This is a nice piece of rhyolite from an erratic (probably not very far-travelled) found on the flank of Carningli in Pembrokeshire. Note the weathering crust, which is everywhere about 10mm thick and in places up to 15mm thick. The crust is formed by a weathering process called oxidation, and it will continue to thicken for thousands of years -- maybe millions -- as long as there is free water in the environment. the rock can be buried or exposed to the atmosphere -- it doesn't seem to matter very much. Different rock types react to chemical weathering in different ways.
One factor which will influence this weathering crust is the annual rainfall received in the location in question: