| Abstrakt: |
One-atmosphere heating experiments were carried out for the 2 cm sized cubic samples of the phenocryst-rich basaltic lava (Suijin lava from Fuji volcano, Japan) to examine how the rheological behavior of the phenocryst-rich lava depends on temperature and time. The fO2 condition was not controlled during the experiments, but oxidation was not significant except for only near the surface of the run sample. At 1150°C, the lava was not deformed by its weight at least within 90 min. At 1155°C, the lava partly deformed at 45 min. At 1160°C, the lava was not deformed at < 20 min, but the deformation onset at 25 ± 5 min and perfectly flowed at > 37 ± 7 min. The onset time of deformation decreased as temperature increased. The rheological behavior of the lava changes drastically from solid to liquid within narrow ranges of temperature and time. Textural analyses of the run samples heated at 1150°C, 1155°C and 1160°C for 45 min suggest that the phenocryst content was almost constant at ~37 vol% regardless of heating temperature. In contrast, crystal content in the groundmass decreases from ~9.5 vol% to ~2.5 vol% as the temperature increases from 1150°C to 1160°C. This behavior can be explained by a crystal content-dependent change of relative viscosity with Φm ~ 0.437‒0.476, where Φm is the critical crystal content at which rheological transition occurs. Present results suggest that in phenocryst-rich lava, a slight increase/decrease in groundmass crystals causes the total crystal content to exceed/fall below the critical crystal content at which the solid-liquid transition occurs. Therefore, cooling has to be suppressed during flow for phenocryst-rich lava in order to flow over long distances. [ABSTRACT FROM AUTHOR] |
