracture-charging mechanism

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B7, PAGES 16,641-16,649, JULY 10, 2000 Volcanic plume electrification: Experimental investigation of a fracture-charging mechanism M. R. James, S. J. Lane, and J. S. Gilbert Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University Lancaster, England Abstract. Although ashfall from particulate volcanic plumes is known to be highly electrically charged, little is known abouthe charging mechanism. We describexperiments 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 specificharges 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 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. 1. Introduction Extensive displays of lightning during many explosive eruptions provide evidence of the highly electrically charged nature of particulate volcanic plumes. The charge resides on the surfaces of silicate particles and on condensedroplets of volcanic gases, and it probably also exists as ions within the plume [Lane and Gilbert, 1992]. Intraplume lightning flashes and plume-to-ground strikes of both polarity [Hoblitt, 1994] provide strong evidence that different areas within plumes can hold opposite net charges. This has been confirmed by the detection of both positive and negative perturbations of the atmospheric potential gradient produced by particulate volcanic plumes [Hatakeyama and Uchikawa, 1952; Lane et al., 1995; Miura et al., 1996; James et al., 1998]. Modeling these perturbations suggests that the electrical structure of plumes can be represented by a dipole (a positive and a negative charge of equal magnitude) with, for the plumes measured, charge magnitudes of between 0.1 and 10 C [Lane et al., 1995; Miura et al., 1996]. It is likely that this dipole structure is formed from the gravitational separation of two components holding opposite polarity net charges. However, it is not known whether these "components" represent either particles of different sizes as postulated by Hatakeyama and Uchikawa [1952] or silicate particles and volcanic gases [Lane and Gilbert, 1992], and the role of particle aggregation in field generation is also unclear. Direct measurements of the charge held on falling ash have detected net specificharges of magnitudes between 4 x 10-7 Copyright 2000 by the American Geophysical Union. Paper number 2000JB /00/2000JB $09.00 C kg ' ([Hatakeyama, 1958] on ashfall 50 km from the vent) and 5 x 10-5 C kg ' ([Gilbert et al., 1991] <5 km from the vent). Net charges of both polarities have been recorded on ashfall from different volcanoes [Hatakeyama, 1958; Gilbert et al., 1991] and also on different ashfalls from the same volcano [Miura et al., 1995]. During ashfall at Sakurajima, Gilbert et al. [ 1991 ] separated particles by their polarity and demonstrated that both positively and negatively charged particles were present. Collected particles of only one polarity held specific charge magnitudes greater than 10-4 C kg -. There have been few experimental investigations into ash particle and volcanic plume electrification, the majority of which have been concerned with phreatomagrnatic and steam plumes. The eruption of Surtsey in 1963 prompted the investigation of a water-boiling mechanism, which was shown to reliably produce charge magnitudes of 10 '5 C kg ' [BjOrnsson et al., 1967; Pounder, 1972] in steam plumes. More recently, Biittner et al. [1997] investigated particle charging in molten fuel coolant interaction explosions of molten olivine melilitite injected with pure water. Net specificharges of up to 10 '7 C kg - were recorded and ascribed to the formation of electric double layers at the interface between the water and the melt. Experiments carded out on "dry" systems have been limited to frictional mechanisms involving particle sliding over one another or down metal chutes. This technique can generate specifi charges in the range of 10-8 to 10 '9 C kg ' [Hatakeyama and Uchikawa, 1952], and although Kikuchi and Endoh [ 1982] did record values of up to 10-5 C kg -, they thoughthe process more analogous to the charging of windblown ash rather than any processes which occur during eruptions. It is also well documented that industrial powders transported pneumatically readily charge through impacts with pipe walls and other particles [Boschung and Glor, 1980]. 16,641

2 16,642 JAMES ET AL.: VOLCANIC PLUME ELECTRIFICATION Further understanding of the electrification of volcanic available on the World Wide Web at A plumes is necessary in order to improve our knowledge of wire mesh above the samples collected any charge released as electrostatic particle aggregation processes, the electrical ions or held on small silicate particles for which electrostatic hazards from eruptions, and our understanding of atmospheric forces dominated over gravitational forces. The charge potential gradient and particle charge measurements made in accumulating on the mesh was measured using a Keithley 614 the field. This would assist the development of volcanic electrometer. A typical experiment consisted of-200 sample plume remote monitoring using potential gradient impacts over a period of-2 min. The data from the measurements [James et al., 1998], a technique which could electrometer and fieldmeter were recorded by a data logger ultimately be used to validate eruption column models. The over a period of 4 min in order to record any drift in the use of lightning detection to indicate the presence of volcanic outputs before the experiment or charge decay after the plumes [Hoblitt, 1994] would also benefit from further experiment had been