سطح زمین مجموعه ای است که عوامل درونی و بیرونی بر آن تأثیرگذار بوده و موجب پیکر تراشی سطح زمین و در نتیجه فرم سازی آن میشود. نواحی خشک و نیمه خشک یکی از صحنههایی است که علی رغم بی عارضه بودن آن، فرایندهای متعدد و پیچیدهای در چهره پردازی آن نقش داشته است به گونه ای که محققان همواره در توجیه چگونه به وجود آمدن آنها با نظریات متعدد و گاه پیچیده رو به رو بودهاند. از جمله پدیدههای عام در مناطق خشک و نیمه خشک دشت سرها هستند که بیکرانگی، هموار بودن و وسعت، آنها تعجب بیندگان را به خود جلب نموده و محققان ژئومرفولوژیست برای توجیه نحوه به وجود آمدن آن نظریات و مکانیسمهای متعددی را ابراز داشته اند. یکی از محققان ژئومرفولوژیست که در مورد به وجود آمدن دشت سرها نظریه جامعی تحت عنوان "تسطیح ورقه ای حرکت غیر متمرکز آب" ارائه نمود ال. سی. کینک (.C .King) است. وی در نظریه خود نحوه به وجود آمدن این دشتهای کم شیب و کم عارضه را به حرکات سفره ای نسبت داده و البته به وجود آمدن آن را مشروط به حاکمیت یک دوره طولانی فرسایش آبی میداند. کارهای دو محقق لاگو (Lago) و المدرسی که بیشتر متکی به تکوین ناهمواریها است سبب شد که نسبت به نظریه کینگ شک و تردیدی به وجود آید. لذا در تحقیقی که در دانشگاه اصفهان به اجرا گذارده شد با باز سازی یک مدل مینیاتوری به تجربه عملی نظریه کینگ مبادرت گردید. در این مدل مینیاتوری که سعی شد با حذف مقیاس زمانی تحولات یک عارضه بازسازی شود نتایج متفاوت و متباینی با آن چه کینگ در نظریه خود به آن اشاره دارد به دست آمد. حاصل تجربی بازسازی دشت سرها در مدل مینیاتوری را میتوان در موارد زیر خلاصه نمود: - استدراج در منحنی مقعر سطوح ارضی گلاسیها برخلاف نظر کینک زائیده تکتونیک جنبا است - تراس بندی پلکانی درسطوح ارضی، نماد عدم تحرک پوسته ارضیدر یک دوره ارام فرسایشی است.
عنوان مقاله [English]
Kingâs theory and its empirical challenges
Land surface is created by a collection of land forms created through internal-external processes during time. The arid and semi-arid areas are the most discussable geomorpholical phenomenon that are theâ glassies about which Johnsonâs sheet erosion (1931-1932) and Kingâs fluvial sheet movement theories prevail (1941-1961).âThe climatic evidence nature of this phenomenon approved by researchers on one hand and the creation of the same during resting period of the geographical cycle on the other are the outstanding points of this analysis. After many years, what lead to a serious reappraisal of Kings Theory are the two reports regarding creation of the irregularities on earth surface in a micro model introduced by Dimitri Lague (2003) and Almodaresi (2007). Here through analyzing the results of micro-scale both the absolute rest and slow constant movement of earth, the glacis formations are compared with the King's findings. The experimental tests here indicate that:
- The gradual curving nature of concave glacis surfaces are the result of active tectonic movement, unlike what King theorizes and in case of stationary state these types of glacis formation with such characteristic would not have been created
- The erosion levels (Terraces form) , the zero time index are inherent in the surface movement
Almodaresi (2007) composed a micro model in Ashk zar -Yazd plain and ran a field study trying to reconstruct and record/register the developing stages of a water-basin. He has defined his attempt on his study as an exposition of ergodicity algorithm of a water-basin. This field study in a sense was considered as a time factor elimination phenomenon yielded in specific results with respect to ergodic discussion. The novelty of this study in geomorphic sense is the formed change pattern in the subject micro water-basin. The afflicted changes on the subject basin surface are specific due to the continuous perpetuation. The form change pattern of the territory, in a geomorphic sense raise many issues the most essential of which are:
- Since the test lasted only for three days, the surface movement is not considered
- The territorial profile of the drainage basin is eroded and changed following a The erosion levels (Terraces form)
- The surface of Terrace constantly change and their number increases
- When the surface change ratio of the whole drainage basin inclines towards a specific number (1:8), eroding process stops and the equilibrium is established.
