An igneous rock is formed from molten rock. Igneous rocks are often associated with volcanos. Intrusive igneous rocks form underground. Crystals in intrusive rocks are often very large. These rocks are found on the Earths' surface when they are exposed by erosion.
All intrusive rocks form slowly.
They cool slowly. Extrusive igneous rocks form on the surface. They cool quite quickly. Because they cool so quickly, they often do not have large crystals. In fact, they often do not have any crystals that can be seen.