Unlike most ancient mountains on the mainland, many important islands in Korea were formed by volcanic activity in the Cenozoic orogeny. Jeju Island, situated off the southern coast, is a large volcanic island whose main mountain Mount Halla or Hallasan () is the highest in South Korea. Ulleung Island is a volcanic island in the Sea of Japan, whose composition is more felsic than Jeju-do. The volcanic islands tend to be younger, the more westward.