Igneous rocks solidify from a liquid magma as it cools.
When a magma cools rapidly, mineral crystals do not have time to grow very large. When a magma cools slowly crystals grow to several millimeters or more in size. Granite and basalt are opposites on these axies for the description of igneous rocks - granite is a slow cooled quartz rich rock, basalt a rapidly cooled mafic rock. Basaltic lava flow, Idaho Basalt is the stuff of the ocean floor. It comes from fresh upwellings from the mantle.