Slab and loose snow avalanches

If you’re looking for the killer, then the slab avalanche is your man. This is the White Death, the Snowy Torrent, the Big Guy in the White Suit. Dry slab avalanches account for nearly all of the avalanche deaths in North America.

A “slab” is a cohesive plate of snow that slides as a unit on the snow underneath. A slab doesn’t have to be hard, it just has to be relatively stronger than the snow underneath. A typical slab is about half the size of a football field, about one to two feet (30–60 cm) deep, and usually reaches speeds of 20 mph (32 km/h) within the first three seconds, quickly accelerating to around 80 mph (128 km/h) after the first, say, six seconds. The bonds holding a slab in place fracture at about 220 mph (352 km/h) and the slab appears to shatter.

Dry slab avalanches can lie teetering on the verge of catastrophe, sometimes for days or even months. The weak layers beneath the slabs are extremely sensitive and the rapid addition of the weight of just one person can easily initiate a fracture on a slope that would not have avalanched otherwise. A slope can sometimes be a giant booby trap—seemingly waiting for just the right person to come along. The crack often forms well above the victim, leaving little room for escape.

Loose snow avalanches
Loose snow sliding down a mountain is called a loose snow avalanche. Small loose snow avalanches are called “sluffs.” Few people are killed by loose snow avalanches because they tend to be smaller, and they tend to fracture beneath you as you cross a slope, rather than above you, as slab avalanches often do. Most of the people killed in loose snow avalanches are climbers, or extreme skiers and boarders in very steep terrain.