Hallasan is the central peak of the gently sloping shield volcano of Jeju Island. It is the highest mountain in South Korea, reaching 1,950 m above sea level. Hallasan is the symbol of Jeju Island and a representative product of the Quaternary volcanism in the Korean Peninsula and adjacent seas.

Hallasan boasts peculiar volcanic landscape, produced by the crater lake Baeknokdam at the summit, the precipitous rocky cliffs of the Yeongsilgiam, and about forty volcanic cones.

Hallasan was designated as a natural monument (no. 182) in 1966 and a national park in 1970 because the mountain preserves the pristine morphology of a shield volcano unaffected by significant weathering or erosion.

The mountain has been protected from human activity since then and is renowned for its unique ecology and biodiversity in addition to volcanic geology and geomorphology.

The mountain could be thus designated as a UNESCO Biosphere Reserve in 2002 and as a UNESCO World Natural Heritage in 2007. Hallasan is composed of numerous basaltic to trachytic lavas and a number of volcanic cones.

It has a small (108 m deep and 550 m wide) crater at its center instead of a caldera, which is named Baeknokdam. Hallasan is interpreted to have formed since the Middle Pleistocene, after about 780,000 years ago, when the dominantly hydrovolcanic eruption in the early stage of Jeju volcanism was replaced by lava effusion.

The volcanic rocks near the Baeknokdam crater at the summit were erupted tens of thousands years ago. Because of its young age, the summit area of Hallasan preserves fresh volcanic landforms and rock formations.

The summit of Hallasan provides different sceneries when viewed from different directions because the summit area was made from two different lavas that have contrastingly different properties. That is, the western half of that area was made from highly viscous trachyte lava, forming a dome-like topography, whereas the eastern half was made from highly fluid trachybasalt lava, resulting in a gently sloping topography.

The southern and northern slopes of the summit are bounded by precipitous rock cliffs because of collapse of the trachytic lava dome. In contrast to the summit area, the flanks of Hallasan were carved by several deep valleys and gorges. The topographic features formed by erosion and collapse of the volcano are especially well developed in the Yeongsilgiam.

 
 
 
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