INFLUENCE OF PARTICLE AGGREGATION ON DEPOSITION OF DISTAL TEPHRA FROM THE MAY 18, 1980, ERUPTION OF MOUNT ST-HELENS VOLCANO
INFLUENCE OF PARTICLE AGGREGATION ON DEPOSITION OF DISTAL TEPHRA FROM THE MAY 18, 1980, ERUPTION OF MOUNT ST-HELENS VOLCANO
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DOI：
10.1029/jb087ib08p07061
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发表时间：
1982-01-01
影响因子：
--
通讯作者：
SIGURDSSON, H
中科院分区：
文献类型：
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作者：
CAREY, SN;SIGURDSSON, H
1980年5月18日，圣海伦斯火山(MSH)喷发在华盛顿、爱达荷州和蒙大拿州产生了一个大范围的火山灰矿床，最小体积为0.55千米3(特弗拉)。该矿床的一个不同寻常的特征是在华盛顿州里茨维尔附近的MSH附近出现了第二厚度最大为325公里的MSH。沿主散散轴方向样品的粒度和组分丰度分析表明，该地区的火山灰颗粒非常细小(平均粒度为22μm)，分选较差，呈多态分布，富含玻璃碎片和浮石碎片。对大气弥散的喷发羽流的灰尘进行了计算机模拟，以评估关于远距离火山灰特征起源的各种假说，特别是厚度与距离的关系。该模式受到喷发柱高度、主要灰尘输送高度、喷发烟羽的横向扩展以及MSH附近大气风结构等观测资料的制约。不同的模拟结果表明，第二个厚度最大值既不能归因于华盛顿市中心风速的降低，也不能归因于对流层顶附近水平风速最大值以上的细火山灰注入。要使模型符合沉积物的观测特征，必须考虑细小于63μm的火山灰的显著颗粒聚集。当小于63μm的火山灰被聚集成直径几百微米的颗粒，沉降速度为0.35m/S时，这一过程是最合适的。5·18喷发期间在华盛顿州普尔曼坠落的类似大小的脆弱火山灰团的观测和收集支持了这一过程(索伦，1982)。作为颗粒集合体的细灰过早落下是5·18 MSH落灰沉积物的粒度特征、组分丰度变化以及厚度与距离关系的基本过程。
The May 18, 1980, eruption of Mount St. Helens (MSH) produced an extensive ashfall deposit in Washington, Idaho, and Montana with a minumum volume of 0.55 km3(tephra). An unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ENE of MSH near Ritzville, Washington. Grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained (mean size, 22 μm), poorly sorted, polymodal, and rich in glass shards and pumice fragments. A computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics, particularly the thickness versus distance relationship. The model was constrained by observations of the eruption column height, elevation of major ash transport, lateral spreading of the eruption plume, and atmospheric wind structure in the vicinity of MSH. Results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central Washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause. For the model to fit the observed characteristics of the deposit, significant particle aggregation of ash finer than 63 μm must be invoked. The best fit occurs when ash less than 63 μm is aggregated into particles several hundred microns in diameter with a settling velocity of 0.35 m/s. Support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at Pullman, Washington, during the May 18 eruption (Sorem, 1982). The premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics, variations in component abundances, and thickness versus distance relationship of the May 18 MSH ash fall deposit.