completed. After each experiment, the understanding of the electrification process and, in particular, particles were retrieved and weighed. Particle size any variations which could occur between different eruptions distributions were determined using an Elzone 280. [Paskievitch et al., 1995]. Currently, owing to the small In contrasto the fracture-dominated impact experiments, number of field experiments, very little is known about the where particles were released from the site of fracture as the factors controlling either net charge magnitude or polarity. samples moved rapidly apart, several experiments were In this paper we describe experiments designed to carried out with particles generated in a friction-dominated investigate charging by fracture, a process which has not environment. For these the solenoid was replaced with a previously been tested for the volcanic scenario. During and geared motor which rotated the "hammer" sample at 60 after material fracture, nuclear particles (electrons, positive revolutions per minute againsthe anvil sample. The samples and negative ions, and neutral atoms) and electromagnetic were kept in contact with each other throughout each radiation are ejected from the fresh crack surfaces in a process experiment by the spring within the anvil apparatus. During called fractoemission [Dickinson et al., 1988]. For silicate these experiments most of the particles produced were particles within a plume, Lane and Gilbert [1992] could maintained in the region of fracture for a period of time ascertaino physical reason why particle polarity should be a (possibly up to -10 s) as they migrated slowly to the free function of particle size [Hatakeyamand Uchikawa, 1952]. edges of the samples under the relative movement of the As an alternative they proposed that the opposite polarity net sample surfaces. Once at the edges, they were free to fall and charges are held on volcanic gases and silicate particles rather were again collected in the Faraday cup. than on different sizes of particles. Thus they suggested that In order to measure the charge on particles of only one ions released by fractoemission during brittle fracture polarity the Faraday cup was replaced by the parallel plates associated with magma fragmentation impart a net charge to apparatus shown in Figure lb. After passing through a the volcanic gases and leave an equal but opposite net charge collimating slot, the falling silicate particles were on the silicate particles. electrostatically separated between two copper plates by In the experiments carried out, silicate particles were maintaining one plate at +3.5 kv and grounding the other produced by fracture during collisions between pumice through a Keithley 614 electrometer. The electrometer samples. The aims were to investigate the net charge held on measured the charge held on particles accumulating on the 0 the particles and any variations of this with different types of V plate, and by changing the polarity of the kilovolt plate pumice. The equipment constructed was also designed to between experiments the charge on particles of either polarity detect any charge released as ions and to allow charge could be recorded. The particles attracted onto the plates measurements of particles of only one polarity. Some adhered (by electrostatic image forces) to aluminium foil assessment of the relative charge on particles separated by strips, allowing them to be recovered for size analysis and their fall velocity also proved possible. In everyday mass determination. Typically, between 10 and 50% of the applications, increased relative humidity (RH) is known to particle -mass which passed through the collimating slot was decrease electrostati charges and hazards [Cross, 1987] by collected on the plates. decreasing surface resistivities. For this reason and because The apparatus was mounted in a vacuum chamber, which volcanic plumes are inherently humid environments, some allowed some experiments to be carried out under lowexperiments were also carried out over a range of RH values. pressure conditions (-lif t Pa) in order to minimize losses from the equipment. Surface charge flow over the insulators of the Faraday cup at atmospheric pressure was 2. Experimental Procedure estimated to reduce the detected charge in an experiment by a Small silicate particles (generally <100 gm in diameter) maximum of 10%. At low pressure this was reduced to 1%. were produced by colliding two centimetre-sized pumice For experiments carried out at atmospheric pressure the samples together [James et al., 1998]. The samples were chamber was utilized to provide a stable, enclosed mounted on sliding rods, one attached to a sprung anvil and environment, mid it also enabled a range of RH to be imposed. the other to a solenoid, which acted as the hammer (Figure 1). Experiments used samples cut from pumice clasts obtained On activation of the solenoid the hammer sample impacted from six different deposits: Mount St. Helens, United States the anvil sample; a small spring then returned the hammer (May 18, 1980, pumice fall deposit; rhyolitic); Sakurajima, sample to its original position ready for the next activation. Japan (Bunmei pumice fall deposit; andesitic); Aira caldera, The net charge held on the silicate particles produced Japan (Osumi pumice fall deposit; dacitic); Soufribre St. during these collisions was measured by allowing them to fall Vincent, Lesser Antilles (Yellow Tuff Formation; andesitic into a calibrated, mesh-screened Faraday cup located below airfall deposit); Santorini, Greece (Minoan pumice fall the samples (Figure 1 a). The potential of the Faraday cup was deposit; rhyodacitc); and Gorge Farm, Kenya (pumice fall detected by a JCI 121 electrostatic fieldmeter (details are deposit from Naivasha; rhyolitic). All samples had densities charge

