A Coastline of Frozen Geometry
Stretching for roughly 2 kilometers along the southern coast of Jeju Island, the Jungmun Daepo Columnar Basalt, also known as the Jusangjeolli Cliff, showcases geological forces. The cliff face is composed of thousands of tightly packed, multi-sided pillars of dark grey basalt. This formation is the result of a thick, basaltic lava flow from Hallasan Mountain, the shield volcano that forms the bulk of Jeju Island. Between 140,000 and 250,000 years ago, during the Pleistocene Epoch, this molten rock flowed into the sea. The rapid cooling caused by the ocean water initiated a process called columnar jointing.
As the lava lost heat to the water and air, it began to contract. This contraction created immense tensile stress within the solidifying rock. To relieve this stress, the rock fractured. The fractures began at the surface and propagated downwards, perpendicular to the cooling front. The most efficient pattern for relieving this stress across a flat plane is a series of cracks intersecting at 120-degree angles, which naturally forms hexagons. While most of the columns here are six-sided, pillars with four to seven sides can also be found. The columns stand up to 40 meters tall in some sections, forming a dramatic wall against the sea.
A Natural Monument
The process of columnar jointing is not unique to this location—it occurs in igneous rocks of various compositions worldwide when cooling conditions are suitable. The scale and clarity of the columns at Jungmun Daepo are exceptional. The formation is considered the largest example of columnar jointing in South Korea. In recognition of its geological value, the South Korean government designated the cliff as Natural Monument No. 443 in 2005.
The constant action of the waves erodes the base of the cliffs, sometimes causing entire sections of columns to collapse into the sea. During high tide or storms, waves can crash against the cliff face, reaching heights of more than 20 meters. This ongoing erosion continually exposes fresh surfaces of the ancient basalt, ensuring the geometric structure remains a prominent coastal feature. The site includes well-maintained observation decks and pathways that allow for safe viewing of the formations.