长白山天池火山千年大喷发空降浮岩碎屑的形貌特征和最终沉降速度

摘要

碎屑的形貌参数是计算碎屑最终沉降速度公式中的一个重要参数,而以往在计算中只是把碎屑假设为球形或椭球形,未对其进行详细研究.通过对长白山天池火山千年大喷发产生的空降碎屑的形貌分析得到一些新的认识:空降碎屑以浮岩为主,浮岩碎屑的形态不规则,从等轴状到拉长状,从次圆状到棱角状都有.随着浮岩碎屑粒径的减小,颗粒拉长现象明显,拉长碎屑的比例从15.02%上升到47.5%.并且,浮岩碎屑的粒度越小,颗粒棱角状越明显.通过对样品的形貌分析,得到其平均形态参数F=0.72.如果假设长白山天池火山千年大喷发浮岩碎屑为球形(F=1)时,计算的最终沉降速度是浮岩碎屑形态参数F=0.72时最终沉降速度的1.52倍,而当假设颗粒为椭球(F=0.5)时,计算的最终沉降速度是浮岩碎屑形态参数F=0.72时最终沉降速度的0.89倍.风速为10m/8时,不同形态参数的碎屑的沉积等厚线图显示,火山碎屑的扩散范围随着形态参数F值的减小而扩大.以上分析充分说明,碎屑的形貌特征对其最终沉降速度和扩散范围都具有较大的影响,今后在模拟火山碎屑扩散和灾害预测时应该充分加以考虑.%The shape parameter of particles is an important parameter in the formula of the terminal settling velocity, but it was usually assumed to be spherical or ellipsoidal without detailed study. According to the morphological characterization of the tephra particles erupted during the Millennium eruption of Changbaishan Tianchi volcano, we got some new recognition: The tephra particles consist mainly of pumice, which have irregular shapes, varying from very equant to elongate and from subrounded to angular. With decreasing of the particle size, the elongation of particles becomes significant, and the proportion of elongated particles increases from 15.02％to 47.5％. Moreover,the smaller the size of particles is, the more angular the shape of particles. An average shape parameter F = 0. 72 has been obtained through the morphological characterization of the pumice particles. The terminal velocity of the pumice particles, which were assumed to be spherical( F= 1 ), erupted during the Millennium eruptionof the Changbaishan Tianchi volcano is about 1.52 times the terminal velocity of the pumice particles of F= 0. 72, and the terminal velocity of the pumice particles, which were assumed to be ellipsoidal ( F = 0. 5 ), is about 0. 89 times the terminal velocity of the pumice particles of F = 0. 72. When the wind speed is taken to be 10m/s,the isopach map of tephra particles with different shape parametersshows that the dispersal range expands with the decrease of the shape parameter F. The aforementioned results show clearly that the morphology of particles influences strongly both the terminal settling velocity and dispersal range, and hence should be fully considered when simulating tephra dispersion and forecasting disasters in the future.