In what different ways can the crystallization process be studied and observed?

• experimentally (melt, cool and quench in lab to arrest process at different stages).
• theoretically (physical chemistry).
• use of rock textures/associations.

What controls the grain size of an igneous rock?

• rate of cooling: faster = finer, slower = coarse.
• temperature contrast between igneous melt and surrounding rock or air or water determines how fast heat flows out of the melt, and hence the speed of cooling. Convection in water and air contributes to faster cooling.
• fluid content: wet = coarser, dry = finer.