Electrical charging of explosive volcanic plumes (Invited)
Abstract
The lightning that accompanies many silicate-rich volcanic plumes is only one indicator of the electrical charging activity that occurs during explosive eruptions. Perturbations to the atmospheric electric gradient and direct measurements of electrical charge on sedimenting particles also indicate that, even in plumes not associated with lightning, significant electrical charging does occur. For a charging process to produce macro-scale electrification and detectable electric fields, the initial charge generation (representing atomic- or micro-scale separation of charge) must be followed by subsequent charge separation. In common with thunderclouds, the gravitational separation of particles or droplets with different fall velocities is assumed to dominate the charge separation process. However, charge generation is much less understood because the complex and poly-phase nature of plumes, which contain solid particles, liquid drops and gases of various chemical compositions, over wide ranges of temperatures and pressures, offers many possibilities in terms of charging mechanisms. We can consider charge generation processes as either primary (i.e. directly associated with the eruption process and dominantly located at the vent) or secondary (i.e. associated with in-plume and dispersion processes, and dominantly at some distance from the vent). For primary charge generation, both observations and measurements have indicated that high degrees of electrification are strongly correlated with events involving either vigorous water boiling or extensive magma fragmentation, or both. Neglecting boiling, since only a relatively few eruptions involve large amounts of external water, primary charging is dominantly attributed to solid-solid processes resulting from magma fragmentation. The most likely charging mechanism is fractoemission, in which charged atomic particles are released from fresh material surfaces during brittle fracture. During magma fragmentation to generate volcanic ash particles, the massive increase in silicate surface area suggests that significant charge can be generated by this mechanism. This is supported by experiments that produced ash particles with a net charge density of up to 10-5 C/kg by fragmenting pumice. In most cases this charge is negative, with an equal magnitude positive charge released as either ions or scavenged onto silicate particles sufficiently small that electrical forces exceed those of gravity. However, these net charges reflect only a small imbalance in much larger individual particle charge densities of up to 10-3 C/kg. Hence, the process of brittle magma fragmentation alone is sufficient to produce the charge densities measured on sedimenting particles during eruptions. Large plumes, which reach sufficient altitudes for ice formation, may also undergo significant secondary charging by mechanisms normally associated with thunderstorms. In such cases, charge generation and separation are intimately interlinked and will further complicate charge distribution within plumes. Studies of charge distribution within plumes are likely to prove valuable for plume detection and hazard mitigation; they may also provide insight into magma fragmentation and particle dispersion, aggregation and sedimentation processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMAE31A..02J
- Keywords:
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- 3304 ATMOSPHERIC PROCESSES / Atmospheric electricity;
- 8428 VOLCANOLOGY / Explosive volcanism