Petra Jordan: A World Heritage Site Under Risk

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Petra is famous for its rock-cut monuments like sanctuaries, tombs, and temples. These monuments are located in the Shera alps of Jordan, situated east of the Wadi Araba Valley.

Approximately 2200 years ago, Nabateans (ancient Arab people who inhabited northern Arabia) had carved more than 900 monuments using sedimentary rocks as they were well-aware of the aesthetical features of sedimentary rock. 

In 1986, the United Nations of Education, Science, and Cultural Organization (UNESCO) emblazoned Petra on the directory of the world heritage site as a work of genius, bearing inimitable testimony to the advancement of civilization that has vanished. Rock-cut monuments of Petra are the epitome of exceptional architecture and represent an important phase of human history. 

In 2007, the rock-cut monuments of Petra were selected as one of the seven wonders of the world. Petra region is also considered as a famous historical site in Jordan because it has impacted the tourism economy of the country. More than 700,000 tourists visit the rock-cut monuments of Petra in a year.

The sedimentary rocks of the Petra region are of lower paleozoic age. Mostly, the sandstone formations of the sedimentary rock are the pertinent stratigraphical units used for carving the monuments. These sandstone formations are the part of the Ordovician disi formation and the Cambrian umm ishrin formation.

Most of the rock-cut monuments of Petra are undergoing weathering due to external agents such as water, humidity, and salt that have contributed to the damage. Also, anthropogenic activities, natural hazards like floods, and earthquakes have worsened the situation.

Thus, by looking at the development of damage, the world monuments fund added the rock-cut monuments of Petra to the list of top 100 endangered monuments of the world in 1998, 2000, and 2002. Therefore, the rock-cut monuments of Petra have become a global concern for preservation. 

In this article, we will discuss the rock types, weathering agents, which are contributing to the degradation process, and preventive measures to reduce the degradation of the rock-cut monuments.

1. Geology of the Petra Site

The ruined city of Petra includes many monuments, tombs, quarries, stairways, and theaters all constructed directly from sandstone rock, especially Cambrian and Ordovician sandstones. More details about the sandstone rock, which is mostly affected by the weathering, are described below: 

1. Most of the sandstone rocks are underlain by igneous rocks, such as granite, gneisses, and schists. 
2. Plinth level of igneous rock displays a well-defined characterized contact plane whereupon the sandstone strata is situated.
3. The shade of Petra sandstone is rosy, that is why the city is called "a rose-red city". 
4. Umm Ishrin Sandstone stratum is of middle to upper Cambrian age, and the thickness of the strata is around 300 m. 
5. The upper-level blanketing formation is comprised of Disi sandstone, and the thickness of this formation is 100 m. This stratum displays a whitish-beige color and is seen throughout the valley. 
6. Sandstones of the Disi formation are critically influenced by the weathering agents. Such strata erode into characteristic domed features that can be recognized from miles away. 
7. Umm Ishrin sandstone formation shows extensive moment jointing and faulting. In contrast, Disi formation sandstone shows less frequent jointing. 
8. Weathering induced sandstone surface recession ranges from 12-50 mm on a horizontal surface and 6-20 mm on vertical surfaces.  

2. Climatic Conditions at Petra Site

The climatic conditions at the Petra region are generally arid with moderate to mild blustery winters and sweltering, dry summers. Regional rainfall occurs due to change in barometric pressure conditions as the peripheral precipitation passes through northern Israel and Jordan. However, when low-pressure fronts get across northern Africa or up through the red sea, heavy rainfall occurs in the Petra region. Such conditions occasionally bring flooding in the region due to a combination of orographic lifting and cyclonic flow.  

Mostly, the precipitation occurs between November and March, and the mean yearly precipitation at the Petra region is around 110-130 mm. Hints of snowfall are also reflected. However, a large portion of the precipitation is observed as rainfall. 

The temperature in the Petra region may dip below 0 degrees during January. However, freezing temperature conditions are not common, and the temperature rises rapidly during the daytime. The maximum and minimum temperature is observed in the months of August and January and the range of temperature is around 18-39⁰C and 6-12⁰C, respectively. 

3. Weathering Factors and Degradation of the Monuments

High-temperature loads are induced on many rock-monuments in Petra. Mostly, the monuments located in the south and west are exposed directly to the sun, for example, the Silk Tomb. Variation due to direct isolation and high-temperature loading affects the monuments and promotes the attack of weathering agents.  

