和歌山県に分布する中新世凝灰岩の急崖に発達するタフォニの分布と形状

概要

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Morphological features of tafoni on Miocene tuff cliffs have been studied in the Kozagawa area, Wakayama, Japan. Distribution of tafoni tends to develop on steep slopes characterized by massive fine-grained acidic tuff. Concave roof of tafoni has been formed with high decreasing rate of rock strength under high humid condition.This supports that concave roofs have been formed due to high rate of salt weathering.Piling up of rock blocks on the foot of such cliffs indicates that evolution of tafoni isclosely related to rock fall from the cliffs. Development of tafoni makes a sort of canopies overhanging on steep cliff, and such unstable portions fall down as rock fall. In addition to development of tafoni, cracks within acidic tuff are also one of causes of rock fall. Such cracks occurred due to rotational moment of overhanging portion.Consequently, weathering rates of rocks due to salt weathering and spacing of these cracks are control the frequency of rock fall in these cliffs.