A Forest Service Guy Just Told Me That..........

Rocks get cracks in them from expasion and contraction, however I think this is more to do with heating and cooling. cracks can fill with water and during cold periods this water can freeze thereby increasing the size of the crack.
I imagine there are certain types of gological structures that we might ordinarily call rocks that do absorb water but I cant give you an example.

Soil will expand and contract as water is removed and replaced. water (rainfall) will erode the soil around a rock and lossen it. I would have said that this is more likely to cause a slide.

That's what I had always thought.

I think a lot depends on whether or not the soil is covered with flourishing vegetation with healthy roots to hold the soil in place.

Where I've seen the most slides it was simply because the hills were so steep, they'd get saturated and just slide, roots or no roots. You don't see rockslides much in dry weather, unless there is water underneath.

Thank you.

What you all are saying makes a lot more sense than what the FS guy told me.