3 JAMES ET AL.' VOLCANIC PLUME ELECTRIFICATION 16,643 electrometer (a) mesh... > ønd data logger (b) mesh - - samples collimating... slot --- samples nylon insulator Faraday cup, I, grounded shielding I > to data logger 1 parallel plates +3.5 kv < grounded shielding to Earth via -> electrometer and data logger Figure 1. Experimental apparatus used to determine the electricharge generated during the fracture of pumice. Both sets of equipment were mounted in a vacuum chamber, so that low pressure could be utilized to minimize charge decay and also to provide a stablenvironment during experiments carried out at atmospheric pressure. (a) Faraday cup apparatus for measurement of the net charge on particles produced by the colliding pumice samples. Adapted from James et al. [1998] with permission from The Geological Society of London. (b) Experimental apparatus showing the parallel plates used to generate the electric field, which separated falling particles by their polarity. For clarity, the diagram shows a crossection with the plates and collimating slot perpendicular to the motion of the samples. During the experiments the plates and slots were oriented parallel to the sample motion in order that initial particle motions did not favor collection on either plate. of <1000 kg m '3 and contained less than 5% phenocrysts. cup charge rapidly decreased to approximately half of its Phenocrysts had a maximum size of 1 mm. maximum value. However, if charge decay over the insulating supports had been responsible, then the measured charge 3. Results would have decayed exponentially (eventually to zero). Calibration of the equipment's decay characteristics showed The pumice collision apparatus proved to be efficient at that under atmosphericonditions the Faraday cup had a producing small silicate particles from the samples used. minimum half-life decay period of 1000 s, which would have Owing to natural inhomogeneities the individual samples produced the trace shown by the dashed line in Figure 3b. and the use of different pumice types, different experiments The rapid apparent charge loss and deviation from an produced particles with slightly varying size distributions. exponential curve indicates that another process must have However, most showed a maximum volume fraction of also been occurring. This is interpreted to be the continuing particles with diameters between 20 and 40 pm, and few, if accumulation of a net positive charge held on slow falling any, showed particles with diameter >70 pm. A typical silicate particles after the impacts stopped. This process is particle size distribution as determined for one experiment is absent in experiments carried out at low pressure, where all shown in Figure 2. particles have the same fall velocity. Although these particles The charge data collected during one experiment carried could not be observed visually through the glass of the out with the Faraday cup apparatus at low pressure (-10- Pa) vacuum chamber, most of the experiments carded out at is shown in Figure 3a. Opposite polarity charges were atmospheric pressure provided this evidence for their detected accumulating in the cup and on the mesh during the production. The magnitude of the charge variations after period of sample collisions. The effectiveness of the isolation impacts ceased was generally small, but the polarity was of the Faraday cup under low-pressure conditions is always opposite to that of the accumulated charge. Data demonstrated by the lack of charge loss after the cessation of collected at low pressure (with a correspondingly smaller impacts. The imbalance of the collected charges indicates that range of fall velocities, Figure 2b) and also during some charge remained on the samples or that there was a experiments carded out with the vacuum chamber removed systematic imbalance in the experimental measurement of (where the apparatus was open to any slight draughts in the charges between the mesh and the Faraday cup. At laboratory) did not demonstrate this feature. atmospheric pressure, charge losses over insulators were increased, and small decreases in the measured charge could be observed once collisions had ceased Ion Production Figure 3b shows an extreme example of apparent charge Direct interpretation of data collected by the mesh was loss during an experiment carried out at atmospheric pressure. unfortunately not possible owing to the electrometer not only After the impacts had stopped, the magnitude of the Faraday detecting charge deposited on the mesh but also being

