Why is Jeju Island geologically different from most of South Korea?

Jeju Island is geologically distinct from the majority of the South Korean mainland, which is primarily composed of ancient Precambrian rocks such as granite and gneiss. In contrast, Jeju Island is a volcanic island formed by a core of volcanic material that rises symmetrically to the peak of Mount Halla.12

The island’s unique landscape features hundreds of crater-formed hills, seaside precipices, and extensive lava tunnels, which are the result of past volcanic activity. These formations, along with the island's volcanic origin, distinguish it from the mountainous, non-volcanic terrain that characterizes most of the Korean peninsula. Due to its significant geological features, the island's volcanic formations were designated a UNESCO World Heritage site in 2007.21