Tephrochronologyによる富士火山とその周辺地域の発達史

-第四紀末期について- (その1)

抄録

Mt. Fuji, the highest peak in Japan, is one of the largest and most typical strato - volcano of basaltic composition. It was shown in a previous paper (Tsuya, 1940) that this volcano consists structurally of three parts, “Komitake”, “Old Fuji”, and “New Fuji”, erupted successively in the order mentioned. This result was obtained from geological and petrological studies of the volcanic ejecta, especially of lava flows and mudflow deposits. As yet few investigations have been carried out on the stratigraphy of pyroclastic fall deposits or “tephrochronology”, although such studies help to clarify some of the remaining problems such as the chronology and the nature of volcanic activity and chronological relationships between the evolution of the landform of the adjacent areas and the development of the volcano itself.
In the present paper, the writer deals first with the stratigraphic division and chronology of the pyroclastic fall deposits on the lower slopes of the volcano and its surroundings, and then applies these results to the chronology of the activities of the volcano.
1) Attention has been paid to the time gaps during deposition as indicated by soil profiles and slight unconformities. Consequently the pyroclastic fall deposits are divided into three groups ; the older tephra, the black humic ash and the younger tephra (Fig. 2). These groups represent the old activity, quiescence, and younger activity respectively. The distribution of the younger tephra is shown in Fig. 4 and certain beds are shown in more detail in Fig. 3. Their volumes are tabulated in Table 1.
2) Both human remains, such as stone instruments and pottery, and the results of carbon - 14 dating afford information regarding the absolute age of the tephra groups. From these results it is inferred that the older tephra were erupted during the upper Pleistocene or the last glacial age, and that the black humic ash was formed from approximately the beginning of Holocene to middle Holocene, and the younger tephra were erupted later.
3) The horizons of various volcanic ejecta were ascertained as follows; The mudflow deposits of the “Old Fuji” volcano (in the Tsuya sense) are found conformably within the layers of older tephra, and many of the earliest lava flows of the “New Fuji” volcano are found in a horizon in the top part of the older tephra or near the base of the black humic ash layers, and finally various new lava flows with a considerable proportion of pyroclastic flow deposits, which are exposed on the flanks, are found within the layers of the younger tephra. These results are illustrated in Table 2.
4) From these facts it is pointed out that the activity of Volcano Fuji are chronologically divided into three stages ; 1) Older Fuji I, 2) Older Fuji II, 3) Younger Fuji. The first stage is characterized by explosive activity, erupting a large amount of pyroclastic “fall” as well as “flow” deposits attended by basaltic lava flows at several altitudes.The second stage of activity comes successively from the predecessor. The activity is considered to be mostly effusive, erupting a large number of basaltic lava flows with a small amount of pyroclastic material. A long quiescence, in which only fine ash fell intermittently, was followed by the last stage of activity, which was a mixed type of eruption in which lava flows and pyroclastic materials both play significant parts. These results are summarized in the right column of Table 2.