4 16,644 JAMES ET AL.' VOLCANIC PLUME ELECTRIFICATION 300 producing the small step-like increase in charge recorded by the mesh (Figure 3b). The positive charge subsequently -. ', _ collected in the Faraday cup indicates that the slowest falling particles, which had spent the longest duration in the cloud, had scavenged sufficient charge to become net positively charged. During low-pressure experiments (- 10-' Pa) the ions have a mean free path of order 1 m, which is much larger than the length scale of the apparatus (of order 0.1 m or less). Thus no charge cloud would be maintained, and interactions : "" 'i :' ;,, ø o - between the ions and silicate particles should not be detected.,, - Consequently, changes in the charge accumulation rate in the o Faraday cup are mirrored by changes in the accumulation rate on the mesh (Figure 3a) and reflect variations in the rate of Diameter (pm) particle production alone. Figure 2. Particle size data collected from one Faraday cup experiment carried out with samples from Naivasha. The 3.2. Net Ash Charge Magnitudes extended tail of fine particles was observed in experiments on For each experiment carried out with the Faraday cup, the all the pumice types, and in order to complete the distribution for surface area calculations it was exponentially extrapolated net specific charge of the collected particles was calculated. to zero at 1! m. The Elzone particle sizing system required The results from experiments carried out with all six pumice that the samples were immersed in an electroyte (4% sodium samples at atmospheric and low pressures are shown in Figure metaphosphate) for measurement, which ensured that any 4a. The data are plotted againsthe specific surface area of the aggregates were properly dispersed and not counted as single particles _ sensitive to the electric field generated by charge located near the mesh. Thus charge accumulating near or on the surfaces of the samples during the experiments influenced the mesh data. The degree to which charge on the samples was detected was a function of the proximity of this charge to the mesh and also its distribution. For the samples used, tests showed that a maximum of 25% of the charge held on a sample would be detected on the mesh. During experiments the charge on the samples was not known; thus it is not possible to tell what proportion of the recorded mesh charge was induced as a result of other nearby charges. However, as was the case for many experiments, if the mesh charge magnitude was >25% of the Faraday cup charge, then simple charge balance analysis demonstrates that some of the mesh charge cannot be accounted for by charge held on the samples. Visual inspections of the mesh were carried out after each experiment to ascertain if any fine particles had adhered to it. During only a few low-pressurexperiments were any significant amounts of particles found on the mesh. Thus, for the majority of experiments, silicate particles are thought to be responsible for depositing only a very small amount of charge on the mesh and most of the recorded charge value is believed to be due to collected ions. o 600 : ', ',. r ', mesa ', _--...,- 200 :-...:... :...:...-- The production of ions can provide a possiblexplanation Figure 3. Data collected during two experiments with the for the decreasing rate of charge collection in the Faraday cup Faraday cup. (a) Here 182 sample impacts (shown by the shown after---60 s in Figure 3b (at atmospheric pressure) when marks on the top axis) carried out under low pressure (---10-' compared with the low-pressurexperiment of Figure 3a. Pa) between samples of Santorini pumice. Negative charge is During the period of sample impacts the rate of charge shown to accumulate in the Faraday cup, and positive charge detected by the mesh in Figure 3b gradually increases then accumulates on the mesh during the period of impacts. Note stabilizes, implying an "equilibrium" of processes concerning the lack of charge loss from the experiment after impacts ions reaching the mesh. We postulate that this indicates the ceased. (b) Here 150 sample impacts carried out at atmospheric pressure (22øC, 44% relative humidity) with buildup of a net positive charge cloud of released ions in the pumice samples from Gorge Farm. The dashed line shows an vicinity of the samples. Particles falling early during the exponential decay for the Faraday cup data (with a decay experiment, before this ion cloud has built up, retain their net constant of 1000 s, a minimum for the apparatus under these negative charge. Particles falling later during the experiment conditions) calculated from the time of the last impact. The scavenge more charge from the cloud and thus hold a deviation of the data from an exponential form implies that diminishing net charge. When impacts cease, the cloud positive charge (held on slow falling particles) must have rapidly dissipates to the mesh and other grounded surfaces, been accumulating the cup during this period. e o F...': '!... Fa-r-ada-; _ ,:1 [ t ' 0:00 0:50 1:40 2:30 3:20 4:1 Time (m:ss) x 0 "' c I x,,, : : Farada y cup. o , "' :00 0:50 1:40 2:30 3:20 4:1 Time (m'ss)

5 _ -, _, JAMES ET AL' VOLCANIC PLUME ELECTRIFICATION 16,645.c: o3 'E 10.7 r _ I I [ h I I - -- [] :42 ' [] 11 rq o 1. ' sl :,_ a : x ' -S... a x---: ,, ' _ -, - a 'v x :... x k i... - _... -,, _, _ -, - -(a) : : 10 O Specific su ace area (m 2 kg ' ) 10-6-,,,, i,,,, i,,,, i,,,, - _,,, _ -,, - ->- - - d - -'-: m ' m - x x : m x x -...,, -,, - - ' ' x x Net Ash Polarity -(b) : : Not all of the experiments produced particles of the same net polarity (Figure 4). The dominant control appeared to be 0 O the sample type, with identical samples consistently Specific su ace area (m 2 kg' ) reproducing the same net polarity particles. An exception to this wer experiments carded out with pumice from Gorge Figure 4. (a) Net specific ash charge results from Farm, which recorded net negative charges at atmospheric experiments with the Faraday cup plotted against the specific pressure but net positive charges when carried out at low surface area of the particles and (b) the calculated net surface pressure ( 10-] Pa). A further complication was encountered charge density plotted against the specific surface area. Each with the results of experiments carried out at low pressure symbol represents the result of one experiment, with solid with pumice from the Aira caldera, the