Flowage of the 1991 Unzen lava; discussions to ‘Rigid migration of Unzen lava rather than flow’ J. Volcanol. Geotherm. Res., 110, 107073

Abstract

This discussion concerns the flowage of the 1991 lava lobes at Unzen that were suggested by Goto et al. (2020) to be moving rigidly. Pictorial evidence of flowage of the 1991 lava lobe of Unzen, such as morphological change of the lava lobe, protrusion of the front of lava lobe, crease structure, and wrinkle surface of lava blocks, are presented. Differences in the interpretation of flow mechanisms may reflect slight difference in temperature and glass composition of the lavas. Temperatures used by Goto et al. (2020) were 780–880 °C, whereas those estimated by Venezky and Rutherford (1999) were 900 ± 30 °C. The groundmass glass compositions of 1991 blocks of Unzen dacite used by Goto et al. (2020) are 1–2 wt% higher in SiO2 than those of Nakada and Motomura (1999) and ours, suggesting that the samples used by Goto et al. (2020) represent the cooler rigid parts of the lava lobes. The lava viscosity is delicately influenced by cooling, dewatering, and crystallization during eruption. The direct application of experimental results to understanding the rheological behavior of natural lavas needs much more careful consideration.