Introduction to Landforms and Geology of Japan

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Fuji Volcano — a beautiful huge body

Eruption history
Cross section of Fuji
Figure 6: Schematic cross section
Based on Nakada et al., 2007; not to scale

Old volcanoes, Pre-Komitake, Komitake, and Ko-Fuji (old Fuji) are buried in the body of Fuji (also called Shin-Fuji [young Fuji] in distinction from Ko-Fuji). The formation of Fuji started about 100,000 years ago on Neogene sedimentary rocks and igneous rocks and Quaternary volcanic products (mainly lava) of Pre-Komitake and Komitake. The history of Fuji is divided into two periods: the Ko-Fuji period and Shin-Fuji period. (The history is based on the description in Nihon University Department of Geosystem Sciences 2006. In other references, it may be different; for example, some papers explain that the eruption of Fuji started 80,000 years ago.)

Before the Fuji volcanic activity, Pre-Komitake and Komitake were formed. The detail histories of these volcanoes are unknown. Pre-Komitake was active from 400,000 to 100,000 years ago. Volcanoes adjacent to Fuji, Ashitaka and Hakone, erupted in the same period. The rock type of Pre-Komitake is andesitic similar to that of Ashitaka, differing from the basaltic lava of Fuji. The activity of Komitake began after the start of Pre-Komitake activity (the beginning period is unknown) and ended about 100,000 years ago. Komitake is a basaltic volcano like Fuji.

The activity of Ko-Fuji started on the south slope of Komitake 100,000 years ago and lasted until 11,000 years ago. Ko-Fuji ejected mainly tephra (lapilli and volcanic ash) and less frequently lavas. Sector collapses repeated to form volcanic fans in the late stage. The crater at the summit and NW-SE fissure craters spouted a massive amount of lava in the end stage (13,000 to 8,000 years ago). The eruptions in this stage were accompanied with tephra fall (until 11,000 years ago).

The develop course of Shin-Fuji is divided into five stages.

Stage 1: Eruptions with a massive amount of lava (11,000 to 8,000 years ago)

The crater at the summit and NW-SE fissure craters spouted a vast amount of lava (about 39 km3). Lavas flowed to expand the skirts except the eastern foot.

Stage 2: Minor eruptions (8,000 to 4,500 years ago)

Fuji intermittently erupted with tephra on a small scale. In the final phase of this stage (5,000 to 4,500 years ago), large explosive eruptions occurred with red scoria and pyroclastic surge.

Stage 3: Summit and flank eruptions (4,500 to 3,200 years ago)

The crater at the summit repeatedly ejected a large amount of lava (about 3 km3), which was block lava with relatively high viscosity. The lavas mainly flowed down on the northwest, west, and southeast slopes. Many parasitic volcanoes were produced in a NW-SE zone (Figure 2). Fuji grew to almost the present height around 3,000 years ago.

Stage 4: Explosive summit eruptions (3,200 to 2,200 years ago)

Explosive eruptions with tephra including scoria repeated on a relatively large scale. Lava was also spouted out at the summit between ejections of tephra. The last large magmatic eruption occurred 2,200 years ago. Scoria deposited on the foot of Fuji and a welded scoria layer formed in the summit area. Fuji has had no large magmatic eruptions at the summit since this eruption, although there were minor phreatic explosions.

Stage 5: Flank eruptions (2,200 years ago to AD 1707)

Small eruptions with lava and tephra formed many parasitic volcanoes on northwest, northeast, and southeast slopes and foots (Figure 2). An eruption in 864 was the largest event in historic times and the volume of ejected magma was about 1.4 km3. On December 16, 1707, an eruption with tephra started on the southeast slope and ended on January 1, 1708 (called the Hoei eruption). A massive amount of tephra (0.7 km3), which was the largest volume in the Shin-Fuji period, covered the south Kanto region. This event formed three explosion craters and one mound, which are found on the flank of the present Fuji (Photo 4). The eruption was a different type from other eruptions. Fuji erupted explosively with an eruption column and spouted dacite pumice first, then andesitic pumice, and finally a large volume of scoria (basalt). Ejecting dacite and andesitic pumice before basalt coming out has not been known in Fuji’s eruptions other than the Hoei eruption. The eruption type was the Plinian eruption, which gushes up bubbled magma containing a large amount of gas to over 10,000 meters high. Plinian eruptions are rare in basaltic volcanoes.

Hoei crater

Photo 4: Hoei crater

Basaltic Fuji

Common volcanoes in island arcs are andesitic (intermediate), but Fuji is a huge basaltic volcano. In Japan, major basaltic volcanoes are only three volcanoes, Fuji, the Izu-oshima volcano, and the Miyake-jima volcano. The Izu-oshima volcano and the Miyake-jima volcano are on the Izu-Bonin Arc, which are 92 km and 160 km to the south of Fuji, respectively.

The evolution process of island arc volcanoes is described in Section Volcano. Basaltic magma provided from the upper mantle into the crust heats up and melts the surrounding crust to produce andesitic or rhyolitic magma because the crust is rich in silica. Also, basaltic magma may change into andesitic or rhyolitic magma by crystallization differentiation (see Volcano, p. 1). Therefore, andesitic volcanoes are common in island arcs even though volcanoes discharge basaltic lava in their early stage. Fuji has ejected a vast amount of basaltic magma for a long time compared to other volcanoes, and the basalt derived from evolved magma (but still basaltic).

The huge body of Fuji is attributed to the provision of a considerable amount of basaltic magma, but the mechanism of the providing system has not been elucidated yet. One of presumptions is that basaltic magma easily comes from the upper mantle through fissures under Fuji (e.g. Nihon University Department of Geosystem Sciences, 2006). Once it was believed that the Philippine Sea Plate rifted under around Fuji based on observations of seismic waves, but afterward, more precise analyses of seismic tomography revealed the Philippine Sea Plate has no rift (Hasegawa et. al., 2010).

Fuji has not eroded very well, maintaining its beautiful edifice. This, however, makes geological survey difficult because there are no gorges cutting the body to expose Fuji’s internal structure. Many pieces of the puzzle are still missing in Fuji.

Future eruption

Eruption and erosion change the shape of stratovolcano (see Volcano, p. 2). Although it is difficult predict how Fuji would change the shape, if an eruption leading to a collapse of a part of the body like the 1980 Mount St. Helens in the US (Wiki) occurs at Fuji, the present beautiful form may be lost. Many Japanese are concerned about Fuji’s next eruption since the 2011 off the Pacific coast of Tohoku earthquake because the 1707 eruption took place 47 days after an strong earthquake of magnitude 8.4 (hypocenter: off the Kii Peninsula). The earthquake may have stimulated the magma chamber and induced the eruption. Therefore, the 2011 earthquake might have affected the stress of the crust around Fuji and increase the risk of eruption. Moreover, experts suggested a huge earthquake is likely to occur at the Nankai Trough in the near future.

References

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