ABSTRACT
Cheju-do, the largest island (1792 km2 in area) of Korea, is located in the Korean Strait, c.90 km south of the Korean Peninsula. The island is a long ellipse, 74 km east-west and 39 km north-south. Mt Halla, the highest peak (1950 m) in South Korea, is located near the centre of the island. Geologically the island is a complex shield volcano, which extends with a gentle slope to the coastal region; it is notable for its numerous parasitic cones. Cheju Island is almost wholly composed of volcanic rocks, mainly trachyte, trachy-andesite, andesite, and basalt, with some sedimentary formations (Won, 1975; Lee, 1982). The pre-volcanic basement of the island is not exposed, but the results of a regional gravity survey together with the presence of granitic xenoliths found in the effusive rocks and in a basaltic breccia near the base of the exposed succession suggest that the basement is composed of pre-Tertiary (probably Bulgugsa) granite. According to Won (1975), the sequences of the volcanic eruptions and some sedimentary rocks in Cheju Island are grouped from oldest to youngest as follows: the Upper Pliocene Sogwipo Formation; the Lower Pleistocene Pyosonri basalt, Sogwipo and Jungmun trachytes; the Lower-Upper Pleistocene Seongsan, Hwasun, and Sinyangri Formations; the Upper Pleistocene Jeju and Hahyori basalts and Beopjeongri (including Sanbansan) trachyte; Siheungri, Seongpanag basalts; Hanlasan basalt and trachy-andesite; Baegrogdam basalt; Lower Holocene Volcanic cone I and Volcanic cone II; and the Upper Holocene AD 1007 and 1002 episodes of volcanic activity; Shell-sand Formation.
