| Autor: |
Genareau, K., Wallace, K. L., Gharghabi, P., Gafford, J. |
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| Zdroj: |
Geophysical Research Letters; 3/28/2019, Vol. 46 Issue 6, p3133-3141, 9p |
| Abstrakt: |
Natural volcanic ashfall samples were examined, and high‐current (~100 kA) electrical impulse experiments were conducted to reveal the changes in grain size that can occur during lightning discharge. Experiments on pseudo ash samples manufactured from volcanic deposits of both rhyolitic and basaltic composition show that aggregates of very fine grained ash particles (<32 μm) melt and degas to form vesiculated pumice fragments >100 μm in size. In some cases, bubbles <5 μm in diameter expand and detach from the outer surface of the pumice to form hollow spheres of glass, one type of lightning‐induced volcanic spherule, while other bubbles fragment. Volcanic ashfall from the 2009 Redoubt eruption and the 2016 Pavlof eruption contains both pumiceous grains and individual spherules. Results of this study reveal that volcanic lightning will alter the grain size distribution of ash through melting, vesiculation, and fragmentation of individual particles or ash aggregates. Plain Language Summary: Volcanic lightning creates very high temperatures and can melt volcanic ash while it is in the atmosphere. This study used lightning simulation experiments to show that the sizes of ash particles will change if exposed to lightning. Particles may be completely destroyed or melted together with other particles to increase the size of the grains. Other particles may form bubbles due to escaping elements. All of these experimentally produced textures were found in deposits from two Alaskan eruptions where volcanic lightning occurred: the 2009 eruption of Redoubt volcano and the 2016 eruption of Pavlof volcano. The effects of volcanic lightning on the sizes of ash grains must be considered when evaluating deposits from eruptions where lightning is documented. Key Points: High‐current (~100 kA) impulse experiments result in melting and degassing of volcanic ash particles and aggregatesAshfall samples from Redoubt and Pavlof volcanoes contain lightning‐induced textures, including single spherules and pumiceous particlesVolcanic lightning will reduce the fine‐grained ash content of fall deposits through destruction of grains or fusing of aggregates [ABSTRACT FROM AUTHOR] |
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