polarity of which symbols representing experiments carried out at atmospheric pressure and open symbols representing experiments at low varied between samples cut from different clasts. A summary pressure (-10-x Pa). The results of impact experiments are of the net charges recorded for the different pumice types (for shown by squares, and those producing positive net charges experiments carded out at atmospheric pressure) is shown in are given by those containing plus symbols. The results from Figure 5, plotting net specificharge against the pumice silica rotational experiments are shown by crosses (net negative) content. Note that only experiments using the lowest silica and plus symbols (net positive) and are all from experiments carried out at low pressure. The dashed lines represent approximate bounds to the loci of the impact experiment results. For the impact experiments the net specificharge and net surface charge density data have correlation coefficients of 0.66 and 0.44, respectively. Both coefficients are significant at the 0.1% level. impact experiments the net surface charge density also increases with decreasing particle size. This demonstrates that the finer particle size distributions are indeed more highly charged than the coarser ones and that the increase observed in Figure 3a is not just a consequence of changing surface area to volume ratio. The data show a reasonable amount of scatter, which is partly a result of the different samples being used. However, even repeated experiments with the same samples showed a range in charge magnitude results. Although this is common in particulate electrostatic problems [e.g., Boschung and Glor, 1980], natural variations within the pumice samples also produced different particle sizes and particle masses per impact between successivexperiments. The net specific charges calculated from the frictiondominated rotational experiments were smaller than those from the impact experiments. All the rotational results shown in Figure 4 are from experiments carried out under low pressure. At atmospheric pressure, charge magnitudes were found to be so small that they were at or below the limit of the equipment's accuracy. particles (calculated from the particle size distribution, assuming spheres of density 2600 kg m -3) and, for the impact experiments carried out at atmospheric pressure, are between 10 -s and 10-6 C kg -]. If the impact experiments carried out at low pressure are also considered, then experiments which produced generally smaller particles (shown by the larger specific surface areas) are shown, by a weak but highly significant correlation, to have generated slightly larger magnitude net specific charges than those which produced generally coarser particles (shown by the smaller specific surface areas). Owing to the fact that charge resides on the particles' surfaces rather than in their bulk it is thus of interest to consider the surface charge density as well as the specific charge. In Figure 4b the net surface charge density (assuming that the measured net charge is evenly distributed over all the particles) is plotted against specific surface area. For the Figure 5. Net specific charge as a function of silica content. For each pumice type, where more than one experiment has been carded out, the average is shown by a symbol (an open square for net negative charges, and a crossed square for net positive), and the bars representhe range of results. These data are only for experiments carded out at atmospheric pressure. Note that during low-pressure experiments the Gorge Farm pumice produced net negative ash, and pumice from the Aira caldera produced different polarity ash from different clasts.

6 _ 16,646 JAMES ET AL.' VOLCANIC PLUME ELECTRIFICATION 1.2 "" Relative humidity (%) Figure 6. The variation of net specific ash charge with Figure 7. Specific charges of particles separated by their relative humidity (RH) and sample impact velocity. Data are polarity as a function of relative humidity (RH). All taken from experiments carried out at atmospheric pressure experiments were carried out at atmospheric pressure and with samples of Mount St. Helens pumice, and the charges are consisted of 50 impacts between the same samples of Mount all negative in polarity. The 12 V data (shown by squares) St. Helens pumice. Crossed squares represent data from represent impacts at--0.7 m s -, and the 6 V data (shown by experiments during which positive particles were collected, crosses) represent impacts at m s -. The bars on the data open squares represent negative charges. The solid squares at low RH show corrections to the initial result if charge on show data from ashfall at Sakurajima volcano from Gilbert et slow falling particles are taken into account. The bars on the al. [1991]. The errors shown by the bars are dominated by data at high RH show corrections to be made to account for uncertainty in the particle mass measurements owing to the the increased rate of charge loss from the Faraday cup under small masses involved. humid conditions. The magnitudes of the 12 V data have a correlation coefficient of-0.96, or-0.97 if the values indicated by the bars are used. Both coefficients are significant at the 0.1% level. detrimental effect on particle charges, mainly at high values (--90% RH). However, results obtained at values of RH <10% were also low, a feature which cannot be easily explained as a content samples (St. Vincent, 55.4 wt % SiO2) produced net dischargeffect of RH but could indicate that fracto-emission positively charged particles under all conditions. Owing to the