Hallasan

Lore & Background

Hallasan is a massive shield volcano that constitutes the majority of Jeju Island. A traditional Korean saying holds that 'Jeju Island is Hallasan, and Hallasan is Jeju.' The mountain is visible from all parts of the island, though its peak is often shrouded in clouds. The volcano began forming in the Pleistocene epoch on the continental shelf, about 100 meters below sea level. Eruptions of basalt and trachyte lava built the island above sea level, reaching a height of 1,950 meters. At the summit lies Baengnokdam, a crater lake whose name means 'white deer lake.' Legend says otherworldly men descend from heaven to play with white deer there. The lake's circumference ranges from approximately 1.5 to 1.7 kilometers and depth up to about 100 meters, varying by season. The eastern and western walls of Baengnokdam differ in composition: the eastern wall is covered with basalt lava and stream sediment, while the western wall is a trachyte lava dome. This suggests Baengnokdam is a pit crater rather than a crater formed by lava eruption. Hallasan's topography significantly influences Jeju Island's weather. When south winds flow, sea breezes form around the mountain and create persistent eddies on the coast, where the highest temperatures occur. When winds are weak, warm air accumulates in the mid-mountainous area, making maximum temperatures there 1–3 °C higher than on the coast, and up to 4 °C higher on the downwind side. Minimum temperatures are affected by cold air descending from Hallasan, which can lower downwind temperatures by 2–5 °C. The deviation between maximum and minimum temperatures increases in spring due to these mountain-driven phenomena.

Reader's Guide

Hallasan is significant as the highest point in South Korea and a defining feature of Jeju Island, both geographically and culturally. Its status as a shield volcano with a disputed active classification underscores the need for further monitoring and study. The mountain's isolation has fostered unique ecosystems: over 50 endemic plant species and numerous animal subspecies have evolved there. The mountain's impact on local climate—creating temperature inversions, eddies, and regional temperature variations—demonstrates its role as a major orographic influence. Hallasan's legacy is as a natural landmark that shapes Jeju's identity, ecology, and weather patterns, while its unresolved eruption history keeps it a subject of scientific attention.

Did You Know?

• The crater lake Baengnokdam has a circumference of approximately 1.5 to 1.7 km and depth up to about 100 m, varying by season.
• The mountain's soil has higher organic content and better drainage than any other soil in South Korea, but is relatively infertile.

Geological Architecture of a Shield Giant

Hallasan is a massive shield volcano that essentially constitutes Jeju Island itself. Its construction began during the Pliocene epoch atop a continental shelf sitting roughly 100 metres below sea level. Over time, successive eruptions of basalt and trachyte lava raised the landmass above the waterline until it reached its present height of approximately 1,950 metres. The summit is crowned by a large volcanic crater exceeding 400 metres in diameter. The soil is composed primarily of volcanic ash, sand, and lapilli, boasting the highest organic content and best drainage of any soil in South Korea, yet remaining relatively infertile.

Baengnokdam and the Puzzle of Its Walls

At the summit of Hallasan sits Baengnokdam, a crater lake whose name translates to white deer lake. Local legend attributes the name to otherworldly figures descending from heaven to play with white deer. The lake formed not through a typical lava eruption but through the collapse of the ridge's top caused by underground magma movement, making it a pit crater rather than an explosive volcanic crater. Its circumference stretches up to 2 kilometres depending on the season, with depths reaching approximately 100 metres. What makes Baengnokdam geologically fascinating is the stark contrast between its walls: the eastern side is capped by 2 to 5 metres of basalt lava overlying a high-concentration stream sediment layer of the Tamna Formation, while the western wall is built from a trachyte lava dome. This asymmetry reveals that the trachyte magma rose to form the dome at the summit, and the clastic sediments on the east confirm the lake's origin as a structural collapse rather than a volcanic explosion.

A Mountain Woven into Jeju's Identity

In Korean tradition, Hallasan is inseparable from Jeju Island itself, captured in the local saying that Jeju Island is Hallasan and Hallasan is Jeju. The peak is visible from every point on the island, though its summit is frequently veiled in clouds. It is commonly ranked among South Korea's three principal mountains alongside Jirisan and Seoraksan. The mountain's naming history is remarkably layered: older English sources called it Mount Auckland, while modern English uses Hanla Mountain or Mount Halla. In Hangul it is written as if it were Hanrasan, yet it is pronounced Hallasan. Over the centuries, Korean speakers have known it by numerous names including Buak, Wonsan, Jinsan, Seonsan, Dumuak, Burasan, Yeongjusan, and Hyeolmangbong, each reflecting different cultural or descriptive associations.

Hallasan as a Climate Engine

Hallasan exerts a profound influence on Jeju's local weather patterns. When southerly winds meet the sea, sea breezes develop around the mountain, and on the downwind coast these winds converge to form a persistent eddy where the highest temperatures are recorded. Under weak-wind conditions in the morning, a reverse layer traps warm lower-layer air, which the sea breeze then lifts into the mid-mountainous zone, raising temperatures there by 1 to 3 degrees Celsius above coastal areas, and by 4 degrees or more on the downwind side. Conversely, Hallasan acts as a cooling source: air chilled on its slopes descends to the coast, and when strong synoptic winds blow, that cold air is pushed downwind, dropping minimum temperatures by 2 to 5 degrees Celsius below surrounding regions, often at night. Weak northwestern winds specifically drive low temperatures into the island's eastern part. These dynamics make the mountain a central variable in Jeju's short-term forecasting.