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Abstract
High-current impulse experiments were performed on volcanic ash samples to determine the magnetic effects that may result from the occurrence of volcanic lightning during explosive eruptions. Pseudo-ash was manufactured through milling and sieving of eruptive deposits with different bulk compositions and mineral contents. By comparing pre- and post-experimental samples, it was found that the saturation (i.e., maximum possible) magnetization increased, and coercivity (i.e., ability to withstand demagnetization) decreased. The increase in saturation magnetization was greater for compositionally evolved samples compared to more primitive samples subjected to equivalent currents. Changes in remanent (i.e., residual) magnetization do not correlate with composition, and show wide variability. Variations in magnetic properties were generally more significant when samples were subjected to higher peak currents as higher currents affect a greater proportion of the subjected sample. The electrons introduced by the current impulse cause reduction and devolatilization of the ash grains, changing their structural, mineralogical, and magnetic properties.
Highlights
Lightning-induced volcanic spherules (LIVS) have been observed in ashfall deposits from explosive volcanic eruptions where lightning was documented[1]
Each sample was subjected to two or three different impulse experiments to ensure that enough material would survive for the variety of analyses performed, and one of each of these was used for subsequent Vibrating sample magnetometry (VSM) analysis, except for the intermediate samples, where two sets were analyzed
50% of ECF2LAP was removed from the post-experimental sample with an ordinary magnet
Summary
Lightning-induced volcanic spherules (LIVS) have been observed in ashfall deposits from explosive volcanic eruptions where lightning was documented[1]. The effects of lightning on geologic materials have been previously documented in a number of locations and several studies have attributed variability in the surficial magnetic characteristics of rock outcrops (a.k.a., anomalous magnetization) to CG lightning strikes[16,17,18,19,20,21]. These lightning-induced magnetic anomalies have posed problems for those attempting to survey regions for unexploded ordnance[22] or archaeological sites[23,24]. This study shows, for the first time, that lightning will cause variations in the magnetic properties of volcanic ash and the amount of that variation will be controlled by both the peak current of the discharge and the bulk composition of the ash
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