雲仙岳1992年噴火における火山豆石の生成条件 : 雲仙岳噴火とその噴出物, 第2報

In the Unzendake eruption since May 24th, 1991, frequent ash falls rich in accretionary lapilli were observed to be originated from ash clouds accompanied with running down pyroclastic flows. To clarify the process and condition for generating fine ash falls including accretionary lapilli from pyroclastic flows, we investigated distribution, occurrences, grain-size, and water content of the ash fall deposits accompanied with pyroclastic flows in April 1st, July 14th, and September 27th, 1992. Grain-size distribution of the ash fall in July 14th is similar to each types of occurrences; mud rain, flattenned and spherical accretionary lapilli, and ash aggregates. The ash fall in September 27th shows decrease in the proportion of particles finer than 6φ (＜16μm) because of sorting during their transport. No visible internal structures are found clearly in many specimens of accretionary lapilli, except for outer layer, thickness of about 40 micron, composed of fine ash, but about 10% of them have obscure core and mantle-like structures. On the basis of the measurement of water content in ash fall deposits rich in accretionary lapilli, difference in occurrences of ash fall deposit depends chiefly on the water content; mud rain formed in water content of 20-25%, flattened accretionary lapilli 15-23%, and spherical accretionary lapilli 8-22%, in the case of April 1st. Water content of ash aggregates is generally much lower than that of spherical accretionary lapilli. Using the water content measured and the weight of ash fall deposits estimated, the amount of water included in all ash fall deposits fallen as accretionary lapilli was calculated at 1,300 t for the April 1st and 250 t for the July 14th, respectively. If the water content of dome lava are assumed to be 1.5 wt%, the total amount of water in dome lava blocks collapsed is only 10%-20% of these. On the other hand, using meteorological data, the amount of water included in the atmospheric air of 1 km^3 is estimated about 11,000 t for the April 1st and 15,000 t for the July 14th, respectively. This amount is much more than the total amount of water included in the ash fall deposit. Thus, even if only a part of the actual amount of water are condensed, its amount may be enough to change for all ash fall deposit to spherical accretionary lapilli. In the September 27th, no accretionary lapilli were generated because the ash cloud didn't reach to the dew point height. Genemte condition of accretionary lapilli in the Unzendake 1992 eruption are summarized as follows: 1: The atmospheric air are risen up vertically over dew point height owing to buoyancy, while mixing with hot ash cloud. 2: Fine ash particles are gathered and adhered to dew drops condensed from vapor included in the air, and form accretionary lapilli. 3: Accretionary lapilli generated are transported by wind and fall mainly in and around Shimabara City and Fukae Town.