Dok island comprises Pliocene volcanic products such as a series of volcanoclastic rocks and lavas ranging in composition from alkali basalts, and trachyandesites to trachytes. Compositional variation of the basaltic rocks can be attributed to fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite. Chemical variations among the trachyandesites are caused by fractionation of clinopyroxene, plagioclase, and magnetite with minor amphibole, while trachytes are controlled mainly by feldspar fractionation. Incompatible element abundance ratios and chondrite normalized LREE/HREE ratios (e.g., (La/Yb)c: 24.8 to 32.8 for basalts, 15.6 to 31.2 for trachyandesites) suggest that the origins of the basalts and trachyandesites involve both different degrees of partial melting and subsequent fractional crystallization processes. Trace element ratios of the basalts from Dok island are characterized by high Ba/Nb, La/Nb, Ba/Th and Th/U and isotopic ratios (Tasumoto and Nakamura, 1991) that are similar to the EM 1 type of oceanic island basalts such as Gough and Tristan da Cunha basalts.

The species composition and phylogeographical features of marine algal flora of Dok island are investigated. A total of 253 species, 26 green, 67 brown, 160 red, were counted cumulatively since first report by Kang and Park (1969) from Dok island. Among them Ectocarpus corticulatus Saunder, Antithamnion okiense Kajimura are first reported from Korea and Feldmannia irregularis, F. globifera, Spatoglossum crassum newly added by this study. And also, a candidated new species, Dictyota sp. (Dictyotaceae, Phaeophyceae) is collected in this study. A remarkable increasing of warm water brown algal species, eg. Nemacystus decipiens, Sphaerotrichia divaricata, Tinocladia crassa, Petrospongium rugosum, Sargassum yezoense, are observed recently studies. This phenomenon is presumed to be closely related with global warming. For phylogeographical study of marine algae of Dok island we selected two species, one red alga Griffithsia japonica Okamura and one brown alga Dictyota coriacae (Holmes) Hwang, Kim et Lee, which have low dispersal capacity by their Oogamious sexual reproduction. The Dok island's haplotype of G. japonica is clearly distinguished from Oki island haplotype but closely related to the Korean type's haplotypes which distributed in south and east coast of Korea. And also, the Dok island's haplotype of D. coreacea is a unique Korean type which discriminated from Japanese and Taiwanese type. From these results, we proposed the marine algae of Dok island, at least in case of warm water species, were originated from Korean coasts rather than western coast of Japan by the Korean warm current.