干旱区生态环境知识资源中心

During a field trip in November 2007 at San Pedro de Atacama (Chile) several centimetre-scale white rims and blisters were observed on the salt crust floor in the famous Valle de la Luna (moon valley). These curious features are composed entirely of pure white salt and have been encountered in a limited area of about 15,000 m(2). A total of 35 of these small structures, named here "salt rims" and "salt blisters", have been located and measured. Their average plan dimensions are 6.5 x 5.5 cm for a mean height of 3.6 cm. Almost 60% of these structures have an opening on top (salt rims), sometimes on their flanks, typically smaller than their diameters. The biggest features always have an opening, being the closed structures, named blisters, normally less than 3 cm in diameter.

A tube-like opening continuing underground for at least a decimetre, mostly deeper than 20 cm, is clearly visible in the inside of the salt rims. The rims perfectly match the edges of these holes. In some cases salt crystals grow on the inner walls of the rims forming curving filaments.

The salt blisters and rims are very similar to homonymous (sam-name) speleothems and form by condensation-evaporation processes after very sporadic rainy episodes. Rainfall (average annual rainfall is < 20 mm), occurs very rarely and enables infiltration of salty water through openings in the salt crusts. This water is conveyed underneath the impervious salt crust presumably in a network of interconnected fissures and underground cavities containing airborne fine sediments and in which moist air flows. According to our genetic model these underground aerosols can escape under the form of vapour only through cracks or openings when outside temperature drops below underground temperature (e.g. during the night). Evaporation processes occur when the vapour exits the holes leading to the formation of a salt rim on the side. When the airflow decreases gradually the rim can get smaller in diameter and eventually closes completely forming a salt blister. Salt rims and blisters are generally seasonal features that are doomed to dissolve by rain, but some appear to be able to survive several years building up a thick salt crust that can reach almost 1 cm.