5:15 PM - 6:30 PM
[ACC25-P02] A discussion of the circumstances and causes of the massive avalanche that occurred at Mt. Kiso Ontake on May 21st 2019
Keywords:Avalanche, Slush, Clay volcanic ash, Mt. Kiso Ontake, Mudflow, Phreatic eruption
A large-scale snow avalanche that occurred on 21st May 2019 on the slope between Ichinoike and Ninoike craters at Mt. Kiso Ontake was unprecedented in terms of location and scale. This avalanche caused damage to the trail and Ninoike mountain lodge. On the same day, a collapse of the snow layer and mudflow occurred in the upper Yukawa River downstream from Ninoke crater. Similar collapse and mud flow were observed at Yukawa River in May 2011, suggesting that small collapse of snow layer and mud flow generation are common at Yukawa in snowmelt season. Ontake was erupted in September 2014, resulted in cohesive muddy volcanic ash deposits around the summit area. Not only appropriate weather and snow conditions but the presence of volcanic ash very likely promote sliding of the snow layer. In order to clarify the generation process of this avalanche, we carried out field observation. The sediments transported by the avalanches and water samples were analyzed. In addition, simple snow block sliding experiments were conducted in order to investigate snow conditions and the effect of volcanic ash.
As a result, several sharp-edged crown fractures were observed at the slope between Ichinoike and Ninoike craters. Snow slabs deposited at Ninoike maintains sedimentary structures formed before the avalanche generation, indicating the slabs simply slid the slope. Although Kunitomo et al. (2019) reported this avalanche was occurred as a slush avalanche, our results indicate the avalanche was generated as a surface-onset full-layer avalanche. At Yukawa River, the gully cracks were formed and slush was deposited on the levees, suggesting the avalanche was transformed to slush flows after entering Yukawa River.
Muddy volcanic ash derived from the 2014 eruption was deposited on the bed surface. The ash contains gypsum and the water sampled at Ichinoike and Ninoike craters showed high concentration of Ca and SO4. These muddy gypsum bearing ash possibly promoted the avalanche generation. Simple snow block sliding experiments indicated that not only appropriate weather and snow conditions but the presence of volcanic ash promote sliding.
We concluded that the avalanche was possibly promoted by muddy water with volcanic ash in addition to appropriate weather and snow conditions (heavy rains and rapid temperature increasing around 0 degrees C).
As a result, several sharp-edged crown fractures were observed at the slope between Ichinoike and Ninoike craters. Snow slabs deposited at Ninoike maintains sedimentary structures formed before the avalanche generation, indicating the slabs simply slid the slope. Although Kunitomo et al. (2019) reported this avalanche was occurred as a slush avalanche, our results indicate the avalanche was generated as a surface-onset full-layer avalanche. At Yukawa River, the gully cracks were formed and slush was deposited on the levees, suggesting the avalanche was transformed to slush flows after entering Yukawa River.
Muddy volcanic ash derived from the 2014 eruption was deposited on the bed surface. The ash contains gypsum and the water sampled at Ichinoike and Ninoike craters showed high concentration of Ca and SO4. These muddy gypsum bearing ash possibly promoted the avalanche generation. Simple snow block sliding experiments indicated that not only appropriate weather and snow conditions but the presence of volcanic ash promote sliding.
We concluded that the avalanche was possibly promoted by muddy water with volcanic ash in addition to appropriate weather and snow conditions (heavy rains and rapid temperature increasing around 0 degrees C).