* Cooling rate: Basalt that cools quickly, like in a volcanic eruption, will have fine-grained textures (also called aphanitic). This is because the crystals have less time to grow. The grains are so small they're often not visible to the naked eye.
* Cooling depth: Basalt that cools slowly, like in a deep intrusion, will have coarse-grained textures (also called phaneritic). This is because the crystals have more time to grow. The grains will be larger and more easily visible.
* Presence of phenocrysts: Some basalts have larger crystals called phenocrysts embedded within the finer groundmass. This indicates a two-stage cooling process.
Common grain size terms used to describe basalt:
* Aphanitic: Very fine-grained, crystals not visible to the naked eye.
* Phaneritic: Coarse-grained, crystals easily visible to the naked eye.
* Porphyritic: Contains phenocrysts (larger crystals) within a finer-grained groundmass.
Example grain sizes:
* Fine-grained basalt (aphanitic): < 1 mm
* Coarse-grained basalt (phaneritic): > 1 mm
* Basalt with phenocrysts (porphyritic): Variable, depending on the size of the phenocrysts.
Therefore, when talking about grain size in basalt, it's important to specify the specific type of basalt and the context.