charging processes are enhanced by small amounts of water low abundance of phenocrysts in the samples and also the fact adsorbed onto the painice surfaces. that the phenocrysts were not generally observed to fracture but were released intact it is thought that they played an insignificant role in the charging process Effect of Relative Humidity and Impact Velocity Several experiments (using only one set of samples from Mount St. Helens) were carried out over a range of Rid values and with different impact velocities (controlled by altering the solenoid operating voltage). The samples were specifically chosen for their homogeneity in order to minimize variation in the results from other factors. Changing the impact velocity between and 0.7 m s -I demonstrated no significant effect in the specific net charge data (Figure 6). The results from the experiments carried out at low and high values of RH show some consistent variation with the humidity, with decreased charges at higher humidity. However, the experiments at low RH demonstrated a larger than normal charge attributed to slow falling particles, and at high Rid the charge decay over the insulators during the experiment was greatly enhanced. If these factors are taken into account (shown by bars on the data in Figure 6), then the apparent effect of Rid on the net specific charge is greatly reduced Particles Separated by Polarity The experiments carried out with the parallel plates (Figure lb) demonstrate that ash produced during pumice collisions consists of particles of both polarities. The collected particles of either polarity held specificharges of up to 10-3 C kg - (Fil ure 7), corresponding to surface charge densities up to 6 x 10 - C m-. Increasing RH was shown to have some,; o ß O " r.f) 0.2 _ 1 i,;- 25 '.00F Relative humidity (%) Approximate sample impact velocity (m s ' ) I ''' I ' '' I ';' I I l,, I' 1 ' i...!!... '-- :, _ 15...,... :... : _... i... : -, J Solenoid voltage (V) Figure 8. The effect of impact velocity on the specific charges of particles separated by their charge polarity. All experiments were carried out at atmospheric pressure, using the same samples of Mount St. Helens pumice. Crossed squares represent data from experiments during which positive particles were collected, and open squares represent negative charges. The greater values for positive particles apparently demonstrate that larger charges are held on some of the positively charged particles than on the negatively charged ones. However, owing to the net negative charge of particles from Mount St. Helens pumice it could also represent an experimental artifact resulting from the collection of aggregates which would have decreased the recorded negative specific charges.

7 JAMES ET AL.: VOLCANIC PLUME ELECTRIFICATION 16,647 Figure 8 shows the way in which the measured specific at low pressure (net positive), demonstrate that other factors charge (of either polarity) varied over an order of magnitude are also involved. This and the more subtle pressure-related as a function of the impact velocity. Each experiment differences shown in Figure 3 may indicate that ion consisted of only 50 sample impacts in order to minimize any scavenging after initial particle generation (the degree of effects of ion scavenging, and the results suggesthat which would vary with atmospheric pressure) could play an positively charged particles are charged to a higher degree important role. than negatively charged particles. However, this effect can During experiments pouring volcanic particles collected also be interpreted as an artifact generated by particle from Mt. Usu down a chute, Kikuchi and Endoh [1982] aggregation within the apparatus. The Mount St. Helens noticed a tendency for net particle polarity to vary with the pumice had been shown, by the Faraday cup experiments, to average particle size of each experiment. Experiments carried produce net negatively charged ash. Thus aggregates out on larger particles (with diameters of up to -1 mm) consisting of many particles (inevitably of both polarities) will recorded net positive charges and experiments using smaller hold a statistically small net negative specific charge when particles (with diameters down to about 10!lm) recorded net compared with the specific charge of individual particles. negative charges. A similar polarity distribution was obtained Therefore collecting aggregates along with other negatively by Hatakeyama and Uchikawa [1952] using particles with charged particles will decrease the specifi charges calculated. average diameters of 300!xm and 40!xm from Mt. Aso. Our Fewer (if any) aggregates would be collected along with net charge results show no polarity variation as a function of positive particles, thus reducing calculated positive specific the particle size distribution produced by each experiment (as charges less or not at all. This hypothesis of increased shown by the specific surface areas in Figure 4). However, the aggregate collection when collecting negative particles was inherent differences (in particle sizes and experimental supported by qualitative observations made during some of method) between the experiments described here and those of the experiments and mass measurements of the particles Hatakeyama and Uchikawa [1952] and Kikuchi and Endoh collected on the parallel plates [James, 1999]. [ 1982] imply that direct comparisons may not be possible. From the data collected, it is not possible to determine 4. Discussion whether there were any initial polarity variations between particles of different sizes within any individual