A multianalytical approach to investigate stone biodeterioration at a UNESCO world heritage site: the volcanic rock-hewn churches of Lalibela, Northern Ethiopia.
A multianalytical approach combining Optical
Microscopy (OM), Backscattered Variable Pressure Scanning
Electron Microscopy + Energy Dispersive X-ray Spectroscopy
(VP-BSEM + EDS), Powder X-ray Diffractometry
(PXRD), Raman Spectroscopy, and Microbiological
techniques has been applied to characterize decay products
and processes occurring at the surface of two rockhewn
churches (Bete Gyorgis and Bete Amanuel) at the UNESCO’s
World Heritage site of Lalibela, Northern Ethiopia.
The two churches were carved into volcanic scoria deposits
of basaltic composition. In their geological history, the Lalibela
volcanic rocks underwent late to post-magmatic hydrothermal
alteration together with partial laterization and
are therefore characterized by a decay-prone highly vesicular
microtexture with late stage to post-magmatic precipitation
of secondary mineral phases (calcite–zeolite–smectite).
The main objective of the study was to gain a better insight
into the weathering products and mechanisms affecting the
surface of the stone monuments and to assess the relative contribution of natural “geological” weathering processes
versus biological/salt attack in stone decay at this unique
heritage site. Results indicate that while the main cause of
bulk rock deterioration and structural failure could be related
to the stone inherited “geological” features, biological
attack by micro- (bacteria) and/or macro- (lichens) organisms
is currently responsible for severe stone surface physical
and chemical weathering leading to significant weakening
of the stone texture and to material loss at the surface of
the churches walls. A prompt and careful removal of the biological
patinas with the correct biocidal treatment is therefore
recommended
