Igneous rocks are records of the thermal history of the Earth.
They tell us about conditions in the interior of the earth - conditions that we can only make inferences about from the rocks we observe. The origin of igneous rocks is closely tied to the theory of plate tectonics in which the rocks themselves are both explained by and help to reinforce the theory. The best known and most spectacular examples of igneous rocks are those that erupt on the surface in the form of molten magma from volcanoes. The Latin for igneous is from ignis meaning fire. Where the upward path of magma is halted and it solidifies below the surface - intrusive rocks form.