Researchers have hypothesized temperature as a mild to moderate weathering agent for degrading the quality of Petra's rock-cut monuments. As indicated by the outcomes gathered from various investigations, the correlation of high-temperature loading and weathering damage is not very severe. However, mild to moderate damages occur on the monuments exposed to temperature loading compared to the monuments insignificantly affected by temperature loading. Thus, high-temperature loading has turned out not to be the principal reason for the degradation of the monuments due to weathering. 

On the other side, a strong correlation exists between water impact and weathering damage. During rainy reason, runoff water flows over the facades of the rock-cut monuments, and flooding conditions were also observed occasionally. Thus, the potential of damage to the monuments due to water impact is high. For example, the effect of water on the Unaishu Tomb during heavy rainfall is illustrated in Figure-2. The portion of monuments protected from the rainfall appears in a lighter color, whereas, the portion of monuments exposed to rain appears in a darker color in the photographs. 

Researchers have observed the changes in exposure characteristics of the monument during periods of rainfall. Such observations helped in mapping the exposed part of the monument, which is vulnerable to damage due to water impact. An example is presented in Figure-3. It was discovered that the south to west exposed monuments are more influenced by water impact due to precipitation compared to the north to east exposed monuments. 

The architectural elements, such as moldings and pediments, provide protection from rain for some of the locally exposed monuments from south to west. Overflowing water affects sites, especially when water channels above the site are filled or eroded with debris. Water flowing from architectural elements is an attribute of many Petra monuments because precipitation often exceeds the water absorption capacity of the rock sediments. The lower sections of many monuments are also influenced by increased humidity.

The predominant weathering factors causing the degradation of the rock-cut monuments are water, salt, and humidity. Water runoff and floods affecting the monuments are characterized by high mechanical water impact potential. Several types and extents of humidity and the impact of salt relate to the weathering patterns formed on the monuments. Mostly, the monuments are categorized into damage index of two degrees. The low damage index represents the monuments sheltered from the rainfall. Thus salt penetration is negligible. The high damage index represents the monuments directly exposed to the rainfall, therefore higher salt penetration is possible.   

Salt weathering has been found to be the main and most detrimental water or moisture-induced weathering activity. The results of mineralization and geotechnical studies have exhibited that all major types of rock separation, such as contour deposits, flaking and granular decomposition, are associated with the presence of salts.

Mostly, halite (NaCl) was found to be the primary salt mineral deposited on the external walls of the monuments. Also, the presence of niter (KNO?), Gypsum (CaSO?.2H?O), and sylvite (KCl) were observed. The type, amount, spatial distribution, and crystallization cycle of salt control the type and strength of stone exfoliation. Chemical analysis of rainwater shows that a significant proportion of the salt component comes from rain, especially with respect to the formation of halite. Nitrate load on the walls of the monument (especially at the bottom) is probably due to the effects of livestock over the decades. 

The wind is observed to be another weathering agent influencing the degradation of the rock-cut monuments. However, the wind is not causing the collapse of stones. Still, it contributes considerably to the erosion of loose stones, and the evaporation of salt fluids and their movement through rocks.

4. Preventive Measures

The following types of preventive measures should be taken into consideration to protect the rock-cut monuments of the Petra region:

1. Stone blocks should be prevented from breaking out. Preservatives should be used to reduce salt weathering. 
2. Loose stone materials should be reinforced again to preserve the originality of the monuments. 
3. For highly damaged monuments, structural stabilization or reinforcement should be provided to keep them structurally sound. 
4. Repair should be done for the damaged stone parts and missing parts should be replaced. 
5. Joints and fissures should be sealed properly to avoid the percolation of water and thus to prevent the disintegration of stone material. 
6. A system should be provided to construct and maintain the control of runoff water. Thus, the capillary rise can be prevented and further the disintegration of stones can be reduced. 
7. The rise in humidity should be controlled by planting more trees at the affected locations. 
8. Desalination of runoff water must be provided because salt weathering is mostly affecting the rock-cut monuments. 
9. The regular cleaning process must be carried out to reduce the deposition of harmful agents on the surface of stones. 
10. External stone wall surfaces, which are exposed to weathering agents, should be protected. Protection can be provided by applying preservative solutions on external surfaces. 

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