Jeju Island, covered with extrusive rocks formed at the 4th Cenozoic era, displays a geographical type of shield volcano as a volcanic island of semi-elliptical form. The direction of the axis is N70°E on the plane, major axis with 73km, minor one with 31km. The island is surrounded by gentle-slope volcanic zones at its sides. The slope becomes steeper as one approaches the center of the island. Hallasan, with a height of 1,950m, lies at the center of the lsland. Paekrokdam, at the top of Hallasan, has a crater that measures 700m in diameter and is 100m deep. The western outer ring of Paekrokdam comprises of cliffs made out of trachyte, and the eastern outer ring is composed of a gentle hill made out of basalt stone. Piedmont districts below the dome-shaped Paekrokdam crater at the top of Mt. Hall shows a form of lava plateau, and Oreum, a tiny volcanic zone, are densely distributed around it. The low grounds of Jeju Island, far from the high grounds of Hallasan, are formed with lowlands of gentle slopes, around which scoria cones are distributed. Various scoria cones are formed in Jeju Island. Usually, scoria cones have a crater at the top of mountain whereas a scoria mound does not. A horse-shoe scoria cone is a crater which was partly eroded at one side due to flowing lava, and some scoria cones have tongue-like forms. Tuff rings, tuff cones made out of porous sedimentary layers, are also heavily distributed at alpine regions of the west, Songaksan at the Southwest, Pyosun, Sungsan, and the Udo area at Udo.
Basin systems are not well established in Jeju Island because of its volcanic features. At the highlands, tiny streams and brooks are distributed through the eroded valleys at the North and South from the top of Hallasan made out of trachyte. Streams and brooks are not noticeable at the east and west areas from the peak, made out of basaltic lava.
Radiometric basin systems are well developed, centering on Hallasan. The mountain's subterranean water source flows out near coastal areas at Gangjeonchun, Oedochun, Yongpochun and forms streams and brooks. At the lowlands, tiny streams are densely formed at the Andyukmyun and Seoguipo areas, which is covered with andesides. Coastal areas, surrounded by cliffs, are limited to the Andyuk and Seoguipo areas, formed by volcanic sedimentary rocks and exposed to trachyte, excluding Songaksan, Sanbangsan, Seongsan Ilchulbong. As an exception, the Namwon and Aewol areas are covered with basalt cliffs. Therefore, most waterfalls in Jeju Island are limited to trachyte cliffs. Small-sized sand beaches, made out of oceanic sand, are sparsely detected at the coastal areas. There are coastal areas where sand dunes, arranged to the direction of wind by southeastern seasonal wind, are densely distributed. Massive sand dune layers are formed at the northeast coastal areas.
Such coastal lines in the south, that connect Sanbangsan-Gunsan, Daepo, Aedolgae-Seogwipo, and Namwon display notable geographical characteristics of an uplifted coast. A portion of a coastal area in Moseulpo shows a feature of marine terrace.