What kind of volcanite the rock-hewn churches of the Lalibela UNESCO's world heritage site are made of?

The extraordinary monumental complex of the 11 rock-hewn churches of Lalibela, included in the UNESCO's world heritage list since 1978, attracted the attention of the conservation science community mainly for their severe chemical weathering, physical decay and structural instability. This study, based on classical modal mineralogy and petrography of samples coming from seven churches (Biet Medhane-Alem, Biet Mariam, Trinity Church, Biet Giyorgis, Biet Amanuel, Biet Abba-Lebanos and Biet Gabriel Rufael), sorts out ambiguous rock-nomenclatures and lithological definitions, which have been found in the literature of the last three decades. We can now affirm that the churches were carved in hydrothermally altered and partially lateritized basaltic scorias (nearly aphyric and highly vesicular). The hewn rock, often reported in literature as “weathered basic tuffs”, can be thus classified as a basaltic scoria deposit, partially welded by syn-post magmatic hydrothermal alteration. Its pyroclastic origin may have strongly enhanced selective weathering. The hewn rock rests on a massive to slightly fractured basalt, still present as bedrock of the Lalibela churches and belonging to lava sequences of the Northern Ethiopian Plateau (continental flood basalts). Despite the severe hydrothermal alteration and partial lateritization of the samples, modal mineralogy, petrography and major-trace elements chemistry strongly suggest that the studied clinopyroxene-olivine transitional basaltic scorias of the churches derive from the same magma type, which gave rise to the Lalibela high-titanium group 2 (HT2) of the Northern Ethiopian Plateau lava flows. The late-stage and post-magmatic phases (smectites, zeolites and calcite) scattered in the groundmass and filling the large subspherical vesicles of the basaltic scorias really represent a typical hydrothermal facies of continental flood basalts. Most of the secondary hydrothermal minerals are pointed out first, as well as appropriate modal mineralogy and petrography, providing useful insights towards unraveling the causes of deterioration of these world heritage monuments. A special emphasis is devoted to the presence of zeolite minerals, never pointed out before this study in the rock-hewn churches of Lalibela, and their possible roles on cyclic adsorbing and release of water.