Abstract: 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 (102) and vesicle irregular shape parameter (10-1) for the three stages of pumices also confirmed for the first time the Plinian or Ultraplinian eruption of the Tianchi volcano.

Key words: Tianchi volcano, vesicle structure, fluid dynamics, shape factor (Ω), capillary number (Ca)