Volcanic rocks are a type of igneous rock that is formed from molten lava or magma that has cooled and solidified. Basalts and lavas are two common types of volcanic rocks. Basalts are dark-colored, fine-grained igneous rocks that are composed mainly of plagioclase and pyroxene minerals. Lavas are molten rock that is expelled from a volcano during an eruption and then cools and solidifies on the surface.