| Research Abstract |
The internal structure of the accretionary lapilli (AL) is divided into the two groups ; concentric (CONC) and ramdom (RAND). The difference of the structure is originated whether the binder of ash particles is ice or water ; in the case of ice, the structure of AL is CONC, while water' that is RAND.On the case of RAND, the 1991 eruption of the Unzen Volcano supports this opinion.
After the detail observation of internal structure and measurement of diameter, deformation degree, porosity and specific gravity of AL, it is clear, that these characteristics of AL relate closely to the effusion types of pyroclastics, that is, pyroclastic flow, pyroclastic fall and base surge.
The CONC occurs in fall and flow deposits, while the RAND does in fall and in surge deposits. In general, the following trends are recognized : in comparison with the CONC, the RAND has smaller porosity and diameter/ larger bulk density/ larger amount of crystals and rockfragments and smaller amount of glass flakes.
In the Goga pyroclastics, Okinawa-jima Island, southwestern Japan, in contrast with the general examples, the two groups of AL were found. The results of chemical analysis of glass flakes show, that the matrics of the pyroclastics and RAND are of the same origin, while CONC is different from them. This suggests, that the CONC was generated in the higher eruption column different from the lower column in which RAND was formed.
AL are very important constituents to know the characteristics of the mode of eruption. On the basis of detal study of AL, the effusion types of the qyroclastics are determined.
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