Search

Volcanic plumes from small and moderate eruptions represent a challenge in the study of plume morphology due to eruption source parameter uncertainties and atmospheric influence. Sakurajima volcano, Japan, features such activity and due to its continuous eruptions in the recent years provides an ideal natural laboratory. A data set of 896 eruptions between 2009 and 2016 with well‐constrained plume heights, estimated erupted mass, and associated atmospheric conditions has been compiled. Plume heights ranged between 1,500 and 5,000 m and mainly developed under stable atmospheric stratification and low background wind speeds. The eruptions presented in the database were used to drive FPLUME, a 1‐D integral volcanic plume model, to study the simulated plume morphology. FPLUME was seen to provide consistent results under stable atmospheric stratification. A method for the real‐time monitoring of erupted mass used in the Sakurajima observatory was seen to provide appropriate first guess estimates for the eruptions, showing agreement with analytical and simulated mass flow rate calculations. Volcanic plumes from Sakurajima show significant influence by the atmospheric environment. The plume scaling parameter (Π) was used to characterize the expected degree of plume bending with results correlating well against modeled plume angles. The vertical wind profile was seen to have a significant impact on the resolved plume. Wind shear characteristics were seen to have a mechanical effect on the plume, aiding or inhibiting bending. Finally, potential issues were identified in simulations under unstable atmospheric conditions as the model either failed to provide a solution or overestimated the plume height. Key Points: A database of 896 eruptions from Sakurajima with associated atmospheric profiles is presentedDifferent plume bending regimes are quantitatively evaluated using the FPLUME modelVertical wind shear mechanically affects plume morphology by aiding or inhibiting plume bending [ABSTRACT FROM AUTHOR]

Abstract: We compare the texture of volcanic ash with the maximum amplitude of explosion earthquakes (A eq) for vulcanian eruptions from Sakurajima volcano. We analyze the volcanic ash emitted by 17 vulcanian eruptions from 1974 to 1987. Using a stereoscopic microscope, we classify the glassy particles into smooth surface particles (S-type particles) and non-smooth surface particles (NS-type particles) according to their surface conditions—gloss or non-gloss appearance—as an indicator of the freshness of the particles. S-type particles are further classified into V-type particles (those including vesicles) and NV-type particles (those without vesicles) by means of examinations under a polarized microscopic of polished thin sections. Cross-correlated examinations against seismological data show that: 1) the number fraction of S-type particles (S-fraction) has a positive correlation with A eq, 2) the number ratio of NV-type particles to V-type particles (the N/V number ratio) has a positive correlation with A eq, and 3) for explosions accompanied with BL-type earthquake swarms, the N/V number ratio has a negative correlation with the duration of the BL-Swarms. BL-Swarms refer to the phenomenon of numerous BL-type earthquakes occurring within a few days, prior to an increase in explosive activity [Kamo, K., 1978. Some phenomena before the summit crater eruptions at Sakura-zima volcano. Bull. Volcanol. Soc. Japan., 23, 53–64]. The positive correlation between the N/V number ratio and A eq could indicate that a large amount of separated gas from fresh magma results in a large A eq. Plagioclase microlite textual analysis of NV-type particles from five explosive events without BL-Swarms shows that the plagioclase microlite number density (MND) and the L/W (length/width) ratio have a positive correlation with A eq. A comparison between textural data (MND, L/W ratio, crystallinity) and the result of a decompression-induced crystallization experiment [Couch, S., Sparks, R.S.J., Carroll, M.R., 2003. The kinetics of degassing-induced crystallization at Soufriere Hills volcano, Montserrat. J. Petrol., 44, 1477–1502.] suggests that a plagioclase microlite texture of volcanic ash from eruptions without BL-Swarms could be generated by a decompression of 100–160 MPa. If the MND is controlled by the water exsolution rate from melt, the positive correlation between the MND and A eq may suggest that A eq becomes large when the effective decompression is large and the water exsolution rate is high (from 6.2×10−5 to 1.9×10−4 wt.%/s). The estimated magma ascent rate ranges from 0.11 to 0.35 m/s, which is one order of magnitude faster than that of an effusive eruption, and one to three orders slower than those for a (sub-) plinian eruption. This suggests that the ascent rate of magma plays an important role in the occurrence of vulcanian eruptions. We propose a simple model for vulcanian eruptions at Sakurajima volcano that takes into account the correlation between the S-fraction and A eq. [Copyright &y& Elsevier]