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Fuji Volcano — a beautiful huge body
Photo 1: Viewed from Lake Kawaguchi
The elegant form of Fuji-san (Mount Fuji) attracting everyone is a typical form of stratovolcanoes commonly seen in island arcs and continents. However, Fuji Volcano (hereinafter “Fuji”) is an uncommon volcano in the formation and size.
Fuji has an almost symmetrical cone with long concave slopes connecting with wide skirts. The symmetry and beautiful curves make Fuji elegant. Although symmetrical cones and concave slopes rising ever steeper to the summit are characteristics of stratovolcanoes, there are few volcanoes with a beautiful cone like Fuji. The forms of many stratovolcanoes are asymmetrical or irregular depending on eruptions and erosion. The shape of Fuji varies with viewpoints. The appearance viewed from Lake Kawaguchi (location shown in Figure 1) is mostly symmetrical (Photo 1). As seen from south, it is noticed that the eastern slope is gentler than the western slope (Photo 2). The strong west wind always blowing above Japan (Westerlies) carried volcanic ejecta eastward. More pyroclastic deposits, therefore, settled on the eastern slope, making it gentler than the western slope. Fuji viewed from Lake Yamanaka have protrusions on the both slopes (Photo 3). The protrusion on the left (south) slope is the edge of crater produced by the 1707 (Hoei) eruption and the protrusion on the other side is a part of old volcano, Komitake, that erupted before the Fuji volcanic activity began. The horizontal cross sections of the cone are circular at heights over 2,500 meters in elevation and elliptic with the northwest-southeast (NW-SE) long axis at lower levels (Figure 2).
Photo 2: Viewed from Fuji-shi (on the south side of Fuji)
Photo 3: Viewed from Lake Yamanaka
The protrusion on the left (south) slope is the edge of crater produced by the 1707 (Hoei) eruption and the protrusion on the other side is a part of old volcano, Komitake.
Figure 2: Contours and parasitic volcanoes (orange
Based on Tsuya, 1944
Fuji is a huge stratovolcano, the largest in Japan. The highest point in Japan is 3,776 meters above sea level, on the summit of Fuji. A few volcanoes have summits over 3,000 meters high, but these volcanoes were formed on the basements with high altitudes. Fuji was built on a place with lower elevation near the sea. The volume is about 300 km3 (Chronological Scientific Tables 2002). Hakone Volcano near Fuji is about 200 km3 in volume and Sakurajima Volcano, a very active volcano in the south of Kyushu, is about 40 km3 in volume (Chronological Scientific Tables 2002). The mean volume of Japanese volcanoes is 37 km3 (Moriya 1983).
Figure 3: Shield volcano (left)
and stratovolcano (right)
Fuji can be divided into a lower part with gentle slopes and wide skirts and upper part with steep slopes; the lower part consists of broad volcanic fans and a shield volcano covering the fans and the upper part is a cone (Nihon University Department of Geosystem Sciences 2006). Such double-decker structure contributes to the loftiness of Fuji. Volcanic mudflow and debris avalanche produced the volcanic fans, and lava flows created the shied volcano. The lavas ejected from Fuji are mainly basaltic. Because basaltic lava has low viscosity and flows easily, it often forms shield volcanoes that looks like a warrior's shield. However, the upper part of Fuji is a cone with steep slopes consisting of basaltic lavas and volcaniclastic materials. This unusual basaltic cone was produced by welding of lava.
How did Fuji grow?
Fuji grew with repeated explosive and non-explosive eruptions with a massive volume of basaltic magma. The system that provides considerable amounts of basaltic magma for a long time is uncommon in island arc volcanoes.
Fuji is at a triple junction where the Eurasia plate, North American, and Philippine Sea Plates meet one another. The Philippine Sea Plate is subducting into under the Eurasian Plate in Suruga Bay (Suruga Trough) and into under the North American Plate in Sagami Bay (Sagami Trough). The Izu Peninsula of the Izu-Bonin Arc on the Philippine Sea Plate collided with the Honshu island. Fuji is situated on the north end of the Izu-Bonin Arc. The location of Fuji is a complex tectonic area. (See Figures 2, 3, and 5 in "Japan in a subduction zone".)
Figure 4: Map around Fuji
Two parallel lines show the arrangement of craters or parasitic volcanoes (i.e. the direction of maximum compression [maximum horizontal principal stress]).
Figure 5: Open crack
In a volcano with a cylindrical vent, rising magma expands the vent of volcano with pressure to produce radial fissures. Also, when the region including the volcano is subjected to stress, fissures regulated by the stress are developed. For example, when rock with cracks is compressed, the cracks open at right angle to the direction of maximum compression (open crack). Such fissures are larger than the radial fissures. Magma rises through these fissures and causes fissure eruptions. Accordingly, the form of volcano is elliptic with the long axis parallel to the stress.
Fuji has many parasitic volcanoes that erupted at the flank and foot (Figure 2). The parasitic volcanoes are distributed radially, but the most frequent eruptions occurred on the northwest and southeast slopes. The cross sections of Fuji under 2,500 meters high are elliptic with the NW-SE long axis as mentioned above. Moreover, in Hakone, Izu-oshima, and Higashi (East) Izu Monogenetic Volcano Group, which are volcanoes near Fuji, eruptive craters or parasitic volcanoes are arranged in the similar direction. These facts suggests major fissures trending northwest under the volcanoes and the northwestward principal axes of maximum compressive stress. This regional stress field is caused by which the Izu-Bonin Arc pushes this area because of the northward movement of the Philippine Sea Plate.