Volcanic Lightning: Global Observations and Constraints on Source Mechanisms
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Bull Volcanol DOI 10.1007/s00445-010-0393-4 RESEARCH ARTICLE Volcanic lightning: global observations and constraints on source mechanisms Stephen R. McNutt & Earle R. Williams Received: 2 June 2008 /Accepted: 4 June 2010 # Springer-Verlag 2010 Abstract Lightning and electrification at volcanoes are is more common at night (56%) and less common in important because they represent a hazard in their own daylight (44%). Reporting also varied substantially from right, they are a component of the global electrical circuit, year to year, suggesting that a more systematic observa- and because they contribute to ash particle aggregation and tional strategy is needed. Several weak trends in lightning modification within ash plumes. The role of water occurrence based on magma composition were found. The substance (water in all forms) in particular has not been bimodal ash plume heights are obvious only for andesite to well studied. Here data are presented from a comprehensive dacite; basalt and basaltic-andesite evenly span the range of global database of volcanic lightning. Lightning has been heights; and rhyolites are poorly represented. The distribu- documented at 80 volcanoes in association with 212 tions of the latitudes of volcanoes with lightning and eruptions. The Volcanic Explosivity Index (VEI) could be eruptions with lightning roughly mimic the distribution of determined for 177 eruptions. Eight percent of VEI=3–5 all volcanoes, which is generally flat with latitude. eruptions have reported lightning, and 10% of VEI=6, but Meteorological lightning, on the other hand, is common in less than 2% of those with VEI=1–2. These findings the tropics and decreases markedly with increasing latitude suggest consistent reporting for larger eruptions but either as the ability of the atmosphere to hold water decreases less lightning or possible under-reporting for small erup- poleward. This finding supports the idea that if lightning in tions. Ash plume heights (142 observations) show a large (deep) eruptions depends on water substance, then the bimodal distribution with main peaks at 7–12 km and 1– origin of the water is primarily magma and not entrainment 4 km. The former are similar to heights of typical thunder- from the surrounding atmosphere. Seasonal effects show storms and suggest involvement of water substance, that more eruptions with lightning were reported in winter whereas the latter suggest other factors contributing to (bounded by the respective autumnal and vernal equinoxes) electrical behavior closer to the vent. Reporting of lightning than in summer. This result also runs counter to the expectations based on entrainment of local water vapor. Editorial responsibility: JC Phillips Keywords Volcanoes . Lightning . Electrification . Electronic supplementary material The online version of this article Ash plumes . Water (doi:10.1007/s00445-010-0393-4) contains supplementary material, which is available to authorized users. S. R. McNutt (*) Introduction Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, P.O. Box 757320, Fairbanks, AK 99775, USA Electrical discharge in volcanic eruptions (i.e., volcanic e-mail: [email protected] lightning) is fairly common yet relatively understudied. Lightning is reported at only a few of the 55–70 volcanoes E. R. Williams that erupt each year (Simkin 1993), so it appears that either Massachusetts Institute of Technology 48-211, Parsons Laboratory, there are systematic problems with reporting, or some Cambridge, MA 02139, USA special set of circumstances must prevail to favor the Bull Volcanol production of lightning, or both. Lightning and electrifica- videos, was also compiled. The bulk of this work was tion at volcanoes are important because they represent a completed before modern computer search engines, such as hazard in their own right (people were killed by volcanic Google™, were in common use. lightning at Paricutin and Rabaul; see McNutt and Davis Key references included the Bulletin of Volcanology, the 2000,p.45–47), they are a component of the global Journal of Volcanology and Geothermal Research, the electrical circuit, and because they contribute to ash particle Smithsonian Institution’s Scientific Event Alert Network aggregation and modification within ash columns (Lane and Global Volcanism Network Bulletins, and the books by and Gilbert 1992; Gilbert and Lane 1994a, b; James et al. McClelland et al. (1989), Taylor (1958), and Newhall and 2003; Textor et al. 2005a, b; Mather and Harrison 2006; Punongbayan (1996), amongst others. We note that books James et al. 2008). Conventional thinking has been that published after about 1984 had better indexes than older interactions between dry silicate ash particles, such as works, probably a result of the increased use of electronic word collisions and fractures, have been dominant processes processing and the ease of making searches by keywords. causing electrification, while the role of water is secondary. Information that was collected included the name of the This is in contrast to early