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Volcanoes
Central Honshu (Kanto and Chubu)
Fig. 13
Central Honshu has extremely complex tectonic settings because the
Northeast Japan Arc meets the Southwest Japan Arc and the Izu-Bonin
Arc. The Japan Trench connects with the Izu-Bonin Trench off the Boso
Peninsula. The Pacific Plate subducts underneath the Philippine Sea
Plate along the Izu-Bonin Trench. The Philippine Sea Plate descends
under the North American Plate along the Sagami Trough and the Suruga Trough and
under the Eurasian Palate along the Nankai Trough. The Sagami and the Suruga
Troughs extend to the land as the plate boundaries and connect with
each other near Fuji-san. The Sagami Trough links with the Japan and
the Izu-Bonin Trenches at their joint, so this point is called the triple
junction. The Philippine Sea Plate subducts in directions splitting it
in the Sagami and the Suruga Troughs. The Philippine Sea Plate subducted along the Sagami Trough
is in contact with the
leading edge of Pacific Plate (Figure 1).
Volcanoes are crowded in central Honshu (Figure
6). The volcanic front extending
from Tohoku curves westward and turns toward the Izu-Bonin Arc near
Asama-yama which repeatedly erupted on a small scale recently. The
volcanic front obliquely crosses the depth contours of the subducting
Pacific Plate near the turning point, meaning that it is not parallel
to the trench, because the movement of Philippine Sea Plate affects it.
Three volcanic zones parallel to the volcanic front lie on the east of
the turning point, continuing from Tohoku. Along the volcanic front
southward, there are two volcanic chains: the volcanic front side chain
including Yatsugatake, Hakone and Izu-oshima, and the back arc side
chain including Fuji-san and Ashitaka-yama. Fuji-san, Hakone, and
volcanoes to the south of them are on the Philippine Sea Plate.
Fuji-san is the largest in volume and Yatsugatake is the second on land
in Japan. The characteristics of Fuji-san, such as basaltic volcano
rather than andesitic, the location on the border of plates, and
particular large volume as an island arc volcano, well demonstrate the
peculiarities of central Honshu.
Monogenetic volcanoes (Izu-tobu Volcano Group) are found in the eastern
Izu Peninsula. It is only a monogenetic volcanic group in the Pacific
side in the Japanese Islands. As mentioned above, monogenetic volcanoes
are formed in areas dominated by weak-compressive or tensile stress.
The stress field in this area is different from that of other areas,
related to the complex tectonic settings of the collision zone.
There is another volcanic zone to the west (back arc side) of above
volcanic chains. This zone including Tate-yama, Norikura-dake, and
Ontake-san is in the Southwest Japan Arc. These volcanoes erupted on
the Hida Range, one of the highest mountain range in Japan.
Furthermore, a volcanic area including Haku-san is to the west of the
volcanic zone.
Izu Islands (Northern Izu-Bonin Arc)
The clear volcanic front runs parallel to the trench. Izu O-shima
and Miyake-jima, which are volcanic islands near the Izu Peninsula, are
420 km3 and 520 km3 in volume, respectively; these are the primary and
secondary volumes in Japan. Nineteen Quaternary volcanoes along the
volcanic front and 250 small volcanoes in back arc basins are found in
the Izu-Bonin Arc.
Basaltic volcanoes dominate this volcanic zone. The crust of the
Izu-Bonin Arc is basaltic and thinner than that of other island arcs,
which is about half thickness of the 30-km thick crust of the Northeast
Japan Arc. Therefore, the density of basaltic magma generated in the
upper mantle is much lower than the basaltic crust, readily ascending
to the surface. However, rhyolitic volcanoes are also
found, such as Nii-jima and Kozu-shima. In the Izu Islands,
stratovolcanoes are common, and lava domes and submarine calderas have
also been formed.
Southwestern Honshu (Kinki and Chugoku)
In this region, only Daisen is a stratovolcano, and monogenetic
volcanic groups, such as the Kan-nabe and the Abu volcano groups, are scattered
in the Sea of Japan side. These groups are small but alive for more
than four million years. Two reasons why monogenetic volcanoes have
been formed in this region are proposed: one is that the volcanism is
related to the plate subduction, and the other is that it is attributed
to the magma rising due to the expansion of the Sea of Japan.
There is no Quaternary volcano in Shikoku.
Kyushu and Nansei Islands
In northern Kyushu, monogenetic volcanoes composed of alkali basalt were formed in the Goto Islanads and Iki Island. In central Kyushu, large volcanoes including Kuju-san, Aso-san, and Unzen-dake are arranged east and west in the Beppu-Shimabara Graben. This volcanic zone is situated in a tensile stress field, rare in Japan. No volcano is found in the Kyushu Mountains about 100 km wide, between the Beppu-Shimabara Graben and the Kirishima volcanoes. In southern Kyushu, volcanoes are distributed parallel to the Ryukyu Trench from the Kirishima volcanoes toward south. This area is characterized by a series of large calderas including Kirishima, Aira, Ata, and Kikai Calderas. These volcanoes catastrophically erupted in the Late Quaternary, with pyroclastic flows covering southern Kyushu thick. Sakura-jima in Kagoshima Bay, which has been formed in the Aira Caldera, frequently erupts and its volcanic ash falls on Kagoshima City on the opposite shore with a population of 600 thousands. The volcanic chain continues about 400 km to the south of Kyushu. There is no volcanic island further south, but submarine volcanoes have been discovered in the backarc basin of the Ryukyu Arc.
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