The inside of the earth is very hot - hot enough to melt rocks. The deeper you get the hotter it gets. Below the surface, the molten rock is called magma; at the earth's surface it becomes lava, nothing has changed only the name of the liquid. Igneous rock is made by fire. Small wonder the magma from which igneous rock is formed can reach temperatures close to 1200 degrees Celsius. As the super hot magma cools, it solidifies to form a rock and that rock is called igneous rock. When most people think of igneous rock they vision a volcano erupting pumice and lava. Igneous rocks form directly by crystallization from materials from a magma melt. Igneous rocks crystalize after the magma reaches the earth's surface.
Igneous rock is divided into 2 groups, extrusive and intrusive. Extrusive rocks form when magma flows onto the surface of the earth or floor of the ocean through deep cracks or fissures and at volcanic vents, and then cools and hardens. Intrusive rock results when magma solidifies beneath the earth's surface in mines and tunnels or at the surface where it has been exposed by geological uplifting and by erosion. In general, extrusive rocks have a finer grained texture than intrusive rocks; intrusive rocks vary from thin sheets to huge, irregular masses. Since magma that forms intrusive rocks solidifies slowly, most intrusive rocks have larger crystals than extrusive rocks.