The basement rocks are of Paleogene as well as Neogene beds which are found in the southern half of Shimabara peninsula. Volcanic activity has begun in Pliocene epoch in the form of mesa or aspite of basalt and pyroxene andesite, both of which are seen only in the southern part of the peninsula. The main activities were played from Pleistocene to recent, having formed many tholoid type volcanoes - Volcano Unzen - in the northern part of the peninsula. Volcano Unzen is entirely made of hornblende andesite, which belongs to the hypersthenic rock series, and often includes many hornblende dolerite xenolith. The chemical analyses show that the lavas are in general rich in MgO, Na2O and K2O and poor in CaO and total (FeO + Fe2O3). The diagrams MgO-(FeO+Fe2O3)-(Na2O+K2O) and norm An-Ab-Or are discussed. The hornblende lavas from Volcano Unzen were probably influenced by the assimilation of the acidic hornblende magma and the basic lava.
