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Outline of landforms and geology of Japan
Landforms
Fig.
Landforms [
]
The Japanese Islands are the emerged parts of island arcs. Mountains including volcanoes occupy 61% of the land. The long axes of major mountain ranges are oriented to the direction of extension of island arcs, or parallel to the trenches, indicating the formation of major mountain ranges is closely related to the development of island arcs in the subduction zone. However, this parallelism is not preserved in collision zones. Moreover, although the Japanese island arcs compressed by plate movement are uplifting on the whole, some places are subsiding or had subsided to form depressions. Accordingly, landform units including mountains and basins are complicatedly distributed in Japan, especially in the inner arcs and the inner zone. Japanese landforms are characterized by volcanic landforms and various types of tectonic relief produced by crustal movement. In addition, heavy rainfall caused by typhoons and rainy seasons leads to active erosion and deposition, rapidly changing landforms. Forms of landslide are commonly found in Japan.
Mountains (nonvolcanic)
Mountains are the most significant landscape component in Japan as they can be viewed everywhere. Their forms are shaped by crustal movement and erosion, features of which vary in different localities. Crustal movement principally restricts the contour of mountain range and erosion carves peaks and valleys.
[Crustal movement producing mountains]
Fig. Warping (In Japan, mountains are built up by several types of crustal movement: warping, folding with faults, reverse faulting, strike-slip faulting, and normal faulting. The right figure shows the distribution of each crustal movement type.
Most mountain ranges in the outer arcs and outer zone are raised by upwarping. Representatives of this type are the Kitakami and Abukuma Mountains in the Northeast Japan outer arc. The Kii and Shikoku Mountains in the outer zone of southwestern Japan have the features of upwarping but their north edges are cut by the MTL. The morphology of these mountain ranges is like a dome or bulge. The Chugoku Mountains in the inner zone of southwestern Japan and the Kitami Mountains in the Kuril inner arc are also classified into mountains upheaved by warping. Both mountains are located in areas of low uplift rates in the Quaternary, characterized by gentle-slope topography. Many mountains in exposed basement rocks regions were developed by upwarping.
Fig. Folding with faults (Kaizuka 1983)
Mountain ranges formed by folding with faults are mainly distributed in Northeast Japan inner arc and the Kuril inner arc. In Northeast Japan inner arc, three uplifted zones, the Ou Range, Dewa Mountains, and Okushiri Ridge in the Sea of Japan, run parallel to the island arc, corresponding with anticlines of folds with the wavelength of several tens of kilometers. Although the Akaishi Range in central Honshu and the Hidaka Mountains in Hokkaido are situated in the outer zone and the Kuril outer arc, respectively, these were uplifted by folding with faults.
Fig. Reverse fault blocks (Kaizuka 1983)
Fig. Normal fault blocks (Kaizuka 1983)
Fig.
Yoro and Suzuka Mountains []
Fault blocks are produced by reverse faults, normal faults, and strike-slip faults. Since the Japanese Islands are dominated by compression, the major fault type is the reverse fault. Normal fault blocks are only found in central Kyushu (Beppu-Shimabara Graben). Many mountains formed by faulting have steep slopes (fault scarps).
The Kiso and Hida Ranges in central Honshu and ranges including the Suzuka and Ikoma Mountains in the Kinki Triangle are reverse-fault mountains with developed steep fault scarps. The Yoro and Suzuka Mountains are a tilted block formed by faulting, characterized by the asymmetrical cross section of which the eastern slopes are steep (fault scarps) and the western slopes are gentle.
