5:15 PM - 6:30 PM
[SVC31-P13] Flowage of the 1991 Unzen lava: Discussion to Goto et al.(2020) Rigid migration of Unzen lava rather than flow. JVGR
Keywords:Unzen volcano, Lava lobe, Viscosity, Flowage of lava, Magma temperature, Groundmass glass composition
Goto et al. (2020) presented new rheological data on the bulk rocks and a groundmass glass synthetic sample of the ejecta of the 1991 Unzen dacite, and together with the previous rheological data of groundmass glass, they suggested that bulk viscosity of Unzen dacite is more than 1 order of magnitude larger than those obtained by Jeffreys model calculated from the apparent velocity of the lava front, slope inclination, lava height and density of the lava by Suto et al.(1993) and Yamamoto et al. (1993). Although they suggested possible effect of water in the lower part of the lava where higher pressure (ca. 1.4 MPa) affords retaining of water in the matrix glass, they suggested rigid migration of the lava from the video image analyses. Our discussion is concerned with more fluidal aspect of the lava from the pictures at the time of eruption, suggesting that at least some part of the lava flowed as viscous fluid, and that shear zone at the base of the lava, which Goto et al.(2020) suggested, is not likely.
Figure 9 of Goto et al.(2020) vaguely show that apparent advance of the lava lobe front is accompanied by crack widening in the lava lobe, and they suggested that apparent advance of the lava lobe represents rotation of the frontal block. However, our sequential pictures show that upstream area also advance nearly parallel to the frontal area, which indicate the deformation of the lava lobe. Other evidence of flowage of the lava lobe includes thickening of the lava lobe in the downstream area, protrusions of the front of the lava lobe, crease structure of the lava lobe near the site of upwelling, and wrinkle structure on the surface of the lava block. Thickening of lava lobe in the downstream area has been illustrated in Figure 7 of Nakada and Fujii (1993)jvgr and our pictures. Protrusive front is also described in Nakada and Fujii (1993) jvgr, Ohta (1992) and our photographs taken from helicopter of the Ground Self-Defence Force in August and November, 1991. Crease structure show viscous deformation of the upwelling lava from the source vent, and are often observed in the 1991 Unzen eruption. Wrinkle texture is sometimes observed on the surface of lava blocks of the Unzen lava lobes including the supplementary movie of Goto et al.(2020). Interval of wrinkles is generally 2-5 meters, and indicate much less viscous interior than the surface of the lava block (Fink and Fletcher, 1978 jvgr).
Several shear zones are observed in the section of lava lobe such as Figure 3 of Suto et al.(1993) and lava lobe-11. The presence of shear zones within the lava lobe together with protrusive front of the lava lobes suggest that basal boundary of the lava lobe is resistant to slide.
The discrepancy of the idea may be caused by slight difference in temperature and glass composition of the lavas: temperature Goto et al.(2020) used was ca. 850°C, whereas experimentally duplicated temperature of Venezky and Rutherford (1999) gave 900±30°C, and glass composition of Nakada and Motomura (1999) gave 1-2wt% lower SiO2 and 0.3-0.4 wt% higher Na2O, which result in the estimated viscosity difference of more than 1 log unit.
The experimental work of Goto et al.(2020) together with those of Cordonnier et al. (2009) and Coats et al. (2018) showed fairly large variation of viscosity on the bulk rock of Unzen lava samples (more than 2 orders of magnitude at 900°C), which may correspond to wide variations of fluidity/rigidity as pictures of the 1991 Unzen lava lobes show. Viscosity of effusing lava is delicately influenced by dewatering, cooling, and crystallization during effusive action, and application of the rheological experimental results to natural lava behavior should be carried out with much more caution.
Figure 9 of Goto et al.(2020) vaguely show that apparent advance of the lava lobe front is accompanied by crack widening in the lava lobe, and they suggested that apparent advance of the lava lobe represents rotation of the frontal block. However, our sequential pictures show that upstream area also advance nearly parallel to the frontal area, which indicate the deformation of the lava lobe. Other evidence of flowage of the lava lobe includes thickening of the lava lobe in the downstream area, protrusions of the front of the lava lobe, crease structure of the lava lobe near the site of upwelling, and wrinkle structure on the surface of the lava block. Thickening of lava lobe in the downstream area has been illustrated in Figure 7 of Nakada and Fujii (1993)jvgr and our pictures. Protrusive front is also described in Nakada and Fujii (1993) jvgr, Ohta (1992) and our photographs taken from helicopter of the Ground Self-Defence Force in August and November, 1991. Crease structure show viscous deformation of the upwelling lava from the source vent, and are often observed in the 1991 Unzen eruption. Wrinkle texture is sometimes observed on the surface of lava blocks of the Unzen lava lobes including the supplementary movie of Goto et al.(2020). Interval of wrinkles is generally 2-5 meters, and indicate much less viscous interior than the surface of the lava block (Fink and Fletcher, 1978 jvgr).
Several shear zones are observed in the section of lava lobe such as Figure 3 of Suto et al.(1993) and lava lobe-11. The presence of shear zones within the lava lobe together with protrusive front of the lava lobes suggest that basal boundary of the lava lobe is resistant to slide.
The discrepancy of the idea may be caused by slight difference in temperature and glass composition of the lavas: temperature Goto et al.(2020) used was ca. 850°C, whereas experimentally duplicated temperature of Venezky and Rutherford (1999) gave 900±30°C, and glass composition of Nakada and Motomura (1999) gave 1-2wt% lower SiO2 and 0.3-0.4 wt% higher Na2O, which result in the estimated viscosity difference of more than 1 log unit.
The experimental work of Goto et al.(2020) together with those of Cordonnier et al. (2009) and Coats et al. (2018) showed fairly large variation of viscosity on the bulk rock of Unzen lava samples (more than 2 orders of magnitude at 900°C), which may correspond to wide variations of fluidity/rigidity as pictures of the 1991 Unzen lava lobes show. Viscosity of effusing lava is delicately influenced by dewatering, cooling, and crystallization during effusive action, and application of the rheological experimental results to natural lava behavior should be carried out with much more caution.