Izu-Bonin (Ogasawara) Islands
Fig.
Topographic map [
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The Izu-Bonin Arc (also called Izu-Ogasawara Arc), 1100 km long and
300 to 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 the 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 the Ogasawara Ridge, the Shichito-Iwojima
Ridge, and the Nishi-Shichito Ridge in the order from east.
The Ogasawara Ridge located in the southern part of the outer arc is a
non-volcanic ridge 400 km long and 50 to 70 km wide. The ridge has a
linear cliff on 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 on the north
side of the tectonic line are distributed at irregular intervals, while
those on 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 the Shichito-Iwojima Ridge. The number of submarine calderas in the
northern part is much more than in the southern part. Small rifts are
intermittently distributed on the west side of the ridge, suggesting
the first stage of the formation process of the backarc basin. The
formation of the rift began one to two 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 en echelon arrangement of northeastward trending
topography (seamounts, troughs, and submarine canyons) from north to
south.
References
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