Growth of volcanic ash aggregates in the presence of liquid water and ice: an experimental approach
Growth of volcanic ash aggregates in the presence of liquid water and ice: an experimental approach
复制标题
液态水和冰存在下火山灰聚集体的生长：一种实验方法
DOI：
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发表时间：
2012
影响因子：
3.5
通讯作者：
C. Cimarelli
中科院分区：
文献类型：
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作者：
A. Eaton;J. Muirhead;C. Wilson;C. Cimarelli
采用振动盘聚集的实验方法，研究了影响火山灰和水成物聚集的关键过程。在18°C和- 20°C的温度下，研究了冰雹和冰球、液态水（≤30 wt%）和混合水相存在下的聚集机制。实验生成的聚集体，经过手工样品、浸渍薄片、扫描电镜图像和x射线显微断层扫描检查，与27 ka Oruanui和2010 Eyjafjallajökull火山喷发的肺岩浆阶段的自然样本密切匹配。激光衍射粒度分析的母灰和骨料也被用来计算第一个实验推导的聚集系数，考虑变化的液态水含量和零度以下的温度。这表明干燥条件（< 5-10 wt%的液体）强烈地促进了小于63 μm颗粒的粒径选择性聚集成聚集体（由聚集系数>1给出）。相比之下，液体饱和条件（15-20 wt%液体）促进较小的尺寸选择性过程。在无液体条件下，结晶冰在静电吸引-冰烧结两阶段过程中也能优先选择<31 μm的火山灰。然而，这并没有在冰表面积聚超过一层的火山灰。当火山云的初始大小分布和含水量已知时，这些定量关系可用于预测聚集的时间尺度和特征，如聚集的大小光谱、密度和组成粒度特征。在火山灰和融化的冰球相互作用以及随后的升华过程中，自然聚集体中心出现了一个形状不规则的毫米级液泡。在低于31 μm灰的干燥混合料中加入湿骨料，通过静电力和稀疏的液桥吸附形成细粒状的边缘。由此，我们推断，天然团聚体的细粒外层反映了湿团聚体在相对干燥且以<31 μm灰为主的火山云区域的再暴露。
Key processes influencing the aggregation of volcanic ash and hydrometeors are examined with an experimental method employing vibratory pan aggregation. Mechanisms of aggregation in the presence of hail and ice pellets, liquid water (≤30 wt%), and mixed water phases are investigated at temperatures of 18 and −20 °C. The experimentally generated aggregates, examined in hand sample, impregnated thin sections, SEM imagery, and X-ray microtomography, closely match natural examples from phreatomagmatic phases of the 27 ka Oruanui and 2010 Eyjafjallajökull eruptions. Laser diffraction particle size analysis of parent ash and aggregates is also used to calculate the first experimentally derived aggregation coefficients that account for changing liquid water contents and subzero temperatures. These indicate that dry conditions (<5–10 wt% liquid) promote strongly size selective collection of sub-63 μm particles into aggregates (given by aggregation coefficients >1). In contrast, liquid-saturated conditions (>15–20 wt% liquid) promote less size selective processes. Crystalline ice was also capable of preferentially selecting volcanic ash <31 μm under liquid-free conditions in a two-stage process of electrostatic attraction followed by ice sintering. However, this did not accumulate more than a monolayer of ash at the ice surface. These quantitative relationships may be used to predict the timescales and characteristics of aggregation, such as aggregate size spectra, densities, and constituent particle size characteristics, when the initial size distribution and water content of a volcanic cloud are known. The presence of an irregularly shaped, millimeter-scale vacuole at the center of natural aggregates was also replicated during interaction of ash and melting ice pellets, followed by sublimation. Fine-grained rims were formed by adding moist aggregates to a dry mixture of sub-31 μm ash, which adhered by electrostatic forces and sparse liquid bridges. From this, we infer that the fine-grained outer layers of natural aggregates reflect recycled exposure of moist aggregates to regions of volcanic clouds that are relatively dry and dominated by <31 μm ash.