The role of gravitational instabilities in deposition of volcanic ash
The role of gravitational instabilities in deposition of volcanic ash
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DOI：
10.1130/g36252.1
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发表时间：
2015-03-01
期刊：
影响因子：
5.8
通讯作者：
Monnard, Helene
中科院分区：
文献类型：
--
作者：
Manzella, Irene;Bonadonna, Costanza;Monnard, Helene
火山灰对火山附近地区和航空来说是一个重大危害。在许多爆炸性喷发中都观察到在扩散的火山云底部形成的重力不稳定性。在这里，我们首次对此类不稳定性的动态进行定量描述，并将其与 2010 年 5 月 4 日埃亚菲亚德拉冰盖（冰岛）喷发观测中的坠落沉积物特征联系起来。引力不稳定性最初以向下传播的指状物的形式出现，在云的底部不断形成，并且似乎以侧风速度被动地平流。指状传播的测量结果与先前的灰云动力学研究推断出的初始条件一致。专门的实验室模拟实验证实，指状向下传播显着超过了单个颗粒的沉降速度，表明重力不稳定性为增强细灰的沉降提供了可能的机制。我们的观察挑战了聚集是近端细灰沉积的主要解释的观点，并直接支持了重力不稳定性在提供可促进聚集的高颗粒浓度区域中的作用。
Volcanic ash is a significant hazard for areas close to volcanoes and for aviation. Gravitational instabilities forming at the bottom of spreading volcanic clouds have been observed in many explosive eruptions. Here we present the first quantitative description of the dynamics of such instabilities, and correlate this with the characteristics of the fall deposit from observations of the 4 May 2010 Eyjafjallajokull (Iceland) eruption. Gravitational instabilities initially took the form of downward-propagating fingers that formed continuously at the base of the cloud, and appeared to be advected passively at the crosswind speed. Measurements of finger propagation are consistent with initial conditions inferred from previous studies of ash cloud dynamics. Dedicated laboratory analogue experiments confirmed that finger downward propagation significantly exceeded the settling speed of individual particles, demonstrating that gravitational instabilities provide a possible mechanism for enhanced sedimentation of fine ash. Our observations challenge the view that aggregation is the primary explanation of proximal fine ash sedimentation, and give direct support for the role of gravitational instabilities in providing regions of high particle concentration that can promote aggregation.