experiment. The Faraday cup experiments produced silicate particles There is no known reason why the initial polarity of particles with specific net charges similar to those previously measured charged by fracture-charging should systematically vary with on ashfall [Hatakeyama, 1958; Gilbert et al., 1991; Miura et particle size because any section of fracture surface has no al., 1995]. Particles produced by sample impacts held net knowledge of the size of particle to which it is attached. We charges substantially greater than those produced during the consider the charge separation detected within the particles friction-dominated rotational experiments. This not only collected by the Faraday cup at atmospheric pressure (Figure demonstrates the effectiveness of fracture charging over 3b) to be due to the secondary process of ion scavenging. "frictional" processes but, because the particles in the However, because ion scavenging is a function of particle size rotational experiments must have been produced by fracture, and fall velocity, then, in some cases, particle polarity can also indicates that some charge recombination must have become a function of particle size. This implies that occurred during these experiments. Along with the mesh data aggregation (which changes the effective particle sizes and this provides evidence for the release of "mobile" charge fall velocities) can also affect particle polarity by changing (ions) during the production of particles. Recombination of ion scavenging efficiencies. Thus, although we propose only this charge with that on the silicate particles was more one, particle size-independent, fundamental charging effective during the rotational experiments, when the particles mechanism (fractoemission), a particle size dependence may were maintained in the location of the fracture sites for longer be detected due to secondary processes involving charge periods of time. recombination. The polarity of the net ash charges recorded by the Faraday The experiments carried out with the parallel plates cup was generally negative, with net positive charges being demonstrate that the net charge held on the particles is due to collected on the mesh. This represents a positive above only a small imbalance in the sums of the (much larger) negative charge distribution which is in agreement with the individual particle charges. Thus net polarity must be models used by Lane and Gilbert [1992], Lane et al. [1995], sensitive to any factor that may alter the ratio of the particle Miura et al. [1996], and James et al. [1998] in order to charges, and could be reversed by only subtle changes (for explain atmospheric potential gradients produced by particle example, ion scavenging). However, despite this apparent rich volcanic plumes. However, Hatakeyama and Uchikawa sensitivity the experimentshowed that for the same samples [1952] detected a negative above positive charge distribution the net polarity was always reproducible under general at Aso volcano, and under certain conditions and for certain laboratory conditions. No change in net polarity was detected samples, net positive particle charges were observed in the between experiments carried out with different impact Faraday cup experiments. The fact that net positively charged velocities, a factor which was shown to increase measured particles were always produced from the lowest silica content particle charges by an order of magnitude. These relatively pumice (55 wt % SiO2) may suggest a possible geochemical large, opposite polarity charges held on individual particles control on net particle polarity. However, as with many are responsible for the electrostatic forces commonly quoted electrostatic systems, surface, rather than bulk, chemistry (and as a major factor in the generation of dry aggregates [Sparks hence the presence or not of adsorbed species, such as water) et al., 1997]. Aggregates were observed in the experiments, may be significant. Additionally, the experiments carried out and at atmospheric pressure some were sufficiently large to with pumice from Gorge Farm, which produced opposite net form circular collapse structures up to several millimeters polarity particles at atmospheric pressure (net negative) than across on the base of the Faraday cup. At low pressure,

8 16,648 JAMES ET AL.: VOLCANIC PLUME ELECTRIFICATION aggregates were generally much smaller but still present. This surface chemistry or ion scavenging processes may also be involved. implies that although electrostatic forces bind the particles together, fall velocity differences are responsible for bringing 5. The measured net charge is the result of a small the particles together [Lane et al., 1993] imbalance in the sum of charges held on both positively and The experiments have confirmed the importance of negatively charged particles. The specific charge held on gravitational separation processes for the electrical structure particles of only one polarity is up to 10-3 C kg -, which is up of plumes but suggesthat the dipole formation may be a to 3 orders of magnitude greater than the net charge. result of complex interactions between ion production and 6. Relative humidity appears to have only a minor effect on scavenging and particle aggregation. The results indicate that net charge magnitudes and only a slightly greater effect on volcanic gases are likely to hold an opposite polarity charge to absolute particle charges. the net ash charge due to the release of ions during magma 7. Varying the impact velocity of the collisions (from 0.3 to fragmentation, as postulated by Lane and Gilbert [1992]. 