Final answer:
Extrusive igneous rocks cool quickly, resulting in a fine-grained texture, while intrusive igneous rocks cool slowly, resulting in a coarse-grained texture.
Step-by-step explanation:
Extrusive igneous rocks cool quickly and as a result these rocks are fine grained or has lack of crystal growth.
Extrusive igneous rocks are formed when molten material flows onto the surface or into cracks near the Earth's surface and cools quickly, not giving enough time for large crystals to grow. As a result, these rocks have a very fine grain.
On the other hand, intrusive igneous rocks are formed when magma cools slowly below the earth's surface. This slow cooling gives individual crystals time and space to grow large, resulting in a coarse-grained texture.