The term 'source of igneous rock' refers to the material from which igneous rocks are formed. This source is typically either LAVA or MAGMA. LAVA is molten rock that has been expelled from a volcano or fissure and cooled on the surface of the Earth. MAGMA is molten rock that is still beneath the surface of the Earth. Both LAVA and MAGMA are composed of minerals and other materials that have melted due to extreme heat and pressure.