Joint spacing, segregation ice and mountain erosion

In some mountainous landscapes, such as the Southern Alps, New Zealand, segregation ice growth in bedrock headwalls may be an important process that produces sediment scree slopes. In contrast, other regions with similar climatic regimes but different lithologic properties, such as the Sierra Nevada Mountains, California, have considerably fewer scree slopes, suggesting that segregation ice growth is less important. Segregation ice forms by water migration to colder regions driven by the reduction in chemical potential associated with intermolecular forces between ice and mineral surfaces. The growth of segregation ice is a function of temperature and the availability of water (a function of precipitation rate, porosity, and permeability of the rock). We modeled the depth dependency of segregation ice growth using a simple finite-difference model. Temperature is conducted using a sinusoidal boundary condition reflecting seasonal variations in atmospheric temperature. In the model, segregation ice grows when three criteria are met: (1) the temperature within the rock is between -3 and -8circC; (2) when water is available to the system (one of the boundaries is above 0oC); and (3) when water can be drawn from warm to cold. For mean annual temperatures (MAT) above 0circC, the growth of segregation occurs during autumn in the top 20 cm of the rock. In contrast, for MAT below 0oC, segregation ice grows between 60 and 200 cm during the spring. These results suggest that for highly fractured rocks with a <20 cm joint spacing segregation ice growth is a highly effective weathering process. In mountains with fractured rocks, such as the Southern Alps, the efficiency of frost weather can result in considerable rockfall that may regulate the elevation of peaks and ridges. However, for regions with sparsely-fractured (or massive) bedrock, our model results suggest that rockfall contributes less to erosion of these mountains. We show that rock properties and temperature exert primary controls on erosion in mountains.