Size limits for rounding of volcanic ash particles heated by lightning

Abstract

Volcanic ash particles can be remelted by the high temperatures induced in volcanic lightning discharges. The molten particles can round under surface tension then quench to produce glass spheres. Melting and rounding timescales for volcanic materials are strongly dependent on heating duration and peak temperature and are shorter for small particles than for large particles. Therefore, the size distribution of glass spheres recovered from ash deposits potentially record the short duration, high‐temperature conditions of volcanic lightning discharges, which are hard to measure directly. We use a 1‐D numerical solution to the heat equation to determine the timescales of heating and cooling of volcanic particles during and after rapid heating and compare these with the capillary timescale for rounding an angular particle. We define dimensionless parameters—capillary, Fourier, Stark, Biot, and Peclet numbers—to characterize the competition between heat transfer within the particle, heat transfer at the particle rim, and capillary motion, for particles of different sizes. We apply this framework to the lightning case and constrain a maximum size for ash particles susceptible to surface tension‐driven rounding, as a function of lightning temperature and duration, and ash properties. The size limit agrees well with maximum sizes of glass spheres found in volcanic ash that has been subjected to lightning or experimental discharges, demonstrating that the approach that we develop can be used to obtain a first‐order estimate of lightning conditions in volcanic plumes.

Lightning Effects on the Grain Size Distribution of Volcanic Ash

“…ECF2LAP was manufactured from pumice lapilli derived from a fall unit of the El Cajete series, the last major explosive event~75 ka from the Valles Caldera (Zimmerer et al, 2016). Although the size of the milled particles cannot be instrument controlled, they are small enough to be melted over the timescales of the current impulse (Wadsworth et al, 2017). Samples utilized here are from the later, violent Strombolian phase of the monogenetic eruption (Genareau et al, 2010;Valentine et al, 2007).…”

“…Although the preexperimental samples were analyzed with the laser diffractometer, the postexperimental samples were analyzed with the Hydro Sight™ peripheral component (video camera), and thus, each different colored curve in (b) and (c) represents a single snapshot of the dispersion when an anomaly (particle of unusual size or shape) is detected. Additionally, modeling studies (Wadsworth et al, 2017) have shown that the timescales of lightning discharge (milliseconds) require grains to be below a certain size (<150 μm) for melting to occur, which is why this size range was examined. Raw data for the curves shown in (a) can be found in the supporting information (Tables S1-S4). samples were available from both eruptions, during which lightning was documented, and both were deposited at a similar distance from the vent (~75 km).…”

“…All four samples were ground into very fine grained ash-sized fragments using a McCrone micronizing mill and sieved through a 32-μm stainless steel mesh to ensure a homogeneous GSD. Although the size of the milled particles cannot be instrument controlled, they are small enough to be melted over the timescales of the current impulse (Wadsworth et al, 2017). These samples were manufactured to ensure that natural magma compositions (rhyolites and basalts) would be utilized, but samples would contain no preexisting lightning-induced textures due to the nature of the deposits (either lavas or pyroclasts too large to be affected by lightning).…”

“…Ash samples were sieved to remove larger grains (>100 μm) prior to mounting, as previous observations of LIVS in ashfall samples indicate they are <100 μm (Genareau et al, 2015). Additionally, modeling studies (Wadsworth et al, 2017) have shown that the timescales of lightning discharge (milliseconds) require grains to be below a certain size (<150 μm) for melting to occur, which is why this size range was examined. BSE images were obtained of the polished epoxy mounts, and SE images were obtained of the grain mounts using an accelerating voltage of 15-20 kV.…”

Effects of Size and Temperature‐Dependent Thermal Conductivity on the Cooling of Pyroclasts in Air

“…Therefore, the knowledge of cooling time scales of pyroclasts is important for better interpretation of pre-to post-eruptive processes. Additionally, interphase heat transfer between pyroclast or lava and surrounding air is an important parameter in subaerial lava flow, eruption column, and pyroclastic flow modeling (e.g., Andrews & Manga, 2012;Dobran et al, 1993;Dufek & Bergantz, 2007;Dufek, 2016;Harris et al, 2007;Patrick et al, 2004;Thomas & Sparks, 1992;Valentine & Wohletz, 1989;Vanderkluysen et al, 2012;Wadsworth et al, 2017;Woods & Bursik, 1991).…”