A new quantitative welding degree classification for ignimbrites

Abstract

As a pyroclastic rock type, ignimbrites may reveal varying degrees of welding depending on the temperature (>535 ℃) and overburden pressure conditions during its formation. The welding degree of ignimbrites increases as the formation temperature and the thickness of the overburden deposit in the depositional environment escalate, which are the most crucial factors controlling the rate of welding in ignimbrites. With the increasing temperature, plastic deformation is observed in ignimbrites and the glassy minerals are being welded. Furthermore, the thickness of the overburden causes the deformation of the ash matrix in ignimbrites at the lower sections and the pumice grains are fattened at diferent rates. An increase in the degree of welding of ignimbrites causes an improvement in the physical and mechanical properties of the rock material as well. Within the scope of this research, petrographical, mineralogical, and geochemical studies were carried out on a total of 16 diferent ignimbrite types, which have diferent color and texture properties, obtained from three diferent regions of Turkey (Kayseri, Nevşehir, Ahlat) where ignimbrites extensively crop out, and the physical and mechanical properties of these samples were revealed. Consequently, a new welding classifcation was developed for ignimbrites considering the uniaxial compressive strength and dry unit weight. The proposed welding classifcation consists of six classes ranging from non-welded to highly welded. When the welding degrees of the selected ignimbrites are evaluated, Kayseri ignimbrites mostly exhibit moderate welding characteristics. Nevşehir ignimbrites, on the other hand, have a low welding degree whereas the degree of welding in Ahlat ignimbrites may vary from low to high. Additionally, long and short axis lengths of pumice grains in the ignimbrite specimens were determined by measuring under the microscope, and shape ratios were determined by diferent shape parameter evaluation methods. As a result, it has been concluded that the pumice grains in Kayseri and Ahlat ignimbrites have a more lenticular structure than the pumice grains in Nevşehir ignimbrites. Eventually, the welding degree classes of ignimbrites and the classifcation developed by using threshold values of the oblateness ratio (OR) values of pumice grains at diferent welding degrees are quite compatible. The proposed welding degree classifcation is of great importance in the selection of ignimbrites widely used as dimension stone and in terms of engineering classifcation of this rock type as well as it will guide to the scientifc studies to be performed on ignimbrites with varying physical and mechanical properties.