Spheroidal, onion skins weathering and granular desintegration ("grusification").
Silicate rocks are not completely soluble in water, they are subject to differential weathering...Spheroidal weathering :
Let's take a look of this kind of weathering
Karlu Karlu Reserve, Northen Territory, Australia. source
We can often observe this kind of structures when some weathered areas have been cleared by road works.
Hydration is the main chemical process breaking granite down, rain water is acid (contains disolved CO2, humic acides...) and affects the rock.
Let see what happens more specifically with these cartoons :
