New Zealand Mountan Safety Council

Online Course - Basics - Avalanche Types

While there are many different types of avalanche, we can place them into two broad categories; Loose, and Slab. Each has different characteristics to tell them apart.

Loose Snow/ Point Release Avalanche

A loose snow avalanche is also often called a point release avalanche. As the names suggest, they involve loose, unconsolidated snow which initiates from a point, gradually fanning out as it moves downhill. This often makes for a ‘teardrop’ shape. The triggered weak layer tends to be the top layer of the snowpack, the surface layer(s).

Loose avalanches start where the stress is applied.

Typical examples can be:

 

 

 

Loose Dry

These are sometimes called sloughs (sluffs), are usually small, but may gain significant mass on long steep slopes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  
  
Loose Wet

These are commonly confined to the warmest part of the day on slopes that get alot of sun.
They are more powerful than loose dry avalanches due to their higher density.

 

 

 

 

 

 



Slab Avalanche

A slab avalanche involves a cohesive block of snow that fractures within the snowpack. A crack spreads out across the slope leaving a tell tale fracture line called a crown wall. The slab will break up into smaller blocks as it moves downhill.

The triggered weak area (layer) is below the surface layer, within the snowpack. Slab avalanches need a relatively strong layer of snow over a relatively weaker layer. When stress becomes too much for the slab to take, it’s ‘back will break’ starting a slab avalanche in motion. Note: Slab avalanches can start above you




Avalanche Path Anatomy 


Avalanche Forecast Regions:
Mountain Safety Council managed websites
Mountainf Safety Council websiteAdventure Smart websiteNew Zealand Avalanche Center