POPULAR SCIENCE: How are holes in sandstone formed?
Honeycombs are usually described as numerous closely fitting holes a few centimetres in diameter. Their evolution has been described in two ways: by case hardening or by the hydraulic hypotheses. The theory of case hardening assumes that the rock surface’s mechanical resistance increases with time due to the precipitation of minerals, while the interior of the rock remains the same. Different resistances would then, in combination with weather effects, cause the gradual deepening of small holes in the surface rock crust until a honeycomb-like surface is formed.
To verify the theory of case hardening, geologists from the Faculty of Science used a variety of methods to measure the porosity, drilling resistance, and tensile strength of the holes and sandstone surfaces. However, the measurement results did not confirm this theory, as the holes and their surroundings mechanically behaved in the same way.
Scientists eventually proved the second hypothesis of honeycomb origin – the hydraulic theory. This hypothesis is described by salt weathering, which is controlled by the flow of water carrying dissolved salts. Sandstone contains rainwater that infiltrates into the rock and, in lower parts, flows to the surface where it evaporates, which can happen even several centimetres below the sandstone surface. Salts present in porous water (e.g. from acid rains) precipitate where evaporation occurs, and their crystallization causes the surrounding material to disintegrate, which in turn leads to erosion of the rock surface. To verify the hydraulic hypothesis, scientists measured the moisture (suction pressure) and the amount of salt in the holes and their protrusions under different weather conditions ranging from dry periods to times after significant rain events. The entire salt precipitation process was also simulated on sandstone samples in the laboratory.
An important part of the methodology was the use of a fluorescein dye, which makes it possible to localise the sites of evaporation by colour changes. The use of the dye has shown that evaporation at low moisture content (a frequent situation on vertical sandstone walls) only occurs at the bottom of the honeycomb holes, while the protrusions around the holes remain completely dry. Salts therefore precipitate in the holes, effectively deepening them, while the honeycomb walls remain dry and are not being damaged by salt weathering. In case of high moisture content (under long-term humid conditions), evaporation takes place from the entire surface, with most of the water evaporating through the protrusions causing their weathering. In such a case, salt preferentially destroys the protrusions and smoothens the sandstone surface. In honeycomb sites, low moisture prevails and results in long-lasting evaporation from the bottoms of the holes, leading to salt precipitation, the gradual deepening of the holes, and the formation of marvellous sandstone honeycombs.
Salt weathering is therefore responsible for honeycomb formations, but at the same time can be the reason of sandstone surface smoothening in case of high moisture contents. That is why we can observe both smooth sandstone walls and rocks with honeycombs. Whether it is one or the other is due to the initial shape of the rock outcrop, its orientation, climate, moisture availability and vegetation present at the site. A change of conditions (e.g. microclimate) can lead to a complete destruction of the honeycombs.