Volcanic Activity and Recent Tephras in the Kuril Islands: Field Result during the
International Kuril Island Project (IKIP) 2000
Yoshihiro ISHIZUKA
31 December, 2000
Division of Earth and Planetary Sciences,
Graduate School of Science,
Hokkaido University,
N10, W8, Kita-Ku Sapporo,
060-0810 Japan
E-mail: yishi@ep.sci.hokudai.ac.jp
1. Introduction
Tephras derived from large explosive eruptions have been widely known to be
good key marker layers not only to construct volcanic histories but also to estimate the
age of remains in archaeological deposits. While many key marker tephra layers related
to the large explosive eruptions have been well investigated in Japan and Kamchatka (e.
g. Machida and Arai 1991; Braitseva et al. 1997), few tephras have been identified in the
Kuril Islands that connect these larger land masses.
The International Kuril Island Project (IKIP), composed of American, Russian,
and Japanese scientists, has studied the biodiversity of the Kuril Islands since 1994.
During the IKIP 2000 of this past summer, archaeological and geological researchers
joined the IKIP project. A preliminary tephrochronological investigation was conducted
around mid to late Holocene archaeological sites on the coast of eleven islands of the
Kuril Islands (Fig. 1). These islands are Shumshu, Paramushir, Onekotan, Kharimkotan,
Shiashkotan, Raikoke, Matua, Ushishir, Simushir, Chirpoi, and Urup. The author reports
here the IKIP 2000 field results relating to recent Kuril tephras in order to evaluate
volcanic activity and estimate the age of remains in the archaeological excavations.
2. General Geology and Volcanic Activity of the Kuril Islands
The Kuril Islands include an older inactive arc (Lesser Kuril Ridge) and a
younger volcanic active arc (Greater Kuril Ridge), the later extending for 1200 km from
Cape Lopatka on Kamchatka to the Nemuro Peninsula on Hokkaido in the NW Pacific
(Nemoto and Sasa, 1960; Gorshkov, 1970; Fig.1). The Lesser Kuril Ridge is located
between the Greater Kuril Ridge and the Kuril-Kamchatka trench. It is composed of
volcanics and sediments from Upper Cretaceous to Paleogene. In contrast, the Greater
Kuril Ridge formed from vigorous submarine volcanic activity beginning in the Miocene
and is composed of many Quaternary volcanoes that consist mainly of composite
volcanoes with lesser calderas.
The Greater Kuril Ridge is divided into three regions, northern, central, and
southern on the basis of the submarine topography and the thickness of the crust. The
Kruzenshtern Strait of 1,900 m below sea level separates the Northern Kuril Islands
(Shumushu to Shiashkotan) from the Central Kuril Islands (Matua to Simushir). The
deepest strait (Boussole Strait) of 2,200 m below sea level divides the Central Kuril
Islands from the Southern Kuril Islands (Chirpoi to Kunashir). The Northern and
Southern Kuril Islands have the greater thickness of the continental crust (15~30 km),
whereas the thinnest oceanic crust (10~15 km) comparable to that of the oceans is in the
Central Kuril Islands (Gorshkov, 1970; Tuyezov, 1971). In the Greater Kuril Ridge 160
Quaternary terrestrial volcanoes and 89 submarine volcanoes are recognized (Gorshkov,
1970; Fig. 1). Thirty-two of these volcanoes have been active in historical time, since the
early 18 century. Nineteen of the active volcanoes have erupted since 1945 (Gorshkov,
1954; Simkin and Siebert, 1994).
[Please check the following paragraph. I deleted some formatting characters that
were mixed with the text, but maybe it is still incorrect. E.g., I do not understand the
"B3" and "B0.01" parts.]
Explosive eruptions of Volcanic Explosivity Index (VEI) = B3 (eruptive volume
of B0.01 km3), that are defined on the basis of their eruptive volumes (Simkin and
Siebert, 1994), have been recorded on 30 volcanoes of the Kuril Islands during the
Holocene (Table 1). Very large explosive eruptions (VEI of 6) with a total volume of
ejecta of 10 to 100 km3 formed three calderas: Tao-Rusyr Caldera on Onekotan Island,
Ushishir Caldera on Ushishir Island, and Lvinaya-Past Caldera on Itrup Island, which
have been dated to 14C ages of 9,400 y.B.P., 9,400 y.B.P., and 7,500 y.B.P, respectively
(Gorshkov, 1970; Braitseva et al., 1995). Since the 18 century, 10 large explosive
eruptions of VEI = 4 have been recorded on composite volcanoes. These inlcude the
1790 and 1981 eruptions of Alaid volcano, the 1853 and 1986 eruptions of Chikurachki
volcano on Paramushir Island, the 1872 eruption on Shaishkotan Island, the 1778 and
1924 eruptions on Raikoke Island, the 1946 eruption on Matua Island, the 1712 eruption
on Chirpoi Island, and the 1973 eruption of Tyatya volcano in Kunashir Island. These
explosive eruptions could have generated tephra deposits across wide regions of the Kuril
Islands.
3. Study Method
Tephrochronological studies focus on soil-tephra sequences, representing a
history of explosive volcanic eruptions. Tephras consist of all pyroclastic materials
ejected from a volcano. Stratigraphically they can be separated by soil, eolian sandy
loams or peats. One sheet of tephra refers to the pyroclastic materials of a series of one
eruption without large-scale interruptions.
During IKIP 2000, soil-pyroclastic sequences were investigated at locations close
to boat landings determined by the research needs of biologists and archaeologists. Cross
sections roughly 2 m deep and 1 m wide were made at the coastal or near coastal cliffs.
