Mount Fuji, an iconic stratovolocano and Japan’s highest mountain 3,776.24 m (12,389 ft), is 100 km south west of Tokyo (and is also one of the few volcanoes I have actually visited -but not climbed!). She is a cultural heritage site. According to UNESCO and Wikipedia, “Mount Fuji has “inspired artists and poets and been the object of pilgrimage for centuries”. The site includes the volcano itself, Fujisan Hongu Sengen Taisha and six other Sengen shrines, two lodging houses, Lake Yamanaka, Lake Kawaguchi, the eight Oshino Hakkai hot springs, two lava tree molds, the remains of the Fuji-kō cult in the Hitoana cave, Shiraito Falls, and Miho no Matsubara pine tree grove” [1,2].
Mount Fuji formed on the foundation of an old quaternary volcano, Komitake, in four phases: the first phase, Sen-komitake, an andesite core was recently discovered deep within the volcano; the second, “Komitake Fuji,” a basalt layer believed to be formed several hundred thousand years ago; the third, about 100,000 years ago, “Old Fuji”, formed over the top of Komitake Fuji; and, fourth and lastly, “New Fuji” formed over the top of Old Fuji .
New Fuji, the current stratovolcano, formed 11,000 to 8,000 years BP with effusive lava flows followed by a three and a half thousand year period of minor explosive eruptions and then major lava flows from 4,500 to 3,000 BP. Later eruptions have been intermittent major eruptions with smaller lava flows and small pyroclastic flows. The Fuji Five Lakes were formed when basaltic lava flows blocked drainage from the Miska Mountains .
Mount Fuji is active. GVP lists 67 eruptions, of which three are VEI 5, two are VEI 4, five are VEI 3, 19 are VEI2, one is VEI 1 and the remaining 37 have no VEI number. The last eruption was in 1707 and a VEI 5 which formed a new crater and also deposited ash on Edo, Tokyo .
Following the 2011 Tōhoku 9.0 Mw earthquake and tsunami  and a 6.2 on 15 March 2011 a few kilometers to the south of Mount Fuji, Mount Fuji was studied in depth to assess the risk of eruption. It was concluded that an eruption was not likely. At the time of writing in May 2014, there is no alert for Mount Fuji.
Mount Fuji’s tectonic setting is classified as continental subduction zone. Plotting the latest 5,000 earthquakes (August 2010 to May 2014) reported by IRIS  we can see that the subduction of the Pacific Plate under the Okhotsk Plate dominates seismic activity in the area:
While being located in a volcanic archipelago, her lavas are: Basalt / Picro-Basalt; and, Andesite / Basaltic Andesite, Dacite . That her major lava type is Basalt / Picro-Basalt, which is similar to those of mid ocean ridges, implies that her lava generation is more complex than straightforward subduction.
Mount Fuji is located at the triple junction of the Amurian Plate, the Okhotsk Plate, and the Philippine Sea Plate. The relative motions of the plates may provide a mechanism to enable large eruptions of basalt.
K Aizawa et al  carried out a wide-band magnetotelluric study on the Mt. Fuji area in 2002 and 2003. They propose that Mount Fuji’s magma chamber is located at a split in the subducting Philippine Sea Plate. Collision of the Izu block with the Honshu block about two million years ago may have caused the split in the Philippine Sea Plate. The resulting tensile stress field in the lower and middle crust beneath Mt. Fuji would allow mantle-derived basaltic magma to ascend easily, similar to the rise of basaltic magma at mid-ocean ridges.
The 1707 eruption
This eruption started after an 8.4 earthquake (the big earthquake) on 28 October 1707 in south west Japan which devastated Honshu Island, followed by smaller earthquakes in December 1707 near Mount Fuji itself. According to Volcano Discovery : the eruptions started on 16 December 1707 with the creation of the new crater and a sub-plinian column of ash and pumice, turning into basaltic lava fountaining six hours after the onset of the eruption. The total volume of lava erupted over 16 days was estimated to 0.68 cubic km. Violent explosions were recorded until 25-27 December, before eruptive activity dropped, ending on 1st January 1708. Tsuya, H  gives a more detailed account of the eruption itself, which I recommend.
Is it possible that the plate movement associated with the big earthquake of 1707 resulted in a tear in the crust which permitted the rapid ascent of magma? Whether or not the big earthquake was sufficient or whether there had been other earthquakes (not reported) is an open question.
And lastly, a brief reminder that there is no alert for Mount Fuji at the moment. Current alerts for Japanese volcanoes can be found here: http://www.jma.go.jp/en/volcano/
Hope you enjoyed reading this. The usual caveats apply: “Not an expert, etc ….”
KarenZ, 15 May 2014
1. Wikipedia: http://en.wikipedia.org/wiki/Mount_Fuji
2. UNESCO: http://whc.unesco.org/en/list/1418/
3. GVP: http://www.volcano.si.edu/volcano.cfm?vn=283030
4. Wikipedia: http://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami
5. Aizawa, K., R. Yoshimura, and N. Oshiman (2004), “Splitting of the Philippine Sea Plate and a magma chamber beneath Mt. Fuji”, Geophys. Res. Lett., 31, L09603, doi:10.1029/ 2004GL019477. http://onlinelibrary.wiley.com/doi/10.1029/2004GL019477/abstract
7. Tsuya, H “Geological and Petrological Studies of Volcano, Fuji, V.: 5 On the 1707 eruption of Volcano Fuji.”, Bulletin of Tokyo University Earthquake Research Institute. 33 books (3), 1955.12.10, pp. 341-383. http://repository.dl.itc.u-tokyo.ac.jp/dspace/handle/2261/11819
8. Earthquake data from IRIS: http://www.iris.edu/ieb/index.html
9. Japan Meteorological Agency: http://www.jma.go.jp/en/volcano/
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