Detailed analysis of particle launch velocities, size distributions and gas densities during normal explosions at Stromboli

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• 作者：Andrew J.L. Harris^a ; ^b ; ^c ; ^{a.harris@opgc.univ-bpclermont.fr} ; Maurizio Ripepe^d ; Elizabeth A. Hughes^a
• 关键词：Plume dynamics ; Particle velocity ; Gas velocity ; Particle size distribution ; Gas density ; Thermal camera
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：2012
• 期刊代码：53_03770273
• 类别：ear
• 出版时间：15 June, 2012
• 卷：231-232
• 期：Complete
• 页码：109-131
• 文件大小：2260 K

摘要

Using high frame rate (33 Hz) thermal video data we describe and parameterize the emission and ascent dynamics of a mixed plume of gas and particles emitted during a normal explosion at Stromboli (Aeolian Islands, Italy). Analysis of 34 events showed that 31 of them were characterized by a first phase characterized by an initial diffuse spray of relatively small (lapilli-sized) particles moving at high velocities (up to 213 m s^鈭?#xA0;1; average 66-82 m s^鈭?#xA0;1). This was followed, typically within 0.1 s, by a burst comprising a mixture of ash and lapilli, but dominated by larger bomb-sized particles, moving at lower exit velocities of up to 129 m s^鈭?#xA0;1, but typically 46 m s^鈭?#xA0;1. We interpret these results as revealing initial emission of a previously unrecorded high velocity gas-jet phase, to which the lapilli are coupled. This is followed by emission of slower moving larger particles that are decoupled from the faster moving gas-phase. Diameters for particles carried by the gas phase are typically around 4 cm, but can be up to 9 cm, with the diameter of the particles carried by the gas jet (D) decreasing with increased density and velocity of the erupted gas cloud (蟻_gas and U_gas). Data for 101 particles identified as moving with the gas jet during 32 eruptions allow us to define a new relation, whereby U_gas = U_particle + a [蟻_gas ]^b. Here, U_particle is the velocity of bombs whose motion is decoupled from that of the gas cloud, and a and b are two empirically-derived coefficients. This replaces the old relation, whereby U_gas = U_particle + k ; a relation that requires a constant gas density for each eruption. This is an assumption that we show to be invalid, with gas density potentially varying between 0.04 kg m^鈭?#xA0;3 and 9 kg m^鈭?#xA0;3 for the 32 cases considered, so that k varies between 54 m^1/2 s^鈭?#xA0;1 and 828 m^1/2 s^鈭?#xA0;1, compared with the traditionally used constant of 150 m^1/2 s^鈭?#xA0;1.