Types of Historic Eruptions and Nature of Magma in Northern Japan

The Plinian eruptions caused by intermediate to felsic magmas at Tarumai, Komaga-take and Usu were the most violent and largest in scale, the volume of the pumice deposits amounting to 0.2 - 3.4 km³ for each eruption. Compared with such activities, eruptions caused by mafic magmas are generally mild and small in scale.

Recent tephrochronological studies on the pyroclastic deposits revealed that the volcanic activity during the past 300 years at Tarumai, Komaga-take and Usu recommenced with a strong Plinian eruption after a long period of quiescence for one or several thousand years, followed by formation of lava domes of more mafic composition. Such a sequence of eruption may be interpreted in terms of compositional zoning within the magma chamber.

In bulk chemical composition, the pumice produced by the Plinian eruptions at Tarumai and Komaga-take are intermediate andesite. Similar andesitic magma has also caused Strombolian eruption as in the case of the 1970 - 1971 activity at Akita-komaga-take. However, in the composition of groundmass, the former is highly silicic, being almost identical to the rhyolitic pumice of Usu, whereas the latter is less silicic but richer in iron. The former magmas represent a trend of fractional crystallization typical of the calc-alkalic series which may be attributed to the higher PH₂O and PO₂ in the magma, though the latter magma has a tholeiitic trend. Accordingly, this fact suggests that type of eruption does not always depend on the bulk composition of the products, but strongly on the composition of the liquid part of the magma.

Explosive eruptions involving production of a large volume of pumice are also a characteristic type of eruption in other island arcs where andesite and dacite of the calc-alkalic series are predominant products. It is suggested that such explosive eruptions are mostly caused by highly silicic liquid magmas rich in volatile matter, even though the bulk composition of the products may be more mafic.

INTRODUCTION

Explosive eruptions involving production of a large volume of pumice and ash are a common and characteristic type of eruption in island arcs where andesite and dacite of the calc-alkalic rock series are predominant products.

It is generally accepted that such explosive eruptions are mostly caused by felsic magmas rich in volatile matter, though the mechanism of such eruptions is very complicated and as yet unexplained. The pumices produced by explosive eruptions, however, are not always felsic in bulk composition; those of intermediate andesite are rather common in island arcs.

In northern Japan, violent pumice eruptions have been frequently recorded in historic times. Petrographical study on these pumices indicates that the composition of the liquid magma which is represented by groundmass composition, is extremely felsic, even though the bulk composition of the products may be more mafic. The significance of such a trend of crystallization of magma will be discussed in this paper along with types of historic eruptions and their sequence.

RECORD OF HISTORIC ERUPTIONS

Many Quaternary volcanoes are distributed along the inner arcs of the Kurile Islands and north Honshu in northern Japan (Fig. 1). Most of them are stratovolcanoes which were formed through central eruption, and lava domes and pyroclastic cones are associated. Some of the volcanoes built calderas as a result of large scale eruptions of pyroclastic flows. The products of these Quaternary volcanoes are comprised in a series of basalt - andesite - dacite - rhyolite suite, of which andesite and dacite of the calc-alkali rock series are most abundant rocks as in other island arcs (Katsui et al. 1974).

During historic times, since the 7th century in north Honshu and the 18th century in east Hokkaido, eruptions have taken place at 15 volcanoes in northern Japan (Fig. 1).

These historic eruptions have been recorded in the Catalogue of the Active Volcanoes of the World, Part XI (Kuno 1962). Since then, collection and interpretation of old documents, and tephrochronological studies of recent pyroclastic deposits have been carried out. On the basis of recent studies, all of the reliable eruptions are listed using the IAVCEI symbols in Table 1.

As shown in Table 1, seven volcanoes recorded phreatic explosions of small scale, of which those at Siretoko-iwo-zan in 1889 - 1890 and 1936 were accompanied by outflow of sulphur, the only unusual phenomenon recorded. Eight other volcanoes erupted essential (magmatic) materials, and their activities were generally larger in scale than the phreatic explosions. Types of this activity show a wide variation from Strombolian to Plinian.

COMPOSITION OF VOLCANIC PRODUCTS AND TYPE OF ERUPTION

Chemical compositions of the essential ejecta and lavas which erupted from the eight volcanoes in historic times are listed in Table 2, in which composition of groundmass as well as bulk composition (whole rock analysis) is shown for some rocks.

Bulk composition of the erupted rocks and type of the activity show a wide variation, as expressed below:

• Basalt and mafic andesite  

-   Tokati : Strombolian eruption preceded by phreatic explosion rarely accompanied by mudflow.

-   Osima-o-sima : Strombolian eruption with lava flow.

-   Iwate : Strombolian eruption with lava flow.

• Andesite

-   Tarumai : Plinian eruption with pumice - fall and flow in 1667 & 1739, and extrusion of lava domes in recent years.

-   Komaga - take: Plinian eruption with pumice - fall and flow, rarely followed by extrusion of lava dome.

-   Akita - komlaga - take: Strombolian eruption with lava flow.

-   Tyokai : Explosion followed by extrusion of lava dome.

• Dacite and rhyolite

-   Usu : Plinian eruption with pumice - fall followed by protrusion of lava dome in 1663, subsequently formation of lava domes and cryptodomes with accompanying nuées ardentes.

