Basalt

Basalt is an extrusive igneous rock. Extrusive (volcanic) igneous rocks form when molten rock, which has erupted from the Earth's interior through a volcano or crack in the Earth, cools rapidly at the surface. The rapid cooling does not typically allow mineral crystals to grow large enough to be seen by the unaided eye, so texture is typically fine grained.

Basalt is dark-gray to almost black. It is very hard and fine grained. The common minerals in basalt are plagioclase (feldspar), pyroxene, and olivine. Some of these minerals may occur as phenocrysts (large grains).

Much of basalt in Texas is exposed in eroded volcanoes. A string of basaltic volcanoes were active in the Late Cretaceous in Central and South Texas, around 80 to 100 million years ago, and much younger basalts occur at several locations in the Trans-Pecos. Basalt is used as roadbed material for railroad tracks in many places in Texas and is commonly called trap rock.

The basalt sample in the Texas Rock Kit was collected in Uvalde County and is Cretaceous in age. In Texas, no volcanoes are considered to be currently active.