A Field of Minor Volcanoes
Jeju Island is the visible peak of a massive dormant shield volcano, Hallasan, which rises 1,950 meters above sea level. Scattered across its flanks are 368 smaller volcanic cones called "oreum," a word from the Jeju dialect meaning a small mountain or peak. This is the highest concentration of such cones found anywhere in the world. These are not random hills; they are monogenetic volcanoes, meaning each one erupted only once. This volcanic activity occurred in phases, starting around 1.8 million years ago, with the oreums forming during later stages of flank eruptions.
The oreums are primarily cinder or scoria cones, built from fragments of frothy, solidified lava ejected during Strombolian-style eruptions. Their highly permeable rock allows rainwater to drain quickly, which has helped preserve their original shapes over millennia. While most are scoria cones, about 10 are classified as tuff cones or rings, formed by explosive interactions between magma and water, and are typically found along the coast. One of the most famous examples is Seongsan Ilchulbong, or "Sunrise Peak," a dramatic tuff cone that erupted from the shallow seabed between 50,000 and 120,000 years ago. An eruption from Geomunoreum between 100,000 and 300,000 years ago created the Geomunoreum Lava Tube System, a UNESCO World Heritage site with some of the world's most impressive lava tube caves, including the 7.4-kilometer-long Manjanggul Cave.