It is noticed that pumice eruptions of Tarumai, Komaga-take and Usu were the most violent and largest in scale. Volume of the pumice deposits is estimated as from 0.2 to 3.4 km³ for each eruption. Compared with such activities, eruptions caused by mafic magmas are rather mild and small in scale.

Factors govering type of volcanic eruption are very complicated (Verhoogen 1951; McBirney & Murase 1970). The physical properties of magma are important factors, and viscosity of magma depends on chemical composition of magma, temperature, etc. Concerning the historic eruptions in northern Japan, it appears that more violent activity is caused by more felsic magma as a rule.

SEQUENCE OF ERUPTION

Recent tephrochronological study of pyroclastic deposits shows that the historic activities during the past 300 years at Tarumai, Komaga-take and Usu were recommenced with a strong Plinian eruption after a long period of quiescence for one or several thousand years. Subsequently, formation of lava domes or cryptodomes was repeated at Tarumai and Usu. A small lava dome was also formed at the final stage of pumice eruption in 1856 at Komaga-take.

The above sequence of eruption suggests that, during a long quiescent time prior to eruption, water in the magma has been concentrated in the upper part of the magma column under lower confining pressures (Verhoogen 1949; Kennedy 1955). For this reason, the Plinian eruptions occurred, throwing high into the air vesicular pumice which originated from the apex of the magma column. Then, the ascending magma became rapidly viscous due to depletion of water in the magma (Shaw 1963), and the eruption ceased with formation of a lava dome or cryptodome.

As shown in Fig. 2, change in bulk chemical composition of volcanic products is noticed as well as change in type of eruption during the past 300 years, i.e. from intermediate andesite to slightly mafic andesite at Tarumai, and from rhyolite to dacite at Usu. A similar variation is also found during the 1929 eruption of Komaga-take. These changes from felsic to mafic can be interpreted in terms of compositional zoning within the magma column.

COMPOSITION OF LIQUID MAGMA AND TYPE OF ERUPTION

In reviewing the historic eruptions in northern Japan, it seems that the more felsic products were formed through the more violent activities such as the Plinian eruptions at Usu, Tarumai and Komaga-take.

It is also noticed that, in bulk chemical composition, the pumice of Tarumai and Komaga-take do not differ singnificantly from the scoria and lava of Akita-komaga-take which are products of the 1970 - 1971 eruptions of the Strombolian type (Soya & Masai 1971). All of these volcanic products are intermediate andesite in composition, ranging from roughly 58 - 62 % in SiO₂ and from 7 - 10 % in total Fe oxides.

However, as shown in Table 2 and Fig. 3, notable differences exist between Tarumai & Komaga-take and Akita-komaga-take in composition of the groundmass and residual glass, the products which represent liquid magma at the time of eruption. The groundmass and glass ejected during violent pumice eruptions are highly silicic (SiO₂ = 69 - 74 %, total Fe oxides = 2 - 3.5 %), being almost identical to the 1663 pumice of Usu (SiO₂ = 73 %, total Fe oxides = 2.6 %), whereas the products of the relatively quiet eruptions from Akita-komaga-take are less silicic but richer in iron (SiO₂ = 61 - 67 %, total Fe oxides = 8 - 9 %) (Aramaki 1971). It is worthy of note that, in the Plinian eruptions, the liquid magma is extremely felsic. Accordingly, type of eruption does not always depend on the bulk composition of the products, but strongly on the composition of the liquid part of the magma and other factors.

Fractional crystallization of the magma of Akita-komaga-take follows a course of the tholeiite series, as shown in Fig. 3. Inferred from the chemical composition, the residual liquid magma may have retained low viscosity as basaltic magma at the time of eruption (Bottinga & Weill 1972), and caused a mild Strombolian eruption.

The pumice produced by the Plinian eruptions at Tarumai and Komaga-take contain a large amount of phenocrysts, as much as 35 - 55 vol. %, viz. abundant calcic plagioclase, hypersthene, augite, and titanomagnetite. As expected from such modal composition, the groundmass glass of the pumice is extremely felsic. The course of fractional crystallization of the magmas of Tarumai and Komaga-take represents a trend typical of the calc -alkali series which is characterized by increase in silica and alkali, and decrease in iron (Fig. 3). It is suggested that such fractional crystallization may have proceeded under conditions of higher water vapor pressure and oxygen partial pressure in magma (Yoder 1969; Osborn 1959). The water vapor pressure in magma as inferred from both composition of plagioclase (Yoder et al. 1957) and equilibrium temperature of pyroxene pairs (Wood & Banno, 1973) on the pumice of Tarumai, was as high as 1.5 - 2.0 kb in the temperature range 1050° - 1070° C (Fig. 4).

A similar result was also obtained on the pumice of Komaga-take. Thus, the residual felsic magma may have been oversaturated with H₂O by succeeding crystallization and ascending of magma, and finally caused the Plinian eruption due to rapid vesiculation of magma.

In northern Japan and other island arcs, andesite and dacite of the calc-alkali series are abundant rocks. The composition of their groundmass, which represents the actual liquid magma, is generally felsic. Explosive eruptions involving production of a large volume of pumice are a characteristic type in these regions. Such explosive eruptions are mostly caused by felsic liquid magmas rich in volatile matters, even though the bulk chemical composition of the products may be more mafic.

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