Abstract
A large scale pyroclastic flow deposit is distributed to the north of the Aira Caldera, South Kyûshû. The deposit, called Ito pyroclastic flow deposit, occupies the area lower than about 400m in altitude surrounded by higher basement mountains.
The deposit attains to 160m or more in maximum thickness, and it looks homogeneous in texture from top to bottom, except the welded zone. Consequently, it is very difficult to separate the deposit into some different flow units.
In the eastern half of the area studied, welding has taken place in the lower part of the deposit (Figs. 3, 4, 7). The welding ranges from incipient to dense. In the welded zone, various features such as color, hardness, texture, etc. change transitionally as exemplified by the systematic vertical variations of dry density (Fig. 7). The features and the pattern of zoning owing to the welding of the deposit indicate that the deposit constitutes a simple cooling unit.
Grain sizes of li.thic fragments contained in the deposit decrease with the increase of the distance away from the Aira Caldera. This fact suggests that the source of the deposit is located there (Fig. 10).
The date of eruption of the Ito pyroclastic flow, determined by the 14C of the carbonized trunk contained in the deposit, is about 16, 000-27, 000 y. B. P.
Although the deposit has been deeply dissected and modified into mesas or buttes (the so-called Shirasu Plateau), some of their surfaces are very flat and even, and thought to be original surfaces. The altitude of the surfaces increases gradually with increasing distances away from the Aira Caldera, i. e., the source of the deposit (Fig. 11). In other words, these surfaces incline gently at an angle of less than about 3 degrees as a whole toward the source. Such a surface of pyroclastic flow deposit inclining toward the source is here named “a reversely inclining surface”. The cause of the reversely inclining surface is discussed below on the basis of topography, general structure of the deposit, general geologic setting, etc.
Firstly, the distribution of the Ito pyroclastic flow deposit is restricted as a whole to the lower area of the basement as mentioned before. Within the lower area, thickness of the deposit before welding as well as that after welding, i. e., that at present, varies in response to the changes in general relief of the basement (Figs. 4, 5, and 6). Thus the area where the deposit is the thickest coincides with the lowest area of the basement. Similarly, not only thickness of the welded zone but also degree of welding of deposit change concordantly in response to the areal variation in thickness of the deposit (Figs. 6 and 8). Moreover, the general configuration of the original surface reflects the underlying topography as a whole (Figs. 5 and 11).
Secondly, welding compaction of the deposit was calculated at several localities, using dry density curves of the deposit, in order to estimate subsidence of the original surface due to welding (Fig. 7). Procedure for calculation of the welding compaction is shown in Fig. 12. The calculation is based upon the assumption that the deposit before welding was homogeneous throughout it in dry density, as inferred from nearly equal dry density of upper and lower nonwelded zone shown in Fig. 7. The restored altitude of the original surfaces before welding in different localities are also shown in Fig. 7. Although the restored contour lines of the original surfaces before welding are somewhat different from those after welding, the surface of the deposit before welding remains reversely inclining (Fig. 13). The deformation (subsidence) of original surfaces due to welding is, therefore, thought qualitatively to be not so great as to vary the original surface from normal inclination to reverse one.
