Mass Estimation and Characteristics of Ejecta from the 2004 Eruption of Asama Volcano(＜Special Section＞The 2004 Eruption of Asama Volcano(2))

Abstract

After 31 years of dormancy, magmatic eruption started at Asama volcano on 1 September 2004. Five major vulcanian explosions, intermittent strombolian explosions and many small-scale explosions were observed from September to December. The initial explosion on 1 September was the largest explosion and the volcanic ash affected areas up to 250km northeast of the volcano. The maximum grain size and the weight of ash collected about 4km NE of the vent were 96mm and 1,000g/m², respectively. Ash from the strombolian explosions during 16-17 September covered most of Karuizawa Town, including areas SE and E of the volcano. Ash fall also affected the Tokyo District on the night of 16 September. The main axes of ash fall on 23, 25, and 29 September, 10 October, and 14 November were dispersed toward NNE to NE, NE, N to NNE, NE, and E, respectively. The distribution of ash fall deposit was thought to be influenced by wind direction and velocity. Ballistics from these explosions were thrown at least 2km away from the vent, and the nature and proportion of rock fragments deposited on the volcano flank differ, depending on the size and type of explosions. The total thickness of proximal ash fall deposits from the 1 September and 14-18 September explosions varies from less than 5mm at the northern rim of the crater to 50mm at the southern rim. Using isopleth maps and excluding ballistic ejecta, the calculated amounts of ash deposited within the area of 1×10⁹m² are as follow; 4.9×10⁷kg, 5.9×10⁷kg, 8.5×10⁶kg, 3×10⁵kg, 1.3×10⁷kg, 2.8×10⁶kg, and 2.5×10⁷kg on 1, 15-18, 23, 25, and 29 September, 10 October, and 14 November, respectively. The total amount of ash generated during the 2004 eruption is more than 1.6×10⁸kg, but still small compared to the lava that formed within the crater.