天池火山三期浮岩气孔形态的复杂性及其动力学成因  

Complexity and dynamics of vesicle shapes of pumices formed in the three Tianchi volcano eruptions

作　　者：彭年[1] 朱晓艳 刘永顺[1] 聂保锋[1] 李苏 PENG Nian;ZHU Xiaoyan;LIU Yongshun;NIE Baofeng;LI Su(College of Resource,Environment and Tourism,Capital Normal University,Beijing 100048,China)

机构地区：[1]首都师范大学资源环境与旅游学院

出　　处：《地学前缘》2019年第6期271-280,共10页Earth Science Frontiers

基　　金：国家自然科学基金项目(41372343,40872062);北京市自然科学基金项目(1082006)

摘　　要：爆炸式喷发过程中,火山碎屑物气孔记录了挥发分出溶、膨胀和合并等信息,其大小、形态、数量密度、空间分布等局域特征是推断火山喷发动力条件的重要参考。文章基于天池火山三期喷发(50000年前大喷发的黄色浮岩、千年大喷发的灰白色浮岩和1668年八卦庙期喷发的黑色浮岩)野外地质工作,以非线性火山喷发动力学为指导,开展了火山通道内气泡生长的流体动力学研究,揭示出岩浆流体黏性力和界面张力的共同作用对于岩浆减压和气泡生长过程的约束。在浮岩气孔结构的定量化分析基础上,进一步研究了天池火山三期喷发的浮岩气孔参数,通过气泡生长流体动力学方程得到了千年大喷发灰白色浮岩毛细管数Ca值为253,明显高于50000年前大喷发黄色浮岩(Ca值为94)和八卦庙期喷发黑色浮岩(Ca值为111),表明了千年大喷发曾发生过明显的成分变化,推测可能与幔源基性岩浆注入有关;而50000年前大喷发黄色浮岩气孔不规则形态参数(1-Ω)值为0.098,大于后两期喷发(分别为0.052和0.064),可能意味着天池火山系统动力学平衡的弛豫周期变小或浮岩气泡生长受动力学、流变学改造过程减弱,这可为进一步研究天池火山活动规律提供参考。三期浮岩毛细管数Ca量级为10^2,气孔不规则形态参数(1-Ω)量级为10^-1,从动力学上首次证实了天池火山属于普林尼型或超普林尼型喷发。In the process of an explosive eruption,the vesicles of volcanic pyroclasts record the information of volatile exsolution and bubble expansion and coalescence.The localization characteristics of bubble size,shape,number density and spatial distribution are important to inferring the dynamic conditions of volcanic eruption.On the basis of field geological work on pumices from the three Tianchi volcano eruptions(yellow pumice from the 50000 year BP eruption,gray-white pumice from the millennium eruption,and black pumice from the 1668 Baguamiao Stage eruption),we studied the fluid dynamics of bubble growth in volcanic conduit under non-linear dynamics of volcanic eruption.The result reveals that the combined effects of magmatic viscous force and surface tension controlled magmatic decompression and bubble growth.According to quantitative analysis of vesicle structures of pumices,we further studied the vesicle parameters of the three pumices from Tianchi volcano eruptions.The results show that the capillary number(Ca)of gray-white pumice from the millennium eruption was 253,which is higher than that of yellow pumice from the 50000 year BP eruption(Ca=94)or of black pumice from the Baguamiao Stage eruption(Ca=111),indicating the millennium eruption had obviously undergone compositional change possibly related to the injection of mantle-derived basic magma.The irregular vesicular shape parameter(1-Ω)of yellow pumice from the 50000 year BP eruption was 0.098,which is higher than that of the two pumices from the later two eruptions(0.052 and 0.064,respectively).It means that the relaxation period of dynamic equilibrium decreased in the Tianchi volcanic system,or the growth of pumice bubbles was weakened by dynamic and rheological modifications,either way it provides a reference for further study of eruptive activity in the Tianchi volcano.The magnitudes of the capillary number(10^2)and vesicle irregular shape parameter(10^-1)for the three stages of pumices also confirmed for the first time the Plinian or Ultraplinian er

关 键 词：天池火山 浮岩气孔结构 流体动力学 形态因子Ω 毛细管数Ca 

分 类 号：P588.14[天文地球—岩石学] P317.3[天文地球—地质学]