When an initial avalanche breaks down into deeper weak layers while in motion.

Credit: Crested Butte Avalanche Center

A slab avalanche steps down if the added stress of the initial avalanche causes deeper layers to fail and a second bed surface forms deeper in the snowpack. Avalanches that step down often surprise people with their size because they suddenly expand to deeper and often wider dimensions than the initial avalanche. Travelers should be extra cautious managing surface instabilities if there is a lurking weak layer deeper in the snowpack. 

A shallow Storm Slab stepped down to produce a very large Deep Persistent Slab. Credit: Flathead Avalanche Center

The skiers who triggered this avalanche were surprised when it stepped down to produce a larger and wider slide. Credit: Colorado Avalanche Information Center

A shallow avalanche that stepped down and triggered an avalanche that broke at the ground. Credit: Crested Butte Avalanche Center