Japan has been an island arc for only the past 16 million years, when it split from the continent during back-arc basin formation. Before that, the Japanese landmass was part of the Eurasian continental coast. Subduction of oceanic plates along this coastline from at least 450 million years ago produced the sediments that were to become the archipelago.
Accretionary Prisms, Accretionary Complexes
Materials bulldozed into the continental shelf by a down-going plate form Accretionary Prisms. These consist of ocean floor sediments, fragments of oceanic crust, sea floor volcanoes and plateaus, trench deposits, and reef limestones — all kinds of displaced and mixed up rocks and sediments. As the prisms build up one against another, they form large complexes of such materials along the landward side of the subduction trench.
Such Accretionary Complexes comprise most of the main geological belts of Japan. Oldest in the west and youngest in the east, they are the Hida and Oki belts which formed in the Middle to Late Palaeozoic; the Akiyoshi, Maizuru and Sangun belts of the Permian; the Mino-Tamba, Ryoke, Chichibu and Kitakami-Oshima belts dating to the Jurassic; and finally the Shimanto belt of the Cretaceous.
A new accretionary complex is being deposited along the eastern continental shelf of southwestern Japan by Philippine Plate subduction even at this very moment, but not every coastline experiences accretion. The northeastern continental shelf of Japan is actually being eroded away by the down-going Pacific Plate.
The Accretionary Complexes in Japan are generally delineated from each other by boundary faults. One might think of the complexes as nigiri-sushi all squashed together to form a larger unit. Each has its own character and contents, even layering, but stands independent of its neighbour, though welded together to form the archipelago.
Volcanoes in the Accretionary Complexes
Some of these Accretionary Complexes have undergone metamorphism, and many have been riddled with volcanic intrusions. Surface volcanics in Japan date only to the Miocene (ca. 22 million years ago), but buried lavas and plutonic granites indicate volcanism along the subduction front in earlier geologic periods. Volcanism is a natural corollary of subduction, but the continental crust from which magma forms and through which it pushes upwards is, in the case of Japan, originally sedimentary. Thus the archipelago results from two balanced formation processes: Accretionary Tectonics and Volcanism.
The most impressive volcanic deposits in Japan are found along the northwestern coast and across the centre of Honshu Island. These resulted from mainly underwater volcanic explosions of ash during rifting of the archipelago from the continent, forming hyaloclastic deposits sometimes up to 10 km (yes, kilometers) thick. They form the Green Tuff zone that gives Sado Island a surreal coastline of sculptured tuff.
Land-based Miocene volcanism occurred as the newly detached archipelago pushed against the new oceanic Philippine Plate in its journey outwards. Subduction of that plate began under southwestern Japan around 14 million years ago, giving rise to andesitic mountain chains along the Inland Sea. None of these volcanoes are now identifiable in the landscape, but their andesites were used by prehistoric peoples for stone tools — and more recently by modern musicians for stone chimes!
The Pliocene and early Pleistocene (ca. 5-2 million years ago) saw a new series of volcanoes formed in northeastern Japan, the Tohoku region, from Pacific Plate subduction. Calderas 20 km in diameter remain from violent explosions at that time. These are overprinted by the current Quaternary volcanic chains, which only date from 700,000 years ago.
Volcanics to the southwest have different causes than the northeast. The central Kanto region volcanoes, including Mt Fuji, are stimulated by the intrusion of the Izu Arc into the middle of Honshu. Kyushu Island has its own series of large volcanoes formed from the subduction of the Philippine Plate. Mt Aso is the largest landward one; Kagoshima Bay at the bottom of Kyushu is the water-filled Aira caldera; the volcanic explosion from this volcano about 25,000 years ago spread ash all the way to Alaska. Mt Sakurajima, a young, active volcano in the bay, sits parasitically on the rim of the old caldera.
Accretionary Complexes vs. Volcanic Products
Geologists assess that the Japanese archipelago is 70% sedimentary, formed mainly from the ancient Accretionary Complexes. This figure might seem misleading, since some have since been metamorphosed to varying degrees and are no longer sedimentary rock but metamorphic rock. However, their origins are not volcanic.
Thus, in Japan, magma percolates upwards through the accretionary substrate, like bubbles in cooking pancakes, and explosive volcanism scatters magmatic products across the land. Virtually every inch of Japanese land surface sports ashfall, but this blanket is of relatively recent geologic production compared to the more ancient sedimentary bedrock.
Though there are many volcanoes in Japan, the archipelago is not of the same volcanic construction as the Izu Arc or any of the hot spot chains: the land itself has an older pedigree of accreted sediments.
For more detail, see:
Barnes, Gina L. (2003) “Origins of the Japanese Islands: the new ‘Big Picture’.” Japan Review 15: 3-48; and (2008) “The making of the Japan Sea and the Japanese mountains: understanding Japan’s volcanism in structural context.” Japan Review 20: 3-52.
Taira, Akihito et al. (1997) "Accretion tectonics of the Japanese islands and evolution of continental crust." Earth & Planetary Sciences 325:467-78.
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