The igneous rock, gabbro, most like formed from the molten material that cooled slowly, deep underground (4). Gabbro found underneath the thin layer of basalt that makes up the top of the earth's crust. Basalt is found at the surface of the earth's crust because the volcanic rock cools quickly. The rock underneath the basalt takes longer to cool and the grain is not as fine as Basalt's grain. This is because the lava cools slower; therefore, larger crystals have time to develop.