Moisture distribution, tensile strength, evaporation rate and origin of coastal honeycombs (Tuscan, Italy)

Cavernous weathering is a typical example of the degradation pattern of both natural outcrops and cultural heritage. It is described from all environments on Earth and also on Mars. The most common examples are honeycombs and tafoni. Honeycombs are known from arid, humid, and cold deserts, but best developed honeycombs are often described from coastal areas. There are many ideas on the origin of cavernous weathering (case hardening, chemical alteration), but currently most authors believe that the origin is caused by salt weathering. Huinink et al. (2004) described a theory that inside the pits, the capillary zone is closer to the surface and therefore the intensity of the evaporation is higher than in walls separation the pits. As more evaporation accumulates more salts the pits enlarges faster than surface outsides the pits is retreating. To verify this theory, in the environment of coastal honeycombs in Tuscan (Italy), the depth of the vaporization plane (interface between dry surface zone and deeper capillary zone) was measured by the "uranine-probe" method (Weiss et al., 2020) inside and outside the ten honeycombs. From the depth of the vaporization plane and climatic conditions on the study site, the intensity of evaporation was calculated and from the mineralization of water the amount of precipitated salts was estimated. To determine the effect of case hardening, the tensile strength of honeycomb pits and walls was measured. The vaporization plane measurements show that for all honeycombs, the vaporization plane was closer to the surface in pits than outside. The evaporation intensity was calculated for the mean depth of vaporization plane inside the honeycombs (2 mm) and the mean depth outside the honeycombs (7 mm). In marine environment a solution on a vaporization plane should be saturated with halite which has an equilibrium relative humidity of 75 %. The evaporation intensity inside the honeycombs is 9.4 mm/year for 75 % RH and 2.7 mm/year outside the honeycombs. Considering that the evaporated water is of the same composition as seawater, 0.1-0.4 g salts precipitate from 1 m², most of which is NaCl. Inside the honeycombs precipitate 3 times more salts than outside. The tensile strength inside the honeycombs is approximately the same as outside considering standard deviation (354±339 and 284±157 kPa, respectively), so case hardening does not have any effect. The results correspond to the theory of origin according to Huinink et al. (2004). For a detailed description of the moisture behavior in future studies, it is necessary to better understand the moisture conditions (especially relative humidity on the vaporization plane) and it is vital to perform repeated measurements during various seasons.

 

References:

Huinink HP, Pel L, Kopinga K., 2004. Simulating the growth of tafoni. Earth Surface Processes and Landforms 29: 1225–1233.

Weiss T, Mareš J, Slavík M, Bruthans J. 2020. A microdestructive method using dye-coated-probe to visualize capillary, diffusion and evaporation zones in porous materials. Science of The Total Environment 704, 135339.