Describe how you can distinguish between intrusive and extrusive igneous rocks?

Crystal size is used to distinguish between intrusive and extrusive igneous rocks. Intrusive rocks such as granite will have coarse crystals whereas extrusive rocks such as basalt will have fine glassy crystals. Extrusive rocks have glassy crystals as this requires rapid cooling under water. Furthermore, intrusive rocks will have baked margins in the country rock at both sides whereas extrusive rocks will have one baked margin in the country rock below only.