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The exposed monoliths present significant problems due to the environmental conditions (Öcal et al. 2009) and show diverse, partly exposure-specific forms of weathering (Fig. 2). Approximately 200 monoliths and stone columns were used to visually assess and characterize the weathering behavior.

Figure 2: Decay phenomena on the monoliths. a) biological colonization and sanding, b) crust formation and salt efflorescence, c) alveolization, d) scaling.

The tropical climate at the ~2000 m a. s. l. El Infiernito area is characterized by two rainy and two dry seasons. The rainiest months are May and October with up to 200 mm, whereas January is the driest month with 40 mm followed by July with 90 mm. In addition to the rain, in the mostly semiarid microclimate, air humidity plays an important role as a water supplier. The relative air humidity fluctuates between 70–85 % depending on the season. Direct water absorption during rain events or capillary suction of surface and soil water, control the humidity regime. The monthly average temperatures are above 20 °C throughout the year, with lowest mean temperatures of 10 °C and highest mean temperatures of 22 °C and can rise to over 30 °C. Especially during the long dry and cloudless seasons, the temperatures on the monoliths’ surface can reach high values.

The monoliths show a large variety of different damage types, such as relief, back-weathering, bursting, rounding, covings and alveolization, which predominantly occurs. Neighboring areas of the stone surface weather back to different degrees, and create a moving relief, e. g. by weathering parallel to the layer or by the loss of components. Often honeycomb-like structures are formed, which are typical for alveolar weathering (Fig. 2c). The causes can be of a very different nature. These can include salt weathering and wind erosion, as well as structural and lithological peculiarities.

The deposition of dirt and dust particles in the pore space leads to a gradual compaction of the rock surface, and may favor biological growth on some monolith surfaces. Gray to brownish crusts (Fig. 2b) can be found on the stone surface as coherent layers, millimeter to centimeters thick. Visually, the limit between the natural patina and the damage pattern crust cannot always be clearly determined. Biological colonization by green algae, lichens and higher plants was observed on all monoliths (Fig. 2a). Loosely adhering whitish salt efflorescence is an indication of the build-up of harmful salts enriched in the stone (Fig. 2b).

Sanding, flaking, peeling (Fig. 2a, d), crumbling and exfoliation as well as transitional forms are evident. In addition to the relief formation, sanding 211is the second most common form of weathering with clearly varying intensities. Like sanding, scaling and flaking from plate-like areas parallel to the surface can lead to total loss. These phenomena are strongly connected to salt crystallization and expansional behavior, as may be fostered by strong sunshine during the day and followed by cold nights. Stress on the rocks’ fabric leads to the formation of cracks. However, the most noticeable damages to the stone surfaces are large, surface-parallel scales. Sometimes, nearly catastrophic damage events may occur on the exposed monoliths, like on Monolith V-0163, which was mapped in 2007 (Fig. 3 and 4), and was considerably damaged with a huge loss of material 12 years later (Fig. 3). In this example, material inhomogeneities like nearly invisible mica- and clay-layers inside the rock may have been an important primary damage factor. Scaling visibly occurs in connection with increased salt pollution, especially in the splash water area. Exposure-related scale formations are clearly recognizable. While on some monoliths only discoloration is observed, others show heavy break outs and material loss (Fig. 3).

Figure 3: Monolith V-0163. Critical condition and strong deterioration between 2007 and 2019.

Figure 4: Damage mapping of Monolith V-0163 in 2007.