Earth

Aggressive waters, saturated waters

Water that comes into contact with rocks initially is undersaturated, i.e. it can dissolve the minerals of the rock and progressively be enriched with the ions freed in the dissolving reaction, till it reaches a point of saturation, i.e. the water solution contains the maximum quantity possible of a particular ion in certain temperature conditions, atmospheric pressure or content of other acids. When this condition of saturation is reached, water no longer has a chemical effect on rock, and therefore it can only affect rock with mechanical erosion processes (as in the case of surface free flowing water). If variations in temperature, in CO₂ content or in concentration of the solution (e.g. in the case of evaporation) occur, saturated water becomes oversaturated, i.e. it contains an excessive quantity of dissolved calcium carbonate that should therefore be deposited in the form of calcite crystals, forming speleothemes, among which stalactites and stalagmites are the better known forms; however speleothemes may exhibit a vast range of forms and colours, at times bizarre and curious (when drops of water that carelessly drip on the sink evaporate, these form the detested calcareous deposits, which are a form of concretion).