Classifying Igneous Rocks
Texture - What does it look like? Refers mainly to mineral size
Composition - What color is it? Rocks are grouped as to whether they are light or dark in color.
Common Igneous Textures |
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Texture |
Definition |
Example |
Aphanitic |
Minerals too small to see |
Basalt |
Phaneritic |
Minerals large enough to see |
Gabbro |
Glassy |
No obvious minerals |
Obsidian |
Pyroclastic |
Fused, glassy volcanic rock fragments and ash |
Tuff |
Vesicular |
Having many holes or pits (escaping gas) |
Pumice |
Porphyritic |
Two distinct mineral sizes |
Olivine Basalt |
Minerals at the top of Bowen’s Reaction Series: |
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| Dark in color | Mafic to ultramafic | Iron & magnesium |
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Minerals at the bottom of Bowen’s Reaction Series: |
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| Light in color | Felsic | Na plagioclase, K plagioclase, quartz |