Investigation about the effect of slope characteristics on soils, forming from granite, using the most appropriate weathering index in semi-arid west of Isfahan province

Granite forming primary minerals and the clay minerals that are formed from weathering of these minerals, determine characteristics of the soils that originate from the granite. Slope characteristics through their effect on soil moisture and temperature, causing changes in soil. The weathering in the soil can be expressed with the most appropriate weathering indices. This study aimed to investigate the soil formation on granite in various aspects, situations and forms of slope, using the most appropriate indices.
Gharreh –Boltagh granitic mass is located in semi-arid west of Isfahan province. In each of the northern and southern aspects of the slope, three profiles on the backslope and three profiles on the footslope were excavated, described and soil and rock samples were taken from their horizons. Physical and chemical properties of the samples were measured. Of the eight weathering indicies, the index with the highest correlation with the ratio of pedogenic iron to total iron, the clay content and bulk density, was selected as the most appropriate index.In order to assess the degree of chemical weathering from subsoil towards topsoil and in various aspects, positions and shapes of the slopes, this index was used.
Significant correlations between the weathering indices and the ratio of pedogenic iron to total iron and bulk density and non-significant correlations between these indices and the clay content were observed. Of the indices, the index of WR was the best index to express the intensity of weathering. The elements Ca, Na, and Mg have been used as mobile elements in the formula of this index. Calcium and sodium in the plagioclase and magnesium in the biotite are present. The average value of WR for granite and for regolith, derived from the weathering of this rock were calculated as 15.8 and 10.9, respectively. On convex slopes, the average values of WR and the regolith thickness were determined as 14.8 and 31 cm, respectively. These values for the concave slopes, respectively were calculated as 13.3 and 90 cm. The average values of WR and the regolith thickness were obtained as 13.9 and 80 cm on the backslopes and as 14.3 and 32.5 cm for the footslopes, respectively. The reason for the higher average value of WR and the lower average value of regolith thickness in the footslopes is presence of a granitic rock outcrop at the northern footslope. On the southern slope, the average values of WR and the regolith thickness were calculated as 14.3 and 43.7 cm and on the northern slope, these values were determined as 14.0 and 69.2 cm, respectively.
Given that the studied area is semi-arid, the amount of rain is at a level that as a result of granite weathering, basic cations are released and leached from the soil. This is the reason why the indices which are determined by the ratio of basic cations oxides to immobile elements oxides, were selected as the most appropriate weathering indices. In addition to degree and aspect, shape of the slope also affects parent rock weathering. Since less water can penetrate the soil on convex slopes, weathering in this type of slope and in concave one was minimum and maximum, respectively. As it was expected, weathering was less on back of the southern slope, relative to its foot. Unexpectedly, on the northern slope, contrary to this phenomenon happened, because there is a rock outcrop at the footslope. At the footslope, the slope is less. As a result, more water penetrates the soil and more weathering occurs. In the southern slope, weathering was less than the northern slope.