Crust occurrence on a Galatian rock-cut dwelling in central Anatolia, Turkey

Mahkemağcin Underground City (MUC) is a four-floored rock-cut dwelling complex carved into ignimbrite tuffs of the Early-Middle Miocene age in central Anatolia. The whole interior of the complex is covered by a 1–5 cm thick alteration crust that is not detected in any cave or outcrop in the region. This crust is investigated using microscopy, scanning electron microscopy, X-ray diffraction and X-ray fluorescence analyses, and the results are discussed from the geomechanical point of view. The colour of the outer surface of the crust is black, which turns red inside and grades into pale yellow above the white parent rock. The compaction and solidity of the crust are relatively high, as much as double that of the parent rock. The crust seems to have prevented and/or significantly decreased the erosion of the surfaces of the room walls. Mineralogical and chemical analyses together with experimental studies on selected samples suggest that such a crust could have formed because of thermal alteration at high temperatures ca. 600–950°C. On examination of the heated samples, the main geochemical changes within heated rocks were (i) an increase in silica concentration by ca. 2%–3% and (ii) the disappearance of zeolites and sericites. Thermoluminescence dating of the crust revealed thermal resetting of quartz crystals within the ignimbrite tuff at 243 ± 260 CE. All the analytical methods applied to the site indicate that the crust was obtained intentionally, possibly by burning woods indoors, aiming to harden the wall surfaces, mainly by Galatian people of the early Medieval time.