Mineralogy of the Mudeungsan Tuff (Republic of Korea) Using Synchrotron X-ray Powder Diffraction and Rietveld Quantitative Analysis

Mudeungsan (Mount Mudeung) is an extinct volcano located in the southwestern part of South Korea that was formed in the Late Cretaceous period. This mountain, 1187 m above sea level, is adjacent to Gwangju Metropolitan City, which has a large population (about 1.4 million) and volcanic rocks, including columnar joints, which form various types of outcrops. Although this mountain was listed as a national geopark in 2014 and a UNESCO Global Geopark in 2018, much basic research has yet to be carried out. In particular, there are no mineralogical studies of volcanic rock samples despite the well-preserved variety of volcanic rocks. For this study, X-ray diffraction analysis was conducted using rock samples from Mudeungsan columnar joints known as tuff. We report that the rocks are mostly dacite, mainly composed of quartz, plagioclase, and sanidine through Rietveld quantitative analysis. In particular, α-cristobalite, a crystalline polymorph of silica, appears in the columnar joint rocks, indicating that Mudeungsan experienced an explosive eruption during the formation of the mountain.

A Study on Evaluation of Slope Stability and Range of Rockfall Hazard of Daljeon-ri Columnar Joint in Pohang, Korea

In this study, we evaluated the slope stability of the Pohang Daljeon-ri columnar joint (Natural Monuments # 415) and calculated the maximum energy, jumping height and moving distance of rockfalls using a simulation. Based on the results, we established the range of rockfall risk. The slopes of the Pohang Daljeon-ri columnar joint have dip directions of 93.79°, 131.99°, 165.54° and 259.84° from left (SW) to right (NE). Furthermore, they have a fan-like shape. The Pohang Daljeon-ri columnar joints are divided into four sections depending on the dip direction. The measurement results of the discontinuous face show that zone 1 is 125, zone 2 is 261, zone 3 is 262, zone 4 is 43. The results of slope stability analyses for each section using a stereographic projection method correspond to the range of planar and toppling failure. Although it is difficult to diagnose the type of failure, risk evaluation of currently falling rocks requires further focus. The maximum movement distance of a rockfall in the simulation was approximately 66 m and the rockfall risk range was the entire area under slope. In addition, it is difficult to forecast where a rock will fall as it rolls in various directions due to topographic factors. Thus, the installation of measures to prevent falling is suggested to secure the stability based on the results of the rockfall simulations and its probabilistic analysis.

Cooling joints of granite as a structural cause of the tors and boulder fields of granite

<p>Typical cooling joints of granite have been believed to be orthogonal, and characteristic topography of granitic rocks like tors and boulder fields are interpreted in combination with the cooling joints and weathering. However, most of the previous studies were performed by the observation on the ground, and the 3D observation of cooling joints and the topographic features was not sufficient. We observed tors and boulder fields of granitic rocks using UAV and 3D modelling and found that columnar joints are typical for the granite that forms tors and that boulder fields are the accumulations of rock columns as well as boulders made by the spheroidal weathering of rock columns. Tors we observed were Mt. Kinabalu of Borneo, Mt. Mizugaki, Mt. Jizo, Mt. Gozaisho, Mt. Konze and 5 other locations in Japan. We observed that tors consist of polygonal rock columns with undulating joints, more irregularly shaped than the columnar joints of volcanic rocks. The cross-sectional areas of rock columns varied from 1 to 130 m^2, much larger than typical rock columns of volcanic rocks. The rock columns of granite are typically polygonal dipyramids, of which shapes may be dependent on the cooling history of granite. Boulder fields we observed was the Kui boulder fields in Hiroshima. We found that the boulder field is the accumulation of prismatic rock columns as well as rounded rock boulders. The prismatic rock columns had basal cross-sectional areas of 0.8 m^2 on average. The rock columns had chamfering cracks at corners, which are assumed to be made during cooling and to form preliminary outlines of core stones. Core stones had surface crusts or rindlets, which exfoliate and leave more rounded core stones.</p><p>  Rainstorm-induced landslides of weathered granite reflect weathering styles of granite: Landslides that occurred recently in Japan had three types, landslides of loosened layers of decomposed granite (or micro-sheeted granite), landslides of core-stone bearing materials, and landslides of saprolite. Landslides with core-stones were particularly destructive because of their inertia. Potential sites of such landslides could be predicted using columnar joints in fresh rocks as a clue.</p>