Schematic diagram of the Deodeoreum lava dome [Provided by Jeju Special Self-Governing Province, resale and DB prohibited]

Traces of an ancient lava dome comparable in scale to Sanbangsan Mountain have been discovered beneath Deodeoreum, an oreum (volcanic cone) in Seogwipo, Jeju.

On the 20th, the Hallasan Research Institute under Jeju’s World Heritage Office announced that it had confirmed the existence of a massive trachyte lava dome during a deep-borehole survey near Deodeoreum in Sangye-dong, Seogwipo.

The research team conducted the survey at a borehole located approximately 200 meters northeast of the peak of Deodeoreum. They confirmed that trachyte—the same rock type that forms Deodeoreum—is continuously distributed throughout a 221-meter section, from 109 meters underground down to the current drilling depth of 330 meters.

By integrating the borehole data with the topography of Deodeoreum, the researchers explained that the lava dome is estimated to be at least 360 meters in scale, extending from a minimum elevation of -132 meters to the peak at 228.4 meters above sea level.

This raises the possibility that Deodeoreum, which currently stands only 48 meters high (relative height of 48m, altitude of 228.4m), was once a massive lava dome comparable to Sanbangsan Mountain (relative height of 345m, altitude of 395m), which was later covered by lava flows over a long period, resulting in its present appearance.

Relative height refers to the difference in elevation between the peak of a mountain or oreum and the surrounding ground level, while altitude indicates the height above the average sea level.

The research team further stated that because the lower boundary of the trachyte body has not yet been identified at the current depth of 330 meters, the actual scale of the volcanic body may be even larger.

Deodeoreum, which stands 48 meters above ground level [Provided by Jeju Special Self-Governing Province, resale and DB prohibited]

The World Heritage Office plans to clarify the formation relationship between volcanic bodies in southwestern Jeju, such as Deodeoreum and Sanbangsan, through precise analysis and dating of rock samples.

The research team stated, "Through this investigation, we have confirmed for the first time that a massive trachyte volcanic body is hidden beneath Deodeoreum, which appears to be just a small oreum on the surface," adding, "This is a significant achievement that demonstrates that the history of volcanic activity in Jeju is far more complex and three-dimensional than what is visible on the surface today."

They further noted, "Drill cores are invaluable research assets that record volcanic activity beneath Jeju, and once lost, they are difficult to recover. This investigation is a clear example of how critical borehole data is to uncovering the history of volcanic activity in Jeju."

A borehole survey involves drilling deep into the ground to directly examine underground rocks and strata.

The Hallasan Research Institute of Jeju’s World Heritage Office is currently conducting a research project titled 'Establishing a Geological Map of the Entire Jeju Region' from 2025 to 2028.

This year, the team is documenting and collecting/analyzing rock samples from lava layers obtained from drill cores acquired from 13 deep boreholes across the province that were drilled for groundwater development and observation. [Yonhap News]