Geological reports of Shimane University

Geological reports of Shimane University 6
1987-06-30 発行


Accretionary lapilli in the Tokibariyama Formation of Dogo,Oki Islands
Yamasaki, Hirofumi
Accretinary lapilli are found involcaniclastic rocks in the Tokibariyama Formation is composed of lava flows and volcaniclastic rocks ranging from intermediate to felsic in property. The volcaniclastic rocks containing the lapilli consist mainly of subaqueous pyroclastic flow deposits. The lapilli are commonly spheroidal and elliptical and range in size from l to 6 mm, but occationally broken. Each of them consits of a main part (an inner coaser part with or without a core and an outer finer part) and a surrounding outer shell part. Concentrically layered structure is usually observed in the cross section.A double lapillus (MOORE & PECK, 1962) and a lapillus with an inner coaser part in unique shape (Fig.5-5) are recognized . The lapilli are composed entirely of clastic volcanic materials similar to those of the matrix, but finer in grain size. Judging from the character of the internal structure and mode of occurrence, the lapilli were formed in the air and falled in to pyroclastic flow deposits during transporta, tion and/or deposition of them. Some of the lapilli are presumed to have been formed by agglutination of particles through the medium of ice.