Granite
The root of a mountain range, cooled slowly enough to grow crystals you can pick out by eye.
How it forms
A body of magma stalls in the crust instead of erupting. It cools from the outside in, and each mineral crystallises as the temperature passes its freezing point — feldspar first, quartz last, filling the gaps between everything else. Tens of kilometres of rock then have to be worn off the top before anyone can stand on it.
Igneous — cooled from molten
Rock that was liquid. Cooled slowly at depth it grows crystals big enough to see; cooled fast at the surface the crystals never get the chance, and the rock comes out fine and dark. Everything else on this page started as one of these.
- What it is
- Magma that never reached the surface. It cooled kilometres down over tens of thousands of years, and that slowness is why its crystals are large enough to see without a lens: glassy grey quartz, blocky pink or white feldspar, and flecks of black mica.
- How to know it
- Speckled, not striped. Look for three colours of grain at once — clear-grey, pink or white, and black — all roughly the same size and packed together with no layering and no preferred direction. If the minerals line up in bands, you are looking at gneiss.
- How it wears
- Feldspar turns to clay and lets go, so the rock crumbles grain by grain into coarse sharp sand. Rounded corners come first: a granite boulder field is a set of spheres because the edges weathered fastest.
- What it means above ground
- Sour, sandy, quick-draining soil that holds neither water nor nutrients well, over rounded outcrops and boulder fields. The water off it is soft, clear and acid, with almost nothing dissolved in it.
- Where you stand on it
- The Canadian Shield from Ontario to Labrador, the Sierra Nevada and the coast ranges of British Columbia, the White Mountains, the Idaho batholith, and the core of the Appalachians.
- What lives on it
- Pine, spruce, oak on the drier ridges, blueberry, sheep laurel and reindeer lichen. Soft acid lakes with lake trout and smallmouth bass but few of the molluscs that need calcium.
- What it's used for
- Countertops, curbstones, monuments and breakwaters — it is hard, takes a polish and does not dissolve. Most of the grand civic buildings of eastern North America are faced with it.
- What it does to the soil
- Sours the soil above it — nothing in it neutralises rain, so the ground stays acid.
Next to it in the cycle
Is this what you're standing on?
Find the rock under your town
Enter a town or postal code and we'll show you the rock beneath it and how old it is, the soil it makes, and the chain of rivers your water joins on the way to the sea.
General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.