Avalanches are often larger and more destructive than they seem from looking at the amount of snow in the initial slab fracture because they entrain all the loose snow in the avalanche track, which can often add up to much more snow than the fractured slab that started the avalanche.
In some avalanches involving deep new snow, researchers have estimated that entrained snow might contribute seven times more mass than the original slab that started the avalanche does. This may be one of the reasons big avalanches seem to jump into warp speed as they descend and go into what calls “hovercraft mode,” running bigger and farther than you can imagine.
Avalanches are often larger than initial slab fracture
The extreme violence inside the flowing debris grinds up all the snow into finer and finer particles, and even if the snow starts out light and fluffy, it can become very dense by the time it finally comes to a stop. It’s not uncommon for a large avalanche that starts out with a density of 5–10 percent ice to air to end up as 30–40 percent ice at the bottom. When everything comes to a stop, the dense snow packs very tightly.
Small grains sinter (bond) much more quickly than large grains, and the tiny grains making up avalanche debris can sinter as much as ten thousand times faster than the larger grains of the initial slab. Finally, all of the kinetic energy liberated on the way down heats up the snow a little and can create small drops of liquid water on the surface of the ice grains.
Combining all these factors
It’s easy to see why avalanche debris in anything but the smallest avalanches seizes up like concrete the instant it comes to a stop. Great beauty and great power also bring great horror and great destruction.
Although avalanches look like flowing whitewater in a river, they do not flow as a liquid. Avalanches exhibit granular flow comprising millions of bouncing particles and since avalanche debris is around 30 percent density and the human body is close to 100 percent density, if avalanches flowed like water, we would always sink like a rock in a lake and end up on the bottom of the debris.
The cool part about granular flow is that larger objects tend to rise to the surface, just as shaking a bag of tortilla chips brings the larger, unbroken pieces to the top. In a cruel twist of fate, this also means that the snowmobile, which in a lake would sink like a rock, often ends up on the surface, while the smaller snowmobile rider often ends up buried.
Granular flow explains the success of the avalanche airbag, which, with one pull of the ripcord, makes a human into a much larger object. The human body is more or less neutrally buoyant in granular flow, and an airbag causes a person to quickly rise to the surface.