تعيين ارتفاع خط تعادل (ELA) در دره ي هراز در آخرين دوره يخچالي

An Assessment of Equilibrium-line Altitudes in the Haraz Valley during the Last Glacial Maximum

گسترش و بسط نظريه ي يخبندان كواترنري به وسيله ي تجزيه و تحليل داده ها و يافتن ارتباط ميان جهت و ارتفاع ناهمواري و چگونگي پديد آمدن اشكال مورفولوژي يخچالي هدفي است كه اين تحقيق بنيادي در تلاش براي نيل به آن است. بر اين اساس، اهداف فرعي زير مطرح مي شوند: الف) شناخت اشكال مورفولوژي يخچالي در دره ي هراز؛ و ب) تعيين مرزهاي يخچالي كواترنري در البرز مركزي. دره ي هراز، از دامنه ي آزادكوه تا دره ي اصلي هراز در زردبند، با روند شرقي غربي محل تشكيل اشكال ژئومورفولوژي يخچالي است كه اين مقاله درصدد است تا با رويكردي آماري، ضمن مطالعه ي حدود گسترش يخبندان هاي دوره ي كواترنري و بازسازي ارتفاع خط تعادل (ELA)، به بررسي توزيع سيرك هاي يخچالي منطقه و ارتفاع برف مرز در دوره ي يخچالي وورم بپردازد. براي بررسي و بيان چگونگي توزيع سيرك هاي يخچالي از روش هاي آمار توصيفي و آزمون خي دو استفاده شده است. روش هاي گوناگوني براي بازسازي ELA وجود دارد كه در اينجا، با روش بررسي كف سيرك، ارتفاع برف مرز دوره ي وورم تعيين مي شود. نتايج تحقيق ثابت مي كنند كه در مورد سيرك هاي يخچالي بين فراواني هاي مشاهده شده و فراواني هاي مورد انتظار، تفاوت معني داري وجود دارد و توزيع اين سيرك ها ناشي از جهت ناهمواري است. در مورد ارتفاع برف مرز دوره ي گذشته نيز، ادغام روش بررسي كف سيرك و روش هاي آماري پيشنهاد مي دهند كه در اين حوضه، ميانگين ارتفاع سيرك هاي يخچالي 2799 متر و مقدار نما 2706 متر است. مقدار نما در سيرك هاي رو به قطب ارتفاع 2874 متر و در سيرك هاي رو به استوا ارتفاع 2941 متر مي باشد.

Of all glaciers used as indicators of climate change، cirque glaciers are probably the optimal ones to use. Due to their small size and volume، their response time to positive or negative changes in mass balance is rapidly manifested، often within a couple of years. Singh (2011: 144) suggested that this implies that a sustained negative mass balance، lasting no less than a decade، will rapidly lead to a reduction in size and vice versa. In the quaternary period، glacial and interglacial conditions have occurred in turn in the earth`s history and laid out by geomorphological evidences are undeniable that can be used to determine for snowline borders and equilibrium line altitudes and so on; for studying climate change in that period. It is possible that the reason for external processes of changing the Earth could not have completely cleared away the glacial landforms that remained from of quaternary glaciation. Material and methods: This research is a basic research; thus the main goal of it is the extension of the quaternary glaciation theory by data analyzing and finding relationship between mountain direction and elevation; and creation of glacier landforms. This research wants to find an answer for these questions: 1) is there correlation between glacial geomorphologic evidences in Haraz valley from altitude and distribution? 2) Is quaternary glacial border in Haraz valley in the last glacial maximum located in different altitude or not? If the answer is yes what are the causes of these differences? 3) What is the relation between volume and distribution of glacial cirques and topography and relieves direction? This research's hypotheses are: 1) Extent، volume and frequency of glacial cirques are more in the northern slopes than southern slopes this issue first of all depends on the precipitation volume. 2) The average of glacial cirques altitude and glacial terminus in northern slopes of Haraz valley is lower than southern slopes. Methods or models which are used to answer this issue can be classified in two groups: first، methods and models which are used in the main part of report، containing contour line methods، this is used to restoring the continental and hydrologic data. In the second part which is used for analyzing data، among the methods that Porter suggested، cirque- floor altitude method is selected for this research. When a glacier just fills a cirque، its steady-state equilibrium line altitudes (ELA) typically lies not far above the average altitude of the cirque floor and cirque floor altitude has sometimes been used as a convenient proxy for former ELAs. This research has been based on the comparison between cirques elevation and determination of the extension of glacial toes in the equator-faced slopes and the pole-faced ones. All data and analyses have been shown with use of ArcGIS and SPSS softwares that illustrated on the different thematic maps. Results and Discussion: The analysis of foundation of glacial cirques in dissimilar altitudes has initiated with reconnoiter of those cirques that are located in 100-meter contour lines، and then they have analyzed with the statistical methods to obtain permanent snowline in last glacial maximum in octagonal aspects in the Haraz valley. To compare the mean altitude of glacial cirques and the altitude that there is the most frequency of the cirques، it has calculated arithmetic mean and “mode”. The results of this study showed that there is a significant difference between the observed frequencies and the expected frequency and distribution of these cirques was promoted by the mountain aspects; i.e. poleward slopes that receive relatively less energy than equatorward slopes are more appropriate location to formation of cirques and other glaciers features. Conclusion: According to the calculated equations، temporary snowline is estimated 4938 m in the equator-faced slopes and 5605 m for the pole-faced ones. Based on the synthetic of statistical analysis and cirque floor altitude method، the former ELA has been 2799 m in the study area and modal value is 2706 m. The modal value in the pole-faced ones is 2874 m and in the equator-faced slopes is 2941 m.