Quaking or "thumper" crust

Near Crystal I observed an extreme example of what I like to call a "thumper" or quaking crust. What I observed was that in many locations on especially SE aspects was that as one skinned up the crust would usually support body weight but would occasionally shake then collapse locally about the skis. In this case, the collapse could be heard and seen to be propagating over a much wider area, indicating much worse than the normal slightly unstable or unconsolidated underlying layer. One could actually hear the propagating wave go out from the disturbed area. On inspection the layer down about 6" was very low density and very large, presumably faceted grains.

Stability wise, we did release one small 6" slab. The progression of instability went like this: When well-frozen, the crust would usually not fail; when beginning to soften the crust would collape over a 25 to 50' radius and settle 1/2" or so; when the crust softened so as to lose integrity then ski penetration increased to 6" in very sloppy, wet snow with large grains at depth. So, there is likely a period of time when the crust is strong enough so as to give good stability, there is also a period of time when the crust is able to store some energy and give the potential for wet slab release, and finally, when the crust breaks down sufficiently, there is the likelihood of wet, loose slides.

Hazard (in this region and above 5500') varies from low when the crust is frozen to moderate or even locally higher as the crust weakens and then moderate when the crust breaks down more completely.

Thanks for the report Gary, you described the conditions saturday at Summerland perfectly. Weird, on some aspects, loud thumps with every steps, radiating waves of sound. Other aspects, crust, feels totally bombroof. we need some radiation to consolidate and ripen up this pack, that's for sure. I need corn!

I am so jealous. That sounds really interesting. Would have loved to have dug pits and checked out with the screen and loup.
Teanaway yielded only boring corn.
Thanks for the report.

These kinds of crusts are very interesting. This one clearly had very little recent load - an inch or two of low density snow, yet there was extensive propagation. Also, it wouldn't propagate once the crust weakened to a certain extent, so the crust itself was a very important agent of the weak layer's failure outward from the center of our additional load.

I think these types of layers, a form of near crust faceting, are the most unpredictable of all layers. With sufficient load it is obvious that they can fail as a persistent weak layer (although the duration of persistence is much shorter in springtime). But it is also interesting and possible that there may be failure without load, but with just weakening (from warming in this case or further faceting).
So that implies that while faceting was occurring when it was quite cold the amount of settlement that took place was minimal. Hence the crust itself was storing elastic energy. Granted there was also a requirement for the weak layer to fail (as it weakened slightly from warming or was overstressed by crustal weakening), but without the crust, the weak layer would merely have settled out rather than collapse suddenly. I've actually seen remote triggering in these types of conditions absent additional load, but just with warming.....

It's not possible to really predict whether failure is very likely or just somewhat likely and digging just shows the weakness. It's unlikely the faceting itself would have been visible with a loupe. It's more the case that "this is weak and hollow and it was cold recently, so it must be faceted..." The grains were unusually large and had little connectivity; apparently enough to propagate the failure, but not much more....

We found the same conditions above 9,000 on the south climb of Mt Hood on Monday, Gary.
I was thinking that with another foot of snow on top of that crust it should be ideal conditions for experimenting with the extended column test, eh?

We found the same conditions above 9,000 on the south climb of Mt Hood on Monday, Gary.
I was thinking that with another foot of snow on top of that crust it should be ideal conditions for experimenting with the extended column test, eh?

Or any other quasi-stability test. The extended column should behave very similarly to the Rutschblock, but with less digging. But I think the proof was in the pudding anyway, the evidence of propagation was very obvious, at least in our case (sounds like yours was too). And the fact that the propagation was extensive, it's progression audible, and that in failure, the crust broke and collapsed 1/2" was definitive as far as failure, at least in certain locations and in certain degrees of warming.

With a foot of new snow, that kind of structure is very scary. The variability is very great, too, obviously with elevation and aspect, but even with just slight changes in slope angle or slope orientation (aspect). A few degrees different and the crust gets stronger so as to minimize the likelihood of release, or gets weaker either to promote failure or to make the layer less able to propagate, depending on the situation. Even the shade of a tree can make a big difference....

The best lesson is to recognize weakness and to acknowledge that the variability is great enough that the behavior of the crust is an unknown in many locations.