Why does rapid cooling produce small grains while slow cooling produces large grains?

Answer 1

In igneous rocks the grains are crystals of different minerals. When a crystal forms the particles take their places in a regular lattice, held together by forces of attraction between the particles. In the hot magma, the particles are moving too rapidly for these forces to prevail. When the liquid magma is cooled the particles slow down, and the forces cause them to align into the pattern we call a lattice. If cooling happens slowly, the particles lose energy slowly enough to allow them to find their places in the existing patterns, so quite large crystals build up. When cooling is fast, the particles slow down very rapidly and they can only align with the particles of the same kind which happen to be near them, so small crystals result. Think of a crowded hall full of soldiers dressed in different colours and all running around at random. They know that when the sergeant-major gives the order they have to form squares with others wearing the same colour. If they are given several minutes to do this, they have time to find the others wearing the same colour and take up their places, so you get a few, big squares. If they only have a few seconds then they can only find a small number of similarly dressed people and you'll get a lot of small groups.