Q: Someone told me they were carving rotten granite. What is that?
A: The term granite brings to mind concepts of hardness, permanence, and intractability of stone outcrops. But boulders can be weathered to a crumbly material, often brown stained that may be easily carved even without carbide or diamond tools.
“Rotten granite” is an informal term used rather accurately to describe this partially decomposed granite.
All rocks at or near the earth’s surface are subject to weathering. Changes may be brought about by physical, chemical, and biological processes. The end product is sediment and soil, although from a geological perspective it’s not the end, but simply a step in a great cycle toward sedimentary rock and beyond. Rotten granite is part way there.
By geologic definition, granite and granitic rocks contain visible grains of quartz, feldspar, and various iron-bearing minerals. These grains all formed from molten material, cooling and crystallizing into a tight, interlocking, three-dimensional puzzle. Subjected to rainwater, groundwater, organic acids, and biological activity the quartz changes very little, but the feldspar begins to alter to clay minerals. The iron-bearing minerals convert to brown, yellow, and red iron oxides: what artists know as ochre. The loosening of the puzzle pieces would hardly be noticeable in a human lifespan.
Weathering moves inward from the rock surface or outward through cracks and fractures, any feature that allows water to contact the rock. With an appropriate set of fractures, granite weathers much as an ice cube melts, from a cubic form to a rounded form—a boulder. A partially weathered boulder may still be coherent enough to carve. It won’t take a polish (it may not even tolerate grinding and sanding) but broad forms with rustic finishes could be carved. However, rotten granite may not be uniformly rotten, a core or other areas of unaltered material may remain. But this offers an opportunity to contrast stained, altered, and unaltered surfaces with only a minimal investment of carving time. Of course there is also the risk of the stone’s simply falling apart with the first (or last) hammer blow.
As weathering progresses beyond any coherence, granite masses may disintegrate to sandy, granular debris called “grus” (from German for grit or gravel). This decomposed granite is used commercially for landscaping, road surfacing, and chicken grit.