Petra revisited: An examination of sandstone weathering research in Petra, Jordan

Abstract

Petra, Jordan, was an important crossroads city occupied during the Nabataean and Roman eras. This paper presents a review of a series of studies conducted from 1990 to 2003 that scrutinized both lithologic (intrinsic) and climatic and anthropogenic (extrinsic) weathering infl uences on the Paleozoic sandstones in Petra, an ideal environmental "laboratory" for the study of weathering features, causes, and rates. However, these structures, which have been stable since their creation two thousand years ago, are deteriorating at an accelerated rate due to natural and human-induced stone decay processes.Comprehensive measurements of surface recession were made in Al-Khazneh, Petra's most celebrated tomb; the Roman Theater; and the Anjar Quarry above this abandoned city. Surface recession rates for sandstone in the Roman Theater were determined to range from 15 to 70 mm/k.y. on horizontal surfaces to 10-20 mm/k.y. on vertical surfaces. Higher iron and silica contents of sandstone matrix were found to decrease overall sandstone weatherability, while calcareous matrix components were found to increase deterioration in areas that receive >5500 MJ/m 2 /yr of solar radiation. Moreover, when iron matrix concentrations exceed 4%-5% (by weight), original stonemason dressing marks are still clearly evident, indicating a nearly unweathered state in 2000 yr. Visitors to Petra have dramatically increased from 100,000 (1990) to 350,000 (1998). Large (and typical) tourist groups entering the chamber of Al-Khazneh were found to raise interior relative humidity levels from 20% to 50%, and interior surfaces have dramatically receded due to visitor touching, as much as 40 mm in less than 50-100 yr.

Chemo-mechanics of salt damage in stone

“…However, it is in conservation science that the consequences of crystallization pressure are most feared because of their impact on monuments, such as the Pharaonic constructions in Luxor 21 , the carved city of Petra 22,23 and the medieval city of Rhodes 24 . Crystallization pressure is even more devastating for wall paintings because of the rapid irreversible damage it can cause to works such as Michelangelo's frescoes in the Sistine Chapel 25 , the tomb of Nefertari 26,27 or prehistoric rock art such as the Albarracin Cultural Park 28 , to name only a few.…”

Revisiting dirt cracking as a physical weathering process in warm deserts

“…Insolation effects (Hobbs, 1918;Blackwelder, 1933;Peel, 1974;Warke and Smith, 1998;Paradise, 2005;Moores et al, 2008), moisture changes (Mabbutt, 1977;Camuffo, 1995;Moores et al, 2008), salt weathering (Mortensen, 1933;McGreevy and Smith, 1982;Viles and Goudie, 2007), exfoliation (Blackwelder, 1925), and fire (Dragovich, 1993) all play a role in generating observed fractures. The processes by which fractures undergo enough additional physical weathering to open crevices wide enough to result in detachment are less well understood.…”

“…The starting point for Ollier's (1965: 237) hypothesis is the same as the starting point of this second iteration: "whether insolation [or other processes] can cause initial cracking of a fresh boulder [or bedrock] need not concern us here." While ongoing research has provided insight on how rock fractures initiate (Friedman, 1975;Engelder, 1987;Paradise, 2005;Viles, 2005;Warke, 2007;Moores et al, 2008;Adelsberger and Smith, 2009;Eppes et al, 2010;Smith et al, 2011), the way that cracks start in desert rocks is beyond the scope of this study because dirt cracking does not initiate fractures.…”

Salt weathering and experimental desalination treatment of building sandstone, Sydney (Australia)

“…The weathering and deterioration of building materials is accomplished by various combinations of biological activity, moisture, humidity, aspect, and temperature (Turkington and Paradise 2003;Mottershead et al 2003;Paradise 2005;Wells et al 2007). In the case of sandstone, its heterogeneous composition is an important control on decay patterns .…”

“…The response of dimension stone to environmental conditions depends on the properties of the rock and the characteristics of the weathering environment to which it has been subjected since emplacement, often referred to as intrinsic and extrinsic variables, respectively (Paradise 2005). Where rocks contain one or more minerals which are susceptible to weathering, or have high permeability or a large number of microfissures, deterioration of the stone may be relatively rapid.…”