columnar basalt definition
basalt. n. 1. (Geological Science) a fine-grained dark basic igneous rock consisting of plagioclase feldspar, a pyroxene, and olivine: the most common volcanic rock and usually extrusive. See flood basalt. 2. (Ceramics) a form of black unglazed pottery resembling basalt. dark, fine-grained, igneous rock consisting mostly of feldspar, iron, and magnesium. Basalt makes up most of the ocean floor. It commonly forms when volcanic lava becomes solid. See Table at rock.