نوع مقاله : مقاله پژوهشی

نویسندگان

1 ژئومورفولوژی، دانشکده جغرافیا، دانشگاه تهران

2 ژئومورفولوژی، دانشگاه پیام نور

چکیده

گسترش و بسط نظریه‌ی یخبندان کواترنری به وسیله‌ی تجزیه و تحلیل داده‌ها و یافتن ارتباط میان جهت و ارتفاع ناهمواری و چگونگی پدید آمدن اشکال مورفولوژی یخچالی هدفی است که این تحقیق بنیادی در تلاش برای نیل به آن است. بر این اساس، اهداف فرعی زیر مطرح می‌شوند: الف) شناخت اشکال مورفولوژی یخچالی در دره‌ی هراز؛ و ب) تعیین مرزهای یخچالی کواترنری در البرز مرکزی. دره‌ی هراز، از دامنه‌ی آزادکوه تا دره‌ی اصلی هراز در زردبند، با روند شرقی‌غربی محل تشکیل اشکال ژئومورفولوژی یخچالی است که این مقاله درصدد است تا با رویکردی آماری، ضمن مطالعه‌ی حدود گسترش یخبندان‌های دوره‌ی کواترنری و بازسازی ارتفاع خط تعادل (ELA)، به بررسی توزیع سیرک‌های یخچالی منطقه و ارتفاع برف‌مرز در دوره‌ی یخچالی وورم بپردازد. برای بررسی و بیان چگونگی توزیع سیرک‌های یخچالی از روش‌های آمار توصیفی و آزمون خی دو استفاده شده است. روش‌های گوناگونی برای بازسازی ELA وجود دارد که در اینجا، با روش بررسی کف سیرک، ارتفاع برف‌مرز دوره‌ی وورم تعیین می‌شود. نتایج تحقیق ثابت می‌کنند که در مورد سیرک‌های یخچالی بین فراوانی‌های مشاهده شده و فراوانی‌های مورد انتظار، تفاوت معنی‌داری وجود دارد و توزیع این سیرک‌ها ناشی از جهت ناهمواری است. در مورد ارتفاع برف‌مرز دوره‌ی گذشته نیز، ادغام روش بررسی کف سیرک و روش‌های آماری پیشنهاد می‌دهند که در این حوضه، میانگین ارتفاع سیرک‌های یخچالی ۲۷۹۹ متر و مقدار نما ۲۷۰۶ متر است. مقدار نما در سیرک‌های رو به قطب ارتفاع ۲۸۷۴ متر و در سیرک‌های رو به استوا ارتفاع ۲۹۴۱ متر می‌باشد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

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

نویسندگان [English]

  • Mojtaba Yamani 1
  • Hamzeh Zamani 2

1 Geomorphology, Faculty of Geography, University of Tehran

2 Geomorphology, Payam Noor University

چکیده [English]

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.

کلیدواژه‌ها [English]

  • The Last Glacial Maximum (LGM)
  • Equilibrium-Line Altitude (ELA)
  • Glacier Cirque
  • Haraz Valley
  • Glacier