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was derived from frequent and intermittent small scale

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pumice (Kashiwabara, 1976), which intercalates humic rich

layers of volcanic ash soil, is the product of multiple

eruptions. This deposit was obtained a C14 age of 59304-180 yr. B. P.(Sasaki et al., 1971). Thus, Yotei volcano continued

its activities with no long quiescence period from Yo-Mk

eruption to the Holocene Kitaoka pumice group.

d. Toya volcano

Toya volcano is a volcano complex with a large caldera and

two post caldera volcanoes (Nakajima and Usu volcanoes). The

Toya pumice flow deposit (Ota,1956) constitutes an extensive

plateau surrounding the caldera. Along the Osarugawa river, a

tephra formation called Osarugawa tephra (Machida et al.,

1987), composed of a pumice fall and a pyroclastic flow

deposit, underlies the Toya pyroclastic flow deposit with 50

cm thick volcanic ash soils in between (Fig. 26-4).

The number of eruptions which resulted in the Toya pumice flow deposit has been a problem. Previous studies regarding

the Toya pumice flow deposit supposed that they were the

products of multiple eruptions which occurred during several tens of thousand years before ca. 10ka (Kondo, 1963; Minato,

48 1966). Ikeda and Katsui (1986) and Machida et al. (1987)

subdivided the deposits into five units, and regarded the

units as products of a series of eruption. Lee (1993) repeated the idea that the pyroclastic flow deposits were derived from

multiple eruptions intervening quiet period owing to the

variation of the mineral and chemical composition within the

deposits. Thus, the number of large scale pyroclastic flow

eruption has been a problem. Machida et al. (1987) also

revealed that co-ignimbrite ash of the Toya pyroclastic flow covered the whole of Hokkaido and the northern part of the northeastern district of Japan. The age of Toya is presumed at 90-120 ka (Machida et al. 1987; Machida and Arai, 1992).

After the caldera formation following the construction of

Nakajima cone, the forming Usu cone started at about ca.10 ka (Oba, 1966). A large scale slope failure occurred on Usu cone at 7-8 ka and formed the Zenkoji debris avalanche deposit,

followed by historical activities after a several thousand

year-long quiescence (Yokoyama et al., 1973). Initially, the

tephras derived from Usu volcano were subdivided into three

layers as Us-a, b and c (Naganuma at al., 1933) . The Us-a was

separated further into Us-al, a2 and a3 by the presence of

humus and peat between them (Yamada, 1958). The stratigraphy and age of the tephras from Usu volcano were examined also by

Kondo (1963), and Yokoyama et a1. (1973). Yokoyama et al.

49

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(1973) subdivided the tephras into following six layers: Us- II a (AD 1910) , Us- III a (AD 1853) , Us- IV a (AD 1822), Us- V a (AD

1769) and Us-b (AD 1663). Including 1977-1978 eruption

deposits, the tephras from Usu volcano were divided into 7

layers (Table 10) .

The absence of volcanic ash soil layers between the

members of the Toya tephra is confirmed by examinations at

many localities, so that the members are regarded to be

derived from a single eruptive cycle. The pyroclastic flow

member, which were considered to be derived from a different

eruption by Lee (1993), are also included in the series of

deposit. The petrographic differences in the tephra sequence

are suppose to have been caused by an inhomogeneous magma

chamber.

At a section of Date (Fig. 26-5), eight tephra layers and one volcanic sand group are found above the Toya pyroclastic flow deposit. Among them, Aso-4, Kt-2, Spfl, B-Tm, and Ko-d were derived from remote volcanoes, and the two pumice fall deposits above and below the Kt-2 are presumed to be derived from Nakajima volcano (Nj-Os, Nj-Dt). Stratified volcanic sand and scoria fall deposits with thickness of fifty centimeters

are observed above Spfl with volcanic ash soils 20-40 cm

thick. This stratified deposit is similar with stratified deposit of Kuttara and Shikotsu volcanoes, so that it is also

50

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ドキュメント内難翁﹄ (ページ 91-96)

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compositions (Table. 8). Consequently the stratified deposit

was derived from frequent and intermittent small scale

eruptions. The latest deposit of Yotei volcano called Kitaoka

pumice (Kashiwabara, 1976), which intercalates humic rich

layers of volcanic ash soil, is the product of multiple

eruptions. This deposit was obtained a C14 age of 59304-180 yr. B. P.(Sasaki et al., 1971). Thus, Yotei volcano continued

its activities with no long quiescence period from Yo-Mk

eruption to the Holocene Kitaoka pumice group.

d. Toya volcano

Toya volcano is a volcano complex with a large caldera and

two post caldera volcanoes (Nakajima and Usu volcanoes). The

Toya pumice flow deposit (Ota,1956) constitutes an extensive

plateau surrounding the caldera. Along the Osarugawa river, a

tephra formation called Osarugawa tephra (Machida et al.,

1987), composed of a pumice fall and a pyroclastic flow

deposit, underlies the Toya pyroclastic flow deposit with 50

cm thick volcanic ash soils in between (Fig. 26-4).

The number of eruptions which resulted in the Toya pumice flow deposit has been a problem. Previous studies regarding

the Toya pumice flow deposit supposed that they were the

products of multiple eruptions which occurred during several tens of thousand years before ca. 10ka (Kondo, 1963; Minato,

48

1966). Ikeda and Katsui (1986) and Machida et al. (1987)

subdivided the deposits into five units, and regarded the

units as products of a series of eruption. Lee (1993) repeated the idea that the pyroclastic flow deposits were derived from

multiple eruptions intervening quiet period owing to the

variation of the mineral and chemical composition within the

deposits. Thus, the number of large scale pyroclastic flow

eruption has been a problem. Machida et al. (1987) also

revealed that co-ignimbrite ash of the Toya pyroclastic flow covered the whole of Hokkaido and the northern part of the northeastern district of Japan. The age of Toya is presumed at 90-120 ka (Machida et al. 1987; Machida and Arai, 1992).

After the caldera formation following the construction of

Nakajima cone, the forming Usu cone started at about ca.10 ka (Oba, 1966). A large scale slope failure occurred on Usu cone at 7-8 ka and formed the Zenkoji debris avalanche deposit,

followed by historical activities after a several thousand

year-long quiescence (Yokoyama et al., 1973). Initially, the

tephras derived from Usu volcano were subdivided into three

layers as Us-a, b and c (Naganuma at al., 1933) . The Us-a was

separated further into Us-al, a2 and a3 by the presence of

humus and peat between them (Yamada, 1958). The stratigraphy and age of the tephras from Usu volcano were examined also by

Kondo (1963), and Yokoyama et a1. (1973). Yokoyama et al.

49

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