Fine-Grained (Aphanitic) Textures

Fine-grained textures generally indicate magmas that rapidly cooled at or near the Earth's surface. Fast cooling prevents crystals from growing very large. The cutoff between fine- and coarse-grained textures is about 1 mm.

All four photos below are of the mafic rock basalt. The more felsic aphanitic rocks tend to have isolated crystals and thus are examples of porphyritic textures with fine-grained groundmasses. (Basalt can also be porphyritic!)

This is a boulder of fine-grained basalt from a barn wall in Wisconsin. The black part of the rock is fresh, the rust and greenish gray parts have been weathered (the minerals have partially broken down due to reaction with water and naturally occurring acids).
You can see in this close up of the boulder shown above a crystalline texture, but the individual grains are less than 1 mm across (and are too small to identify by eye). Thus, this is a fine-grained texture.
Here is another fine-grained basalt boulder that happens to be darker gray than the one above. Color variations are normal.
The bulk of this basalt from California is clearly fine-grained. The irregular white masses are mostly vesicles that have been filled in with a white mineral; they are examples of amygdaloids. The white mass to the upper left is a bird dropping. Note the cracked weathering rind along the lower right edge.

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