When salt spray or mist lands on rock, then evaporates, salt crystals grow (as sugar crystals do in maple syrup when it sits around a long time) and pry apart the other tiny grains of mineral. With enough time, moisture and drying, big caves can be dug into what seems like hard rock. There can be a tiny amount of regular salt in granite, and there are other salts that can form (gypsum), though I don't know that our gneiss has enough of either to do the wonderful sculpting we have at Snowy Mt. However, there often is a thin coating of a white material on the rock, and I found a thicker layer of white mineral in three small hollows in one boulder.
Recently I took a group of hardy and surprisingly trusting people to refind one glacial erratic near Thirteenth Lake. We did a lot of difficult bushwhacking, but they were great sports, especially the ones who had been lost with me before! But this boulder is easy to find though not to get to. It is different from the other tafoni because it has eroded into about five sections, unlike the others that have virtually no cracks in them. And it is covered with moss whereas the others are barren except for a little green algae.
This tafone also is unique because it is a favorite roosting spot of some tiny bird that sits on a small projection, to judge by the droppings beneath it. I would love to know what kind of bird it is and and see it enshrined in its very special shelter!