The Weathering State of Volcanic Ashes and Clay Mineral Formation at Kuchinoerabujima Island

Kuchinoerabujima is a small volcanic island which is situated at Tokara Islands, southern part of Kyushu, Japan. And the climate of the island, one of the factors affecting weathering, is warm and very wet.<BR>In the island, alteration phenomena of the sedimentary volcanic ashes which belong to augite-hyperthene andesite were observed. And the sequence of changes of the sediments from upper to lower layer was studied mineralogically and chemically, and following results were obtained.<BR>1) At surface layer, amorphous silico-alumina and hydrous sesquioxides as secondary minerals are detected. The amount of those clay-size materials is very little in quantity.<BR>2) In the second layer, the decomposition of plagioclase obviously proceeds and allophane, halloysite and gibbsite are formed. The grain size of those minerals is generally very small except gibbsite, which cementizes the particles of volcanic ashes and grows about 10-20μin size.<BR>3) In the third layer which developes at the part of 2-3 m from the surface (vertical width 0.5-2 m), halloysite, illite, 14Å-mineral and iron hydroxide are found. This layer is very clayey and is dark-brownish in color and also it represents an illuvial zone. The amounts of Si0₂, Fe₂0₃, K₂0 and Mg0 of the clay separated from this layer are relatively rich compared with the components of the clays obtained from other layers.<BR>4) Under the third layer, allophane and gibbsite are mainly formed when the layer is composed of coarse particles. And as the layer becomes more clayey, halloysite content is more rich. The molar ratio, Si0₂/Al₂0₃, of the clay is about 2.