波食棚上の節理沿いに発達するリムの形成プロセス

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Raised rims are well developed along polygonal joints with spacings of 30-40cm on sandstone layers forming intertidal shore platforms at Aoshima Island on the Nichinan Coast, Japan. The rims have widths of 2-3cm and have relative heights of 1-5cm from the bottom of adjacent depressions (called pools). To explore the process of rim formation, measurements of the hardness and moisture contents of the sandstone surface were conducted at the rims and pools. An Equotip tester and an infrared optical moisture meter were used for the former and latter measurements, respectively. The Equotip testing showed that the rims and pools have no significant difference in hardness under dry conditions. This result does not support the existing explanation that the elevated rims are formed due to case hardening, i.e., cementation by concentration of calcium, silica and/or iron. Temporal changes in the measured moisture content during ebb tide showed that the value at the pools decreased drastically from 6-7% to 1-2% in low tide due to strong drying caused by direct insolation, while rims maintained higher moisture contents of 5-6% by capillarity in narrow joints with interstices of 10-30μm. These results suggest that (1) the higher moisture content prevents drying and thus salt weathering on the surface of sandstone along the joints, which results in little deterioration of the surface strength, and (2) the surface lowering occurs as a result of strength reduction due to salt weathering prevailing on the place apart from the joints, i.e., on the pools. This leads to the conclusion that the rims and pools are landforms controlled by the difference in fluctuation of moisture contents.
日本地形学連合の論文
2008-10-25

波食棚上の節理沿いに発達するリムの形成プロセス

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