Coastal honeycombs (Tuscany, Italy): Moisture distribution, evaporation rate, tensile strength, and origin
Abstrakt
Cavernous weathering (honeycombs, tafoni) is a common weathering feature of both natural and artificial exposures. Honeycombs are known from various environments but are best developed in coastal areas.
There are several theories as to their origin, with salt weathering currently being the most favoured by the geomorphological community. To test if the drying pattern of salt-laden moisture results in honeycombs (the theory of Huinink et al., Earth Surface Processes and Landforms, 29(10), 1225-1233, 2004), coastal honeycombs in the metasandstone of Tuscany (Italy) were studied both in the field and with a laboratory evaporation experiment.
The depth of the evaporation front was measured by the 'uranine-probe' method in the honeycomb pits and lips. The evaporation rate was calculated from the depth of the evaporation front as well as the climatic conditions at the study site.
Lastly, the amounts of precipitated salts were estimated based on the evaporation rate of seawater. In the evaporation experiment, the evaporation front retreated faster in the lips than in the pits, and the field measured evaporation front was closer to the surface in the pits (2 mm) than in the lips (7 mm).
Thus, the calculated evaporation rate was higher in the pits than in the lips (16.1 and 4.6 mm/yr, respectively). Similarly, the amount of salts precipitated was also higher in the pits (0.7 kg/m(2)/yr compared to 0.2 kg/m(2)/yr in lips).
Faster salt deposition in the pits as well as the evaporation front position fits well with the theory of Huinink et al. Based on surface tensile strength measurements, case hardening is not protecting the honeycomb lips.