The present geomorphic feature of Northeast Japan, which is characterized by three rows of elevated zone and narrow depressions placed among them, has been established by Neogene Green Tuff activity. Pre-Tertiary basement rocks are developed in the elevated zones, and in particular are widely distributed in the outer row (Abukuma- Kitakami Mountains) occupying almost all of the area. The basement rocks are composed of two Jurassic accretionary bodies and a few Siluru-Jurassic micro-continents. These geologic bodies were collided and accreted in late Jurassic and early Cretaceous in the marginal area of east Asia. After the collision and accretion, Northeast Japan had been released from extensive tectonic movement for about 80 million years. 23 million years ago, Green Tuff activity was happened with andesitic volcanism. It has been emphasized that Neogene tectonism of Northeast Japan was closely related with volcanic activity. Many models explaining Green Tuff activity and the tectonic development have been proposed. Plate tectonic theory of active margine seems to be fairly effective for the explanation. From middle Jurassic, Northeast Japan has been situated in the convergent boundary area. Oceanic plate subduction triggered off the separation of Northeast Japan from Asian continent in Palaeogene. Eastward migration of Northeast Japan caused the opening of the Sea of Japan. Neogene intense volcanism and thick sediment accumulation is supposed to be closely related with the migration. Quaternary andesitic volcanos were formed under west-east compressional field, and most of them are situated in the middle row (Ou backbone ranges).
