Crown profile of skier triggered avalanche 12-23-09.  Willows drainage, NW facing, 38-39 degrees, 1.5-2' crown, 80' wide.  There was about 100' of hangfire above the crown.  Snow was thin and weak and while descending to the crown while rappelling on a rope, I was punching through the slab into the extremely weak depth hoar.  The slide descended into trees, so the person who triggered the slide was extremely lucky not to be injured or killed being strained through trees.