Assessing weathering and resistance of rock units against erosion in the West Islamabad region, Kermanshah province

Introduction
The effects of weathering and erosion on Quaternary rocks and deposits depend on several factors, some of which are related to the nature of the rock and other factors are related to the external environment. Investigating all of the erosive factors is difficult and complex, therefore, the effective factors should be prioritized and taken into account in order to be able to determine a statistical relationship between the erosive factors and sediment statistics. Investigating the weathering and erodability of rock units is very important because of scientific and experimental knowledge of the rock's erodability in the region and as a result, their ranking and also their resistance to erosive factors can be a great help in evaluating the erodibility of rocks and for decision-making about the erosion control project in the region. For this reason, this study was conducted with the aim of investigating the weathering and rating of the rock mass resistance of geological formations to erosion in the West Islamabad region.
 
Materials and Methods
In this research, the role of weathering was investigated with the Lewis Peltier method using isotemperature and isohyetal maps that were produced in the ArcGIS environment using climatic data. Various parameters can be effective in weathering processes, the most important of which are average temperature and annual precipitation, and Peltier models are also created based on these two parameters. For analyzing the resistance of stone units, the inherent characteristics of the parent materials which include mineralogical composition and texture were used. The types of stones and formations in the region were identified using geological maps and Google Earth. In the last stage, the analysis and interpretation of the results and finally the classification of rocks and formations in the study area were done using a qualitative method.
 
Results and Discussion
According to the Peltier diagram and the precipitation and temperature of the study area, the results showed that the prevailing regime in the area is moderate chemical weathering. Also, out of the nine morphogenetic states in the Peltier model, two semi-arid and savanna states occur in the climatic conditions of the region. The study area is diverse in terms of rainfall due to its topography condition. The highest rainfall and the lowest temperature belong to the northern part of the region, and the highest annual rainfall is about 700 mm, which is considered one of the Savan regions in terms of morphogenetic regions. The dominant feature of the Savan region can be called the activity of runoff and the moderate influence of the prevailing winds in the region. Other parts of the study area are considered as part of the semi-arid region due to rainfall and average temperature. The most important geomorphodynamic features of these areas are the influence of wind and moderate to intense running water activity. Water erosion occurs due to heavy rainfall in an area. So, if the amount of runoff is high, water and river processes can be intensive, but this situation in the diagram may be somewhat misleading because in areas with high rainfall and high temperature, this type of erosion is weak. Pelletier probably drew this diagram based on the vegetation, precipitation, and temperature condition, because in areas with high temperature and high precipitation, rich vegetation is formed, which prevents soil erosion due to the presence of foliage and an extensive root system. The erodibility of rock and sedimentary units has been divided into six classes considering lithological factors. Each of the sedimentary units that have the highest level of sensitivity and erodibility is classified in the sixth class, that is, very weak, and according to the degree of sensitivity of the units located in the next classes, it decreases with the decrease of the class number, so that the facies located in the first class has the lowest sensitivity. According to the evaluation carried out in the study area, the least erosivity is related to limestone outcrops. Dolomite sensitivity changes from one to four in different areas, and the most sensitivity is related to discontinuous debris deposits, including The types of alluvial sediments of river beds and floodplains, the alluvium and alluvial cones are related to Qt2 and have a level of sensitivity of 6. The comparison of the results of the classification method presented in this research with the amount of observed sediment yield indicated that considering other effective parameters on erosion, it can be trusted with a high and acceptable level of confidence and it can be used to rate the stone units and formations susceptibility to erosion.
 
Conclusion
As a result of weathering processes, the connected rocks will change to large and small separated materials. These materials rarely remain fixed on the slopes and they move on the slopes due to the gravity and the weight of the mass of materials or under the influence of the transport processes and are transferred to the foot of the mountain in different forms. In the West Islamabad region, due to the heavy rainfalls, mostly moderate chemical weathering occurs. Chemical weathering includes a set of chemical processes and interactions as a result of which substances in the atmosphere such as water, carbon dioxide, and oxygen show a chemical reaction against the minerals in nature, and as a result, new materials and minerals with different characteristics from the primary minerals will be created. Based on field observations and studies, dissolution is one of the most important chemical reactions common in the region. Considering that the dominant regime in the study area is chemical weathering and more than 90% of the area is in the semi-arid area, which is affected by the moderate to the severe activity of running water, as well as the inherent sensitivity of the rocks in the area to erosion, various types of erosion can be seen in the area, which makes it necessary to have suitable soil conservation programs in the region.