Petra revisited: An examination of sandstone weathering research in Petra, Jordan
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 influences 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/m2/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.