Classification and Distribution of the Islands of Japan

Abstract

The total number of small islands that make up Japan amount to about 5529 (Table I), excluding the 623 in the mandated territory of the South Sea and the 108 islands of the Kwantung leased territory. In this papas, only islands smaller than Etorofu (3220 km2) of the Kuriles are considered, the large islands of Honsiu, Sikoku, Kiusiu, and Hokkaidô forming Japan proper, and Formosa being treated as mainlands. At first we classified the islands into continental and oceanic according only to their distances from the mainland. There are two kinds, viz., isolated islands and island groups. In addition to these we have many islands forming the centers of lakes, calderas, and coral lagoons. The second classification of islands was made on a genetic and topo-graphic basis. Volcanic, coral, and non-volcanic islands were genetically classified. Types intermediate between these three are the reef encircled volcanic islands, reef encircled non-volcanic islands, and partially volcanic islands with volcano (ordinary islands but with one or more volcanoes on parts of the island). Fig. 1 shows that distribution of these six types. These six types can be subdivided further according to their forms, the stages of erosion and the crustal movements they have suffered. Such a division is shown in Table III. Since the oceanic and continental islands are generally in groups, they have been classed as island groups, as in Fig. 2. The characteristic features, such as the nature of the bed rock, the volcanoes, ; coral reefs, whether elevated or living, the erosion stages, the abrasion terraces and drowned valleys, of. every island of every group were studied. Table IV shows the data thus obtained and Fig. 3-6 the distri-bution of these topographical forms. Finally we considered some topographical agents which affect the shape, the largeness and even the number of islands, and we knew the diminishing effect by deposition is very large. In Table V, twice the islands number of each group is written, excluding the smaller ones than 100m in length.