Prehistoric megalithic temples of the Maltese archipelago : causes and mode of deterioration of Globigerina Limestone

Abstract

The causes and mode of deterioration of the Globigerina Limestone have been studied within the framework of a multidisciplinary research project on the prehistoric megalithic temple complexes of the Maltese arcipelago. The temple complexes ofTarxien and Hagar-Qim (Malta) were entirely built in this particular lithotype, which was also used to make decorative elements in Ggantija (Gozo). Approximately one hundred samples (surface fragments and cores) have been studied in all; studies have included: mineralogical investigations (principal composition. that of the insoluble residue and of the clay fraction), petrographic examination in thin section, phisical tests (determination of the specific gravities, total open porosity, porosimetric distribution, absorption capacity and degree of saturation), chemical analysis (soluble salts) and morphological observations by SEM. The sum total of the data obtained has led to an understanding of the mode of deterioration, divided into distinct phases having a precise temporal succession: the formation of a thick and compact superficial crust due to the reprecipitation of calcite dissolved from within the stone by imbibed water (and which contains variable quantities of NaCl in solution); lifting of the crust and the start of intense deterioration phenomena including loss of cohesion, corrosion and powdering within the substrate which has become weakened by the dissolution of calcite; loss of the crust, often followed by initiation of the process of alveolisation. This latter process depends on the number, orientation and size of the sedimentary structures (bioturbations) present. This mode of deterioration, widespread in many other analogous "soft limestones" of the Mediterranean Basin (e.g. "Pietra Leccese" and Calcare di Noto") is directly related to the associated carbonatic composition and elastic structure and with certain distinctive characteristics: a very high porosity (up to and over 40%) and water absorption which is not extremely fast on first contact with water, but which continues for over 5 months after immersion.