11:30 AM - 11:45 AM
[SVC36-09] Mt. Fuji volcanic edifice and magmatic activity during the Hoshiyama stage inferred from the Mabusegawa debris avalanche deposits
Keywords:Mt.Fuji, debris avalanche deposits, lava blocks, magma
Mt. Fuji is the largest active volcano in Japan located in the northern end of the Izu arc. It became active about 0.1 Ma and formed a body covering the south side of the Komitake volcano, which was active until ~0.1 Ma. With a volume of approximately 400 km3 and an elevation of 3,776 m, the present volcanic edifice has been formed through several collapses and more than several hundred eruptions, repeated effusive of basaltic lava flows, and scattering of scoria and ash associated with explosive eruptions. The active history of Mt. Fuji can be divided into the following three stages based mainly on the differences in eruption style (Takada et al., 2016): the Hoshiyama stage from 0.1 Ma to 17 ka, the Fujinomiya stage from 17 ka to 8 ka, and the Subashiri stage after ~8 ka. In addition, the Subashiri stage is further divided into following four stages: Sb-a from 8 ka to 5.6 ka, Sb-b from 5.6 ka to 3.5 ka, Sb-c from 3.5 ka to 2.3 ka, and Sb-d after ~2.3 ka. Most of the volcanic deposits after the Fujinomiya period, known as Younger-Fuji, are exposed on the surface, and the eruption history is being clarified based on geological surveys. The Hazard Map of Mt. Fuji, revised in March 2021, is based on detailed eruption history since the Sb-b stage. On the other hand, the most of magmatic activity and eruption history of the Hoshiyama stage, known as Older-Fuji, have not been fully understood. The main reason for this is that most of the volcanic edifice and deposits of Older-Fuji are covered by Younger-Fuji.
At the point about 20 km east of the summit of Mt. Fuji, a large outcrop appeared on the slope that had been denuded due to road construction. The outcrop is U-shaped and consists of three sides, northwest, northeast, and southeast, and is about 10 m high and over 200 m long. The top of outcrop is composed of pumice and scoria layer deposited during the AD1707 Hoei eruption. Beneath them are dozens of layers of scoria and ash deposited before the Subashiri stage. These tephra layers are divided into the Subashiri and Fujinomiya stages by the Fuji Black Soil layer, which is about 1 m-thick and suggests a period of marked decline in volcanic activity. In addition, a debris avalanche deposits with a thickness of 1 to 3 m are exposed at the bottom of the outcrop (about 7 to 8 m below the surface). The upper boundary slopes gently toward the east. The layer contains numerous subangular to angular lava blocks ranging in size from several cm to tens of cm. The layers are massive and non-stratified, but some parts have red oxidized structures. On the northwestern side, there are several cavities ~30 cm wide and ~1 m long in random directions, with wood chips and leaf fragments inside, which might be the remains of trees that were caught in the debris avalanche and fell. This layer is correlated with the Mabusegawa debris avalanche deposits in terms of its stratigraphy and distribution. To investigate the origin of the formation, we collected samples of the lava blocks, wood chips, and leaves.
First, Carbon-14 age of the leave is 18,931–18,764calBP (2σ calendar age range). This is the same period as the depositional age of the Lake Tanuki debris avalanche deposits distributed at the southwestern foot of the Mt. Fuji, which suggests that the southwestern to southeastern foot of Older-Fuji edifice collapsed several times at the end of the Hoshiyama stage. In addition, it was found that the lava blocks in the layer were derived from the lava flows that formed the volcanic edifice of Older-Fuji. About 50 lava blocks collected are 0.2 to 5.2 kg in weight and are 6 to 23 cm in size, mostly fresh basaltic lava blocks, but some red oxidized. We conducted thin-section observation and petrographic descriptions of the lava blocks. The texture, modal compositions, and size of phenocrysts are different in each block, which suggests that the blocks were derived from several different lava flows of Older-Fuji. On the other hand, there are many blocks containing plagioclase, olivine, and pyroxene phenocrysts. In particular, plagioclase phenocrysts are mixed with both normal and dusty zoning types, suggesting magma mixing prior to the eruption. Such characteristics have been reported in the many ejecta of Younger-Fuji, which suggests the universal mixing of large amount of basaltic magma originating from the deep part and a small amount of more felsic magma in the shallow part also occurred during the Hoshiyama stage.
At the point about 20 km east of the summit of Mt. Fuji, a large outcrop appeared on the slope that had been denuded due to road construction. The outcrop is U-shaped and consists of three sides, northwest, northeast, and southeast, and is about 10 m high and over 200 m long. The top of outcrop is composed of pumice and scoria layer deposited during the AD1707 Hoei eruption. Beneath them are dozens of layers of scoria and ash deposited before the Subashiri stage. These tephra layers are divided into the Subashiri and Fujinomiya stages by the Fuji Black Soil layer, which is about 1 m-thick and suggests a period of marked decline in volcanic activity. In addition, a debris avalanche deposits with a thickness of 1 to 3 m are exposed at the bottom of the outcrop (about 7 to 8 m below the surface). The upper boundary slopes gently toward the east. The layer contains numerous subangular to angular lava blocks ranging in size from several cm to tens of cm. The layers are massive and non-stratified, but some parts have red oxidized structures. On the northwestern side, there are several cavities ~30 cm wide and ~1 m long in random directions, with wood chips and leaf fragments inside, which might be the remains of trees that were caught in the debris avalanche and fell. This layer is correlated with the Mabusegawa debris avalanche deposits in terms of its stratigraphy and distribution. To investigate the origin of the formation, we collected samples of the lava blocks, wood chips, and leaves.
First, Carbon-14 age of the leave is 18,931–18,764calBP (2σ calendar age range). This is the same period as the depositional age of the Lake Tanuki debris avalanche deposits distributed at the southwestern foot of the Mt. Fuji, which suggests that the southwestern to southeastern foot of Older-Fuji edifice collapsed several times at the end of the Hoshiyama stage. In addition, it was found that the lava blocks in the layer were derived from the lava flows that formed the volcanic edifice of Older-Fuji. About 50 lava blocks collected are 0.2 to 5.2 kg in weight and are 6 to 23 cm in size, mostly fresh basaltic lava blocks, but some red oxidized. We conducted thin-section observation and petrographic descriptions of the lava blocks. The texture, modal compositions, and size of phenocrysts are different in each block, which suggests that the blocks were derived from several different lava flows of Older-Fuji. On the other hand, there are many blocks containing plagioclase, olivine, and pyroxene phenocrysts. In particular, plagioclase phenocrysts are mixed with both normal and dusty zoning types, suggesting magma mixing prior to the eruption. Such characteristics have been reported in the many ejecta of Younger-Fuji, which suggests the universal mixing of large amount of basaltic magma originating from the deep part and a small amount of more felsic magma in the shallow part also occurred during the Hoshiyama stage.