Quaternary Volcanoes > Active Volcanoes > Fuji

5: Petrological characteristics of the eruption products - 6: Conclusions

5: Petrological characteristics of the eruption products
  The majority of eruption products from Fuji Volcano are basalt, with much smaller amounts of basaltic andesite, andesite, and dacite. Various types of basaltic rock have been erupted throughout the volcano’s eruptive history; these range widely from aphyric basalt to porphyritic basalt with phenocrysts comprising various combinations of plagioclase, olivine, clinopyroxene, and orthopyroxene. Meanwhile, basaltic andesite rocks only occur in volcanic products from the Subashiri-c Stage, and andesite and dacite rocks only occur in specific stratigraphic units, namely Subashiri-c Stage S-13 (Zunasawa) Scoria Fall Deposits (Machida, 1964, Miyaji, 1988, not included in the new geological map) and Subashiri-d Stage Hoei EP. The phenocryst assemblages of these rocks are highly uniform, i.e., lacks diversity.

  Whole-rock chemical compositions of basalt products during the whole eruption stages show a range of SiO2 (48.7 to 51.9 wt.%), MgO (4.2 to 6.5 wt.%), K2O (0.47 to 1.00 wt.%), FeO*/MgO ratio (1.7 to 2.8). As basalt rocks, MgO contents tend to decrease and K2O contents tend to increase with increasing SiO2, and wide variability is observed in K2O contents and the FeO*/MgO ratio (Togashi et al., 1991; Takahashi et al., 1991 2003; Yoshimoto et al., 2004; Ishizuka et al., 2007; Kaneko et al., 2010, 2014), indicating that highly differentiated basaltic magmas have erupted over the course of the volcano’s eruptive history (Fujii, 2007). The mean whole-rock SiO2, MgO, and K2O contents of each stratigraphic unit included in the new geological map are presented in Appendix 1.

  The petrography and whole-rock chemical compositions of products vary by eruption stage, but these differences appear to be the result of incremental changes. Hoshiyama Stage products predominantly comprise olivine basalt with small (3 mm or less) plagioclase phenocrysts and low incompatible element contents (Togashi, 1991; Takahashi et al., 1991; Togashi and Takahashi, 2007). Analysis of boring core samples collected on the western and southern foot of the volcano indicates that the magma compositions of ejecta changed incrementally from the end of the Hoshiyama Stage to the Fujinomiya Stage (Togashi et al., 1997; Miyaji et al., 2001). With regard to the magma plumbing systems of the Hoshiyama Stage, Kaneko et al. (2010) analyzed the whole-rock chemical compositions and melt-inclusions of Old-Tephra layers from Ko-Fuji Volcano and proposed a model wherein large amounts of basaltic magma from a deep magma chamber mixes with smaller amounts of andesitic magma from a shallow magma chamber before being erupted.

  The petrography of Fujinomiya Stage eruption products differs from that of the underlying Hoshiyama Stage products and is characterized as basalt containing large (4 to 12 mm) abundant plagioclase phenocrysts with occasional aphyric basalt. Variability in K2O contents and FeO*/MgO ratio of the products increases with increasing SiO2 contents. Analysis of boring core samples collected on the southern and western foot of the volcano reveals clear differences in FeO*/MgO ratio between older (1.7 to 2.3) and younger (2.2 to 2.8) Fujinomiya Stage LFs. This compositional change is estimated to have occurred during the Fujinomiya Stage, sometime between 9600 and 8600 cal BC (Yamamoto et al., 2007).

  Subashiri Stage scoria fall deposits exhibit systematically higher SiO2 content than the Old-Tephra Layers analyzed by Machida (1964). This change is attributed to the advancement of magmatic differentiation in the shallow magma chamber during the quiescent period of the Subashiri-a Stage (Kaneko et al., 2014).

6: Conclusions
  Fuji Volcano has not erupted since the Hoei eruption of 1707 AD and appears, at least superficially, to be inactive. Indeed, as of 2015, no fumarolic activity is observed. However, repeated deep low-frequency (DLF) earthquake activity has been recorded at a depth of 10 to 20 km below Fuji Volcano since the 1980s. Between September, 2000 and May, 2001, the occurrence rate of DLF earthquake activity increased by more than one order of magnitude over previous periods, resulting in a so-called “earthquake swarm” (Ukawa, 2005, 2007). Although DLF earthquake activity has since returned to previous levels, the earthquake swarm left a strong impression regarding the potential risk of an eruption in Fuji Volcano. It is in this context that the Volcanic Disaster Mitigation Maps (Hazard Maps) of Fuji Volcano was developed and published in 2004 under the leadership of the Cabinet Office (Aramaki, 2007).

  It goes without saying that detailed knowledge of a volcano’s eruptive history is essential for predicting future eruptions. Miyaji (2007) showed the change in Fuji volcano’s eruption frequency (Cumulative volume for the total products) over its 110 ka eruptive history, while providing greater detail for activity in the last 2,200 years. As is evident from the new geological map, it is possible to delineate distinct periods, or stages, of continuous activity (e.g. Subashiri-a, -b, -c, and-d Stages) lasting anywhere from one thousand to several thousand years during which similar volcanic activity occurs repeatedly. The dominant type of eruptive activity changes as the activity stage changes. Furthermore, it is difficult to predict in what direction such change will occur. Effusive lateral eruptions resulting in LFs, which began to dominate around 300 BC, clearly began to lose prominence around 1200 AD. The explosive Hoei eruption of 1707 AD was unique given the prior activity during the Subashiri-d Stage. Taking into consideration past changes in dominant type of volcanic activity, there is a strong possibility that Fuji Volcano is entering or has already entered a new activity stage. This only increases the difficulty of predicting what type of activity will occur next. The hazard map published by the Cabinet Office, which attempts to address the various eruption types that could occur, reflects the eruptive history and characteristics of Fuji Volcano and provides a realistic basis on which to develop disaster prevention measures. Although the detailed eruptive history provided by this geological map may end up unintentionally highlighting the uncertain nature of predictions regarding eruption type, it is hoped that future disaster prevention plans for Fuji Volcano will be developed with a firm understanding of this uncertainty.

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