عوامل كنترل كننده هوازدگي حفره اي (تافوني) در گرانيت هاي جنوب مشهد و سازنده شمشك واقع در بخش شمالي بينالود

Controlling factors of cavernous weathering (Tafoni) in granite rocks of Southern Mashhad and Shemshak formation on Northern parts of Binaalood

The most common weathering features on granite landforms are cavernous weathering that are known as tafone. Tafoni are deep cavities which are produced by a variety of different weathering mechanisms. This study investigates the formation of tafoni in the granitic rocks of South Mashhad and Shemshak formation in semi-arid climatic conditions. Based on petrographical studies, this granite consists of crystal quartz, k-spar, plagioclase and mica. In the study area, tafoni formed by both physical and chemical weathering processes and are mainly controlled by mineralogical and structural characteristics of the rocks and variations in the local climate and microclimate within the caves. Due to granular fabric of these rocks, one of the most important processes in the formation of these features is granular disintegration of unstable minerals such as feldspar and mica. The in situ weathering soil (under lichens) from granite rocks of Southern Mashhad was studied by X-Ray diffraction (XRD) and consists of quartz, orthoclase, albite, calcite and clay minerals (kaolinite, illite, montmorillonite). Joints, pegmatite veins, xenoliths, exfoliation, core softening and case hardening of rocks outcrops are other important factors that control the tafoni initiation and growth. Therefore, the writers suggest the lithology, structure of the rock and environmental effects are the most important control factors of tafoni occurrence in this region. In Shemshak formation, dissolution of calcareous cement and salt weathering are the most roles of tafoni formation in this area.