Introduction to the Landforms and Geology of Japan



Izu-Bonin (Ogasawara) Islands

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 highest point is at the northernmost area of the ridge. The elevation decreases toward the south along the volcanic front and increases from the lowest point around 29°N (to the west of the northernmost Ogasawara Ridge). A tectonic line, Sofugan Tectonic Line, trending north-northeast is found at the lowest point of the ridge. This tectonic line 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 landforms (seamounts, troughs, and submarine canyons) from north to south.



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