Reconstruction of Forests buried by Fuji 1707 Tephra on the Southeastern Slope of Fuji Volcano, Central Japan1) Naomichi MIYAJI2), Shuichi NOSHIRO3) a

Size: px

Start display at page:

Download "Reconstruction of Forests buried by Fuji 1707 Tephra on the Southeastern Slope of Fuji Volcano, Central Japan1) Naomichi MIYAJI2), Shuichi NOSHIRO3) a"

ようじろうはかまや
4 years ago
Views:

1 Reconstruction of Forests buried by Fuji 1707 Tephra on the Southeastern Slope of Fuji Volcano, Central Japan1) Naomichi MIYAJI2), Shuichi NOSHIRO3) and Mutsuhiko MINAKI4) The eastern and southeastern slopes of Fuji Volcano which are covered with Fuji 1707 Tephra lack subalpine coniferous forests. At three sites on the southeastern slope, geological and paleobotanical studies were made on the fossil forests buried under this tephra. This pyroclastic fall deposit ejected from the Hoei Craters of December 16, 1707 A.D. consists of a lower pumice bed (Ho-Ia) and upper scoria beds (Ho-Ib) in this area. The wood fossils and other plant macrofossils are buried in or beneath the Ho-Ia, which consists of many bombs and coarse pumice fragments, with a maximum thickness of 70 cm. The occurrence of wood fossils and other plant macrofossils indicate that leaves, branches and stems were buried in that order in the lower pumice bed very rapidly. Most of the wood fossils were carbonized by the heat of the pumice fragments. The forests which existed until December 16, 1707 A.D. were reconstructed. At site MK-4 ( m in alt.), there were stands composed of subalpine conifers such as Abies veitchii, Tsuga diversifolia and Picea jezoensis, one stand of Larix and one small stand of deciduous broad-leaved trees, that were distributed in patches. At site MK-2 (1680 m in alt.), there was a forest of Abies, Tsuga diversifolia, and Picea jezoensis with a few broadleaved trees. At site MK-5 (1630 m in alt.), there was a forest of Abies, Picea jezoensis, and Picea cf. maximowiczii with a few montane-zone elements such as Pterocarya or Pourthiaea. These facts indicate that, prior to the eruption, subalpine coniferous forests and montane ones existed on the southeastern slope of Fuji Volcano, and that the boundary between the subalpine zone and the montane zone was at about 1650 m in alt. around the three sites. The elevation of this boundary is lower than that of the present one on the southern slope of Fuji Volcano. Since then, the forests have been replaced by a Larix forest or a Quercus and Betula forest at sites MK-2 and MK-5 where the surfaces have become relatively stable; whereas, a volcanic desert still exists around site MK-4 situated in the course of an avalanche chute.

3 Fig. 2 Columnar sections at sites MK-2, 4, and 5. 1: scoria. 2: pumice. 3: lithic fragment. 4: coarse volcanic ash. 5: weathered tephra. 6: soil. 7: lava. 8: wood.

6 Taxon Tax: Taxus A: Abies T: Tsuga PA: Picea sp. A PB: P. sp. B P: P. sp. L: Larix Bet: Betula Aln: Alnus Sor: Sorbus Ace: Acer Til: Tilia

13 Explanation of the Plates Plate I Occurrence of buried woods and plant macrofossils, and the embedding Fuji 1707 Tephra. ST: secondarily-deposited pyroclastics. S: scoria bed. P: pumice bed. WS: weathered tephra and soil. 1. Outcrop at site MK-4. Fuji 1707 Tephra overlies the weathered tephra and soil. Buried woods are seen along the boundary. Secondarily deposited pyroclastics cover Fuji 1707 Tephra. lower weathered tephra and soil. The pumice bed is thicker where pumice fragments accumulate. 3. Plant macrofossils in the lower part of the pumice bed at site MK-4. Shoots of Picea jezoensis are seen. 4. Buried, upright Betula stem with roots at site MK-4. The portion above the soil burned and became charcoal. The pumice bed creeps up around the stem. 5. Buried, fallen Taxus stem at site MK-4. The upper surface is burned; whereas, the rest is unburned. Plate III Plant macrofossils obtained from sites MK-2, 4, and 5. (I) T: trichome. R: resin duct. V: vascular bundle. Numbers in the parentheses correspond to Minaki's specimen numbers at Osaka City University.

14 Plate IV Plant macrofossils obtained from sites MK-2, 4, and 5. (II) R: resin duct. V: vascular bundle. Numbers in the parentheses correspond to Minaki's specimen numbers as in Plate III.

15 宮地能城南木: 図版I

宮地能城南木: 図版II 1 2 3 4 5 6 7 8 9 10 11 12 13

16 宮地能城南木: 図版II

地質調査研究報告/Bulletin of the Geological Survey of Japan

地質調査研究報告/Bulletin of the Geological Survey of Japan Shun Nakano, Akira Takada, Yoshihiro Ishizuka, Yusuke Suzuki, Tatsuro Chiba, Ken-ichi Arai, Makoto Kobayashi and Yasuhisa Tajima (2007) Eruption ages of younger-stage lava flows and olderstage pyroclastic