Rockfall activity from an alpine cliff during thawing periods

Rockfall activity during thawing periods was investigated by collecting rockfall debris fallen on the snow-covered talus slope in a cirque in the Japanese Alps. Near-surface rock temperature was also monitored on the cirque wall. Maximum rockfall activity occurs on average about 10 days after the meltout of the cirque wall. The intensive activity is rarely associated with precipitation events or diurnal frost cycles on the rockface. A thermal conduction model suggests that rockfalls at this site occur most frequently when the seasonal thawing front penetrates to a depth of about 1 m in the bedrock. While the freeze-thaw penetration controls the maximum dimensions of detachable rock mass, joint spacing on the rockface affects the size distribution of rockfall debris. Boulder falls resulting from seasonal frost weathering are considered to be the most important process responsible for the contemporary modification of the cirque wall. The rockfall volume during a thawing period is usually 1-3 m(3), which is equivalent to the rate of cirque wall retreat of the order of 0.01 mm year(-1). However, big boulder falls that occur once per decade can increase the retreat rate significantly. (C) 1999 Elsevier Science B.V. All rights reserved.