rock crystals query

[amyskinner]

Why do intrusive igneous rocks have larger crystals than extrusive ones?

[steveb]

If i remember geography correctly its because intrusive rocks cool slower which causes larger crystals to form than when the cool quickly. Im sorry i dont remember anything more than that

[Clanad]

Although steve is essentially correct (by the way, geography is the study of the Earth and its lands, features, inhabitants, and phenomena, while geology is the science and study of the solid and liquid matter that constitute the Earth... entirely pedantic, I know) but knowing where and how intrusive rocks form as opposed to extrusive rocks is instructive.

Intrusive igneous rocks form deep within the Earth, hence the slower cooling time, while extrusive rocks make there way to the surface, (such as magma in the form of lava) and cool much quicker, substantiating steve's reply.