Rock IdentifierBasalt
Key takeaways
- Basalt is a fine-grained, dark mafic volcanic rock that cooled fast at the surface.
- Gas bubbles can leave it full of holes (vesicular) or filled with later minerals (amygdaloidal).
- It forms ocean floors, lava plains, and famous columnar formations.
The world's workhorse rock
Basalt is the most common volcanic rock on Earth, paving the ocean floors and vast lava plateaus. It's the fast-cooled, surface-erupted counterpart to slow-cooled gabbro. Rich in iron and magnesium (mafic), it's dark grey to black and so fine-grained that individual crystals are hard to see with the naked eye.
How to recognize it
Look for a dense, dark, even-textured rock with no visible layering. Escaping gas often leaves rounded holes (vesicles) near the top of a flow; when later minerals like quartz, calcite, or zeolites fill those holes, the texture is called amygdaloidal. Slow, uniform cooling can also crack cooling lava into the dramatic hexagonal columns seen at places like the Giant's Causeway.
Look-alikes
Other dark fine-grained rocks (andesite, some basaltic glass) resemble it, and telling them apart precisely can need lab work. But for field purposes, a hard, heavy, dark, fine igneous rock — especially with gas holes or columns — is very likely basalt. It's usually harder and denser than dark sedimentary shale, which splits into flat layers.
