28,000-Year-Old Rhyolitic Rock Field on Mt. Halla Designated as Natural Monument

28,000-Year-Old Rhyolitic Rock Field on Mt. Halla Designated as Natural Monument: Travel / 연합뉴스 / 2025-07-15 16:21:13

▲ This photo provided by the Korea Heritage Service (KHS) on July 15, 2025, shows that the “Rhyolitic Breccia Field at Mosewat, Mt. Halla," designated a Natural Monument. (PHOTO NOT FOR SALE) (Yonhap)

SEOUL, July 15 (Yonhap) -- The Korea Heritage Service announced on Tuesday that the “Rhyolitic Breccia Field at Mosewat, Mt. Halla” has been designated a Natural Monument.

Located on the southwestern outer slope of Baengnokdam Crater, the site features a vast accumulation of rhyolitic rock fragments of various sizes. Spanning approximately 2.3 kilometers in length and up to 500–600 meters in width, the field was formed around 28,000 years ago by volcanic debris flows following the collapse of a small lava dome.

Unlike the basaltic rocks commonly found in Jeju, the rhyolitic rocks here are light-colored due to their high silica (SiO₂) content, highlighting the site’s distinct geological characteristics and scientific value.

In volcanic processes, rocks typically solidify in sequence from basaltic to andesitic to rhyolitic compositions as magma cools. Although basalt has long been considered the only rock type present in Jeju, the discovery of this rhyolitic breccia field marks the first confirmed presence of rhyolite on the island.

Significantly, the rhyolitic breccia fragments—whose formation period is precisely identified—are widely found on the surface, providing crucial clues to deciphering the volcanic sedimentary sequences of Mt. Halla’s highlands.

The name “Mosewat” comes from the Jeju dialect meaning “sandy field,” although the area is, in fact, more accurately described as a rocky field due to its stony terrain.