All igneous rock comes from molten rock — but where it cools splits the family into two very different kinds. First, two words people constantly mix up:

• Magma — molten rock while it's still underground.
• Lava — the very same molten rock once it reaches the surface (out of a volcano). Same stuff, different address.

The two kinds of igneous rock

• Intrusive rock (also called plutonic) forms when magma pushes up into older rock and cools slowly, underground — never reaching the surface. Slow cooling gives mineral crystals time to grow large, so the rock looks coarse and grainy. Granite is the classic example.
• Extrusive rock (also called volcanic) forms when lava erupts and cools quickly at the surface. Fast cooling means tiny crystals or none at all, so the rock looks fine and smooth. Basalt is the classic example.

This hands geologists a neat trick: big crystals means it cooled slowly underground (intrusive); tiny crystals means it cooled fast at the surface (extrusive).

So what's an "intrusion"?

An intrusion is simply that underground event given a name: a body of magma that pushes ("intrudes") into the surrounding rock and hardens there. The hardened result is intrusive rock. You'll often hear of an intrusion cutting across older layers — and that's a useful clue, because the intrusion must be younger than the rock it cuts through.