The size of crystals in igneous rocks can tell us about the physical environment under which the rock formed.
Igneous rocks are rocks that are derived from molten magma formed within the crust.
When large crystals are formed, it suggests that the magma has cooled slowly without any disturbance. Slow cooling of magma makes minerals to surround nucleation centers easily an take their time to form.
When small crystals are formed, we can infer that cooling was rapid. Magma had to cool rapidly and several nucleation centers resulted.
Large crystals in rocks are usually produced deep beneath the earth crust forming a rock type known as plutonic igneous rocks.
Those with small crystals that forms on the surface are called extrusive or volcanic igneous rocks.
