关键词: 火山碎屑流, 粒度特征, 搬运机制, 长白山天池火山

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