بررسي نقش گُلسنگها در هوازدگي زيستي سنگ‌هاي مخروطي شكل روستاي كندوان

Influence of biological weathering by lichens on the cone-shaped rocks of Kandovan village

سنگ‌هاي مخروطي شكل (كله‌قندي) روستاي تاريخي كندوان در اثر عوامل مختلفي از جمله كلني هاي زيستي هوازده مي‌شوند. در مطالعه حاضر با نمونه برداري و تهيه مقاطع نازك ميكروسكوپي، انجام آناليز اشعه X و تصوير برداري با ميكروسكوپ نور بازتابشي و فلورسنس، تغييرات فيزيكي و شيميايي ناشي از فعاليت هاي زيستي گُلسنگ بر روي سنگ‌هاي روستاي كندوان بررسي شده است. بر اساس مقاطع ميكروسكوپي و آناليز اشعه X توده سنگ روستاي كندوان داراي تركيب اسيدي بوده و عمدتاً از كاني‌هاي كوارتز، پلاژيوكلاز و هورنبلند تشكيل‌شده است. تقريباً تمام كاني ها سالم بوده و هوازدگي و دگرساني به كاني‌هاي رسي كمتر ديده مي‌شود. تصاوير ميكروسكوپي نشان مي دهند كه گُلسنگها در تلاش براي جذب مواد معدني ريسه‌هاي خود را در سنگ‌هاي كندوان نفوذ داده و باعث تخريب بافت سنگ تا عمق حدود 5 ميلي متري مي‌شوند. ضمناً اسيد اگزاليك ترشح‌شده از گُلسنگ نيز باعث هوازدگي شيميايي قشر سطحي سنگ مي‌شود. اقليم سرد و نيمه‌خشك و ارتفاع بالاي روستاي كندوان مانع از فعاليت زيستي زياد و هوازدگي شديد ناشي از گُلسنگ شده و تركيب اسيدي سنگ نيز باعث مقاومت آن در برابر اسيدهاي آلي گرديده است. با اين حال اين نوع از هوازدگي و فرسايش و تخريب‌هاي ناشي از آن در طولاني مدت توانسته باعث شكل گيري مورفولوژي خاص روستا شود. با اين حال ادامه روند هوازدگي باعث تخريب بيشتر سنگ‌هاي مخروطي شكل مي‌شود كه در اين راستا انجام اقدامات كنترلي و حفاظتي ضروري به نظر مي‌رسد.

Introduction Kandovan village with cone-shaped rocks is one of the tourist attractions in East Azarbaijan province of Iran. The materials of these houses are types of pyroclastic rocks (Ignimbrite) with acidic composition. The cone-shaped rocks are due to rock erosion and weathering along fractures. In terms of geography, Kandovan is located in a region with 2220 altitude and semi dry-cold climate. Unfortunately, the cone-shaped rocks of this village are exposed to various deterioration factors such as biological weathering caused by the growth of lichens on them. Lichens are described as an association between a fungus and photosynthetic partners, generally green algae or cyanobacteria. Weather and climate condition of the area are one of the most important factors for lichen’s growth. The wet, light and so warm environments have best condition to its growth. In suitable condition lichen’s photosynthesis is more intense. On the other hand, characteristics of the highland such as high temperature fluctuations, high levels of ultraviolet light (UV-B), wind high speed, variable snow cover and short growing seasons are inappropriate condition for lichens. They affect their substrate rocks, both in their natural state and when used as building stones have long been recognized. Studies show there is a direct relationship between the growth of lichens and rock weathering. The interface between lichen and rock is known as a place of considerable physical and chemical activity. Generally, the physical weathering of rocks by lichens proceeds by the penetrating of hyphae through intergranular voids and mineral cleavage planes and the absorbing of rock minerals. One of the chemical effects of lichens on rocks and minerals is rock’s weathering and dissolving due to lichen’s acidic secretions such as oxalic acid that during this processes, mineral and components of rock change to clay minerals and metallic oxides. Methodology The present study has dealt with the physical and chemical changes caused by the biological activities of the lichens on rocks using microscopic thin sections, X-ray analysis and imaging by epi-illumination and fluorescence light microscopy. For this purpose several samples have been taken from 1cm deep of rock surface that covered by lichens for X-ray analysis and 5 cm of it for making tin section. For using epi-illumination and fluorescence light microscopy, samples have been taken of rock surface that covered by lichens same as other samples. All samples have been sampled in the spring because the most bio-active step for lichen is in this time. In fluorescence light microscopy method, rock samples have been painted by aniline blue. All studies on weathered rocks have been repeated on fresh rocks for comparing the results. Discussion and results According to study of rock thin sections via microscope, the rocks of Kandovan have an acidic chemical composition and it is composed mainly of the fresh Quartz, Plagioclase and Hornblende minerals and there are no traces of clayey minerals. The results of X-ray analysis are shown that same as rock thin sections, all components of rocks are fresh. Thus, the chemical weathering caused by the growth of lichens on rocks should be less than 5 mm. The prepared microscopic images show there are secretions on lichens thus they are in bio-active step. The physical weathering is shown as a number of cracks, powdered and soft form under the lichen coverage. These changes are caused by the penetrating of hyphae through intergranular voids and mineral cleavage planes and the absorbing of rock minerals. The images obtained by fluorescence light microscopy method are shown penetration of lichen’s hyphae through the rock components. Despite lichen’s acidic secretions, all rock’s minerals are fresh and there isnʹt considerable chemical weathering that be determined by X-ray analysis. In a vertical sequence of samples that covered by lichen, biological weathering intensity reduced of the surface to a depth and after 5 mm deep of rock surface is not founded no deterioration. Conclusion Cold and dry climate of Kandovan village and its high altitude have prevented the high biological activity of lichens and severe weathering caused by them and also acidic chemical composition of rock have caused it to be persistent against organic acids. Nevertheless, the weathering of the thin layer of these rocks by biological factors was considered an important agent for forming this specific morphology in the long term and is considered a serious destructive agent and it is necessary to clean and protect against them.