Why are some igneous rocks coarse and others are smooth

The reason why some igneous rocks are smooth and some are coarse is the manner in which they formed. We can simply divide the igneous rocks into intrusive and extrusive ones.

The intrusive igneous rocks are formed deep into the ground. The magma that has formed this rocks has been exposed to higher temperatures and pressures. That has led to a very slow cooling off of the magma. Because the magma has been cooling off slowly, the crystals had time and the right conditions to grow larger, thus making the intrusive igneous rocks coarse.

The extrusive igneous rocks are formed on the surface. They have been formed by the lava that once it come out on the surface has been exposed to much lower temperatures and pressures. That has led to very quick cooling off of the lava. Because the lava has cooled off quickly, the crystals didn't had time to grow in size, thus they have remained very small, almost unnoticeable, thus making the extrusive igneous rocks smoother.