Direct observation of frost wedging in alpine bedrock

Width and temperature of rock joints were automatically monitored in the Ja panese Alps. Three years of monitoring on a sandstone rock face shows two s easonal peaks of joint widening in autumn and spring. The autumn events are associated with short-term freeze-thaw cycles, and the magnitude of wideni ng reflects the freezing intensity and water availability. The short-term f reezing can produce wedging to a depth of at least 20 cm. The spring events follow a rise in the rock surface temperature to 0 degreesC beneath the se asonal snowcover, and likely originate from refreezing of meltwater enterin g the joint. Some of these events contribute to permanent enlargement of th e joint. Two other joints on nearby rock faces experience only sporadic wid ening accompanying freeze-thaw cycles and insignificant permanent enlargeme nt. Observations indicate that no single thermal criterion can explain fros t weathering. The temperature range at which wedging occurs varies with the bedrock conditions, water availability and duration of freezing. Copyright (C) 2001 John Wiley & Sons, Ltd.