Final answer:
A fast cooling rate in igneous rocks results in the formation of fine-grained crystals, while a slow cooling rate leads to the formation of coarse-grained crystals.
Step-by-step explanation:
A fast cooling rate in igneous rocks has the effect of forming fine-grained crystals. When magma cools quickly, such as when it reaches the Earth's surface as lava, there is little time for the crystals to grow, resulting in smaller grains. This is in contrast to slow cooling rates which allow for the formation of larger mineral grains, resulting in coarse-grained rocks.