Alveolar Weathering

Author: Heiner Siedel

English Version by Sandra Leithäuser

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Salts and Alveolar Weathering

Alveolization is also known as honeycomb weathering and was first described by natural scientists of the 19th century, on natural outcrops of rocks. These bizarre, but often regularly composed shapes consist of deeply eroded cavities (alveoli), ranging from centimeters to decimeters. The alveoli are separated by walls or ridges of stone, which have been unaffected or nearly unaffected by weathering. The phenomenon of alveolization has attracted the interest of geo-scientists very early on, who aimed to study the question of its origin. In Germany, the typical areas for research were the Palatine Forest (Pfälzer Wald) [Haeberle:1915]Title: Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine
Author: Häberle, Daniel
and the Elbe Sandstone Mountains (Kreidesandstein, [Beyer:1911]Title: Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets
Author: Beyer, O.
). As early as 1915 Häberle [Haeberle:1915]Title: Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine
Author: Häberle, Daniel
points to examples of honeycombe weathering on building stone.

Figure 1: Alveolar weathering on cretaceaus sandstone , Rauenstein, Saxon Switzerland, Germany

Figure 2: Alveolar weathering on the limestone of Caen, church St. Pierre in Caen, Normandie, France

A possible cause for the development of this pattern is the chemical decomposition of the stone [Beyer:1911]Title: Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets
Author: Beyer, O.
, [Haeberle:1915]Title: Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine
Author: Häberle, Daniel
. Otto Beyer, as a result of his investigations, suggests that salt induced weathering plays an important role, naming the salt minerals alum (KAl(SO₄)₂·12H₂O) and gypsum (CaSO₄*2H₂O) in the title of his paper. He sees the different effects of the more readily soluble alum and the poorly soluble, often crust forming salt gypsum as a cause for the development of cavity and honeycombe shaped structures on sandstone rocks. While alum crystallizes in the pores of the sandstone, giving rise to a blasting pressure that destroys the fabric, gypsum blocks up the pores, “fills drip holes and fissures while covering seepage points with a crust, or bulges over them, and achieves with the hereby developed strong ribs, rinds and other cementations, the conservation of the sandstone”([Beyer:1911]Title: Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets
Author: Beyer, O.
, S. 466).

Soluble salts in alveoli have later been detected on many other outcrops in the world, and suggestions have been made that they advance the weathering process. For instance in coastal, but also in arid regions, chlorides often appear in such structures. Alongside, other influencing factors like the induration of the surface are still being discussed, e.g., due to local precipitation of reworked silicic acid on the surface of the rock. In nature such silica weathering processes on rock surfaces proceed very slowly and take a very long time to develop.

In buildings or monuments, several types of stones are prone to alveolar weathering. Amongst these are particularly porous sedimentary rocks (such as sandstones, limestones and tuff). Often visible by the naked eye, the inhomogeneity in the fabric of natural stone effectively causes this weathering pattern. The structure of the sedimentary layers, foreign inclusions, bioturbations or other such disruptions, can cause during the weathering process the relief development at differing speeds. The structurally influencing factors relating to the specific stone, which are responsible for the formation of alveoli, are generally displayed in the developed weathering patterns. Causal differences in the micro-structure can be determined quantitatively, for example, in the contents of minerals with a low resistance to weathering or in the pore size distribution. These differences generate the preconditions for a relief to develop on a previously level surface. An essential role for the progress of weathering in nearly all investigated cases, is played by salts. Salts are usually introduced into the stone via a contaminated environment and by reactions with mortar.

Recent studies on the appearance of alveolization, explored the kind of salts present and their exact spatial distribution in alveoli and the adjoining walls ([Siedel:2008]Title: Salt-induced alveolar weathering of rhyolite tuff on a building: causes and processes
Author: Siedel, Heiner
[Siedel:2010]Title: Alveolar weathering of Cretaceous building sandstones on monuments in Saxony, Germany
Author: Siedel, Heiner
). These investigations showed the interaction of readily and poorly soluble salts in sandstones and tuffs, beginning with localized erosion that resulted in minor differences in relief due to the presence of fossil structures, or, in the case of sandstone, the diagenetically conjunction with ferrous mineral impregnated stone sections, or due to lapilli inclusions in tuff. During the research, salts with significantly differing solubility, such as magnesium sulfate and gypsum, were heterogeneously distributed in the stone following wet-dry cycling. The poorly soluble gypsum stayed mainly near the surface of the stone while the more soluble magnesium sulfate, mostly still in solution, concentrated in the subsurface of the depressions (alveoli) that remained damp for longer times.

The high concentration of salts in the depressions will crystallize upon further drying, and when rewetted it leads to the hydration of salts and to detachment of grains from the stone´s surface. This process enlarges the initially small depression. Often loose accumulations of detached stone grains and salts can be found inside the larger cavities. They repeatedly reappear after removal. Now the alveoli deepen and connect with neighboring cavities, because walls or ridges are eroded. The gypsum encrusted surface, meanwhile, remains stable due to a higher resistance to weathering. The decay pattern of advanced alveolization has been established through detailed investigations on facades. The findings are in accordance with the suggestions made by Beyer in 1911, based on his observations about the combined effect of readily and poorly soluble salts on natural outcrops.

Further detailed investigations, considering other case studies are necessary in order to fully understand the dynamic of weathering and the influencing factors of alveoli formation on building stone.

Literature

[Beyer:1911]	Beyer, O. (1911): Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets. In: Zt. dt. geol. Ges., 63 (4), 429-467
[Haeberle:1915]	Häberle, Daniel (1915): Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine. In: Geologische Rundschau, 6 (4), 264-285, Url, 10.1007/BF01797404
[Huinink.etal:2004]	Huinink, H. P.; Pel, L.; Kopinga, K. (2004): Simulating the growth of tafoni. In: Earth Surface Processes and Landforms, 29 (10), 1225--1233, Url, 10.1002/esp.1087
[Pauly:1976]	Pauly, J.-P. (1976): Le Rôle des chlorures dans les maladies alvéolaire et desquamante. In: Skoulikidis, T. (eds.): Proceedings of the 2nd International Symposium on Deterioration of Building Stones, Athens 1976,Universite Technique Nationale 79-91.
[Pauly:1976a]	Pauly, Jean-Pierre (1976): Maladie Alveolaire Conditions de Formation et d'Evolution. In: Rossi-Manaresi, Raffaella (eds.): The Conservation of Stone I, Proceedings of the International Symposium, Bologna, June 1975,Centro per la conservazione delle sculture all'aperto, Bologna 55-80.
[Siedel:2008]	Siedel, Heiner (2008): Salt-induced alveolar weathering of rhyolite tuff on a building: causes and processes. In: Ottosen, Lisbeth M.; Rörig-Dalgaard, Inge; Larsen , Poul Klenz; Brajer, Isabelle; Bøllingtoft, Peder; Marciniak, Mette; Svane, Maja (eds.): Salt Weathering on Buildings and Stone Sculptures, Technical University of Denmark, Lyngby, Denmark, 79-88.
[Siedel:2010]	Siedel, Heiner (2010): Alveolar weathering of Cretaceous building sandstones on monuments in Saxony, Germany. In: Prikryl, R; Torok, A. (eds.): Natural Stone Resources for Historical Monuments. Geological Society, London, Special Publications, Geological Society, London, 11-23, Url, 10.1144/SP333.2.