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A new quantitative welding degree classification for ignimbrites

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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 flattened at different 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 different ignimbrite types, which have different color and texture properties, obtained from three different 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 classification was developed for ignimbrites considering the uniaxial compressive strength and dry unit weight. The proposed welding classification 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 different 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 classification developed by using threshold values of the oblateness ratio (OR) values of pumice grains at different welding degrees are quite compatible. The proposed welding degree classification is of great importance in the selection of ignimbrites widely used as dimension stone and in terms of engineering classification of this rock type as well as it will guide to the scientific studies to be performed on ignimbrites with varying physical and mechanical properties.

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This study was financially supported by the Scientific Research Projects Coordination Office of the Nevşehir Hacı Bektaş Veli University (project number BAP18F25). We deeply thank the Nevşehir Hacı Bektaş Veli University Scientific Research Projects Coordination Office for the financial support.


Nevşehir Hacı Bektaş Veli Üniversitesi, BAP18F25.

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Authors Mutluhan Akin, Tamer Topal, İsmail Dinçer, Muge K. Akin and Ayse Orhan wrote the main manuscript text. The figures are prepared by Muge K. Akin, Ayse Orhan, Ali Ozvan and Mutluhan Akin. The laboratory tests were performed by Mutluhan Akin, İsmail Dinçer, Ahmet Orhan and Ali Ozvan. All authors reviewed the manuscript.

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Correspondence to Mutluhan Akin.

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Akin, M., Topal, T., Dinçer, İ. et al. A new quantitative welding degree classification for ignimbrites. Environ Earth Sci 82, 345 (2023).

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