2. Geology of the Japanese Islands: An Outline

The geological history of the Japanese Islands began from the breakup of the Super-continent Rodinia about 750 Ma. After a passive margin setting of about 250 million years, the Paleo-Pacific Ocean started to subduct beneath the Paleo-Asian continent about 500 Ma. The tectonic setting of this archipelago is an active convergent margin, where an oceanic plate has been continually subducting for over 500 million years. Plate subduction formed accretionary complexes based on sediment supply from mountains developed in the active continental margins. The accretionary complex is characterized by “ocean plate stratigraphy” which is basalt, limestone, chert, siliceous shale, and terrigenous turbidite in the ascending order. Non-metamorphosed accretionary complexes range from Carboniferous to the present in age. The accretionary complexes are metamorphosed into metamorphic rocks of low-temperature and high-pressure type and of high-temperature and low-pressure type. Extensive igneous activities occurred during Cretaceous to Paleogene, and arc volcanism has been very active throughout Cenozoic. The major tectonic events in Cenozoic are back-arc spreading and arc–arc collision. These events formed the Sea of Japan as a back-arc basin and a complicated island arc system around Japan. Plate subduction also causes frequent earthquakes and volcanic activities in the Japanese Islands. Although Japanese Islands suffer various geological hazards, these geological processes are fundamental for the formation of land for us to live on and for beautiful sceneries like Mt. Fuji.