关键词: 火山碎屑, 细火山灰, 粒度分析, 概率累积曲线, 长白山地区

Abstract: The pyroclastic deposits from tephra fall,pyroclastic-flow,base-surge,and lahar are well developed in Changbaishan Mountain area where volcanic activity is active in the Holocene.These different pyroclastic deposits are difficult to distinguish from each other in the field because of their similar physical and geological appearance.Through field investigation of pyroclastic deposit sections in the study area,we systematically collected different pyroclastic deposit samples at different sites,and then calculated the grain size parameters and plotted out the probability cumulative curves of all samples in the laboratory.Our results indicate that the pyroclastic samples from different origins can be distinguished by characteristic grain-size diagrams,e.g.median diameter(Md_Ф)versus sorting(σ_Ф),F1 versus F2 diagram.In terms of texture,most pyroclastic deposits show similar characters.However,with the increase of distance from the volcano crater,grain size decreases correspondingly.Finally,Laser Particle Size analyzer is utilized at first time to analyze the subpopulation of fine ash(≤62.5μm)of our samples.On the basis of the fine ash subpopulation histogram distribution and weight cumulative curves,we found that the fine ashes with grain-size from 0.02μm to 62.5μm in pyroclastic-flow deposits and pyroclastic-surge deposits are significantly different due to their different formation origin and different transportation processes.

Key words: pyroclastic deposit, fine ash, grain-size analyses, cumulative curve, Changbaishan area