Izu-Bonin (Ogasawara) Islands
Fig.
Topographic map [
]
The Izu-Bonin Arc (also called Izu-Ogasawara Arc), 1100 km long and 300-400 km wide, collides with
Central Honshu at the northern end and connected with the Mariana Arc
at the southern end. The straight volcanic front clearly runs in the
center of the island arc, dividing into the outer arc and inner arc.
The outer arc has the non-volcanic landforms with gentle slopes, and
the inner arc has volcanoes and complicated landforms including ridges,
seamounts, and basins. Three ridge lines and basins between the lines
are arranged north-south in parallel; the ridge lines are, from east,
the Ogasawara Ridge, Shichito-Iwojima Ridge, and Nishi-Shichito Ridge.
The Ogasawara Ridge located in the southern part of the outer arc is a
non-volcanic ridge 400 km long and 50-70 km wide. The ridge has a
linear cliff in the west side and is separated from the
Shichito-Iwojima Ridge by the Ogasawara Trough. The Ogasawara Ridge is
thought to be formed by volcanism during the Paleozene.
The Shichito-Iwojima Ridge situated in the center of the island arc
consists of active volcanoes, such as the Izu-Oshima, Miyakejima, and
Iwojima Volcanoes, along the volcanic front. Some volcanoes emerged to
be islands and the others are below sea level. The elevation of the
ridge that is the highest in the northernmost area decreases toward the
south along the volcanic front and increases from the point to the west
of the northernmost Ogasawara Ridge (around 29°N). A tectonic line (Sofugan
Tectonic Line), which trends north-northeast, at
the lowest elevation point divides the island arc into two parts. Volcanoes in the north side of the tectonic line are
distributed at irregular intervals, while those in the south side are
arranged at even intervals. The irregular intervals of the volcanoes
probably result from the formation of tectonic blocks dividing the
island arc. Submarine calderas are found in this ridge. The number of
submarine calderas in the northern part is much more than in the
southern part. Small rifts intermittently are distributed in the west side
of the ridge, supposing the first stage of the formation process of the backarc basin. The formation began
1-2 million years ago and is in
progress accompanied with active volcanism.
The Nishi-Shichito Ridge in the western margin of the Izu-Bonin Arc
comprises seamounts which are Tertiary volcanoes. This ridge is
characterized by abreast arrangement of northeast-trending topography (seamounts, troughs, and
submarine canyons) from north to south.
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