THE VOLCANIC ACTIVITY AND VOLCANOGENIC IMPACT ON NATURAL ENVIRONMENT OF MT FUJI, BASED ON THE LAKE BOTTOM SEDIMENTS FROM FUJI FIVE LAKES AROUND FUJI VOLCANO, CENTRAL JAPAN

Mount Fuji volcanic activity began about 100,000 years ago, and pyroclastic materials and lava flows had erupted actively in the past 10,000 years. A giant latest eruption happened about 300 years ago in the Edo Period, and pyroclastic falls occurred in the capital Edo. Five lakes (Fuji-goko), such as Motosu Lake, Shoji Lake, Sai Lake, Kawaguchi Lake and Yamanaka Lake existed on the north foot of Fuji volcano, dammed by lava flows from Mt Fuji.

Research has been done in our Institute from 1997 to study the volcanic activity from the lake sediments of Motosu Lake, Kawaguchi Lake and Yamanaka Lake in Fuji-goko at the north foot of Mt Fuji and which then can be read, an influence on the natural environment of that and environment change history from the viewpoint concerning study of the Quaternary stratigraphy.

Tephra such as scoria falls and lava flows of Mt Fuji origin were deposited in the lake sediments of Motosu Lake, Kawaguchi Lake and Yamanaka Lake. The importance of tephra and lava flows vary with the location of each lake. Lava flows dominate the lake bottom sediemnts of Motosu Lake located at the northwestern foot where many parasitic volcanoes occur, and there are a few tephra and lacustrine deposits. Fine and coarse grain-size sediments are dominant in such cases as mud and sand and gravel, and the lava flows in central part and tephra occur in the lake sediments of Kawaguchi Lake located at the north foot. Tephra such as scoria and lapilli are dominant in the lake sediments of Yamanaka Lake located at the northeastern foot, and lava flows are not found.

A microfossil assemblage analysis, especially fossil pollen, opal phytolith and diatom, was carried out to explain changes in natural environment around these each lakes and the influence of the volcanic activity. The local pollen zone is subdivided into four pollen zones in Mount Fuji north foot area in descending order, based on pollen analysis of bottom sediment from Kawaguchi Lake, as follows;

I. pollen zone; the increase of Pinus subgenus Diploxylon

II. Upollen zone; the increase tendency of Cryptomeria and the appearance of Cyclobalanopsis

III. pollen zone; the increase and decrease of Quercus, accompanying of the broadleaf tree pollen

IV. pollen zone; the appearance of the conifer pollen of the high rate and the increase and decrease of the pollen of the broadleaf tree.