A coarse-grained rock and a fine-grained rock probably had different cooling rates during their formation.

Here's why:

* Coarse-grained rocks (like granite) form when magma cools slowly deep underground. This slow cooling allows large crystals to grow.

* Fine-grained rocks (like basalt) form when lava cools quickly on the surface. Rapid cooling doesn't give crystals much time to form, resulting in smaller crystals.

So, the size of the crystals in an igneous rock is a direct result of the rate at which the magma or lava cooled.