Basalt is a mafic extrusive igneous rock consisting of a glassy matrix with phenocrysts of plagioclase and sometimes olivine or pyroxene. Basalt containing many gas bubbles (vesicles) is called scoria. Basalt is commonly extruded along rift zones (mid-oceanic spreading centers) and crustal hot spots (like Hawaii). It’s typically dark grey to black in color but sometimes weathers to brown due to the high content of magnetite.