Double slab avy?

**Fatty** · 03-23-2007, 05:35 PM

My tivo grabbed an episode of TGR's Untracked where a bunch of snowboarders went down to Chile for some access to turns, etc.

What caught my eye was this:

It appears that there is a double slab separation under the ridge with debris down in the lower right. (in addition to the slab that looks to have ripped from lower left all the way to upper right

)

From the (low quality camera-phone on rear projection tv) picture, what can you say about the causes of this? How normal is a double release like this? Think it was concurrent or the top layer releasing first then the second releasing later? Am I just seeing this wrong?

Avy jong here - saw something unexpected and wanted to hear some thoughts

**mrkristofo** · 03-23-2007, 05:48 PM

This is called a step-down avalanche. There's 2 weak layers present here, say the first storm came in and dropped some graupel, followed by hard winds and made a slab. Follow that with some cold clear nights for some surface hoar growth on top, and then another storm with some wind making another slab. What happens is, the first layer fails, and as it gains momentum the extra stress it places on the second layer causes it to fail as well.

A really primo example if this is the massive Ruapehu (sp?) avalanche video. The first slide is a small dry slab triggered with the charge from the heli, and after it picks up some speed the whole mountainside rips loose. Look on youtube if you havent seen it.

EDIT: http://patrol.mammothmountain.com/mo...pehu030906.mpg (96MB), 34 seconds in.

**eirikainersharp** · 03-25-2007, 11:11 PM

I have seen fracture mechanics like this more than once. The are generally attributed to a complex snow pack. Generally there is a shallower weaker layer above and a stronger layer bellow. Initially the shallow layer fails and the when it builds up sufficient momentum iniciates a failure in the lower layer. Essentially you can tink of it like a compression test when you get a sudden planar on 5 taps at the base of the storm snow and then another sudden planar on 15 taps on a month old crust further down.