Igneous Rocks
Formed by the cooling and solidification of magma or lava.
Igneous rocks form when molten rock (magma underground, lava on the surface) cools and solidifies.
Intrusive igneous rocks (e.g. granite):
- Magma cools SLOWLY underground (inside the crust).
- Slow cooling β minerals have time to form LARGE crystals.
- Result: coarse-grained texture.
- Example: granite, diorite, gabbro.
Extrusive igneous rocks (e.g. basalt):
- Lava erupts onto the Earth's surface and cools RAPIDLY.
- Rapid cooling β minerals form SMALL crystals (or no crystals β glassy).
- Result: fine-grained or glassy texture.
- Example: basalt, obsidian, pumice.
Why does crystal size matter? Slow cooling allows atoms to arrange into ordered crystal lattices over longer periods. Rapid cooling "locks in" atoms before large crystals can form.
Economic importance: Igneous rocks are mined for valuable minerals. Magma intruding into existing rocks creates hydrothermal veins that concentrate valuable metal ores (gold, copper, tin). Kimberlite pipes β a type of igneous intrusion β are the source of most of the world's diamonds.
- Intrusive = slow cooling = large crystals (granite).
- Extrusive = fast cooling = small crystals (basalt).
- Hydrothermal veins associated with igneous activity concentrate metal ores.