Weathering and River Erosion: Insights from the Variation of Intact Rock Strength in Rhyodacites

Abstract

The relationship between weathering and fluvial erosion is important in shaping the incisions of bedrock channels. In rhyodacites from Paraná Volcanic Province, plucking and macro-abrasion processes are predominant in the fluvial incision process, according to field observations. Using the intact rock strength as a proxy for the weathering, this paper analyzes how the strength of rhyodacites behave along a cross section of a bedrock channel and seeks to understand how this affects erosion. Topography and intact rock strength were surveyed continuously along a 30 m cross section using a Schmidt hammer, model N. Mean, median, and standard deviation were calculated for each meter of the section. The topographic amplitude of the bed in the section is 30 cm, featuring an almost flat geometry. The average strength values range from 30 to 59 R (7.65 to 58.15 MPa). Despite the small topographical amplitude, the general behavior of the rock strength follows the macrotopography of the bed, increasing in the high zones and decreasing in the low zones. The standard deviation, interpreted as directly proportional to the degree of weathering, follows the microtopography of the bed (amplitude ≈ 10 cm), being greater in low areas and smaller in high areas. This association between standard deviation of strength and microtopography reveals the seasonal variation of the water level in the channel, whereby higher moisture in low-lying areas intensifies the weathering of the rock. Weathering affects the density of rhyodacites in the area with a minimum reduction of 11.2% being calculated. This enhances the erosion processes by plucking and macroabrasion by reducing the intact rock strength. The susceptibility of the rhyodacites to weathering, despite the differences within the section, favors the maintenance of the rectangular geometry of the channel and shows how erosive processes are affected by the condition of the rock. The study revealed that the notion of decreasing the intact rock strength towards the banks associated with greater weathering in higher areas, cannot be generalized, as it depends on the geometry of the channel section, the lithological nature of the bed, and the frequency of flows.