3) What makes weak layers persistent?

Really, just their very weak nature.

From Cookiemonster

3) What makes weak layers persistent?

* Large grain sizes and other structural characteristics such as thickness.
* Anisotropic strength characteristics ( stronger in compression than shear ).
* Metamorphic phenomena at various spatial and temporal scales.

Not only do weak layers (PWKL's) have weak structure, but the very large and angular ice crystals they consist of are so very different than the rest of the snowpack (which ususally consists of much smaller grains with relatively good bonds) that the metamorphic process takes a very long time to break down the enlarged ice crystals. The structure often favors sudden collapse over slow settlement.

I've been posting a lot of collections of general statements because there are so many possible situations and outcomes that's it's impossible to do anything but generalise. Going in-depth usually requires speculation, or the construction of edge cases, which I try to avoid when discussing snow safety.

I'll elaborate below. Please note that some of this discussion is speculative and may not reflect the snowpack at any particular place and time.

1. Large grain sizes and other structural characteristics such as thickness.

* For a given aggregate of large ice grains, the pore space will be correspondingly large.
* Large pore spaces provide room for further crystal growth.
* Overburden pressure is one of the primary factors in reducing pore space.
* Most crystal forms associated with persistent weaknesses exhibit anisotropic strength characteristics.
* This means that the crystal forms in weak layers resist overburden pressure, which allows the pore space to remain large.

Of course there are other variables, such as the nature of the bonds between individual ice grains, the relationship between crystal morphology and stress concentration at interfaces, and other synoptic-meso-micro-scale metamorphic phenomena in the snowpack.

2. Anisotropic strength characteristics ( stronger in compression than shear ).

* See #1.

3. Metamorphic phenomena at various spatial and temporal scales.

* I don't want to write 100 more bullet points today!