Genesis of the columnar joints from welded tuff in Mount Mudeung National Geopark, Republic of Korea

Abstract

Mount Mudeung National Geopark (MMNG), Gwangju, Republic of Korea (1187 masl), is known for its huge, broad occurrences of columnar jointed colonnades in the Cretaceous Mudeungsan Tuff. To understand the genesis of columnar joints infilling a volcanic vent, integrated geochemical and geophysical studies were conducted. Most colonnades in the Geopark are located in regions higher than 700 masl and show elevation-dependent variations in the mean face width of the columns. These mean widths are approximately 1.7, 1.3, 2.9, and 1.2 m for the summit (>1100 masl), upper (950–1100 masl), lower (850–950 masl), and lowest (700–850 masl) colonnades, respectively. This variation implies that columnar jointing resulted from discontinuous tuff depositions on relatively planar ground, which is associated with caldera structures and landslides from the emptied caldera rim. Geochemical analyses of major, minor, and trace elements show that the andesitic-dacitic Mudeungsan Tuff resulted from the arc magmatism associated with subduction of the Izanagi Plate and involved crustal components. Few geochemical variations of the Mudeungsan Tuff indicate that tuff originated from a single magma chamber. U-Pb age dating indicates that the Mudeungsan Tuff was deposited at ~85 Ma except for the Chotdae-bong colonnades, which were dated to ~87 Ma. However, a scaling analysis of the elevation-dependent variations of face widths and simplified numerical model calculations indicate that the variation in face widths with elevation indicates the sequential emplacement, cooling, and compaction of separate welded tuffs. Thus, the welded tuffs of MMNG can be thought to be formed by three discontinuous large explosive eruptions with periods of repose after each eruption. These tuffs include 1) the highest-elevation columnar joints of colonnades in the summit area (>1100 m), corresponding to the lower colonnades of the third tuff; 2) the intermediate-elevation columnar joints of colonnades, corresponding to the upper (950–1100 m) and lower (850–950 m) colonnades of the second tuff; and 3) the lowest columnar joints of colonnades (700–850 masl), corresponding to the upper colonnades of the first tuff.