Extrusive igneous rocks form when lava reaches the Earth's surface a volcano and cools quickly. Most extrusive (volcanic) rocks have small crystals. Examples include basalt, rhyolite, andesite, and obsidian. Metamorphism is a process that changes preexisting rocks into new forms because of increases in temperature, pressure, and chemically active fluids. Metamorphism may affect igneous, sedimentary, or other metamorphic rocks.