Abstract: Pyroclastic flow deposits have become a research topic of great interest in volcanology,because they may bring the humanity devastating hazards.This paper demonstrates the results of grain-size sieve analyses of the millennium pyroclastic flow deposits from Tianchi volcano.The result shows that the pyroclastic flow deposits are poorly sorted and the ash cloud surge deposits accompanied with pyroclastic flow are well sorted.The average greatest grain-sizes of lithic fragments and pumice fragments in pyroclastic flow deposits decrease with increasing distance from the crater,indicating that gravity sorting and mechanical wearing did occur during the transportation of the pyroclastic flow.In addition,fluidization also occurred during the transportation of pyroclastic flow.The rate of fluidization decreases with increasing distance from the crater,indicating the occurrence of degassing during the transportation process of the pyroclastic flow.This process might cause the increase of viscosity and yield strength of the pyroclastic flow,resulting in the deposition of the flow.The rate of fluidization,therefore,is an important factor controlling the transportation distance.The confluence of multi-channel pyroclastic flows at distal part may increase the rate of fluidization of the pyroclastic flow,as a result,the transportation distance becomes longer and the hazard area becomes greater.

Key words: pyroclastic flow, grain-size characteristics, transportation mechanism, Tianchi volcano

摘要： 火山碎屑流堆积因其巨大的危害性而成为火山学研究的重要课题之一。文中对长白山区天池火山千年大喷发火山碎屑流堆积进行了粒度分析。分析结果表明,火山碎屑流堆积分选差,伴生的灰云浪堆积分选性好。火山碎屑流堆积中的岩屑和浮岩的平均最大粒度随着离开火山口距离的加大而减小,反映火山碎屑流搬运过程中存在重力分选和机械磨损作用。火山碎屑流在搬运过程中发生了流体化作用,离开火山口越远流体化速度越小,反映火山碎屑流在搬运过程中发生了流体化去气作用,这种作用使火山碎屑流的粘度和屈服强度增加而导致沉积。流体化速度是控制搬运距离的因素之一,远源多渠道火山碎屑流的汇合使流体化速度增大,搬运距离更远,造成的灾害范围也增大。

关键词: 火山碎屑流, 粒度特征, 搬运机制, 长白山天池火山