Deep in the crust and upper mantle, rock can melt into magma. When magma or lava cools, mineral crystals grow and lock together into igneous rock ("igneous" means "from fire").
| Where it cooled | How fast | Texture | Examples |
|---|---|---|---|
| Deep underground (intrusive) | Slowly, over thousands of years | Coarse: crystals big enough to see | Granite (light), gabbro (dark) |
| At the surface (extrusive) | Fast, in days to years | Fine: crystals too small to see | Rhyolite (light), basalt (dark) |
| At the surface, almost at once | In seconds to minutes | Glassy: no crystals | Obsidian |
| At the surface, full of gas | Fast, with bubbles | Full of holes (vesicular) | Pumice (light), scoria (dark) |
Key idea
Slow cooling grows large crystals (intrusive rock like granite); fast cooling makes small crystals (extrusive rock like basalt); very fast cooling makes glass.
Why it works
Crystals grow as atoms in the melt find their places and join a growing crystal. That takes time. Deep underground, surrounding rock keeps magma warm for thousands of years, so a few crystals grow large. Lava at the surface loses heat quickly, so many tiny crystals start at once and none gets big. If lava freezes in seconds, the atoms stay jumbled: glass.
Worked example: Big crystals
A rock has large, interlocking pink, white and gray crystals. Where did it form?
- Large crystals need slow cooling.
- Slow cooling happens deep underground, so this is an intrusive rock (it sounds like granite).
Answer: Deep underground, cooling slowly
Worked example: Two sizes
A rock has a few large crystals scattered in a fine-grained background. What happened?
- The big crystals grew slowly while the magma was still deep.
- Then the magma rose and erupted, and the rest cooled fast into tiny crystals.
- This two-stage texture is called porphyritic.
Answer: Slow cooling underground, then fast cooling at the surface
Common mistake
Expecting fast cooling to make big crystals. Crystals need time to grow, so it is the reverse: slow cooling makes big crystals, and lava that cools in seconds can become glass with no crystals at all. Check: the coarse rocks (granite, gabbro) are the ones found where deep rock has been exposed.