Earth and Space Science / Unit 2: Minerals and Rocks
2.4 Igneous Rocks
Notes
A rock is a solid mixture of minerals (or other materials). Igneous rocks form from cooled magma (underground) or lava (at the surface).
- Intrusive (plutonic) rocks cool slowly underground and have large crystals: granite, gabbro.
- Extrusive (volcanic) rocks cool quickly on the surface and have tiny crystals or glass: basalt, obsidian, pumice.
- Felsic rocks (granite) are light-colored and rich in silica; mafic rocks (basalt, gabbro) are dark and rich in iron and magnesium.
- Texture shows cooling history: coarse-grained means slow, fine-grained means fast, glassy means almost instant cooling.
Practice
Granite has large visible crystals. It most likely formed:
Large crystals indicate slow cooling, which happens underground.
Which igneous rock is extrusive and dark-colored?
Basalt is a fine-grained mafic volcanic rock.
Obsidian looks like black glass because it:
Extremely fast cooling gives a glassy texture.
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