0.7 m s - ) produces an order of magnitude increase However, variations of particle charge polarity with particle measured particle charges, but this is not accompanied by a (or aggregate) fall velocity are also likely to be detected as a detectable change in net charges. result of ion scavenging and aggregation processes. 8. The Faraday cup experiments provided some evidence The demonstrated effectiveness of fracture charging for for the importance of ion scavenging. This secondary process silicate particlesupports the theory that plume electrification of charge recombination can be capable of producing particles may be a consequence of fractoemission processes during the whose net polarity is a function of their fall velocity. brittle fragmentation of magma and pumice. Although our In conclusion, (1) fractoemission processes operating results indicate that this process alone is sufficiento account during the brittle fragmentation of magma and pumice are for the potential gradient and charge data recorded at capable of producing the observed electrification of volcanic volcanoes, it is possible that charging due to liquid plumes. (2) During fragmentation an equal and opposite net fragmentation also plays a role. However, the electrification charge to that held on the silicate particles will be released as of plumes from pyroclastic flows generated by the brittle ions, effectively charging the volcanic gases. (3) If failure of magma lobes [Miura et al., 1996] demonstrate that fractocharging is the dominant mechanism behind plume liquid fragmentation is certainly not required. If brittle electrification, then this implies that charge generation is fracture is indeed the dominant charging mechanism, then it concentrated within the upper regions of the conduit and the implies that during explosiveruptions, electrification mainly jet portion of the plume where magma strain rates are highest occurs in the upperegions of the conduit and in the jet region and thus so are the rates of brittle fragmentation. (4) The high of the plume, where magma strain rates and the rates of brittle levels of charge held on individual silicate particles will fracture are highest. In the case of the production of inevitably promote aggregation which will, in rum, partly pyroclastic flows, charge generation will be maintained within control the distribution of net charges within plumes and the flows as the continuing clast-clast impacts prolong affect the particle fallout behavior. fragmentation. The large specific charges held on particles, even under conditions of high relative humidity, suggest that Acknowledgments. This work was supported by PPARC grant We thank J. Toothill, S. Marshall, and S. Black for plume chargeshould persist for a considerable time. Coupled pumice samples and geochemical data, and I.F.E. Windermere are with dipole formation from the separation of silicate particles acknowledged for the use of their Elzone facility. Thanks also go to and volcanic gases and between the particles themselves, this L. Glaze and T. Koyaguchi, whose reviews substantially improved supports atmospheric potential gradient monitoring as a this manuscript. sensitive plume detection and tracking technique. References 5. Summary The results of the experiments can be summarized as the following. 1. Small silicate particles (<100 tm in diameter) produced by the l[yacture of pumice hold a net specific charge similar to that measured on ashfall from volcanic plumes (-10-5 to 10-6 C kg- ). 2. Charge magnitudes generated during fracture-dominated impact experiments are significantly larger than those produce during friction-dominated rotation experiments. 3. During experiments the mesh detected a charge of opposite polarity to that of the falling silicate particles, which is interpreted to be ions release during the fracture process. 4. The main control on net particle polarity appears to be sample type, with the majority of samples producing net negatively charged particles. The most silica-poor sample (55 wt % SiO2) always produced net positively charged particles, suggesting a possible bulk geochemical control on net polarity. However, polarity variations with other samples demonstrated that this cannot be the only factor and that Bj6rnsson, S., D.C. Blanchard, and A. T. Spencer, Charge generation due to contact of saline waters with molten lava, J. Geophys. Res., 72, , Boschung, P., and M. Glor, Methods for investigating the electrostatic behaviour of powders, J. Electrost., 8, , Biittner, R., H. ROder, and B. Zimanowski, Electrical effects generated by experimental volcanic explosions, Appl. Phys. Lett., 70, , Cross, J. A., Electrostatics: Principles, Problems and Applications, 500 pp, IOP, Bristol, England, Dickinson, J. T., S.C. Langford, L. C. Jensen, G. L. McVay, J. F. Kelso, and C. G. Pantano, Fractoemission from fused silica and sodium silicate glasses, J. Vac. Sci. Technol. A, 6, , Gilbert, J. S., S. J. Lane, R. S. J. Sparks, and T. Koyaguchi, Charge measurements on particle fallout from a volcanic plume, Nature, 349, , Hatakeyama, H., On the disturbance of the atmospheric electric field caused by the smoke-cloud of the volcano Asama-yama, Pap. Meteorol. Geophys., 8, , Hatakeyama, H., and K. Uchikawa, On the disturbance of the atmospheric potential gradient caused by the eruption smoke of the volcano Aso, Pap. Meteorol. Geophys., 2, 85-89, Hoblitt, R. P., An experiment to detect and locate lightning associated with eruptions of Redoubt volcano, J. Volcanol. Geotherm. Res., 62, , 1994.

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