studies of Surtsey, Iceland, in volcano, the date (and time, where noted), the Volcanic which seawater played a prominent role (Blanchard 1964; Explosivity Index (VEI; Newhall and Self 1982; Simkin Anderson et al. 1965; Blanchard and Björnsson 1967). and Siebert 1994) of the eruption, the plume height Further, the existence and role of ice particles in ash maximum, the magma composition, whether it was day or plumes, now recognized as a fundamental aspect of night, any significant ancillary observations, and references thundercloud electrification, has been noted in only a few (Appendix). Latitudes and longitudes for volcanoes are studies at volcanoes (e.g. Thorarinsson 1966; Rose et al. available from Volcanoes of the World (Simkin and Siebert 1995a, b, 2000; Hoblitt 2000; Thompson 2000). In this 1994). VEI values were generally taken from Simkin and report, the expansion of steam and its subsequent conden- Siebert (1994) after checking that the date of the lightning sation and freezing are emphasized as primary processes observations coincided with the date of an eruption that was whose magnitude and effects have been understudied with assigned a VEI. We also estimated many VEI values respect to volcanic lightning. ourselves based mainly on reports of tephra volumes and Despite the frequent occurrence of volcanic lightning, plume heights. and many spectacular photographs, only two systematic compilations of basic lightning facts have been published (McNutt and Davis 2000; Mather and Harrison 2006). Here Analyses and results we present results of a more comprehensive literature search on the occurrence of volcanic lightning and attempt Reporting problems to summarize the effects of observed parameters such as height of plume, volume of tephra and gases (mainly water We first discuss reporting problems because these must be vapor), and magma composition. We also discuss reporting understood before other features of the data can be parameters because these affect the historical record of interpreted. Considerations of the detectability of volcanic lightning occurrence. Few instrumental studies of volcanic lightning over the full range of sunlight conditions must be lightning have been undertaken (Hoblitt 1994; McNutt and determined. Of the 131 eruptions for which the time of day Davis 2000; Paskievitch et al. 1995; Thomas et al. 2007, was known, we found that 44% occurred during daylight 2010), but this study is complementary to all of them. hours and 56% at night (Appendix). This suggests that Accordingly, some recommendations are included regard- lightning is more easily seen against a dark background and ing desirable electrical studies that will be necessarily may be missed during bright daytime conditions. As a aimed at constraining mechanisms for volcanic lightning. specific example of this effect, the eruption of Mount Spurr, Alaska, on September 17, 1992, occurred at night and had dramatic displays of lightning readily noted by people on Data and methods the ground. However, the amount of lightning for this eruption (7 flashes recorded instrumentally) was nearly an The main information source for this study is a compre- order of magnitude less than for the other two 1992 hensive literature compilation that was carried out over a eruptions (61 and 57 flashes, also recorded instrumentally), period of about 10 years. A standard bibliography keyword which were of similar size but occurred during the day, search yielded only a handful of studies, so instead the when ground observations of lightning were lacking literature was searched systematically, colleagues were (McNutt and Davis 2000). All three Spurr eruptions used polled, photographs were collected, and information from the same instrumental data to determine the number of newspapers and other sources, such as poster displays and flashes; details are given in McNutt and Davis (2000). Bull Volcanol An additional reporting problem with regard to daytime level for eruptions with VEI=3 and larger following the observations has to do with the fact that many volcanoes standard conventions (Simkin and Siebert 1994). For are located in tropical areas where lightning is common comparison, a histogram of all known VEI is shown in from thunderstorms. Thus volcanic lightning may not be Fig. 1 (top). There are many more small eruptions than noticed if an observer is not aware of an eruption, because large ones, so the numbers increase from right to left. The the lightning would be dismissed as being due to normal drop off for VEI=1 is likely due to under reporting (Simkin weather patterns. Conversely, thunderstorm lightning that and Siebert 1994). happens to occur near an erupting volcano might be The number of occurrences of volcanic lightning at associated with the eruption even if it is not caused by the various VEI values is also shown in Table 2 along with all eruption, depending on the interpretation of the observer. known VEI and percent of cases showing lightning versus For some regions, such as the Alaska/Aleutian arc, all VEI. The percent of eruptions with lightning is nearly lightning is not common (J. Painter, National Weather the same for VEI=3, 4, and 5.