3.1. Geological and rock distribution
The geology of the Korean peninsula is very diverse, and its rocks are often well exposed in the mainly mountainous terrain. More than half of the peninsula comprises granite and granite gneiss. The former mainly intruded during the Triassic, Jurassic, and Cretaceous periods, whereas the latter formed because of metamorphism and granitization of sedimentary rocks during the Precambrian. In addition, volcanic rocks can be found mainly in strata from the Cretaceous period of the Mesozoic era in the southern part of the Korean peninsula and are exposed as tuff, andesite, and rhyolite. The geological characteristics of the Korean peninsula are closely related to the materials and construction techniques used to shape the stone heritages of the country (Lee and Jo, 2016).
The geology of the area around the five-story stone pagoda was formed by volcanic activity during the Cretaceous period, and it mainly comprises quartz porphyry, porphyry tuff, acidic tuff, and porphyrite (Fig. 2). Geumgolsan Mountain comprises acidic tuff, which is the same material used to build the stone pagoda. In general, tuff is soft, highly porous, and absorbent, and it has the advantage of being easily dressed. Therefore, tuff has long been widely used as a material for buildings and stonework. The stone pagoda appears to have been constructed from tuff because of its proximity to Geumgolsan Mountain and the ease of dressing this material. This is evidenced by the many traces of quarrying that have been observed in large outcrops on Geumgolsan Mountain.
3.2. Lithological characteristics
The five-story stone pagoda was built from lithic tuff that is light yellow to light gray. The rock contains many fragments of light green pumice with sizes of several millimeters to several centimeters (Fig. 3a). Because pumice has weak physical properties compared to the rock substrate, these fragments detached from the original rock of the stone pagoda owing to physical and chemical weathering (Fig. 3b). A horizontal bedding structure was clearly observed for the pagoda, which was attributed to the material characteristics of lithic tuff (Fig. 3c). In general, masonry structures require a horizontal bedding to prevent cracking due to compressive forces. However, some members of the stone pagoda exhibited a vertical bedding structure around which microcracks were distributed (Fig. 3d); this was presumed to indicate that the material characteristics were not sufficiently considered at the time of construction.
To supplement the petrographic features of the stone pagoda, a slab analysis was conducted on lithic tuff collected from Geumgolsan Mountain. These samples clearly contained pumice, which was not well observed on the surface of the stone pagoda, and phenocrysts such as quartz, plagioclase, K-feldspar, and biotite were identified. In addition, iron-containing minerals were discolored to light brown because of chemical weathering, and dark-colored contaminants caused by manganese hydroxide were partially extracted from the surface (Fig. 3e).
The mineral compositions of the lithic tuff of the stone pagoda and Geumgolsan Mountain were analyzed by using a polarizing microscope (Fig. 4a) and X-ray diffraction (Fig. 4b). The results showed that the rocks contained pumice and phenocrysts such as quartz, K-feldspar, plagioclase, and mica based on plagioclase substrates. In modal analysis based on the mineral composition, the rocks were plotted in the area corresponding to lithic tuff with a composition of 36.2–42.4% glass, 5.8–9.2% crystal, and 51.9–57.1% rock fragment (Fig. 4c).
The main components of the lithic tuff in the stone pagoda and Geumgolsan Mountain were plotted on total alkalis–silica (TAS) and SiO2-(Zr/TiO2 × 0.001) classification diagrams (Figs. 4d, 4e). The rocks from the two sources were in the area corresponding to rhyolite. In addition, the rocks showed almost the same trends in their geochemical characteristics regarding the major, rare earth, and compatible and incompatible elements (Fig. 4f). These results indicate that the stone pagoda and lithic tuff of Geumgolsan Mountain have a genetically common source.
3.3. Weathering characteristics
The soft and porous characteristics of lithic tuff are convenient for stone dressing but are also the main factors for the weak physical properties. In addition, the fragments of pumice cause differential weathering (Germinario and Török 2020), and the bedding structure causes cracks because of anisotropy (Wang et al. 2017). Therefore, a slab cross-sectional analysis and simple test of the physical properties were performed to examine the weathering characteristics of lithic tuff.
First, lithic tuff showing the same weathering tendencies as the stone pagoda was collected from Geumgolsan Mountain, and its cross-section was polished to produce a slab sample. The slab face comprised two weathered layers discolored to black and a fresh layer (Fig. 5). The outermost first weathering layer had cavities that formed because of the erosion or detachment of pumice, and the layer showed relatively severe discoloration because of oxides and hydroxides. The second weathering layer below the rock surface was discolored because of chemical weathering, similar to the surface layer, but pumice fragments remained within the layer. In the innermost fresh layer, the light green color of the original rock was maintained well without discoloration, and the pumice fragments remained within the layer. However, the pumice fragments were so soft that they could be detached even by fingernails.
The water absorption rate and porosity of a specimen detached from the stone pagoda and fresh rock from Geumgolsan Mountain were calculated to compare their physical properties (Fig. 6a). On average, the water absorption rate and porosity of the stone pagoda were 9.5% and 19.2%, respectively, which were greater than the water absorption rate (8.6%) and porosity (17.8%) of the rock from Geumgolsan Mountain. Thus, the stone pagoda showed poorer physical properties than the fresh lithic tuff. To identify the weathering characteristics of the stone pagoda, fresh lithic tuff was submerged in distilled water for 24 h and dried at 105° C for another 24 h. Then, the change in weight was measured (Fig. 6b). The lithic tuff showed relatively rapid changes in weight for one cycle and then gradually decreased in weight thereafter. The decrease in weight was attributed to weak minerals including pumice reacting with water, which cause them to dissolve partially or detach physically. After 5 days of submersion, the pumice in the lithic tuff detached and precipitated in the beaker. This indicates that the pumice is sensitive to moisture.
To examine the chemical weathering characteristics, SEM equipped with EDS was conducted on small samples of the stone pagoda. The pumice surface was observed to be covered with cubic sodium chloride crystals (Na 34.3 wt.%, Cl 53.8 wt.%) (Fig. 7a), which was attributed to the sea breeze because the stone pagoda is about 3 km from the ocean. In addition, the brown discoloration on the surface of the stone pagoda appears to have been caused by microcrystalline clay minerals and iron oxide (Fig. 7b) with some coexisting microcrystalline gypsum crystals (Figs. 7c and 7d). The maximum iron oxide content for the brown discoloration was 26.5 wt.%. In contrast, the black discoloration consisted of diverse structures such as a thick crust of amorphous material (Figs. 7e and 7f), dendrites (Figs. 7g–7j), and acicular-columnar microtexture aggregates (Fig. 7k). EDS analysis showed that the black discoloration materials mainly comprised Mn (19.4–71.7 wt.%) and Fe (1.2–26.5 wt.%). High levels of Ca (13.7 wt.%) and S (12.9 wt.%) were detected in the acicular-columnar microtextures, which indicated the coexistence of gypsum crystals (Figs. 7l–7n). Clay minerals, dust, fine soil particles, and microorganisms were also identified.