| The products of volcanic eruption record a large number of magma physical and chemical processes throughout the volcanic eruption, and thus based on the quantitative study on the microstructure of the volcanic eruption products, important information about magma eruptive process can obtained that many macro researches cannot access. It has become an effective way to obtain eruption dynamics from quantitative microstructural research. In addition, it has become a quick and efficient way to determine the scope of the ash diffusion and deposition from the numerical simulation, thus which can be used to predict the disaster degree and range of a future volcanic eruption.In this paper, the component and quantitative microstructure of pumices from three explosive eruptions in Holocene (the BP5000eruption, the millennium eruption and the late eruption) of Changbaishan Tianchi volcano were studied in detail. The results show that, the pumices from BP5000eruption and the millennium eruption are all pantellerite in composition, but those from the millennium eruption are more acid than those from BP5000eruption. The pumices from late eruption are high potassium trachyte in composition. The pumices from these three eruptions compose of vesicle with different sizes, vesicle wall and a small amount of phenocrysts (<15%). The yellow pumices from BP5000eruption are small, and have the smallest vesicles (about1μn) but the greatest number density (3.1×1015/m3). The sizes and numbers of vesicles in gray pumice from BP5000eruption and gray white pumice from millennium eruption are relatively similar, the smallest vesicles are several micrometers, and the number densities are1.12×1015～1.9×1015/m3. The vesicles in black pumice from modern eruption enlarge to centimeters, and are generally larger than10μm in size. The vesicle number (1.09×1014/m3) decreases and vesicle walls thicken. The quantitative analyses of vesicle also show that the vesicle has experienced at least three times of nucleation and growth. Though they have the similar growth rate, the vesicle nucleation rate during the late eruption of5000years ago was much faster than that in any other periods. Finally, according to the components and vesicle number, the decompression rateswere obtained. Then weestimated that theeruption column height HT of5000years ago was34～46km, the magma recharge ratio was2.7×108～7×108kg/s, and the eruption strengthened from the gray pumice to yellow pumice. The intensity of the late eruption is weaker, the eruption column height HT was29～36km, and the magma recharge ratio was1.3X108～3.2×108kg/s.Based on the field investigation and experimental analysis, we confirm the simulated parameters for the millennium eruption of Changbaishan Tianchi volcano. The simulated results are in good agreement with the results from measurement in situ, indicating that the model is reliable and the parameters used in the model are reasonable. We improved the software of diffusion model, which now can be used to simulatethe probability exceeding a tephra thickness threshold under different magnitude eruptionin a particular district. According to wind profiles of29years (1980～2008),20,000simulations under different wind profile were carried out, and then probabilistic hazard maps of tephra fallout were constructed for tephra thickness thresholds,20cm and70cm. This study can provide an important scientific basis to the volcanic hazard analysis and risk mitigation countermeasure in Changbaishan volcanic area. |
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