The sites of the cross sections were chosen at the relatively heights and plains [chosen to
include terraces and bluffs at different elevations?]. Features of stratigraphic
relationship, color, thickness, sorting, and maximum diameters of essential ejecta of each
individual tephra were described at each sections. The tephras were divided into two
genetic types, pyroclastic flow deposits and pyroclastic fall deposits according to their
sorting. The pyroclastic fall deposits were further subdivided into three types -- ash,
pumice, and scoria fall deposits -- in terms of their grain size and color.
4. Preliminary Results of the Tephras in the Shumshu Island
Despite an absence of Quaternary volcanoes on Shumshu, 11 layers of tephras
were found in a 130 cm deep (from the surface) cross section of a 12 m above sea level
(a.s.l.) sea cliff of on the western coast (Fig. 2). The tephras consist of brown to
yellowish ash fall deposits and black to reddish scoria fall deposits.
The most remarkable tephra in this sequence can be recognized as a yellowish
layer of ash fall deposits 83 to 102 cm below surface. Under the stereoscope, it has the
grain diameters of < 1.0 mm and the minerals of plagioclase and hornblende with a small
amount of pyroxenes. Most of glasses in this tephra are classified into bubble wall types
that are frequently found in the tephra derived from large explosive eruptions that
typically form calderas (Machida and Arai, 1991). Judging from color, isopach maps,
and mineral assemblages (Braitseva et al., 1997), the remarkable tephra could be
equivalent to the KO tephra derived from Kuril Lake Ilinsky caldera, southern
Kamchatka erupted in 7666 y.B.P.
Five other ash fall deposits can be recognized as layers varying in thickness from
2 to 5 cm. Considering the main direction of the ash fall deposits, the tephras may be
partly originated from explosive eruptions from southern Kamchatka volcanoes such as
Khodutkinsky Maar, which erupted in 2805 y.B.P., or Ksudach volcano, which erupted in
1090 y.B.P. (Melekestsev et al.,1996; Braitseva et al., 1997).
5. Preliminary Results of the Tephras in the Matua Island
On Matua Island, in the central Kurils, at least 19 tephra layers were found in a
150 cm deep cross section from 15 m a.s.l on the surface of a Tertiary basement hill at the
southern end of the island (Fig. 3). These layers consist of diverse fall deposits of
pumices, scoriae, and ashes as well as pyroclastic flow deposits. Most of the layers were
probably derived from the composite cone, Sarychev Peak, in the center of the island
because the cone had been intermittently erupted in the historical time (Simkin and
Siebert, 1991). The uppermost thick tephra layer (14 cm thick) that consists of black
scoria fall deposits from 14 to 28 cm in depth from the surface may be derived from
explosive eruptions such as the historical eruptions in 1930, 1946, or 1960 (Table 1).
The cultural layer of an archaeological site has been also found from 96 to 110 cm
depth from the surface at this section (Fig. 3). The layer is visible as dark brown loams
with the shards of earthenware and many carbonized woods up to a diameter of 4 cm.
This layer is interbedded between light gray ash fall deposits and brownish yellow
pyroclastic flow deposits.
6. Preliminary Results of the Tephras in the Chirpoi Island
Eight tephra layers have been found along a 300 cm deep cross section 18 m a.s.l.
on the northern coast of in the Chirpoi Island, in the southern Kurils (Fig. 4). The tephra
layers consist of ash and pumice fall deposits. In this section, thick, poorly continuous
epiclastic sands are also accumulated, probably as dune deposits.
Two remarkable tephra layers can be recognized: yellowish brown pumice fall
deposits and yellow ash fall deposits. The upper pumice layer that has 6 cm thick and the
grain diameters of <5 cm could be derived from the volcanoes of the Chirpoi Island,
probably andesitic to dacitic Chernyi volcano in the center of the island. The lower
yellow ash layer of 3 cm thick is interbedded with cultural layers of archaeological sites
that are visual as dark brown loams. Both cultural layers have the shards of earthenware
and many charcoals.
The uppermost tephra layers at this section consist of white and gray ash fall deposits
from 45 to 60 cm deep from the surface. These layers may be derived from historical
explosive eruptions of Chernyi volcano, those that are the 1712 eruption of VEI of 4, and
1857 eruption of VEI of 3 (Table 1).
7. Preliminary Conclusions
During IKIP 2000, at least 70 tephra layers have been found on the eleven Kuril
Islands of Shumshu, Paramushir, Onekotan, Kharimkotan, Shiashkotan, Raikoke, Matua,
Ushishir, Simushir, Chirpoi, and Urup Islands. These tephras consist mainly of ash fall
deposits with lesser pumice and scoria fall deposits and pyroclastic flow deposits. On
Shumshu Island, three tephra layers of ash fall deposits may be compared with the
tephras derived from volcanoes of the southern Kamchatka erupted before ca. 1000 to
8000 years. On Matua and Chirpoi Islands, notable tephra layers together with cultural
layers of archaeological sites have been found. The age of these layers will be estimated
using by the 14C dating of the charcoals and relative age of the earthenware.
ACKNOWLEDGMENTS
I am indebted to Ben Fitzhugh and Theodore Pietsch of the University of
Washington for the invitation to participate in the IKIP 2000. I thank the captain and
crews of R/V OCEAN of Russian Academy of Science for supporting this research. I
would like to thank Kaoru Tezuka, Carole Mandryk, Christopher Lockwood, Scotty
Moore, Cristie Boone, and Valery Shubin and his colleagues of the archaeological team
of the IKIP 2000 for gracious assistance. I thank Mitsuhiro Nakagawa of Hokkaido
University for helpful discussions.
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