Abstract
This study demonstrates the Fourier transform infrared (FTIR), Raman spectroscopy andscanning electron microscope - energy dispersive X-ray spectrometer (SEM-EDS) techniques which has been applied to characterize decay products and processes occurring at the surface of rock-hewn churches at the UNESCO’s World Heritage site of Lalibela, Northern Ethiopia. Spectroscopic analyses comparing the observed frequencies with the available literature, minerals were identified. A special emphasis is devoted to the presence of whewellite/weddellite and kaolinite minerals, never pointed out before this study in the rock hewn churches of Lalibela, and their possible roles on rock decay. The identification of montmorillonite as a weathering product up to 20 mm from the rock/atmosphere interface beyond the level at which kaolinite, whewellite and gypsum are found, suggests that montmorillonite may penetrate even deeper into the rock substrate causing major rock decay. Results indicate that biological attack by green algae and lichens is currently responsible for severe stone surface, physical and chemical weathering leading to considerable weakening of the churches walls. A prompt and careful removal of the biological and other deposits with the non-destructive method (laser cleaning) treatment is therefore recommended. Key words: Fourier transform infrared (FTIR), Raman, whewellite, kaolinite, rock hewn churches, mineral characterization.
Highlights
Lalibela is known throughout the world for its extraordinary complex of 11 monolithic churches (Figure 1), carved, some 800 years ago during the Empire of King Lalibela
This study demonstrates the Fourier transform infrared (FTIR), Raman spectroscopy and scanning electron microscope - energy dispersive X-ray spectrometer (SEM-EDS) techniques which has been applied to characterize decay products and processes occurring at the surface of rock-hewn churches at the UNESCO’s World Heritage site of Lalibela, Northern Ethiopia
Results indicate that biological attack by green algae and lichens is currently responsible for severe stone surface, physical and chemical weathering leading to considerable weakening of the churches walls
Summary
Lalibela is known throughout the world for its extraordinary complex of 11 monolithic churches (Figure 1), carved, some 800 years ago during the Empire of King Lalibela. Water absorption represents the main contributing factor of damage for high exposed rock materials and structures, in terms of direct rainfall, soil infiltration, capillarity and diffuse humidity, for the alteration of basalt (Delmonaco et al, 2009). Another study (Asrat and Ayallew, 2011) reported material loss due to deep weathering activated by rain water penetration and the leakage of groundwater affects most of the Lalibela churches. This exposure has resulted in the severe degradation of the churches, most of which are considered to be in a critical condition. Most of the churches are protected by shelters to prevent the erosive effects of rainfall, especially on the roof rock cover, and temporary scaffolding to prevent collapse of the most exposed structures
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