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Outline of landforms and geology of Japan
Rocks older than Permian
Zones including various types of rocks older than Permian are within Jurassic complex zones: the Kurosegawa Belt (also called Kurosegawa Tectonic Zone), the Hida Marginal Belt, and the Southern Kitakami Belt (locations). The Kurosegawa Belt lying from the eastern Kii Peninsula to western Kyushu yields different types and ages of rocks including serpentinite, high-pressure type metamorphic rock, 400-million-year-old granite, limestone with Silurian and Devonian fossils, and felsic volcanic rock. These rocks are found as fragments and small blocks in discontinuous narrow zone about 5 km wide. The Hida Marginal Belt in central Honshu consists of high-pressure type metamorphic rock, serpentinite, and Ordovician-Permian sedimentary rocks. The Southern Kitakami Belt includes Ordovician-Devonian shallow-sea deposits, Early Paleozoic high-pressure type metamorphic rock, and Mesozoic shallow-sea deposits. These belts have fossils similar to those found in Australia and the Yangzi Platform. The occurrence of the various rocks and fossils imply that the belts were exotic blocks (see “Formation history of the Japanese Islands”).
Plutonic rocks
Photo: Granite (Nezamenotoko) [
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Granite is broadly distributed in the Japanese Islands, occupying 10% of the islands, while gabbro and diorite are in limited areas. Granite in Japan is classified into four: Paleozoic, Permian-Jurassic, Cretaceous-Paleogene, and Neogene granites. Cretaceous-Paleogene granite is most widespread, found from central Honshu (Chubu) to western Honshu (Chugoku and the Seto Inland Sea). Granite of this age metamorphosed Jurassic accretionary complex in the Ryoke and Abukuma metamorphic belts by intrusion. The similar igneous activity is known in the Maritime Province of Siberia and the circum-Pacific area. The ages of granites are younger toward the Pacific side except Paleozoic granite, indicating that the continental crust developed toward the outside.
Paleogene deposits (coal seams)
The distribution areas of Paleogene deposits in Japan are much smaller than other rock areas, but some of the deposits are characterized by coal seams. Large coal fields are in central Hokkaido and northern Kyushu.
Neogene and Quaternary rocks (after the islands separated from the continent)
Neogene deposits are broadly distributed in northeastern Japan and the Fossa Magna (central Honshu), while, in southwestern Japan, they are found only along the coast of the Sea of Japan and in some areas on the Pacific coast. This is because northeastern Japan was under sea for a long period after the islands separated from the Asian continent.
The arc-trench system has depositional areas including forearc and backarc basins. Neogene forearc deposits are found in the Pacific coast areas including the Boso Peninsula (Kanto), Shizuoka (central Honshu), Miyazaki (southeastern Kyushu), and the Nansei Islands. Sediments accumulated in the backarc basin during the formation of the Sea of Japan (Miocene) are distributed in the Sea of Japan side of the Japanese Islands and the Fossa Magna. Deposits underneath coastal plains are extremely thick, producing petroleum and natural gas in Akita and Niigata Prefectures. The deposits have considerable volcanic rocks extruded during the Sea of Japan expanding (Early and Middle Miocene). These rocks are volcanic and volcaniclastic rocks of basalt, andesite and rhyolite. Most of them were altered into greenish rocks, known as “green tuff”. The green tuff area yields kuroko (black ore) including sphalerite, galena, barite, tetrahedrite, and pyrite. Kuroko deposits are hydrothermal deposits produced by reaction between seawater and magma attributed to submarine volcanic activity 17 million to 14 million years ago.
Quaternary deposits are mainly distributed in plains and low hill areas. Volcanic products are usually observed only around volcanoes, but massive pyroclastic materials from large-scale eruptions of Quaternary volcanoes cover broad areas. For example, the Ito pyroclastic flow ejected from the Aira Caldera 25000 years ago expanded in southern Kyushu. The deposit of this pyroclastic flow is found to the 90 km north of the Caldera. In the Kanto Plain, the Kanto loam consisting of volcaniclastic material and its eolian secondary sediment is well-known, derived from volcanoes around the Kanto Plain such as Fuji, Hakone, Asama, and Akagi volcanoes.
