Tidal wetting and drying on shore platforms: An experimental assessment

Rocks were subjected to wetting and drying cycles with de-ionized water under real time conditions. Using tidal simulators, one-third of the samples experienced 11 h of exposure and 1 h of inundation over a 12-h tidal cycle (representing high tidal conditions), one-third experienced 6 h of exposure and 6 h of inundation (representing mid-tidal conditions), and one-third experienced 1 h of exposure and 11 h of inundation (representing low tidal conditions). We obtained 324 cores from igneous, metamorphic, and sedimentary rocks from Ontario, which were exposed to more than 930 wetting and drying cycles over 16 months. We obtained 675 cores and cubes from basalts, sandstones, and argillites from shore platforms in eastern Canada, which were exposed to about 700 cycles over 12 months. Many of the cores and cubes experienced very little to no breakdown, particularly those from the igneous and metamorphic rocks of Ontario. Sandstones from a sloping platform in the macrotidal Bay of Fundy and argillites from a horizontal, mesotidal platform in Gaspé, Québec, were the most susceptible rock types, with equivalent surface downwearing rates ranging from 0 to 4 mm year− 1. The results suggest that downwearing rates decrease with elevation within the intertidal zone, probably because rocks require much longer to desorb than to absorb water. The experiments demonstrated that wetting and drying can be an important process on shore platforms in some types of rock, helping to lower the height and reduce the gradient of sloping macrotidal platforms and to reduce the height of horizontal micro- and mesotidal platforms.