Relationships between Formative Processes of Notches and Tafoni Developing on Steep Slopes of Volcanic Breccia and Occurrences of Rock Falls: A Case at Mt. Gokenzan, Takamatsu City, Kagawa Prefecture, Japan

Abstract

 Morphological features of notches and tafoni, which are well developed on volcanic breccia cliffs of Miocene Setouchi volcanic rocks in Kagawa, Shikoku, Japan, are described. The moisture content of a rock surface shows the highest value at the concave roof portion. The presence of a salt, such as gypsum, on notch walls suggests these notches are formed by salt weathering under a high humidity. Notches and tafoni develop to produce a visor overhang on a steep cliff, and such unstable blocks occasionally collapse as rock falls. In addition to the development of notches and tafoni, cracks within volcanic breccia can also cause rock falls. Consequently, the rate at which notches deepen due to salt weathering and the spacing of tension cracks may control the frequency of rock falls from these cliffs.