IgneousSoil pH 5.0–6.5, depending on the member

Igneous rock

The family of rocks that were once molten, whether they cooled underground or in the open air.

How it forms

Rock melts where the crust is hot enough or the pressure drops enough, rises because the melt is lighter than what surrounds it, and crystallises wherever it stops. Where it stops is the whole difference between granite and basalt.

Igneous — cooled from molten

Magma rising from depth: cooling slowly underground to make coarse crystals, or erupting and cooling fast to make fine onessurfaceerupts — cools fast,crystals too small to seestalls and cools slowly —crystals grow large

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
Rock formed by the cooling of magma. Some maps say only this much, which still tells you a good deal: it will be hard, unlayered, and it will not have fossils in it.
How to know it
Interlocking crystals with no layering and no bedding — grains that grew into each other rather than settling on each other. Coarse grains mean it cooled deep; grains you cannot see mean it cooled fast.
How it wears
By the breakdown of its feldspars into clay, which releases whatever else was locked between them as sand and grit.
What it means above ground
Coarse, gritty soil and rounded outcrops, generally acid and generally hungry, though the dark iron-rich members are much more fertile than the pale ones.
Where you stand on it
Everywhere there has ever been a mountain range or a volcano, which over a long enough span is most places.
What lives on it
Depends entirely on which member: pine and blueberry on the pale acid ones, rich forest and farmland on the dark ones.
What it's used for
Building stone, aggregate, and roughly all of the metal ore ever mined.
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

  • Granite

    the coarse pale one, cooled at depth

  • Basalt

    the fine dark one, cooled at the surface

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.