Evaluation of the Relationship between Welding Degree and the Shape of Lithic Fragment for İgnimbrites

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Akın M.(Executive), Topal T., Dinçer İ., Akın M., Özvan A. , Orhan A., et al.

Project Supported by Higher Education Institutions, 2018 - 2020

  • Project Type: Project Supported by Higher Education Institutions
  • Begin Date: July 2018
  • End Date: September 2020

Project Abstract

Ignimbrite, a pyroclastic rock type, may have different welding degrees depending on the temperature (> 535˚C) and pressure conditions during formation. The welding degree in ignimbrites increases as the formation temperature and thickness of the overburden load in the depositional environment, which are the most important factors controlling the welding, in ignimbrites. With the increasing formation temperature, plastic deformation is observed in the ignimbrites and welding occurs in the glassy minerals. Moreover, the increasing thickness of overburden due to deposition deforms the matrix of ignimbrites as well as the pumice grains at the lower sections. During this deformation, lithic fragments are flattened at different rates. With high deformation, pumice grains of ignimbrites acquire a lens-shaped structure and the rock becomes more welded. The increase in the welding degree of ignimbrites causes the physical-mechanical properties of the rock to improve.

Within the scope of this research, petrographic, mineralogical and geochemical studies were carried out on 16 different ignimbrite types, which have different color and texture properties, obtained from three different regions (Kayseri, Nevşehir, Ahlat) where ignimbrites are widely observed in our country, and the physical and mechanical properties of these samples were revealed. In addition, the long and short axis lengths of lithic fragments consisting of pumice and other rock grains found in the same ignimbrite samples were determined by measuring under the microscope, and shape ratios were revealed by different shape parameter evaluation methods. When evaluated in general, it was revealed that pumice grains in Kayseri and Ahlat ignimbrites have a more lens-shaped structure. In Nevşehir ignimbrites, pumice grains do not have an oblate structure. On the other hand, in all ignimbrite samples, it was revealed that there is no significant flattening of the lithic grains belonging to other rock fragments. From the point of material strength, Kayseri ignimbrite mostly has higher strength than other ignimbrite types. Nevşehir ignimbrites have the lowest strength.

As one of the aims of the study, the statistical relationships between different physical and mechanical properties of ignimbrites and flattening rates in pumice grains were investigated. As a result of simple regression analysis, it was determined that the most significant relationship was between the aspect and oblateness ratio of the pumice grains and the uniaxial compressive strength of ignimbrites. According to this relationship, the higher the oblateness ratio (flatness) of the pumice, the higher the material strength.

On the other hand, in order to evaluate the welding degree of ignimbrites as a result of this research project, other research data obtained in the region were also included in the database, and a welding degree classification was developed for ignimbrites by using uniaxial compressive strength and dry unit weight. The suggested welding classification consists of six classes ranging from non-welded to very well-welded. When the welding degree of the studied ignimbrites is evaluated, Kayseri ignimbrites mostly exhibit moderate welding degree. Nevşehir ignimbrites have a less welded structure. In Ahlat ignimbrites, the degree of welding may vary from low to good. At the same time, a great similarity was achieved between the welding degree classes of ignimbrites and the classification using the threshold values of the oblateness ratio (OR) values of pumice grains at different welding degrees.