In the duration of controlled erosion process the change pattern of the drainage basin is appealing since in such environmental conditions that are the consistency in land shape and lack of its crust motion. Whatever occurs in the natural arena is the creation of staircase embankment the number of which increases with an increase in erosion time and a decrease in Terraces surface area. This process begins from the base-surface and gradually rises to the highest of the drainage basin.
The result of such land evaluation is a Terrace profile of the surfaces next to one another at different heights. This process contradicts with that of the Kingâs theory. This contradiction is verified through another micro modeling conducted by Demitry Lague where the potential for base-level change was provided in the experiment duration.
Demitri Lague.et.al (2003) presented a micro model and traced the surface changes. The outstanding difference of this model with that of the Almodaresiâs is lack of constant bases surface determined for indicating the erosion
His model is designed in such a manner that the movement of the taste plate containing soil pile and the showering process occurs simultaneously which leads to the gradual uplift of the surface in the precipitation simulator chamber.
His work was captured by LIDAR cameras and then analyzed. The manner by which a chosen surface is developed after water erosion implementation together with base surface change is presented clearly.
The result of the selected photographed surface of the test stages and drawing the land profile shown the 8- step changes clearly and this evolutionary trend especially the 8th step corresponds to Kingâs profile
Bearing in mind that in his test the base surface was uplifted by controlled changes unlike Almodaresiâs test where the crust movement was kept at Zero and appeared completely different and the obtained profiles have special characteristics that on one hand differ from Almodaresiâs model and on the other, it verifies the nearness and similarity of this model with that of the Kingâs.
These characteristics are:
- The evolutionary pattern of land profiles in this test was from convex to concave in a sense that in profile five no more convex profile is seen
-The equilibrium in land evolution is evident based on Daviesâs theory
-The gradual change in the gradient is well traceable
-In profile 8 all the characteristics of a glacis profile is observed according to Kingâs definition, moreover -even Kingâs mountain line is evident in this model.
Said otherwise the Kingâs âgeomorphic setâ that consists of Incellbrge, Kenic, low gradient concave range and the ending playa line are established.
The results indicate that the necessary condition for Kingâs model accomplishment depends on a kind of movement in the surface during the prevailing erosion cycle which contradicts Kingâs theory that is based on the occurrence of such land evolution in a non-movement condition of land evaluation in a resting cycle.
The ordered gradient changes in this model are provable. Lago (2003), was able to draw the gradient changes during the test stages.
Common similarities exist between Kingâs and Lagoâs models and as established, the aspects and trends of a glacis that has occurred and is summarized in the concave range and gradual gradient changes in this model. Here it can be concluded that what King was seeking to established by inducting a kind of surface movement could be accomplished. Said otherwise, creation of boundless glacises is the outcome of harmonized erosive factors and surface movements.
Almodaresi and Lago have examined one process where in one model the surface movement is exerted while in the other no surface movement is involved this difference indicates that contrary to Kings Theory glacises are never created due to sheet gradation of a constant surface during time but due to availability of slow surface movement.
It is obvious that the crust mobility, direction and intensity and its relation to level of erosion are different issues, while both contribute to the formation of glacises. The issue of mobility is not discussed in this study.
The case investigations and studies on glacises in Iran indicate that the great glacis belts like Dagh Sorkh range, Tabas and Gavkhooni playas have fractures on the main drainage channels by cutting along their beds on their slops and the rang fronts. One sample is the Nahrain river bed in Tabas that with no doubt points to crust movement in the region.
Based on the obtained data analysis of two tests conducted by Almodaressi and Lago in micro scale it is deduced that contrary to Kingâs theory where the premise is that the appearance of glossy is due to a prevailing motionless crust resting period, the micro experimental models show that the creation of concave and glacis ranges are the results of fluvial sheet movements accompanied with tectonic movements. In fact if these surface movements are not considered in this equation, the formed sheet gradation would appear in stair case embankment formations. That is, the tectonic movement contributes to the glacis panorama essentially despite the duration of erosion period although the movements in these instances are very rare and slow.