Understanding some of the effects of shelter design on deterioration at the Mnajdra Megalithic Temples of Malta

The site of Mnajdra, one of the UNESCO-listed Megalithic Temples of Malta (https://whc.unesco.org/en/list/132)  dating back to 3600 - 2400 BC, is located on the southern coast of the Maltese archipelago. This site, along with two other similar sites, was sheltered in 2009 with reversible, open-sided shelters, with the aim of mitigating the critical impact of the aggressive marine environment on the conservation of the limestone megaliths.  Environmental conditions such as rain, wind, salt damage and direct insolation - triggering thermoclastism (thermal stress)- were identified as a key factor contributing to the progressive stone weathering and are currently being monitored by a multidisciplinary study.

This abstract will focus on understanding the impact of these protective, open-sided shelters on the temperature variations of the Mnajdra’s façade, with particular attention to diurnal and seasonal fluctuations, comparing to data available from the pre-sheltering period. Other studied environmental factors will not be addressed here.

The Mnajdra complex is widely recognized for its astronomical alignment, with the main (eastern) façade oriented to mark the equinoxes, solstices, and other solar events. In order to retain the association of the Temples with these alignments and to continue to observe these events, the shelters were designed to remain more open on the eastern side. As a result, surface temperature variations are currently monitored on the eastern façade, where direct sunlight could cause significant temperature fluctuations with possible subsequent deterioration effects such as microcracks formation and progressive material weakening.

Thermal imaging data was thus collected across two seasons - autumn (9th October 2023) and summer (19th June 2024) at 10-minute intervals during morning hours to identify trends of the fluctuations.

Results from these two campaigns revealed significant surface temperature fluctuations in autumn and lower variations in summer. Surface temperature gradients were observed, with a more intense gradient in autumn (from 27,8°C at 7.50 am to over 35°C at 8.50am), and less intense fluctuation in summer (from 27,3°C at 6.50am to 30,4°C at 7.50am). In both seasons, hotspots were identified particularly in areas of different megaliths (of the same stone type) where prolonged exposure (approximately 1h) to direct solar radiation occurred.

Taking into account the difference in air temperature recorded during the acquisition (19-25°C in October and 26-32°C in June) this difference can possibly be attributed to the inclination of the sun. The more significant surface temperature fluctuations observed in autumn are likely attributed to the angle of solar incidence at that time. The sun reaches the studied megaliths more directly in autumn and less directly in the summer solstice, resulting in greater variations in surface temperature.

This study forms part of a broader multidisciplinary project integrating non-invasive analytical techniques and environmental parameter modelling to evaluate the efficacy of sheltering systems. All the findings will provide scientific data to inform conservation strategies also aiming at mitigating the progressive weathering on these unique heritage sites, and ensuring their long-term preservation sites also in projection of increasing challenges due to the impacts of climate change.