風化火山灰土の母材の起源(＜特集＞堆積物による火山噴火史研究)

抄録

Successive terrestrial coverbeds called “volcanic ash soils” are widely distributed in Japan. In this paper, the origin of parent material of volcanic ash soils is discussed from the stratigraphy and the mineralogy of volcanic ash soils to clarify the accumulation of volcanic ash soils. In the Hakkoda Mountains and in the southern part of Tokachi Plain, thickness of tephra interbeds recognized within peaty soils are almost the same as those within the volcanic ash soils. No more distinct tephra interbed is observed in the volcanic ash soils with the exception of the marker tephra interbeds. This fact indicates that the accumulation of the volcanic ash soils have occurring through the periods without tephra deposition. Moreover, the existence of soil developed on the latest Hokkaido-Komagatake tephra of 1929, also suggests that the accumulation of volcanic ash soils is almost independent of the tephra deposition. Mineralogical composition of the fraction coarser than 0.02 mm in the volcanic ash soils are mainly derived from the underlying tephra interbeds. On the other hand, the finer fraction contains amorphous clay minerals and non-volcanogeneous materials such as quartz and illite. The former is weathering products of tephra and the latter would be derived from eolian dust. In particular, several studies on oxygen isotope composition have revealed that the origin of fine quartz is the eolian dust. Accumulation rate of fine materials in the volcanic ash soils ranges from 10^-1 to 10⁰ mg cm^-2 y^-1. It agrees well with those of paleosols intercalated into sand dunes along the coast of the Japan Sea, of pelagic sediment of the Japan Sea, and of eolian dust fall calculated from atmospheric dust density. From the evidence mentioned above, parent material of volcanic ash soils is originated from the reworked materials of tephra with the fine materials of the eolian dust.